U.S. patent application number 10/514638 was filed with the patent office on 2006-06-29 for light emitting device structure having nitride bulk single crystal layer.
Invention is credited to Roman Doradzinski, Robert Dwilinski, Jerzy Garczynski, Yasuo Kanbara, Leszek Sierzputowski.
Application Number | 20060138431 10/514638 |
Document ID | / |
Family ID | 36610366 |
Filed Date | 2006-06-29 |
United States Patent
Application |
20060138431 |
Kind Code |
A1 |
Dwilinski; Robert ; et
al. |
June 29, 2006 |
Light emitting device structure having nitride bulk single crystal
layer
Abstract
The object of this invention is to provide a high-output type
nitride light emitting device. The nitride light emitting device
comprises an n-type nitride semiconductor layer, a p-type nitride
semiconductor layer and an active layer therebetween, wherein the
light emitting device comprises a gallium-containing nitride
semiconductor layer prepared by crystallization from supercritical
ammonia-containing solution in the nitride semiconductor layer.
Inventors: |
Dwilinski; Robert; (Warsaw,
PL) ; Doradzinski; Roman; (Warsaw, FR) ;
Garczynski; Jerzy; (Lomianki, PL) ; Sierzputowski;
Leszek; (Union, NJ) ; Kanbara; Yasuo;
(Tokushima, JP) |
Correspondence
Address: |
WESTERMAN, HATTORI, DANIELS & ADRIAN, LLP
1250 CONNECTICUT AVENUE, NW
SUITE 700
WASHINGTON
DC
20036
US
|
Family ID: |
36610366 |
Appl. No.: |
10/514638 |
Filed: |
December 11, 2002 |
PCT Filed: |
December 11, 2002 |
PCT NO: |
PCT/JP02/12969 |
371 Date: |
August 22, 2005 |
Current U.S.
Class: |
257/79 ;
257/E33.003 |
Current CPC
Class: |
B82Y 20/00 20130101;
H01S 5/3216 20130101; H01S 5/34333 20130101; H01L 33/16 20130101;
H01S 5/22 20130101; H01S 2304/00 20130101; H01S 5/2231 20130101;
H01S 5/2232 20130101; H01L 33/38 20130101; H01L 33/007 20130101;
H01S 5/2206 20130101 |
Class at
Publication: |
257/079 |
International
Class: |
H01L 33/00 20060101
H01L033/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 17, 2002 |
US |
10/147,318 |
Claims
1. A light emitting device structure comprising an n-type nitride
semiconductor layer, an active layer comprising an In-containing
nitride semiconductor, and a p-type nitride semiconductor layer,
formed on a substrate for growth, wherein said light emitting
device structure comprises a gallium-containing nitride
semiconductor layer prepared by crystallization from supercritical
ammonia-containing solution formed on ternary or quaternary nitride
layer in said nitride semiconductor layer.
2. The light emitting device structure according to claim 1,
wherein said substrate is a gallium-containing nitride bulk single
crystal prepared by crystallization from supercritical
ammonia-containing solution.
3. The light emitting device structure according to claim 1,
wherein said substrate has at least one plane selected from the
group comprising A-plane, M-plane, R-plane, C-plane, {1-10n (n is a
natural number)}, and {11-2m (m is a natural number)} of the
gallium-containing nitride bulk single crystal, as its own
surface.
4. The light emitting device structure according to claim 1,
wherein said substrate is not a substrate of nitride
semiconductor.
5. The light emitting device structure according to claim 1,
wherein said substrate has dislocation density of 10.sup.6/cm.sup.2
or less.
6. The light emitting device structure according to claim 1,
wherein said n-type nitride semiconductor layer is deposited on the
substrate directly or through the Al.sub.xGa.sub.1-xN
(0.ltoreq.x.ltoreq.1) buffer layer.
7. The light emitting device structure according to claim 1,
wherein said substrate is a composite substrate (template), which
comprises gallium-containing nitride grown on a heterogeneous
substrate by crystallization from supercritical ammonia-containing
solution.
8. The light emitting device structure comprising an n-type nitride
semiconductor layer, an active layer comprising an In-containing
nitride semiconductor, and a p-type nitride semiconductor layer,
formed on a substrate for growth, wherein said light emitting
device structure comprises a single crystal Al.sub.xGa.sub.1-xN
(0.ltoreq.x.ltoreq.1) layer doped with impurities prepared by
crystallization from supercritical ammonia-containing solution.
9. The light emitting device structure comprising an n-type nitride
semiconductor layer, an active layer comprising an In-containing
nitride semiconductor, and a p-type nitride semiconductor layer,
formed on a substrate for growth, wherein said light emitting
device structure comprises a high-resistance single crystal
Al.sub.xGa.sub.1-xN (0.ltoreq.x.ltoreq.1) layer prepared by
crystallization from supercritical ammonia-containing solution as a
current confinement layer.
10. The light emitting device structure according to claim 8,
wherein said active layer is a quantum well layer structure
comprising at least one of InGaN well layer or InAlGaN well
layer.
11. The light emitting device structure according to claim 1,
wherein said light emitting device structure is used for a
semiconductor laser device and formed on A-plane or M-plane of the
bulk single crystal gallium-containing nitride semiconductor
substrate, and the light emitting surface of the resonator is
M-plane of A-plane.
Description
TECHNICAL FIELD
[0001] The present invention relates to a structure wherein a
single crystal nitride layer prepared by crystallization from
supercritical ammonia-containing solution is used as a substrate or
an intermediate layer of light emitting devices such as a laser
structure etc.
BACKGROUND ART
[0002] In the nitride semiconductor laser, crystal defect or
dislocation of a waveguide causes electron-hole pairs to make
non-radiative recombination therein. Ideally, considering the laser
function, the dislocation density in the waveguide may be
10.sup.6/cm.sup.2 or less, preferably 10.sup.4/cm.sup.2 or less.
However, in the present situation, the dislocation density can not
be reduced to less than 10.sup.6/cm.sup.2 by using a vapor phase
epitaxial growth (MOCVD and HVPE) or by using a repeated ELOG
(Epitaxial lateral overgrowth), because the waveguide is grown on a
heterogeneous substrate, such as sapphire substrate or SiC
substrate.
[0003] To form a light emitting device comprising nitride
semiconductor on a sapphire substrate or a SiC substrate without
crack, the nitride semiconductor having the reduced dislocation
density is required to be grown in the form of a thin layer on a
sapphire substrate or a SiC substrate. If the nitride semiconductor
is grown in the form of a thick layer on the substrate such as
sapphire substrate etc, the curving of the substrate will be
bigger, which leads to higher probability of crack occurrence.
