U.S. patent application number 11/149302 was filed with the patent office on 2005-10-13 for production method for light emitting element.
This patent application is currently assigned to Shin-Etsu Handotai Co., Ltd.. Invention is credited to Ishizaki, Jun-ya.
Application Number | 20050224825 11/149302 |
Document ID | / |
Family ID | 27482244 |
Filed Date | 2005-10-13 |
United States Patent
Application |
20050224825 |
Kind Code |
A1 |
Ishizaki, Jun-ya |
October 13, 2005 |
Production method for light emitting element
Abstract
In a first invention, a p-type Mg.sub.xZn.sub.1-xO-type layer is
grown based on a metal organic vapor-phase epitaxy process by
supplying organometallic gases which serves as a metal source, an
oxygen component source gas and a p-type dopant gas into a reaction
vessel. During and/or after completion of the growth of the p-type
Mg.sub.xZn.sub.1-xO-type layer, the Mg.sub.xZn.sub.1-xO-type
thereof is annealed in an oxygen-containing atmosphere. This is
successful in forming the layer of p-type oxide in a highly
reproducible and stable manner for use in light emitting device
having the layer of p-type oxide of Zn and Mg. In a second
invention, a semiconductor layer which composes the light emitting
layer portion is grown by introducing source gases in a reaction
vessel having the substrate housed therein, and by depositing a
semiconductor material produced by chemical reactions of the source
gas on the main surface of the substrate. A vapor-phase epitaxy
process of the semiconductor layer is proceed while irradiating
ultraviolet light to the main surface of the substrate and the
source gases. This is successful in sharply enhancing reaction
efficiency of the source gases when the semiconductor layer for
composing the light emitting layer portion is formed by a
vapor-phase epitaxy process, and in readily obtaining the
semiconductor layer having only a less amount of crystal defects.
In a third invention, a buffer layer having at least an
Mg.sub.aZn.sub.1-aO-type oxide layer on the contact side with the
light emitting layer portion is grown on the substrate, and the
light emitting layer portion is grown on the buffer layer. The
buffer layer is oriented so as to align the c-axis thereof to the
thickness-wise direction, and is obtained by forming a metal
monoatomic layer on the substrate based on the atomic layer
epitaxy, and then by growing residual oxygen atom layers and the
metal atom layers. This is successful in obtaining the light
emitting portion with an excellent quality. In a fourth invention,
a ZnO-base semiconductor active layer included in a double
heterostructured, light emitting layer portion is formed using a
ZnO-base semiconductor mainly composed of ZnO containing Se or Te,
so as to introduce Se or Te, the elements in the same Group with
oxygen, into oxygen deficiency sites in the ZnO crystal possibly
produced during the formation process of the active layer, to
thereby improve crystallinity of the active layer. Introduction of
Se or Te shifts the emission wavelength obtainable from the active
layer towards longer wavelength regions as compared with the active
layer composed of ZnO having a band gap energy causative of shorter
wavelength light than blue light. This is contributive to
realization of blue-light emitting devices.
Inventors: |
Ishizaki, Jun-ya;
(Annaka-shi, JP) |
Correspondence
Address: |
SNIDER & ASSOCIATES
P. O. BOX 27613
WASHINGTON
DC
20038-7613
US
|
Assignee: |
Shin-Etsu Handotai Co.,
Ltd.
Tokyo
JP
100-0005
|
Family ID: |
27482244 |
Appl. No.: |
11/149302 |
Filed: |
June 10, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11149302 |
Jun 10, 2005 |
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10475489 |
Oct 22, 2003 |
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10475489 |
Oct 22, 2003 |
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PCT/JP02/04127 |
Apr 25, 2002 |
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Current U.S.
Class: |
257/94 ;
257/E21.463 |
Current CPC
Class: |
H01L 21/02507 20130101;
H01L 21/02579 20130101; H01L 21/02483 20130101; H01L 33/0083
20130101; C30B 29/16 20130101; C30B 29/16 20130101; H01L 21/02554
20130101; C30B 25/105 20130101; H01L 21/02477 20130101; C30B 25/105
20130101; C23C 16/40 20130101; C30B 29/16 20130101; C30B 25/02
20130101; H01L 21/02565 20130101; C30B 25/183 20130101; H01L
21/0262 20130101; H01L 21/0242 20130101; H01L 21/02472 20130101;
C30B 25/02 20130101; H01L 21/0248 20130101 |
Class at
Publication: |
257/094 |
International
Class: |
H01L 029/22 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 27, 2001 |
JP |
2001-131328 |
Apr 27, 2001 |
JP |
2001-131376 |
Jul 31, 2001 |
JP |
2001-232608 |
Aug 30, 2001 |
JP |
2001-260742 |
Claims
1-19. (canceled)
20. A light emitting device having a light emitting layer portion
composed of an Mg.sub.aZn.sub.1-aO-type (where,
0.ltoreq.a.ltoreq.1)-oxide and formed on a substrate, and having a
buffer layer formed between the substrate and the light emitting
layer portion, the buffer layer having at least an
Mg.sub.aZn.sub.1-aO-type oxide layer on the contact side with the
light emitting layer portion; the Mg.sub.aZn.sub.1-aO-type oxide
layer has wurtzite crystal structure in which metal atom layers and
oxygen atom layers are alternatively stacked in the direction of
the c-axis; and the buffer layer has the c-axis of the wurtzite
crystal structure oriented to the thickness-wise direction, has one
metal atom layer as a metal monoatomic layer formed in contact with
the substrate, and has the residual oxygen atom layers and the
metal atom layers alternatively stacked successive to the metal
monoatomic layer.
21. A light emitting device having a double heterostructured light
emitting layer portion which comprises an active layer and cladding
layers, wherein the active layer is composed of a Group II-VI
compound semiconductor containing Zn as a Group II element, and
containing O together with Se or Te as a Group VI element, and the
cladding layers are composed of Mg.sub.xZn.sub.1-xO-type (where,
0.ltoreq.x.ltoreq.1) oxide.
22. The light emitting device as claimed in claim 21, wherein the
active layer has a multi-layered structure in which sub-layers
composed of ZnSe or ZnTe are inserted in a main layer composed of
ZnO so as to be distributed over the thickness-wise direction.
23. The light emitting device as claimed in claim 22, wherein the
sub-layer has a width not larger than that of a unimolecular layer
of the active layer.
24. The light emitting device as claimed in claim 21, wherein the
cladding layers are composed of ZnO.
Description
TECHNICAL FIELD
[0001] This invention relates to a light emitting device and a
method of fabricating the same.
BACKGROUND ART
[0002] There have long been demands for high-luminance, light
emitting device capable of causing short-wavelength emission in the
blue light region. Such light emitting device has recently been
realized by using AlGaInN-base materials. Rapid progress has also
been made in applying the device to full-color, light emitting
apparatuses or to display apparatuses by combining it with red and
green high-luminance, light emitting devices. Use of the
AlGaInN-base material, however, inevitably raises the costs because
the material contains Ga and In as major components, both of which
are relatively rare metals. One of other major problems of the
material is that the growth temperature thereof is as high as 700
to 1,000.degree. C., and thus consumes a considerably large amount
of energy for the production. This is undesirable not only in terms
of cost reduction, but also in terms of being against the stream of
the times where discussions on energy saving and suppression of
global warming are prevailing. Japanese Laid-Open Patent
Publication No. 2001-44500 proposes a light emitting device having
a more inexpensive ZnO-base compound semiconductor layer
heteroepitaxially grown on a sapphire substrate. Japanese Laid-Open
Patent Publication No. 11-168262 discloses a two-dimensional-array
planar light emitting device using a light emitting layer portion
composed of oxides of Zn and Mg, or alloy thereof.
[0003] In addition, an InAlAsP/InGaAsP compound semiconductor laser
typically for use in transponders for submarine optical fiber
cables, of which specifications such as crystal defect density or
the like are very strictly regulated in order to realize a high
output and an high durability.
[0004] In all of these devices, semiconductor layers composing the
light emitting layer portion are formed by a vapor-phase epitaxy
process such as sputtering, molecular beam epitaxy (MBE) or metal
organic vapor phase epitaxy (MOVPE).
[0005] There is a problem that oxide layers of Zn and Mg are very
likely to cause oxygen deficiency, and they inevitably tend to have
an n-type conductivity, so that it is intrinsically difficult to
obtain the crystal having only a less amount of n-type carrier
(electrons) as a conductive carrier. Nevertheless, in the
fabrication of the electronic devices disclosed in the
above-described patent publications, it is essentially necessary to
form oxide layers of Zn and Mg having a p-type conductivity. These
oxide crystals, however, tend to have an n-type conductivity due to
oxygen deficiency as described in the above, and it has long been
believed as very difficult to form the p-type crystal or non-doped,
semi-insulating crystal used for the active layer. One possible
method may be such as adding p-type dopant, but conversion of an
n-type conductivity of a material into a p-type conductivity needs
a large amount of dopants in order to compensate the whole portion
of the existing n-type carriers and to excessively generate p-type
carriers, so that problems in stability, reproducibility and
uniformity of the electric characteristics remain unsolved.
[0006] Even for the case where the light emitting device is to be
fabricated by a vapor-phase epitaxy process using any compound
semiconductors other than the oxides of Zn and Mg (referred to as
ZnO-base oxide or MgZnO-base oxide, hereinafter), only a tiny
crystal defect ascribable to variation in reaction efficiency of
the source gases may cause failure especially in the aforementioned
InAIAsP/InGaAsP compound semiconductor laser, for which a very high
level of quality is required, and may considerably lower the
production yield.
[0007] ZnO-base oxide can be obtained by a vapor-phase epitaxy in a
vacuum environment, where heteroepitaxial growth process using a
substrate of a different origin, such as sapphire, is
unconditionally adopted because of difficulty in bulk single
crystal growth. It is therefore necessary to interpose an
appropriate buffer layer between the substrate and the light
emitting layer portion in order to attain a desirable crystallinity
of the light emitting layer portion as described in the above. The
aforementioned Japanese Laid-Open Patent Publication No. 2001-44500
discloses a method in which the buffer layer (contact layer) is
formed by MBE (Molecular Beam Epitaxy) process or MOVPE
(Metalorganic Vapour Phase Epitaxy) process similarly to the light
emitting layer to be formed in succession.
[0008] The MBE process, however, cannot readily suppress generation
of the oxygen deficiency due to its low pressure in the growth
atmosphere, so that it is very difficult for the process to form
the ZnO-base oxide layer which is indispensable for composing the
light emitting device. On the other hand, the MOVPE process can
arbitrarily vary partial pressure of oxygen during the growth, and
thus can suppress generation of the oxygen elimination or oxygen
deficiency by raising the atmospheric pressure to some extent. In
the MOVPE process generally proceeded in a continuous manner, even
if any accidental irregularity such as deficiency or dislocation of
the atoms should occur, the layer growth for the next layer and
thereafter continuously proceed while leaving the irregularity
unrepaired, so that the process could not always ensure a desirable
quality of the buffer layer which governs the crystal quality of
the light emitting layer portion, and this has consequently been
making it difficult to obtain the device having an excellent light
emission efficiency.
[0009] The aforementioned ZnO-base oxide will have a larger band
gap energy as alloy composition x of MgO (magnesium oxide) to ZnO
(zinc oxide) increases. For the case where the ZnO-base
semiconductor light emitting device, which comprises an MgZnO-type
oxide, is configured based on the double heterostructure, it is
therefore a general practice to compose the active layer with ZnO
in view of ensuring more effective confinement of carriers injected
thereto. The MgZnO-type oxide can be formed by the MOVPE process or
MBE process as described in the above, but the formation process
thereof is highly causative of oxygen deficiency of the MgZnO-type
oxide and can readily result in degradation of crystallinity of the
active layer composed of ZnO. This consequently expands total half
value width of the emission wavelength range ascribable to the
active layer, reduces the emission intensity, and suppresses the
emission efficiency for specific wavelength to be desired.
[0010] A first subject of the invention is, therefore, to provide a
method of fabricating a light emitting device having a ZnO-base
oxide layer, capable of growing the p-type oxide layer in a
reproducible and stable manner.
