U.S. patent application number 09/337404 was filed with the patent office on 2002-04-25 for a layered structure including a nitride compound semiconductor film and method for making the same.
Invention is credited to UETA, YOSHIHIRO, YUASA, TAKAYUKI.
Application Number | 20020048964 09/337404 |
Document ID | / |
Family ID | 16019358 |
Filed Date | 2002-04-25 |
United States Patent
Application |
20020048964 |
Kind Code |
A1 |
YUASA, TAKAYUKI ; et
al. |
April 25, 2002 |
A LAYERED STRUCTURE INCLUDING A NITRIDE COMPOUND SEMICONDUCTOR FILM
AND METHOD FOR MAKING THE SAME
Abstract
A method for forming a nitride compound semiconductor film of
the present invention includes the steps of: providing a substrate
having a portion which acts as a growth suppressing film on a
outermost surface thereof; forming a growth promoting film
partially on the substrate; and forming a nitride compound
semiconductor on the growth promoting film.
Inventors: |
YUASA, TAKAYUKI; (IKOMA-GUN,
JP) ; UETA, YOSHIHIRO; (TENRI-SHI, JP) |
Correspondence
Address: |
NEIL A DUCHEZ ESQ
RENNER OTTO BOISSELLE & SKLAR PLL
NINETEENTH FLOOR
1621 EUCLID AVENUE
CLEVELAND
OH
44115
|
Family ID: |
16019358 |
Appl. No.: |
09/337404 |
Filed: |
June 21, 1999 |
Current U.S.
Class: |
438/763 ; 257/82;
257/E21.131; 438/767; 438/791 |
Current CPC
Class: |
H01L 21/02458 20130101;
C30B 25/18 20130101; H01L 21/02433 20130101; H01L 21/0242 20130101;
H01L 21/0254 20130101; H01L 21/02639 20130101; H01L 33/0075
20130101; H01S 2304/12 20130101; B82Y 20/00 20130101; C30B 25/02
20130101; H01L 21/02381 20130101; H01S 5/34333 20130101; C30B
29/403 20130101; H01L 33/007 20130101; C30B 29/406 20130101 |
Class at
Publication: |
438/763 ;
438/791; 438/767; 257/82 |
International
Class: |
H01L 027/15; H01L
031/12; H01L 021/20; C30B 001/00; H01L 033/00; H01L 021/36; H01L
021/31; H01L 021/469 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 24, 1998 |
JP |
10-176761 |
Claims
What is claimed is:
1. A method for forming a nitride compound semiconductor film, the
method comprising the steps of: providing a substrate having a
portion which acts as a growth suppressing film on a outermost
surface thereof, forming a growth promoting film partially on the
substrate; and forming a nitride compound semiconductor on the
growth promoting film.
2. A method for forming a nitride compound semiconductor film
according to claim 1, wherein the growth suppressing film is an
amorphous film.
3. A method for forming a nitride compound semiconductor film
according to claim 1, wherein the substrate is a crystal substrate
of a cubic crystal structure having a surface along a (110)
orientation or a (110) orientation.
4. A method for forming a nitride compound semiconductor film
according to claim 1, wherein the growth promoting film is Zno or
In.sub.sGa.sub.wAl.sub.1-s-wN(0.ltoreq.s.ltoreq.1,
0.ltoreq.w.ltoreq.1, and 0.ltoreq.s+w.ltoreq.1).
5. A method for forming a nitride compound semiconductor film
according to claim 1, wherein a thickness of the growth promoting
film is equal or greater than about 0.2 .mu.m.
6. A method for forming a nitride compound semiconductor film
according to claim 1, wherein the growth promoting film is in a
form of a plurality of stripes separated from one another by an
interval of about 20 .mu.m or less.
7. A nitride compound semiconductor light-emitting diode,
comprising: a substrate having a portion which acts as a growth
suppressing film on a top surface thereof; a growth promoting film
formed partially on the substrate; and a nitride compound
semiconductor layered structure including a light-emitting layer
formed on the substrate, the light-emitting layer having a
light-emitting region into which an electric current injected,
wherein the light-emitting region is formed above a region of the
substrate where no growth promoting film is formed.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a layered structure in
which a nitride compound semiconductor film of an excellent quality
can be obtained, and a method for forming the same.
[0003] 2. Description of the Related Art
[0004] Conventionally, a nitride compound semiconductor has been
employed and studied as a light-emitting diode and a device with a
high-temperature resistance. By adjusting its composition, a
nitride compound semiconductor can be employed as a light-emitting
diode for a wide range of short wavelengths from blue to
orange.
[0005] It has been known in the art that there is a need to reduce
threading dislocation and cracks in a crystal for realizing a
nitride compound semiconductor with excellent characteristics and a
high reliability. As a conventional method for reducing the
threading dislocation and cracks, the following method has been
suggested (Jpn. J. Appl. Phys., Vol.36 (1997), pp. L899-L902).
First, a thin film of GaN is epitaxially grown on a substrate by a
metal organic chemical vapor deposition (MOCVD). After a striped
selective growth mask of SiO.sub.2 is formed on the substrate as a
growth suppressing member, GaN is epitaxially grown on the wafer.
According to this method, a flat epitaxial film is formed due to
lateral crystal growth which occurs over the selective growth
mask.
[0006] FIGS. 7A to 7D illustrate the steps of the above-mentioned
method. A thin GaN film 702 with a thickness of about 0.5 .mu.m to
about 2 .mu.m is deposited on a sapphire substrate 701 via a
low-temperature buffer layer (not shown) of a GaAlN type material
(FIG. 7A). Next, a SiO.sub.2 film is formed on the thin GaN film
702, and patterned by a common photolithography technique so as to
provide a striped SiO.sub.2 selective growth mask 703 (width: about
5 .mu.m, pitch: about 7 .mu.m) (FIG. 7B).
