U.S. patent application number 12/193992 was filed with the patent office on 2009-08-20 for nitride semiconductor light emitting device and method of manufacturing the same.
This patent application is currently assigned to OPNEXT JAPAN, INC.. Invention is credited to Aki TAKEI, Akihisa TERANO.
Application Number | 20090206360 12/193992 |
Document ID | / |
Family ID | 40954273 |
Filed Date | 2009-08-20 |
United States Patent
Application |
20090206360 |
Kind Code |
A1 |
TERANO; Akihisa ; et
al. |
August 20, 2009 |
NITRIDE SEMICONDUCTOR LIGHT EMITTING DEVICE AND METHOD OF
MANUFACTURING THE SAME
Abstract
The present invention provides a nitride semiconductor light
emitting device having an n-electrode that has an Au face excellent
in ohmic contacts to an n-type nitride semiconductor and excellent
in mounting properties, and a method of manufacturing the same. The
nitride semiconductor light emitting device uses an n-electrode
having a three-layer laminate structure that is composed of a first
layer containing aluminum nitride and having a thickness not less
than 1 nm or less than 5 nm, a second layer containing one or more
metals selected from Ti, Zr, Hf, Mo, and Pt, and a third layer made
of Au, from the near side of the n-type nitride semiconductor in
order of mention. The n-electrode thus formed is then annealed to
obtain ohmic contacts to the n-type nitride semiconductor.
Inventors: |
TERANO; Akihisa; (Hachioji,
JP) ; TAKEI; Aki; (Sayama, JP) |
Correspondence
Address: |
MILES & STOCKBRIDGE PC
1751 PINNACLE DRIVE, SUITE 500
MCLEAN
VA
22102-3833
US
|
Assignee: |
OPNEXT JAPAN, INC.
|
Family ID: |
40954273 |
Appl. No.: |
12/193992 |
Filed: |
August 19, 2008 |
Current U.S.
Class: |
257/103 ;
257/E21.09; 257/E33.023; 372/43.01; 438/46 |
Current CPC
Class: |
H01S 5/34333 20130101;
H01L 33/40 20130101; H01L 33/32 20130101; B82Y 20/00 20130101 |
Class at
Publication: |
257/103 ;
372/43.01; 438/46; 257/E33.023; 257/E21.09 |
International
Class: |
H01L 33/00 20060101
H01L033/00; H01S 5/00 20060101 H01S005/00; H01L 21/20 20060101
H01L021/20 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 18, 2008 |
JP |
2008-035868 |
Claims
1. A nitride semiconductor light emitting device, comprising: an
n-type nitride semiconductor layer provided over a substrate; a
light emitting layer provided over the n-type nitride semiconductor
layer, for emitting light of a predetermined wavelength; a p-type
nitride semiconductor layer provided over the light emitting layer;
an n-electrode electrically connected to the n-type nitride
semiconductor layer; and a p-electrode electrically connected to
the p-type nitride semiconductor layer, wherein the n-electrode has
a laminate structure composed of a first layer, a second layer, and
a third layer, from the near side of the n-type conductive
substrate in order of mention, the first layer contains aluminum
nitride and has a thickness not less than 1 nm or less than 5 nm,
the second layer contains one or more metals selected from Ti, Zr,
Hf, Mo, and Pt, and the third layer is made of Au.
2. The nitride semiconductor light emitting device according to
claim 1, wherein an electrical property making a junction between
the n-electrode and the n-type conductive substrate is an ohmic
property.
3. The nitride semiconductor light emitting device according to
claim 1, wherein the second layer of the n-electrode has a
two-layer structure having Ti provided at the near side of the
n-type conductive substrate and Pt provided at the far side.
4. The nitride semiconductor light emitting device according to
claim 1, wherein the aluminum nitride is provided in form of
islands, so as to expose at least part of the surface of the n-type
conductive substrate.
5. The nitride semiconductor light emitting device according to
claim 1, wherein the substrate includes an n-type conductive
nitride semiconductor, and the n-electrode is provided onto the
surface near the n-type nitride semiconductor layer of the n-type
conductive substrate, or onto the rear side opposite to the
surface.
6. The nitride semiconductor light emitting device according to
claim 1, wherein the n-type conductive substrate is provided over
an insulation material, and the n-electrode is provided onto the
surface near the n-type nitride semiconductor layer of the n-type
conductive substrate.
