U.S. patent application number 12/822516 was filed with the patent office on 2010-10-21 for photonic devices formed of high-purity molybdenum oxide.
Invention is credited to Takashi KATODA.
Application Number | 20100265978 12/822516 |
Document ID | / |
Family ID | 33128299 |
Filed Date | 2010-10-21 |
United States Patent
Application |
20100265978 |
Kind Code |
A1 |
KATODA; Takashi |
October 21, 2010 |
PHOTONIC DEVICES FORMED OF HIGH-PURITY MOLYBDENUM OXIDE
Abstract
The present invention is directed to photonic devices which emit
or absorb light with a wavelength shorter than that GaN photonic
devices can emit or absorb. The devices according to the present
invention are formed using molybdenum oxide of a high purity as a
light emitting region or a light absorbing region. New inexpensive
photonic devices which emit light with a wavelength from blue to
deep ultraviolet rays are realized. The devices according to the
present invention can be formed at a temperature relating low such
as 700.degree. C.
Inventors: |
KATODA; Takashi; (Kochi-shi,
JP) |
Correspondence
Address: |
Robinson Intellectual Property Law Office, P.C.
3975 Fair Ridge Drive, Suite 20 North
Fairfax
VA
22033
US
|
Family ID: |
33128299 |
Appl. No.: |
12/822516 |
Filed: |
June 24, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10848145 |
May 19, 2004 |
7759693 |
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12822516 |
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Current U.S.
Class: |
372/45.01 ;
257/43; 257/E29.1; 257/E31.026; 257/E33.037 |
Current CPC
Class: |
H01L 33/26 20130101;
H01S 5/32 20130101; H01S 5/327 20130101; H01L 31/0264 20130101 |
Class at
Publication: |
372/45.01 ;
257/43; 257/E29.1; 257/E33.037; 257/E31.026 |
International
Class: |
H01S 5/30 20060101
H01S005/30; H01L 29/24 20060101 H01L029/24; H01L 33/26 20100101
H01L033/26; H01L 31/032 20060101 H01L031/032 |
Foreign Application Data
Date |
Code |
Application Number |
May 30, 2003 |
JP |
2003-154107 |
Claims
1. A semiconductor photo-device wherein molybdenum oxide is used in
at least one layer which converts electrical energy to light or
light to electrical energy.
2. The semiconductor photo-device according to claim 1, wherein
said at least one layer comprises at least a part of a
light-emitting or light-absorbing region in a photo-conductive
device, and said light-emitting or light-absorbing region composes
at least a part of a photo-conductive device, a photo-diode, a
photo-transistor, a light-emitting diode, a semiconductor laser, a
solar cell or a CCD.
3. The semiconductor photo-device according to claim 1, wherein
said molybdenum oxide has a high purity property so that efficient
conversion from electrical energy to light or from light to
electrical energy occurs in said molybdenum oxide region.
4. The semiconductor photo-device according to claim 1, wherein
said molybdenum oxide is a high purity molybdenum oxide which is
formed by vapor phase deposition at a temperature lower than
700.degree. C.
5. The semiconductor photo-device according to claim 1, wherein
said molybdenum oxide is crystalline having a high purity and has a
bandgap of 3.45-3.85 eV.
6. A light emitting diode comprising: a layer of molybdenum oxide
on a substrate; a layer of n-type molybdenum oxide; and a layer of
p-type molybdenum oxide so that said layer of n-type molybdenum
oxide and said layer of p-type molybdenum oxide forms a pn junction
from which light is emitted.
7. A light emitting diode comprising: a layer of molybdenum oxide
on a substrate; a buffer layer of molybdenum oxide on said
molybdenum oxide layer; a layer of n-type molybdenum oxide on said
buffer layer; and a layer of p-type molybdenum oxide on said n-type
layer.
8. Laser diodes comprising a layer of molybdenum oxide on a
substrate; a first cladding layer of n-type semiconductor on said
molybdenum oxide layer, said first cladding layer having a bandgap
larger than that of said molybdenum oxide; an active layer of
p-type molybdenum oxide on said first cladding layer; and a second
cladding layer of p-type semiconductor on said active layer, said
second cladding layer having a bandgap larger than that of said
molybdenum oxide.