[0007] Thereafter, a thick GaN film 704 with a thickness in the
range of about 10 .mu.m to about 300 .mu.m is deposited by a
hydride vapor phase epitaxy (HVPE) or MOCVD method. At the initial
growth stage of the thick GaN film 704, the GaN crystal grows only
in a window area 704 of the thin GaN film 702 on which no selective
growth mask 703 is formed, as shown in FIG. 7C. At this stage,
crystal deposition is locally suppressed by the selective growth
mask 703, thereby selectively growing the crystal as shown in FIG.
7C. However, as the GaN crystal continues to grow, the GaN crystal
on the window region 704 starts to laterally extend over the
selective growth mask 703 (in this specification, this will be
referred to as "lateral growth"). As a result, GaN crystals growing
from adjacent window regions 704 attach to each other, thereby
forming a thick GaN film 705 exhibiting a single layer structure
(FIG. 7D).
[0008] Curve 201 in FIG. 2 illustrates density of threading
dislocation on a wafer surface for various thicknesses (about 10
.mu.m, about 50 .mu.m, about 100 .mu.m, and about 300 .mu.m) of the
thick GaN film 705 obtained by the conventional method. Threading
dislocations are evenly distributed in all regions of the thick GaN
film 705. The density is reduced as the thickness increases, and
with the thick GaN film 705 having a thickness of about 100 .mu.m
or more, the density of threading dislocations is reduced to about
5.times.10.sup.7 cm .sup.-2. No crack is observed in the thick GaN
film 705 irrespective of the thickness thereof.
[0009] However, the conventional method and the thick GaN film 705
obtained by the conventional method have the following
problems.
[0010] (1) It is impossible to reduce the density of threading
dislocation on the surface of the thick GaN film 705 to a density
on the order of 10.sup.6cm.sup.-2 or less as needed for a device
used with a large current density, such as a light-emitting diode
and particularly a semiconductor laser device. Thus, if a GaN-type
semiconductor laser device is formed on the wafer as shown in FIG.
7D, the operating life of the device will be as short as about 400
hours (under conditions of 60.degree. C. and 5mW), failing to
realize the commercially required operating life of about 5000
hours.
[0011] (2) The manufacturing method is tedious because it is
necessary to perform two epitaxial growth steps for the thin GaN
film 702 and for the thick GaN film 705.
SUMMARY OF THE INVENTION
[0012] According to one aspect of this invention, a method for
forming a nitride compound semiconductor film includes the steps
of: (1)providing a substrate having a portion which acts as a
growth suppressing film on a outermost surface thereof; (2)forming
a growth promoting film partially on the substrate; and (3) forming
a nitride compound semiconductor on the growth promoting film.
[0013] In one embodiment of the invention, the growth suppressing
film is an amorphous film.
[0014] In another embodiment of the invention, the substrate is a
crystal substrate of a cubic crystal structure having a surface
along a (110) orientation or a (110) orientation.
[0015] In still another embodiment of the invention, the growth
promoting film is ZnO or
In.sub.sGa.sub.wAl.sub.1-s-wN(0.ltoreq.s.ltoreq.1,
0.ltoreq.w.ltoreq.1, and 0.ltoreq.s+w.ltoreq.1).
[0016] In yet another embodiment of the invention, a thickness of
the growth promoting film is equal or greater than about 0.2
.mu.m.
[0017] In another embodiment of the invention, the growth promoting
film is in a form of a plurality of stripes separated from one
another by an interval of about 20 .mu.m or less.
[0018] According to another aspect of the present invention, a
nitride compound semiconductor light-emitting diode includes: a
substrate having a portion which acts as a growth suppressing film
on a top surface thereof; a growth promoting film formed partially
on the substrate; and a nitride compound semiconductor layered
structure including a light-emitting layer formed on the substrate,
the light-emitting layer having a light-emitting region into which
an electric current injected. The light-emitting region is formed
above a region of the substrate where no growth promoting film is
formed.
[0019] Thus, the invention described herein makes possible the
advantages of (1) providing a convenient method for forming a
GaN-type semiconductor layer with a low density of threading
dislocations, and (2) improving the operating life of a
semiconductor laser device.
[0020] These and other advantages of the present invention will
become apparent to those skilled in the art upon reading and
understanding the following detailed description with reference to
the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIGS. 1A to 1D illustrate a method for forming a nitride
compound semiconductor film according to Example 1 of the present
invention.
[0022] FIG. 2 is a graph showing a thickness dependency of the
threading dislocation density of the thick GaN film according to
Example 1 of the present invention.
[0023] FIG. 3 is a graph illustrating changes in surface
irregularity of a thick GaN film with respect to an etching width
of a growth promoting film.
[0024] FIG. 4 is a graph illustrating changes in surface
irregularity of a thick GaN film with respect to a stripe width of
a growth promoting film.
[0025] FIG. 5 illustrates a structure of a thick GaN film when a Si
substrate having a surface along a (111) orientation is used
according to a comparative example.
[0026] FIG. 6 is a cross-sectional view illustrating a structure of
a semiconductor laser device according to Example 6 of the present
invention.
[0027] FIGS. 7A to 7D show one conventional method for forming a
thick nitride compound semiconductor film.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] First, the basic concept of the present invention will be
described.
[0029] In the present invention, a growth promoting film is
partially formed on a substrate having a portion which acts as a
growth suppressing film on a surface thereof, and a nitride
compound semiconductor film of a single crystal is grown thereon.