7. The nitride semiconductor light emitting device according to
claim 1, wherein the nitride semiconductor light emitting device is
a light emitting diode (LED).
8. The nitride semiconductor light emitting device according to
claim 1, wherein the nitride semiconductor light emitting device is
a laser diode (LD).
9. A method of manufacturing a nitride semiconductor light emitting
device, the method including the steps of: forming, over a
substrate, an n-type nitride semiconductor layer containing at
least an n-type impurity; forming, over the n-type nitride
semiconductor layer, a light emitting layer for emitting light of a
predetermined wavelength; forming, over the light emitting layer, a
p-type nitride semiconductor layer containing a p-type impurity;
forming, in contact with the p-type nitride semiconductor layer, a
p-electrode; laminating, over one surface of the n-type conductive
substrate and from the near side of the substrate in order of
mention, a first layer containing aluminum nitride and having a
thickness not less than 1 nm or less than 5 nm, a second layer
containing one or more metals selected from Ti, Zr, Hf, Mo, and Pt,
and a third layer made of Au; and after the lamination process,
carrying out an annealing process on the substrate.
10. The method of manufacturing a nitride semiconductor light
emitting device according to claim 9, wherein the annealing is
carried out at a temperature range of 400.degree. C. to 600.degree.
C.
11. The method of manufacturing a nitride semiconductor light
emitting device according to claim 9, wherein the aluminum nitride
is formed by sputtering.
Description
CLAIM OF PRIORITY
[0001] The present application claims priority from Japanese Patent
Application No. JP2008-035868 filed on Feb. 18, 2008, the content
of which is hereby incorporated by reference into this
application.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a nitride semiconductor
light emitting device such as light emitting diode (LED), laser
diode (LD) or the like that operates in the visible to ultraviolet
wavelength region, and a method of manufacturing the same.
[0004] 2. Description of the Related Arts
[0005] Nitride semiconductors represented by gallium nitride (GaN)
have been used as a material for light emitting elements capable of
generating a light in the edge to ultraviolet region. In general, a
light emitting element using a nitride semiconductor is provided
with a light emitting layer (typically known as an active layer)
having a multiple quantum well structure, and p-type and n-type
nitride semiconductor layers for current feeding that are disposed
above and below the light emitting layer.
[0006] With recent advances in the development of GaN substrate,
laser diodes now demonstrate high performance of laser, high
quality and high yield, which have been made possible by
epitaxially growing a n-type nitride semiconductor layer, a light
emitting layer, and a p-type nitride semiconductor layer
sequentially on a n-type conductive GaN substrate such that
dislocation density or defects within an epilayer can be reduced
compared with epitaxial growth on a conventional sapphire substrate
and the cleaved end-face of a flat resonator can easily be
formed.
[0007] Moreover, the introduction of an n-type GaN substrate has
shorten the manufacturing process of laser diodes by bringing a
change in the structure of them, i.e., forming an n-type ohmic
electrode on the rear side of the n-type GaN substrate, not on the
exposed surface of an n-type nitride semiconductor layer provided
to the core of an epitaxial growth layer by the conventional
semiconductor process.
[0008] As an example, Japanese Patent Application Publication No.
07-45867 disclosed the primary use of Ti/Al electrode as an ohmic
electrode at the bottom of an n-type nitride semiconductor, and
explained that desirable ohmic contact with an n-type layer could
be obtained by annealing the adhered electrode at high
temperature.
[0009] However, when an electrode having Al on the uppermost
surface undergoes annealing at high temperature, Al is oxidized.
Thus, in the mount process for a device, if the electrode having Al
on the outermost surface was used, bonding by Au wire or Au-based
(e.g., Au--Sn alloy) soldering was not so strong but easily
separated. As a result, sufficient junction strength between both
sides was hard to obtain.
[0010] To resolve the above problem, Japanese Patent Application
Publication No. 2006-59933 disclosed an ohmic electrode to be
formed on the surface of an n-type nitride semiconductor, the ohmic
electrode being provided with, from the near side the n-type
nitride semiconductor, a first layer with thickness of 10 to 70 nm
consisting of Al and/or an Al alloy, a second layer with thickness
of 10 to 300 nm consisting of one or more metals selected from Pd,
Ti, Nb, Mo and W for example, which have higher melting point than
that of the first layer (Al, Al alloy) and the third layer (Au),
and a third layer with thickness of 100 to 1000 nm consisting of
Au, in sequential order. The ohmic electrode was then subjected to
annealing at 350 to 600.degree. C. to obtain desirable ohmic
property against the n-type nitride semiconductor as well as a
smooth and lustrous surface and desirable wire bonding
property.
