U.S. patent application number 09/947346 was filed with the patent office on 2002-05-09 for light emitting nitride semiconductor device, and light emitting apparatus and pickup device using the same.
Invention is credited to Ito, Shigetoshi, Tsuda, Yuhzoh.
Application Number | 20020053665 09/947346 |
Document ID | / |
Family ID | 18758613 |
Filed Date | 2002-05-09 |
United States Patent
Application |
20020053665 |
Kind Code |
A1 |
Tsuda, Yuhzoh ; et
al. |
May 9, 2002 |
Light emitting nitride semiconductor device, and light emitting
apparatus and pickup device using the same
Abstract
There is provided a light emitting device having high luminous
efficacy or emission intensity. The device includes a light
emitting layer provided between n- and p-type layers of nitride
semiconductor formed on a GaN substrate. The light emitting layer
is formed of a well layer or a combination of well and barrier
layers. The well layer is made of a nitride semiconductor
containing an element X, N and Ga, wherein X is As, P or Sb. The
ratio of the number of the atoms of element X to the sum of the
number of the atoms of element X and N, is not more than 30 atomic
percent. The well layer contains Mg, Be, Zn, Cd, C, Si, Ge, Sn, O,
S, Se or Te as an impurity for improving the crystallinity of the
well layer.
Inventors: |
Tsuda, Yuhzoh; (Tenri-shi,
JP) ; Ito, Shigetoshi; (Ikoma-shi, JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
18758613 |
Appl. No.: |
09/947346 |
Filed: |
September 7, 2001 |
Current U.S.
Class: |
257/14 |
Current CPC
Class: |
G11B 7/127 20130101;
H01L 33/325 20130101; H01S 5/34333 20130101; B82Y 20/00 20130101;
H01L 33/32 20130101; H01S 5/04253 20190801; H01S 5/22 20130101 |
Class at
Publication: |
257/14 |
International
Class: |
H01L 029/06 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 8, 2000 |
JP |
2000-272513(P) |
Claims
What is claimed is:
1. A light emitting nitride semiconductor device, comprising: a
substrate made of nitride semiconductor crystal or a substrate
having a nitride semiconductor crystal film grown on a crystalline
material other than said nitride semiconductor crystal; an n-type
layer and a p-type layer made of nitride semiconductor and formed
on said substrate; and a light emitting layer provided between said
n- and p-type layers, wherein said light emitting layer is formed
of a well layer or a combination of well and barrier layers; of
said layer or layers forming said light emitting layer, at least
said well layer is made of a nitride semiconductor containing an
element X, N and Ga, said element X being at least one selected
from the group consisting of As, P and Sb; in said nitride
semiconductor forming said light emitting layer, the ratio of the
number of the atoms of said element X to the sum of said number of
the atoms of said element X and the number of the atoms of said N,
is not more than 30 atomic percent; and of said layer or layers
forming said light emitting layer, at least said well layer
contains as an impurity at least one element selected from the
group consisting of Mg, Be, Zn, Cd, C, Si, Ge, Sn, O, S, Se and
Te.
2. The light emitting nitride semiconductor device of claim 1,
wherein said impurity is contained so as to improve crystallinity
of said well layer.
3. The light emitting nitride semiconductor device of claim 1,
wherein a total content of said impurity is 1.times.10.sup.17 to
5.times.10.sup.20/cm.sup.3.
4. The light emitting nitride semiconductor device of claim 1,
wherein said nitride semiconductor crystal or said nitride
semiconductor crystal film of said substrate or said light emitting
nitride semiconductor device has a threading dislocation density of
not more than 3.times.10.sup.7/cm.sup.2 or an etch pit density of
not more than 7.times.10.sup.7/cm.sup.2.
5. The light emitting nitride semiconductor device of claim 3,
wherein said nitride semiconductor crystal or said nitride
semiconductor crystal film of said substrate or said light emitting
nitride semiconductor device has a threading dislocation density of
not more than 3.times.10.sup.7/cm.sup.2 or an etch pit density of
not more than 7.times.10.sup.7/cm.sup.2.
6. The light emitting nitride semiconductor device of claim 1,
wherein said light emitting layer is a multi-quantum well
layer.
7. The light emitting nitride semiconductor device of claim 3,
wherein said light emitting layer is a multi-quantum well
layer.
8. The light emitting nitride semiconductor device of claim 4,
wherein said light emitting layer is a multi-quantum well
layer.
9. A light emitting apparatus, comprising said light emitting
nitride semiconductor device as recited in claim 1 and having an
emission wavelength of 360 nm to 650 nm.
10. An optical pickup apparatus, comprising a light emission
apparatus comprising said light emitting nitride semiconductor
device as recited in claim 1 and having an oscillation wavelength
of 360 nm to 420 nm.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to light emitting nitride
semiconductor devices having high luminous efficacy, and light
emitting apparatus and optical pickup devices employing the
same.
[0003] 2. Description of the Background Art
[0004] Japanese Patent Laying-Open No. 8-64870 discloses a light
emitting device having a stack of gallium nitride based compound
semiconductor layers, wherein an active layer is formed of a
gallium nitride based compound semiconductor in which the nitrogen
is partially replaced with phosphorus and/or arsenic and the active
layer is doped with at least one dopant selected from the group
consisting of Mg, Zn, Cd, Be, Ca, Mn, Si, Se, S, Ge and Te. In this
prior art, the active layer is doped for the purpose of making the
emission wavelength longer. According to the publication, the
dopant introduced into the active layer allows a different emission
level to be generated in the energy band of the crystal, resulting
in a longer emission wavelength. The half-width of the emission
intensity spectrum relative to the emission wavelength can also be
changed depending on the combination of the dopants. If, for
example, Zn and Se are the dopants, an emission level as ZnSe can
be established, which is different from the level produced by
doping Zn or Se and also different from a simple sum of the levels
by Zn and Se. The publication, however, does not disclose any
specific amount of the dopant(s) introduced into the active layer.
The publication only discloses a light emitting device formed on a
sapphire substrate specifically. The publication also fails to note
the crystallinity of each layer formed on the substrate.
[0005] Japanese Patent Laying-Open No. 10-270804 discloses a light
emitting nitride semiconductor device having a light emitting layer
(an active layer) with a multi quantum well structure formed of
GaNAs, GaNP or GaNSb well layer/GaN barrier layer. In this
publication, a sapphire (.alpha.-Al.sub.2O.sub.3) substrate and a
SiC substrate are examples of the substrate. The publication does
not disclose any doped active layer.
[0006] The light emitting layer formed of GaNAs, GaNP or GaNSb
crystal can provide smaller effective mass of electrons and holes
as compared with InGaN crystal conventionally used. This suggests
that a population inversion for lasing can be obtained by a small
carrier density (and that a lasing threshold current value can be
reduced). If, however, As is contained in the light emitting layer
of nitride semiconductor, the light emitting layer can readily be
separated into a higher nitrogen content region and a higher As
content region. Hereinafter this phenomenon will be referred to as
"composition separation". The crystal system can further be
separated into a hexagonal system of the higher nitrogen content
region and the cubic system of the higher As content region. Such a
separation into different crystal systems (hereinafter referred to
as "crystal system separation") can result in a reduced luminous
efficacy due to the degraded crystallinity. Such crystal system
separation can be occurred in a P- or Sb-containing light emitting
nitride semiconductor layer as well as in the As-containing layer.
Thus, it has been desired that such crystal system separation
should be prevented for improved luminous efficacy (or emission
intensity).
SUMMARY OF THE INVENTION
[0007] An object of the present invention is to provide a light
emitting device having a higher luminous efficacy or emission
intensity by clarifying a structure capable of enhancing the
performance of the light emitting device using a light emitting
layer of nitride semiconductor containing at least one of As, P and
Sb.
[0008] The present inventors have found that the crystal system
separation due to As, P or Sb contained in the light emitting
nitride semiconductor layer can be reduced by doping the light
emitting layer with an impurity of at least one element of Mg, Be,
Zn, Cd, C, Si, Ge, Sn, O, S, Se and Te, so that the light emitting
nitride semiconductor device can have a good crystallinity and a
high luminous efficacy (or emission intensity).
[0009] Thus, the present invention is directed to a light emitting
nitride semiconductor device including: one of a substrate made of
nitride semiconductor crystal and a substrate having a nitride
semiconductor crystal film grown on a crystalline material other
than the nitride semiconductor crystal; an n-type layer and a
p-type layer each made of nitride semiconductor formed on the
substrate; and a light emitting layer provided between the n- and
p-type layers. The light emitting layer is formed of a well layer
or a combination of well and barrier layers. Of the layer(s)
forming the light emitting layer, at least the well layer is made
of a nitride semiconductor containing an element X, N and Ga,
wherein the element X is at least one selected from the group
consisting of As, P and Sb. In the nitride semiconductor forming
the light emitting layer, the ratio of the number of element X
atoms to the total number of the element X atoms and N atoms, is
not more than 30 atomic percent. Of the layer(s) forming the light
emitting layer, at least the well layer contains as an impurity at
least one element selected from the group consisting of Mg, Be, Zn,
Cd, C, Si, Ge, Sn, O, S, Se and Te.
[0010] In the present invention, the total content of the impurity
is 1.times.10.sup.17 to 5.times.10.sup.20/cm.sup.3.
[0011] In the present invention, preferably, the nitride
semiconductor crystal or the nitride semiconductor crystal film of
the substrate, or the light emitting nitride semiconductor device
has a threading dislocation density of not more than
3.times.10.sup.7/cm.sup.2 or an etch pit density of not more than
7.times.10.sup.7/cm.sup.2.
[0012] In the present invention, typically, the light emitting
layer may be a multi-quantum well layer.
[0013] The present invention is also directed to a light emitting
apparatus comprising the light emitting nitride semiconductor
device as aforementioned and having an emission wavelength of 380
nm to 650 nm.
[0014] The present invention is also directed to an optical pickup
apparatus including a light emission apparatus comprising the light
emitting nitride semiconductor device as aforementioned and having
an oscillation wavelength of 380 nm to 420 nm.