As a method for growing the crystal, any commonly-used epitaxial
growth method can be used, such as metal organic chemical vapor
deposition (MOCVD), hydride vapor phase epitaxy(HVPE), molecule
beam epitaxial (MBE), and the like.
[0030] The term "growth suppressing film" as used herein refers to
a film on which the initial epitaxial growth of a nitride compound
semiconductor occurs with difficulty, in other words, a film on
which a nuclear generation is hard to initiate. The term "growth
promoting film" as used herein refers to a compound film on which
the initial growth of a nitride compound is facilitated, in other
words, a compound film on which nuclear generation easily
occurs.
[0031] The growth suppressing film for a nitride compound
semiconductor includes an amorphous film or a crystalline film of a
cubic crystal structure having a (100) orientation or a (110)
orientation. The crystal film of a cubic crystal structure having a
(100) orientation or (110) orientation may be GaP, .beta.-SiC,
GaAs, or MgAl.sub.2O.sub.4. The amorphous film may be a glass, an
amorphous Si, SiO.sub.x (x=1-2), or Si.sub.2N.sub.y (y=1-3). The
growth promoting film for the nitride compound semiconductor may be
ZnO, GaN, AlN, InN or a mixture thereof
(In.sub.sGa.sub.wAl.sub.1-s-wN: 0.ltoreq.s.ltoreq.1,
0.ltoreq.w.ltoreq.1, 0.ltoreq.s+w.ltoreq.1) .
[0032] According to the present invention, a nitride compound
semiconductor is grown on a wafer including a growth suppressing
film and a convex growth promoting film in the form of stripes each
having a width of about 100 .mu.m or less. Accordingly, a nitride
compound semiconductor crystal is selectively grown on the growth
promoting film. The nitride compound semiconductor crystal grows
not only on the top surface of the growth promoting film, but also
on the side surface thereof. Therefore, immediately after the
growth starts, the nitride compound semiconductor crystal grows in
a lateral direction (i.e., a direction parallel to the wafer
surface) at a certain growth rate, whereby the lateral growth rate
is substantially enhanced in comparison to that of the conventional
method.
[0033] Accordingly, the nitride compound semiconductor crystals
growing from adjacent stripes of the growth promoting film attach
to each other into a single nitride compound semiconductor layer in
a shorter growth time as compared with that of the conventional
method (thus, a single nitride compound semiconductor layer having
a smaller thickness than that achieved by the conventional method
can be obtained). Because of the substantially enhanced lateral
growth rate, it is possible to obtain a region having threading
dislocation density on the order of about 10.sup.6cm.sup.-2 or less
in the single nitride compound semiconductor layer where the
crystal is grown in the lateral direction. Moreover, since the
width of the growth promoting film is about 100 .mu.m or less, even
if the wafer temperature is lowered to room temperature after
growing the nitride compound semiconductor crystal at a high
temperature near 1000.degree. C., heat distortion of the nitride
compound semiconductor layer is small, thereby suppressing cracks
therein.
[0034] When a semiconductor laser device is formed on the region of
the nitride compound semiconductor layer with reduced threading
dislocation (i.e., "the lateral growth region" or a region above
the growth suppressing film), it is possible to realize a device
which exhibits a operating life of about 5000 hours or more in a
reliability test under the conditions of 60.degree. C. and 5mW.
[0035] FIGS. 1A to 1D illustrate one example of a method for
forming a nitride compound semiconductor according to the present
invention. First, selected regions of a substrate 101 which
functions as a growth suppressing film for a nitride compound
semiconductor are covered with a growth promoting film 102 for the
nitride compound semiconductor (FIG. 1A). The selective covering
process may be achieved by first covering the entire substrate
surface with the growth promoting film 102 by chemical vapor
deposition, and the like, and then partially etching the film 102
by a simple photolithography method. Alternatively, the selective
covering process may be achieved by depositing or sputtering the
growth promoting film 102 using a patterned mask.
[0036] Next, the substrate 101 partially covered with the growth
promoting film 102 is introduced into an apparatus for growing a
nitride compound semiconductor, and the crystal is grown under
normal conditions. The method for growing the nitride compound
semiconductor may be any nitride compound semiconductor growth
method known in the art, such as MOCVD, MBE, HVPE, and the like.
When a crystal is grown on the substrate 101 of the present
invention, substantially no nitride compound semiconductor grows on
a portion 103 of the substrate 101 where no growth promoting film
102 is formed, i.e., a portion where the substrate 101 made of the
growth suppressing film is exposed, while the nitride compound
semiconductor 104 starts to epitaxially grow on the growth
promoting film 102. The nitride compound semiconductor 104 grows
not only on the top surface of the growth promoting film 102, but
also on a side surface thereof. As a result, the nitride compound
semiconductor crystal starts to extend over the exposed portion of
the growth promoting film 101 (FIG. 1B).
[0037] As the crystal continues to grow from a state shown in FIG.
1B, the nitride compound semiconductor crystal further grows in the
thickness direction as well as in the lateral direction. As a
result, the selectively grown nitride compound semiconductor
crystals attach to each other (FIG. 1C). Thereafter, the
semiconductor crystal only grows in the vertical direction, thereby
forming a nitride compound semiconductor layer 106 having a flat
surface. The threading dislocation density of the resultant nitride
compound semiconductor layer 106 is smaller than that of the
conventionally obtained layer by an order of magnitude or more.
