U.S. patent application number 11/135236 was filed with the patent office on 2005-09-22 for gallium-containing light-emitting semiconductor device and method of fabrication.
This patent application is currently assigned to Sanken Electric Co., Ltd.. Invention is credited to Murofushi, Hitoshi, Takeda, Shiro.
Application Number | 20050205886 11/135236 |
Document ID | / |
Family ID | 34985305 |
Filed Date | 2005-09-22 |
United States Patent
Application |
20050205886 |
Kind Code |
A1 |
Murofushi, Hitoshi ; et
al. |
September 22, 2005 |
Gallium-containing light-emitting semiconductor device and method
of fabrication
Abstract
An LED comprising a light-generating semiconductor region having
an active layer sandwiched between two confining layers of opposite
conductivity types. A cathode is arranged centrally on one of the
opposite major surfaces of the semiconductor region from which is
emitted the light. An array of discrete gold regions are formed via
transition metal regions on the other major surface of the
semiconductor region at which is exposed one of the confining
layers which is of n-type AlGaInP semiconductor material. The gold
is thermally diffused into the confining layer via the transition
metal regions at a temperature less than the eutectic point of gold
and gallium, thereby creating an array of ohmic contact regions of
alloyed or intermingled gold and gallium, which are less absorptive
of light than their conventional counterparts, to a thickness of 20
to 1000 angstroms. After removing the transition metal regions and
gold regions from the surface of the light-generating semiconductor
region, a reflective layer of aluminum is formed so as to cover
both the ohmic contact regions and the exposed surface portions of
the AlGaInP confining layer. An electroconductive base-plate of
doped silicon is then bonded to the reflective layer.
Inventors: |
Murofushi, Hitoshi;
(Niiza-shi, JP) ; Takeda, Shiro; (Niiza-shi,
JP) |
Correspondence
Address: |
WOODCOCK WASHBURN LLP
ONE LIBERTY PLACE, 46TH FLOOR
1650 MARKET STREET
PHILADELPHIA
PA
19103
US
|
Assignee: |
Sanken Electric Co., Ltd.
Niiza-shi
JP
|
Family ID: |
34985305 |
Appl. No.: |
11/135236 |
Filed: |
May 23, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11135236 |
May 23, 2005 |
|
|
|
PCT/JP03/14890 |
Nov 21, 2003 |
|
|
|
Current U.S.
Class: |
257/98 ;
257/E21.086; 257/E21.138; 257/E33.068 |
Current CPC
Class: |
H01L 33/0093 20200501;
H01L 21/182 20130101; H01L 21/2215 20130101; H01L 33/20 20130101;
H01L 33/387 20130101; H01L 33/405 20130101 |
Class at
Publication: |
257/098 |
International
Class: |
H01L 029/22 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 29, 2002 |
JP |
2002-348416 |
Claims
What is claimed is:
1. A light-emitting semiconductor device of improved efficiency,
comprising: (a) a light-generating semiconductor region having a
first major surface from which light is emitted and a second major
surface which is opposite to the first major surface, the
light-generating semiconductor region comprising a plurality of
compound semiconductor layers including a gallium-containing
compound semiconductor layer which is exposed at the second major
surface of the semiconductor region; (b) an electrode on the first
major surface of the semiconductor region; (c) an ohmic contact
region held in ohmic contact with at least part of the second major
surface of the light-generating semiconductor region, the ohmic
contact region being made from a mixture of at least two metals
including gallium; and (d) a reflective layer of electrically
conducting material held against at least either of the ohmic
contact region and that part, if any, of the gallium-containing
compound semiconductor layer of the light-generating semiconductor
region which is exposed at the second major surface of the
semiconductor region through the ohmic contact region, for
reflecting the light from the semiconductor region back toward the
semiconductor region for emission from the first major surface
thereof.
2. A light-emitting semiconductor device as defined in claim 1,
wherein the ohmic contact region is of a mixture of gallium and
gold.
3. A light-emitting semiconductor device as defined in claim 1,
wherein the ohmic contact region is from abut 20 to about 1000
angstroms in thickness.
4. A light-emitting semiconductor device as defined in claim 1,
wherein the gallium-containing compound semiconductor layer of the
light-generating semiconductor region is made from one of the
following three compound semiconductors and a conductivity-type
determinant: (a) a first compound semiconductor that is generally
expressed as Al.sub.xGa.sub.yIn.sub.1-x-yP where the subscript x is
a numeral that is equal to or greater than zero and less than one;
the subscript y is a numeral that is greater than zero and equal to
or less than one; and the sum of x and y is greater than zero and
equal to or less than one; (b) a second compound semiconductor that
is generally expressed as Al.sub.xGa.sub.yIn.sub.1-x-yAs where the
subscript x is a numeral that is equal to or greater than zero and
less than one; the subscript y is a numeral that is greater than
zero and equal to or less than one; and the sum of x and y is
greater than zero and equal to or less than one; and (c) a third
compound semiconductor that is generally expressed as
Al.sub.xGa.sub.yIn.sub.1-x-yN where the subscript x is a numeral
that is equal to or greater than zero and less than one; the
subscript y is a numeral that is greater than zero and equal to or
less than one; and the sum of x and y is greater than zero and
equal to or less than one.
5. A light-emitting semiconductor device as defined in claim 1,
wherein the reflective layer is made from a metal that is more
reflective than the metals of the ohmic contact region.
6. A light-emitting semiconductor device as defined in claim 5,
wherein the reflective layer is of aluminum.
7. A light-emitting semiconductor device as defined in claim 1,
further comprising an electroconductive baseplate attached to the
reflective layer.
8. A light-emitting semiconductor device as defined in claim 7,
wherein the baseplate is of doped silicon, and wherein a second
electrode is formed on the baseplate.
9. A light-emitting semiconductor device as defined in claim 1,
wherein the ohmic contact region is open-worked to expose part of
the second major surface of the light-generating semiconductor
region, and wherein the reflective layer covers both the ohmic
contact region and the exposed part of the second major surface of
the semiconductor region.