[0011] In particular, according to Patent Document 2, it is
important that Pd with thickness of 50 nm is used as metal for the
second layer, the ohmic property is checked on using, as a
parameter, the thickness of a metal film containing Al of the first
layer as a main ingredient, and the first layer film thickness is
limited to a range of 10 nm to 70 nm so as to reduce contact
resistance.
[0012] Further, it described that the relationship between the film
thickness of the first layer metal and the contact resistance
remained the same when the second layer was made of Pd metal and
when the second layer was made of one of Ti, Nb, Mo and W, instead
of Pd.
[0013] As yet another example, Japanese Patent Application
Publication No. 2004-221493 disclosed an electrode to be laminated
on the surface of an n-type nitride semiconductor layer, the
electrode being provided with, from the bottom, an Ti layer (e.g.,
30 nm), an Al layer (e.g., 150 nm), a Mo layer (e.g., 30 nm), a Pt
layer (e.g., 15 nm), and a Au layer (e.g., 200 nm) in sequential
order, such that delamination of the Au layer is suppressed and
diffusion of the Au layer into the semiconductor layer side can be
nearly completely suppressed.
SUMMARY OF THE INVENTION
[0014] Based on the kinds of metals used for the diffusion barrier
layer described in Japanese Patent Application Publication Nos.
2006-59933 and 2004-221493, the inventors formed an electrode
having a five-layer laminate structure on an n-type nitride
semiconductor layer, which is provided with an Al layer as the
first layer with thickness of 100 nm, a diffusion barrier layer
having a three-layer structure composed of a Mo layer with
thickness of 50 nm, a Ti layer with thickness of 100 nm and a Pt
layer with thickness of 50 nm, and lastly a Au layer with thickness
of 300 nm in sequential order. The electrode then went though an
annealing process at 500.degree. C. under nitrogen atmosphere. It
turned out that, as shown in FIG. 7, the electrode surface had a
severely rugged or uneven area accompanied by discoloration.
[0015] The surface area was analyzed with the application of Auger
electron spectroscopy, the analysis result of which is shown in
FIG. 2.
[0016] From the drawing of the uneven surface area, the inventors
identified Al, which was supposed to be at the undermost layer of
the electrode, on the uppermost layer of the electrode instead and
observed the presence of oxygen in addition to Al there, which
implies that oxidized Al was formed on the uppermost layer of the
electrode.
[0017] On the contrary, Au having been at the uppermost layer
seemed to be diffused towards the semiconductor.
[0018] In effect, the uneven surface area was also found even when
only the Al film of the first layer was made thinner to about 30
nm, and the uneven, discolored area has expanded in size if
annealing temperature was raised.
[0019] As noted before, if an oxidized Al area is created on the
electrode surface, sufficient junction strength cannot be obtained
between the electrode surface and Au wire or soldering material
during the mount process. This can actually be a serious problem
when mounting a device.
[0020] From the facts mentioned above, one may assume that if Al
metal essential for acquiring ohmic property exists in the
uppermost Au layer of an electrode, it is difficult to completely
suppress the diffusion of Al caused by high-temperature annealing
process, no matter how thin the diffusion barrier can be prepared
within a reasonable film thickness allowed in the existing
semiconductor manufacturing techniques.
[0021] To resolve the foregoing problems, it is, therefore, an
object of the present invention to provide a nitride semiconductor
light emitting device having an n electrode which demonstrates a
satisfactory ohmic contact to an n-type nitride semiconductor and
unlike in the related art techniques, which does not make the
electrode surface rough even after a high-temperature annealing
process, and a method of manufacturing the same.
[0022] To resolve the problems, it is essential to have the
n-electrode which demonstrates a satisfactory ohmic contact to an
n-type nitride semiconductor, without using Al metals.
[0023] The present invention therefore presents several embodiments
of such a device and its manufacturing method in order to obtain
satisfactory ohmic contacts to an n-type nitride semiconductor, and
some of them are as follows.
