U.S. patent application number 11/080509 was filed with the patent office on 2006-03-02 for reflective electrode and compound semiconductor light emitting device including the same.
This patent application is currently assigned to Samsung Electro-mechanics Co., Ltd.. Invention is credited to Joon-seop Kwak, Tae-yeon Seong, June-o Song.
Application Number | 20060043388 11/080509 |
Document ID | / |
Family ID | 36139625 |
Filed Date | 2006-03-02 |
United States Patent
Application |
20060043388 |
Kind Code |
A1 |
Kwak; Joon-seop ; et
al. |
March 2, 2006 |
Reflective electrode and compound semiconductor light emitting
device including the same
Abstract
Provided are a reflective electrode and a compound semiconductor
light emitting device, such as an LED or an LD, including the same.
The reflective electrode, which is formed on a p-type compound
semiconductor layer, includes: a first electrode layer forming an
ohmic contact with the p-type compound semiconductor layer; a
second electrode layer disposed on the first electrode layer and
formed of transparent conductive oxide; and a third electrode layer
disposed on the second electrode layer and formed of an optical
reflective material.
Inventors: |
Kwak; Joon-seop;
(Gyeonggi-do, KR) ; Seong; Tae-yeon; (Gwangju-si,
KR) ; Song; June-o; (Gwangju-si, KR) |
Correspondence
Address: |
BUCHANAN INGERSOLL PC;(INCLUDING BURNS, DOANE, SWECKER & MATHIS)
POST OFFICE BOX 1404
ALEXANDRIA
VA
22313-1404
US
|
Assignee: |
Samsung Electro-mechanics Co.,
Ltd.
Gyeonggi-do
KR
Gwangju Institute of Science and Technology
Gwangju-si
KR
|
Family ID: |
36139625 |
Appl. No.: |
11/080509 |
Filed: |
March 16, 2005 |
Current U.S.
Class: |
257/80 ;
257/E33.068 |
Current CPC
Class: |
H01S 5/04257 20190801;
H01S 5/18305 20130101; H01S 5/04253 20190801; H01S 5/183 20130101;
H01L 33/405 20130101; H01L 33/32 20130101; H01S 5/18375
20130101 |
Class at
Publication: |
257/080 |
International
Class: |
H01L 33/00 20060101
H01L033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 31, 2004 |
KR |
10-2004-0069151 |
Claims
1. A reflective electrode of a compound semiconductor light
emitting device, which is formed on a p-type compound semiconductor
layer, the electrode comprising: a first electrode layer forming an
ohmic contact with the p-type compound semiconductor layer; a
second electrode layer disposed on the first electrode layer and
formed of transparent conductive oxide; and a third electrode layer
disposed on the second electrode layer and formed of an optical
reflective material.
2. The electrode of claim 1, wherein the first electrode layer is
formed of indium oxide to which at least an additive element
selected from the group consisting of Mg, Cu, Zr, and Sb is
added.
3. The electrode of claim 2, wherein an addition ratio of the
additive element to the indium oxide is in the range of 0.001 to 49
atomic percent.
4. The electrode of claim 2, wherein the thickness of the first
electrode layer ranges from 0.1 to 500 nm.
5. The electrode of claim 1, wherein the first electrode layer is
formed of Ag and an Ag-based alloy.
6. The electrode of claim 5, wherein the Ag-based alloy is an alloy
of Ag and at least one selected from the group consisting of Mg,
Zn, Sc, Hf, Zr, Te, Se, Ta, W, Nb, Cu, Si, Ni, Co, Mo, Cr, Mn, Hg,
Pr, and La.
7. The electrode of claim 5, wherein the thickness of the first
electrode layer ranges from 0.1 to 500 nm.
8. The electrode of claim 1, wherein the transparent conductive
oxide is formed of one selected from the group consisting of ITO,
ZITO, ZIO, GIO, ZTO, FTO, AZO, GZO, In.sub.4Sn.sub.3O.sub.12, and
Zn.sub.1-xMg.sub.xO (0.ltoreq.x.ltoreq.1).
9. The electrode of claim 8, wherein the thickness of the second
electrode layer ranges from 0.1 to 500 nm.