[0015] The foregoing and other objects, features, aspects and
advantages of the present invention will become more apparent from
the following detailed description of the present invention when
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] In the drawings:
[0017] FIG. 1 is a schematic cross section of an example of a light
emitting diode device grown on a nitride semiconductor
substrate;
[0018] FIG. 2 is a schematic cross section of an example of a quasi
GaN substrate;
[0019] FIG. 3(a) is a schematic cross section showing the process
of another example of the quasi GaN substrate, and
[0020] FIG. 3(b) is a schematic cross section showing the completed
structure thereof;
[0021] FIG. 4 is a schematic cross section of another example of
the light emitting diode device according to the present
invention;
[0022] FIG. 5 is a top view of the light emitting diode device
shown in FIG. 4;
[0023] FIG. 6 is a schematic cross section of an example of a laser
diode device according to the present invention;
[0024] FIG. 7 schematically shows an optical disc apparatus as one
example of an information recording apparatus; and
[0025] FIG. 8 represents a relationship between the amount of an
impurity added to the light emitting layer, and the crystal system
separation and the emission intensity of the device;
[0026] In the figures, an n-type GaN substrate is represented by a
reference numeral 100, a low temperature GaN buffer layer by 101,
an n-type GaN layer by 102, a light emitting layer by 103, a p-type
Al.sub.0.1Ga.sub.0.9N carrier block layer by 104, a p-type GaN
contact layer by 105, a transparent electrode by 106, a p electrode
by 107, an n electrode by 108, a dielectric film by 109, a quasi
GaN substrate by 200 and 200a, a seed substrate by 201, a low
temperature buffer layer by 202, an n-type GaN film by 203, a first
n-type GaN film by 203a, a second n-type GaN film by 203b, an
anti-growth film by 204, an n-type GaN thick film by 205, the
center of the width of the anti-growth film by 206, the center of
the width of the anti-growth film free portion by 207, the center
of the width of a groove by 208, the center of the width of the
groove free portion, i.e., a plateau by 209, a substrate by 300, an
n-type GaN substrate by 400, a low temperature GaN buffer layer by
401, an n-type Al.sub.0.05Ga.sub.0.95N layer by 402, an n-type
In.sub.0.07Ga.sub.0.93N anti-crack layer by 403, an n-type
Al.sub.0.1Ga.sub.0.9N clad layer by 404, an n-type GaN optical
guide layer by 405, a light emitting layer by 406, a p-type
Al.sub.0.2Ga.sub.0.8N carrier block layer by 407, a p-type GaN
optical guide layer by 408, a p-type Al.sub.0.1Ga.sub.0.9N clad
layer by 409, a p-type GaN contact layer by 410, an n electrode by
411, a p electrode by 412, and a SiO.sub.2 dielectric film by
413.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] The device according to the present invention has a
substrate made of nitride semiconductor crystal (hereinafter
referred to as a "nitride semiconductor substrate") or a substrate
having a nitride semiconductor crystal film grown on a crystalline
material other than the nitride semiconductor crystal (hereinafter
referred to as a "quasi nitride semiconductor substrate"). The
nitride semiconductor substrate generally has a low dislocation
density such as 10.sup.7/cm.sup.2 or less. Thus the nitride
semiconductor substrate may be used to fabricate a light emitting
nitride semiconductor device having a small threading dislocation
density of at most approximately 3.times.10.sup.7/cm.sup.2, or a
small etch pit density of at most approximately
7.times.10.sup.7/cm.sup.2 and hence good crystallinity. Such an
effect can also be obtained by employing the quasi nitride
semiconductor substrate. If the quasi nitride semiconductor
substrate is used, the nitride semiconductor crystal film grown on
the different crystalline material preferably has a dislocation
density of at most 10.sup.7/cm.sup.2 to reduce the dislocation
density of the device. The dislocation density may be represented
by an etch pit density or a threading dislocation density. The etch
pit density can be obtained by measuring a pit density on the
surface of a test piece such as a substrate which has been immersed
in an etchant of 1:3 of phosphoric acid and sulfuric acid (at
250.degree. C.) for 10 minutes. The threading dislocation density
can be measured with a transmission electron microscope.
[0028] For the light emitting layer containing As, P, Sb, in
particular, a high threading dislocation density results in a
reduced luminous efficacy and hence an increased threshold current
value. This is possibly because As, P, or Sb segregates in the
vicinity of the threading dislocation so that the crystallinity can
be degraded in the light emitting layer. The use of the nitride
semiconductor substrate or the quasi nitride semiconductor
substrate can prevent such an increase of a threshold current value
and the degradation of the crystallinity in the light emitting
layer. The nitride semiconductor substrate is also preferable as it
can provide good resonator ends with a small mirror loss through a
cleavage process. The nitride semiconductor substrate has high
thermal conductivity and thus serves as a good heat sink. The
nitride semiconductor substrate and the nitride semiconductor film
formed thereon can have substantially the same thermal expansion
coefficient, so that the wafer can have little distortion and the
yield of chips through the dividing process can be improved. Thus
it is particularly preferable to use the nitride semiconductor
substrate for the device of the present invention.
[0029] Principle of the Present Invention
[0030] In the conventional GaNAs well layer, crystal system
separation can readily be caused by the As contained in the layer,
resulting in degraded crystallinity and reduced luminous efficacy
of the light emitting nitride semiconductor device. This crystal
system separation can be generated in a GaNP or GaNSb well layer as
well as in the GaNAs well layer.
[0031] The crystal system separation may be caused by the fact that
the rate of the adsorption of As, P or Sb to Ga is extremely higher
than that of N to Ga and the fact that the volatility of N is
extremely higher than that of As, P or Sb, i.e., N can readily be
removed from the crystal. When a Ga source material and an N source
material are supplied to grow GaN crystal, at the outermost surface
(the epitaxial growth surface) of the growing GaN crystal, a part
of the supplied N material is combined with the Ga material to form
the GaN, while most of N, having high volatility, readily
re-evaporates. The re-evaporation of N results in some Ga failing
to form GaN crystal and diffusing in the epitaxial growth surface
and then re-evaporating. In such a process, if a source material of
As, P or Sb is supplied, the Ga diffusing in the epitaxial growth
surface can readily be adsorbed to As, P or Sb, since the rate of
the adsorption of As, P or Sb to Ga is extremely higher than that
of N to Ga. Thus the Ga--As, Ga--P or Ga--Sb bond is formed more
preferentially than the Ga--N bond. In addition, Ga has a long
surface migration length, which can give a high probability of the
collision of the Ga--As, Ga--P or Ga--Sb bonds. At the collision,
the bonds are fixed to facilitate crystallization. This is a
segregation effect due to the above-mentioned bond. This
segregation effect results in a composition separation into a
portion with a high content of the bond and a portion with a low
content of the bond. When the composition separation has advanced,
the portion with the high content of the bond finally forms a cubic
crystal system and the portion with the low content of the bond
finally forms a hexagonal crystal system. This is referred to as
crystal system separation.
[0032] In the present invention, the crystal system separation is
prevented by doping the light emitting nitride semiconductor layer
with at least one of Mg, Be, Zn, Cd, C, Si, Ge, Sn, O, S, Se and
Te. The impurity is distributed uniformly across the entire
epitaxial growth film to form a nucleus for crystal growth. This
nucleus traps the Ga--As, Ga--P or Ga--Sb bond. More specifically,
the introduction of the impurity to form the nucleus substantially
reduces the surface migration length of Ga. Thus, doping the entire
surface of the epitaxial growth film uniformly with the impurity
can reduce the collision of the Ga--As, Ga--P or Ga--Sb bonds, so
that localized significant crystallization can be prevented (i.e.,
the segregation effect can be reduced). Thus, the crystal system
separation can be prevented to improve the crystallinity of the
light emitting layer. The adsorption of the Ga atom to the Group V
is described in the above by way of illustration. The
above-described mechanism is also applicable to other Group III
atoms than Ga.
[0033] Relationship Between Doping and Crystal Defects in Light
Emitting Layer in the Present Invention
[0034] The relationship between the doping according to the present
invention and the crystal defects (mainly the threading
dislocation) will be described referring to FIG. 8. FIG. 8 shows
the degree of the crystal system separation generated in a Si-doped
GaN.sub.0.92P.sub.0.08 well layer having an emission wavelength of
520 nm, and the emission intensity. Herein the "degree of crystal
system separation" refers to the ratio by volume (in percentage) of
the portion with the crystal system separation to the crystal
system separation free portion (that is formed with the average
composition ratio) in a unit volume of the well layer. In FIG. 8,
the horizontal axis represents the amount of Si dopant introduced,
and the left vertical axis represents the degree (%) of the crystal
system separation and the right vertical axis represents the
emission intensity in an arbitrary unit. In FIG. 8, the emission
intensity is standardized to have a level of one when the well
layer is not doped with the impurity. In FIG. 8, the circle
represents the characteristics of the light emitting device with
the well layer grown on a GaN substrate (an example of the nitride
semiconductor substrate), and the square represents the
characteristics of the light emitting device with the well layer
grown on a sapphire substrate. As shown in the figure, the light
emitting device grown on the GaN substrate has a threading
dislocation density of approximately 1.times.10.sup.7/cm.sup.2 and
an etch pit density of not more than approximately
5.times.10.sup.7/cm.sup.2, and the light emitting device grown on
the sapphire substrate has a threading dislocation density of
approximately 1 to 10.times.10.sup.9/cm.sup.2 and an etch pit
density of not less than approximately 4.times.10.sup.8/cm.sup.2.
The etch pit density can be obtained by measuring a pit density on
the surface of the epitaxial wafer (the light emitting device)
which has been immersed in an etchant of 1:3 of phosphoric acid and
sulfuric acid (at 250.degree. C.) for 10 minutes. The etch pit
density is obtained as a result of measuring the surface of the
epitaxial wafer of the light emitting nitride semiconductor device,
which is not a result of directly measuring the defects in the
light emitting layer. However, if the etch pit density is high, the
light emitting layer has a high defect density, and therefore the
measured etch pit density will indicate whether the active layer
has a large number of defects.
[0035] FIG. 8 demonstrates that the crystal system separation can
be reduced by the doping according to the present invention more
effectively in the light emitting device grown on the GaN substrate
than in that grown on the sapphire substrate. The figure also shows
that the device on the GaN substrate has greater emission
intensity.
[0036] As well as the GaN substrate as described above, a substrate
having a structure with a GaN crystal film grown on a crystalline
material other than GaN crystal (hereinafter referred to as a
"quasi GaN substrate") is also preferable. The quasi GaN substrate
may be produced as described in detail below. In the nitride
semiconductor films grown on the quasi GaN substrate, the smallest
threading dislocation density is not more than approximately
3.times.10.sup.7/cm.sup.2 and the smallest etch pit density is not
more than approximately 7.times.10.sup.7/cm.sup.2. These values are
close to those of the nitride semiconductor film grown on the GaN
substrate. However, the quasi GaN substrate has different portions
with low and high threading dislocation densities in a mixed manner
and therefore it can provide a lower yield of the light emitting
device than the GaN substrate (the nitride semiconductor
substrate). In the light emitting device grown on the quasi GaN
substrate, the relationship between the amount of Si dopant
introduced and the degree of the crystal system separation and the
device's emission intensity is almost the same as that in the GaN
substrate as shown in FIG. 8. If the quasi GaN substrate is used,
to obtain such a result as shown in FIG. 8, the light emitting
device is desirably grown on portions with less crystal defects (or
less threading dislocations).