[0038] As described above, by applying the technique of the present
invention, threading dislocations can be reduced from that achieved
by the conventional method. The reason for this is believed to be
as follows. In the conventional method, the growth promoting
region, on which a GaN film is selectively grown, is exposed
between adjacent stripes of the growth suppressing film. Therefore,
at the initial stage, the nitride compound semiconductor crystal
grows only between the adjacent stripes of the growth suppressing
film, and the side surfaces of the growing nitride compound
semiconductor layer are covered with the growth suppressing film.
It is believed that a high density of crystal dislocation occurs at
an interface between the growing nitride compound semiconductor
film and the growth suppressing film, and extends in the nitride
compound semiconductor crystal along the crystal growth direction
(FIG. 7C). After the thickness of the grown crystal exceeds the
thickness of the growth suppressing film, the crystal starts to
grow laterally. Then, the threading dislocation generated from the
interface between the side surface of the growth suppressing film
and the nitride compound semiconductor crystal extends, while also
spreading laterally, across the nitride compound semiconductor
crystal, finally reaching the surface of the nitride compound
semiconductor layer. As a result, a threading dislocation density
as high as about 5.times.10.sup.7/cm.sup.2 will exist even in
regions without the growth suppressing film. In addition,
substantially the same threading dislocation density results in
portions of the nitride compound semiconductor crystal which are
grown laterally over the growth suppressing film. The threading
dislocation continues to extend toward the crystal surface even
after the laterally grown portions of the nitride compound
semiconductor attach to each other into a single film (FIG. 7D).
Therefore, a threading dislocation density as high as
5.times.10.sup.7/cm.sup.2 results on the surface of the nitride
compound semiconductor layer formed according to the conventional
method.
[0039] On the other hand, according to the present invention, the
lateral growth of the crystal starts immediately after the nitride
compound semiconductor layer starts to grow, and there is no growth
suppressing film beside the growing nitride compound semiconductor
crystal. This is believed to be the reason for the fact that a high
threading dislocation density as that of the conventional method
occurrs at the interface between side surfaces of the growth
suppressing film and the growing nitride compound semiconductor
crystal.
[0040] Hereinafter, specific examples of the present invention will
be described.
EXAMPLE 1
[0041] Referring to FIGS. 1A to 1D, a nitride compound
semiconductor film according to Example 1 of the present invention
will be described. In the present example, zinc oxide (ZnO) as a
compound is sputtered on a glass substrate, and a nitride compound
semiconductor film is grown by MOCVD.
[0042] A glass substrate 101 having a diameter of about 2 inches is
subjected to organic cleaning and etching by fluorine, and washed
in deionized water for several minutes. Then, a ZnO film 102 is
sputterred on the glass substrate 101. The ZnO film 102 may be
provided by planar magnetron sputtering in a mixed gas (50% argon
and 50% oxygen), under a pressure of about 0.05 Torr, with the
power supplied from a high frequency power source being adjusted to
about 200 W. The substrate 101 is heated to about 300.degree. C.
with a heater of a resistance heating type. The ZnO film 102 grown
for about 30 minutes under the above conditions has a thickness of
about 1 .mu.m. The ZnO film 102 is partially etched by
photolithography into a striped pattern with a width of about 4
.mu.m and an interval of about 8 .mu.m between the center of a
stripe to the center of the next stripe. HCl diluted to about 10%
is used as an etchant.
[0043] The glass substrate 101 having the striped ZnO film 102
which is formed by the method above is introduced into an MOCVD
apparatus and a GaN film is formed therein. The film forming
temperature of the MOCVD film may be about 1000.degree. C. About 5
l/min of NH.sub.3 as a V group material gas, about 100 .mu.mol/min
of trimethylgallium (TMG) as a III group material gas, and about 20
l/min of H.sub.2 as a carrier gas are introduced into the
apparatus. Under the above-described conditions, after about 1
hour, the grown GaN film has a thickness of about 6 .mu.m. A length
m of the GaN film 104 projecting from the sides of the growth
promoting layer 102 is about 1.5 .mu.m (FIG. 1B). As the crystal
continues to grow from a state shown in FIG. 1B, the nitride
compound semiconductor crystal further grows in the thickness
direction as well as in the lateral direction. As a result, the
selectively grown nitride compound semiconductor crystals attach to
each other (FIG. 1C). Thereafter, the semiconductor crystal only
grows in the vertical direction, thereby forming a nitride compound
semiconductor layer 106 having a flat surface.
[0044] The threading dislocation density of GaN film 104 on the
growth promoting layer 102 grown as described above is about
5.times.10.sup.9/cm.sup.2, which is lower than that of the
conventional method. Moreover, the threading dislocation density of
the laterally grown portion of the GaN film 104 is only about
3.times.10.sup.3/cm.sup.2- , which is lower than that of the
conventional method by about three orders of magnitude.
[0045] Curves 202 and 203 in FIG. 2 show threading dislocation
densities of GaN films of various thicknesses which are grown
according to the method of the present invention over various
growth times. Curves 202 and 203 correspond to a portion of a GaN
film over the growth promoting film, and another portion of the GaN
film over the growth suppressing film, respectively. Over the ZnO
film as a growth promoting film, the threading dislocation density
is smaller than that of the conventional method by about an order
of magnitude. When the GaN film has a thickness of about 100 .mu.m
or more, the threading dislocation density is reduced to be as
small as 4.times.10.sup.6/cm.sup.2. In a region with no ZnO film,
that is, in a region over the glass substrate made of a growth
suppressing film material, a dramatically small threading
dislocation density of about 3-10.times.10.sup.3/cm.sup.2 is
realized across the thickness of a continuous film (about 3 .mu.m
in this case). As described above, the threading dislocation
density is reduced in all regions in comparison to that of the
conventional method. In particular, in a region with no ZnO film,
as the growth promoting film (i.e., in a region where the substrate
as the growth suppressing film is exposed), a significant reduction
in the threading dislocation density is realized.