However, the nitride semiconductor in the form of a thin layer, in
which the dislocation density is reduced, has not been realized by
the vapor phase epitaxial growth.
[0004] To summarize the above, there has been a limitation to form
a nitride semiconductor light emitting device (especially a laser
device) by the vapor phase growth. Moreover, regarding the light
emitting diode, in case that the higher luminance and higher output
are required, the crystal dislocation of the substrate and of the
intermediate layer will be a serious problem.
DISCLOSURE OF INVENTION
[0005] The first object of the present invention is to provide a
light emitting device structure, which comprises a light emitting
device comprising an n-type nitride semiconductor layer, an active
layer comprising an In-containing nitride semiconductor, and a
p-type nitride semiconductor layer, formed on a substrate for
growth, wherein the light emitting device comprises a
gallium-containing nitride semiconductor layer prepared by
crystallization from supercritical ammonia-containing solution,
instead of the so-far used vapor phase growth. The
gallium-containing nitride semiconductor layer as one of the layers
in the light emitting device is prepared by crystallization from
supercritical ammonia-containing solution so that the crystalline
quality of the layers formed on the gallium-containing nitride
semiconductor layer can be recovered.
[0006] The second object of the present invention is to form a
substrate for growth having low dislocation density by using a
gallium-containing nitride bulk single crystal prepared by
crystallization from supercritical ammonia-containing solution.
Accordingly, the nitride semiconductor device formed on the
substrate can be a nitride semiconductor with lower dislocation
density. Concretely, this object is to form a nitride substrate
having a lower dislocation density, i.e. 10.sup.5/cm.sup.2 or less
and more preferably 10.sup.4/cm.sup.2 or less and to form thereon a
light emitting device (laser structure etc.) having less crystal
dislocation causing non-radiative recombination.
[0007] The third object of the present invention is to provide a
light emitting device structure, such as a laser device etc, which
comprises a high-resistance layer prepared by crystallization from
supercritical ammonia-containing solution as a current confinement
layer.
[0008] The inventors of the present invention found the following
matters by using a technique wherein a gallium-containing nitride
is recrystallized by crystallization from supercritical
ammonia-containing solution, so-called AMMONO method:
[0009] the ratio of Ga/NH.sub.3 can remarkably be improved (over 20
times), compared with the ratio set by MOCVD vapor phase
growth,
[0010] the bulk single crystal having a lower dislocation density
can be obtained by AMMONO method at a very low temperature
(600.degree. C. or less), while the bulk single crystal is prepared
by the vapor phase growth of the nitride at 1000.degree. C. or
more,
[0011] the lower dislocation density and recovery of the
crystalline quality thereof can be realized despite the thin layer
growth of the gallium-containing nitride, and
[0012] the single crystal substrate wherein the single crystal
substrate is formed on A-plane or M-plane as an epitaxial growth
face can be obtained, while such substrate would not be prepared by
the so-far vapor phase growth.
[0013] The first invention is to provide a light emitting device
structure comprising a gallium-containing nitride single crystal
substrate, an n-type nitride semiconductor layer, an active layer
comprising an In-containing nitride semiconductor, and a p-type
nitride semiconductor layer, formed on the substrate, for growth
prepared by the vapor phase growth, wherein a gallium-containing
nitride semiconductor layer is formed to preserve the crystalline
quality which would be degraded during the deposition of the layers
in the light emitting device in the form of quaternary or ternary
compound, such as InAlGaN, InGaN or AlGaN etc. on the substrate.
Moreover, it is possible to recover the crystalline quality which
would be detracted by newly occurred dislocation or impurity
dopants during the depositing process of nitride semiconductor. The
first invention is characterized in that the gallium-containing
nitride semiconductor layer is formed by crystallization from
supercritical ammonia-containing solution, so that the layer can
become an epitaxial growth plane whose dislocation density thereon
is 10.sup.6/cm.sup.2 or less, preferably 10.sup.4/cm.sup.2.
[0014] Specifically, the gallium-containing nitride has to be grown
at the temperature which does not damage the active layer
comprising an In-containing nitride semiconductor. In the AMMONO
method, the nitride is grown at 600.degree. C. or less, preferably
550.degree. C. or less, therefore the single crystal GaN or AlGaN
layer can be deposited on the active In-containing layer without
the detraction of the active layer, although the growth temperature
of 1000.degree. C. or more is required in the vapor phase growth.
The active layer comprising an In-containing nitride semiconductor
is usually formed at 900.degree. C., as lower temperature does not
damage to the active layer from degradation etc. Furthermore, the
crystalline quality can be recovered by the thin layer of less than
several am, preferably several hundreds .ANG. and the dislocation
density can also be reduced, so that the resulting laser device
etc. is not subject to the stress.
[0015] The second invention is characterized in that the substrate
is the gallium-containing nitride bulk single crystal prepared by
crystallization from supercritical ammonia-containing solution,
which leads to a light emitting device with lower dislocation
density by the combination of the first invention and the second
invention. Moreover, the substrate in the light emitting device
structure has at least one plane selected from the group comprising
A-plane, M-plane, R-plane, C-plane, {1-10n (n is a natural
number)}, and {11-2m (m is a natural number)} of the
gallium-containing nitride bulk single crystal, as its own
surface.
[0016] According to the present invention, a nitride bulk single
crystal shown in Drawings can be prepared by applying the AMMONO
method, therefore A-plane or M-plane which is parallel to C-axis of
hexagonal structure for an epitaxial growth can be obtained. (FIG.
9) In the present invention, an epitaxial growth required by a
device structure can be carried out in case that the plane has the
area of 100 mm.sup.2. A-plane and M-plane are non-polar, unlike
C-plane. In case that A-plane or M-plane of the gallium-containing
nitride is used as a plane for depositing of layers, there can be
obtained a laser device having no cause of the deterioration of the
performance such as the red shift of light emitting, recombination
degradation and increase of the threshold current. According to the
present invention, when the nitride semiconductor laser device is
grown on A-plane of the GaN substrate prepared by crystallization
from supercritical ammonia-containing solution, the active layer of
the laser device is not subject to the polarization effect. In such
a case, the light emitting face of the resonator will be M-plane,
on which M-plane end face film can be formed and thus cleavage is
easily performed. In case that the nitride semiconductor laser
device is grown on M-plane of the GaN substrate prepared by
crystallization from supercritical ammonia-containing solution, the
active layer is not subject to the polarization effect and A-plane
end face film being non-polar can be obtained on the light emitting
face of the resonator.
[0017] According to the present invention, a substrate for growth
means not only a substrate of only gallium-containing nitride but
also a composite substrate (template) which comprises
gallium-containing nitride grown on a heterogeneous substrate. In
case that the gallium-containing nitride is formed on a
heterogeneous substrate by crystallization from supercritical
ammonia-containing solution, first GaN, AlN or AlGaN layer is
preformed on the heterogeneous substrate and then the
gallium-containing nitride is formed thereon.