[0024] 1. A nitride semiconductor light emitting device, including:
an n-type nitride semiconductor layer provided over a substrate; a
light emitting layer provided over the n-type nitride semiconductor
layer, for emitting light of a predetermined wavelength; a p-type
nitride semiconductor layer provided over the light emitting layer;
an n-electrode electrically connected to the n-type nitride
semiconductor layer; and a p-electrode electrically connected to
the p-type nitride semiconductor layer, wherein the n-electrode has
a laminate structure composed of a first layer containing aluminum
nitride and having a thickness not less than 1 nm or less than 5
nm, a second layer containing one or more metals selected from Ti,
Zr, Hf, Mo, and Pt, and a third layer made of Au, from the near
side of the n-type nitride semiconductor in order of mention, and
wherein junction between the n-electrode and the n-type nitride
semiconductor show ohmic properties.
[0025] 2. A method of manufacturing a nitride semiconductor light
emitting device, the method including the steps of: forming, over a
substrate, an n-type nitride semiconductor layer containing at
least an n-type impurity; forming, over the n-type nitride
semiconductor layer, a light emitting layer for emitting light with
a predetermined wavelength; forming, over the light emitting layer,
a p-type nitride semiconductor layer containing a p-type impurity;
forming, in contact with the p-type nitride semiconductor layer, a
p-electrode; forming, in contact with the n-type nitride
semiconductor, an n-electrode having a laminate structure composed
of a first layer containing aluminum nitride and having a thickness
not less than 1 nm or less than 5 nm, a second layer containing one
or more metals selected from Ti, Zr, Hf, Mo, and Pt, and a third
layer made of Au, from the bottom in order of mention; and carrying
out an annealing process.
[0026] By using such an n-electrode, satisfactory ohmic contacts to
an n-type nitride semiconductor were obtained, without using Al
metals. At the same time, even when the n-electrode undergoes a
high-temperature annealing process, diffusion of Al as in the
related art techniques does not occur any more. Accordingly, the
n-electrode and the Au-based solder/the Au wire are bonded in
practically sufficient junction strength during the device mount
process, and this in turn makes it possible to manufacture nitride
semiconductor light emitting devices at a high yield.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 shows a cross-sectional view of a nitride
semiconductor laser device in accordance with a first embodiment of
the present invention;
[0028] FIG. 2 graphically shows the result of analysis on an rugged
or uneven surface of an electrode accompanied by discoloration
after annealing at 500.degree. C., the electrode having a laminate
structure of Al (film thickness=100 nm)/Mo/Ti/Pt/Au provided based
on a related art technique;
[0029] FIG. 3A shows I-V characteristics between electrodes of a
sample prepared to verify functions and effects of the present
invention;
[0030] FIG. 3B shows I-V characteristics between electrodes of a
sample prepared to verify functions and effects of the present
invention;
[0031] FIG. 3C shows I-V characteristics between electrodes of a
sample prepared to verify functions and effects of the present
invention;
[0032] FIG. 3D shows I-V characteristics between electrodes of a
sample prepared to verify functions and effects of the present
invention;
[0033] FIG. 4 graphically shows the result of evaluation on the
dependence of non-contact resistivity (.rho..sub.c) of a sample
prepared to verify functions and effects of the present invention
upon annealing temperature;
[0034] FIG. 5 is a schematic view of a nitride semiconductor laser
device in accordance with the first embodiment of the present
invention;
[0035] FIG. 6 is a schematic cross-sectional view of a nitride
semiconductor light emitting diode in accordance with a second
embodiment of the present invention; and
[0036] FIG. 7 graphically shows the observation result of the
surface of an electrode after annealing at 500.degree. C., the
electrode having a laminate structure of Al (film thickness=100
nm)/Mo/Ti/Pt/Au provided based on a related art technique.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0037] A preferred embodiment of the present invention will now be
described with reference to the accompanying drawings.
Embodiment I
[0038] FIG. 1 is a schematic cross-sectional view of a nitride
semiconductor laser in accordance with one embodiment of the
present invention.
[0039] Since the main gist of the present invention lies in the
structure of an n-electrode formed in contact with an n-type
nitride semiconductor, any of conventional laminate structures was
adopted for the structure of epitaxial growth layers of laser, to
which the present invention is not limited.
[0040] The following will now explain an overall procedure of
manufacturing a nitride semiconductor laser.
[0041] Referring to FIG. 1, on an n-type GaN substrate 1, an n-type
buffer layer 2 made of Si-doped GaN, an n-type clad layer 3 made of
Si-doped AlGaN, an n-type guide layer 4 made of Si-doped GaN, an
active layer 5 made of InGaN in a multiple quantum well structure,
an electronic block layer 6 made of Mg-doped AlGaN (composition
ratio of Al is 7%), a p-type clad layer 7 made of Mg-doped AlGaN
(composition ratio of Al is 4%), and a p-type contact layer 8 made
of Mg-doped GaN were grown sequentially in the order of mention by
molecular organic chemical vapor deposition (MOCVD).
