U.S. patent application number 09/813570 was filed with the patent office on 2001-08-02 for method of fabricating a group iii-v semiconductor light emitting device with reduced piezoelectric fields and increased efficiency.
Invention is credited to Akasaki, Isamu, Amano, AichiHiroshi, Takeuchi, Tetsuya, Yamada, Norihide.
Application Number | 20010010372 09/813570 |
Document ID | / |
Family ID | 17415444 |
Filed Date | 2001-08-02 |
United States Patent
Application |
20010010372 |
Kind Code |
A1 |
Takeuchi, Tetsuya ; et
al. |
August 2, 2001 |
Method of fabricating a group III-V semiconductor light emitting
device with reduced piezoelectric fields and increased
efficiency
Abstract
An optical semiconductor device having a plurality of GaN-based
semiconductor layers containing a strained quantum well layer in
which the strained quantum well layer has a piezoelectric field
that depends on the orientation of the strained quantum well layer
when the quantum layer is grown. In the present invention, the
strained quantum well layer is grown with an orientation at which
the piezoelectric field is less than the maximum value of the
piezoelectric field strength as a function of the orientation. In
devices having GaN-based semiconductor layers with a wurtzite
crystal structure, the growth orientation of the strained quantum
well layer is tilted at least 1.degree. from the {0001} direction
of the wurtzite crystal structure. In devices having GaN-based
semiconductor layers with a zincblende crystal structure, the
growth orientation of the strained quantum well layer is tilted at
least 1.degree. from the {111} direction of the zincblende crystal
structure. In the preferred embodiment of the present invention,
the growth orientation is chosen to minimize the piezoelectric
field in the strained quantum well layer.
Inventors: |
Takeuchi, Tetsuya;
(Kanagawa, JP) ; Yamada, Norihide; (Tokyo, JP)
; Amano, AichiHiroshi; (Aichi, JP) ; Akasaki,
Isamu; (Aichi, JP) |
Correspondence
Address: |
Rachel V. Leiterman
Skjerven Morrill MacPherson LLP
Suite 700
25 Metro Drive
San Jose
CA
95110
US
|
Family ID: |
17415444 |
Appl. No.: |
09/813570 |
Filed: |
March 20, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09813570 |
Mar 20, 2001 |
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09162708 |
Sep 29, 1998 |
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6229151 |
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Current U.S.
Class: |
257/79 ; 257/94;
257/E33.003 |
Current CPC
Class: |
H01S 5/343 20130101;
H01S 5/0035 20130101; H01L 33/24 20130101; H01S 5/320275 20190801;
H01S 5/34333 20130101; H01S 5/04257 20190801; H01L 33/16 20130101;
H01S 5/0213 20130101; H01S 5/021 20130101; H01L 33/18 20130101;
H01L 33/32 20130101; H01S 5/32025 20190801; B82Y 20/00 20130101;
H01S 5/3201 20130101 |
Class at
Publication: |
257/79 ;
257/94 |
International
Class: |
H01L 027/15; H01L
033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 30, 1997 |
JP |
09-265311 |
Claims
What is claimed is:
1. In an optical semiconductor device having a plurality of
GaN-based semiconductor layers containing a strained quantum well
layer, said strained quantum well layer having a piezoelectric
field therein having a field strength that depends on the
orientation of said strained quantum well layer when said quantum
layer is grown, the improvement comprising growing said strained
quantum well layer with an orientation at which said piezoelectric
field is less than the maximum value of said piezoelectric field
strength as a function of said orientation.
2. The optical semiconductor device of claim 1 wherein said
GaN-based semiconductor layers have a wurtzite crystal structure
and wherein said growth orientation of said strained quantum well
layer is tilted at least 1.degree. from the {0001} direction of
said wurtzite crystal structure.
3. The optical semiconductor device of claim 2 wherein at least one
other layer of said GaN-based semiconductor has a growth
orientation in the {0001} direction.