[0037] Thus it has been found that the emission intensity of the
light emitting device with less crystal defects (mainly threading
dislocations) is greater than that with more crystal defects, even
if both devices contain the impurity in the same concentration.
Thus the crystal defects may also trap the Ga--As, Ga--P or Ga--Sb
bond as well as the nucleus formation by the impurity. However, the
function of the crystal defects to trap the bond is significantly
greater than that of the nucleus formation by the impurity and
therefore the crystal defects may promote the segregation rather
than reduce the crystal system separation. The crystal defects are
not uniform and the threading dislocation, which is a main defect
of the crystal defects, is in the form of a pipe having a diameter
on the order of several nm to several tens nm. Such crystal defects
may cause a significant segregation effect. In contrast, the
nucleus formation by the impurity should be distributed uniformly
across the entire epitaxial growth film.
[0038] As can be seen from the foregoing, in order to improve the
emission intensity, it is desirable that the light emitting layer
is doped with the impurity and the GaN substrate (the nitride
semiconductor substrate) or the quasi GaN substrate is used for the
light emitting device. It has also been found that a threading
dislocation of not more than approximately
3.times.10.sup.7/cm.sup.2 or an etch pit density of not more than
approximately 7.times.10.sup.7/cm.sup.2 significantly improve the
effect of introducing the impurity in the present invention.
[0039] The obtained characteristics of the device having a light
emitting nitride semiconductor layer containing any one of As, P
and Sb are similar to those as shown in FIG. 8. A similar effect
can also be obtained when Si dopant is replaced by Mg, Be, Zn, Cd,
C, Ge, Sn, O, S, Se or Te dopant.
[0040] Impurity in Well Layer According to the Present Invention
and Its Amount
[0041] A description will now be provided of the impurity and its
amount to be introduced to produce the effect of the present
invention.
[0042] Initially, experiments are carried out to reveal the amount
of As, P or Sb for causing the above-described crystal system
separation. As a result, when the GaN crystal is doped with As, P
or Sb of 1.times.10.sup.18/cm.sup.2 or more, the crystal system
separation starts (with a crystal system separation degree of
approximately 2 to 3%), and the degree attains to approximately 12
to 13% when the number of the element atoms introduced amounts to
approximately 10 atomic percent of the total number of the Group V
element atoms in the nitride semiconductor.
[0043] Referring again to FIG. 8, the relationship between the
amount of the impurity introduced, and the crystal system
separation and the emission intensity will be described. In the
figure, as indicated by hollow circles, the degree of the crystal
system separation started to decrease (to 10% or less) at a dopant
amount of approximately 1.times.10.sup.17/cm.sup.3, was
approximately 6% or less at approximately
5.times.10.sup.17/cm.sup.3, started to gradually increase at
approximately 2.times.10.sup.19/cm.sup.3, abruptly increased at
more than 1.times.10.sup.20/cm.sup.3, and was 10% or more at more
than 5.times.10.sup.20/cm.sup.3. On the other hand, as indicated by
solid circles, similarly, the emission intensity started to
increase at a dopant amount of approximately
1.times.10.sup.17/cm.sup.3, abruptly increased at approximately
5.times.10.sup.17/cm.sup.3, had a peak around
5.times.10.sup.18/cm.sup.3, started to gradually decrease around
2.times.10.sup.19/cm.sup.3, abruptly decrease at more than
1.times.10.sup.20/cm.sup.3, and was no longer superior at more than
5.times.10.sup.20/cm.sup.3. This shows that there is a correlation
between the crystal system separation and the emission
intensity.
[0044] In FIG. 8, as indicated by the circles, the crystal system
separation was not prevented at a dopant amount of less than
1.times.10.sup.17/cm.sup.3. This may be because at such a dopant
amount, the residual crystal defects can trap As, P or Sb more
strongly than the impurity. On the other hand, the crystal system
separation started to gradually increase at a dopant amount of more
than 2.times.10.sup.19/cm.sup.3. This may be because the
crystallinity of the light emitting layer is degraded by the doping
itself.
[0045] As can be seen from the detail of FIG. 8, also in the light
emitting device grown on a sapphire substrate, which is indicated
by squares, the degree of the crystal system separation gradually
decreases as the impurity is introduced, and the emission intensity
accordingly gradually increases. Such an effect of reducing the
crystal system separation is, however, different from that of the
device on the GaN substrate. The light emitting device grown on the
sapphire substrate has a threading dislocation density higher than
that on the GaN substrate, so that it cannot efficiently exhibit
the effect of the introduced impurity. If the impurity
concentration is not higher than that in the device on the GaN
substrate, the crystal system separation cannot be reduced
effectively. As for the squares, it was expected that the crystal
system separation would further be reduced by doping at not less
than approximately 2.times.10.sup.19/cm.sup.3. In fact, however,
the degree of the crystal system separation increases as shown in
FIG. 8. The excessively introduced impurity seems to degrade the
crystallinity so significantly that the crystal system separation
increases.
[0046] Thus, for a high emission intensity or luminous efficacy in
the light emitting device using a GaN substrate or a quasi GaN
substrate with a low threading dislocation, the degree of the
crystal system separation is preferably not more than 10%, and more
preferably not more than approximately 6%. Such a degree of the
crystal system separation can be obtained by introducing the
impurity of 1.times.10.sup.17/cm.sup.3 to
5.times.10.sup.20/cm.sup.3 and preferably
5.times.10.sup.17/cm.sup.3 to 1.times.10.sup.20/cm.sup.3.
[0047] A similar effect was also obtained when the Si dopant was
replaced by Mg, Be, Zn, Cd, C, Ge, Sn, O, S, Se or Te dopant. When
a plurality kinds of the above dopants were introduced, a similar
effect was also obtained. If a plurality kinds of dopants are
introduced, the total amount thereof is desirably
1.times.10.sup.17/cm.sup.3 to 5.times.10.sup.20/cm.sup.3.
[0048] A barrier layer may be doped or may not be doped, since the
barrier layer does not directly contribute to the light emitting
through the recombination of the injected carriers. If the barrier
layer contains at least one of As, P and Sb, however, preferably it
is doped with the impurity as well as the well layer. By doing so,
the crystallinity of the barrier layer can also be improved, as
described above.
[0049] Impurity in the Present Invention
[0050] The impurity suitable for the present invention is Mg, Be,
Zn, Cd, C, Si, Ge, Sn, O, S, Se or Te. These impurity elements are
generally classified into Group II elements Be, Mg, Zn, Cd, Group
IV elements C, Si, Ge, Sn, and Group VI elements O, S, Se, Te.
[0051] The ionic bonding of Group IV elements is weaker than that
of Group II or VI elements (to be close to covalent bond) and it
mainly, simply inhibits a Ga--As, Ga--P or Ga--Sb bond from
diffusing in the surface (or substantially reduces the surface
migration length). A part of the ionic bonding of Group IV elements
is less substituted by the impurity as compared with the case of
the Group II or VI elements. In the case of the Group IV elements,
therefore, the sift level of the emission wavelength is not so
significant, and the collision of the bonding and the localized
large crystallization can be prevented (i.e., the effect of
reducing the crystal system separation can be obtained) only by
controlling the amount of the impurity.
[0052] Of the Group IV elements, Si is particularly preferable and
C and Ge are the next preferable elements in order of decreasing
the single-bond energy to N. If the element has a higher
single-bond energy to N, such an element hardly combines with N. In
the present invention, the crystal system separation is prevented
by reducing the segregation of As, P or Sb. In the present
invention, the impurity that adsorbs As, P, Sb rather than N is
preferably used.
[0053] The Group II elements forms positive ions and thus not only
inhibit the Ga--As, Ga--P or Ga--Sb bond from diffusing in the
surface but attract and adsorb the bond. Therefore, the amount of
the Group II element introduced for efficiently preventing the
crystal system separation can be smaller than that of the Group IV
element. In the case of the Group II elements, the degree of the
crystal separation started to decrease at a dopant amount of
approximately 5.times.10.sup.16/cm.sup.3, was minimum around
1.times.10.sup.18/cm.sup.3, and started to increase around
1.times.10.sup.20/cm.sup.3 or more. If the amount of the impurity
introduced is small, the degradation of the crystallinity by the
doping itself can be reduced.
[0054] The Group III elements (such as Al, Ga or In) that can
combine with As, P or Sb can be replaced with the Group II
impurity. When such replacement is occurred, As, P or Sb remains in
the crystal and the other Group III elements can re-evaporate, so
that the emission wavelength of the light emitting device can be
sifted to a somewhat longer wavelength. In the case that the Group
II elements are used for doping, therefore, it can be more
difficult to obtain the targeted emission wavelength through the
process as compared with the case that the Group IV elements are
used for doping. If, however, the light emitting layer should
contain so much As, P or Sb that the mixed crystal content can be
high (i.e., a long-wavelength light emitting device of
approximately 450 nm or more should be produced), the control of
the amount of the Group II element introduced can be easier than
the control of the amount of the supplied As, P or Sb source
material. This is because in such a case, a proportional
relationship cannot be established between the amount of the
supplied As, P or Sb material and the composition ratio of the
light emitting layer.
[0055] The Group VI elements forms negative ions and therefore they
not only inhibit the Ga--As, Ga--P or Ga--Sb bond from diffusing in
the surface but attract and adsorb the bond effectively. Therefore,
the amount of the Group VI element introduced for efficiently
preventing the crystal system separation can be smaller than that
of the Group IV element. In the case of the Group VI elements, for
example, the degree of the crystal separation started to decrease
at a dopant amount of approximately 5.times.10.sup.16/cm.sup.3, was
minimum around 1.times.10.sup.18/cm.sup.3, and started to increase
around 1.times.10.sup.20/cm.sup.3 or more. If the amount of the
impurity introduced is small, the degradation of the crystallinity
by the doping itself can be reduced.