[0011] A second subject of the invention is to provide a method of
fabricating a light emitting device capable of drastically raising
reaction efficiency of the source gases when the semiconductor
layer composing the light emitting layer portion is formed by a
vapor-phase epitaxy process, and of readily realizing semiconductor
layers having a conductivity type which have not conventionally
been obtainable, and having only a less amount of crystal defects
and being high in quality.
[0012] A third subject of the invention is to provide a method of
fabricating a light emitting device capable of realizing a
high-quality, light emitting layer portion composed of a ZnO-base
oxide, and to provide also a light emitting device obtainable by
the method.
[0013] A fourth subject of the invention is to provide a light
emitting device using a ZnO-base oxide, of which active layer can
be formed with a high quality in an exact manner, and is further to
provide a high-performance, blue-color light emitting device at low
costs.
DISCLOSURE OF THE INVENTION
[0014] (First Invention)
[0015] A first invention is to solve the aforementioned first
subject, and is to provide a method of fabricating a light emitting
device having a light emitting layer portion which includes a
p-type Mg.sub.xZn.sub.1-xO (where, 0.ltoreq.x.ltoreq.1) layer,
wherein the p-type Mg.sub.xZn.sub.1-xO layer is grown by a metal
organic vapor-phase epitaxy process while supplying organometallic
gases, an oxygen component source gas and a p-type dopant gas into
a reaction vessel, and is annealed during and/or after completion
of the growth thereof in an oxygen-containing atmosphere.
[0016] In the first invention, the p-type Mg.sub.xZn.sub.1-xO layer
grown by a metal organic vapor-phase epitaxy process is annealed in
the oxygen-containing atmosphere during and/or after completion of
the growth. This effectively prevents the oxygen deficiency from
occurring, and successfully obtains a crystal having a less amount
of n-type carrier. It is therefore no more necessary to add an
excessive amount of p-type dopant for compensating the n-type
carrier, and this makes it possible to obtain the light emitting
device containing the p-type Mg.sub.xZn.sub.1-xO layer, excellent
in the stability, reproducibility and uniformity in the electrical
characteristics.
[0017] In order to obtain a high-luminance, light emitting device,
it is effective to compose the light emitting layer portion so as
to have a double heterostructure as described in the next. That is,
the light emitting layer portion is configured so as to have a
structure in which an n-type cladding layer, an active layer, and a
p-type Mg.sub.xZn.sub.1-xO (where, 0.ltoreq.x.ltoreq.1) layer are
stacked in this order. The method of fabricating a light emitting
device according to the first invention herein characteristically
comprises:
[0018] an n-type cladding layer growing step for growing the n-type
cladding layer; and
[0019] an active layer growing step for growing the active layer;
and
[0020] a p-type cladding layer growing step for growing the p-type
cladding layer by a metal organic vapor-phase epitaxy process while
supplying organometallic gases, an oxygen component source gas and
a p-type dopant gas into a reaction vessel, and annealing the
p-type cladding layer during and/or after completion of the growth
thereof in an oxygen-containing atmosphere. This method is
successful in realizing, a device showing high emission intensity
specific to the double heterostructure.
[0021] The light emitting layer portion can be configured so that
the n-type cladding layer composed of an n-type Mg.sub.zZn.sub.1-zO
(where, 0.ltoreq.z.ltoreq.1) layer, the active layer composed of a
Mg.sub.yZn.sub.1-yO (where, 0.ltoreq.y<1, x>y) layer, and the
p-type cladding layer composed of a p-type Mg.sub.xZn.sub.1-xO
(where, 0.ltoreq.x.ltoreq.1) layer are stacked in this order. In
the n-type cladding layer formation step herein, organometallic
gases and an oxygen component source gas are supplied into the
reaction vessel so as to allow the n-type cladding layer to grow on
the substrate based on a metal organic vapor-phase epitaxy process.
The active layer growing step herein is a step for growing the
active layer on a substrate by a metal organic vapor-phase epitaxy
process while supplying organometallic gases and an oxygen
component source gas into the reaction vessel, and includes a step
for annealing the layer during and/or after completion of the
growth thereof in an oxygen-containing atmosphere.
[0022] In the above-described method in which the active layer
composed of the Mg.sub.yZn.sub.1-yO layer and the p-type cladding
layer composed of the p-type Mg.sub.xZn.sub.1-xO layer are formed
by a metal organic vapor-phase epitaxy process, the annealing
carried out during and/or after completion of the growth of these
layers can effectively prevent the oxygen deficiency from occurring
within the layers, and is successful in readily obtaining the
crystal having only a less amount of n-type carrier. It is
therefore no more necessary for the p-type cladding layer to be
added with an excessive amount of p-type dopant for compensating
the p-type carrier, and it is made possible for the active layer to
suppress the carrier concentration and to raise the emission
recombination efficiency. This is also advantageous in that it can
largely reduce the costs, because all layers composing the light
emitting layer portion can be composed using the inexpensive
MgZnO-base oxide material. On the other hand, the growth process
for the n-type cladding layer does not adopt the aforementioned
annealing to thereby intentionally produce the oxygen deficiency
(note that a composite oxide obtained by partially replacing Zn in
ZnO with Mg is occasionally abbreviated as MgZnO in the description
below, this by no means indicates a condition of Mg:Zn:O=1:1:1,
where the same will apply also to the second to fourth
inventions).
[0023] It is now preferable to suppress the oxygen deficiency
concentration in the p-type MgZnO layer or MgZnO active layer to as
low as 10 sites/cm.sup.3 or below (0 site/cm.sup.3 not precluded).
In this case, it is very difficult for RF sputtering and molecular
beam epitaxy (MBE) to suppress generation of the oxygen deficiency,
since pressure in the growth atmosphere in these processes are as
low as 10.sup.-4 Torr to 10.sup.-2 Torr (1.3332.times.10.sup.-2 Pa
to 1.3332 Pa), so that it is substantially impossible for these
methods to grow the p-type MgZnO layer. On the contrary, a
vapor-phase epitaxy process based on the MOVPE process can
arbitrarily vary oxygen partial pressure during the growth, and
thus can suppress generation of the oxygen elimination or oxygen
deficiency by raising the atmospheric pressure to some extent.
[0024] When the annealing for suppressing generation of oxygen
deficiency is carried out, it is preferable to reduce as possible
the amount of supply of the organometallic gases than the amount of
supply adopted for the case where the layer growth is a matter of
preference, and it is more preferable to interrupt the supply, in
view of suppressing generation of the oxygen deficiency in the
layers. The oxygen-containing atmosphere during the annealing can
be created by introducing the oxygen component source gas (same as
that used for the layer growth based on the MOVPE process) into the
reaction vessel, which is efficient because the annealing can be
completed within the same reaction vessels used for the layer
growth.
[0025] The annealing may be carried out after completion of the
layer growth, but it may be difficult for the annealing after the
completion to fully remove the oxygen deficiency which remains deep
inside the layer if the oxygen deficiency is accidentally formed in
the process of the layer growth. It is therefore effective to carry
out the annealing during the layer growth, and more preferably to
alternatively repeat the intermittent layer growth and the
annealing in the oxygen-containing atmosphere for the purpose of
more effective suppression of the oxygen deficiency. In this case,
the aforementioned repetition of the intermittent layer growth and
annealing will be more efficient if the layer to be annealed is
grown while continuously supplying the oxygen component source gas
and intermittently interrupting supply of the organometallic gases,
to thereby make use of the time duration of interrupted supply of
the organometallic gases as an effective duration of the
annealing.
[0026] Next, the annealing for suppressing the oxygen deficiency
for the p-type MgZnO layer or the MgZnO active layer must be
carried out in the oxygen-containing atmosphere having an oxygen
partial pressure higher than dissociation oxygen pressure of MgZnO
(where, oxygen-containing molecules other than O.sub.2 are to be
included after converting the component oxygen into O.sub.2). In an
atmosphere having an oxygen partial pressure lower than the
dissociation oxygen pressure of MgZnO, it is impossible to prevent
the oxygen deficiency from occurring due to promoted decomposition
of MgZnO. The oxygen partial pressure adaptable to the annealing is
more preferably 1 Torr (133.32 Pa) or above. While there is no
special limitations on the upper limit of the oxygen partial
pressure, the pressure is preferably set within a range not
causative of unnecessary rise in costs of the annealing facility
(typically set at 7,600 (1.013 MPa) Torr or around for the
annealing in the reaction vessel).
[0027] (Second Invention)
[0028] A second invention is to solve the aforementioned second
subject, and is to provide a method of fabricating a light emitting
device having a step of growing a semiconductor layer for composing
a light emitting layer portion in vapor phase by introducing source
gases in a reaction vessel having a substrate disposed therein, and
by allowing a semiconductor material generated based on chemical
reactions of the source gases to deposit oh the main surface of the
substrate, wherein a vapor-phase epitaxy process of the
semiconductor layer is proceeded while irradiating ultraviolet
light to the source gases introduced in the reaction vessel.
[0029] Because the chemical reactions for producing the
semiconductor material from the source gases is promoted by
ultraviolet irradiation in the second invention, the semiconductor
material will be less causative of crystal defects or the like
during deposition on the main surface of the substrate, and will
readily realize the semiconductor layer having only a less amount
of crystal defects.
[0030] In the production of the semiconductor material through the
chemical reactions of the source gases, a reaction system
containing the source gases needs be transferred into a reactive
transition state having a high enthalpy. If the amount of energy
required for causing transfer to the transition status is not
supplied, unreacted or incompletely-reacted components of the
source gases will increase components causing adsorption within the
layer and will be causative of the crystal defects. Although the
necessary energy might be supplemented by heat energy, this
requires rise in the temperature of the system. An excessive rise
in the temperature of the substrate however ruin adsorption ratio
of the semiconductor material contributable to the crystal growth,
and undesirably results in formation of the layers only having a
large amount of crystal defects. In contrast, combined use of the
ultraviolet irradiation described in the above is successful in
securing a necessary and enough energy for completing the
generation reactions of the semiconductor material without
excessively raising the temperature of the system, and in forming
the semiconductor layer having only a less amount of crystal
defects.
[0031] In this case, one possible system is such as having a
ultraviolet light source disposed so as to oppose with the main
surface of the substrate, in which the source gases are supplied
between the substrate and the ultraviolet light source while
irradiating ultraviolet light towards the main surface. This is
successful in selectively accelerating the generation reactions of
the semiconductor material from the source gases in the vicinity of
the main surface of the substrate. Ultraviolet light irradiated to
the substrate is once absorbed by the substrate, and can highly
activate the outermost portion of the layers under growth based on
the light excitation effect. More specifically, it is supposed that
a highly activated status similar to that obtainable by the layer
growth under a high temperature is locally realized in the
outermost portion of the layers, and this makes it possible to
efficiently proceed the layer growth while suppressing thermal
decomposition of the source gas components in the vapor phase.
[0032] In one rational method of irradiating ultraviolet light to
the source gases or the substrate in the reaction vessel, a part of
the wall portion of the reaction vessel opposing to the main
surface of the substrate is configured as a transparent wall
portion, the ultraviolet light source is disposed outside the
reaction vessel, and ultraviolet light from the ultraviolet light
source is irradiated towards the main surface through the
transparent wall portion. According to this configuration, the
ultraviolet light source can be disposed outside the reaction
vessel, and this prevents the light source per se from being
adversely affected by corrosion or deposited reaction products, and
elongates the service life of the apparatus.
[0033] Although any vapor-phase epitaxy processes may be applicable
so far as they can correlate the chemical reactions to the layer
growth, a metal organic vapor-phase epitaxy (MOVPE) process is
particularly preferable because of its potential of efficiently
growing a high-quality oxide semiconductor or compound
semiconductor. While the MBE process is one possible method other
than the MOVPE process, the MOVPE process can more advantageously
be adopted to the formation of the oxide semiconductor layer
described below because it is more unlikely cause the oxygen
deficiency.