4. The optical semiconductor device of claim 2 wherein said
strained quantum well layer is tilted at 40.degree., 90.degree., or
140.degree. from said {0001} direction.
5. The optical semiconductor device of claim 1 wherein said
GaN-based semiconductor layers have a zincblende crystal structure
and wherein said growth orientation of said strained quantum well
layer is tilted at least 1.degree. from the {111} direction of said
zincblende crystal structure.
6. The optical semiconductor device of claim 5 wherein at least one
other layer of said GaN-based semiconductor has a growth
orientation in the {111} direction.
7. A method for fabricating a GaN-based optical semiconductor
device, said method comprising the steps of: growing a first
semiconductor layer on a substrate, said first semiconductor layer
being grown with a first facet orientation; altering the surface of
said first semiconductor layer that is not in contact with said
substrate such that said altered surface provides a growth
orientation having a second facet orientation for a subsequent
semiconductor layer grown thereon, said second facet orientation
differing from said first facet orientation; and growing a strained
quantum well layer on said altered surface.
8. The method of claim 7 wherein said step of altering said surface
of said first semiconductor layer comprises selectively etching
said first semiconductor layer or selective diffusion of said first
semiconductor layer.
9. The method of claim 7 further comprising the step of growing a
second semiconductor layer on said strained quantum well layer,
said second semiconductor layer being grown with a facet
orientation equal to said first facet orientation.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to optical semiconductor
devices, and particularly, to a structure for improving the
efficiency of light emitters and photodetectors fabricated from
GaN-based semiconductors.
BACKGROUND OF THE INVENTION
[0002] In the following discussion a III-N semiconductor is a
semiconductor having a Group III element and nitrogen. III-N
semiconductors such as GaN are useful in fabricating light emitting
elements that emit in the blue and violet regions of the optical
spectrum. These elements include light emitting diodes and laser
diodes. Laser diodes that use semiconductor material based on GaN
that emit in the blue and violet regions of the spectrum hold the
promise of substantially improving the amount of information that
can be stored on an optical disk. However, higher efficiencies are
needed for both semiconductor light emitters and photodetectors.
This is a particularly urgent problem in GaN-based optical
semiconductor devices using BN, AlN, GaN, or InN, which are
compounds of nitrogen and Group III elements such as B, Al, Ga, and
In and their mixed crystal semiconductors (hereinafter, called
GaN-based semiconductors).
[0003] Light emitting elements based on III-N semiconductors are
typically fabricated by creating a p-n diode structure having a
light generating region between the p-type and n-type layers. The
diode is constructed from layers of III-N semiconducting materials.
After the appropriate layers are grown, electrodes are formed on
the p-type and n-type layers to provide the electrical connections
for driving the light-emitting element.
[0004] One class of blue and green light-emitting diodes (LEDs) or
short-wavelength laser diodes (LDs) use GaInN/GaN strained quantum
wells or GaInN/GaInN strained quantum wells located between the
n-type and p-type layers to generate light by the recombination of
holes and electrons injected from these layers. In prior art
devices, a strained GaN-based semiconductor layer is constructed by
growing a {0001} plane of a normal GaN-based crystal. The resulting
layer has a large piezoelectric field. For example, in a
Ga.sub.0.9In.sub.0.1N strained layer, an extremely large
piezoelectric field of around 1 MV/cm is generated.
[0005] Usually, when an electric field exists in a quantum well,
the energy band of the quantum well layer tends to tilt
substantially as the electric field increases. As a result, the
wave functions of the electrons and holes separate from one
another, and the overlap integrals of both wave functions decrease.
Since the optical properties such as the light emission and
absorption efficiencies depend on these overlap integrals, the
efficiency of these devices decreases with increasing electric
fields.
[0006] Broadly, it is the object of the present invention to
provide an improved III-N semiconductor device in which the
efficiency of light generation or detection is increased relative
to prior art devices.