[0018] The third invention is characterized by a light emitting
device structure which comprises a light emitting device comprising
an n-type nitride semiconductor layer, an active layer comprising
an In-containing nitride semiconductor, and a p-type nitride
semiconductor layer, formed on a substrate for growth, wherein the
light emitting device comprises a layer in the form of
high-resistance single crystal having a general formula
Al.sub.xGa.sub.1-xN (0.ltoreq.x.ltoreq.1), prepared by
crystallization from supercritical ammonia-containing solution as a
current confinement layer. Accordingly, it is possible to limit the
flowing position of the electric current and confine the current
without forming the ridge in the laser device. Higher mixture ratio
of Al in the crystal leads to the lower refraction index so as to
confine the light efficiently. The current confinement layer made
of AlN is preferred.
[0019] According to the present invention, aforementioned single
crystal layer is usually in the form of a non-doped crystal. Even
if AlGaN layer has non-uniform mixture ratio of crystal in
direction of the thickness and then a tendency of decreased mixture
ratio from the beginning of forming step is shown, there is no
hindrance to the function as a current confinement layer.
Furthermore, the layer can attain its function in the form of thin
layer, i.e. several to several tens nm. Accordingly, when the
AMMONO method is applied, alkali metal such as Na, K or Li etc, or
alkali metal compound such as azide, amide, imide, amide-imide or
hydride may be used as a mineralizer. Considering dissolving of the
current confinement layer with the supercritical ammonia at the
beginning of the AMMONO method, it is preferable that the thickness
of the lower layer of the current confinement layer is set thicker
than usual. When the current confinement layer or
gallium-containing nitride semiconductor layer is prepared by the
AMMONO method, it is recommended that a mask may be formed having
the lower or same solubility in the supercritical ammonia. The
formation of the mask can prevent the dissolution in the
supercritical ammonia from the end face of the other layers of the
nitride semiconductor, especially the dissolution of the active
layer. The mask may be selected from the group consisting of SiO,
SiN, AlN, Mo, W, and Ag. In the supercritical ammonia these
materials for mask are more stable than GaN and the contact surface
covered with the mask material can be prevented from the
dissolution. In a later process, i.e. the process of formation of a
ridge, the mask can be easily removed.
[0020] In the AMMONO method using the supercritical ammonia, a
nitride semiconductor is grown in the supercritical ammonia wherein
a gallium-containing nitride has the negative solubility curve.
Detailed explanation of the method is disclosed in Polish Patent
Application Nos. P-347918, P-350375 and PCT Application No.
PCT/IB02/04185, so that those skilled in the art can easily carry
out the present invention with reference to the abstract and
examples explained below.
[0021] In the present invention, gallium-containing nitride or
nitride is defined as below and as the general formula
Al.sub.xGa.sub.1-x-yIn.sub.yN, where 0.ltoreq.x<1,
0.ltoreq.y<1, and 0.ltoreq.x+y<1, and may contain a donor, an
acceptor, or a magnetic dopant, as required. For example, if the
donor is doped, the nitride can be changed into n-type, so that the
gallium-containing nitride semiconductor layer can be formed on a
part of n-type nitride semiconductor layer. If acceptor is doped,
the nitride can be changed into p-type, so that the
gallium-containing nitride semiconductor layer is formed on a part
of p-type nitride semiconductor layer. If a substrate for growth is
a conductive substrate, a laser device (FIG. 1) or LED device (FIG.
4) having a pair of opposite electrodes can be obtained. It enables
to introduce the huge electric current thereto.
[0022] As will be defined later, the supercritical solvent may
contain NH.sub.3 and/or a derivative thereof. The mineralizer may
contain alkali metal ions, at least, ions of lithium, sodium or
potassium. On the other hand, the gallium-containing feedstock can
be mainly composed of gallium-containing nitride or a precursor
thereof. The precursor can be selected from an azide, imide,
amidoimide, amide, hydride, intermetallic compound, alloy or metal
gallium, each of which may contain gallium, as it is defined
later.
[0023] According to the present invention, seeds for forming the
substrate for growth can be comprised with GaN prepared by HVPE,
crystals formed on the wall in the autoclave by spontaneous growth
during crystallization from supercritical ammonia-containing
solution, crystals prepared by flux method or crystals prepared by
high-pressure method. It is preferable that a heterogeneous seed
has a lattice constant of 2.8 to 3.6 with respect to a.sub.o-axis
and a nitride semiconductor having a surface dislocation density of
10.sup.6/cm.sup.2 or less is formed on the seed. Such a seed is
selected from a body-centered cubic crystal system Mo or W, a
hexagonal closest packing crystal system .alpha.-Hf or .alpha.-Zr,
a tetragonal system diamond, a WC structure crystal system WC or
W.sub.2C, a ZnO structure crystal system SiC, especially
.alpha.-SiC, TaN, NbN or AlN, a hexagonal (P6/mmm) system
AgB.sub.2, AuB.sub.2, HfB.sub.2 or ZrB.sub.2, and a hexagonal
(P6.sub.3/mmc) system .gamma.-MoC .epsilon.-MbN or ZrB.sub.2. In
order to make the surface property, the appropriate condition for
crystal growth, Ga irradiation, NH.sub.3 process and Oxygen plasma
process may be carried out as required, so that the heterogeneous
seed has the Ga-polarity or N-polarity. Moreover, HCl process, HF
process may be carried out, as required, to purify the surface. Or
a GaN or AlN layer is formed on the heterogeneous seed by the vapor
phase growth, so that the crystallization can effectively be
carried out by crystallization from supercritical
ammonia-containing solution. After such processes,
gallium-containing nitride grown on the seed by polishing or wire
saw, so as to prepare by a wafer for a substrate for growth.
[0024] In the present invention, the crystallization of
gallium-containing nitride is carried out at a temperature of 100
to 800.degree. C., preferably 300 to 600.degree. C., more
preferably 400 to 550.degree. C. Also, the crystallization of
gallium-containing nitride is carried out under a pressure of 100
to 10,000 bar, preferably 1,000 to 5,500 bar, more preferably 1,500
to 3,000 bar.
[0025] The concentration of alkali metal ions in the supercritical
solvent is adjusted so as to ensure the specified solubilities of
feedstock and gallium-containing nitride, and the molar ratio of
the alkali metal ions to other components of the supercritical
solution is controlled within a range from 1:200 to 1:2, preferably
from 1:100 to 1:5, more preferably from 1:20 to 1:8.