[0034] In a metal organic vapor-phase epitaxy process, the
semiconductor layer composed of the metal oxide can be formed by
using organometallic gases and an oxygen component source gas as
the source gases, based on chemical reactions of the organometallic
gases with the oxygen component source gas. In the formation of the
oxide semiconductor, any unreacted or incompletely-reacted oxygen
component source gas incorporated into the layer by adhesion will
be causative of the oxygen deficiency after elimination of the
component. The oxygen deficiency emits an electron as a carrier,
and thus inevitably makes the conductivity type of the resultant
layer n-type. This is a serious non-conformity in formation of
p-type layer or insulating (non-doped) layer indispensable for
forming the light emitting layer portion. Adoption of the second
invention herein is successful in effectively suppressing
generation of the oxygen deficiency. The oxide semiconductor layer
thus formed is exemplified by Mg.sub.xZn.sub.1-xO (where,
0.ltoreq.x.ltoreq.1) layer. Use of the Mg.sub.xZn.sub.1-xO layer
makes it possible to readily form a light emitting device capable
of ensuring high luminance light emission in the blue light region
or ultraviolet region.
[0035] Adoption of the second invention is successful in
effectively suppressing the oxygen deficiency, and is consequently
successful in readily obtaining the crystal having only a less
amount of n-type carrier. It is therefore no more necessary to add
an excessive amount of p-type dopant for compensating the n-type
carrier, and this makes it possible to obtain the light emitting
device containing the p-type Mg.sub.xZn.sub.1-xO layer, excellent
in the stability, reproducibility and uniformity in the electrical
characteristics.
[0036] More specifically, the light emitting layer portion can be
configured so as to have a double heterostructure in which the
n-type cladding layer composed of an n-type Mg.sub.zZn.sub.1-zO
(where, 0.ltoreq.z.ltoreq.1) layer, the active layer composed of a
Mg.sub.yZn.sub.1-yO (where, 0.ltoreq.y<1, x>y) layer, and the
p-type cladding layer composed of a p-type Mg.sub.xZn.sub.1-xO
(where, 0.ltoreq.x.ltoreq.1) layer are stacked in this order. In
this case, the n-type cladding layer can readily be formed by
supplying the organometallic gases and an oxygen component source
gas into the reaction vessel, without specifically irradiating
ultraviolet light. The active layer can be formed by supplying the
organometallic gasses and an oxygen component source gas into the
reaction vessel with irradiating ultraviolet light. The p-type
cladding layer can be formed by additionally supplying a p-type
dopant gas in the process similar to that for the active layer.
[0037] Also in the second invention, it is preferable to suppress
the oxygen deficiency concentration in the p-type MgZnO layer or
MgZnO active layer to as low as 10 sites/cm.sup.3 or below (0
site/cm.sup.3 not precluded), and a vapor-phase epitaxy process
based on the MOVPE process is preferable in view of suppressing the
oxygen deficiency.
[0038] The second invention is also applicable to fabrication of
compound semiconductor light emitting devices other than those
using the MgZnO-base oxide, such as InAIAsP/lnGaAsP compound
semiconductor light emitting device (laser device, in
particular).
[0039] (Third Invention)
[0040] A third invention is to solve the aforementioned third
subject, and includes a method of fabricating a light emitting
device and thus-fabricated, light emitting device. The method of
fabricating a light emitting device of the third invention is such
as fabricating a light emitting device having a light emitting
layer portion composed of an Mg.sub.aZn.sub.1-aO-type (where,
0.ltoreq.a.ltoreq.1) oxide, wherein a buffer layer is formed on a
substrate, the buffer layer having at least an
Mg.sub.aZn.sub.1-aO-type oxide layer on the contact side with the
light emitting layer portion, and the light emitting layer portion
is grown on the buffer layer;
[0041] the Mg.sub.aZn.sub.1-aO-type oxide layer has wurtzite
crystal structure in which metal atom layers and oxygen atom layers
are alternatively stacked in the direction of the c-axis, the
buffer layer is grown so as to orient the c-axis of the wurtzite
crystal structure to the thickness-wise direction, and so as to
form a metal atom layer as a metal monoatomic layer on the
substrate by the atomic layer epitaxy, and then to form the
residual oxygen atom layers and metal atom layers.
[0042] The light emitting device of the third invention is such as
having a light emitting layer portion composed of an
Mg.sub.aZn.sub.1-aO-type (where, 0.ltoreq.a.ltoreq.1) oxide and
formed on a substrate, and having a buffer layer formed between the
substrate and the light emitting layer portion, the buffer layer
having at least an Mg.sub.aZn.sub.1-aO-type oxide layer on the
contact side with the light emitting layer portion;
[0043] the Mg.sub.aZn.sub.1-aO-type oxide layer has wurtzite
crystal structure in which metal atom layers and oxygen atom layers
are alternatively stacked in the direction of the c-axis; and
[0044] the buffer layer has the c-axis of the wurtzite crystal
structure oriented to the thickness-wise direction, has a single
atom layer portion as a metal monoatomic layer formed in contact
with the substrate, and has the residual oxygen atom layers and
metal atom layers alternatively stacked successive to the metal
monoatomic layer.
[0045] In the third invention, the entire portion or at least a
portion on the contact side with the light emitting layer portion
of the buffer layer formed on the substrate is composed of an
Mg.sub.aZn.sub.1-aO-type oxide (where, alloy composition a is not
always same with that of the light emitting layer portion, and the
oxide may occasionally be referred to as MgZnO-type oxide or simply
as MgZnO, while omitting indication of the alloy composition a).
Because the portion on the junction interface side of the buffer
layer and the light emitting layer portion have basically the same
crystal structure (wurtzite crystal structure) and the same
component system, local irregularity of the crystal structure due
to interaction between the components over the junction interface
becomes less likely to occur, and this is advantageous in realizing
the light emitting layer portion having a desirable crystallinity.
Typically the entire portion of the buffer layer may be composed of
the MgZnO-type oxide. This makes it possible to carry out a
vapor-phase epitaxy process of the buffer layer and light emitting
layer portion in the same facility in an extremely simple
manner.
[0046] In the third invention, the buffer layer is formed
particularly so as to form a metal atom layer as a metal monoatomic
layer on the substrate by the atomic layer epitaxy (ALE) process,
and then to form the residual oxygen atom layers and metal atom
layers. By adopting the ALE process, formation of the metal atom
layer can be saturated once a single atomic layer is completed
(so-called, self-termination function), and the atoms arranged in
the layer are less likely to cause any irregularity such as
deficiency or dislocation. By forming a single layer of the
less-irregular metal atom layer and then forming the succeeding
metal atomic layers and oxygen atom layers, it is made possible to
obtain the buffer layer having an excellent crystallinity. This
consequently improves the crystallinity of the light emitting layer
portion formed thereon, and is advantageous in realizing a
high-performance, light emitting device. By adopting the
above-described method, the light emitting device of the third
invention will have the c-axis of the wurtzite crystal structure
oriented to the thickness-wise direction, will have the single atom
layer portion in contact with the substrate formed as a metal
monoatomic layer, and will have the residual oxygen atom layers and
metal atom layers alternatively formed in succession to the metal
monoatomic layer. Thus-configured buffer layer has an excellent
crystallinity, and this makes it possible to realize the light
emitting layer portion having only a less amount of defects and
irregularity, and having a desirable emission efficiency.
[0047] The ALE process can be carried out in a form of a metal
organic vapor-phase epitaxy (MOVPE) process in which an
organometallic compound gas and an oxygen component source gas are
supplied in a reaction vessel having a substrate disposed therein.
More specifically, only an organometallic compound gas, which
serves as a source material for the metal atom layer, is allowed to
flow through the reaction vessel to thereby form the first metal
atom layer for composing the buffer layer so as to be saturated by
a single atom layer, to thereby form a metal monoatomic layer. As
shown in FIG. 16A, organometallic compound (MO) molecule causes
decomposition or elimination of organic groups bound thereto, and
allows its metal atom to chemically adsorb onto the substrate.
Under the ALE process, the metal atom is adsorbed while keeping a
part of its organic groups unremoved, and as shown in FIG. 16B,
forms the metal atom layer so as to orient the residual organic
group towards the upper-surface. Once the first single atomic layer
is completed, thus-oriented organic groups can inhibit adhesion of
newly-coming metal atoms and can fully exhibit the self-termination
function, so that the atoms arranged in the layer will become very
unlikely to cause irregularities such as deficiency and
dislocation.
[0048] In the MOVPE process, oxygen partial pressure during the
growth can arbitrarily be varied, so that generation of the oxygen
elimination or oxygen deficiency is effectively avoidable by
raising the atmospheric pressure to some extent. This consequently
makes it possible to form the p-type Mg.sub.aZn.sub.1-aO layer
indispensable for the light emitting device, in particular the
p-type Mg.sub.aZn.sub.1-aO layer such as having a density of oxygen
deficiency of as small as 10 sites/cm.sup.3 or below. The smaller
density of oxygen deficiency is the better (that is, 0
site/cm.sup.3 not precluded).
[0049] When the MOVPE process is adopted, composition of the entire
portion of the buffer layer using MgZnO-type oxide is advantageous,
because the buffer layer and light emitting layer portion can be
grown sequentially in the same reaction vessel only by adjusting
ratio of the organometallic gasses and oxygen component source gas.
This is also advantageous in that the purging of the vessel between
growth processes for the buffer layer and light emitting layer
portion needs only a short time as compared with the case where the
buffer layer is formed using different materials such as GaN, or
the purging per se is omissible.
[0050] Also in the third invention, it is effective to grow the
light emitting layer portion so as to have a double heterostructure
as described below, in order to obtain a high-luminance, light
emitting device. That is, the double heterostructured, light
emitting layer portion is formed on the buffer layer by
sequentially stacking a first-conductivity-type cladding layer
(p-type or n-type) composed of Mg.sub.aZn.sub.1-aO-type oxide, an
active layer, and a second-conductivity-type cladding layer (n-type
or p-type) having a conductivity type different from that of the
first-conductivity-type cladding layer, in this order.
[0051] (Fourth Invention)
[0052] A fourth invention is to provide a light emitting device for
solving the fourth subject. The light emitting device has a double
heterostructured, light emitting layer portion which comprises an
active layer and cladding layers, wherein the active layer is
composed of a Group Il-VI compound semiconductor containing Zn as a
Group II element, and containing O together with Se or Te as a
Group VI element, and the cladding layers are composed of
Mg.sub.xZn.sub.1-xO-type (where, 0.ltoreq.x.ltoreq.1) oxide.
[0053] In the double hetero-type, ZnO-base semiconductor light
emitting devices composed of an MgZnO-base oxide, those having the
active layer composed of ZnO, having a band gap energy of 3.25 eV,
causes light emission in near violet color. To adjust the band gap
energy suitable for blue-color light emission, it is necessary to
add some impurity to the ZnO active layer to thereby form impurity
levels, or to configure the active layer using a ZnO-base alloyed
compound semiconductor having a smaller band gap energy than ZnO
has.
[0054] To achieve blue-color light emission with high emission
efficiency, it is necessary for the active layer to satisfy the
above-described constitutional conditions, and to stabilize the
crystallinity. In view of stabilizing the crystallinity of the
active layer composed of a ZnO-base semiconductor mainly containing
ZnO, an essential point resides in that how successfully the oxygen
deficiency can be suppressed when the active layer is stacked by
epitaxially growing the ZnO-base semiconductor typically based on
the MOVPE or MBE process.
[0055] The active layer in the fourth invention is formed using
Group II-VI compound semiconductor (aforementioned ZnO-base
semiconductor) containing Zn (zinc) as a Group II element, and
containing O (oxygen) together with Se (selenium) or Te (tellurium)
as a Group VI element, and this makes it possible to introduce Se
or Te, which belongs to the same Group with oxygen, into the
oxygen-deficient sites. For the case where the introduced Se or Te
acts as an impurity, Zn--Se pair or Zn--Te pair is supposed to form
a deeper impurity level than ZnO forms, so that blue-color light
emission With a higher efficiency than that given by ZnO-base
semiconductor can be obtained.
[0056] In the active layer composed of ZnO-base semiconductor, Se
or Te introduced into the oxygen-deficient sites may not exist in a
form of impurity, but may form a local crystal structure of ZnOSe
or ZnOTe which is different from ZnO. Both of the ZnOSe crystal and
ZnOTe crystal have smaller band gap energies as compared with that
of ZnO crystal, and can form the active layer capable of blue-color
light emission at a higher efficiency. The emission possibly
obtained via the impurity levels results in saturation of effect of
improving the emission efficiency due to a limited range of
formation of Zn--Se pair or Zn--Te pair which is causative of the
impurity levels. On the other hand, the emission possibly obtained
via the bands formed by the ZnOSe crystal or ZnOTe crystal results
in further increase in the emission efficiency.