9. A laser diode comprising: a layer of molybdenum oxide on a
substrate; a buffer layer of molybdenum oxide on said layer; a
first cladding layer of n-type semiconductor on said buffer layer,
said first cladding layer having a bandgap larger than that of said
molybdenum oxide; an active layer of p-type molybdenum oxide on
said first cladding layer; and a second cladding layer of p-type
semiconductor on said active layer, said second cladding layer
having a bandgap larger than that of said molybdenum oxide.
10. The light emitting diode according to claim 6, wherein said
substrate is composed of molybdenum.
11. The laser diode according to claim 8, wherein said substrate is
composed of molybdenum.
12. The laser diode according to claim 8, wherein said first and
second cladding layers are composed of chromium molybdenum
oxide.
13. The light emitting diode according to claim 7, wherein said
substrate is composed of molybdenum.
14. The laser diode according to claim 9, wherein said substrate is
composed of molybdenum.
15. The laser diode according to claim 9, wherein said first and
second cladding layers are composed of chromium molybdenum
oxide.
16. The laser diode according to claim 11, wherein said first and
second cladding layers are composed of chromium molybdenum
oxide.
17. The laser diode according to claim 14, wherein said first and
second cladding layers are composed of chromium molybdenum oxide.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a semiconductor photonic
devices formed of high-purity molybdenum oxide which emit or absorb
light with a short wavelength.
[0003] More particularly, the present invention relates to new
light emitting diodes which emit blue light and have possibility to
overcome problems accompanying to devices made up of known
semiconductors such as gallium nitride (GaN) or silicon carbide
(SiC). Moreover, the invention relates to photonic devices which
emit light with a wavelength shorter than 361 nm in which GaN
light-emitting diodes can emit or selectively absorb light having a
wavelength shorter than 361 nm.
[0004] 2. Related Background Art
[0005] Light emitting diodes which emit blue light have developed
recently in order to realize three primary colors of light and to
obtain light with a shorter wavelength for digital video disc
(DVD). Developed blue-light emitting devices use gallium nitride
(GaN) as an active region which is very important to emit light.
The bandgap of GaN is about 3.43 eV which corresponds to a
wavelength of 361 nm. Although blue light can be obtained from GaN
devices, there are some difficult problems. At first, bulk crystal
of GaN has not been obtained because an equilibrium vapor pressure
of nitrogen is very high relative to that of gallium. Therefore,
substrates made up of sapphire or silicon carbide (SiC) are used.
GaN cannot be formed directly on a sapphire substrate because there
is lattice mismatch of 16% between sapphire and GaN. Therefore a
buffer layer of aluminum nitride (AlN) is formed on a sapphire
substrate before growth of GaN. AlN is resistive because it is
difficult to dope impurities into AlN. A structure and its
fabrication process, therefore, are severely restricted. On the
other hand, SiC substrates are very expensive because bulk crystal
of SiC can be grown at a very high temperature of 2200-2400.degree.
C.
[0006] Zinc oxide (ZnO) has possibility to be used to form a
blue-light emitting device. However, its bandgap is 3.2 eV which
corresponds to a light wavelength of 387 nm which is larger than
that GaN devices emit. Moreover, ZnO has many problems to be solved
to realize practical devices.
[0007] The shortest wavelength of light which semiconductor
photonic devices can emit at present is that GaN devices can emit.
The maximum density of DVD memory is decided by the wavelength.
Therefore, a new photonic device which can emit light with a
shorter wavelength is expected in order to increase the maximum
density of DVD memory or to replace gas lasers such as He--Cd
laser. In addition, a new blue-light emitting device made up of new
material is expected because present blue-light emitting devices
have many problems as described above. Moreover, a new device which
can emit light with a wavelength shorter than 361 nm which GaN
devices can emit or a shorter wavelength of deep ultraviolet rays
such as 250-350 nm is expected.
[0008] The problem to be solved to realize a new device is to
obtain a new substrate which replaces expensive substrate such as
sapphire or SiC.