10. A light-emitting semiconductor device as defined in claim 1,
wherein the gallium-containing compound semiconductor layer of the
light-generating semiconductor region exposed at the second major
surface thereof is a lower cladding, and wherein the semiconductor
region further comprises: (a) an active layer of gallium-containing
compound semiconductor material on the lower cladding; and (b) an
upper cladding of gallium-containing compound semiconductor
material on the active layer, the upper cladding being opposite in
conductivity type to the lower cladding.
11. A method of making a light-emitting semiconductor device of
improved efficiency, which comprises: (a) providing a
light-generating semiconductor region having a first major surface
from which light is emitted and a second major surface which is
opposite to the first major surface, the light-generating
semiconductor region comprising a plurality of compound
semiconductor layers including a gallium-containing compound
semiconductor layer which is exposed at the second major surface of
the semiconductor region; (b) creating a transition metal layer
containing a transition metal on at least part of the second major
surface of the light-generating semiconductor region; (c) creating
a diffusible metal layer on the first metal layer, the diffusible
metal layer containing a metal that can be thermally diffused into
the gallium-containing compound semiconductor layer of the
light-generating semiconductor region through the transition meal
layer; (d) creating an ohmic contact region in the
gallium-containing compound semiconductor layer of the
light-generating semiconductor region by causing thermal diffusion
of the diffusible metal from the diffusible metal layer into the
gallium-containing compound semiconductor layer through the
transition metal layer at a temperature less than the eutectic
point of elements constituting the gallium-containing compound
semiconductor layer and the diffusible metal; (e) removing the
transition metal layer and the diffusible metal layer from the
light-generating semiconductor region; and (f) creating a
reflective layer of electrically conducting material on at least
either of the ohmic contact region and that part, if any, of the
gallium-containing compound semiconductor layer of the
light-generating semiconductor region which is exposed at the
second major surface of the semiconductor region through the ohmic
contact region.
12. A method of making a light-emitting semiconductor device as
defined in claim 11, wherein the transition metal layer together
with the diffusible metal layer thereon is formed in discrete
regions on the second major surface of the light-generating
semiconductor region, covering parts, and uncoverig the rest, of
the second major surface, so that ohmic contact regions are created
only in those parts of the gallium-containing compound
semiconductor layer of the light-generating semiconductor region
which have been covered by the discrete regions of the transition
metal layer and the diffusible metal layer.
13. A method of making a light-emitting semiconductor device as
defined in claim 12, wherein the reflective layer is created on the
complete second major surface of the light-generating semiconductor
region.
14. A method of making a light-emitting semiconductor device as
defined in claim 11, wherein the gallium-containing compound
semiconductor layer of the light-generating semiconductor region is
made from one of the following three compound semiconductors and a
conductivity-type determinant: (a) a first compound semiconductor
that is generally expressed as Al.sub.xGa.sub.yIn.sub.1-x-yP where
the subscript x is a numeral that is equal to or greater than zero
and less than one; the subscript y is a numeral that is greater
than zero and equal to or less than one; and the sum of x and y is
greater than zero and equal to or less than one; (b) a second
compound semiconductor that is generally expressed as
Al.sub.xGa.sub.yIn.sub.1-x-yAs where the subscript x is a numeral
that is equal to or greater than zero and less than one; the
subscript y is a numeral that is greater than zero and equal to or
less than one; and the sum of x and y is greater than zero and
equal to or less than one; and (c) a third compound semiconductor
that is generally expressed as Al.sub.xGa.sub.yIn.sub.1-x-yN where
the subscript x is a numeral that is equal to or greater than zero
and less than one; the subscript y is a numeral that is greater
than zero and equal to or less than one; and the sum of x and y is
greater than zero and equal to or less than one.
15. A method of making a light-emitting semiconductor device as
defined in claim 11, wherein the gallium-containing compound
semiconductor layer of the light-generating semiconductor region is
made from a conductivity-type determinant and a compound
semiconductor that is generally expressed as
Al.sub.xGa.sub.yIn.sub.1-x-yP where the subscript x is a numeral
that is equal to or greater than 0.4 and less than 1.0; the
subscript y is a numeral that is greater than zero and equal to or
less than one; and the sum of x and y is greater than zero and
equal to or less than one.
16. A method of making a light-emitting semiconductor device as
defined in claim 15, wherein the gallium-containing compound
semiconductor layer of the light-generating semiconductor region
contains the conductivity-type determinant with a concentration of
not less than 10.sup.18 cm.sup.-3.
17. A method of making a light-emitting semiconductor device as
defined in claim 11, wherein the transition metal layer is selected
from among: (a) a layer of at least either of Cr, Ti, Ni, Sc, V,
Mn, Fe, Co, Cu, Zn, and Be; (b) a lamination of an Au sublayer, Cr
sublayer, and another Au sublayer; (c) a lamination of a Cr
sublayer, Ni sublayer, and Au sublayer; and (d) a lamination of a
Cr sublayer, AuSi sublayer, and Au sublayer.
18. A method of making a light-emitting semiconductor device as
defined in claim 11, wherein the diffusible metal layer is selected
from among: (a) a gold layer: (b) a lamination of an Au sublayer,
Cr sublayer, and another Au sublayer; (c) a lamination of a Cr
sublayer, Ni sublayer, and Au sublayer; and (d) a lamination of a
Cr sublayer, AuSi sublayer, and Au sublayer.
19. A method of making a light-emitting semiconductor device as
defined in claim 11, wherein the diffusible metal is gold, which is
so diffused into the gallium-containing compound semiconductor
layer of the light-generating semiconductor region that the ohmic
contact region created is of an alloy of gallium and gold.
20. A method of making a light-emitting semiconductor device as
defined in claim 11, wherein the ohmic contact region is from about
20 to about 1000 angstroms thick.