[0057] The double heterostructure adopted for the light emitting
device of the fourth invention is such as having the active layer,
which is composed of the aforementioned Se- or Te-containing,
ZnO-base semiconductor, sandwiched between the cladding layers
which are composed of Mg.sub.xZn.sub.1-xO-type
(0.ltoreq.x.ltoreq.1) oxide having a band gap energy larger than
that of the active layer. The Mg.sub.xZn.sub.1-xO-type-
-(0.ltoreq.x.ltoreq.1) oxide will have a larger band gap energy as
MgO alloy composition x increases, but will also have a larger
insulating property. Increase in MgO alloy composition x,
therefore, makes it difficult to dope an effective number of
carriers into the cladding layer. It is in particular difficult for
ZnO, having an n-type conductivity in a non-doped status, to form
the p-type cladding layer which should be doped with p-type
carriers. In contrast to that the active layer has been formed by
using ZnO, the active layer in the fourth invention is formed by
using the Se- or Te-containing, ZnO-base semiconductor having a
band gap energy smaller than that of ZnO, so that it is made
possible to configure the cladding layer using the
Mg.sub.xZn.sub.1-xO-base oxide of which ZnO or MgO alloy
composition x is suppressed to a low level. This consequently makes
it possible to dope an effective number of carriers into the
cladding layer, to dope an effective number of carriers also into
the active layer, and to improve the emission efficiency.
[0058] When the active layer is composed of ZnOSe crystal or ZnOTe
crystal, the ZnOSe crystal or ZnOTe crystal will have a smaller
band gap energy as the ratio of Se or Te to O increases, and thus
the emission wavelength becomes shorter. A band gap energy suitable
for blue-color light emission falls within a range from 2.52 to
3.15 eV, where the largest band gap energy of 3.15 eV suitable for
blue-color light emission can be attained typically by adjusting a
ratio of O and Se to 61:39 for the ZnOSe crystal, and by adjusting
a ratio of O and Te to 81:19 for the ZnOTe crystal. Because ZnO has
a band gap energy of 3.25 eV, the cladding layer can be formed by
using ZnO without suppressing the carrier confinement effect in the
active layer. By composing the cladding layer with ZnO, the
cladding layer and active layer will have ZnO as a major
constituent thereof, and this not only makes it possible to improve
working efficiency in the fabrication, but also makes it
unnecessary to use excessive Mg, and contributes to cost
reduction.
[0059] Beside the above-described blue-color light emission, it is
also possible to obtain band gap energy suitable for emission at
longer wavelength regions such as blue-green to green regions, by
adjusting ratio of Se and Te to O in the ZnOSe crystal or ZnOTe
crystal. Since the band gap energy of the active layer in this case
is smaller than that suitable for blue-color light emission, the
cladding layer can be composed by using ZnO.
[0060] The active layer of the ZnO-base semiconductor light
emitting device of the fourth invention can be configured as having
a multi-layered structure in which sub-layers composed of ZnSe or
ZnTe are inserted in a main layer composed of ZnO so as to be
distributed over the thickness-wise direction.
[0061] As described in the above, when the active layer is formed
by epitaxially growing ZnO-base semiconductor, crystallinity of the
active layer can be improved by introducing Se or Te, which belongs
to the same Group with oxygen, to oxygen-deficient sites. It is
also possible to shift the emission wavelength of the active layer
to the longer wavelength region. While the active layer may be
configured as a single layer composed of Se- or Te-containing
ZnO-base semiconductor, adoption of the above-described,
multi-layered structure, which is typified by a structure in which
the sub-layers composed of ZnSe or ZnTe, and having a width not
larger than that of a unimolecular layer of the active layer, are
inserted in a main layer composed of ZnO, ensures the effects
described in the next. Thus-formed sub-layer can function as a
.delta. doped layer and can localize Se or Te in the thickness-wise
direction, and this makes it possible to enhance effect of
introducing Se or Te to the oxygen-deficient sites. This enhances
binding tendency with Zn in the closest vicinity, and raises
probability of forming Zn--Se pair or Zn--Te pair, or of forming
ZnOSe crystal or ZeOTe crystal. Even if the devices are not
introduced into the oxygen-deficient sites, it is made possible to
prevent non-luminescent center caused by unmatched interface or
dislocation, by suppressing formation of different crystal phases
such as ZnSe and ZnTe. If the coverage ratio of the sub-layer is
controlled so as to be smaller than a unimolecular layer of the
active layer, Se or Te is successfully prevented from depositing as
an impurity rather than being incorporated into the
oxygen-deficient sites.
[0062] Because the number of layers of the sub-layers to be
inserted into the active layer can properly be selected depending
on the band gap energy, and more specifically on the ratio of Se or
Te to O in the ZnOSe crystal or ZnOTe crystal for composing the
active layer, and is not specifically limited. It is, however,
preferable of course that effect of introduction of Se or Te can
uniformly extend over the active layer in view of obtaining a
uniform light emission therefrom. It is therefore preferable to
form the sub-layers so as to be distributed over the thickness-wise
direction, and typically in a periodical manner.
[0063] Other conditions commonly applicable to the first to fourth
inventions will be described.
[0064] The growth of the p-type MgZnO layer or MgZnO active layer
based on the MOVPE process can more advantageously be proceeded
under an atmosphere conditioned at a pressure of 10 Torr (1.3332
kPa) or above, so as to more effectively suppress generation of the
oxygen deficiency during the film formation, and to obtain the
p-type MgZnO layer or MgZnO active layer having desirable
characteristics. It is more preferable herein to adjust oxygen
partial pressure (including any other oxygen-containing molecules
other than O.sub.2, after converting component oxygen to O.sub.2)
to 10 Torr (1.3332 kPa) or above. For the case where the n-type
MgZnO layer is formed on the buffer layer, and further thereon the
MgZnO active layer and p-type MgZnO layer is formed, any oxygen
deficiencies generated in the n-type MgZnO may be causative of
irregularity or the like in the MgZnO active layer and p-type MgZnO
layer formed thereafter, so that it is preferable that also the
n-type MgZnO layer is grown so as to suppress the oxygen deficiency
as possible. In this case, the n-type MgZnO layer is added with an
n-type dopant so as to have the conductivity type of n-type. On the
other hand, for the case where the p-type MgZnO layer is formed on
the buffer layer, and further thereon the MgZnO active layer and
n-type MgZnO layer are formed, it is also allowable to
intentionally form the oxygen deficiency in the n-type MgZnO layer
so as to have an n-type conductivity.
[0065] To make Mg.sub.aZn.sub.1-aO to have a p-type conductivity,
it is necessary to add an appropriate p-type dopant as described in
the above. As the p-type dopant, either one of, or two or more of
N, Ga, Al, In, Li, Si, C, and Se are available. Among these, use of
N is particularly preferable in view of obtaining desirable p-type
characteristics. As the metal element dopant, either one of, or two
or more of Ga, Al, In and Li are available, where Ga is
particularly effective. Combined addition of these dopants with N
can ensure desirable p-type characteristics in a more reliable
manner.
[0066] To ensure sufficient emission characteristics, p-type
carrier concentration in the p-type Mg.sub.aZn.sub.1-aO layer
preferably falls within a range from 1.times.10.sup.16
sites/cm.sup.3 to 8.times.10.sup.13 sites/cm.sup.3. The p-type
carrier concentration less than 1.times.10.sup.16 sites/cm.sup.3
may make it difficult to obtain a sufficient emission luminance. On
the other hand, the p-type carrier concentration exceeding
8.times.10.sup.18 sites/cm.sup.3 may excessively increase the
amount of p-type carriers injected to the active layer, and this is
causative of increase in p-type carrier not contributable to the
light emission due to reverse diffusion into the p-type
Mg.sub.aZn.sub.1-aO layer, or injection into the n-type cladding
layer after getting over the potential barrier, to thereby lower
the emission efficiency. Also for the n-type Mg.sub.aZn.sub.1-aO
layer, it is preferable to adjust n-type carrier concentration
within a range from 1.times.10.sup.16 sites/cm.sup.3 to
8.times.10.sup.18 sites/cm.sup.3 based on the same reason.
[0067] Examples of materials available for substrate include
aluminum oxide, gallium oxide, magnesium oxide, gallium nitride,
aluminum nitride, silicon, silicon carbide, gallium arsenide,
indium-tin composite oxide and glass. Particularly preferable forms
of the substrate include the followings. As shown in FIG. 2,
Mg.sub.aZn.sub.1-aO-type oxide has wurtzite crystal structure
comprising metal atom layers and oxygen atom layers alternatively
stacked in the direction of c-axis, where the oxygen atoms follow a
hexagonal atomic arrangement. The substrate is, therefore,
preferably an oxide single crystal substrate in which oxygen atoms
follow the hexagonal atomic arrangement, and the C-plane ((0001)
plane) of the hexagonal atomic arrangement is exposed to the main
surface, in terms of improving crystal matching with the buffer
layer, and of obtaining the light emitting layer portion with a
desirable crystallinity. In this case, the buffer layer is composed
of the Mg.sub.aZn.sub.1-aO-type oxide over the entire portion
thereof, and is formed on the main surface of the oxide single
crystal substrate so as to orient the c-axis of its wurtzite
crystal structure in the thickness-wise direction. Examples of such
oxide single crystal substrate include those composed of
corundum-structured oxide, where a sapphire substrate is one
specific example thereof.
[0068] As shown in FIG. 15, in an oxide having corundum-type
structure, a lattice of oxygen (O) atoms has a hexagonal atomic
arrangement, and in the direction of c-axis thereof, O atom (ion)
layers and metal atom (ion: shown as Al in the drawing) layers are
alternatively stacked. In this crystal structure, one of both
atomic layers appearing on both ends in the direction of c-axis
will always be an oxygen atom layer plane, and the other will
always be a metal atom layer plane. The O atom layer plane has the
same O atomic arrangement with the O atom layer in the wurtzite
crystal structure except for difference in the lattice constants.
For the case where the main surface of the oxide single crystal
substrate having such crystal structure will have formed thereon
the buffer layer comprising Mg.sub.aZn.sub.1-aO-type oxide having
the wurtzite crystal structure, a junction structure having better
matching property can be obtained by stacking the metal atom layer
of the buffer layer on the main surface of the substrate composing
the O atom layer plane.
[0069] It is to be noted that it is also allowable to grow the
light emitting layer portion on the A-plane of the sapphire
substrate as disclosed in Japanese Laid-Open Patent Publication No.
2001-44500, and this is effective to a certain extent in terms of
planarization of the crystal growth surface. Because the A-plane of
the sapphire substrate has metal atoms and oxygen atoms exposed
thereon in a mixed manner, the general continuous-growth-type MOVPE
process may raise probability of causing adsorption of oxygen atoms
and zinc atoms at the same time on the A-plane ((11-20) plane).
This is more likely to cause irregularity in the stacking of the
buffer layer grown based on the c-axis orientation, and is not
always successful in obtaining a high-quality buffer layer and
light emitting layer portion. Use of the ALE process, as in the
third invention, is now successful in obtaining a high-quality
buffer layer and, consequently, the light emitting layer portion in
a highly reproducible manner, because the metal monoatomic layer
can be formed also on the A-plane in a forced manner.
BRIEF DESCRIPTION OF THE DRAWINGS
[0070] FIG. 1 is a conceptual drawing of a double heterostructured,
light emitting layer portion including a p-type MgZnO layer;
[0071] FIG. 2 is a schematic drawing of a crystal structure of
MgZnO;
[0072] FIG. 3 is a schematic drawing of an arrangement of the metal
ions and oxygen ions in the MgZnO layer;
[0073] FIG. 4 is a schematic band chart of a light emitting device
using a junction structure of Type-I band lineup;
[0074] FIG. 5A is schematic drawings for explaining a growth
process of the light emitting layer portion of the light emitting
device having a type shown in FIG. 4 in Embodiment 1 of the
invention;
[0075] FIG. 5B is a schematic sectional view of a reaction vessel
shown in FIG. 5A;
[0076] FIG. 6 is a drawing for explaining an exemplary fabrication
process of the light emitting device having a type shown in FIG.