[0046] In Example 1, a glass substrate is used as an amorphous
substrate, but other substrates can be used, alternatively, such as
a wafer in which amorphous Si is deposited to a thickness of about
1 .mu.m on a Si substrate, or a wafer in which a SiO.sub.x film
(x=1-2) or a Si.sub.2N.sub.y film (y=2-3) with a thickness of about
0.2-2 .mu.m is formed on a Si substrate. The selective growth of
the nitride compound semiconductor from the ZnO film portion, and
the effects associated therewith have been confirmed also with
these alternative substrates. In such cases, the amorphous Si film,
the SiO.sub.x film, and the Si.sub.2N.sub.y film, function as the
growth suppressing film for the nitride compound semiconductor.
EXAMPLE 2
[0047] Hereinafter, Example 2 of the present invention will be
described where HVPE is used as a method for growing a GaN film,
instead of the MOCVD used in Example 1.
[0048] A ZnO film is formed and a substrate is etched in a manner
similar to that of Example 1, and the ZnO film is introduced into a
HVPE apparatus to form a GaN film thereon. The HVPE film is formed
at a temperature of about 1000.degree. C., and with the following
gases being introduced into the apparatus: about 1000 cc/min of
NH.sub.3 as a material gas of V group, about 20 cc/ min of HCl
which is passed along a molten Ga metal to supply a III-group
material, and about 3000 cc/ min of H.sub.2 as a carrier gas. The
temperature of the molten Ga metal is about 800.degree. C. The
growth rate of a GaN film under the conditions above is 100 .mu.m/h
in the direction perpendicular to the substrate.
[0049] The relationship between the thickness of the GaN film
formed and the threading dislocation density on the surface of the
GaN film has been examined for various growth times while
maintaining the above-mentioned conditions. In a region over the
ZnO film, when a thickness of the film is about 6 .mu.m, the
threading dislocation density is about 1.times.10.sup.9/cm.sup.2.
When the thickness is increased to about 10 .mu.m, the threading
dislocation density is reduced to about 1.times.10.sup.8/cm.sup.2.
When the thickness is further increased to about 20 .mu.m, the
threading dislocation density is reduced to about
3.times.10.sup.7/cm.sup.2. As the growth further continues for
about 3 more hours, the GaN thick layer has a thickness of 300
.mu.m, and the threading dislocation density on the surface of the
thick film is about 8.times.10.sup.6/cm.sup.2. The threading
dislocation density characteristic is substantially the same as
that of curve 202 in FIG. 2, and the threading dislocation density
of the GaN thick layer of the present example is smaller than that
of the conventional method by an order of magnitude. Moreover, in a
region with no ZnO film, i.e., in a region where the glass
substrate is exposed, the threading dislocation density of the GaN
film surface is substantially the same as that of curve 203
obtained in Example 1. It has thus been confirmed that HVPE as a
method for growing the GaN film provides substantially the same
effect as that provided by MOVCD.
[0050] No stripes and no irregularity which may possibly occur due
to the stripe pattern of the ZnO film are observed on the surface
of the thick GaN film provided according to Example 2 of the
present invention, thereby providing a single, continuous, and flat
film. In addition, a microscopic measurement has shown no cracks or
pits on the film. As in Example 1, the cause of the reduction in
threading dislocation density is believed to be the lateral growth
of the GaN film from the sides of ZnO film (a growth promoting
film) being promoted from the beginning of the growth process.
EXAMPLE 3
[0051] In Example 3, results obtained by changing the interval
between adjacent stripes of the growth promoting film will be
described. In a manner similar to that of Example 1, a ZnO film is
sputtered over the glass substrate and then is etched by
photolithography.
[0052] Samples are produced with the stripe width of the ZnO film
(i.e., the width of the ZnO film which is not etched away but
remains after the etching process) being fixed to about 5 .mu.m,
and with the stripe interval (i.e., the width of the ZnO film which
is etched away between adjacent remaining stripes) being varied
from about 1 .mu.m, to about 2 .mu.m, about 5 .mu.m, about 10
.mu.m, about 15 .mu.m, about 20 .mu.m, about 25 .mu.m, and about 30
.mu.m. The substrate is subjected to HVPE for 3 hours to grow a GaN
film in a manner similar to that of Example 2. Herein, the
thickness of the GaN film is about 300 .mu.m irrespective of the
other conditions.
[0053] The GaN film of all the above stripe intervals shows a
constant threading dislocation density of about
7-10.times.10.sup.6/cm.sup.2 on a surface of the GaN film over the
ZnO film region, and no crack is observed in any case. Thus, it is
confirmed that for all the above stripe intervals, the threading
dislocation density of the GaN film can be reduced from that of the
conventional method by about an order of magnitude. A measurement
of the threading dislocation density on the GaN film surface in a
region where the glass substrate is exposed has shown that a wafer
having a stripe interval of about 10 .mu.m to 20 .mu.m has a very
small threading dislocation density as that represented by curve
203, but wafers having stripe intervals of about 25 .mu.m and about
30 .mu.m, respectively, have a region with a high, localized
threading dislocation density of about 10.sup.6/cm.sup.2
substantially along the center line of the etching width/cm. This
is believed to occur as follows. When the etching width exceeds
about 20 .mu.m, the crystal axes of adjacent GaN crystals growing
laterally toward each other are shifted by about 1.degree. from
each other, thereby resulting in the high threading dislocation
density substantially along the center line of the etching width
where the adjacent GaN crystals attach to each other.