21. A method of making a light-emitting semiconductor device as
defined in claim 11, wherein the reflective layer is made from a
metal selected to possess a higher reflectivity than does the ohmic
contact region.
22. A method of making a light-emitting semiconductor device as
defined in claim 21, wherein the reflective layer is made from
aluminum.
23. A method of making a light-emitting semiconductor device as
defined in claim 11, which further comprises joining an
electroconductive baseplate to the reflective layer.
24. A method of making a light-emitting semiconductor device as
defined in claim 23, wherein the electroconductive baseplate is of
doped silicon, and wherein the method further comprises joining an
electrode to the baseplate.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation of Application PCT/JP2003/014890,
filed Nov. 21, 2003, which claims priority to Japanese Patent
Application No. 2002-348416 filed Nov. 29, 2002.
BACKGROUND OF THE INVENTION
[0002] This invention relates to a light-emitting semiconductor
device, or light-emitting diode (LED) according to more common
parlance, and more particularly to such devices employing
gallium-containing compound semiconductors. The invention also
concerns a method of making such light-emitting semiconductor
devices.
[0003] The LED has been known which has a light-generating
semiconductor region grown on a substrate of electrically
conducting material such as gallium arsenide. Typically, the
light-generating semiconductor region has an active layer
sandwiched between an n-type cladding or lower confining layer,
which overlies the substrate, and a p-type cladding or upper
confining layer. An anode is mounted centrally atop the upper
confining layer whereas a cathode underlies the substrate. The
light generated at the active layer partly traverses directly
through the upper confining layer and issues from that part of the
surface of the semiconductor region which is left uncovered by the
anode. The rest of the light is radiated toward the substrate via
the lower confining layer. How to reflect this light most
effectively back toward the light-emitting surface of the
semiconductor region is of critical importance for the highest
possible efficiency of the LED.
[0004] One conventional solution to this problem is a reflective
film known as the Bragg reflector interposed between the substrate
and the light-generating semiconductor region. The Bragg reflector
is easy to fabricate by epitaxial growth, the method adopted for
subsequent formation of the semiconductor region. Offsetting this
advantage is the lack of sufficient reflectivity with respect to
the light having a wide spectrum of wavelengths.
[0005] Another prior art method calls for the removal of the
gallium arsenide substrate following the epitaxial growth of the
semiconductor region thereon. A transparent baseplate is then
bonded to the semiconductor region in place of the substrate that
has been removed, by way of a mechanical support for the LED. Then
a reflective electrode is attached to the transparent baseplate.
The reflective electrode serves not only as electrode but to
reflect the light back through the transparent baseplate toward the
light-emitting surface of the semiconductor region. This known
remedy is objectionable for a relatively high forward voltage
required between anode and cathode as a result of additional
resistance at the interface between light-generating semiconductor
region and transparent baseplate.
[0006] Japanese Unexamined Patent Publication No. 2002-217450,
filed by the assignee of the instant application, represents an
improvement over the more conventional devices listed above. It
teaches the creation of isolated ohmic contact regions of
gold-germanium-gallium alloy on the underside of the
light-generating semiconductor region. These ohmic contact regions,
as well as the surface of the semiconductor region left uncovered
thereby, are covered by a reflective layer of aluminum or other
metal. An electroconductive baseplate is bonded to the underside of
the reflective layer. Making good ohmic contact with the
light-generating semiconductor region of, say, aluminum gallium
indium phosphide, the ohmic contact regions of
gold-germanium-gallium alloy serve for reduction of the forward
voltage of the LED.
[0007] The last cited prior art LED proved to possess its own
weaknesses, however. The gold-germanium-gallium ohmic contact
regions were rather inconveniently absorptive of light by reasons
of their germanium content and thickness in particular. The total
reflectivity of the ohmic contact regions and reflective layer was
therefore as low as 30 percent or thereabouts, making it difficult
for the LED to gain sufficiently high efficiency. Another
shortcoming concerned the morphology of the gold-germanium-gallium
ohmic contact regions: Their surfaces were so uneven that
difficulties were experienced in bonding the electroconductive
baseplate thereto via the reflective layer.
SUMMARY OF THE INVENTION
[0008] The present invention has it as an object to further enhance
the efficiency of the light-emitting semiconductor device of the
kind incorporating the gold-germanium-gallium ohmic contact
regions, or, for a given intensity of light produced, to make the
forward voltage of the device lower than hitherto.
[0009] Briefly stated in one aspect thereof, the invention concerns
a light-emitting semiconductor device of improved efficiency
comprising a light-generating semiconductor region having a first
major surface from which light is emitted and a second major
surface which is opposite to the first major surface. The
light-generating semiconductor region is constituted of a plurality
of compound semiconductor layers in lamination including a
gallium-containing compound semiconductor layer which is exposed at
the second major surface of the semiconductor region. The invention
particularly features an ohmic contact region held in ohmic contact
with at least part of the second major surface of the
light-generating semiconductor region. The ohmic contact region is
made from a mixture of at least two metals including gallium and is
pervious to the light generated by the light-generating
semiconductor region. Also included is a reflective layer of
electrically conducting material held against at least either of
the ohmic contact region and that part, if any, of the
gallium-containing compound semiconductor layer of the
light-generating semiconductor region which is exposed at the
second major surface of the semiconductor region through the ohmic
contact region, for reflecting the light from the semiconductor
region back toward the semiconductor region for emission from the
first major surface thereof.