4;
[0077] FIG. 7A is a drawing for explaining operation of the method
of fabricating the light emitting device of the Embodiment 1 of the
invention;
[0078] FIG. 7B is an explanatory drawing as continued from FIG.
7A;
[0079] FIG. 7C is an explanatory drawing as continued from FIG.
7B;
[0080] FIG. 8A is a drawing of a first example of a supply sequence
for an organometallic gas and an oxygen component source gas in the
process shown in FIG. 5A;
[0081] FIG. 8B is a drawing of a second example of the same;
[0082] FIG. 8C is a drawing of a third example of the same;
[0083] FIG. 8D is a drawing of a fourth example of the same;
[0084] FIG. 9A is a drawing of a fifth example of the same;
[0085] FIG. 9B is a drawing of a sixth example of the same;
[0086] FIG. 10A is a schematic band chart of a light emitting
device using a junction structure of Type-I and Type-II band
lineups;
[0087] FIG. 10B is a schematic band chart of another example;
[0088] FIG. 11A is a schematic drawing for explaining a vapor phase
growth apparatus for growing the light emitting layer portion using
a ultraviolet lamp based on the MOVPE process in Embodiment 2 of
the invention;
[0089] FIG. 11B is a schematic drawing of a modified example of
FIG. 11A;
[0090] FIG. 12 is a conceptual drawing of a vapor-phase epitaxy
process for forming the light emitting layer portion using
ultraviolet laser beam;
[0091] FIG. 13 is a schematic drawing showing a specific example of
the light emitting device of Embodiment 3 of the invention;
[0092] FIG. 14A is a drawing for explaining an exemplary
fabrication process of the light emitting device shown in FIG.
13;
[0093] FIG. 14B is a drawing of a process step as continued from
FIG. 14A;
[0094] FIG. 14C is a drawing of a process step as continued from
FIG. 14B;
[0095] FIG. 15 is a schematic drawing of corundum-type crystal
structure;
[0096] FIG. 16A is a drawing for explaining operation of a method
of fabricating a light emitting device of Embodiment 3 of the
invention;
[0097] FIG. 16B is a drawing for explaining operation as continued
from FIG. 16A;
[0098] FIG. 16C is a drawing for explaining operation as continued
from FIG. 16B;
[0099] FIG. 16D is a drawing for explaining operation as continued
from FIG. 16C;
[0100] FIG. 17 is a drawing exemplifying a temperature control
sequence and gas supply sequence in the process steps shown in
FIGS. 14A to 14C;
[0101] FIG. 18A is a drawing for explaining effect of configuring a
mixed metal atom layer as a metal monoatomic layer grown by the ALE
process;
[0102] FIG. 18B is a drawing for explaining the effect as continued
from FIG. 18A;
[0103] FIG. 18C is a drawing for explaining the effect as continued
from FIG. 18B;
[0104] FIG. 19A is a drawing for explaining an example in which a
metal composition gradient layer is configured as a buffer
layer;
[0105] FIG. 19B is a drawing for explaining the metal composition
gradient layer shown in FIG. 19A;
[0106] FIG. 20A is a schematic sectional view showing a stacked
structure of a first example of the ZnO-base semiconductor light
emitting device of Embodiment 4 of the invention;
[0107] FIG. 20B is a schematic sectional view showing a stacked
structure of a second example of the same;
[0108] FIG. 21 is a schematic sectional view showing a stacked
structure of an exemplary electrode formation status of the
ZnO-base semiconductor light emitting device of the fourth
embodiment of the invention; and
[0109] FIG. 22 is a schematic sectional view showing a stacked
structure of another exemplary electrode formation status differed
from FIG. 4.
BEST MODE FOR CARRYING OUT THE INVENTION
[0110] Best modes for carrying out the invention will be explained
referring to the drawings.
Embodiment 1
[0111] FIG. 1 is a drawing schematically showing a stacked
structure of the essential portion of the light emitting device of
the first invention, and the device has a light emitting layer
portion in which an n-type cladding layer 34, an active layer 33
and a p-type cladding layer 2 are stacked in this order. The p-type
cladding layer 2 is composed as a p-type Mg.sub.xZn.sub.1-xO layer
(0.ltoreq.x.ltoreq.1: may occasionally be referred to as p-type
MgZnO layer 2, hereinafter). In the p-type MgZnO layer 2, a trace
amount of either one of, or two or more of N, Ga, Al, In and Li,
for example, are contained as a p-type dopant. The p-type
carrier-concentration is adjusted within a range from
1.times.10.sup.16sites/cm.sup.3 to 8.times.10.sup.18 sites/cm.sup.3
as described in the above, and more specifically within a range
from 10.sup.17 sites/cm.sup.3 to 10.sup.18 sites/cm.sup.3 or
around.
[0112] FIG. 2 is a schematic drawing of a crystal structure of
MgZnO, where so-called wurtzite structure is shown. In this
structure, oxygen-ion-packed planes and metal-ion (Zn ion or Mg
ion) packed planes are stacked along the direction of the c-axis
alternatively, and as shown in FIG. 3, the p-type MgZnO layer 2 is
formed so as to align the c-axis thereof along the thickness-wise
direction. Formation of a vacancy due to omission of an oxygen ion
causes oxygen deficiency, and consequently produces electrons as
n-type carriers. Excessive formation of such oxygen deficiency
undesirably increases the n-type carriers, to thereby ruin p-type
conductivity. It is therefore important that how completely the
oxygen deficiency can be suppressed in order to form the p-type
MgZnO layer.
[0113] The p-type MgZnO layer 2 can be formed by the MOVPE process.
Principle of the MOVPE process per se is publicly known. The
aforementioned p-type dopant is added during a vapor-phase epitaxy
process. The p-type MgZnO layer 2 is annealed during or after
completion of a vapor-phase epitaxy process in an oxygen-containing
atmosphere. The annealing is successful in suppressing elimination
of oxygen ions, and in obtaining a desirable p-type MgZnO layer 2
having only a small amount of oxygen deficiency. It is also
effective to carry out the growth of the p-type MgZnO layer 2 under
an atmospheric pressure of 10 Torr (1.3332 kPa) or above in terms
of suppressing generation of the oxygen deficiency.
[0114] Now referring back to FIG. 1, the active layer 33 is
composed of a material having an appropriate band gap depending on
desired emission wavelength. For example, for those available for
visible light emission, materials having band gap energies E.sub.g
(3.10 eV to 2.18 eV or around), capable of causing light emission
in a wavelength range of 400 nm to 570 nm, are selected. Although
this range covers emission wavelength from violet region to green
region, those having band gap energies E.sub.g (2.76 eV to 2.48 eV
or around) capable of causing light emission in a wavelength range
of 450 to 500 nm are selected in particular for the case where
blue-color light emission is desired. On the other hand, those
having band gap energies E.sub.g (4.43 eV to 3.10 eV or around)
capable of causing light emission in a wavelength range of 280 nm
to 400 nm are selected in particular for the case where ultraviolet
emission is desired.
[0115] The active layer 33 can be formed typically using a
semiconductor capable of forming a Type-I band lineup between
itself and the p-type MgZnO layer. An example of such active layer
33 is an Mg.sub.yZn.sub.1-yO layer (where, 0.ltoreq.y<1, x>y:
referred to as MgZnO active layer, hereinafter). It is to be noted
now that "a Type-I band lineup is formed between the active layer
and the p-type MgZnO layer" means a junction structure, as shown in
FIG. 4, in which the individual energy levels of the bottom of the
conduction band and the upper end of the valence band E.sub.cp,
E.sub.vp of the p-type cladding layer (p-type MgZnO layer 2), and
the individual energy levels of the bottom of the conduction band
and the upper end of the valance band E.sub.ci, E.sub.vi of the
active layer satisfy the following relations of inequality:
E.sub.ci<E.sub.cp (1)
E.sub.vi>E.sub.vp (2)
[0116] In this structure, specific barrier will appear for both of
the forward diffusion of holes from the active layer 33 to the
n-type cladding layer 34, and the forward diffusion of electrons
(n-type carriers) to the p-type cladding layer 2. If the material
for the n-type cladding layer 34 is appropriately selected so as to
form Type-I band lineup between the active layer 33 and the n-type
cladding layer 34, similarly to as shown in FIG. 4, the active
layer will have formed therein well-formed potential barriers both
at the bottom of the conduction band and the upper end of the
valence band, and will enhance confinement effect both for
electrons and holes. This consequently results in more enhanced
effects of promoting carrier recombination and of improving
emission efficiency. While AlGaN or the like is available for the
n-type cladding layer 34, n-type Mg.sub.zZn.sub.1-zO layer (where
0.ltoreq.z.ltoreq.1:occasionally also referred to as n-type MgZnO
layer", hereinafter) is more advantageous, because this makes it
possible to form all layers composing the light emitting layer
portion with MgZnO-base oxide material (such light emitting layer
portion will be referred to as "full-oxide-type, light emitting
layer portion, hereinafter), so that it is no more necessary to use
rare metals such as above-described Ga and In (dopants excluded),
which contributes to a considerable cost reduction. Height of the
potential barriers on both sides of the active layer can be
equalized by making the alloy compositions of the n-type MgZnO
layer 34 and the p-type MgZnO identical. Thickness t of the active
layer 33 is selected so as to avoid decrease in the carrier density
in the active layer 33 and excessive increase in the amount of
carriers passing through the active layer 33 based on the tunneling
effect, and is typically adjusted within a range from 30 nm to
1,000 nm.
[0117] In the MgZnO active layer 33, a value of alloy composition y
can, also serve as a factor which determines band gap energy
E.sub.g. For example, the value is selected in a range of
0.ltoreq.y.ltoreq.0.5 for the case where ultraviolet emission over
a wavelength of 280 nm to 400 nm is desired. The potential barrier
height thus formed is preferably 0.1 eV to 0.3 eV or around for
light emitting diode, and 0.25 eV to 0.5 eV or around for
semiconductor laser light source. This value can be determined by
selecting the alloy compositions x, y and z of the p-type
Mg.sub.xZn.sub.1-xO layer 2, Mg.sub.yZn.sub.1-yO active layer 33,
and n-type Mg.sub.zZn.sub.1-zO layer 34.
[0118] The following paragraphs will describe one exemplary process
for fabricating the light emitting device having the aforementioned
full-oxide-type, light emitting layer portion. First, as shown in
(a) of FIG. 6, a GaN buffer layer 11 is epitaxially grown on a
sapphire substrate 10, and a p-type MgZnO layer 52 (typically of 50
nm thick), an MgZnO active layer 53 (typically of 30 nm thick), and
an n-type MgZnO layer 54 (typically of 50 nm thick) are formed in
this order (inverted order of the growth also acceptable). The
epitaxial growth of the individual layers can be carried out by the
MOVPE process as described in the above. It is to be noted that,
MBE in the context of this specification include not only MBE in a
narrow sense in which both of a metal element component source and
a non-metal element component source are used in solid forms, but
also include MOMBE (Metal Organic Molecular Beam Epitaxy) using the
metal element component source in a form of organometallic compound
and the non-metal element component source in a solid form; gas
source MBE using the metal element component source in a solid form
and the non-metal element component in a gas form; and chemical
beam epitaxy (CBE) using the metal element component source in a
form of organometallic compound and the non-metal element component
source in a gas form.
[0119] All of the p-type MgZnO layer 52, MgZnO active layer 53 and
the n-type MgZnO layer 54 can continuously be formed by the MOVPE
process using the same source materials and in the same reaction
vessel as shown in FIG. 5A. In this case, the growth is preferably
allowed to proceed at slightly lower temperatures, typically at
300.degree. C. to 400.degree. C., so as to reduce reactivity with
the GaN buffer layer (not shown in FIG. 5A), and to raise the
lattice matching property. The substrate can be heated using a
heater embedded in a susceptor for holding the substrate, as shown
in FIG. 5B.
[0120] Examples of the major materials for composing the individual
layers are such as follows:
[0121] oxygen component source gas: preferably supplied in a form
of oxidative compound gas in view of suppressing an excessive
reaction with organometallic compounds described later, although
oxygen gas is allowable, typified by N.sub.2O, NO, NO.sub.2 and CO,
where N.sub.2O (nitrous oxide) adopted in this embodiment;
[0122] Zn source (metal component source) gas: dimethyl zinc
(DMZn), diethyl zinc (DEZn), etc.; and
[0123] Mg source (metal component source) gas: bis-cyclopentadienyl
magnesium (Cp.sub.2Mg), etc.