[0054] As a result of evaluating the regularity (i.e., flatness) of
the GaN film surface, those films having a stripe width over about
20 .mu.m have considerable irregularity along the center line of
the etching width, because the adjacent laterally-growing crystals
do not sufficiently attach to each other. FIG. 3 shows a
relationship between an etching width and a surface regularity. In
FIG. 3, the vertical axis represents the surface irregularity
amplitude of various wafers normalized based on the surface
irregularity amplitude of a GaN film having a thickness of about
300 .mu.m which is obtained when the etching width of the ZnO film
is about 10 .mu.m. It has been shown that a substantially constant
irregularity amplitude is obtained for etching widths of up to
about 20 .mu.m, but the irregularity amplitude is increased by
about 4-6 fold when the etching width is about 25 .mu.m or about 30
.mu.m, thereby deteriorating the flatness of the surface.
Therefore, when producing a light-emitting diode, it is important
to set the etching width to about 20 .mu.m or less.
[0055] In a manner similar to that of Example 2 of the present
invention, various GaN films have been grown while varying the
stripe width of the ZnO film from about 1 .mu.m, to about 2 .mu.m,
about 5 .mu.m, about 10 .mu.m, about 15 .mu.m, about 20 .mu.m,
about 25 .mu.m, and about 30 .mu.m and keeping etching width
constant at about 5 .mu.m. Each GaN film shows defect density
characteristics similar to those shown in curve 202 and 203 (FIG.
2). FIG. 4 shows the irregularity amplitude on the surface of each
of the GaN films which is normalized based on the irregularity
amplitude of a wafer having a stripe width of about 10 .mu.m. As
can be seen from FIG. 4, the wafers of the various stripe widths
have no significant difference in their surface irregularity.
EXAMPLE 4
[0056] In Example 4 of the present invention, a material for the
growth promoting film is examined. A thick GaN film is grown by
HVPE in a manner similar to that of Example 2, but GaN, AlN, InN,
and a mixed crystal thereof grown at a low temperature are used as
the growth promoting film instead of ZnO used in Example 1.
[0057] First, a case where GaN is used for the growth promoting
film will be described. A thin GaN film having a thickness of about
0.5 .mu.m is grown on a glass substrate by MOCVD. The growth
temperature is about 600.degree. C., NH.sub.3 and TMG are used as
material gases, and H.sub.2 is used as a carrier gas. The thin GaN
film formed in such a manner is subjected to a normal
photolithography process and an etching process with hot nitric
acid (about 200.degree. C.), thereby partially removing a region of
the thin GaN film with a width of about 10 .mu.m, and leaving
stripes of the thin GaN film each having a width of about 10 .mu.m
at a pitch of about 20 .mu.m on the glass substrate. Thereafter, a
thick GaN film having a thickness of about 20 .mu.m is grown on the
wafer formed as described above by HVPE in a manner similar to that
of Example 2 of the present invention.
[0058] The threading dislocation density of the GaN thick layer is
about 5.times.10.sup.7/cm.sup.2 in regions over the above-described
thin GaN film in the form of stripes remaining on the glass
substrate. Thus, it is possible to obtain the thick GaN film having
substantially the same threading dislocation density as that
obtained in the case where the ZnO film is used as the growth
promoting film. A dislocation density characteristic substantially
the same as that in the case of using ZnO (shown by curve 202 in
FIG. 2) has been confirmed also when the thickness of the thick GaN
film is varied. Also, the threading dislocation density of the
thick GaN film surface in a region where the glass substrate is
exposed is substantially the same as that obtained when ZnO is used
as the growth promoting film (shown in curve 203).
[0059] When AlN, InN, or a mixed crystal of InGaAlN are used as a
compound film, instead of the GaN film, the threading dislocation
density on the thick GaN film surface with a thickness of about 20
.mu.m is about 3-8.times.10.sup.7/cm.sup.2, thereby confirming an
effect of reducing the threading dislocation density which is
substantially the same as that obtained when the GaN film is
used.
[0060] In the present example, a glass substrate is used as an
amorphous substrate, but any other substrate is applicable as long
as it has a high heat resistance such that the substrate can
withstand the GaN growth process. It is especially effective to
provide a certain crystalline seed in a substrate so that the
crystal orientation of the compound film to be formed on the
substrate is aligned in a certain direction, or to employ a
graphoepitaxial method so as to scratch the substrate in a certain
direction. The glass substrate used in the present example has
shallow grooves (depth: about 10 nm to about 300 nm) etched on the
surface thereof in a stripe pattern having an interval of about 0.5
.mu.m.
[0061] As the amorphous substrate, any of the following substrates
may be used instead of the glass substrate as described above: a
substrate having a SiO.sub.x (x=1-2) film having a thickness in the
range of about 0.5 .mu.m to about 2 .mu.m deposited by a thermal
oxidation method or a CVD method on any of a Si wafer, a sapphire
wafer, and a GaAs wafer; a substrate having a SiN.sub.y (y=1-3)
film having a thickness in the range of about 0.5 .mu.m to about 2
.mu.m deposited by a plasma CVD method on any of the
above-mentioned wafers; and a substrate having an amorphous Si film
having a thickness in the range of about 0.5 .mu.m to about 5 .mu.m
deposited on any of the above-mentioned wafers. It has been
confirmed that each of the above-listed substrates functions as a
growth suppressing film as the glass substrate, and provides an
effect of reducing the threading dislocation density which is
substantially the same as those shown by curves 202 and 203 (FIG.
2).