[0010] Preferably, the gallium-containing compound semiconductor
layer of the light-generating semiconductor region is made from one
of the following three compound semiconductors and a
conductivity-type determinant:
[0011] (a) a first compound semiconductor that is generally
expressed as Al.sub.xGa.sub.yIn.sub.1-x-yP where the subscript x is
a numeral that is equal to or greater than zero and less than one;
the subscript y is a numeral that is greater than zero and equal to
or less than one; and the sum of x and y is greater than zero and
equal to or less than one;
[0012] (b) a second compound semiconductor that is generally
expressed as Al.sub.xGa.sub.yIn.sub.1-x-yAs where the subscript x
is a numeral that is equal to or greater than zero and less than
one; the subscript y is a numeral that is greater than zero and
equal to or less than one; and the sum of x and y is greater than
zero and equal to or less than one; and
[0013] (c) a third compound semiconductor that is generally
expressed as Al.sub.xGa.sub.yIn.sub.1-x-yN where the subscript x is
a numeral that is equal to or greater than zero and less than one;
the subscript y is a numeral that is greater than zero and equal to
or less than one;
[0014] and the sum of x and y is greater than zero and equal to or
less than one.
[0015] Preferably, the ohmic contact region is made, by the method
of this invention to be summarized subsequently, from a mixture or
alloy of gallium and gold to a thickness of from about 20 to about
1000 angstroms. Further the ohmic contact region is divided into an
array of discrete regions, which are embedded in the second major
surface of the light-generating semiconductor region. Thus the
discrete ohmic contact regions substantially "cover" parts of the
second major surface of the light-generating semiconductor region
and leave the rest of the surface exposed. The reflective layer is
held against both the ohmic contact regions and the exposed part of
the second major surface of the light-generating semiconductor
region. The reflective layer may be made from aluminum or like
metal for higher reflectivity than the ohmic contact regions.
[0016] Made from a mixture or alloy of gallium and gold in
particular, the ohmic contact regions according to the invention
are far less absorptive of light than the prior art
gold-germanium-gallium ohmic contact regions. The less absorptive
ohmic contact regions permit, of course, a correspondingly higher
percentage of the light from the light-generating semiconductor
region to pass therethrough for reflection by the reflective layer
back toward the light-emitting first major surface of the
semiconductor region. Thus, for a given voltage applied, the device
will emit light of greater intensity than heretofore.
[0017] Conversely, for a given output light intensity, the ohmic
contact region or regions may be greater in surface area with
respect to the area of the second major surface of the
semiconductor region, to an extent corresponding to the increased
amount of light reflected back through the ohmic contact regions.
That is to say that the same output light intensity is obtainable
if the ohmic contact region or regions are made greater than
hitherto. Such larger ohmic contact region or regions lead to less
resistance to current flow through the device, to less forward
voltage, to less power loss, and hence to higher efficiency of
light production.
[0018] It is also recommended that the ohmic contact region or
regions be made as aforesaid from about 20 to about 1000 angstroms
in thickness. Made so thin, the ohmic contact region or regions
will absorb even less light and permit an even higher proportion of
the incoming light to be reflected back toward the light-emitting
surface of the device.
[0019] Another aspect of the invention pertains to a method of
making the light-emitting semiconductor device of the above
summarized construction, with a particular emphasis on how to
create the ohmic contact region or regions. There is first prepared
the light-generating semiconductor region including the
gallium-containing compound semiconductor layer which is exposed at
the second major surface of the semiconductor region. By way of
preparation for creation of the ohmic contact region or regions in
this second major surface of the semiconductor region, a transition
metal layer is formed on at least part or parts of the second major
surface. Then, on this transition metal layer, a diffusible metal
layer is formed which contains a metal, preferably gold, that can
be thermally diffused into the gallium-containing compound
semiconductor layer of the light-generating semiconductor region
through the transition metal layer. The desired ohmic contact
region or regions are created in the gallium-containing compound
semiconductor layer of the light-generating semiconductor region by
the thermal diffusion of gold or the like as the article is
subsequently heated to a temperature less than the eutectic point
of the metals concerned, which are gallium and gold in the
illustrated embodiments. Then the transition metal layer and
diffusible metal layer are both removed from the light-generating
semiconductor region. Then the reflective layer is created on at
least either of the ohmic contact region or regions and that part,
if any, of the gallium-containing compound semiconductor layer of
the light-generating semiconductor region which is exposed at the
second major surface of the semiconductor region through the ohmic
contact region or regions.
[0020] Particular attention may be paid to the transition metal
layer through which gold is diffused into the gallium-containing
compound semiconductor layer of the light-generating semiconductor
region. The transition metal is capable both of solid-phase
decomposition of the compound semiconductor into the individual
elements and of cleansing the semiconductor surface. These inherent
capabilities of the transition metal enable solid-phase gold
diffusion into the gallium-containing compound semiconductor layer
at a temperature as low as below the eutectic point of gallium and
gold. Formed in this manner by low-temperature solid-phase
diffusion, the ohmic contact region or regions are of minimal
thickness and are highly favorable in surface morphology, besides
being free from germanium or other metal that interferes with the
passage of light therethrough.
[0021] The above and other objects, features and advantages of this
invention will become more apparent, and the invention itself will
best be understood, from a study of the following description and
appended claims, with reference had to the attached drawings
showing the preferred embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a cross section through an LED embodying the
principles of this invention.
[0023] FIG. 2 is a transverse section through the LED, taken along
the line II-II in FIG. 1.
[0024] FIG. 3 is a cross section through the light-generating
semiconductor region of the LED, shown by way of a first step for
fabricating the LED of FIG. 1 by the method of this invention.
[0025] FIG. 4 is a view similar to FIG. 3 but additionally showing
a transition metal layer and diffusible metal layer formed in
discrete regions on the light-generating semiconductor region.
[0026] FIG. 5 is a view similar to FIG. 4 but additionally showing
the ohmic contact regions formed in the light-generating
semiconductor region by thermal diffusion of the diffusible metal
through the transition metal layer.
[0027] FIG. 6 is a view similar to FIG. 5 except that the
transition metal layer and diffusible metal layer of FIGS. 4 and 5
are not shown because they have been removed upon creation of the
ohmic contact regions as in FIG. 5.
[0028] FIG. 7 is a view similar to FIG. 6 but additionally showing
the reflective layer subsequently formed on the FIG. 6 article.