[0056] The Group V elements (such as P, As or Sb) that can combine
with the Group III element such as Ga can be replaced with the
Group VI impurity. When such replacement is occurred, As, P or Sb
re-evaporates from the crystal, so that the emission wavelength of
the light emitting device can be sifted to a somewhat shorter
wavelength. In the case that the Group VI elements are used for
doping, therefore, it can be more difficult to obtain the targeted
emission wavelength through the process as compared with the case
that the Group IV elements are used for doping. If, however, the
content of As, P or Sb in the light emitting layer should be so low
that the mixed crystal content can be low (i.e., a short-wavelength
light emitting device of approximately 450 nm or less should be
produced), the control of the amount of the Group VI element
introduced can be easier than the control of the amount of the
supplied As, P or Sb source material. This is because As, P or Sb
is less volatile that N and can be easily incorporated into the
light emitting layer.
[0057] Preferable impurities for the As or P containing light
emitting layer will be described below.
[0058] Impurities for As-containing Light Emitting Layer
[0059] If the light emitting layer contains As, Ge or Si is most
preferable dopant, which is the Group IV element. Because the
covalent bond radii of Ge and Si (approximately 0.122 nm and
approximately 0.117 nm, respectively) are close to that of As
(approximately 0.121 nm), Ge and Si seems to be able to trap As
readily and appropriately.
[0060] Second preferable is Mg or Zn, which is the Group II
element. The ionic radii of Mg and Zn are approximately 0.065 nm
and 0.074 nm, respectively, which are close to that of Ga (0.062
nm), which is a Group III element and a main component of the light
emitting layer. Thus if Ga is replaced with Mg or Zn as described
above, defects or distortion can preferably be reduced in the light
emitting layer.
[0061] Third preferable is C, which is the Group IV element. The
covalent bonding radius of C is approximately 0.077 nm, which is
significantly close to that of N (0.070 nm), which a Group V
element and a main component of the light emitting. Therefore, if
the light emitting layer is made of C dopant containing nitride
semiconductor crystal, distortion or defects of the crystal due to
the doping can be reduced due to its covalent bonding radius
significantly close to that of N, a main component of the light
emitting layer.
[0062] Impurities for P-containing Light Emitting Layer If the
light emitting layer contains P, Si is most preferable dopant,
which is the Group IV element. Because the covalent bond radius of
Si (approximately 0.117 nm) is close to that of P (approximately
0.110 nm), Si seems to be able to trap P readily and
appropriately.
[0063] Second preferable is Mg or Zn, which is the Group II
element. The ionic radii of Mg and Zn are approximately 0.065 nm
and 0.074 nm, respectively, which are close to that of Ga (0.062
nm), which is a Group III element and a main component of the light
emitting layer. Thus if Ga is replaced with Mg or Zn as described
above, defects or distortion can preferably be reduced in the light
emitting layer.
[0064] Third preferable is C, which is the Group IV element. The
covalent bonding radius of C is approximately 0.077 nm, which is
significantly close to that of N (0.070 nm), which a Group V
element and a main component of the light emitting. Therefore, if
the light emitting layer is made of C dopant containing nitride
semiconductor crystal, distortion or defects of the crystal due to
the doping can be reduced due to its covalent bonding radius
significantly close to that of N, a main component of the light
emitting layer.
[0065] Process of Introducing the Impurity
[0066] In the process of introducing the impurity, the impurity may
be introduced before the light emitting layer (a well layer or a
barrier layer) is formed, or the impurity may be introduced in the
growth process of the light emitting layer (a well layer or a
barrier layer).
[0067] In the former manner, i.e., when the impurity is used before
the growth process of the light emitting layer, the impurity can
form a crystal nucleus before the light emitting layer is grown.
Thus, the crystal system separation can be prevented efficiently
from the initial stage of forming the light emitting layer. This
can prevent the light emitting layer from being affected by the
underlying layer and from having the crystal system separation. The
former manner is effective in the case that the As, P or Sb
containing light emitting layer has a relatively small
thickness.
[0068] In the latter manner, i.e., when the impurity is introduced
during the growth of the light emitting layer, the impurity forms a
crystal nucleus in the process of the light emitting layer growth,
so that the crystal system separation slightly remains in the light
emitting layer. In this case, however, the introduction of the
impurity synchronized with the growth of the light emitting layer
can provide respective layers with the crystal system separations
prevented in the same manner in the crystal growth direction. That
is, in the latter manner, the local generation of an intensive
crystal system separation can be prevented in the light emitting
layer, so that the crystallinity can be uniform over the layer.
Thus the latter manner is effective in growing a thick light
emitting layer.
[0069] Preferably, the impurity is dispersed in the light emitting
layer uniformly. This can effectively reduce the substantial
surface migration length of the Ga--As, Ga--P or Ga--Sb bond and
thus prevent the bond from being locally fixed into segregation.
The impurity is preferably introduced by the method using a gaseous
impurity material, such as MOCVD.
[0070] Light Emitting Layer for the Present Invention
[0071] In the present invention, the light emitting layer may be
formed simply of a well layer, or it may have a structure in which
well and barrier layers are stacked alternately. In the present
invention, of the layers constituting the light emitting layer, at
least the well layer is made of a nitride semiconductor containing
an element X that is at least one selected from the group
consisting of As, P and Sb. If the light emitting layer is composed
of the combination of well and barrier layers, only the well
layer(s) may be formed of such a nitride semiconductor or the well
and barrier layers may be formed of such a nitride semiconductor.
Such a nitride semiconductor further contains Ga and N. In such a
nitride semiconductor, element X has an atomic fraction smaller
than N. Additionally, in such a nitride semiconductor, the ratio of
the number N.sub.1 of element X to the total number of number
N.sub.1 and the number N.sub.2 of element N, is not more than 30
atomic percent, preferably not more than 20 atomic percent. The
layer formed of such a nitride semiconductor (the well layer or the
well and barrier layers) preferably has an element X concentration
of not less than 1.times.10.sup.18/cm.sup.3. If
{N.sub.1/(N.sub.1+N.sub.2)}.times.100(%) exceeds 30%, the
introduction of the impurity cannot achieve sufficient prevention
of the crystal system separation and the crystallinity of the light
emitting layer is degraded. If
{N.sub.1/(N.sub.1+N.sub.2)}.times.10- 0(%) is not more than 20%,
the impurity introduced can prevent the crystal system separation
more effectively. If the element X concentration is not less than
1.times.10.sup.18/cm.sup.3, the introduction of the impurity can
significantly restrain the crystal system separation. Such a
composition of the nitride semiconductor can restrain the crystal
system separation as described above to make the crystallinity of
the well layer good, resulting in a high emission intensity or a
low lasing threshold current density. A similar mechanism can be
applied to the barrier layer, although the barrier layer is not
required to contain As, P or Sb and it is only required to have a
greater band gap energy than the well layer.
[0072] In the present invention, the nitride semiconductor forming
at least the well layer can typically be represented by the formula
In.sub.xAl.sub.yGa.sub.1-x-yN.sub.tAs.sub.uP.sub.vSb.sub.z, wherein
0.ltoreq.x<1, 0.ltoreq.y<1, and 0<u+v+z<t. In the
formula, t+u+v+z may be one. At least one of u, v and z is not
zero. (u+v+z)/(u+v+z+t) is not more than 0.3 and preferably not
more than 0.2.
[0073] Thickness of Light Emitting Layer
[0074] In the present invention, the well layer is preferably 0.4
nm to 20 nm in thickness. If the well layer has a thickness smaller
than 0.4 nm, a carrier confinement level by the quantum well effect
can be too high such that the luminous efficacy can be reduced. If
the well layer has a thickness greater than 20 nm, crystallinity is
degraded, depending on the As, P, Sb content in the well layer.
[0075] In the present invention, the barrier layer is preferably 1
nm to 20 nm in thickness. If the barrier layer has a thickness
smaller than 1 nm, carriers might be confined insufficiently. If
the barrier layer has a thickness greater than 20 nm, it would be
difficult to form a subband structure for the multi quantum well
layer.
[0076] Structure of Light Emitting layer
[0077] In the present invention, the light emitting layer is
typically composed of the combination of the well and barrier
layers as shown in Table 1. Alternatively, the light emitting layer
may have a composition containing the Group III element(s)
presented in Table 1 and N, and two or more additional elements
selected from the group consisting of As, P and Sb. The total
content of the additional elements relative to all of the Group V
elements forming the light emitting layer, is not more than 30
atomic percent, preferably not more than 20 atomic percent. In
Table 1, for the light emitting layer in the present invention, the
triangle indicates applicable combinations, the circle indicates
preferable combinations, and the double circle indicates most
preferable combinations. If the light emitting layer has a
mono-quantum well structure formed simply of a well layer, the well
layers in Table 1 that contain Sb can be marked by the triangle and
the remainder can be marked by the double circles.
1TABLE 1 Barrier layer GaN GaNAs GaNP GaNSb InGaN InGaNAs InGaNP
InGaNSb AlGaN GaNAs .circleincircle. .largecircle. .largecircle.
.DELTA. .circleincircle. .largecircle. .largecircle. .DELTA.
.largecircle. GaNP .circleincircle. .largecircle. .largecircle.
.DELTA. .circleincircle. .largecircle. .largecircle. .DELTA.
.largecircle. GaNSb .DELTA. .DELTA. .DELTA. .DELTA. .DELTA. .DELTA.
.DELTA. .DELTA. .DELTA. InGaNAs .circleincircle. .largecircle.
.largecircle. .DELTA. .circleincircle. .largecircle. .largecircle.
.DELTA. .largecircle. InGaNP .circleincircle. .largecircle.
.largecircle. .DELTA. .circleincircle. .largecircle. .largecircle.
.DELTA. .largecircle. InGaNSb .DELTA. .DELTA. .DELTA. .DELTA.
.DELTA. .DELTA. .DELTA. .DELTA. .DELTA. AlGaNAs .circleincircle.
.largecircle. .largecircle. .DELTA. .circleincircle. .DELTA.
.DELTA. .DELTA. .circleincircle. AlGaNP .circleincircle.
.largecircle. .largecircle. .DELTA. .circleincircle. .DELTA.
.DELTA. .DELTA. .circleincircle. AlGaNSb .DELTA. .DELTA. .DELTA.
.DELTA. .DELTA. .DELTA. .DELTA. .DELTA. .DELTA. InAlGaNAs
.circleincircle. .largecircle. .largecircle. .DELTA.
.circleincircle. .DELTA. .DELTA. .DELTA. .circleincircle. InAlGaNP
.circleincircle. .largecircle. .largecircle. .DELTA.
.circleincircle. .DELTA. .DELTA. .DELTA. .circleincircle. InAlGaNSb
.DELTA. .DELTA. .DELTA. .DELTA. .DELTA. .DELTA. .DELTA. .DELTA.