[0026] The present invention relates to a technique of an
ammono-basic growth of crystal which comprises a chemical transport
in a supercritical ammonia-containing solvent containing at least
one mineralizer for imparting an ammono-basic property, to grow a
single crystal of gallium-containing nitride. This technique has a
very high originality, and therefore, the terms herein used should
be understood as having the meanings defined as below in the
present specification.
[0027] The term "gallium-containing nitride" in the specification
means a compound which includes at least gallium and nitrogen atom
as a consistent element. It includes the binary compound GaN,
ternary compounds such as AlGaN, InGaN or also quaternary compounds
AlInGaN, where the range of the other elements to gallium can vary,
in so far as the crystallization growth technique of ammonobasic is
not hindered.
[0028] The term "gallium-containing nitride bulk single crystal"
means a gallium-containing nitride single crystal substrate on
which an optic or electronic device such as LED or LD can be
prepared by an epitaxial growth process such as MOCVD, HVPE or the
like.
[0029] The term "a precursor of gallium-containing nitride" means a
substance which may contain at least gallium, and if needed, an
alkali metal, an element of the Group XIII, nitrogen and/or
hydrogen, or a mixture thereof, and examples of such a precursor
include metallic Ga, an alloy or an intermetallic compound of Ga,
and a hydride, amide, imide, amido-imide or azide of Ga, which can
form a gallium compound soluble in a supercritical
ammonia-containing solvent as defined below.
[0030] The term "gallium-containing feedstock" means a
gallium-containing nitride or a precursor thereof.
[0031] The term "supercritical ammonia-containing solvent" means a
supercritical solvent which may contain at least ammonia, and ion
or ions of at least one alkali metal for dissolving
gallium-containing nitride.
[0032] The term "mineralizer" means a supplier for supplying one or
more of alkali metal ions (Li, K, Na or Cs) for dissolving
gallium-containing nitride in the supercritical ammonia-containing
solvent. Concretely, the mineralizer is selected from the group
consisting of Li, K, Na, Cs, LiNH.sub.2, KNH.sub.2, NaNH.sub.2,
CsNH.sub.2, LiH, KH, NaH, CsH, Li.sub.3N, K.sub.3N, Na.sub.3N,
CS.sub.3N, Li.sub.2NH, K.sub.2NH, Na.sub.2NH, Cs.sub.2NH,
LiNH.sub.2, KNH.sub.2, NaNH.sub.2, CsNH.sub.2, LiN.sub.3, KN.sub.3,
NaN.sub.3 and CsN.sub.3.
[0033] The phrase "dissolution of gallium-containing feedstock"
means a reversible or irreversible process in which the feedstock
takes the form of a gallium compound soluble in the supercritical
solvent such as a gallium complex compound. The gallium complex
compound means a complex compound in which a gallium atom as a
coordination center is surrounded by ligands, e.g., NH.sub.3 or
derivatives thereof such as NH.sub.2.sup.- and NH.sup.2-.
[0034] The term "supercritical ammonia-containing solution" means a
solution including a soluble gallium-containing compound formed by
dissolution of gallium-containing feedstock in the supercritical
ammonia-containing solvent. Based on our experiments, we have found
that there is an equilibrium relationship between the
gallium-containing nitride solid and the supercritical solution
under a sufficiently high temperature and pressure conditions.
Accordingly, the solubility of the soluble gallium-containing
nitride can be defined as the equilibrium concentration of the
above soluble gallium-containing compound in the presence of solid
gallium-containing nitride. In such a process, it is possible to
shift this equilibrium by changing in temperature and/or
pressure.
[0035] The phrase "negative temperature coefficient of solubility"
shown in the gallium-containing nitride in the supercritical
ammonia means that the solubility is expressed by a monotonically
decreasing function of the temperature, when all other parameters
are kept constant. Similarly, the phrase "positive pressure
coefficient of solubility" means that the solubility is expressed
by a monotonically increasing function of the temperature, when all
other parameters are kept constant. Based on our research, the
solubility of gallium-containing nitride in the supercritical
ammonia-containing solvent has a negative temperature coefficient
at least within the range of 300 to 550.degree. C., and a positive
pressure coefficient at least within the range of 1 to 5.5
kbar.
[0036] The phrase "supersaturation of the supercritical
ammonia-containing solution of gallium-containing nitride" means
that the concentration of the soluble gallium compounds in the
above supercritical ammonia-containing solution is higher than the
concentration in the equilibrium state, i.e., the solubility of
gallium-containing nitride. In case of the dissolution of
gallium-containing nitride in a closed system, such supersaturation
can be achieved, according to the negative temperature coefficient
or a positive pressure coefficient of solubility, by raising the
temperature or reducing the pressure.
[0037] The chemical transport from the lower temperature
dissolution zone to higher temperature crystallization zone is
important for gallium-containing nitride in the supercritical
ammonia-containing solution. The phrase "the chemical transport"
means a sequential process including the dissolution of
gallium-containing feedstock, the transfer of the soluble gallium
compound through the supercritical ammonia-containing solution, and
the crystallization of gallium-containing nitride from the
supersaturated supercritical ammonia-containing solution. In
general, a chemical transport process is carried out by a certain
driving force such as a temperature gradient, a pressure gradient,
a concentration gradient, difference in chemical or physical
properties between the dissolved feedstock and the crystallized
product, or the like. Preferably, the chemical transport in the
process of the present invention is achieved by carrying out the
dissolution step and the crystallization step in separate zones,
provided that the temperature of the crystallization zone is
maintained higher than that of the dissolution zone, so that the
gallium-containing nitride bulk single crystal can be obtained by
the processes of this invention.
[0038] The term "seed" has been described above. According to the
present invention, the seed provides a region or area on which the
crystallization of gallium-containing nitride is allowed to take
place. Seed may be a laser device or LED device, whose surface is
exposed for forming a current confinement layer. Moreover, the
growth quality of the crystal depends on the quality of the seed
for forming the substrate for growth. Thus, the seed of a high
quality should be selected.
[0039] The term "spontaneous crystallization" means an undesirable
phenomenon in which the formation and the growth of the core of
gallium-containing nitride from the supersaturated supercritical
ammonia-containing solution occur at any site inside the autoclave,
and the spontaneous crystallization also includes disoriented
growth of the crystal on the surface of the seed.
[0040] The term "selective crystallization on the seed" means a
step of allowing the crystallization to take place on the surface
of the seed, accompanied by substantially no spontaneous growth.
This selective crystallization on the seed is essential for the
growth of a bulk single crystal, it is also one of the conditions
to form aforementioned gallium-containing nitride semiconductor
layer, electric current confinement layer and a substrate for
growth by applying crystallization from supercritical
ammonia-containing solution.