[0009] The second problem is to realize new semiconductor which can
be grown at a lower temperature at which GaN or SiC layers are
formed. Large energy is necessary to form semiconductor layers at a
high temperature. In addition, there are possibilities that atoms
move between layers and a composition is disturbed or dopants move
near the interface between layers. It is necessary to form layers
of GaN or SiC at a temperature higher than 1000.degree. C.
SUMMARY OF THE INVENTION
[0010] The present invention is directed to photonic devices which
emit or absorb light with a wavelength shorter than that GaN
photonice devices can emit or absorb.
[0011] The devices according to the present invention are formed
using molybdenum oxide of a high purity as a light emitting region
or a light absorbing region. New inexpensive photonic devices which
emit light with a wavelength from blue to deep ultraviolet rays are
realized.
[0012] The devices according to the present invention can be formed
at a temperature relatively low such as 700.degree. C.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 shows the optical reflection characteristics of the
molybdenum oxide formed by oxidation of high-purity molybdenum at
550.degree. C.
[0014] FIG. 2 shows the Raman scattering spectra from molybdenum
oxides formed by oxidation of high-purity molybdenum at various
temperatures from 450 to 650.degree. C.
[0015] FIG. 3 shows the X-ray diffraction spectra from molybdenum
oxides formed by oxidation of high-purity molybdenum at various
temperatures from 450 to 650.degree. C.
[0016] FIG. 4 shows temperature dependence of the electrical
resistance of molybdenum oxide formed by oxidation of high-purity
molybdenum at 550.degree. C.
[0017] FIG. 5 is a schematic view of a structure of the
light-emitting diode according to an embodiment of the present
invention.
[0018] FIG. 6 is a schematic view of a structure of the laser diode
according to the second embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0019] Reference will now be made in greater detail to preferred
embodiments of the invention.
[0020] The problems described above were resolved by using
high-purity molybdenum oxide as a light emitting region of photonic
devices.
[0021] Molybdenum oxide has been studied for catalyst and its
properties are shown for example in the following paper. Martin
Lerch, Reinhard Schmacker, Robert Schlogl, "In situ Resonance Raman
Studies of Molybdenum Oxide Based Selective Oxidation Catalysts"
Fachbereich Chemie der Technischen Universitat Berlin zur Erlongung
des akademischen Grades, Marz 2001, Berlin.
[0022] The paper is included as a reference literature of this
specification. However, application of molybdenum oxide to photonic
devices, such as a light emitting diode or a laser diode is not
proposed in the paper. Although the bandgap of molybdenum oxide is
reported as 2.9-3.15 eV in page 8 of the paper, any effects
obtained by using molybdenum oxide in photonic devices are not
described. The values of the bandgap, 2.9-3.15 eV, are the results
for molybdenum oxide formed by physical method such as sputtering
or deposition in vacuum. In addition, a purity of the sample, that
is molybdenum oxide, is not shown in the paper. In general,
semiconductor material used in photonic devices is high-purity
crystal and its bandgap is measured for such crystal. However the
bandgap shown in the above paper is that of molybdenum oxide formed
by deposition in vacuum because molybdenum oxide is considered as
catalyst in the paper. Material formed by deposition is usually
amorphous and it is well know to the peoples in the art that the
material has disordered structure. In addition, a thickness of a
film formed by deposition in vacuum is generally small such as 100
nm and a thickness of 1 .mu.m is too large to be formed by
deposition in vacuum. When a thickness is small size such as 100
nm, properties such as a bandgap of a film are affected by a
substrate and change with a thickness of a film or material of a
substrate. The bandgap shown above was obtained for such films with
small thicknesses and was not necessarily identical to that
inherent to crystalline molybdenum oxide with a larger thickness
such as 1 .mu.m. The reason why a bandgap was not measured for
crystalline molybdenum oxide with a thickness larger than 100 nm in
the paper described above is considered that application of
molybdenum oxide to photonic devices such as a light emitting or
laser diodes was not intended in the paper.
[0023] The inventor of this invention measured properties of the
molybdenum oxide formed by oxidation of a molybdenum plate with a
purity of 99.99% in oxygen atmosphere with a purity of 99.9995%.