[0007] It is a further object of the present invention to provide a
strained quantum well layer having a reduced piezoelectric
field..
[0008] These and other objects of the present invention will become
apparent to those skilled in the art from the following detailed
description of the invention and the accompanying drawings.
SUMMARY OF THE INVENTION
[0009] The present invention is an optical semiconductor device
having a plurality of GaN-based semiconductor layers containing a
strained quantum well layer in which the strained quantum well
layer has a piezoelectric field that depends on the orientation of
the strained quantum well layer when the quantum layer is grown. In
the present invention, the strained quantum well layer is grown
with an orientation at which the piezoelectric field is less than
the maximum value of the piezoelectric field strength as a function
of the orientation. In devices having GaN-based semiconductor
layers with a wurtzite crystal structure, the growth orientation of
the strained quantum well layer is tilted at least 1.degree. from
the {0001} direction of the wurtzite crystal structure. In devices
having GaN-based semiconductor layers with a zincblende crystal
structure, the growth orientation of the strained quantum well
layer is tilted at least 1.degree. from the {111} direction of the
zincblende crystal structure. In the preferred embodiment of the
present invention, the growth orientation is chosen to minimize the
piezoelectric field in the strained quantum well layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 illustrates the crystal structure of a WZ-GaN-based
semiconductor.
[0011] FIG. 2 is a graph of the piezoelectric field generated in
the quantum well with respect to the growth orientation of the
WZ-GaN-based semiconductor quantum well.
[0012] FIG. 3 illustrates the crystal structure of a ZB-GaN-based
semiconductor.
[0013] FIG. 4 is a graph of the piezoelectric field strength
generated in the quantum well with respect to the first path shown
in FIG. 3.
[0014] FIG. 5 is a cross-sectional view of an edge emitting laser
diode according to one embodiment of the present invention.
[0015] FIG. 6 is a graph of the relative light generation
efficiency of quantum wells in a semiconductor device of the
present invention and a prior art semiconductor device as functions
of the well width.
[0016] FIG. 7 is a cross-sectional view of an edge emitting laser
diode according to a second embodiment of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0017] The present invention is based on the observation that the
piezoelectric field in a strained quantum well layer depends on the
orientation of the crystal structure of the quantum well layer, and
hence, by controlling the facet orientation, the piezoelectric
field can be minimized. The manner in which this is accomplished
may be more easily understood with reference to two types of
strained quantum well structures, those based on a wurtzite crystal
structure and those based on a zincblende crystal structure.
[0018] Refer now to FIG. 1 which illustrates a wurtzite crystal GaN
(WZ-GaN) structure 10. The piezoelectric field generated in a
crystal having a facet orientation along arc 11 in FIG. 1 is shown
in FIG. 2 as a function of the angle .theta. between the {0001}
direction and the facet orientation. The data shown in FIG. 2 is
for Ga.sub.0.9In.sub.0.1N strained quantum well layers. The
piezoelectric field reaches maxima in the {0001} direction or the
{000-1} direction, and has three orientations at which the
piezoelectric field is zero. The same result is obtained for other
arcs, e.g., arc 12. That is, the piezoelectric field is uniquely
determined by the difference in the angle between the {0001}
direction and the facet orientation of the concerned plane, i.e,
the piezoelectric field is independent of .phi..
[0019] Hence it is clear from FIG. 2 that there are three sets of
planes for which there is no piezoelectric field. For example, the
planes at 90.degree. to the C-axis, i.e., the A-plane, {2-1-10},
the M plane {0-110}, etc. The planes around 40.degree. and
140.degree. to the C-axis also provide planes with a zero
piezoelectric field, i.e., the R planes {2-1-14}, {01-12}, etc.