[0029] FIG. 8 is a view similar to FIG. 7 but additionally showing
the electroconductive silicon baseplate bonded to the FIG. 7
article.
[0030] FIG. 9 is a graph plotting the relationship between the
reflectivity of the LED according to the invention in comparison
with that according to the prior art.
[0031] FIG. 10 is a view similar to FIG. 1 but showing an alternate
form of LED according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0032] The present invention is believed to be best embodied in the
LED shown completed in FIGS. 1 and 2 and in successive states of
fabrication in FIGS. 3-8. Generally designated 1 in FIG. 1, the
representative LED broadly comprises:
[0033] 1. A light-generating semiconductor region 2 where light is
produced and which is constituted of several layers in lamination
to be set forth shortly.
[0034] 2. An anode 3 of open-worked or meshed design on a first 15
of the pair of opposite major surfaces of the semiconductor region
2, the first major surface 15 being hereinafter referred to as the
top surface of the semiconductor region.
[0035] 3. An array of isolated ohmic contact regions 4, to which
the present invention is specifically directed, on the other major
surface 16, hereinafter referred to as the bottom surface, of the
semiconductor region 2.
[0036] 4. A reflective layer 5 covering the whole array of ohmic
contact regions 4 as well as that part of the bottom surface of the
semiconductor region 2 which is left exposed through the ohmic
contact regions.
[0037] 5. An electroconductive silicon baseplate 8 joined to the
underside of the reflective layer 5 via bonding metal layers 6 and
7.
[0038] 6. A cathode 9 underlying the electroconductive silicon
baseplate 8.
[0039] 7. A current blocking layer 10 formed centrally on the top
surface of the semiconductor region 2 in underlying relationship to
part of the anode 3.
[0040] The light-generating semiconductor region 2 as the
semiconductor body is shown as a lamination of an n-type
semiconductor lower confining layer or cladding 11, an active layer
12, a p-type semiconductor upper confining layer or cladding 13,
and a current spreading layer 14 of a p-type compound
semiconductor, all grown by epitaxy in that order on a substrate,
not shown, of any known or suitable composition. The substrate for
growing the layers 11-14 of the light-generating semiconductor
region 2 is not shown because it is removed following the
fabrication of all these layers and is absent from the completed
LED 1. The current spreading layer 14 provides the above defined
top surface 15 of the semiconductor region 2 whereas the n-type
lower cladding 11 provides the bottom surface 16 of the
semiconductor region. Generated in the active layer 12, the light
traverses the upper cladding 13 and current spreading layer 14 and
issues from the top surface 15 of the semiconductor region 2.
[0041] The n-type lower cladding 11 of the light-generating
semiconductor region 2 is made from, in addition to an n-type
dopant such as silicon, any of gallium-containing III-V compound
semiconductors that are generally defined as:
Al.sub.xGa.sub.yIn.sub.1-x-yP
[0042] where the subscript x is a numeral that is equal to or
greater than zero and less than one; the subscript y is a numeral
that is greater than zero and equal to or less than one; and the
sum of x and y is greater than zero and equal to or less than one.
Preferably, the aluminum proportion x is from about 0.15 to about
0.45, and most desirably from about 0.2 to about 0.4. The gallium
proportion y is preferably from about 0.15 to about 0.35, and most
desirably from about 0.4 to about 0.6. The n-type impurity may be
added to this lower cladding 11 to the concentration of
5.times.10.sup.17 cm.sup.-3 or more. The gallium content of this
lower cladding 11 is conducive to the formation of the ohmic
contact regions 4 of alloyed or intermingled gallium and gold, as
will be later explained in more detail. As is well known, the lower
cladding 11 has a greater bandgap than does the active layer
12.
[0043] As required or desired, an n-type contact layer of Groups
III-V compound semiconductor material might be formed under the
n-type lower cladding. The semiconductor materials for this n-type
contact layer can also be expressed as
Al.sub.xGa.sub.yIn.sub.1-x-yP.
[0044] The active layer 12 of the light-generating semiconductor
region 2 is made from any of p-type Group III-V compound
semiconductors that are generally defined as:
Al.sub.xGu.sub.yIn.sub.1-x-yP
[0045] where the subscripts x and y are both numerals that are
equal to or greater than zero and equal to or less than one, and
the sum of x and y is equal to or greater than zero and equal to or
less than one. Preferably, the aluminum proportion x is not less
than about 0.1.
[0046] No conductivity determinant is added to the active layer 12
in this embodiment of the invention. In practice, however, the
active layer 12 may be doped with a p-type determinant to a
concentration less than that of the p-type upper cladding 13, or
with an n-type determinant to a concentration less than that of the
n-type lower cladding 11. The showing of the single active layer 12
in FIG. 1 is for the sake of simplicity only; in practice, it may
take the form of either multiple or single quantum well
configuration which is per se well known in the art.
[0047] The p-type upper cladding 13 overlying the active layer 12
may be fabricated from any of the p-type Group III-V compound
semiconductors that are generally defined as:
Al.sub.xGa.sub.yIn.sub.1-x-yP
[0048] where the subscripts x and y are both numerals that are
equal to or greater than zero and equal to or less than one, and
the sum of x and y is equal to or greater than zero and equal to or
less than one. Preferably, the aluminum proportion x of the upper
cladding 13 is from about 0.15 to about 0.45. The p-type impurity
(e.g. zinc) of the upper cladding 13 is not less than
5.times.10.sup.17 cm.sup.-3. As is well known, the bandgap of this
upper cladding 13 is greater than that of the active layer 12.
[0049] The current spreading layer 14 on the upper cladding 13 is
designed to serve the triple purpose of enhancing uniformity in the
distribution of the forward current flowing through the
light-generating semiconductor region 2, assuring ohmic contact
with the anode 3, and permitting unimpeded passage of the light
therethrough for emission from the LED. The current spreading layer
14 can be made from any such p-type Group III-V compound
semiconductor as GaP, Ga.sub.xIn.sub.1-xP, or Al.sub.xGa.sub.1-xAs.