[0042] Next, a desired area on an opened substrate surface by a
resist pattern was etched by a well-known photolithography
technique and a drying etching method using a chlorine based gas
into the middle of the p-type clad layer 7, to be more specific, to
a depth so as to leave 30 to 40 nm of the p-type clad layer 7.
Accordingly, a 1.5 .mu.m-wide ridge resonator having the p-type
contact layer 8 as a core region is formed.
[0043] Next, a SiO.sub.2 film 9 with thickness of 250 nm was formed
over the front side of the substrate by a well-known insulation
film formation method such as CVD or sputtering. Then, a
photoresist pattern was formed by photolithography in a manner that
only a region at the uppermost portion of the ridge was open. With
this photoresist pattern as an etching mask, the open region was
etched to expose the p-type contact layer 8 at the core portion of
the ridge.
[0044] At this time, the etching process is carried out, either by
wet etching in use of HF-based solution or by dry etching in use of
fluorine-based gas (e.g., CF.sub.4 or the like).
[0045] After the photoresist pattern was formed in a manner to open
a desired region including the open, core portion of the ridge, a
Ni (nickel) film, a Mo (molybdenum) film and Au were adhered
sequentially onto the entire surface side of the substrate 1 by
vacuum deposition for example, and unnecessary metal film(s) and
the photoresist pattern were removed by a well-known lift-off
method. In result, a p-electrode 10 made of Ni/Mo/Au is formed over
the p-type contact layer 8 at the core portion of the ridge and
over the SiO.sub.2 film around it.
[0046] Next, the n-type GaN substrate 1 was polished and thinned,
starting from the rear side of the substrate, by a well-known
polishing technique until the substrate has a thickness of about
100 .mu.m.
[0047] Next, on the entire rear side of the polished and thinned
n-type GaN substrate 1, a 3 nm-thick aluminum nitride (AlN) 101 was
first adhered by sputtering, and then a 50 nm-thick Ti (titanium)
film 102, a 50 nm-thick Pt (platinum) film 103, and a 500 nm-thick
Au (gold) film 104 were adhered sequentially in order of mention by
electron beam evaporation for example. Later, the substrate was
annealed at 500.degree. C. for 10 minutes under nitrogen
atmosphere. In this way, a four-layer n-electrode 11 making an
ohmic contact to the n-type GaN substrate 1 is formed in a
four-layer laminate structure composed of AlN/Ti/Pt/Au from the
bottom in order of mention.
[0048] Next, the n-electrode 11 was cleaved perpendicularly to the
length direction of the ridge to form about 600 .mu.m long
bar-shaped resonator cross-sections on both sides, and a
single-side coating film 12 having a desired reflectance and
transmission factor is formed on both cross-sections.
[0049] Further, the bars were made into a chip by cleavage to
complete manufacturing of a nitride semiconductor laser that has
the cross-sectional structure of FIG. 1 and the configuration shown
in FIG. 5. With the p-electrode side of the laser chip as a mount
face, the laser chip was mounted, by dye-bonding, on a sub-mount
face that is made of SiC coated with a Au--Sn solder, and the
sub-mount having the laser chip mounted thereon is mounted further
on a stem. Lastly, the p-electrode side of the laser chip facing
upward and the electrode side on the sub-mount to which the
n-electrode is electrically connected are properly bonded and wired
by Au wires, thereby completing the manufacture of a nitride
semiconductor laser device.
[0050] Among fifty of nitride semiconductor laser devices that were
prepared following the manufacturing process described above, no
connection defect occurred in the process of wire bonding, and the
outer appearance of the junction was also satisfactory.
[0051] In order to evaluate adhesion between the Au wire and the
n-electrode surface of each, the inventors conducted a pull test on
the Au wire bonded to the n-electrode surface. It turned out that
breaking strength for all the 50 laser devices was 5 g or more, and
all the broken portions, if any, were found along the Au wires.
[0052] Further, I-V (current-voltage) characteristics of each were
evaluated through an external input terminal to which Au wire of
each laser device is connected. There was no sharp increase in
direct resistance or non-uniformity in increasing voltage, and
electrical conduction between the electrode and the Au wire was
also satisfactory.