[0041] The autoclave to be used in the present invention is a
closed system reaction chamber for carrying out the ammono-basic
growth of the crystal and any form of the autoclave is
applicable.
[0042] In this regard, the temperature distribution in the
autoclave, as described later in the part of Examples, is
determined by using an empty autoclave, i.e. without the
supercritical ammonia, and thus, the supercritical temperature is
not the one actually measured. On the other hand, the pressure in
the autoclave is directly measured, or it is determined by the
calculation from the amount of ammonia initially introduced, and
the temperature and the volume of the autoclave.
[0043] It is preferable to use an apparatus as described below, to
carry out the above process. An apparatus according to the present
invention provides an autoclave for preparing the supercritical
solvent, characterized in that a convection control means for
establishing a convention flow is arranged in the autoclave, and a
furnace unit is equipped with a heater or a cooler.
[0044] The furnace unit includes a higher temperature zone,
equipped with a heater, which corresponds to the crystallization
zone in the autoclave, and a lower temperature zone, equipped with
a heater or a cooler, which corresponds to the dissolution zone in
the autoclave. The convection control means may be composed of at
least one horizontal baffle having a central opening and/or a
periphery space and dividing the crystallization zone from the
dissolution zone. Inside the autoclave, the feedstock is located in
the dissolution zone, and the seed is located in the
crystallization zone, and convection flow in the supercritical
solution between two zones is controlled by the convection control
means. It is to be noted that the dissolution zone is located above
the horizontal baffle, and the crystallization zone, below the
horizontal baffle.
[0045] Crystallization from supercritical ammonia-containing
solution (AMMONO method) is summarized as follows. In the reaction
system, the negative dissolution curve (negative temperature
coefficient of solubility) means that the solubility of the nitride
semiconductor is lower in the higher temperature zone and the
solubility thereof is higher in the lower temperature zone. When
the temperature difference is controlled properly in the higher
temperature zone and the lower temperature zone inside the
autoclave, the nitride are dissolved in the lower temperature zone
and it is recrystallized in the higher temperature zone. Due to the
generated convection flow from the lower temperature zone to the
higher temperature zone, a predetermined concentration of nitrides
can be kept in the higher temperature zone and the nitrides can be
selectively grown on the seed. Moreover, the aspect ratio
(longitudinal direction/lateral direction) in the reaction system
inside the autoclave is preferably set 10 or more, so that the
convection flow does not stop. The convection control means is
located within the range from 1/3 to 2/3 of the total length of the
inner chamber of the autoclave. The ratio of opening in the
horizontal baffle on the cross-sectional area is set at 30% or
less, so that the spontaneous crystallization can be prevented.
[0046] The wafer is thus placed in the higher temperature zone, and
the feedstock in the lower temperature zone in the reaction system
inside the autoclave. Dissolution of the feedstock in the lower
temperature zone leads to the supersaturation. In the reaction
system, a convection flow is generated, due to which the dissolved
feedstock flows to the higher temperature zone. Due to a lower
solubility at the higher temperature zone, the dissolved feedstock
becomes recrystallized on the wafer which is the seed.
Recrystallization carried out in this way results in forming a bulk
single crystal layer. Moreover, a characteristic feature of this
method, as compared to the methods by which nitride semiconductor
is formed from the vapor phase growth at temperature over
900.degree. C., is the fact that it allows growth of nitride
semiconductor at a temperature preferably 600.degree. C. or less,
and more preferably 550.degree. C. or less. Due to this, in the
wafer placed in the higher temperature zone a thermal degradation
of the active In-containing layer does not occur.
[0047] The material of the feedstock depends on the composition of
the single crystal layer. In case that GaN is used, GaN single
crystal, GaN poly crystal, GaN precursor or metallic Ga can
generally be used, GaN single crystal or GaN poly crystal can be
formed and then recrystallized. GaN prepared by the vapor phase
growth, such as HVPE method or MOCVD method, by AMMONO method, by
flux method or by high pressure method can be used. GaN powder in
the form of a pellet can also be used. The precursor of GaN may
contain gallium azide, gallium imide, gallium amide or the mixture
thereof. In case of AlN--similarly as GaN--AlN single crystal, AlN
poly crystal, AlN precursor or metallic Al is used, AlN single
crystal or AlN poly crystal can be formed and then recrystallized.
AlGaN is a mixed crystal of AlN and GaN, and the feedstock thereof
may be mixed appropriately. Moreover, the usage of metal and single
crystal or poly crystal (for example, metallic Al and GaN single
crystal or poly crystal) and preferably adding more than two kinds
of mineralizer etc. can lead to a predetermined composition.
[0048] It is possible to use alkali metals, such as Li, Na, K, Cs
or complexes of alkali metals, such as alkali metal azide, alkali
metal amide, alkali metal imide as a mineralizer. A molar ratio of
the alkali metal to ammonia ranges from 1:200 to 1:2. Li is
preferably used. Li is a mineralizer, for which the solubility of
nitride is low, which leads to restraint of dissolution of the
uncovered nitride semiconductor device, preventing the spontaneous
crystallization and effective formation of the thin layers of the
thickness from ten to several tens nm.
BRIEF DESCRIPTION OF DRAWINGS
[0049] FIG. 1 is a schematic cross-sectional view of the end face
of the nitride semiconductor laser device according to the present
invention.
[0050] FIG. 2A-2E represent the schematic cross-sectional view
illustrating a manufacturing process of the nitride semiconductor
laser device, in case of the preferred embodiment according to the
present invention.
[0051] FIGS. 3A and 3B represent the schematic cross-sectional view
of the end face of the nitride semiconductor laser device according
to the present invention.
[0052] FIG. 4A to 4F represent the schematic cross-sectional view
illustrating a manufacturing process of the nitride semiconductor
laser device, in case of the preferred embodiment according to the
present invention.
[0053] FIG. 5 is a schematic cross-sectional view of the nitride
semiconductor LED device according to the present invention.
[0054] FIG. 6 is a schematic view of the nitride semiconductor LED
device according to the present invention.
[0055] FIG. 7 is a schematic view of the nitride semiconductor LED
device according to the present invention.
[0056] FIG. 8 is a schematic view of the nitride semiconductor LED
device according to the present invention.
[0057] FIG. 9 presents a frame format of the substrate in which
A-plane being parallel to c-axis is cut out from the bulk single
crystal and a light emitting end face is formed on M-plane.
BEST MODE FOR CARRYING OUT THE INVENTION
[0058] Further herein a detailed description of the embodiments of
the present invention is provided.