[0020] The strength of the piezoelectric field depends on the
composition of the GaInN strained quantum well layer. However, the
plane orientations in which the field is zero are, at most, only
slightly altered. In particular, the 90.degree. facet orientation
measured from the {0001} direction where the piezoelectric field
becomes 0 does not depend on the ratio of Ga to In. The plane
orientations corresponding to the 40.degree. and 140.degree.
orientations discussed above change by no more than a maximum of
5.degree. from the 40.degree. and 140.degree. values determined for
the composition shown in FIG. 2.
[0021] A similar analysis can be applied to other crystal
structures. Consider a zincblende crystal structure GaN-based
semiconductor layer, referred to as ZB-GaN in the following
discussion. A ZB-Ga.sub.0.9In.sub.0.1N strained quantum well layer
can be formed on GaN in a manner analogous to the WZ-GaN-based
semiconductor strained quantum well layer discussed above. FIG. 3
shows the crystal structure 20 of the ZB-GaN-based semiconductor.
To simplify the discussion, the spherical coordinate system used
with reference to FIG. 1 will also be used here. The radius vector
has a polar angle .theta. measured from the {001} direction and a
cone angle, .phi., about the {001} direction. First and second
paths having a constant azimuth angle .phi. are shown at 21 and
22.
[0022] Refer now to FIG. 4, which is a plot of the piezoelectric
field in the strained quantum well layer with respect to the polar
angle .theta. for various orientations of the strained quantum well
layer on path 21. In FIG. 4, .phi.=45.degree. and the {001}
direction corresponds to .theta.=0.degree.. The {111} direction
corresponds to .theta.=54.7.degree., the {110} direction
corresponds to .theta.=90.degree., and the {11-1} direction
corresponds to .theta.=125.3.degree.. It is clear from FIG. 4, that
the piezoelectric field has maxima in the {111} direction (.theta.
around 55.degree.) and the {11-1} direction (.theta. around
125.degree.). More importantly, the piezoelectric field goes to
zero for .theta.=0, 90.degree., and 180.degree..
[0023] A similar analysis with respect to path 22 shows that the
piezoelectric field is essentially 0 for all points along this
path. Path 22 corresponds to a Ga.sub.0.9In.sub.0.1N strained
quantum well layer in which the growth orientation corresponds to
.theta. and .phi.=90.degree.. Hence, in a strained quantum well
crystal of ZB-GaN-based semiconductor, almost no piezoelectric
field is generated in the strained quantum well layer that has
growth planes beginning in the {001} plane or {011} plane and a
facet orientation angle .theta. on path 22. A similar result holds
for planes that are equivalent to these.
[0024] The manner in which the above-described observations are
used in the fabrication of a light emitter will now be explained
with the aid of FIG. 5 which is a cross-sectional view of a laser
30 according to the present invention. If the crystal growth
orientation is excluded, the composition of each deposited layer is
essentially that used in a conventional laser diode.
[0025] Laser 30 is constructed from a number of layers. An n-type
GaN contact layer 33, an n-type AlGaN cladding layer 34, a strained
multiple quantum well layer 35, a p-type AlGaN cladding layer 36,
and a p-type GaN contact layer 37 are successively deposited on a
substrate 31 which is typically, sapphire, SiC, or GaN. An
n-electrode 38 and a p-electrode 39 are deposited as shown.
[0026] The strained multiple quantum well layer 35 is typically
constructed from GaInN/GaN or GaInN/GaInN. In a laser diode
according to the present invention, the layers of the quantum well
are caused to grow such that the piezoelectric field generated by
the layers is negligible. In conventional laser diodes, the {0001}
plane of a sapphire substrate is used to grow the various layers.
As noted above, this leads to a high piezoelectric field and poor
efficiency.
[0027] As noted above, there are a number of planes for which the
piezoelectric field is substantially zero. One of these is utilized
in a laser diode according to the present invention. The particular
plane will depend on the type of crystal. For example, in the case
of a WZ-GaN light emitter, the {2-1-10} plane of the strained
quantum layer material can be caused to grow by selecting the
appropriate growing surface of substrate 31. If the substrate is
sapphire, the sapphire is cut such that the {01-12} plane is used
for growing layer 33. In the case of SiC, the {2-1-10} plane is
used. In the preferred embodiment of the present invention, SiC
with a growth plane of {2-1-10} is preferred.