[0124] Examples of the p-type dopant gas include the
followings:
[0125] Li source gas: n-butyl lithium, etc.;
[0126] Si source gas: silicon hydrides such as monosilane;
[0127] C source gas: hydrocarbons (typically alkyl containing one
or more C atoms); and
[0128] Se source gas: hydrogen selenide, etc.
[0129] One or more selected from the group consisting of Al, Ga and
In can be allowed to function as excellent p-type dopants when
added together with N. Examples of the dopant gas include the
followings:
[0130] Al source gas: trimethyl aluminum (TMAI), triethyl aluminum
(TEAI), etc.;
[0131] Ga source gas: trimethyl gallium (TMGa), triethyl gallium
(TEGa), etc.; and
[0132] In source gas: trimethyl indium (TMIn), triethyl indium
(TEln), etc.
[0133] For the case where N is used as a p-type dopant together
with a metal element (Ga), the p-type MgZnO layer is grown while
supplying a gas which serves as an N source together with an
organometallic gas which serves as a Ga source. In particular in
this embodiment, N.sub.2O used as an oxygen component source also
serves as an N source.
[0134] The individual source gases are fed into the reaction vessel
after being appropriately diluted with a carrier gas (nitrogen gas,
for example). Ratio of flow rates of the organometallic compound
gases MO which respectively serves as Mg source and Zn source is
controlled using mass flow controllers MFC or the like,
corresponding to variety in the alloy composition of the individual
layers. Also flow rates of N.sub.2O, which is an oxygen component
source gas, and a p-type dopant source gas are controlled by the
mass flow controllers MFC.
[0135] The n-type MgZnO layer 54 can be grown by a method in which
oxygen deficiency is intentionally produced so as to attain n-type
conductivity, where it is effective to lower the atmospheric
pressure (lower than 10 Torr (1.3332 kPa), for example) than that
in the cases where the MgZnO active layer 53 and the p-type MgZnO
layer 52 is formed. It is also allowable to form the layer by
separately introducing an n-type dopant. It is still also allowable
to increase ratio of Group II to Group VI elements (supply II/VI
ratio) of the source materials.
[0136] For the growth of the MgZnO active layer 53 and p-type MgZnO
layer 52, a unique method capable of suppressing oxygen deficiency
as described in the next is adopted. That is, as expressed by two
patterns (a) and (b) shown in FIG. 5A, the layer is grown while
continuously supplying an oxygen component source gas (N.sub.2O),
whereas intermittently interrupting supply of the organometallic
gases, to thereby make use of the time duration of interrupted
supply of the organometallic gases as an effective duration of the
annealing for suppressing generation of the oxygen deficiency, or
for repairing the undesirably generated oxygen deficiency.
[0137] The oxygen deficiency is caused by elimination of oxygen
during the layer growth. To suppress the oxygen deficiency, it is
therefore essential to fully react metal ions (Zn and Mg) derived
from the organometallic gases with oxygen derived from the oxygen
component source gas. Because bond energy between oxygen and Zn or
Mg is relatively large, oxygen once bound with the metals in a
stoichiometric manner will become less likely to be eliminated
again. It is, however, considered that oxygen tends to be
eliminated in an intermediate state where the reaction is not fully
completed, and that the layer growth at the lower temperature
region as described in the above is particularly causative of the
oxygen deficiency due to the incomplete reaction.
[0138] It is therefore preferable, as shown in FIG. 7A, to proceed
the layer growth only to an extremely small thickness so as to
prevent the oxygen deficiency from being incorporated deep inside
the layer, and then, as shown in FIG. 7B, to anneal the layer while
interrupting the supply of the organometallic gases but continuing
only the supply of the oxygen component source gas (N.sub.2O),
because the reaction between unreacted portions of the oxygen
component source gas and organometallic metal gases is promoted,
and the formation of the oxygen deficiency is suppressed. Even if
the oxygen deficiency should generate, it is expected that the
oxygen component source gas decomposes and generated oxygen is
adsorbed so as to repair the oxygen deficiency. After completion of
the annealing over a duration of time necessary and sufficient for
fully expressing these effects, the supply of the organometallic
compound gas is restarted as shown in FIG. 7C, to thereby further
continue the layer growth. These processes are repeated thereafter.
FIG. 8A shows an exemplary supply sequence of the organometallic
gases (MO) and the oxygen component source gas. Growth of the MgZnO
active layer 53 and the p-type MgZnO layer 52 can be proceeded
basically in a similar manner, except that the dopant gas is not
supplied for the former, but supplied only for the latter.
[0139] In this case, it is necessary that the surface of the layers
during the annealing is kept at a temperature higher by 100.degree.
C. or more than the layer growth temperature and lower than the
melting point of the oxide (700.degree. C. in this embodiment), in
order to promote decomposition of the oxygen component source gas,
rearrangement of the adsorbed oxygen for repairing the oxygen
deficiency, and binding reaction with metal ions already
incorporated within the layer. The temperature higher by less than
100.degree. C. than the layer growth temperature may result in only
an insufficient effect of suppressing the oxygen deficiency. On the
other hand, it is self-evident that the temperature exceeding the
melting point of the oxide is nonsense. Because the annealing
temperature is set higher than the substrate temperature in the
layer growth, it is convenient to use a separate heater specialized
for the annealing, besides a heater for heating the substrate. The
separate heater is exemplified by an infrared lamp in FIG. 5A.
[0140] Once the oxygen deficiency is formed in the newly-grown
portion of the layer, it is advantageous to anneal the layer before
the oxygen deficiency is buried in view of smoothly repairing it
under milder conditions. It is therefore effective to set a unit of
the discontinuous (intermittent) layer growth to monoatomic layer
(adjacent oxygen packing layer and metal ion packing layer are
deemed to comprise monoatomic layer) or around. Introduction period
s for the organometallic compound gas is thus set so as to afford
an amount of introduction of the gas necessary for the growth of
the monoatomic layer.
[0141] The introduction of the organometallic compound gases may be
effected in a period s' longer than the period s for forming a
complete monoatomic layer as shown in FIG. 9A, or may be effected
in a shorter period s" as shown in FIG. 9B, so far as it falls
within a range from 0.5 atomic layers to 2 atomic layers. The
introduction period s less than 0.5-atomic-layers-equivalent time
may lower the fabrication efficiency, and the exceeding
2-atomic-layers-equivalent time may reduce the merit of the
intermittent layer growth, because time of annealing for
suppressing the oxygen deficiency becomes too long. The
introduction time s of the organometallic compound gases is,
therefore, preferably set considering the time required for
reaction of oxygen atoms with the metal atoms, and relaxation of
strain in the crystal lattice.
[0142] On the other hand, the annealing time needs some
consideration. The reaction per se between the metal atom and
oxygen atom completes within a relatively short time, but an
additional time is substantially necessary for purging of the
organometallic gas out from the reaction vessel in order to ensure
uniform reaction (while actual variation pattern of flow rate
should always show transient periods in which flow rate of the
organometallic gas varies with time, when switched from the
annealing period including the purge-out time, the transient
periods are not illustrated in FIGS. 8A to 8C, and FIGS. 9A and 9B
for simplicity). Assuming now that the sectional area of the
reaction vessel allowing the gas flow as 20 cm.sup.2 as shown in
FIG. 5B, a total gas volume as 50 liters/min (converted value for
the standard state), and a length of the heated portion including
the substrate along the gas flow direction as 5.0 cm, a minimum
necessary time for the purging is calculated as 0.002 seconds.
However, the time for purging of 0.002 seconds is practically
insufficient, because it is technically difficult to keep a signal
input/output cycle of a gas sequencer precisely as short as less
than 0.1 seconds, and a stagnation layer is formed in the vicinity
of the inner wall of the reaction vessel and at the heated portion
including the substrate, where the flow rate is slower. It is
therefore preferable to set an interruption time for the
introduction of the organometallic compound as long as 1 second or
more so as to tolerate the mechanical accuracy. Specific conditions
for the annealing typically relate to a nitrogen flow rate of 10
liters/min (converted value for the standard state), N.sub.2O flow
of 1 liter/min (converted value for the standard state), a layer
surface temperature of 700.degree. C., a pressure of 760 Torr
(101.3 kPa), and a retention period for one cycle of 5 to 15
seconds.
[0143] It is also allowable to keep supply of a small amount of the
organometallic compound gas during annealing period, as shown in
FIG. 8B, rather than completely interrupting the supply, so far as
the suppressive effect for the oxygen deficiency will not largely
be ruined. It is also allowable to reduce the supply volume of the
oxygen component source gas from the supply volume during the layer
growth as shown in FIG. 8C, because oxygen during the annealing
period is necessary only in an amount consumed for suppressing or
repairing the oxygen deficiency. It is still also allowable to
gradually increase or decrease the amount of supply of the
organometallic compound gas as shown in FIG. 8D, instead of the
step-wise variation shown in FIG. 8A.
[0144] During the layer growth while introducing the organometallic
gases, it is effective to keep pressure in the reaction vessel at
10 Torr (1.3332 kPa) or above. This is more successful in
suppressing the oxygen elimination, and in growing the MgZnO layer
having a less amount of oxygen deficiency. In particular for the
case where N.sub.2O is used as the oxygen component source, the
above-described setting of the pressure successfully prevents
N.sub.2O from being rapidly dissociated, and this makes it possible
to more effectively suppress generation of the oxygen deficiency.
The higher the atmospheric pressure rises, the larger a suppressive
effect for the oxygen elimination becomes, where a pressure of only
as high as 760 Torr (1 atm, or 101.3 kPa) or around may be
sufficient for obtaining the effect. Adoption of a pressure of 760
Torr (101.3 kPa) or below means that the reaction vessel is
conditioned at normal pressure or reduced pressure, and this
requires only a relatively simple seal structure of the vessel. On
the contrary, adoption of a pressure exceeding 760 Torr (101.3 kPa)
means that the vessel is pressurized, and this requires a slightly
stronger seal structure in order to prevent leakage of the internal
gases, and further requires a pressure-proof structure or the like
for the case where the pressure is considerably high, where the
suppressive effect for the oxygen elimination becomes more
distinctive in anyway. The upper limit of the pressure in this case
should be determined to an appropriate value considering a balance
between the cost of the apparatus and attainable suppressive effect
for the oxygen elimination (typically 7,600 Torr (10 atm, or 1.013
MPa) or around).
[0145] After completion of the growth of the light emitting layer
portion, a metal reflective layer 22 is formed on the n-type MgZnO
layer 54 as shown in (b) of FIG. 6, the sapphire substrate 10 is
separated as shown in (c) of FIG. 6, and a transparent conductive
material layer 25 (e.g., ITO film) is formed on the p-type MgZnO
layer 52. Thereafter as shown in (d) of FIG. 6, the light emitting
device 104 is obtained by dicing. It is also allowable herein to
leave the growth substrate such as sapphire substrate unseparated,
and to use it as a part of the device.
[0146] The annealing for suppressing the oxygen deficiency for the
MgZnO active layer 53 and p-type MgZnO layer 52 may collectively be
carried out after the layer growth completed. In this case, it is
also allowable to carry out the annealing after the substrate is
transferred to a separate furnace specialized for annealing
different from the reaction vessel. The annealing is preferably
carried out each time the MgZnO active layer 53 and p-type MgZnO
layer 52 are grown. In view of repairing the oxygen deficiency
incorporated into the layer, the annealing is preferably carried
out at a temperature range slightly higher than that in the case
where the layer growth and annealing are repeated in an
intermittent manner. Specific conditions for the annealing
typically relates to nitrogen flow rate of 10 liters/min (converted
value for the standard state), N.sub.2O flow rate of 1 liter/min
(converted value for the standard state), a layer surface
temperature of 800.degree. C., a pressure of 760 Torr (101.3 kPa),
and an annealing period of 30 minutes.