FIG. 1 shows the optical reflection characteristics of the
molybdenum oxide formed by oxidation of the molybdenum plate at
550.degree. C. for 120 minutes. A thickness of the molybdenum oxide
was 10.2 .mu.m. The longest wavelength at which absorption begins,
that is at which reflection is zero which is obtained by
extrapolating the spectra shown in FIG. 1 gives the bandgap of the
molybdenum oxide. Light with a wavelength shorter than 388 nm was
absorbed for this sample. It means that the bandgap of the sample
was 3.66 eV. Because a thickness of the sample was 10.2 .mu.m,
there is no effect of the substrate and the value of the bandgap
must be one inherent to molybdenum oxide. The reason why the value
of the bandgap 3.66 eV is larger than that 2.9-3.15 eV reported by
Martin Lerch et al as shown in the above paper is considered as
follows. It is well known in the art that material with disordered
structure such as a film formed by deposition in vacuum forms
so-called band tail in the forbidden region of the energy band
structure and its effective bandgap is decreased. The value
reported by Martin Lerch et al was obtained for samples with
disordered structure. On the other hand, the value obtained by the
inventor is that for the high-purity crystalline molybdenum oxide.
Therefore the value of the bandgap measured by this inventor was
larger than those reported by Martin Lerch et al. Following data
show in detail the results for high-purity crystalline molybdenum
oxide obtained by this inventor.
[0024] FIG. 2 shows the Raman scattering spectra and FIG. 3 shows
the X-ray diffraction spectra from the molybdenum oxide formed
similarly to that shown in FIG. 1 except that the molybdenum oxide
was obtained by oxidation at a temperature from 450 to 650.degree.
C. The spectra shown in FIGS. 2 and 3 mean that the main
composition of the molybdenum oxide was MoO.sub.3. However it is
possible that other compositions were included under the detection
limit. The bandgap obtained from the optical reflection spectra as
described for FIG. 1 was 3.45-3.85 eV for the molybdenum oxide
formed at 450-650.degree. C.
[0025] A bandgap is affected by structure, that is crystal or
amorphous, disorder of crystal, a size of crystalline particle if
the material is poly-crystalline, or strain even the material has
the same composition.
[0026] Therefore it should be notified that molybdenum oxide with a
composition of MoO.sub.3 does not have always the bandgap of
3.45-3.85 eV. In other words, the bandgap of 3.45-3.85 eV depends
on structure and strain as well as composition. The spectra shown
in FIG. 3 consist of sharp peaks and it means that the sample is
pure crystal. Moreover, there is possibility that a larger bandgap
will be obtained by making quality of the crystal better.
[0027] FIG. 4 shows temperature dependence of electrical resistance
of the molybdenum oxide whose optical reflectance property is shown
in FIG. 1. As shown in the figure, resistance decreases with
increase of temperature. It means that a carrier density increases
with increase of temperature and it is phenomenon only
semiconductor shows. That is, electrical conductivity which is
reciprocal to resistance is determined by a carrier density and
carrier mobility. Carrier mobility decreases with increase of
temperature because effects of lattice vibration increase with
temperature. Therefore if a carrier density does not increases with
temperature such as metal or insulating material, conductivity
decreases with increase of temperature and resistance will
increase. FIG. 4 shows as well as FIG. 1 that the molybdenum oxide
is semiconductor.
[0028] As shown above, crystalline molybdenum oxide can be obtained
by oxidizing a molybdenum plate at a temperature lower than
650.degree. C. A high-quality molybdenum oxide layer can be grown,
for example, by vapor phase growth on a buffer layer of molybdenum
oxide which has been grown previously on molybdenum oxide, for
example, by vapor phase deposition on molybdenum oxide formed by
oxidation of a molybdenum plate. Vapor phase growth of molybdenum
oxide can be done at a temperature lower than 650.degree. C. by a
method which will be described in the other patent application.