[0059] The schematic cross-sectional view of the semiconductor
laser according to the present invention is shown in FIG. 1. On the
substrate 1 for growth, the n-type nitride semiconductor layer 2
and the p-type nitride semiconductor layer 4 are formed. Between
them there is the active layer 3 of a single quantum well or a
multi quantum well structure in the form of an In-containing
nitride semiconductor. This results in the laser device having a
good light emitting efficiency at the wavelength region between
near-ultraviolet and green visible light (from 370 nm to 550 nm).
The n-type nitride semiconductor layer 2 is composed of an n-type
contact layer 21, a InGaN crack-preventing layer 22, an n-type
AlGaN clad layer 23 and an n-type GaN optical guide layer 24. The
n-type contact layer 21 and the crack-preventing layer 22 can be
omitted. The p-type nitride semiconductor layer 4 is composed of a
cap layer 41, a p-type AlGaN optical guide layer 42, a p-type AlGaN
clad layer 43 and a p-type GaN contact layer 44. According to the
present invention, gallium-containing nitride semiconductor layer
prepared by the crystallization from supercritical
ammonia-containing solution can be used in the n-type nitride
semiconductor layer 2 or p-type nitride semiconductor layer 4. The
substrate 1 is comprised with a bulk single crystal and the
dislocation thereof is remarkably low, i.e. about
10.sup.4/cm.sup.2. Therefore, the n-type contact layer 21 can be
formed without ELO layer for decreasing dislocation, AlGaN layer
for decreasing the pits or buffer layer. The substrate is a
conductive substrate and n-type electrode is formed below the
substrate so that the p-type electrode and the n-type electrode
compose a face-type electrodes structure. In the above embodiment,
the resonator of the semiconductor laser device is composed of the
active layer 3, the p-type optical guide layer 24, n-type optical
guide layer 42 and the cap layer 41.
[0060] Further herein the typical manufacturing method of the
nitride semiconductor laser device of the present embodiment is
provided.
[0061] FIG. 2A to 2E illustrate the process which comprises the
steps of forming a laser device on the C-plane using a conductive
GaN substrate as a substrate for growth and a n-type electrode
below the substrate.
[0062] FIG. 4A to 4E illustrate the process which comprises the
steps of forming a n-type nitride semiconductor layer 2, an active
layer 3 and a first p-type nitride semiconductor layer 4A of a
laser device, and then forming a current confinement layer 5 by
crystallization from supercritical ammonia-containing solution, and
finally forming a second p-type nitride semiconductor layer 4B.
Next, after growing a nitride semiconductor layer, a p-type
electrode is formed on the second p-type nitride semiconductor
layer 4B and n-type electrode is formed below the substrate for
growth so that a laser device can be obtained.
[0063] The first method shown in FIG. 2, the conductive substrate
for growth is first prepared. (FIG. 2A) Next, the wafer is prepared
on the C-plane of Substrate 1 by depositing successively the n-type
nitride semiconductor layer 2 composed of an n-type contact layer
21, a crack-preventing layer 22, an n-type clad layer 23 and an
n-type optical guide layer 24, then the active layer 3 and finally
the p-type nitride semiconductor layer 4 composed of a protective
layer 41, a p-type optical guide layer 42, a p-type clad layer 43
and a p-type contact layer 44. (FIG. 2B) According to the present
invention, a gallium-containing nitride semiconductor layer
prepared by crystallization from supercritical ammonia-containing
solution is intercalated in the n-type nitride semiconductor layer
and/or the p-type nitride semiconductor layer so that the
crystalline quality of the laser device can be recovered. In this
process, since the substrate for growth is used, the dislocation of
the epitaxial layer can be decreased without forming the n-type
nitride semiconductor layer 2 through a buffer layer prepared at
the low temperature an ELO layer. The n-type contact layer 21 or
the crack preventing layer 22 can be omitted.
[0064] Next, the wafer is etched and a ridge is formed. Then the
buried layer 70 is formed to cover the ridge and next the p-type
electrode 80 is formed. The ridge stripe which performs the optical
wave guide is formed in the direction of the resonator. The width
of the ridge is from 1.0 .mu.m to 20 .mu.m and the ridge reaches
the p-type clad layer or the p-type guide layer. The buried layer
is made of SiO.sub.2 film or ZrO.sub.2 film etc. A p-type ohmic
electrode 80 is formed to be in contact with the p-type contact
layer 43 which is on the top surface of the ridge. Both of single
ridge and plural ridges can be used. A multi-stripe-type laser
device can be obtained by plural ridges. Next, a p-type pad
electrode is formed. Moreover, a SiO.sub.2/TiO.sub.2 serves as a
reflecting film for laser oscillation due to an alternate
arrangement and a patterning of the SiO.sub.2 and TiO.sub.2 layers.
Finally, each nitride semiconductor laser device is cut out from
the wafer by scribing. In this way a finished nitride semiconductor
laser device is obtained. (FIG. 2E, FIG. 1)
[0065] FIG. 4A to 4E illustrate the process of manufacturing a
laser device comprising a current confinement layer. A-plane of the
substrate 1 is cut out from the bulk single crystal as illustrated
in FIG. 9 and used as a substrate, and a light emitting end face is
M-plane so that a laser device can be obtained by cleavage. On the
substrate 1 for growth the n-type nitride semiconductor layer 2 and
the active layer 3 are deposited successively. Next, the first
p-type nitride semiconductor layer 4A is formed. (FIG. 4A) The same
reference numeral is given to the same element to omit the
explanation. Next, the first p-type nitride semiconductor layer 4A
is prepared by etching in the form of convex-shape. (FIG. 4B) Then,
the current confinement layer 5 is prepared by crystallization from
supercritical ammonia-containing solution using the
gallium-containing nitride semiconductor layer. (FIG. 4C) Further,
the second p-type nitride semiconductor layer 4B is formed. (FIG.
4D) The p-type ohmic electrode 80 is formed to be in contact with
the second p-type nitride semiconductor layer 4B. Next, the n-type
electrode 90 is formed below the substrate 1. (FIG. 4E) Next, the
p-type pad electrode 110 is formed. Next, the light emitting end
face is formed by cleavage, so that the wafer becomes in the form
of a bar. After such process, the light emitting film may be formed
on the light emitting end face so as to obtain a laser device by
cleaving. The current confinement layer 5 can be arranged at the
side of the p-type nitride semiconductor layer (FIG. 3A) or n-type
nitride semiconductor layer (FIG. 3B)
[0066] In case that the current confinement layer 5 is formed, the
single crystal AlGaN layer can be formed at a low temperature, i.e.
from 500.degree. C. to 600.degree. C., by applying crystallization
from supercritical ammonia-containing solution. P-type nitride
layer can be formed without degradation of the active In-containing
layer.
[0067] FIG. 5 illustrates the obtained LED device having a
gallium-containing nitride semiconductor layer prepared by
crystallization from supercritical ammonia-containing solution.