[0028] The relative efficiency of a laser diode according to the
present invention and a conventional laser diode grown on the
{0001} plane of a sapphire substrate is shown in FIG. 6 as a
function of the width of the quantum well. Curve A is the
efficiency for the device discussed above with reference to FIG. 5,
and curve B is the efficiency of the conventional device. It will
be appreciated from this figure that the present invention provides
a substantial improvement in the efficiency of light
generation.
[0029] The present invention may also be utilized to provide
improved performance from photodetectors. Photodetectors fabricated
by growing the device on the {0001} plane of a sapphire substrate
exhibit an efficiency and absorption band that depend on light
intensity. In particular, the efficiency of conversion increases
with light intensity while the useful wavelength range
decreases,
[0030] In a photodetector according to the present invention, the
device is grown on a substrate that results in little or no
piezoelectric field in the strained quantum well layer. Hence, the
increase in efficiency and decrease in absorption band are
substantially reduced or eliminated. In general, the growing
technique for a photodetector is the same as that used to construct
a light emitter; however, thicker strained quantum well layers are
utilized to improve the absorption of the incident light.
[0031] It would be advantageous in many circumstances to utilize a
sapphire or SiC substrate in which the layers, except for strained
quantum wells, are grown on the {0001} plane, since substrates cut
to provide growth on a {0001} plane are commercially available.
Refer now to FIG. 7 which is a cross-sectional view of the optical
semiconductor device 50 according to another embodiment of the
present invention in which only the layers related solely to light
emission and absorption have the desired facet orientation. Device
50 is constructed by growing an n-type GaN contact layer 53 and an
n-type Al GaN cladding layer 54 on the {0001} plane orientation on
the substrate 51 such as SiC or GaN based on conventional
technology. Next, by selective growing or selective etching, the
{2-1-14} plane or {01-12} plane is formed. The GaInN/GaN or
GaInN/GaInN strained multiple quantum well layer 55 is then formed
by repeating the crystal growth.
[0032] Next, the remaining p-type AlGaN cladding layer 56 and the
p-type GaN contact layer 57 are successively deposited and formed.
The p-type Al GaN cladding layer 56 and the p-type GaN contact
layer 57 change the crystal structure back to that corresponding to
the {0001} plane from the facet orientation of the well layer 55
and become layers with specific thicknesses. The n-electrode 58 and
the p-electrode 59 are formed as the electrodes on the n-type GaN
contact layer 53 and the p-type GaN contact layer 57, respectively.
The growing surfaces 55A, 55B on both sides of the GaInN strained
multiple quantum well layer 55 are the {01-12} plane or the
{2-1-14} plane. The p-type AlGaN cladding layer 56 and the p-type
GaN contact layer 57 become flat growing surfaces. To simplify the
next process, it is advisable that they be several microns thick.
In the preferred embodiment of the present invention, an AlN buffer
layer 52 is grown on the substrate 51.
[0033] As noted above, the specific plane selected for growing the
quantum well layer depends on the crystal type. In WZ-GaN-based
optical semiconductor devices, the {0001} plane may be utilized,
since this plane has excellent crystal quality and generates almost
no piezoelectric field. For devices based on different compound
semiconductors such as AlN, it can be shown that the piezoelectric
field as a function of the facet orientation behaves similarly to
that described above if the crystal type is the same. The
orientation inclination, .theta., for which the piezoelectric field
of 0 may, however, change by as much as 10.degree..
[0034] Various modifications to the present invention will become
apparent to those skilled in the art from the foregoing description
and accompanying drawings. Accordingly, the present invention is to
be limited solely by the scope of the following claims.
* * * * *