[0053] The following will explain the structure, function and
effect of an n-type ohmic electrode in accordance with preferred
embodiments of the present invention, on the basis of the
experimental results provided by the inventors.
[0054] For the experiment, a low-temperature buffer layer
consisting of GaN, an undoped GaN layer with film thickness of 5000
nm, and a Si-doped GaN (doping
concentration=1.0.about.2.0.times.10.sup.18 cm.sup.-3, film
thickness=1000 nm) were epitaxially grown by molecular organic
chemical vapor deposition (MOCVD).
[0055] As for the evaluation on the ohmic property in used of the
manufacturing of the nitride semiconductor laser, a conventional
transmission line model (TLM) pattern was formed by a photoresist
to complete a sample prior to the electrode formation.
[0056] Mesa width (=electrode width) of the TLM pattern composed of
the Si-doped GaN layer is 100 .mu.m , and photoresist openings for
adhering an n-electrode are arranged at the surface of the earth
formed by mesa-etching in such a manner that interelectrode gaps
are 20 .mu.m, 40 .mu.m, 80 .mu.m, 160 .mu.m, and 320 .mu.m.
[0057] Aluminum nitride to become the undermost layer of an
electrode was formed by sputtering at three different film
thicknesses, e.g., 3 nm, 5 nm, and 8 nm, so as to prepare three
kinds of samples. At this time, the substrate was not heated.
[0058] Next, on each of four samples, i.e., three samples described
above and one sample (sample #1) without having aluminum nitride
adhered thereto, a Ti (titanium) layer with film thickness of 50
nm, a Pt (platinum) layer with film thickness of 50 nm, and a Au
(gold) layer with film thickness of 300 nm were deposited by
electron beam evaporation. Later, the photoresist, unnecessary
metal films and the aluminum nitride film were removed by a
well-known lift-off method, such that the samples were prepared to
get ready for TLM measurement.
[0059] The four samples were then annealed at a temperature range
of 400.degree. C. to 550.degree. C. under nitrogen atmosphere, and
I-V characteristics between electrodes with the interelectrode gap
of 20 .mu.m were evaluated (refer to FIGS. 3A through 3D).
[0060] More specifically, FIG. 3A shows characteristics of only the
Ti/Pt/Au electrode without having aluminum nitride, and FIGS. 3B,
3C, and 3D show characteristics of an electrode that is formed by
laying Ti/Pt/Au over aluminum nitride (hereinafter abbreviated as
AlN in the case of describing a laminate structure) of the
undermost layer in different film thicknesses of 3 nm, 5 nm, and 8
nm, respectively.
[0061] The following will now explain characteristics of each
electrode.
[0062] In the case of the Ti/Pt/Au electrode of FIG. 3A, it showed
non-ohmic properties before annealing, but a small increase in
current was observed as voltage increased. After the electrode was
annealed at 400.degree. C. or higher, however, nearly no current
flew.
[0063] In the case of the AlN (3 nm)/Ti/Pt/Au electrode of FIG. 3B,
there was no increase in current before annealing, but
comparatively good ohmic properties were shown after annealing at
400.degree. C. or higher. It demonstrated the best I-V
characteristics after it was annealed at 450 to 500.degree. C.
Though not shown, .rho..sub.c after annealing at 600.degree. C. was
less than 1.0.times.10.sup.-4 .OMEGA.cm.sup.2, and the lustrous
electrode surface was tarnished due to slight unevenness. However,
a noticeably discolored area as in the related art was not
observed, and the electrode surface well retained Au (gold)
color.
[0064] In the case of the AlN (5 nm)/Ti/Pt/Au electrode of FIG. 3C,
there was no increase in current prior to annealing until after
annealing at 400.degree. C., and ohmic properties started showing
after the electrode was annealed at 450.degree. C. or higher. The
best I-V characteristics were demonstrated after the electrode was
annealed at 500 to 550.degree. C., but its current value was small
compared with that of (b).
[0065] In the case of the AlN (8 nm)/Ti/Pt/Au electrode of FIG. 3D,
there was no increase in current prior to annealing until after
annealing at 450.degree. C., and ohmic properties started showing
only after the electrode was annealed at 500.degree. C. Even after
annealing at 550.degree. C., any large current value was not
obtained, compared with that of (b) or (c).