.DELTA. Barrier layer AlGaNAs AlGaNP AlGaNSb InAlGaN InAlGaNAs
InAlGaNP InAlGaNSb GaNAs .DELTA. .DELTA. .DELTA. .largecircle.
.DELTA. .DELTA. .DELTA. GaNP .DELTA. .DELTA. .DELTA. .largecircle.
.DELTA. .DELTA. .DELTA. GaNSb .DELTA. .DELTA. .DELTA. .DELTA.
.DELTA. .DELTA. .DELTA. InGaNAs .DELTA. .DELTA. .DELTA.
.largecircle. .DELTA. .DELTA. .DELTA. InGaNP .DELTA. .DELTA.
.DELTA. .largecircle. .DELTA. .DELTA. .DELTA. InGaNSb .DELTA.
.DELTA. .DELTA. .DELTA. .DELTA. .DELTA. .DELTA. AlGaNAs
.largecircle. .largecircle. .DELTA. .largecircle. .DELTA. .DELTA.
.DELTA. AlGaNP .largecircle. .largecircle. .DELTA. .largecircle.
.DELTA. .DELTA. .DELTA. AlGaNSb .DELTA. .DELTA. .DELTA. .DELTA.
.DELTA. .DELTA. .DELTA. InAlGaNAs .largecircle. .largecircle.
.DELTA. .circleincircle. .largecircle. .largecircle. .DELTA.
InAlGaNP .largecircle. .largecircle. .DELTA. .circleincircle.
.largecircle. .largecircle. .DELTA. InAlGaNSb .DELTA. .DELTA.
.DELTA. .DELTA. .DELTA. .DELTA. .DELTA. .circleincircle. a most
preferable combination of well + barrier layers for the light
emitting layer of the present invention .largecircle. a preferable
combination of well + barrier layers for the light emitting layer
of the present invention .DELTA. a combination of well + barrier
layers for the light emitting layer of the present invention
[0078] As described above, the addition of the impurity to the As,
P or Sb containing light emitting layer can reduce the crystal
system separation and improve the sharpness of the interface
between the well and barrier layers. This facilitates fabricating
the combinations of the well and barrier layers presented in Table
1 (multi-quantum well structures). In contrast, a conventional,
impurity-free light emitting layer contains portions having
different crystal systems in a mixed manner, so that the sharpness
of the interface between its well and barrier layers gets
significantly worse as the number of the stacked light emitting
layers increases. Such degradation in the sharpness of the
interface makes it difficult to fabricate a multilayered structure
(a multi-quantum well structure) itself and the light emitting
layer also provides an unevenness of color and reduced emission
intensity. According to the present invention, such disadvantage of
the prior art can be overcome by the addition of the impurity to
the light emitting nitride semiconductor containing at least one of
As, P and Sb and the multi-quantum well structure can easily be
formed. Preferably, the multi-quantum well structure provides an
emission intensity greater than a mono-quantum well structure and
provides a laser diode with a smaller threshold current density.
More specific compositions of the well and barrier layers forming
the light emitting layer will be described below.
[0079] GaNX Well Layer (X is As, P, Sb or Any Combination
Thereof)
[0080] If a well layer is formed of GaNX crystal, it does not
contain In and is thus free of the In segregation-induced
composition separation. The In composition separation herein means
that a single layer is separated into a region with a high In
content and a region with a low In content (and the regions are
mixed in the layer). The well layer free of the In-induced
composition separation does not have a non-light emitting region
caused by a high In content and it can preferably be free of a
factor increasing the threshold current value of the device.
[0081] Of GaNX crystals, the 3-element mixed crystal of GaNAs, GaNP
or GaNSb has a composition easier to control than the 4-element
mixed crystal of GaNAsP and the 5-element mixed crystal of
GaNAsPSb. Thus the targeted wavelength can be obtained in a good
reproducibility. Of P, As and Sb, P has a atomic radius (a Van der
Waals radius or covalent bond radius) closest to that of N and
therefore it can displace a portion of the N atoms in the mixed
crystal more easily than As and Sb. Thus GaN with P added thereto,
or GaNP, can have good crystallinity. This suggests that an
increased P content in GaNP may not so severely degrade the
crystallinity of the mixed crystal. When the light emitting device
uses a GaNP well layer, the GaNP crystal can cover a wide emission
wavelength range from ultra violet light emission to red light
emission.
[0082] Of P, As, and Sb, Sb has the largest atomic radius (or Van
der Waals radius or covalent bond radius) relative to that of N,
and as compared to As and Sb, it has a weaker tendency to displace
a portion of the N atoms in the mixed crystal. However, the Sb
atomic radius greater than that of As and P can prevent the removal
of highly volatile N atoms from the mixed crystal and thus make the
crystallinity of GaNSb good.
[0083] The atomic radius of As is intermediate between those of P
and Sb and therefore GaNAs can preferably have both characteristics
of GaNP and GaNSb.
[0084] The emission wavelength of the light emitting layer
employing the GaNX well layer can be modified by the modulation of
the As, P or Sb content ratio in the well layer. For example, to
obtain a emission wavelength around UV 380 nm, in
GaN.sub.1-xAs.sub.x x should be 0.001, in GaN.sub.1-yP.sub.y y
should be 0.01, and in GaN.sub.1-zSb.sub.z z should be 0.02. To
obtain an emission wavelength around 410 nm of blue-violet color,
in GaN.sub.1-xAs.sub.x x should be 0.02, in GaN.sub.1-yP.sub.y y
should be 0.03, and in GaN.sub.1-zSb.sub.z z should be 0.01. To
obtain a wavelength around 470 nm of blue color, in
GaN.sub.1-xAs.sub.x x should be 0.03, in GaN.sub.1-yP.sub.y y
should be 0.06, and in GaN.sub.1-zSb.sub.z z should be 0.02. To
obtain a wavelength around 520 nm of green color, in
GaN.sub.1-xAs.sub.x x should be 0.05, in GaN.sub.1-yP.sub.y y
should be 0.08, and in GaN.sub.1-zSb.sub.z z should be 0.03. To
obtain a wavelength around 650 nm of red color, in
GaN.sub.1-xAs.sub.x x should be 0.07, in GaN.sub.1-yP.sub.y y
should be 0.12, and in GaN.sub.1-zSb.sub.z z should be 0.04. The
above composition ratios or near ratios can complete the targeted
emission wavelength.
[0085] When Al is added to the GaNX well layer, the As, P or Sb
content should be higher than that for the aforementioned emission
wavelengths, because the Al added increases the band gap energy.
The addition of Al to the GaNX well layer is preferable, however,
because the crystallinity of the well layer can be improved. The N
element in the GaNX well layer is significantly more volatile than
As, P and Sb, and N can readily be removed from the crystal, so
that the crystallinity of the well layer can be degraded. When Al
is added to the GaNX well layer, Al that is highly reactive can
strongly combine with N, so that the removal of N from the well
layer can be prevented and the degradation in crystallinity can be
reduced.
[0086] The GaNX well layer is preferably combined with a barrier
layer of GaN, GaNAs, GaNP, InGaN, InGaNAs, InGaNP, AlGaN or
InAlGaN. Particularly, in GaN, InGaN, AlGaN, which are a 2-element
mixed crystal or a 3-element mixed crystal composed of two types of
Group III elements and one type of a Group V element, the
composition can readily be controlled and therefore desired
compounds can be formed in a good reproducibility. In particular,
InGaN is preferable as its crystallinity can be better than that of
GaN or AlGaN when it is produced at the temperature range for
growing the GaNX well layer, such as 600.degree. C. to 800.degree.
C. When the barrier layer is made of GaN, the crystallinity of
which can be better than that of AlGaN, the interface between the
well and barrier layers can be so flat that the luminous efficacy
can be improved.
[0087] InGaNX Well Layer
[0088] When the well layer is formed of InGaNX crystal, it can have
the composition separation due to the effect of the In segregation.
Like In, however, As, P or Sb can reduce the band gap energy of the
well layer, and therefore the In content in the InGaNX well layer
can be lower that that in the conventional InGaN well layer to give
the targeted emission wavelength. When at least one of As, P and Sb
is added to the In-containing well layer, the content of In can be
low (so that the composition separation can be reduced) while the
well layer can have moderate In segregation. The moderate In
segregation can provide a localized level for the trap of the
carriers of the electrons and holes, so that the luminous efficacy
can be improved and the threshold current value can be lowered.
[0089] Of InGaNX crystals, the 4-element mixed crystal of InGaNAs,
InGaNP or InGaNSb can have a composition easier to control than the
5-element mixed crystal of InGaNAsP and the 6-element mixed crystal
of InGaNAsPSb, so that the targeted emission wavelength can be
provided in a good reproducibility.
[0090] Of P, As, and Sb, P has an atomic radius (a Van der Waals
radius or covalent bond radius) closest to that of N, and as
compared to As and Sb, it has a stronger tendency to displace a
portion of the N atoms in the mixed crystal. Thus InGaN with P
added thereto, or InGaNP, can have a good crystallinity. This
suggests that an increased P content in InGaNP may not so severely
degrade the crystallinity of the mixed crystal. When the light
emitting device uses a InGaNP well layer, the InGaNP crystal can
cover a wide emission wavelength range from ultra violet light
emission to red light emission.
[0091] Of P, As, and Sb, Sb has the largest atomic radius (or Van
der Waals radius or covalent bond radius) relative to that of N,
and as compared with As or Sb, it has a weaker tendency to displace
a portion of the N atoms in the mixed crystal. However, the Sb
atomic radius greater than that of As and P can prevent the removal
of highly volatile N atoms from the mixed crystal and thus make the
crystallinity of InGaNSb good.
[0092] The atomic radius of As is intermediate between those of P
and Sb and therefore InGaNAs can preferably have both
characteristics of InGaNP and InGaNSb.
[0093] The emission wavelength of the light emitting layer
employing the InGaNX well layer can be modified by the modulation
of the As, P or Sb content in the well layer. For example, Table 2
presents a relationship between the compositions of InGaNAs and
InGaNP, and the emission wavelength. In preparing the well layer,
the compositions shown in Table 2 or near compositions can complete
the targeted emission wavelength.