Therefore light emitting devices using molybdenum oxide can be
fabricated fundamentally at a temperature lower than 650.degree. C.
using a molybdenum plate. Other materials such as aluminum (Al)
crystal or Zinc sulfide (ZnS) can be used as a substrate. Lattice
mismatchs between molybdenum oxide and aluminum and between
molybdenum oxide and zinc sulfide are 2.0% and 3.1%. They are much
smaller than lattice mismatch between sapphire and gallium nitride,
which is 16%. The problems accompanying to the present blue-light
emitting devices, which are use of expensive substrates, growth at
a very high temperature and complicated structures and fabrication
process, are resolved by forming light emitting devices using
fundamentally molybdenum oxide, and light with a wavelength shorter
than 361 nm can be obtained. In addition, molybdenum oxide is used
to form devices for which a smaller bandgap is preferable, the
bandgap of the devices being controlled, for example, by doping of
impurity.
[0029] FIG. 5 shows schematically a structure of a light emitting
diode (1) according to the first embodiment of the present
invention. In this embodiment, a substrate (2) is a plate of
molybdenum. However other material can be used as a substrate as
far as it is electrically conductive. A layer (3) consists of
molybdenum oxide formed by oxidizing a surface region of the
molybdenum substrate (2).
[0030] The layer (3) was formed by oxidizing a molybdenum plate
with a purity of 99.99% at 550.degree. C. in an atmosphere of
oxygen with a purity of 99.9995% and its thickness is 6.0 .mu.m.
Although the layer (3) is not intentionally doped, it is n type. It
is considered that oxygen vacancies act as donors. A buffer layer
(4) is formed on the layer (3) in order to confine disorder in the
layer (3) which originates because the layer (3) has a different
composition from the substrate (2). For example, the layer (4)
consists of molybdenum oxide formed, for example, by vapor phase
deposition at 630.degree. C. and is n type with a carrier density
of 3.times.10.sup.17 cm.sup.-3. It's thickness is 4.0 .mu.m. A
layer (5) of molybdenum oxide is formed on the layer (4). The layer
(5) is formed, for example, by vapor phase deposition at
600.degree. C. and consists of crystal whose quality is better than
that of the layer (4). The layer (5) is n type with a carrier
density of 6.times.10.sup.16 cm.sup.-3. A thickness of the layer
(5) is 3.0 .mu.m. It is not necessary to form the layer (5) when it
is not necessary to make efficiency of the light emitting diode (1)
as high as possible. A layer (6) of p-type molybdenum oxide is
fanned on the layer (5). The layer (6) is doped, for example, with
magnesium to a hole density of 1.0.times.10.sup.17 cm.sup.-3. A
thickness of the layer (6) is 2.0 .mu.m and formed for example, by
vapor phase deposition. An electrode (7) is formed on the layer
(6). The electrode (7) has a shape of doughnut (ring-shape) in
order not to obstruct emission of light. Although the electrode is
made up of gold in this embodiment, other metals can be used for
the electrode. The electrode (7) is the upper electrode of the
light emitting diode and the conductive molybdenum substrate acts
as the bottom electrode. Characteristics of the light emitting
diode (1) obtained by simulation are as follows. A voltage at the
forward vias was 10V when current was 20 mA, a light power was 60
.mu.w when current was 20 mA, and a peak wavelength was 330 nm.
[0031] FIG. 6 shows a laser diode (100) according to the second
embodiment of the present invention. Although a substrate (101) is
a molybdenum plate, other materials can be used as substrates as
far as they are conductive. The substrate (101) is desirable to be
conductive. A layer (102) is formed by oxidizing a surface region
of the substrate and consists of molybdenum oxide. The layer (102)
was formed by oxidizing the molybdenum substrate with a purity of
99.99% in an atmosphere of oxygen with a purity of 99.995% at
550.degree. C. for 40 minutes. The layer (102) shows n type
although it is not intentionally doped. As described for the first
embodiment, it is considered that oxygen vacancies act as donors. A
buffer layer (103) is formed on the layer (102) in order to confine
disorder in the layer (102). The disorder is introduced because the
layer (102) has a different composition to the substrate (101). The
layer (103) consists of molybdenum oxide formed by, for example,
vapor phase deposition at 630.degree. C. and is n type with a
carrier density of 3.times.10.sup.17 cm.sup.-3. A thickness of the
layer (103) is 3.0 .mu.m. A layer (104) of chromium molybdenum
oxide (Cr.sub.0.1Mo.sub.0.9O.sub.3) is formed on the layer (103).