The p-type impurity concentration of this current spreading layer
14 is made higher than that of the upper cladding 13. A p-type
contact layer could be laid over the current spreading layer 14 for
better ohmic contact with the anode 3.
[0050] Arranged centrally on the top surface 15 of the
semiconductor region 2 in underlying relationship to part of the
anode 3, the current blocking layer 10 is made from an electrically
insulating material in order to preclude concentrated current flow
through the underlying central part of the semiconductor
region.
[0051] The open-worked anode 3 as the first electrode covers the
entire top surface 15 of the semiconductor region 2 as well as the
current blocking layer 10 formed thereon, making ohmic contact with
the current spreading layer 14. In practice the anode 3 may be a
lamination of a chromium and a gold layer. As seen in a direction
normal to the top surface 15 of the semiconductor region 2, the
anode 3 is meshed or latticed in shape in order to permit emission
of the light therethrough and to cause uniform flow of forward
current through the entire semiconductor region. A transparent
material might be employed for the anode 3.
[0052] The ohmic contact regions 11 are made from a mixture of at
least two metals including gallium and are pervious to the light
generated by the light-generating semiconductor region 2. FIG. 2
better reveals the array of discrete ohmic contact regions 4
embedded in the bottom surface 16 of the semiconductor region 2, or
of its n-type lower cladding 11. It is therefore both ohmic contact
regions 4 and lower cladding 11 that define the bottom surface 16
of the semiconductor region 2. Made practically solely from a
mixture or alloy of gallium and gold to a thickness of
approximately 20 to 1000 angstroms, the ohmic contact regions 4
make ohmic contact with both lower cladding 11 and underlying
reflective layer 5. The ohmic contact regions 4 would not make
proper ohmic contact with the neighboring layers if less than about
20 angstroms thick, and would not be sufficiently pervious to light
if more than about 1000 angstroms thick.
[0053] Made from a mixture of gold and gallium, the ohmic contact
regions 4 will be less absorptive of light than the prior art
gold-germanium-gallium alloy regions on the underside of the
light-generating semiconductor region of the LED according to
Japanese Unexamined Patent Publication No. 2002-217450, supra.
Further, if made from an alloy of gold and gallium, the ohmic
contact regions 4 are more permeable to light than are the same
prior art gold-germanium-gallium alloy regions. These prior art
alloy regions contain light-blocking germanium and are as thick as
2000 angstroms or more, blocking or absorbing an inconveniently
high proportion of the light impinging thereon. By contrast the
ohmic contact regions 4 according to the invention do not contain
germanium and are as thin as about 20 to 1000 angstroms, permitting
the permeation of a much greater proportion of light.
[0054] The reflective layer 5, covering the surfaces of the ohmic
contact regions 4 and that of the n-type lower cladding 11 exposed
therethrough, is higher in reflectivity than the ohmic contact
regions 4. Part of the light that has been radiated from the active
layer 12 toward the bottom surface 16 of the semiconductor region 2
is reflected by the surface of the reflective layer 5 that is
exposed through the ohmic contact regions 4, back toward the top
surface 15 of the semiconductor region 2. The rest of the light
that has been radiated toward the semiconductor region bottom
surface 16 is in part reflected by the surfaces of the ohmic
contact regions 4 and in part, after traversing these ohmic contact
regions, reflected by the underlying surface portions of the
reflective layer 5, both again back toward the semiconductor region
top surface 15.
[0055] In this embodiment of the invention the total reflectivity
of the gold-gallium ohmic contact regions 4 and the underlying
parts of the reflective layer 5 is approximately 60 percent, which
is twice as much as that of the prior art gold-germanium-gallium
alloy regions and the underlying parts of the reflective layer.
[0056] The reflective layer 5 has the bonding metal layer 6 of gold
formed on its complete underside. The other bonding metal layer 7,
which also is of gold, is formed on the complete top surface of the
electro-conductive silicon baseplate 8. This baseplate is bonded to
the underside of the reflective layer 5 as the two bonding metal
layers 6 and 7 are joined to each other under heat and pressure.
Made from silicon doped with impurifies, the baseplate 8 serves as
a mechanical support of the LED, in addifion to as a heat radiator
and current path. The baseplate 8 is different from the substrate,
not shown, that is used for growing thereon the constituent layers
11-14 of the light-generating semiconductor region 2, as will be
better understood from the subsequent disclosure of the method of
manufacturing this LED 1.
[0057] The cathode 9 as the second electrode is formed on the
entire bottom surface of the baseplate 8. This cathode will be
unnecessary, however, if a metal-made baseplate is employed in
place of the silicon baseplate, as the metal-made baseplate will
perform the function of a cathode as well.
Method of Fabrication
[0058] The fabrication of the LED 1 shown in FIGS. 1 and 2 started
with the preparation of a gallium arsenide substrate, not shown.
The semiconductor region 2 was formed on this GaAs substrate by
successively growing by metal organic chemical vapor deposition
(MOCVD) the n-type lower cladding 11, active layer 12, p-type upper
cladding 13, and current spreading layer 14. Then the gallium
arsenide substrate was removed from under the completed
semiconductor region 2. FIG. 3 shows the thus completed
semiconductor region 2.
[0059] Then, for creation of the array of isolated ohmic contact
regions 4, FIGS. 1 and 2, there were first formed on the entire
bottom surface 16 of the semiconductor region 2 a transition metal
layer of chromium (Cr) and, thereon, a diffusible meal layer of
gold, both by vacuum deposition. Then an etchant-resist mask was
laid over the diffusible meal layer and transition metal layer by
known photolithography, and these layers were selectively etched
away through the etchant-resist mask thereby leaving an array of
isolated regions of transition metal layer 17, FIG. 4, and of
diffusible metal layer 18 on the now exposed bottom surface 16 of
the semiconductor region 2. The transition metal layer 17 could be
from about 10 to 500 angstroms thick, and the diffusible metal
layer 18 from about 200 to 10,000 angstroms thick.