2TABLE 2 In.sub.yGa.sub.1-yN.sub.1-xAs.sub.x In(y = 0.01) In(y =
0.02) In(y = 0.05) In(y = 0.1) In(y = 0.2) In(y = 0.35) Emission
wavelength 380 nm 0.005 0.004 0.001 400 nm 0.012 0.011 0.008 0.003
410 nm 0.016 0.015 0.011 0.006 470 nm 0.034 0.033 0.029 0.024 0.014
0.001 520 nm 0.046 0.045 0.041 0.036 0.025 0.012 650 nm 0.07 0.069
0.065 0.059 0.048 0.034 P content (x) for wavelength of
In.sub.yGa.sub.1-yN.sub.1-xAs.sub.x crystal
In.sub.yGa.sub.1-yN.sub.1-xP.sub.x In(y = 0.01) In(y = 0.02) In(y =
0.05) In(y = 0.1) In(y = 0.2) In(y = 0.35) Emission wavelength 380
nm 0.008 0.006 0.001 400 nm 0.02 0.018 0.013 0.004 410 nm 0.025
0.023 0.018 0.01 470 nm 0.055 0.053 0.047 0.038 0.022 0.001 520 nm
0.075 0.073 0.067 0.058 0.041 0.019 650 nm 0.116 0.114 0.107 0.097
0.079 0.055 P content (x) for wavelength of
In.sub.yGa.sub.1-yN.sub.1-xP.sub.x crystal
[0094] When Al is added to the InGaNX well layer, the In content
and the As, P or Sb content should be higher than those for the
emission wavelengths as shown in Table 2, because the Al added
increases the band gap energy. The addition of Al to the InGaNX
well layer is preferable, however, because the crystallinity of the
well layer can be improved. The N element in the InGaNX well layer
is significantly more volatile than As, P and Sb, and N can readily
be removed from the crystal, so that the crystallinity of the well
layer can be degraded. When Al is added to the InGaNX well layer,
Al that is highly reactive can strongly combine with N, so that the
removal of N from the well layer can be inhibited.
[0095] The InGaNX well layer is preferably combined with a barrier
layer of GaN, GaNAs, GaNP, InGaN, InGaNAs, InGaNP, AlGaN or
InAlGaN. Particularly, in GaN, InGaN, AlGaN, which are a 2-element
mixed crystal or a 3-element mixed crystal composed of two types of
Group III elements and one type of a Group V element, the
composition can readily be controlled and therefore desired
compounds can be formed in a good reproducibility. In particular,
InGaN is preferable as its crystallinity can be better than that of
GaN or AlGaN when it is produced at the temperature range for
growing the InGaNX well layer, such as 600.degree. C. to
800.degree. C. When the barrier layer is made of GaN, the
crystallinity of which can be better than that of AlGaN, the
interface between the well and barrier layers can be so flat that
the luminous efficacy can be improved.
EXAMPLE 1
[0096] A light emitting device having the structure as shown in
FIG. 1 was fabricated. Referring to FIG. 1, the light emitting
nitride semiconductor diode device is composed of an n-type GaN
substrate 100 having the C (0001) plane, a low temperature GaN
buffer layer 101 (of 100 nm in thickness), an n-type GaN layer 102
(having a thickness of 3 .mu.m and a Si impurity concentration of
1.times.10.sup.18/cm.sup.3), a light emitting layer 103, a p-type
Al.sub.0.1G.sub.0.9N carrier block layer 104 (having a thickness of
20 nm and a Mg impurity concentration of
6.times.10.sup.19/cm.sup.3), a p-type GaN contact layer 105 (having
a thickness of 0.1 .mu.m and a Mg impurity concentration of
1.times.10.sup.20/cm.sup.3), a transparent electrode 106, a p
electrode 107, and an n electrode 108. The device was fabricated by
the following process.
[0097] First, in a metal-organic chemical vapor deposition (MOCVD)
apparatus, n-type GaN substrate 100 was placed, and NH.sub.3
(ammonia) that is a Group V source material, and TMGa
(trimethylgallium) or TEGa (tryethylgallium) that is a Group III
source material, were used to grow low temperature GaN buffer layer
101 at 550.degree. C. to have a thickness of 100 nm. Then at
1050.degree. C. SiH.sub.4 (silane) was added to the source
materials and n-type GaN layer 102 (having a Si impurity
concentration of 1.times.10.sup.18/cm.sup.3) of 3 .mu.m in
thickness was formed. Then the substrate temperature was decreased
to 800.degree. C., and while SiH.sub.4 was introduced as a Si
impurity source, PH.sub.3 or TBP (t-butylphosphine) was introduced
as a P source material to grow GaN.sub.0.92P.sub.0.08 light
emitting layer 103 of 4 nm thick. This light emitting layer has a
single quantum well structure.
[0098] If As is added to the light emitting layer, AsH.sub.3 or
TBAs (t-butylarsine) may be used. If Sb is added to the light
emitting layer, TMSb (trimethylantimony) or TESb (triethylantimony)
may be used. In forming the light emitting layer, dimethylhydrazine
may be used in place of NH.sub.3 as the N source material.
[0099] Then the substrate temperature was increased again to
1050.degree. C. and TMAl (trimethylaluminum) or TEAl
(triethylaluminum) that is a Group III source material was used to
grow p-type Al.sub.0.1Ga.sub.0.9N carrier block layer 104 of 20 nm
thick and subsequently grow p-type GaN contact layer 105 of 0.1
.mu.m thick. As the p-type impurity, Mg was added in a
concentration of 5.times.10.sup.19/cm.sup.3 to
2.times.10.sup.20/cm.sup.3. The source of Mg was EtCP.sub.2Mg
(bisethylcyclopentadienylmagnesium). Preferably, p-type GaN contact
layer 105 has a p-type impurity concentration increasing as it
approaches the location at which transparent electrode 106 is
formed. Such an impurity distribution can prevent the crystal
defects from being increased by the impurity introduction, and can
reduce the contact resistance of the p electrode. A small amount of
oxygen may also be added to the p-type layer being grown to remove
the hydrogen remaining in the p-type layer, because the hydrogen
can interfere with the activation of Mg serving as the p-type
impurity.
[0100] After p-type GaN contact layer 106 was grown, the atmosphere
in the reactor of the MOCVD apparatus was replaced by absolute
nitrogen carrier gas and NH.sub.3 and the temperature was decreased
at a rate of 60.degree. C./minute. After the substrate temperature
reached 800.degree. C., the NH.sub.3 supply was stopped and the
substrate was allowed to stand at 800.degree. C. for five minutes
and its temperature was then lowered to room temperature. In this
process, the substrate may preferably be held at a temperature of
650.degree. C. to 900.degree. C. and allowed to stand for three to
ten minutes. The temperature may also be reduced preferably at a
rate of not less than 30.degree. C./minute.
[0101] The grown film was evaluated by Raman spectroscopy and it
was found that the film already had p-type characteristics (i.e.,
Mg was already activated) without annealing, a conventional
technique for making nitride semiconductor films have p-type
conductivity. The contact resistance had already been reduced
enough for forming the p electrode. When the conventional annealing
to give p-type conductivity was also used, the rate of activated Mg
was preferably improved.
[0102] Then the epi-wafer was taken out from the MOCVD apparatus
and electrodes were formed. Since n-type GaN substrate 100 was
used, Hf and Au metal films were deposited on the back surface of
substrate 100 in this order to form n electrode 108. The n
electrode materials may be replaced by Ti/Al, Ti/Mo, Hf/Al or the
like. In particular, Hf is preferably used to reduce the contact
resistance of the n electrode.
[0103] In forming the p electrode, Pd of 7 nm thick was
vapor-deposited for transparent electrode 106 and Au was
vapor-deposited for p electrode 107. Alternatively the material for
the transparent electrode may be Ni or Pd/Mo, Pd/Pt, Pd/Au, Ni/Au
or the like.
[0104] Finally, a scriber was used to divide the product into
chips. In doing so the scriber was applied on the back surface of
n-type GaN substrate 100 (the side having n electrode 108 deposited
thereon) to prevent debris from adhering, in the scribing step, to
the transparent electrode side for taking light. In the scribing
step, the product was divided into chips in such a manner that at
least one side of the chip has a cleavage plane of the nitride
semiconductor substrate. This prevents the chips from having an
abnormal geometry due to chipping, cracking and the like and thus
increases yield per wafer.
[0105] In the above process, the light emitting nitride
semiconductor diode device as shown in FIG. 1 was prepared, with
different amounts of dopant Si in the light emitting layer. The
obtained device was examined for the emission intensity and the
crystal system separation degree in the light emitting layer and
the relationship as shown in FIG. 8 was obtained. When the light
emitting layer was doped with the impurity (Si) in a concentration
of 1.times.10.sup.18/cm.sup.3, 5.times.10.sup.18/cm.su- p.3,
2.times.10.sup.19/cm.sup.3, or 1.times.10.sup.20/cm.sup.3, the
device had significantly high emission intensity. Preferable
results were obtained in the impurity concentration range from
1.times.10.sup.17 to 5.times.10.sup.20/cm.sup.3.
[0106] In the device as shown in FIG. 1, the low temperature GaN
buffer layer may be replaced with a low temperature
Al.sub.xGa.sub.1-xN buffer layer, wherein 0.ltoreq.x.ltoreq.1.
Alternatively, the low temperature GaN buffer layer may not be
used. If the GaN substrate does not have a preferable surface
morphology, however, the low temperature Al.sub.xGa.sub.1-xN buffer
layer, wherein 0.ltoreq.x.ltoreq.1, is preferably provided to
improve the surface morphology. Herein the "low temperature buffer
layer" refers to a buffer layer grown at a temperature of
approximately 450 to 600.degree. C. The buffer layer grown in such
a temperature range is polycrystalline or amorphous.
[0107] In the device shown FIG. 1, the light emitting layer may
have a multi-quantum well structure in place of the single quantum
well structure. If the layer has a multi-quantum well structure, it
may have a structure starting and ending with a barrier layer or a
structure starting and ending with a well layer. Not more than 10
well layers preferably gave high intensity of the light emitting
diode.
[0108] In the device shown in FIG. 1, the p-type
Al.sub.0.1Ga.sub.0.9N carrier block layer may be replaced with an
AlGaN layer having an Al content other than 0.1. An increased Al
content can preferably enhance the carrier confinement in the well
layer. In contrast, the Al content may be reduced within a certain
range that the carrier confinement is maintained. In such a case,
the carrier mobility in the carrier block layer can preferably be
increased and the electrical resistance can preferably be lowered.
The carrier block layer is not limited to the 3-element mixed
crystal of AlGaN and it may be a 4-element mixed crystal such as
AlInGaN, AlGaNP, AlGaNAs or the like.
[0109] In the device according to the present invention, the n
electrode may be formed on the n-type GaN layer exposed on the side
of the p electrode by dry etching, as shown in FIG. 4.