10. The electrode of claim 1, wherein the optical reflective
material is one selected from the group consisting of Ag, an
Ag-based alloy, Al, an Al-based alloy, and Rh.
11. The electrode of claim 10, wherein the thickness of the third
electrode layer ranges from 10 to 5000 nm.
12. The electrode of claim 1, further comprising a fourth electrode
layer formed on the third electrode layer using a predetermined
material to prevent agglomeration caused by an annealing process
from occurring on the surface of the third electrode layer.
13. The electrode of claim 12, wherein the fourth electrode layer
is formed of one selected from the group consisting of Cu, Cu/Ru,
Cu/Ir, a Cu-based alloy, Cu-based alloy/Ru, and Cu-based
alloy/Ir.
14. The electrode of claim 13, wherein the thickness of the fourth
electrode layer ranges from 1 to 500 nm.
15. A compound semiconductor light emitting device comprising an
n-type electrode, a p-type electrode, and an n-type compound
semiconductor layer, an active layer, and a p-type compound
semiconductor layer, which are interposed between the n-type
electrode and the p-type electrode, wherein the p-type electrode
comprises: a first electrode layer forming an ohmic contact with
the p-type compound semiconductor layer; a second electrode layer
disposed on the first electrode layer and formed of transparent
conductive oxide; and a third electrode layer disposed on the
second electrode layer and formed of an optical reflective
material.
16. The device of claim 15, wherein the first electrode layer is
formed of indium oxide to which at least an additive element
selected from the group consisting of Mg, Cu, Zr, and Sb is
added.
17. The device of claim 16, wherein an addition ratio of the
additive element to the indium oxide is in the range of 0.001 to 49
atomic percent.
18. The device of claim 16, wherein the thickness of the first
electrode layer ranges from 0.1 to 500 nm.
19. The device of claim 15, wherein the first electrode layer is
formed of one of Ag and an Ag-based alloy.
20. The device of claim 19, wherein the Ag-based alloy is an alloy
of Ag and at least one selected from the group consisting of Mg,
Zn, Sc, Hf, Zr, Te, Se, Ta, W, Nb, Cu, Si, Ni, Co, Mo, Cr, Mn, Hg,
Pr, and La.
21. The device of claim 19, wherein the thickness of the first
electrode layer ranges from 0.1 to 500 nm.
22. The device of claim 15, wherein the transparent conductive
oxide is formed of one selected from the group consisting of ITO,
ZITO, ZIO, GIO, ZTO, FTO, AZO, GZO, In.sub.4Sn.sub.3O.sub.12, and
Zn.sub.1-xMg.sub.xO (0.ltoreq.x.ltoreq.1).
23. The device of claim 22, wherein the thickness of the second
electrode layer ranges from 0.1 to 500 nm.
24. The device of claim 15, wherein the optical reflective material
is one selected from the group consisting of Ag, an Ag-based alloy,
Al, an Al-based alloy, ad Rh.
25. The device of claim 24, wherein the thickness of the third
electrode layer ranges from 10 to 5000 nm.
26. The device of claim 15, further comprising a fourth electrode
layer formed on the third electrode layer using a predetermined
material to prevent agglomeration caused by an annealing process
from occurring on the surface of the third electrode layer.
27. The device of claim 26, wherein the fourth electrode layer is
formed of one selected from the group consisting of Cu, Cu/Ru,
Cu/Ir, a Cu-based alloy, Cu-based alloy/Ru, and Cu-based
alloy/Ir.
28. The device of claim 27, wherein the thickness of the fourth
electrode layer ranges from 1 to 500 nm.
Description
BACKGROUND OF THE INVENTION
[0001] Priority is claimed to Korean Patent Application No.
10-2004-0069151, filed on Aug. 31, 2004 in the Korean Intellectual
Property Office, the disclosure of which is incorporated herein in
its entirety by reference.
[0002] 1. Field of the Invention
[0003] The present invention relates to a reflective electrode and
a compound semiconductor light emitting device, and more
particularly, to a reflective electrode with low contact
resistance, high reflectance, and improved electrical conductivity,
and a compound semiconductor light emitting device including the
same.