[0147] The active layer 33 shown in FIG. 1 can also be formed using
a semiconductor capable of forming a Type-II band lineup between
itself and the p-type MgZnO layer 2. An example of such active
layer 33 is an InGaN layer (referred to as InGaN active layer,
hereinafter). It is to be noted now that "a Type-II band lineup is
formed between the active layer and the p-type Mg.sub.xZn.sub.1-xO
layer" means a junction structure as shown in FIG. 10A, in which
the individual energy levels of the bottom of the conduction band
and the upper end of the valence band E.sub.cp, E.sub.vp of the
p-type cladding layer (p-type Mg.sub.xZn.sub.1-xO layer 2), and the
individual energy levels of the bottom of the conduction band and
the upper end of the valance band E.sub.ci, E.sub.vi of the active
layer satisfy the following relations of inequality:
E.sub.ci>E.sub.cp (3)
E.sub.vi>E.sub.vp (4)
[0148] In this structure, no specific barrier will appear for the
forward diffusion of electrons (n-type carriers) from the active
layer to the p-type cladding layer, but a relatively high potential
barrier is formed for the reverse diffusion of holes (p-type
carriers) from the active layer to the p-type cladding layer. This
promotes carrier recombination in the active layer, and can achieve
high emission efficiency. Assuming now that the layer is expressed
as In.sub..alpha.Ga.sub.1-.alpha.N, where .alpha. is an InN alloy
composition, a relation of 0.34.ltoreq..alpha..ltoreq.0.47 is
preferably adopted for blue visible light emission, and a relation
of 0.ltoreq..alpha..ltoreq.0.19 is preferably adopted for
ultraviolet emission.
[0149] In this case, the n-type cladding layer 34 preferably uses a
semiconductor capable of forming a Type-I band lineup between
itself and the active layer. An example of such n-type cladding
layer 34 is an n-type AlGaN (Al.sub..beta.Ga.sub.1-.beta.N) layer.
It is to be noted now that "a Type-I band lineup is formed between
the n-type cladding layer and the active layer" means a junction
structure, as shown in FIG. 10A, in which the individual energy
levels of the bottom of the conduction band and the upper end of
the valence band E.sub.ci, E.sub.vi of the active layer, and the
individual energy levels of the bottom of the conduction band and
the upper end of the valance band E.sub.ci, E.sub.vi of the n-type
cladding layer (n-type AlGaN layer 4) satisfy the following
relations of inequality:
E.sub.ci<E.sub.cn (5)
E.sub.vi>E.sub.vn (6)
[0150] In this structure, a relatively high potential barrier is
formed for the reverse diffusion of electrons from the n-type
cladding layer to the active layer, and a well-type potential
barrier is formed at the upper end of the valence band
corresponding to the position of the active layer, to thereby
enhance the confinement effect of holes. All of these promote
carrier recombination in the active layer, and consequently achieve
high emission efficiency.
[0151] In the structures shown in FIG. 10A, a suppressive effect of
reverse diffusion of holes from the active layer to the p-type
cladding layer can successfully be raised by increasing the energy
barrier height (E.sub.vi-E.sub.vp) at the upper end of the valence
band. For this purpose, it is effective to raise MgO alloy
composition of the p-type Mg.sub.xZn.sub.1-xO layer 2 (that is,
value of x) composing the p-type cladding layer. The alloy
composition x is determined depending on desired current density,
so as not to cause excessive leakage of the carriers towards the
p-type cladding layer. In a typical case where the active layer 33
is composed of an InGaN layer, the alloy composition x is
preferably set within a range from 0.05 to 0.2 or around for light
emitting diode, and 0.1 to 0.4 or around for semiconductor laser
light source.
[0152] The bottom of the conductive band descends in a step-wise
manner from the active layer towards the p-type cladding layer, and
the electrons not contributed to the emissive recombination in the
active layer then flow into the p-type cladding layer having a
higher carrier concentration, and become no more contributable to
light emission due to Auger recombination or the like. In order to
raise the emission efficiency, it is therefore necessary that
electrons as much as possible recombine with holes before they flow
into the p-type cladding layer, and it is therefore effective to
increase the thickness t of the active layer to a certain level or
above (e.g., 30 nm or above). As shown in FIG. 10B, too small
thickness t of the active layer increases electrons possibly flow
into the p-type cladding layer and become not contributable to the
light emission, and this results in lowered emission efficiency. On
the other hand, increase in the thickness t of the active layer
beyond a necessary level results in lowered carrier density in the
active layer and thus lowers the emission efficiency. The thickness
is thus typically set to 2 .mu.m or below.
[0153] In FIG. 10A, it is advantageous in view of suppressing
non-emissive recombination at the junction boundary that a relation
of E.sub.cp>E.sub.vi is satisfied similarly for the case where
the InGaN active layer is used, that is, the p-type cladding layer
and the active layer have forbidden bands which overlap with each
other.
Embodiment 2
[0154] The next paragraphs will describe an embodiment of the
second invention. Since the essential portion of the light emitting
device to which the second invention is applicable is same as
described in Embodiment 1, detailed description will be omitted
(see FIGS. 1 to 4, and FIGS. 10A and 10B). As shown in (a) of FIG.
6, the GaN buffer layer 11 is epitaxially grown again on the
sapphire substrate 10, and further thereon the p-type MgZnO layer
52 (typically of 50 nm thick), the MgZnO active layer 53 (typically
of 30 nm thick) and the n-type MgZnO layer 54 (typically of 50 nm
thick) are formed in this order (inverted order of the growth also
acceptable). The epitaxial growth of the individual layers in this
embodiment can be carried out by the MOVPE process similarly to as
described in Embodiment 1, where differences reside in the
following points. More specifically, in the growth of the MgZnO
active layer 53 and p-type MgZnO layer 52 herein, a ultraviolet
lamp (e.g., excimer ultraviolet lamp) as a ultraviolet light source
is disposed opposing to the main surface of the substrate in order
to suppress the generation of the oxygen deficiency, and the source
gases are supplied between the substrate and ultraviolet light
source while irradiating ultraviolet light from the ultraviolet
lamp towards the main surface of the substrate.
[0155] FIGS. 11A and 11B show an apparatus used for vapor-phase
epitaxy process of the light emitting layer portion using the
ultraviolet lamp based on the MOVPE process. Similarly to as
described previously referring to FIG. 5A, all of the p-type MgZnO
layer, MgZnO active layer and n-type MgZnO layer can sequentially
be formed in the same reaction vessel using the same source gases.
In this case, it is preferable to proceed the growth at slightly
lower temperatures, typically at 300 to 400.degree. C. so as to
reduce the reactivity with the GaN buffer layer and raise the
lattice matching property. The substrate can be heated using a
heater embedded in a susceptor for holding the substrate.
[0156] The wall portion of the reaction vessel is configured as a
transparent wall portion composed of a quartz glass or the like,
and the ultraviolet lamp is disposed outside the reaction vessel,
so as to effect the ultraviolet irradiation through the transparent
wall portion towards the substrate. The ultraviolet lamp available
herein has an emission wavelength of approximately 172 nm, and an
output power density of approximately 8 mW/cm.sup.2 when the flow
rates of N.sub.2O and organometallic compound gas are within a
range from 100 cm.sup.3/min to 1,000 cm.sup.3/min and 10
cm.sup.3/min to 100 cm.sup.3/min, respectively.
[0157] It is supposed that ultraviolet light irradiated to the
substrate is once absorbed by the substrate, and can highly
activate the outermost portion of the layers under growth based on
the light excitation effect. That is, a highly activated status
similarly to as obtained in the layer growth under high
temperatures can locally be realized in the outermost portion of
the layer. Also a part of the source gases is brought into a
high-energy transition status (radical, etc.) by the ultraviolet
irradiation. As a consequence, the organometallic gases and oxygen
component source gas (N.sub.2O) can react in the vicinity of the
activated outermost portion of the layer, in a stoichiometric
manner without causing unreacted components or the like, and the
layer growth is promoted in a manner less causative of the oxygen
deficiency.
[0158] Radicals of the organometallic gases and oxygen component
source gas are unstable in general, and the radicals ascribable to
these components will be converted into other decomposition
products not contributable to the oxide formation reaction, if a
status in which these radicals are brought into a close vicinity
enough for causing reaction is not realized for a long duration of
time. While this kind of decomposition reaction is more likely to
proceed as temperature of the system elevates, this can be
suppressed to a certain extent typically by lowering the substrate
temperature to as relatively low as 400.degree. C. or below. The
ultraviolet irradiation can enhance reaction activity in the
vicinity of the main surface of the substrate, and this makes it
possible to readily form the oxide semiconductor layer having only
a less amount of oxygen deficiency even when the substrate
temperature cannot be raised so high for various reasons.
[0159] On the other hand, probability of the oxide formation
reaction contributable to the layer growth is higher in the
boundary layer (in which mass transfer is governed by diffusion,
also referred to as stagnation layer), and lower in an area outside
the boundary layer and having a large gas flow rate. It is thus
understood that the larger the flow rate of the gas flowing through
the reaction vessel grows, the thinner the boundary layer becomes,
and the growth speed of the oxide is depressed. Adjusting now the
flow rate of the source gases supplied between the substrate and
the ultraviolet lamp (ultraviolet light source) so as to be faster
on the ultraviolet light source side than that on the main surface
side as shown in FIG. 11B, the reaction products become less likely
to deposit on the wall portion of the reaction vessel in the
vicinity of the ultraviolet lamp, and this makes it possible to
avoid a nonconformity such that the deposit shadows ultraviolet
light from the light source to thereby degrade the reaction
efficiency. More specifically, as shown in FIG. 11B, a gas intake
port and a gas discharge port of the reaction vessel are formed so
as to be divided into a first gas intake/discharge port, and a
second gas intake/discharge port, and the flow rate is adjusted so
as to make a gas flow rate .lambda..sub.1 on the first gas
intake/discharge port side is faster than a gas flow rate
.lambda..sub.2 on the second gas intake/discharge port side.
[0160] The ultraviolet lamp is advantageous in view of ensuring a
large irradiation area, and of allowing the reaction for the oxide
layer formation to proceed in a uniform and efficient manner. On
the other hand, it is also allowable to irradiate a ultraviolet
laser beam in a two-dimensional scanning manner over the substrate
as shown in FIG. 12. This system can use a light convergence
density larger than that available from the ultraviolet lamp, and
thus further enhance the reaction efficiency. In an exemplary
configuration shown in FIG. 12, a laser light source-composed as an
excimer laser light source or a semiconductor laser light source is
scanned in the X direction with the aid of a polygon mirror, and in
synchronization therewith a susceptor holding the substrate is
driven in the Y direction, which crosses the X direction, with the
aid of a Y-scanning table, so as to scan over the entire portion of
the main surface of the substrate with the laser beam in a
two-dimensional manner.
[0161] The process steps after completion of the growth of the
light emitting layer portion are same as those described in
Embodiment 1 referring to (b) to (d) of FIG. 6.
Embodiment 3
[0162] The next paragraphs will describe an embodiment of the third
invention. Although the essential portion of the light emitting
device to which the third invention is applicable is almost the
same as described in Embodiment 1 (see FIGS. 1 to 5A, and FIGS. 10A
and 10B), it is essential in the third invention to form the buffer
layer as described below. That is, the buffer layer has the c-axis
of the wurtzite crystal structure oriented to the thickness-wise
direction, has a single metal atom layer as a metal monoatomic
layer formed in contact with the substrate, and has the residual
oxygen atom layers and metal atom layers alternatively stacked
successive to the metal monoatomic layer. An exemplary fabrication
process will be explained below.
[0163] First as shown in FIG. 13, a buffer layer 111 composed of
MgZnO is epitaxially grown on the sapphire substrate 10, and
further thereon an n-type MgZnO layer 34 (typically of 50 nm
thick), an MgZnO active layer 33 (typically of 30 nm thick) and a
p-type MgZnO layer 32 (typically of 50 nm thick) are formed in this
order (inverted order of the growth or layers 32 to 35 also
acceptable). These layers can be grown by the MOVPE process.
[0164] By the MOVPE process, all of the buffer layer 111, n-type
MgZnO layer 34, MgZnO active layer 33 and p-type MgZnO layer 32 can
continuously be formed by the MOVPE process using the same source
materials and in the same reaction vessel as shown in FIGS. 14A to
14C. Temperature in the reaction vessel is adjusted using a heating
source (an infrared lamp in this embodiment) so as to promote the
chemical reactions for the layer growth. Major source materials for
the individual layers and style of feeding thereof are the same as
those described in Embodiments 1 and 2.