[0110] As regards the crystal plane on which the device structure
should be formed, the GaN substrate C (0001) plane may be replaced
by the C (000-1) plane, the A {11-20} plane, the R {1-102} plane,
the M {1-100} or {1-101} plane. Furthermore, a substrate surface
forming an offset angle within two degrees with the above crystal
plane can preferably have a good surface morphology. In the present
invention, any substrate made of nitride semiconductor,
particularly including an Al.sub.xGa.sub.yIn.sub.z- N substrate,
wherein 0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1,
0.ltoreq.z.ltoreq.1 and x+y+z=1, may be used. The substrate may be
doped with Si, O, Cl, S, C, Ge, Zn, Cd, Mg or Be. For an n-type
nitride semiconductor substrate, Si, O and Cl are particularly
preferable.
[0111] While the present device is produced by the MOCVD, it may be
produced by molecular beam epitaxy (MBE), hydride vapor phase
epitaxy (HVPE) or the like.
EXAMPLE 2
[0112] A light emitting nitride semiconductor diode was fabricated
as in Example 1 except that the GaN substrate 100 shown in FIG. 1
was replaced by a quasi GaN substrate 200 shown in FIG. 2 or a
quasi GaN substrate 200a shown in FIG. 3(b) and that, as shown in
FIG. 4, the n electrode was formed on the same side as the p
electrode. The quasi GaN substrate will be described with reference
to FIGS. 2 and 3 and then will be described a light emitting diode
employing the quasi GaN substrate.
[0113] The quasi GaN substrate 200 shown in FIG. 2 is composed of a
seed substrate 201, a low temperature buffer layer 202, an n-type
GaN film 203, an anti-growth film 204, and an n-type GaN thick film
205. Quasi GaN substrate 200 has seed substrate 201 other than a
nitride semiconductor substrate and seed substrate 201 is used as a
base for growing n-type GaN thick film 205. The anti-growth film
refers to a film that inhibits nitride semiconductor crystal from
being grown directly thereon.
[0114] FIG. 3(a) shows an intermediate step in a process of
producing quasi GaN substrate 200a and FIG. 3(b) shows complete
quasi GaN substrate 200a. The quasi GaN substrate 200a shown in
FIG. 3(b) is composed of a seed substrate 201, a low temperature
buffer layer 202, a first n-type GaN film 203a and a second n-type
GaN film 203b. As shown in FIG. 3(a), initially on seed substrate
201, low temperature buffer layer 202 is formed and thereon the
first n-type GaN film 203a is formed and then the surface of GaN
film 203a is dry-etched or wet-etched to have a groove. The product
is then transported again to the crystal growth apparatus and the
second n-type GaN film 203b is deposited to complete quasi GaN
substrate 200a (FIG. 3(b)). While, as shown in FIG. 3(a), the
substrate has a groove only reaching an intermediate portion of the
first n-type GaN film 203a, it may have a groove reaching low
temperature buffer layer 202 or seed substrate 201.
[0115] When a nitride semiconductor film was grown on quasi GaN
substrate 200 or 200a, the obtained film had a dislocation density
(a threading dislocation density of approximately
3.times.10.sup.7/cm.sup.2 and an etch pit density of approximately
7.times.10.sup.7/cm.sup.2), which is lower than that of the film
grown on a sapphire substrate, a SiC substrate or the like (a
threading dislocation density of approximately 1 to
10.times.10.sup.9/cm.sup.2 and an etch pit density of approximately
4.times.10.sup.8/cm.sup.2).
[0116] The quasi GaN substrate shown in FIG. 2 has a higher
threading dislocation density at a portion 206 located exactly
above the center of an anti-growth film having a predetermined
width and at a portion 207 located exactly above the center of the
anti-growth film free portion having a predetermined width.
Similarly, the quasi GaN substrate shown in FIG. 3(b) has a higher
threading dislocation density at a portion 208 located exactly
above the center of the groove having a predetermined width and at
a portion 209 located exactly above the center of the groove free
portion (plateau portion) having a predetermined width. In
contrast, in FIG. 2 a portion located at or near the center between
portions 206 and 207 has a lowest threading dislocation density and
so does a portion shown in FIG. 3 that is located at or near the
center between portions 208 and 209. Thus the quasi GaN substrate
has a portion with a high threading dislocation density and a
portion with a low threading dislocation density in a mixed manner.
Therefore, the quasi substrate is inferior in device yield to the
GaN substrate. It is recommendable that a light emitting device
should be fabricated at a low threading dislocation density region
of the quasi GaN substrate.
[0117] Specific examples of seed substrate 200 include C-plane
sapphire, M-plane sapphire, A-plane sapphire, R-plane sapphire,
GaAs, ZnO, MgO, spinel, Ge, Si, 6H-SiC, 4H-SiC, 3C-SiC and the
like. Specific examples of anti-growth film 204 can include
dielectric films such as SiO.sub.2 film, SiN.sub.x film, TiO.sub.2
film and Al.sub.2O.sub.3 film, and metal films such as tungsten
film. Alternatively, a hollow may be provided in place of the
anti-growth film.
[0118] If the seed substrate is a conductive substrate of SiC, Si
or the like, the n electrode may be formed on the back surface of
the substrate as shown in the FIG. 1. In this case, however, a high
temperature buffer layer should be substituted for the low
temperature buffer layer. The high temperature buffer layer refers
to a buffer layer grown at not lower than 900.degree. C. It should
also contain at least Al, otherwise the nitride semiconductor film
with good crystallinity cannot be formed on the SiC or Si
substrate. A most preferable material for the high temperature
buffer layer is AlN.
[0119] The crystal plane of the main surface of the seed may
typically be the C {0001} plane, the A {11-20} plane, the R {1-102}
plane, the M {1-100} plane, or the {l-101} plane. The substrate
surface may preferably form an offset angle within two degrees with
the above crystal planes to have a good surface morphology.
[0120] The quasi GaN substrate was used to fabricate a light
emitting diode as shown in FIGS. 4 and 5. FIG. 4 is a cross section
of the light emitting diode and FIG. 5 is a top view thereof. As
shown in FIG. 4, the light emitting diode is composed of a
substrate 300, a low temperature GaN buffer layer 101 (of 50 nm in
thickness), an n-type GaN layer 102, a light emitting layer 103, a
p-type Al.sub.0.1Ga.sub.0.9N carrier block layer 104, a p-type GaN
contact layer 105, a transparent electrode 106, a p electrode 107,
an n electrode 108, and a dielectric film 109. In this diode,
substrate 300 has the structure of the quasi GaN substrate 200 in
FIG. 2 or that of the quasi GaN substrate 200a in FIG. 3.
[0121] The light emitting diode is fabricated in such a manner that
at least the portions 206 and 207 in FIG. 2 or the portions 208 and
209 in FIG. 3 are excluded from the diode structure. Preferably,
the formation of the light emitting diode starts at a position 1
.mu.m distant in the lateral direction from each centerline of
portions 206 and 207 or each centerline of portions 208 and 209. At
the portions less than 1 .mu.m distant from each centerline, the
threading dislocation density can be relatively high and cracks can
readily be caused.
[0122] In the device shown in FIG. 4, the low temperature buffer
layer may be a low temperature Al.sub.xGa.sub.1-xN buffer layer,
wherein 0.ltoreq.x.ltoreq.1. Alternatively, the low temperature GaN
buffer layer may be omitted. If the quasi GaN substrate does not
have a preferable surface morphology, however, the low temperature
Al.sub.xGa.sub.1-xN buffer layer, wherein 0.ltoreq.x.ltoreq.1, is
preferably provided to improve the surface morphology.
[0123] Alternatively, seed substrate 201 may be removed from quasi
GaN substrate 200 or 200a by means of a grinder and the obtained
substrate may be used as substrate 300 to fabricate the light
emitting device as shown in FIG. 4. Alternatively, low temperature
buffer layer 202 and the underlying layer(s) may all be removed
from quasi GaN substrate 200 or 200a by means of a grinder and the
light emitting device may similarly be fabricated. Alternatively,
anti-growth film 204 and the underlying layer(s) may all be removed
from quasi GaN substrate 200 or 200a by means of a grinder and the
light emitting device may similarly be fabricated. When seed
substrate 201 is removed, n electrode 111 can be formed on the back
surface of the substrate as shown in the FIG. 1. Alternatively,
seed substrate 200 may be removed after the light emitting device
is completed.
[0124] The crystal system separation that can be caused by doping
the light emitting layer with the impurity can efficiently be
inhibited in the light emitting device formed on the quasi GaN
substrate as shown in FIG. 4. The light emitting device of this
example also exhibited characteristics similar to those obtained in
Example 1.
EXAMPLE 3
[0125] A nitride semiconductor laser diode was fabricated, as shown
in FIG. 6. The laser diode shown in FIG. 6 is composed of a C plane
(0001), n-type GaN substrate 400, a low temperature GaN buffer
layer 401, an n-type Al.sub.0.05Ga.sub.0.95N layer 402, an n-type
In.sub.0.07Ga.sub.0.93N anti-crack layer 403, an n-type
Al.sub.0.1Ga.sub.0.9N clad layer 404, an n-type GaN optical guide
layer 405, a light emitting layer 406, a p-type
Al.sub.0.2Ga.sub.0.8N carrier block layer 407, a p-type GaN optical
guide layer 408, a p-type Al.sub.0.1Ga.sub.0.9N clad layer 409, a
p-type GaN contact layer 410, an n electrode 411, a p electrode
412, and a SiO.sub.2 dielectric film 413.
[0126] While SiH.sub.4 was adding to both of the barrier and well
layers (in a Si concentration of 1.times.10.sup.18/cm.sup.3), 4
nm-thick In.sub.0.05Ga.sub.0.95N.sub.0.98P.sub.0.02 well layer and
6 nm-thick In.sub.0.05Ga.sub.0.95N barrier layer were grown in
three cycles in the order of
barrier/well/barrier/well/barrier/well/barrier to form light
emitting layer 406 of a multi-quantum well structure. The crystal
system separation that can restrained in the obtained semiconductor
laser. The semiconductor laser of this example exhibited a low
threshold current density.
[0127] Low temperature GaN buffer layer 401 may be replaced with a
low temperature Al.sub.xGa.sub.1-xN buffer layer, wherein
0.ltoreq.x.ltoreq.1. Alternatively, the low temperature GaN buffer
layer may be omitted. If the GaN substrate does not have a
preferable surface morphology, the low temperature
Al.sub.xGa.sub.1-xN buffer layer, wherein 0.ltoreq.x.ltoreq.1, is
preferably provided to improve the surface morphology.