[0004] 2. Description of the Related Art
[0005] Compound semiconductor light emitting devices, for example,
semiconductor laser diodes such as light emitting diodes (LEDs) and
laser diodes (LDs), convert electric signals into optical signals
using the characteristics of compound semiconductors. Laser beams
of the compound semiconductor light emitting devices have
practically been applied in the fields of optical communications,
multiple communications, and space communications. Semiconductor
lasers are widely used as light sources for data transmission or
data recording and reading in the field of optical communications
and such apparatuses as compact disk players (CDPs) or digital
versatile disk players (DVDPs).
[0006] A compound semiconductor light emitting device can be
categorized into a top-emitting light emitting diode (TLED) and a
flip-chip light emitting diode (FCLED) according to the emission
direction of light.
[0007] The TLED emits light through a p-type electrode, which forms
an ohmic contact with a p-type compound semiconductor layer. The
p-type electrode includes a Ni layer and an Au layer, which are
sequentially stacked on a p-type compound semiconductor layer.
However, since the p-type electrode formed of the Ni layer and the
Au layer is translucent, the TLED including the p-type electrode
has low optical efficiency and low brightness.
[0008] In the case of the FCLED, light emitted from an active layer
is reflected by a reflective electrode formed on a p-type compound
semiconductor layer, and the reflected light is emitted through a
substrate. The reflective electrode is formed of a material having
good optical reflectance, such as Ag, Al, and Rh. The FCLED
including this reflective electrode can have high optical
efficiency and high brightness. However, owing to a relatively high
contact resistance between the reflective electrode and the p-type
compound semiconductor layer, a light emitting device including the
reflective electrode has a shortened life span and unreliable
characteristics.
[0009] To solve these problems, research on materials and
structures for an electrode having low contact resistance and high
reflectance has progressed.
[0010] International Patent Publication No. WO 01/47038 A1
discloses a semiconductor light emitting device including a
reflective electrode, which is provided with an ohmic contact layer
disposed between the reflective electrode and a p-type compound
semiconductor layer. However, the ohmic contact layer is formed of
a material having low optical transmissivity, such as Ti or Ni/Au,
thus degrading optical efficiency and brightness.
SUMMARY OF THE INVENTION
[0011] Embodiments of the present invention provides a reflective
electrode, which reduces contact resistance and has high
reflectance and improved electrical conductivity, and a compound
semiconductor light emitting device including the same.
[0012] The present invention can be embodied as a reflective
electrode of a compound semiconductor light emitting device, which
is formed on a p-type compound semiconductor layer. The electrode
includes, for example, a first, second and third electrode layer.
The first electrode layer forms an ohmic contact with the p-type
compound semiconductor layer. The second electrode layer is
disposed on the first electrode layer and is formed of transparent
conductive oxide. The third electrode layer disposed on the second
electrode layer and formed of an optical reflective material, in
this embodiment.
[0013] The first electrode layer may be formed of indium oxide to
which at least an additive element selected from the group
consisting of Mg, Cu, Zr, and Sb is added, and an addition ratio of
the additive element to the indium oxide is in the range of 0.001
to 49 atomic percent, for example. The thickness of the first
electrode layer can range from 0.1 to 500 nm.
[0014] Alternatively, the first electrode layer can be formed of Ag
and an Ag-based alloy, and the Ag-based alloy can be an alloy of Ag
and at least one selected from the group consisting of Mg, Zn, Sc,
Hf, Zr, Te, Se, Ta, W, Nb, Cu, Si, Ni, Co, Mo, Cr, Mn, Hg, Pr, and
La. The thickness of the first electrode layer can range from 0.1
to 500 nm, for example.
[0015] The transparent conductive oxide can be formed of a material
selected from the group consisting of ITO, ZITO, ZIO, GIO, ZTO,
FTO, AZO, GZO, In.sub.4Sn.sub.3O.sub.12, and Zn.sub.1-xMg.sub.xO (0
.ltoreq..times..ltoreq.1), and the thickness of the second
electrode layer can range from 0.1 to 500 nm.