[0060] Speaking more broadly, the transition metal layer could be
any of:
[0061] 1. A layer of at least either of Ti, Ni, Sc, V, Mn, Fe, Co,
Cu, Zn, and Be in addition to Cr;
[0062] 2. A lamination of an Au sublayer, Cr sublayer, and another
Au sublayer;
[0063] 3. A lamination of a Cr sublayer, Ni sublayer, and Au
sublayer; and
[0064] 4. A lamination of a Cr sublayer, AuSi sublayer, and Au
sublayer.
[0065] Again speaking broadly, the diffusible metal layer could be
any of the following in addition to the exemplified layer of gold
only:
[0066] 1. A lamination of an Au sublayer, Cr sublayer, and another
Au sublayer;
[0067] 2. A lamination of a Cr sublayer, Ni sublayer, and Au
sublayer; and
[0068] 3. A lamination of a Cr sublayer, AuSi sublayer, and Au
sublayer.
[0069] There is an alternative method of creating the array of
isolated regions of transition metal layer 17 and diffusible metal
layer 18 as in FIG. 4. The bottom surface 16 of the semiconductor
region 2 may first be covered with a mask having an array of
windows formed therein. Then the transition metal layer 17 and
diffusible metal layer 18 may be formed one after the other by
vacuum deposition on the mask as well as on those parts of the
semiconductor region bottom surface 16 which are exposed through
the windows in the mask. Then the mask may be removed together with
the overlying parts of the transition metal layer 17 and diffusible
metal layer 18 thereby leaving an array of isolated regions of
layers 17 and 18 on the semiconductor region bottom surface 16 as
in FIG. 4.
[0070] Then the FIG. 4 article was heated to a temperature (e.g.
300.degree. C.) that is lower than the eutectic temperature
(345.degree. C.) of gallium in the n-type lower cladding 11 and
gold in the diffusible metal regions 18 and at which gold (or some
other diffusible metal in cases where such a metal is employed in
lieu of gold) can be diffused into the lower cladding with the aid
of the transition metal regions 17. The desired ohmic contact
regions 4 of a gallium-and-gold mixture were thus created as in
FIG. 5 as the gold was diffused into the lower cladding 11 through
the transition metal layer 17.
[0071] The heat treatment above may be effected at such a
temperature, and for such a period of time, that the ohmic contact
regions 4 may be formed to a thickness ranging from about 20 to
about 1000 angstroms. The temperature in particular may be so
determined as to create the ohmic contact regions 4 that are
unvarying in thickness and low in resistance and that make good
ohmic contact with the lower cladding 11.
[0072] Tests were conducted in order to ascertain the reflectivity
of the Au--Ga ohmic contact regions 4 according to the invention in
comparison with that of the noted prior art Au--Ge--Ga ohmic
contact regions. The curve A in the graph of FIG. 9 plots the total
reflectivity of the Au--Ga ohmic contact regions 4 and the
underlying parts of the reflective layer 5 according to the
invention against the temperature of the heat treatment. The curve
B in the same graph plots the total reflectivity of the prior art
Au--Ge--Ga ohmic contact relations and the underlying parts of the
reflective layer against the temperature of the heat treatment for
creation of the ohmic contact regions. The reflectivities were
measured for red light with a wavelength of 650 nanometers.
[0073] The reflectivity at 300.degree. C. according to the
invention is approximately 30 percent, twice as high as that
according to the prior art. It will also be observed from this
graph that the lower the temperature of the heat treatment, the
higher is the reflectivity. But the contact resistance between the
ohmic contact regions and the lower cladding 11 grows
inconveniently high if the temperature of the heat treatment is too
low. The temperature of the heat treatment should therefore be from
about 250.degree. to about 340.degree., preferably from about
290.degree. to about 330.degree. C., in order to keep the contact
resistance not more than 2.times.10.sup.-4 ohm-cm.sup.2.
[0074] The transition metal regions 17, FIGS. 4 and 5, are
effective both to decompose the AlGaInP of the lower cladding 11
into the individual elements, imparting greater mobility thereto,
and to cleanse the surface of the lower cladding. These functions
of the transition metal regions 17 are conducive to gold diffusion
into the lower cladding 11 at a temperature less than the eutectic
point of gallium and gold, with the consequent creation of the
extremely thin ohmic contact regions 4 of a mixture or alloy of
these metals.
[0075] Then the transition metal regions 17 and diffusible metal
regions 18 were etched away, leaving the ohmic contact regions 4
which were embedded in the lower cladding 11. FIG. 6 shows the
resulting semiconductor region 2 with the ohmic contact regions 4.
Fabricated by the above described method, the Au--Ga ohmic contact
regions 4 according to the was far better in surface morphology
than the prior art Au--Ge--Ga regions. The bottom surface 16 of the
semiconductor region 2, including the exposed surfaces of the ohmic
contact regions 4, was therefore flatter than the corresponding
surface of the prior art.
[0076] Then, on this bottom surface 16 of the semiconductor region
2, aluminum was vacuum deposited to a thickness of one to 10
micrometers, and the deposit was heated with an infrared lamp,
thereby completing the reflective layer 5 shown in FIG. 7. The
electroconductive reflective layer 5 makes ohmic contact with the
exposed surfaces of the ohmic contact regions 4. Further, as the
reflective layer 5 makes Schottky contact with the exposed surface
of the lower cladding 11, the forward current of the LED 1 does not
flow from the lower cladding to the reflective layer. The
reflective layer 5 was highly favorable in flatness thanks to the
improved surface morphology of the ohmic contact regions 4.
[0077] Then, as shown also in FIG. 7, the bonding metal layer 6 was
formed on the surface of the reflective layer 5 by vacuum
deposition of gold.