[0165] The buffer layer 111 is grown as descried in the next. FIG.
17 shows a control sequence of temperature in the reaction vessel
and introduction of the individual gases in this embodiment. The
substrate 10 on which the layers are grown is a sapphire (i.e.,
single crystal alumina) substrate having the c-axis as the
principal crystal axis, where the main surface on the
oxygen-exposed plane side shown in FIG. 15 is used as a layer
growth plane. Prior to the layer growth, the substrate 10 is
thoroughly annealed under an oxidative gas atmosphere. The
oxidative gas may be any of those selected from O, CO and N.sub.2O,
where N.sub.2O is selected in this embodiment so as to be used also
as the oxygen component source gas in the layer growth described
later. For the case where the annealing is carried out in the
reaction vessel for the MOVPE process, preferable conditions for
the annealing relate to a temperature of 750.degree. C. or above
(but lower than the melting point of the substrate), and an
annealing time of 30 minutes or more. It is, however, also
allowable to shorten the above-described annealing time if the
surface of the substrate can be cleaned to a satisfactory level by
wet cleaning or the like.
[0166] After completion of the annealing, the substrate temperature
is lowered to a first temperature which is set to 250 to
300.degree. C. (set to 350.degree. C. herein) as shown in FIG. 17
in order to suppress generation of the oxygen deficiency, while
keeping the oxidative gas atmosphere. After the temperature is
stabilized at a set value, supply of the oxidative gas is
interrupted, and the gas is then thoroughly purged out by replacing
the inner atmosphere of the reaction vessel with nitrogen gas. It
is preferable to set the purging time to 5 seconds or longer,
although variable depending on shape and capacity of the reaction
vessel.
[0167] Next, as shown in FIGS. 14A and 16A, the organometallic gas
MO is supplied into the reaction vessel, and the first metal atomic
layer which composes a part of the buffer layer 111 is formed as a
monoatomic layer by the ALE process. As previously explained in the
above, growth of the monoatomic layer in the ALE process saturates
once a single atomic layer is completed based on the
self-termination function, and no more growth of the metal atomic
layer would occur even if the supply of the organometallic compound
gas MO is continued.
[0168] Thereafter the supply of the organometallic gas MO is
interrupted, the gas is thoroughly purged out by replacing the
inner atmosphere of the reaction vessel with nitrogen gas, and as
shown in FIG. 16C, N.sub.2O is introduced as the oxygen component
source gas (and also as a gas for creating the oxidative
atmosphere), and the oxygen atom layer is formed only by a single
atomic layer by the ALE process. This results in the formation of
the MgZnO layer only by a single atomic layer on the substrate
10.
[0169] The temperature in the reaction vessel is thereafter
increased to a second temperature which is set to 400 to
800.degree. C. (set to 750.degree. C. herein) as shown in FIG. 17
while keeping the oxidative gas atmosphere, and also keeping the
organometallic gas continuously supplied, so as to form the
residual portion of the buffer layer by the general MOVPE process
as shown in FIGS. 14B and 16D. In this process, the buffer layer
111 having an excellent planarity can be obtained by growing the
layer at a speed of 0.1 nm/sec or around until a thickness of 10 nm
or around is attained, and thereafter at a speed of 1 nm/sec. In
view of obtaining the buffer layer excellent both in the
crystallinity and planarity, it is also preferable to grow a
plurality of layers from the first layers by the ALE process.
[0170] Although the buffer layer 111 of this embodiment is formed
as a simple oxide layer comprising ZnO, it may also be formed as a
composite oxide layer of MgZnO having an appropriate alloy
composition harmonized with the alloy composition of the adjacent
layer on the light emitting-portion side. The Al atom layer located
just below the outermost oxygen atom layer of the sapphire
substrate comprises, as shown in FIG. 18A, two Al atom sites Al-1
and Al-2, which differ from each other in the distance to the
oxygen layer. Assuming now that the metal atom layer formed on the
oxygen layer is a Zn atom layer, both sites Al-1 and Al-2 differ in
the Coulomb repulsive force between Zn atom and Al atom located
while placing the oxygen layer in between. For this reason, Zn
atoms corresponding to both sites will have different displacement
in the direction normal to the plane of the oxygen atom layer, and
this may causative of irregularity in stacking of the later-coming
layers. To relieve this effect, as shown in FIGS. 18B and 18C, it
is effective to form the first single atomic layer (or a plurality
of layers) as a composite oxide layer which contains Group II atom
(e.g., Mg) having a smaller ionic radius than Zn, or Group II atom
(e.g., Ca, Sr, Ba) having a larger ionic radius by an appropriate
ratio, and this can improve the crystallinity of the light emitting
layer portion to be obtained. It is now also effective, in view of
enhancing the above-described effect, to dispose a
composition-gradient layer, having metal cation composition
gradated in the thickness-wise direction, between such composite
oxide layer (having a metal cation composition A) and the cladding
layer (having a metal cation composition B: n-type MgZnO layer 54
herein) formed in contact with the buffer layer 111, in order to
ensure continuity between both compositions A and B, as shown in
FIG. 19A. In an exemplary case where both of the composite oxide
layer and cladding layer are composed of MgZnO, the
composition-gradient layer can be formed so that composition
parameter .nu. varies continuously between A and B typically as
shown in FIG. 19B, where composition parameter .nu. represents
metal cation composition and is given by
.nu..ident.N.sub.Mg/(N.sub.Mg+N.sub.Zn), where N.sub.Mg is molar
content of Mg, and N.sub.Zn is molar content of Zn; A is an
expression of .nu. for the composite oxide layer, and B is that for
the cladding layer.
[0171] After the buffer layer 111 is completed, as shown in FIG.
14C, the n-type MgZnO layer 34, MgZnO active layer 33 and p-type
MgZnO layer 32 are formed in this order by the MOVPE process. These
process steps are basically same as those described in Embodiments
1 and 2.
[0172] In this embodiment, after completion of the growth of the
light emitting layer portion, the active layer 33 and the p-type
MgZnO layer 32 are partially removed by photolithography or the
like as shown in FIG. 13, a transparent electrode 125 comprising
indium tin oxide (ITO) or the like is formed, a metal electrode 122
is formed on the residual p-type MgZnO layer 32, and the layers are
then diced together with the substrate 10 to thereby produce the
light emitting device 1. It is thus self-evident that the light
emitting device 1 is configured so that the buffer layer 111
composed of MgZnO is formed on the substrate 10, and further
thereon the light emitting layer portion again composed of MgZnO is
formed. Light extraction is therefore available mainly on the
transparent sapphire substrate 10 side.
[0173] It is to be noted that the light emitting device can of
course be configured as shown in FIG. 6. In this case, the layers
are formed on the buffer layer 111 in an order inverted from that
shown in FIG. 13, that is, the p-type MgZnO layer 32, MgZnO active
layer 33 and n-type MgZnO layer 34 are formed in this order. This
configuration is advantageous in obtaining the device having an
improved weatherability, because a metal layer of MgZnO composing
the light emitting layer portion is exposed only after the
substrate 10 is separated.
Embodiment 4
[0174] FIGS. 20A and 20B schematically show a stacked structure of
the essential portion of the light emitting device in order to
explain one embodiment of the fourth invention. As shown in FIG.
20A, on a substrate 210, a ZnO buffer layer 211, an n-type
MgZnO-type oxide layer 234, a ZnO-base semiconductor active layer
233 and a p-type MgZnO-type oxide layer 232 are stacked by the
epitaxial growth process while keeping lattice matching, to thereby
form a double hetero, light emitting layer portion 200. The
ZnO-base semiconductor active layer (also simply referred to as
active layer) 233 is composed of a ZnO-base semiconductor
containing Zn as a Group II element, and containing O together with
Se or Te as a Group VI element. FIG. 20A shows the active layer 233
configured as a single layer, whereas FIG. 20B shows the active
layer 233 having a multi-layered structure in which sub-layers 237
composed of ZnSe or ZnTe are periodically inserted in a ZnO main
layer 236 while keeping an area width equivalent to or less than
one molecular layer of the active layer 233.
[0175] As shown in FIG. 20A, by composing the active layer 233
using a ZnO-base semiconductor containing Se or Te, it is made
possible to introduce Se or Te, which belongs to the same Group
with oxygen, to oxygen-deficient sites, and this is successful in
improving the crystallinity of the active layer 233 and making the
band gap energy thereof well suited to blue-color light emission as
described in the above. On the other hand, as shown in FIG. 20B, by
composing the active layer 233 so as to have a multi-layered
structure in which sub-layers 237 composed of ZnSe or ZnTe are
periodically inserted in a ZnO main layer 236, it is made possible
to enhance binding property of thus introduced Se or Te with the
closest Zn. Although FIG. 20B illustrates the sub-layer 237 as
having a coverage ratio of 1, it is also allowable to reduce the
coverage ratio to as smaller than 1 in order to prevent Se or Te
from being deposited rather than being introduced into the
oxygen-deficient sites. The number of formation of the sub-layers
237 can properly be adjusted depending on desired emission
wavelength in the active layer 233.
[0176] The substrate 210 shown in the FIGS. 20A and 20B may be such
as those used in Embodiments 1 to 3. Although the ZnO buffer layer
211 can epitaxially be formed by stacking ZnO crystal, it is also
allowable to epitaxially grow either one of ZnS, ZnSe and ZnTe, and
then convert them to obtain the ZnO buffer layer 211 by annealing
under the oxygen-containing atmosphere.
[0177] N-type dopant added to the n-type, MgZnO-type oxide layer
234 (also simply referred to as n-type MgZnO layer 234,
hereinafter) and p-type dopant added to the p-type, MgZnO-type
oxide layer 232 (also simply referred to as p-type MgZnO layer 232,
hereinafter) may be such as those used in Embodiments 1 to 3.
[0178] The epitaxial growth of the individual layers shown in FIG.
20A can be carried out based on the MOVPE or MBE process. It is to
be noted that MBE in the context of this patent specification
include not only MBE in a narrow sense in which both of a metal
element component source and a non-metal element component source
are used in solid forms, but also include MOMBE (Metal Organic
Molecular Beam Epitaxy) using the metal element component source in
a form of organometallic compound and the non-metal element
component source in a solid form; gas source MBE using the metal
element component source in a solid form and the non-metal element
component in a gas form; and chemical beam epitaxy (CBE) using the
metal element component source in a form of organometallic compound
and the non-metal element component source in a gas form.
[0179] Also the ZnO main layer 236 shown in FIG. 20B can be formed
by the epitaxial growth process similarly to as described in the
above. On the other hand, the sub-layer 237, which is composed of
ZnSe or ZnTe, and must be adjusted to have an area width equivalent
to or less than a single molecular layer of the active layer 233,
can be formed by the ALE (Atomic Layer Epitaxy) process in which a
Zn-source gas and S- or Se-source gas, both serve major source
materials, are alternatively supplied. A proper adjustment of flow
rates of thus supplied source gases makes it possible to reduce the
coverage ratio of the sub-layer 237 smaller than 1.
[0180] Major source materials for the individual layers, except the
Se source and Te source, may be such as those used in the MOVPE
process in Embodiments 1 to 3, and also the basic process steps are
the same as those described in Embodiments 1 to 3. Available
Se-source gases include H.sub.2Se, and available Te-source gases
include H.sub.2Te.
[0181] After completion of the growth of the light emitting layer
portion 200, the substrate 210 is lapped and etched, as shown in
FIG. 21, a p-type electrode 223 composed of In and n-type
electrodes 224 composed of Au are respectively formed, the stack is
diced, and the individual electrodes are bonded with Al wirings, so
as to obtain the ZnO-base semiconductor light emitting device.
Light extraction is therefore available mainly on the p-type MgZnO
layer 232 side. In FIG. 21, the light extraction is, however, not
available from the area where the p-type electrode 223 is formed.
It is therefore advantageous to partially remove the active layer
233 and p-type MgZnO layer 232 by photolithography or the like as
shown in FIG. 22, a transparent electrode 225 comprising indium tin
oxide (ITO) or the like is formed, a metal electrode 222 is formed
on the residual p-type MgZnO layer 232, and the layers are then
diced together with the sapphire substrate 221 to thereby produce
the ZnO-base semiconductor light emitting device. Light extraction
is therefore available mainly on the transparent sapphire substrate
221 side.
* * * * *