[0016] The optical reflective material is one selected from the
group consisting of Ag, an Ag-based alloy, Al, an Al-based alloy,
and Rh, and the thickness of the third electrode layer can range
from 10 to 5000 nm, for example.
[0017] An optional fourth electrode layer can be formed on the
third electrode layer using a predetermined material to prevent
agglomeration caused by an annealing process from occurring on the
surface of the third electrode layer. Examples of the fourth
electrode layer material include one selected from the group
consisting of Cu, Cu/Ru, Cu/Ir, a Cu-based alloy, Cu-based
alloy/Ru, and Cu-based alloy/Ir. The fourth electrode layer ranges
from 1 to 500 nm, for example.
[0018] The present invention can also be embodied in a compound
semiconductor light emitting device, for example. The compound
semiconductor light emitting device includes an n-type electrode, a
p-type electrode, and an n-type compound semiconductor layer, an
active layer, and a p-type compound semiconductor layer, which are
interposed between the n-type electrode and the p-type electrode,
The p-type electrode is structured in accordance with the
above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The above features and advantages of the present invention
will become more apparent by describing in detail exemplary
embodiments thereof with reference to the attached drawings in
which:
[0020] FIG. 1 is a cross-sectional view of a reflective electrode
according to an embodiment of the present invention;
[0021] FIG. 2 is a cross-sectional view of a reflective electrode
according to another embodiment of the present invention;
[0022] FIG. 3 is a cross-sectional view of a compound semiconductor
light emitting device including the reflective electrode shown in
FIG. 1;
[0023] FIG. 4A is a graph showing a current-voltage (I-V)
characteristic of the reflective electrode (Ag/ITO/Ag) shown in
FIG. 1; and
[0024] FIG. 4B is a graph showing an I-V characteristic of an InGaN
blue light emitting diode (LED) including the reflective electrode
(Ag/ITO/Ag) shown in FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
[0025] The present invention will now be described more fully with
reference to the accompanying drawings, in which exemplary
embodiments of the invention are shown.
[0026] FIG. 1 is a cross-sectional view of a reflective electrode
22 according to an embodiment of the present invention.
[0027] Referring to FIG. 1, the reflective electrode 22 is formed
on a p-type compound semiconductor layer 20. The reflective
electrode 22 includes a first electrode layer 22a, a second
electrode layer 22b, and a third electrode layer 22c, which are
sequentially stacked on the p-type compound semiconductor layer
20.
[0028] The first electrode layer 22a is formed of a material, which
can form an ohmic contact with the p-type compound semiconductor
layer 20, to a thickness of about 0.1 to 500 nm.
[0029] In the present embodiment, the first electrode layer 22a is
formed of indium oxide (e.g., In.sub.2O.sub.3) to which at least an
additive element selected from the group consisting of Mg, Cu, Zr,
and Sb is added.
[0030] The additive element controls the band gap, electron
affinity, and work function of the indium oxide, thereby improving
the ohmic contact characteristic of the first electrode layer 22a.
Specifically, the additive element increases the effective carrier
concentration of the p-type compound semiconductor layer 20 and
readily reacts with elements constituting the p-type compound
semiconductor layer 20 except nitrogen.
[0031] For example, when the p-type compound semiconductor layer 20
is formed of a GaN-based compound, the additive element may react
to Ga prior to N. In this case, Ga of the p-type compound
semiconductor layer 20 reacts to the additive element, thus
generating Ga vacancies in the surface of the p-type compound
semiconductor layer 20. As the Ga vacancies function as a p-type
dopant, an effective concentration of p-type carriers in the
surface of p-type compound semiconductor layer 20 increases.
[0032] The indium oxide to which the additive element is added
reacts to a Ga.sub.2O.sub.3 layer, which is a native oxide layer
that remains on the p-type compound semiconductor layer 20, thus
generating a transparent conductive oxide (TCO) between the p-type
compound semiconductor layer 20 and the first electrode layer 22a.
The Ga.sub.2O.sub.3 layer serves as a barrier to the flow of
carriers at an interface between the p-type compound semiconductor
layer 20 and the first electrode layer 22a. Thus, a tunneling
conduction phenomenon may occur at the interface between the first
electrode layer 22a and the p-type compound semiconductor layer 20,
thus improving the ohmic contact characteristic of the first
electrode layer 22a.