[0062] Moreover, GaN films have been formed according to the steps
of Example 1 on ZnO growth promoting films of various thicknesses
of about 0.1 .mu.m, about 0.2 .mu.m, about 2 .mu.m, about 3 .mu.m,
and about 5 .mu.m, rather than about 1 .mu.m as in Examples 1 to 4.
It has been confirmed that a reduced threading dislocation density
on the surface of the thick GaN film as those shown by curves 202
and 203 can be obtained when the thickness of the growth promoting
film is about 0.2 .mu.m or more. However, in the case where the
growth promoting film has a thickness of about 0.1 .mu.m, the
threading dislocation density is similar to or higher than that
shown by 201 of the GaN film obtained by the conventional method.
This can be understood from the fact that the lateral growth of the
crystal from the sides of the growth promoting film has a
significant influence on the effect of reducing the threading
dislocation density of the present invention, and that the area of
the side of the growth promoting film (ZnO film) is proportional to
the thickness thereof. Accordingly, it is understood that the
thickness of the growth promoting film of the present invention
should be equal to or greater than about 0.2 .mu.m. Moreover, as
the thickness of the growth promoting film is increased, the
threading dislocation density can be reduced more, but it will then
become necessary to increase the GaN crystal growth time in order
to obtain a flat thick GaN film. Accordingly, it is preferred to
set the thickness of the growth promoting film to about 3 .mu.m or
less.
EXAMPLE 5
[0063] In Example 5, a case where a Si substrate is used as a
non-amorphous substrate (crystalline substrate) which functions as
a growth suppressing film will be described. A Si substrate is
washed for a few minutes with a mixed solution of sulfuric acid and
hydrogen peroxide. Then the Si substrate is subjected to a soft
etching process with diluted hydrogen fluoride, and a striped ZnO
film is formed as in the above-described examples. Then, a GaN film
is grown thereon. The GaN film is grown on several types of
substrates having particular orientations, respectively. For any of
the substrates, the GaN film can be easily grown on the striped ZnO
film.
[0064] When the Si substrate having a surface along a (100) or
(110) orientation, the threading dislocation density on the surface
of the GaN film in a region directly above the ZnO film is
substantially the same as that shown by curve 202. In any region
where the Si substrate is exposed (i.e., where the ZnO film is
etched away), the threading dislocation density is about
1.times.10.sup.4/cm.sup.2, which is lower than that of the
conventional method, as in Examples 1 to 4 which use an amorphous
substrate.
[0065] In the present example, the Si substrate having a surface
along a (100) or (110) orientation is used as the growth
suppressing film. However, also when a substrate having a cubic
crystal structure, such as GaP, GaAs, or the like, having a surface
along a (100) or (110) orientation is used, a function as the
growth suppressing film for a GaN film can be obtained, and a good
GaN film can be obtained which has a threading dislocation density
smaller than that of the conventional method by an order of
magnitude or more.
COMPARATIVE EXAMPLE 1
[0066] In this comparative example, a thick GaN film is grown in a
manner similar to Example 5 by using a Si substrate having a
surface along a (111) orientation. As shown in FIG. 5, the thick
GaN film having a single flat surface cannot be obtained because a
GaN crystal is deposited in a region where the (111) oriented Si
substrate is exposed at the same time when the lateral growth of a
crystal starts to occur (at the beginning of the growth process)
from the sides of the growth promoting film. Reference numeral 501
denotes a Si substrate having a surface along a (111) orientation;
502 denotes a ZnO film as a growth promoting film; 503 denotes a
GaN crystal selectively growing upwardly from the ZnO film 502; and
504 denotes multi-crystalline GaN growing directly from the (111)
oriented Si substrate 501.
[0067] As illustrated in FIG. 5, the Si substrate 501 of a (111)
orientation does not function as the growth suppressing film, and
thus a single crystal thick GaN film cannot be obtained. Moreover,
because of the multi-crystalline GaN 504, the lateral growth from
the sides of the growth promoting film is inhibited. As a result,
the threading dislocation density in the vicinity of the surface of
the GaN film formed partially on the growth promoting film 502 is
very high (on the order of about 10.sup.9/cm.sup.2) irrespective of
the thickness of the GaN film. Thus, a GaN film of an excellent
quality cannot be obtained.
[0068] Example 5 and Comparative Example 1 will be compared with
each other in terms of the dependency of the threading dislocation
density of the thick GaN film in a region directly above the growth
promoting film on the surface orientation of the substrate used as
a growth suppressing film, for a GaN film thickness of about 100
.mu.m. When a Si substrate having a surface orientation of (100) or
(110) is used, the threading dislocation density is about
4.times.10.sup.6/cm.sup.2. On the other hand, when a Si substrate
of a (111) surface orientation is used, the threading dislocation
density is about 3.times.10.sup.9/cm.sup.2. As is evident from the
above, when the Si substrate is used, it is important to select a
surface orientation of (100) or (110) in order to reduce the
threading dislocation density of the thick GaN film.
[0069] When a GaN thick layer is formed by the steps as those of
Example 5 on a GaAs substrate or a GaP substrate having a (111)
orientation, the formation will be as illustrated in FIG. 5, and a
continuous, single GaN film cannot be obtained. It is thus shown
that it is preferred to use a substrate having a surface along a
(100) orientation or a (110) orientation when a crystalline
substrate is used as a growth suppressing film.
[0070] Although formation of a GaN film as a nitride compound
semiconductor film has been described in the examples above, the
present invention is also applicable to formation of a
semiconductor film other than a GaN film, e.g., a film of a nitride
containing aluminum or indium as a III group element, and a film of
a compound in which the nitride element is partially substituted
with another V group element (e.g., P, As, and the like).