[0068] After the gallium-containing nitride semiconductor layer 202
is formed directly on the conductive substrate 201 without forming
buffer layer prepared at low temperature, a modulation doped layer
203 composed of undoped GaN/Si doped GaN/undoped GaN and an active
layer 205 composed of InGaN well layer/GaN barrier layer through a
superlattice layer 204 are formed. LED is obtained by successively
depositing a p-type clad layer 206, an undoped AlGaN layer 207 and
a p-type contact layer 208 on the top surface of the active layer
205. The p-type electrode 209 and n-type electrode 210 are
simultaneously formed on the p-type contact layer 208 and below the
substrate 201, respectively.
[0069] According to the present invention, the gallium-containing
nitride semiconductor layer 202 can be formed instead of the
modulation doped layer 203 and the superlattice layer 204, while
the n-type contact layer is formed on one bottom side, and the
gallium-containing nitride semiconductor layer 202 can be formed on
the active layer. As described above, AMMONO method which enables
to form the single crystal at a low temperature allows simplifying
the device structure as well as recovering the crystalline quality
and decreasing the dislocation density.
[0070] The following examples are intended to illustrate the
present invention and should not be construed as being
limiting.
EXAMPLE 1
[0071] The GaN substrate 1 doped with Si of 2 inch diameter on
C-plane as a growth face is placed in a MOCVD reactor. Temperature
is set at 1050.degree. C. Hydrogen is used as a carrier gas, and
ammonia and TMG (thrimethylgallium) are used as gaseous
materials.
[0072] On the substrate, the following layers are deposited one
after the other:
[0073] (1) 4 .mu.m thickness n-type GaN contact layer, doped with
Si at the level of 3.times.10.sup.18/cm.sup.3.
[0074] (2) n-type clad layer, in the form of the superlattice of
the total thickness being 1.2 .mu.m, formed by alternate deposition
of 25 angstroms thickness undoped Al.sub.0.1Ga.sub.0.9N layer and
n-type GaN layer doped with Si at the level of
1.times.10.sup.19/cm.sup.3.
[0075] (3) a wafer is introduced into the reactor (autoclave),
inside which is filled with the supercritical ammonia. Having been
filled with the feedstock in the form of GaN of 0.5 g, ammonia of
14.7 g and mineralizer in the form of Li of 0.036 g, the autoclave
(36 cm.sup.3) is tightly closed at a temperature 500.degree. C. or
less inside the autoclave. The internal chamber of the autoclave is
divided into two zones: the higher temperature zone and the lower
temperature zone. In the higher temperature zone of 550.degree. C.
there is a wafer, whereas in the lower temperature zone of
450.degree. C. there is feedstock in the form of GaN and Ga metal.
The sealed autoclave is left for three days. Under the low
temperature condition, the layer for recovering the crystalline
quality of 100 angstrom thickness in the form of single crystal GaN
is grown in supercritical ammonia.
[0076] (4) Then the wafer is taken out from the autoclave and set
in the MOCVD reactor device at a temperature of 1050.degree. C. 0.2
.mu.m thickness undoped GaN n-type optical guide layer.
[0077] (5) an active layer of the total thickness being 380
angstroms in the form of layers alternately arranged, i.e. barrier
layer/well layer/barrier layer/well layer/barrier layer, wherein
100 angstroms thickness with Si doped In.sub.0.05Ga.sub.0.95N layer
forms a barrier layer, and 40 angstroms thickness undoped
In.sub.0.1Ga.sub.0.9N layer forms a quantum well layer.
[0078] (6) 0.2 .mu.m thickness undoped GaN p-type optical guide
layer.
[0079] (7) p-type clad layer in the form of the superlattice of the
total thickness being 0.6 .mu.m, formed by alternate deposition of
25 angstroms thickness undoped Al.sub.0.16Ga.sub.0.84N layer and 25
angstroms thickness undoped GaN layer.
[0080] (8) 150 angstroms thickness p-type contact layer of p-type
GaN doped with Mg at the level of 1.times.10.sup.20/cm.sup.3.
[0081] After the above layers are deposited, the formed wafer is
subject to annealing at 700.degree. C. in the MOCVD reactor device
under the nitrogen atmosphere, due to which the resistance of the
p-type nitride semiconductor layer is additionally reduced.
[0082] After annealing, the wafer is taken out from the reactor and
a protective film (mask) in the form of SiO.sub.2 stripe is
deposited on the surface of the top p-type contact layer. Next, by
using RIE method, the wafer is etched and a stripe is formed,
uncovering thereby end faces of the resonator and the surface of
the n-type contact layer. The SiO.sub.2 protective film (mask)
formed on the surface of the p-type contact layer is removed by
using the wet etching method.
[0083] Next, under the low temperature condition, in the
supercritical ammonia 100, angstrom thick single crystal GaN end
face film is grown on the stripe end face, stripe lateral face and
uncovered surfaces of the p-type contact layer.
[0084] After a single crystal GaN end face film, which can be
omitted, is formed, the single crystal GaN formed on the surface of
the top p-type contact layer is removed by etching. Next, the
surface of the p-type contact layer is covered with the SiO.sub.2
mask in the form of 1.5 .mu.m wide strips and etching of the p-type
clad layer is continued until a ridge is formed on the strip part.
Etching is carried out until thickness of the p-type clad layer
becomes 0.1 .mu.m on both sides of ridge.
[0085] In this way a ridge part of 1.5 .mu.m width is formed.
[0086] Next, by use of the ion sputtering method, a 0.5 .mu.m
thickness ZrO.sub.2 film is formed so that it would cover stripe
surfaces over the SiO.sub.2 mask.
[0087] After the thermal processing, the buried layer 70 in the
form of the ZrO.sub.2 film is deposited on the top stripe surface,
on the lateral face of ridge and on the surface of the p-type clad
layer located on both sides of ridge. This ZrO.sub.2 film allows
stabilizing a lateral mode at the moment of laser oscillation.
[0088] Next the p-type electrode 80 in the form of Ni/Au is formed
on the p-type contact layer, so that an ohmic contact would appear,
and the n-type electrode 90 in the form of Ti/Al below the
substrate 1. Then, the wafer is subject to the thermal processing
at 600.degree. C. Next, pad electrode in the form of Ni(1000
.ANG.)-Ti(1000 .ANG.)-Au(8000 .ANG.) is laid on the p-type
electrode. After a reflecting film 100 in the form of SiO.sub.2 and
TiO.sub.2 is formed, each nitride semiconductor laser device is cut
out from the wafer by scribing.