[0033] An addition ratio of the additive element to indium oxide is
in the range of 0.001 to 49 atomic percent.
[0034] In another embodiment, the first electrode layer 22a may be
formed of Ag or an Ag-based alloy. The Ag-based alloy is an alloy
of Ag and at least one selected from the group consisting of Mg,
Zn, Sc, Hf, Zr, Te, Se, Ta, W, Nb, Cu, Si, Ni, Co, Mo, Cr, Mn, Hg,
Pr, and La. The Ag or Ag-based alloy may form an ohmic contact with
the p-type compound semiconductor layer 20, as described above.
That is, the Ag and alloy elements, which may form the first
electrode layer 22a, increase the effective carrier concentration
of the p-type compound semiconductor layer 20 and readily react
with elements constituting the p-type compound semiconductor layer
20 except nitrogen. A detailed description thereof will be omitted
here.
[0035] The second electrode layer 22b is formed of TCO to a
thickness of 0.1 to 500 nm. The TCO may be one selected from the
group consisting of indium tin oxide (ITO), zinc-doped indium tin
oxide (ZITO), zinc indium oxide (ZIO), gallium indium oxide (GIO),
zinc tin oxide (ZTO), fluorine-doped tin oxide (FTO),
aluminum-doped zinc oxide (AZO), gallium-doped zinc oxide (GZO),
In.sub.4Sn.sub.3O.sub.12, and zinc magnesium oxide
(Zn.sub.1-xMg.sub.xO, 0.ltoreq..times.23 1). The TCO may be, for
example, Zn.sub.2In.sub.2O.sub.5, GaInO.sub.3, ZnSnO.sub.3, F-doped
SnO.sub.2, Al-doped ZnO, Ga-doped ZnO, MgO, or ZnO.
[0036] The third electrode layer 22c is formed of an optical
reflective material to a thickness of about 10 to 5000 nm. The
optical reflective material is one selected from the group
consisting of Ag, an Ag-based alloy, Al, an Al-based alloy, and Rh.
Here, the Ag-based alloy refers to an alloy of Ag and any alloy
material, and the Al-based alloy refers to an alloy of Al and any
alloy material.
[0037] The first, second, and third electrode layers 22a, 22b, and
22c can be formed using an electronic beam (e-beam) & thermal
evaporator or a dual-type thermal evaporator. Also, the first,
second, and third electrode layers 22a, 22b, and 22c can be formed
by physical vapor deposition (PVD), chemical vapor deposition
(CVD), or plasma laser deposition (PLD). Each of the first, second,
and third electrode layers 22a, 22b, and 22c can be deposited at a
temperature of about 20 to 1500.degree. C. inside a reactor that is
maintained under an atmospheric pressure to 10.sup.-12 Torr.
[0038] After the third electrode layer 22c is formed, the resultant
structure is annealed. Specifically, the resultant structure where
the third electrode layer 22c is formed is annealed in an
atmosphere containing at least one of N, Ar, He, O.sub.2, H.sub.2,
and air. The annealing process is performed at a temperature of
about 200 to 700.degree. C. for 10 seconds to 2 hours.
[0039] Another annealing process may be additionally performed
under the same conditions after the second electrode layer 22b is
formed. That is, after each of the second and third electrode
layers 22b and 22c is formed, an annealing process may be
performed. Thus, the formation of the reflective electrode may
comprise performing an annealing process twice.
[0040] FIG. 2 is a cross-sectional view of a reflective electrode
23 according to another embodiment of the present invention.
[0041] In the present embodiment, only different characteristics
than in the first embodiment will be described, and the same
reference numerals are used to denote the same elements as in the
first embodiment.
[0042] Referring to FIG. 2, the reflective electrode 23 further
includes a fourth electrode layer 22d disposed on a third electrode
layer 22c in comparison with the reflective electrode 22 shown in
FIG. 1.