The layer (104) of chromium molybdenum oxide has a larger bandgap
than molybdenum oxide and acts as a cladding layer which confines
carrier and light in an active layer of the laser diode. Although
the layer (104) is not intentionally doped, it is n type with a
carrier density of 6.times.10.sup.16 cm.sup.-3. It is formed, for
example, by vapor phase deposition at 600.degree. C. and its
thickness is 3.0 .mu.m. A layer (105) of a p type molybdenum oxide
is formed on the layer (104) as an active layer of the laser diode
(100). The layer (105) is formed, for example, by vapor phase
deposition with doping to a hole density of 1.times.10.sup.17
cm.sup.-3. A thickness of the layer (105) is 0.5 .mu.m. A layer
(106) of chromium molybdenum oxide (Cr.sub.0.1Mo.sub.0.9O.sub.3) is
formed on the layer (105). A layer (106) has a larger bandgap than
the active layer (105) of molybdenum oxide and acts as a cladding
layer of the laser diode (100). The layer (106) is formed, for
example, by vapor phase deposition and has a thickness of 3.0
.mu.m. The layer (106) is doped, for example, with magnesium to p
type with a hole density of 4.0.times.10.sup.17 cm.sup.-3. A layer
(107) of silicon dioxide is formed on the layer (106) except a
central stripe region (108). Because silicon dioxide is resistive,
current is limited in the stripe region (108). The silicon dioxide
layer (107) is formed, for example, by sputtering and has a
thickness of 100 nm. An electrode layer (109) is formed on the
layer (107) and in the stripe region (108). Although the electrode
layer (109) is formed by vacuum deposition in an embodiment, other
materials and other deposition methods can be used. The layer (109)
is the upper electrode of the laser diode (100) while the substrate
(101) acts as the bottom electrode because the substrate is
conductive. A width of the stripe region (108) is, 20 .mu.m in this
embodiment. A length of the stripe region is 500 .mu.m in this
embodiment.
[0032] FIG. 6 shows one edge surface of the laser diode (100) and
another edge surface is parallel to the edge surface apart from it
by a length of the stripe (108). A pair of the parallel surfaces
form a Fabry-Perot resonator of the laser diode (100). Function of
a Fabry-Perot resonator in a laser diode is well known in the art.
The two edge surfaces are half mirror in order to form a
Fabry-Perot resonator. In this embodiment, the edge surfaces were
formed by reactive ion etching using CF.sub.4 and H.sub.2 gases
because cleavage cannot be used since the substrate (101) is
molybdenum which is not crystal and hard. However other methods can
be used to form the edge surfaces.
[0033] Characteristics of the laser diode (100) were shown by
simulation as follows. A threshold current density and a threshold
voltage were 5.05 kA/cm.sup.2 and 16.2V, respectively at pulse
oscillaton of 5 .mu.s/1 kHz. A peak wavelength was 330 nm.
[0034] FIG. 6 shows only essential elements of a laser diode and
other elements can be added to improve characteristics of the laser
diode. For example, a low resistive p type layer is formed on one
cladding layer (106) in order to improve characteristics of an
electrode.
[0035] Although in the embodiment shown in FIG. 6 the cladding
layers (104) and (106) consist of chromium molybdenum oxide
(Cr.sub.0.1Mo.sub.0.9O.sub.3), chromium molybdenum oxide with other
compositions (Cr.sub.xMo.sub.1-xO.sub.3, X>0.1) or other
materials can be used as far as they have larger bandgaps than that
of molybdenum oxide.
[0036] Details of the present invention have been described with
reference to the embodiments of a light emitting diode and a laser
diode. Merits obtained from the fact that high-purity molybdenum
oxide has a large bandgap are useful in other photonic devices
based on the principle of the present invention. Such applications
of the present invention are easily derived in the art and they are
included in the scope of the present invention.
[0037] For example, molybdenum oxide is used in devices such as
photo-conductive devices, photo-diodes, photo-transistors, CCD and
solar cells. Molybdenum oxide is used in photo-absorption regions
of such devices.
* * * * *