EXAMPLE 6
[0071] A method for manufacturing a light-emitting diode according
to the present invention will now be described. A light-emitting
diode in the present example is a semiconductor laser device. FIG.
6 shows a cross-sectional view thereof.
[0072] The wafer used in this example is provided with a silicon
oxide (SiO.sub.x) film 602 having a thickness of about 2 .mu.m
formed by thermal oxidation on a Si substrate 601. Deposited on the
wafer are: a striped AlN growth promoting film 603 (width: about 3
.mu.m, thickness: about 0.8 .mu.m); an n-type thick GaN film 604
(thickness: about 15 .mu.m); an n-type Al.sub.0.08Ga.sub.0.92N
cladding layer 605; an n-type GaN guide layer 606; a multiple
quantum well active layer 607 including two In.sub.0.15Ga.sub.0.85N
well layers each having a thickness of about 2 nm and three
In.sub.0.05Ga.sub.0.95N barrier layers each having a thickness of
about 3 nm; a p-type GaN guide layer 608; a p-type
Al.sub.0.08Ga.sub.0.92N cladding layer 609 having a mesa stripe
structure having width of about 2 .mu.m; a p-type GaN contact layer
610; an n-type Al.sub.0.1Ga.sub.0.9N current constriction layer
611; a p-type buried layer 612; an n-side electrode 621; and a
p-side electrode 622. Crystal growth of these layers can be
achieved by MOCVD.
[0073] In the semiconductor laser device of this example, a current
is injected into, and light is emitted from, a portion of the
active layer 607 located below the mesa stripe 620. The light
emitting portion is arranged above a region where the SiO.sub.x
film 602 is exposed after the growth promoting film 603 is
partially removed. This is because the threading dislocation
density of the GaN thick layer 604 is lower and the crystallinity
is better in a region with no growth promoting film 603, as can be
seen from a comparison between curves 202 and 203 in FIG. 2. A
reliability evaluation of the device according to the present
example has shown a threshold current of about 35 mA and a
differential emission efficiency of about 0.7 W/A, indicating a
high efficiency emission characteristic at a low current. A good
reliability has been confirmed, lasting for about 5000 hours or
more, under conditions of an atmospheric temperature of about
60.degree. C. and an optical output of about 5 mW.
[0074] A measurement of the characteristics of the device obtained
in the case where the mesa stripe 620 is formed above the AlN
growth promoting film 602 has shown a threshold voltage of about 57
mA, and a differential emission efficiency of about 0.5 W/A,
indicating an increase in the threshold current and a decrease in
the emission efficiency. However, a test under the conditions as
described above has shown that the operating life of the device is
about 3500 hours, which is still significantly better than that
obtained by the conventional method (see Comparative Example 2
below). Such an improvement in the operating life of the device is
obtained by the effect of reducing the threading dislocation
density which is provided by the present invention (as can be seen
from a comparison between curves 201 and 202 in FIG. 2).
COMPARATIVE EXAMPLE 2
[0075] As a comparison with Example 6, a device has been produced
by providing a structure corresponding to a part of the structure
of Example 6 including the n-type AlGaN cladding layer 605 and
other layers thereabove, on the GaN thick layer 705 which is
produced by the conventional method described above with reference
to FIGS. 7A-7D. A measurement of the characteristics of the device
in this comparative example has shown a threshold current equal to
or greater than about 80 mA, and a differential emission efficiency
less than or equal to about 0.3 W/A, indicating a higher threshold
current and a lower efficiency than those of the device of Example
6, irrespective of the location of the mesa stripe 620. An
operating life test under the same conditions as those of Example 6
has shown that the device of this comparative example has a short
operating life of about 250 hours or less, indicating that the
device will have a significant problem in practical use.
[0076] According to the present invention, crystal growth of an
excellent nitride compound semiconductor with low threading
dislocation density can be provided: (1) by using a substrate
(growth suppressing film) on which the epitaxial growth of the
nitride compound semiconductor is difficult to occur; (2) by
selectively covering a number of locations of the substrate with a
growth promoting film (where initial growth of the nitride compound
semiconductor easily occurs) in the form of stripes each having a
width of about 100 .mu.m or less; and (3) by epitaxially growing
the nitride compound semiconductor on the substrate, thereby
promoting the lateral growth of the crystal from the sides of the
growth promoting film. By growing a thick film having a thickness
of about 100 .mu.m or more by the method of the present invention,
it is possible to grow a nitride compound semiconductor film having
a further reduced threading dislocation density with no cracks.
[0077] Furthermore, it is possible to provide a nitride compound
semiconductor with a further reduced threading dislocation density
by utilizing the process in which a nitride compound semiconductor
crystal grows horizontally to the substrate and allowing nitride
compound semiconductor crystals from adjacent growth promoting
films to attach to each other. When the horizontal crystal growth
process is utilized, a compound film is preferably provided in a
stripe pattern on the substrate with a stripe interval of about 20
.mu.m or less, so that the adjacent crystals are well attached to
each other, thereby providing a nitride compound semiconductor
having reduced threading dislocation density and irregularity.
[0078] Moreover, when a semiconductor laser device is produced on
the nitride compound semiconductor of the present invention, it is
possible to achieve excellent characteristics such as a lower
threshold current, a higher emission efficiency, and a longer
operating life. In addition, when a light-emitting region of the
semiconductor laser, into which a current is injected, is provided
above a region with no growth promoting film, the operating life of
device can be further improved.
[0079] Various other modifications will be apparent to and can be
readily made by those skilled in the art without departing from the
scope and spirit of this invention. Accordingly, it is not intended
that the scope of the claims appended hereto be limited to the
description as set forth herein, but rather that the claims be
broadly construed.
* * * * *