[0043] The fourth electrode layer 22d is formed of one selected
from the group consisting of Cu, Cu/Ru, Cu/Ir, a Cu-based alloy, a
Cu-based alloy/Ru, and a Cu-based alloy/Ir. The fourth electrode
layer 22d is formed to a thickness of about 1 to 500 nm. Here, the
Cu-based alloy refers to an alloy Cu and any alloy material.
[0044] The fourth electrode layer 22d prevents an agglomeration
caused by an annealing process from occurring on the surface of the
third electrode layer 22c.
[0045] Specifically, there is a great difference in surface energy
between a p-type compound semiconductor layer 20 and a metal
constituting the third electrode layer 22c, for example, Ag, an
Ag-based alloy, Al, an Al-based alloy, or Rh. It is generally known
that the difference in surface energy allows agglomeration to
occur, and this can occur on the surface of the third electrode
layer 22c during the annealing process. When the agglomeration
occurs on the surface of the third electrode layer 22c, the
reflectance of the third electrode layer 22c is degraded, thus
reducing an optical output of a compound semiconductor light
emitting device including the reflective electrode 22.
[0046] In the present embodiment, the material forming the fourth
electrode layer 22d has a relatively similar surface energy to that
of the p-type nitride semiconductor layer 20 and an excellent
electrical conductivity. Thus, the fourth electrode layer 22d
formed on the third electrode layer 22c serves as both an
agglomeration preventing layer (APL) and an electrode layer.
[0047] The fourth electrode layer 22d can be formed by PVD, CVD, or
PLD using an e-beam & thermal evaporator or a dual-type thermal
evaporator. The fourth electrode layer 22d is deposited at a
temperature of about 20 to 1500.degree. C. inside a reactor that is
maintained under an atmospheric pressure to 10.sup.-12 Torr.
[0048] After the fourth electrode layer 22d is formed, the
resultant structure may be annealed. Specifically, the resultant
structure where the fourth electrode layer 22d is formed is
annealed in an atmosphere containing at least one of N, Ar, He,
O.sub.2, H.sub.2, and air. The annealing process is performed at a
temperature of 200 to 700.degree. C. for 10 seconds to 2 hours.
[0049] FIG. 3 is a cross-sectional view of a compound semiconductor
light emitting device including the reflective electrode shown in
FIG. 1.
[0050] Referring to FIG. 3, the compound semiconductor light
emitting device includes at least an n-type compound semiconductor
layer 102, an active layer 104, and a p-type compound semiconductor
layer 106 between an n-type electrode 120 and a p-type electrode
108. The p-type electrode 108 is the same as the reflective
electrode 22 shown in FIG. 1. That is, the p-type electrode 108
includes the first electrode layer 22a, the second electrode layer
22b, and the third electrode layer 22c shown in FIG. 1 of which
operations and effects are the same as described above.
[0051] The n-type compound semiconductor layer 102 includes a first
compound semiconductor layer as a lower contact layer, which is
stacked on a substrate 100 and has a step difference, and a lower
clad layer stacked on the first compound semiconductor layer. The
n-type lower electrode 120 is disposed in a stepped portion of the
first compound semiconductor layer.
[0052] The substrate 200 is typically a sapphire substrate or a
freestanding GaN substrate. The first compound semiconductor layer
may be an n-GaN-based III-V group nitride compound semiconductor
layer, preferably, an n-GaN layer. However, the present invention
is not limited thereto, but the first compound semiconductor layer
may be formed of any other III-V group compound semiconductor that
enables laser oscillation (lasing). The lower clad layer may be an
n-GaN/AlGaN layer having a predetermined refractive index, but it
is possible to use any other compound semiconductor layer that
enables lasing.
[0053] The active layer 104 may be formed of any material that
enables lasing, preferably, a material that can oscillate laser
beams having a small critical current and a stable transverse mode
characteristic. The active layer 104 may be a GaN-based III-V group
nitride compound semiconductor layer, which is InxAlyGa1-x-yN
(0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, and x+y.ltoreq.1). The
active layer 104 may have one of a multiple quantum well (MQW)
structure and a single quantum well (SQW) structure, and the
technical scope of the present invention is not limited by the
structure of the active layer 104.