[0089] Each nitride semiconductor laser device manufactured in this
way is equipped with a heat sink and the laser oscillation is
carried out. Due to an increased COD level, prolonged continuous
oscillation time is expected--with threshold current density: 2.0
kA/cm.sup.2, power output: 100 mW, preferably 200 mW, and 405 nm
oscillation wavelength.
EXAMPLE 2
[0090] GaN substrate 1 for growth doped with Si is prepared by
crystallization from supercritical ammonia-containing solution,
whereas other stages of production of the nitride semiconductor
laser device are carried out similarly as in Example 1.
[0091] Each laser device manufactured in this way is equipped with
a heat sink and the laser oscillation is carried out. Prolonged
laser lifetime in continuous oscillation mode is expected--with
threshold current density: 2.0 kA/cm.sup.2, power output: 100 mW
and 405 nm oscillation wavelength--similar as in Example 1.
EXAMPLE 3
[0092] First, a GaN substrate 1 doped with Si of 2 inch diameter on
C-plane as a growth face is placed in a MOCVD reactor. Temperature
is set at 1050.degree. C. Hydrogen is used as a carrier gas, and
ammonia and TMG (thrimethylgallium) are used as gaseous
materials.
[0093] On the substrate, the following layers are deposited one
after the other:
[0094] (1) 4 .mu.m thickness n-type GaN contact layer, doped with
Si at the level of 3.times.10.sup.18/cm.sup.3.
[0095] (2) n-type clad layer, in the form of the superlattice of
the total thickness being 1.2 .mu.m, formed by alternate deposition
of 25 angstroms thickness undoped Al.sub.0.1Ga.sub.0.9N layers and
n-type GaN layers doped with Si at the level of
1.times.10.sup.19/cm.sup.3.
[0096] (3) 0.2 .mu.m thickness undoped GaN n-type optical guide
layer.
[0097] (4) an active layer of the total thickness being 380
angstroms in the form of layers alternately arranged, i.e. barrier
layer/well layer/barrier layer/well layer/barrier layer, wherein
100 angstroms thickness with Si doped In.sub.0.05Ga.sub.0.95N layer
forms a barrier layer, and 40 angstroms thickness undoped
In.sub.0.1Ga.sub.0.9N layer forms a quantum well layer.
[0098] (5) the p-type optical guide layer undoped GaN of 0.2 .mu.m
as a first p-type nitride semiconductor layer.
[0099] (6) Next, the first p-type nitride semiconductor layer
except the area for the passage of a current is removed by etching.
(FIG. 4B)
[0100] (7) the wafer is introduced into the reactor (autoclave),
inside which is filled with supercritical ammonia. Having been
filled with the feedstock in the form of Al of 0.5 g, ammonia of
14.7 g and mineralizer in the form of Li of 0.036 g, the autoclave
(36 cm.sup.3) is tightly closed at a temperature 500.degree. C. or
less inside the autoclave. The internal chamber of the autoclave is
divided into two zones: the higher temperature zone and the lower
temperature zone. In the higher temperature zone of 550.degree. C.
there is a wafer, whereas in the lower temperature zone of
450.degree. C. there is feedstock in the form of Al metal. The
sealed autoclave is left for three days. Under the low temperature
condition the current confinement layer 5 of 100 angstrom thickness
in the form of Al is grown in supercritical ammonia.
[0101] (8) the wafer is taken out from the autoclave and set in the
MOCVD reactor device at a temperature of 1050.degree. C. The p-type
clad layer as the second p-type nitride semiconductor layer in the
form of the superlattice of the total thickness being 0.6 .mu.m,
formed by alternate deposition of 25 angstroms thickness undoped
Al.sub.0.16Ga.sub.0.84N layer and 25 angstroms thickness undoped
GaN layer.
[0102] (9) 150 angstroms thick p-type contact layer of p-type GaN
doped with Mg at the level of 1.times.10.sup.20/cm.sup.3.
[0103] After the above layers are deposited, the formed wafer is
subject to annealing at 700.degree. C. in the MOCVD reactor device
under the nitrogen atmosphere, due to which the resistance of the
p-type nitride semiconductor layer is additionally reduced.
[0104] After annealing, the wafer is taken out from the
reactor.
[0105] After a single crystal GaN end face film, which can be
omitted, is formed on the light emitting face, the single crystal
GaN formed on the surface of the top p-type contact layer is
removed by etching. Next, the p-type electrode 80 in the form of
Ni/Au is formed on the surface of the p-type contact layer so that
an ohmic contact would appear, and the n-type electrode 90 in the
form of Ti/Al below the substrate 1. Then, the wafer is subject to
the thermal processing at 600.degree. C. Next, pad electrode in the
form of Ni(1000 .ANG.)-Ti(1000 .ANG.)-Au(8000 .ANG.) are laid on
the p-type electrode. After a reflecting film 100 in the form of
SiO.sub.2 and TiO.sub.2 is formed, each nitride semiconductor laser
device is cut out from the wafer by scribing.
[0106] Each nitride semiconductor laser device manufactured in this
way is equipped with a heat sink and the laser oscillation is
carried out. Due to the increase of a COD level, prolonged
continuous oscillation time is expected--with threshold current
density: 2.0 kA/cm.sup.2, power output: 100 mW, preferably 200 mW,
and 405 nm oscillation wavelength.
[0107] Each laser device manufactured in this way is equipped with
a heat sink and the laser oscillation is carried out. Prolonged
laser lifetime in continuous oscillation mode is expected--with
threshold current density: 2.0 kA/cm.sup.2, power output: 100 mW
and 405 nm oscillation wavelength--similar as in Example 1.
INDUSTRIAL APPLICABILITY
[0108] As described above, since the nitride semiconductor light
emitting device according to the present invention comprises a
gallium-containing nitride semiconductor layer prepared by
crystallization from supercritical ammonia-containing solution, the
crystalline quality can be recovered, while otherwise it would be
degraded after forming the layer of quaternary or ternary compound.
As the result there can be provided a laser device which is
excellent in the lifetime property and current resistant
property.
[0109] Moreover, non-polar nitride A-plane or non-polar nitride
M-plane is cut out from the bulk single crystal, the substrate for
growth is prepared in this way, and the laser device can be formed
on the A-plane or M-plane as an epitaxial growth face. Thus, there
can be obtained the laser device wherein the active layer is not
influenced by the polarization and there is no cause of the
deterioration of the performance such as the red shift of light
emitting, recombination degradation and increase of the threshold
current.
[0110] Furthermore, in case that the current confinement layer is
formed at a lower temperature, the laser device can be obtained
without the device degradation, and the process for forming the
ridge can be omitted.
[0111] Moreover, the nitride layer can be formed in the form of
single crystal at low temperature, so that the active In-containing
layer is not influenced by degradation or damaged. Therefore the
function and lifetime of the device can be improved.
* * * * *