[0066] FIG. 4 shows evaluation results on the dependency of
non-contact resistivity (.rho..sub.c) calculated in use of
resistance value obtained from the I-V characteristics upon
annealing temperature.
[0067] As can be seen from characteristic line #1 (AlN film
thickness=3 nm), characteristic line #2 (AlN film thickness=5 nm),
and characteristic line #3 (AlN film thickness=8 nm) in the
drawing, .rho..sub.c seems to increase along with an increase in
the film thickness of aluminum nitride, and the aluminum nitride
film should be at least 5 nm thick to show the I-V characteristics
described above and to obtain ohmic properties applicable to a
light emitting device such as a laser device.
[0068] On the contrary, one might presume that satisfactory ohmic
properties cannot be obtained as the aluminum nitride film gets
thinner. According to the experimental results, ohmic properties do
not show at all if the aluminum nitride film is not provided.
[0069] Therefore, the inventors believed that a lower limit of the
thickness of an aluminum nitride film to be adhered would be around
1 nm which is the same as the limit set based on the conventional
evaluation on the film thickness. In other words, ohmic properties
are expected to improve even with the presence of thin aluminum
nitride islands, not necessarily in film form.
[0070] Next, the inventors examined a change in the laminate
structure of the AlN (film thickness=3 nm)/Ti/Pt/Au electrode
having demonstrated the best characteristics in the previous
experiment before annealing and after annealing at 500.degree. C.,
by cross-section TEM observation and EDX surface analysis. It
turned out that there was no significant change in aluminum nitride
of the first layer even after annealing at 500.degree. C., and its
presence was observed very clearly. Moreover, except for an
occurrence of slight interdiffusion across the Ti/Pt/Au interface
of the upper layer, the electrode well maintained its original
laminate state.
[0071] The observation facts noted above indicate that in order to
obtain ohmic contacts to an n-type nitride semiconductor without
using Al metals, it is important to provide aluminum nitride with a
designated film thickness between the n-type nitride semiconductor
and a metal film made of a metal other than Al. Also, by overlaying
and adhering metal films during sputtering used as part of the
aluminum nitride formation method and by carrying out an annealing
process at a temperature at least between 400 and 600.degree. C. in
conformation to the experiment results, it becomes possible to
obtain a satisfactory ohmic contact to an n-type nitride
semiconductor and to maintain the uppermost Au surface in a
suitable condition for the mount process, without hetero-metal
diffusion therein by annealing.
[0072] Having learned from the experiment results that I-V
characteristics change dramatically about a certain annealing
temperature and that no significant change was observed in the
aluminum nitride film after annealing at 500.degree. C., the
inventors believed a hetero interface could have been formed as the
n-type nitride semiconductor bonds epitaxially, during annealing,
with the less-crystalline aluminum nitride film that has simply
been deposited on the n-type nitride semiconductor, such that ohmic
contacts were realized by tunnel effects between the n-type nitride
semiconductor and the metal film on aluminum nitride. The same can
also be predicted from the observation result that the resistance
value between n-electrodes has a tendency to increase as the
aluminum nitride film having a band gap greater than that of GaN
gets thicker.
[0073] On the other hand, in order to grow aluminum nitride on a
substrate by a conventional epitaxial growth method such as MOVPE,
the substrate needs to be heated at 1200.degree. C. or higher. If a
metal film is formed on that substrate surface, or if the substrate
has been thinned, it is highly possible that the growth path is
contaminated by metal(s) or the substrate is cracked or split.
Because of these or other adverse effects, the method is not likely
to be selected in reality.
[0074] As a further example, if a substrate having an active layer
containing In (indium) is annealed at 1200.degree. C. or higher,
the active layer could be broken due to an occurrence of diffusion
and segregation of In.
[0075] The present invention is characterized in that the same
function and effect are obtained from aluminum nitride that is
formed by a film formation method such as sputter in general as
part of the semiconductor processing process, and that aluminum
nitride can be formed by other methods like CVD or sublimation
besides the sputtering method.
[0076] In addition, as for the electrode metal(s) formed on
aluminum nitride, one can conclude, based on the inferential
principle of obtaining ohmic properties, any kind of metal
materials or metal compound materials may be used as long as they
are adhesive to aluminum nitride.
Embodiment 2
[0077] FIG. 6 is a schematic view of a nitride semiconductor light
emitting diode in accordance with another embodiment of the present
invention. An overall manufacturing method thereof will now be
explained below.