[0078] There was separately prepared the baseplate 8 of doped
silicon which, as has been stated, is to function as a mechanical
support for the other constituent parts of the LED. The bonding
metal layer 7 of gold was formed on one of the opposite major
surfaces of the baseplate 8 by vacuum deposition. This layer 7 was
held under pressure against the bonding metal layer 6 on the
underside of the reflective layer 5, and both layers 6 and 7 were
heated to a temperature not exceeding 300.degree. C. thereby
causing both layers to unite with each other by the mutual welding
of gold. The baseplate 8 was thus integrally joined to the
reflective layer 5.
[0079] Then, referring back to FIG. 1, the current blocking layer
10 and anode 3 were conventionally formed on the top surface 15 of
the semiconductor region 2. The cathode 9 was formed on the
underside of the baseplate 8. Thus was completed the fabrication of
the LED 1.
[0080] The advantages gained by this particular embodiment of the
invention may be recapitulated as follows:
[0081] 1. The ohmic contact regions 4 and the underlying parts of
the reflective layer 5 are as high in total reflectivity as 60
percent thanks to the absence of light-absorbing germanium from the
ohmic contact regions and to their reduced thickness. A much
greater proportion than heretofore of the light that has been
radiated from the active layer 12 toward the reflective layer 5 is
sent back toward the light-emitting surface 15 of the LED,
resulting in a significantly higher efficiency of light production
per unit of electric power consumed.
[0082] 2. The ohmic contact regions 4 can be made larger than their
conventional counterparts for a given area of the bottom surface 16
of the light-generating semiconductor region 2 and for a given
optical output by reason of the improved total reflectivity of the
ohmic contact regions and the underlying parts of the reflective
layer 5. Such larger ohmic contact regions lead to less forward
resistance, less forward voltage drop and power loss, and higher
efficiency. The maximum efficiency of the red light emitting diode
1 was 47 lumens per watt at a current density of 40 amperes per
square centimeter.
[0083] 3. The ohmic contact regions 4 of gold and gallium are
capable of creation at a temperature less than their eutectic point
by thermal diffusion of gold from the diffusible metal regions 18
into the n-type lower cladding 11 via the transition metal regions
17.
[0084] 4. The electroconductive silicon baseplate 8 can be firmly
bonded to the reflective layer 5 by virtue of the improved surface
morphology of the ohmic contact regions 4.
Embodiment of FIG. 10
[0085] Another preferred form of LED 1.sub.a shown in FIG. 1 has an
integral ohmic contact region 4.sub.a covering the entire bottom
surface 16 of the lower cladding 11. A relatively high efficiency
is nevertheless obtainable because the total reflectivity of the
ohmic contact region 4.sub.a and reflective layer 5 is as high as
60 percent. As the ohmic contact region 4.sub.a is larger than all
the isolated ohmic contact regions 4 of FIGS. 1 and 2 combined, so
much is reduced the resistance to forward current flow, with a
corresponding diminution of power loss.
[0086] Another feature of the alternative LED 1.sub.a resides in a
metal-made baseplate 8.sub.a which is affixed to the reflective
layer 5 under heat and pressure in place of the silicon baseplate 8
of the previous embodiment. No dedicated cathode is provided as the
baseplate 8.sub.a serves as both mechanical support and
cathode.
[0087] The alternative LED 1.sub.a is akin to the first disclosed
LED 1 in all the other details of construction. The integrated
ohmic contact regions 4a are created by the same method, and with
the same composition and to the same thickness, as their FIGS. 1
and 2 counterpart 4, so that the LED 1.sub.a offers substantially
the same advantages as the LED 1.
Possible Modifications
[0088] Notwithstanding the foregoing detailed disclosure it is not
desired that the present invention be limited by the exact showings
of the drawings or the description thereof. The following is a
brief list of possible modifications, alterations or adaptations of
the illustrated embodiments of the invention which are all believed
to fall within the purview of the claims annexed hereto:
[0089] 1. The silicon baseplate 8, FIG. 1, and metal-made baseplate
8a, FIG. 10, may both be omitted if the light-generating
semiconductor region is sufficiently sturdy and self-supporting.
The electroconductive reflective layer 5 will then serve also as
cathode, so that the dedicated cathode 9 may also be omitted from
the FIG. 1 construction.
[0090] 2. The ohmic contact regions 4 need not be rectangular as in
FIG. 2 but may be circular or otherwise in shape. They need not be
separated into discrete units, either, but may be latticed or
otherwise joined to one another, besides being wholly combined into
a single, closed layer as indicated at 4.sub.a in FIG. 10.
[0091] 3. An n-type contact layer of AlGaInP and/or n-type buffer
layer of AlGaInP could be interposed between the n-type lower
cladding 11 and the reflective layer, and the ohmic contact regions
4 or region 4.sub.a could be formed in contact therewith instead of
with the lower cladding.
[0092] 4. The ohmic contact regions 4 or region 4.sub.a may be made
from Au--Ge--Ga alloy or some such material other than Au--Ga alloy
or mixture, provided that the resulting ohmic contact region or
regions are permeable to the light generated by the LED. The total
reflectivity of such ohmic contact region or regions and the
reflective layer will be raised to a satisfactory level if the
thickness of the ohmic contact region or regions are limited to the
range of from about 20 to about 1000 angstroms.
[0093] 5. The diffusible metal regions 18, FIGS. 4 and 5, could be
of a gold-based alloy or Ga-based alloy.
[0094] 6. The reflective layer 5 of the LED 1, FIG. 1, could be
provided only under the ohmic contact regions 4 or under the
exposed surface portions of the lower cladding 11.
[0095] 7. The active layer 12 can be made from any such III-V
compound semiconductor as Al.sub.xGa.sub.yIn.sub.1-x-yAs or
Al.sub.xGa.sub.yIn.sub.1-x-yN. Also, the p-type upper cladding 13
can be made from any such III-V compound semiconductor as
Al.sub.xGa.sub.yIn.sub.1-x-yAs or
Al.sub.xGa.sub.yIn.sub.1-x-yN.
* * * * *