[0128] In.sub.0.07Ga.sub.0.93N anti-crack layer 403 may be replaced
with another InGaN layer having an In content of other than 0.07.
Alternatively, the InGaN anti-crack layer may be omitted. If there
is a significant lattice mismatch between the clad layer and the
GaN substrate, InGaN anti-crack layer should be provided.
[0129] The structure of the light emitting layer starting and
ending with a barrier layer may be replaced by the structure
starting and ending with a well layer. The number of the well
layers is not limited to three, and ten or less well layers were
able to provide a low threshold current density and to generate
continuous oscillation at room temperature. In particular, the
devices having two to six well layers preferably had a low
threshold current density.
[0130] In the process of forming the multi-quantum well structure
of the light emitting layer, the 4 nm-thick
In.sub.0.05Ga.sub.0.95N.sub.0.98P.su- b.0.02 well layer and 6
nm-thick In.sub.0.05Ga.sub.0.95N barrier layer may be replaced with
different material layers (see the section entitled Structure of
Light Emitting Layer). The well layer and the barrier layer may
have a thickness of 0.4 nm to 20 nm and a thickness of 1 nm to 20
nm respectively, so that they can have good crystallinity to
achieve the effect of the present invention sufficiently.
[0131] Since the barrier layer in this example does not contain any
of As, P or Sb, the barrier layer may be free of the impurity.
Alternatively, as far as the above-described requirements for the
impurity are satisfied, an impurity other than Si may be used, or
the dose of the impurity may be changed.
[0132] P-type Al.sub.0.2Ga.sub.0.8N carrier block layer 407 may be
replaced with an AlGaN layer having an Al content of other then
0.2. Alternatively, the carrier block layer may be omitted.
However, the carrier block layer was able to contribute to a lower
threshold current density. The carrier block layer can serve to
confine the carriers in the light emitting layer. A higher Al
content in the carrier block layer can preferably enhance the
carrier confinement. On the other hand, the Al content may be
reduced within a certain range that the carrier confinement is
maintained. In such a case, the carrier mobility in the carrier
block layer can preferably be increased and a low electrical
resistance can preferably be obtained in the device.
[0133] As for the p- and n-type clad layers, Al.sub.0.1Ga.sub.0.9N
may be replaced with another 3-element crystal of AlGaN having an
Al content of other than 0.1. A higher mixing ratio of Al can
provide a larger energy gap and a larger difference of index of
refraction between the clad layer and the light emitting layer. In
such a case, the carriers and the light can efficiently be confined
in the light emitting layer so that the lasing threshold current
density can be reduced. The Al content may be reduced within a
certain range that the carrier and light confinement is maintained.
In such a case, the carrier mobility in the clad layer can
preferably be increased and a low electrical resistance can
preferably be obtained in the laser device, so that a low operating
voltage can be achieved in the device.
[0134] Preferably, the AlGaN clad layer has a thickness of 0.7
.mu.m to 1.5 .mu.m. The clad layer with such a thickness can
provide the laser devise with a good unimodal, vertical transverse
mode and an increased light confinement efficiency, so that the
optical characteristics of the laser can be improved and the lasing
threshold current density can be reduced.
[0135] The clad layer is not limited to the 3-element mixed crystal
of AlGaN and it may be a 4-element mixed crystal such as AlInGaN,
AlGaNP or AlGaNAs. Alternatively, the p-type clad layer may have a
superlattice structure composed of a p-type AlGaN layer and a
p-type GaN layer, or of a p-type AlGaN layer and a p-type InGaN
layer to reduce the device resistance.
[0136] In this example, the effect of the C {0001} plane GaN
substrate was similar to that in Example 1. On the other hand, the
effect of the quasi GaN substrate in place of the GaN substrate was
similar to that in Example 2. If the quasi GaN substrate is used,
the ridged stripe as shown in FIG. 6 is preferably formed at a
position separate from the portions 206 and 207 in FIG. 2 or
separate from the portions 208 and 209 in FIG. 3. More preferably,
the ridged stripe is formed at a portion 1 .mu.m distant, in the
lateral direction, from each centerline of portions 206 and 207 or
from each centerline of portions 208 and 209. The portions less
than 1 .mu.m distant in the lateral direction from each centerline
can have a high threading dislocation density and can be liable to
cause cracks.
EXAMPLE 4
[0137] A device was fabricated, as in Examples 1 to 3, except that
the light emitting layer was doped with carbon (C) impurity in a
concentration of 1.times.10.sup.20/cm.sup.3. Similar
characteristics were obtained.
EXAMPLE 5
[0138] A device was fabricated, as in Examples 1 to 3, except that
the light emitting layer was doped with Mg impurity in a
concentration of 1.times.10.sup.17/cm.sup.3. Similar
characteristics were obtained.
[0139] Light Emitting Apparatus
[0140] The light emitting nitride semiconductor diode of the
present invention can be used to provide a light emitting
apparatus, such as a display device, white-light source device or
the like. For example, the light emitting diode of the present
invention can be employed for at least one of the three primary
colors of light, i.e., red, green and blue to provide a display
devise.
[0141] The amber-color light emitting diode employing a
conventional InGaN well layer is not marketable for its poor
reliability and low emission intensity. The In content in the
conventional InGaN well layer is so high that significant
composition separation can be caused by In (i.e., a high In content
portion and a low In content portion can be formed). On the other
hand, As, P or Sb contained in the light emitting layer can serve
to reduce the band gap energy of the light emitting layer (the well
layer), like In. Therefore, In can be reduced or omitted by the
addition of As, P or Sb to the light emitting layer (the well
layer). The conventional nitride semiconductor layer containing at
least one of As, P and Sb, however, has crystal system separation
as described above, and its crystallinity degraded can result in a
low emission intensity. Thus the conventional device cannot derive
substantial advantage from As, P or Sb. The crystal system
separation and the like can also disturb the interface between the
well and barrier layers. Thus the multi-quantum well structure can
hardly be fabricated or the light emitting device can have
increased unevenness of color or decreased emission intensity.
[0142] In the present invention, the impurity added to the light
emitting nitride semiconductor layer containing at least one of As,
P and Sb can reduce the crystal system separation to overcome the
above-described disadvantages. According to the present invention,
the crystallinity of the light emitting layer can be improved and
the light emitting diode can derive the advantage from As, P or Sb
contained in the light emitting layer. The light emitting device
according to the present invention can have any emission wavelength
in the range of 360 nm to 650 nm. The wavelength and the
composition of the light emitting layer are exemplarily presented
in the above section entitled Structure of Light Emitting
Layer.
[0143] According to the present invention, light emitting diodes of
the three primary colors can be combined together to provide a
white-light source device. Alternatively, the light emitting diode
of the present invention having an emission wavelength from the w
range to the violet-color range (from 360 nm to 420 nm) may have
fluorescent paint applied thereon to provide the white-light source
device. Such white-light sources can replace a halogen light source
for conventional liquid crystal displays and serve as a backlight
with low power consumption and high intensity for the displays. It
can be used as a backlight for a liquid crystal display allowing
man-machine interface via mobile notebook personal computers,
cellular phones and the like. It can provide a miniaturized and
clear liquid crystal display.
[0144] Optical Pickup Device
[0145] The nitride semiconductor laser of the present invention is
applicable to optical pickup devices.
[0146] The light emitting nitride semiconductor layer according to
the present invention contains at least one of As, P and Sb. These
elements contained in the light emitting layer can reduce the
effective mass of the electrons and the holes in the light emitting
layer, and increase the mobility of the electrons and holes. The
former suggests that the carrier population inversion for lasing
can be generated by a smaller current injection. The latter
suggests that if the electrons and holes are consumed by the
radiative recombination in the light emitting layer, new electrons
and holes can rapidly be injected through diffusion. Thus it was
believed that the nitride semiconductor laser with As, P or Sb
could have a lower threshold current density and superior
self-oscillation characteristics (or lower noise characteristics)
as compared with the nitride semiconductor laser completely free of
As, P and Sb. If the crystal system separation occurs in the As, P
or Sb containing light emitting layer of the nitride semiconductor
device, however, it will be difficult to obtain such
advantages.
[0147] In the present invention, the impurity added to the light
emitting nitride semiconductor layer can reduce the crystal system
separation. According to the present invention, the light emitting
layer can improve in crystallinity and the semiconductor laser can
have a low threshold current density accompanied by higher output,
and a long life. According to the present invention, a
semiconductor laser having superior noise characteristics can be
fabricated. For example, a nitride semiconductor laser of the
present invention having an oscillation wavelength of 380 to 420 nm
can have a lower lasing threshold current density, a smaller amount
of spontaneous emission light in the laser light, and less
susceptible to noise as compared with a conventional InGaN-based
nitride semiconductor laser. The present semiconductor laser can
reliably work under a high power (e.g. 50 mW) and a
high-temperature ambient. Such a laser is suitable for an optical
disc for high density recording and reproduction.
[0148] FIG. 7 shows an optical disc device employing the nitride
semiconductor laser diode device according to the present
invention. In the optical disc device, the nitride semiconductor
laser emits laser beam, which is transmitted via an optical
modulator, a splitter, a follow-up mirror and a lens to illuminate
an optical disc. The beam from the splitter is detected by a
photodetector. The photodetector outputs a signal which is in turn
transmitted to a control circuit. The control circuit sends signals
to a motor actuating the disc, the semiconductor laser, the optical
modulator and the follow-up mirror, respectively. The laser beam is
modulated by the optical modulator in response to information input
and recorded on the disc via the lens. In reproduction, laser beam
optically changed by pit arrangement on the disc is transmitted
though the splitter and detected by the photodetector to form a
reproduced signal. This series of operations are controlled by the
control circuit. Normally, a laser output of approximately 30 mW is
provided in recording and that of approximately 5 mW is provided in
reproduction.
[0149] Besides the optical disc device as described above, the
device according to the present invention is also applicable to
laser printers, barcode readers, and projectors using three primary
color (blue, red, green) laser diodes.
[0150] In the present invention, the impurity added to the light
emitting layer can reduce the crystal system separation in the
light emitting layer. According to the present invention, the light
emitting nitride semiconductor device with a high luminous efficacy
can be provided. According to the present invention, such a device
is applied to a light emitting apparatus and optical pickup
device.
[0151] Although the present invention has been described and
illustrated in detail, it is clearly understood that the same is by
way of illustration and example only and is not to be taken by way
of limitation, the spirit and scope of the present invention being
limited only by the terms of the appended claims.
* * * * *