[0078] On a sapphire substrate 20 is formed a multiple layer
structure including a buffer layer 21 made of undoped GaN, a
Si-doped n-type GaN layer 22 having carrier concentration of
2.times.10.sup.18 cm.sup.-3 and film thickness of 5 .mu.m, Si-doped
n-type AlGaN clad layer 23, an active layer 24 made of
In.sub.bGa.sub.1-bN (0<b.ltoreq.0.1), a p-type clad layer 25
made of Mg-doped AlGaN having Mg doping concentration of
3.0.times.10.sup.19 cm.sup.-3 and film thickness of 40 nm, and a
p-type contact layer 26 made of Mg-doped GaN, the layers being
sequentially grown by molecular organic chemical vapor deposition
(MOCVD). Next, a desired region was etched, starting from the
surface side of the substrate 20, by a well-known photolithography
technique and by dry etching using a chlorine based gas, so as to
expose the Si-doped n-type GaN layer 22.
[0079] Next, a 1.5 nm-thick aluminum nitride (AlN) 201 was adhered
by sputtering to a desired position of the exposed Si-doped n-type
GaN layer 22. After that, a 30 nm-thick Hf (hafnium) film 202, a 50
nm-thick Mo (molybdenum) film 203, a 100 nm-thick Zr (zirconium)
film 204, a 100 nm-thick Pt (platinum) film 205, and a 500 nm-thick
Au (gold) film 206 were further deposited on the AlN film 201 by
electron beam evaporation, and then annealed at 450.degree. C.
under nitrogen atmosphere. In this manner, an n-electrode 27,
having the six-layer laminate structure of AlN/Hf/Mo/Zr/Pt/Au and
making an ohmic contact to the Si-doped n-type GaN layer 22, is
thus formed.
[0080] Next, a p-type ohmic electrode 28 is formed by forming a
laminate composed of 30 nm-thick Pd film/70-nm-thick Pt film/300-nm
Au film at a desired position on a second p-type clad layer 26 in a
non-etched region.
[0081] At this time, the uppermost surface of the n-electrode 27
and the uppermost surface of the p-electrode 28 are roughly the
same height.
[0082] After that, the rear side of the sapphire substrate 20 was
made thinner by diamond polishing particles until it becomes as
thin as 200 .mu.m, and, as a final process, the polished face was
subjected to mirror-like finishing and was made into a chip of
desired size, thereby completing the manufacture of a nitride
semiconductor LED.
[0083] Since the LED uses light that is emitted from the
mirror-like polished rear side through the sapphire substrate,
patterning, by the Au--Sn solder, should be performed in advance on
the mount where the LED is to be mounted, and for the patterned
Au--Sn solder, the mount process was carried out in a manner that
the Au--Sn solder pattern was matched with each of the p- and
n-electrodes.
[0084] A total of 30 LED devices were manufactured following the
above process, and I-V characteristics of each LED were evaluated
through an external input terminal. It turned out that an average
voltage necessary to obtain 50 mA current was 3.25V. Also, there
was no sharp increase in voltage and current was obtained for
each.
[0085] By conducting the shear failure test on each of the mounted
LED chips, the inventors learned that all of them had shear
strength of 400 g or more, and observed no occurrence of a problem
during the mount process.
[0086] Although the embodiments having been explained so far
utilized a sapphire substrate as a substrate material, it is
needless to say that any substrate material, say, GaN, SiC, Si, or
the like can also be used as long as a nitride semiconductor can
grow reasonably thereon. Further, a variety of substrate materials
may be used, depending on the structure of LED to be
manufactured.
[0087] While the embodiments having been explained so far utilized
the Ti/Pt laminate and the Hf/Mo/Zr/Pt laminate, from the bottom,
as metal films with aluminum nitride on the n-electrode and with
Au, the present invention is not limited thereto but any suitable
metal laminate of different film thickness depending on the
material used for bonding, e.g., Au wire or Au--Sn solder, can also
be used.
[0088] The embodiments of the present invention have been mainly
focused on the manufacturing method of a nitride semiconductor
light emitting device, but the structure of a specific nitride
semiconductor is not limited to those embodiments but can be varied
based on the configuration or required function for a device to be
manufactured.
[0089] By applying the n-electrode of the invention to a nitride
semiconductor light emitting device, satisfactory ohmic contacts to
an n-type nitride semiconductor can be obtained, and the uppermost
surface of the electrode even after high-temperature annealing
still retains Au suitable for the mount process.
* * * * *