[0054] An upper waveguide layer and a lower waveguide layer may be
further formed on and under the active layer 104, respectively. The
upper and lower waveguide layers are formed of a material having a
low refractive index, preferably, a GaN-based III-V group compound
semiconductor. The lower waveguide layer may be an n-GaN layer,
while the upper waveguide layer may be a p-GaN layer.
[0055] The p-type compound semiconductor layer 106 is stacked on
the active layer 104 and includes an upper clad layer, which has a
lower refractive index than the active layer 104, and a second
compound semiconductor layer, which is an ohmic contact layer
stacked on the upper clad layer. The second compound semiconductor
layer may be a p-GaN-based III-V group nitride compound
semiconductor layer, preferably, a p-GaN layer. However, the
present invention is not limited thereto, but the second compound
semiconductor layer may be any other III-V group compound
semiconductor layer that enables laser oscillation (lasing). The
upper clad layer may be a p-GaN/AlGaN layer having a predetermined
refractive index, but it is possible to use any other compound
semiconductor layer that enables lasing.
[0056] An n-type electrode 120 is disposed in a stepped portion of
the first compound semiconductor layer, which is a lower ohmic
contact layer. Alternatively, the n-type electrode 120 may be
formed on a bottom surface of the substrate 100 opposite the p-type
electrode 108. In this case, the substrate 100 may be formed of
silicon carbide (SiC) or gallium nitride (GaN).
[0057] FIG. 4A is a graph showing a current-voltage (I-V)
characteristic of the reflective electrode (Ag/ITO/Ag) shown in
FIG. 1.
[0058] The reflective electrode includes a first electrode layer
formed of Ag, a second electrode layer formed of ITO, and a third
electrode layer formed of Ag, which were sequentially stacked on a
substrate. The first, second, and third electrode layers were
formed to a thickness of about 3, 100, and 250 nm,
respectively.
[0059] The electrical characteristics of the reflective electrode
(Ag/ITO/Ag) were measured as deposited and as annealed at
530.degree. C., respectively. The annealing process was performed
in an O.sub.2 or N atmosphere for 1 minute after the second
electrode layer was formed. After the third electrode layer was
formed, an annealing process was additionally performed under the
same conditions.
[0060] FIG. 4B is a graph showing an I-V characteristic of an InGaN
blue light emitting diode (LED) including the reflective electrode
(Ag/ITO/Ag) shown in FIG. 1.
[0061] As can be seen from FIGS. 4A and 4B, the annealed reflective
electrode and the light emitting device including the same
exhibited an excellent I-V characteristic.
[0062] Hereinafter, experimental examples, which were conducted by
the inventors in connection with the reflective electrode according
to the present invention, will be described. The scope of the
present invention is not limited by the following exemplary
processes.
[0063] At the outset, the surface of a structure, in which a p-type
GaN-based compound semiconductor layer is formed on a substrate,
was washed in an ultrasonic bath at a temperature of 60.degree. C.
using trichloroethylene (TCE), acetone, methanol, and distilled
water, respectively, for 5 minutes each time. Then, the resultant
structure was hard baked at a temperature of 100.degree. C. for 10
minutes to remove the remaining moisture from this sample.
[0064] Thereafter, a photoresist layer was spin-coated on the
p-type compound semiconductor layer at 4,500 RPM. The resultant
structure was soft baked at a temperature of 85.degree. C. for 15
minutes. To develop a mask pattern, the sample was aligned with a
mask, exposed to ultraviolet rays (UV) of 22.8 mW for 15 seconds,
and dipped in a solution containing a mixture of a developing
solution with distilled water in a ratio of 1:4 for 25 seconds.
[0065] Thereafter, the developed sample was dipped in a buffered
oxide etchant (BOE) solution for 5 minutes to remove a contaminated
layer from the sample. Then, a first electrode layer was formed on
the resultant structure using an e-beam evaporator. The first
electrode layer was deposited by mounting Ag as an object of
reaction on a mounting stage.
[0066] After the first electrode layer was deposited, a second
electrode layer was deposited using ITO, a lift-off process was
carried out using acetone, and the sample was loaded into a rapid
thermal annealing (RTA) furnace and annealed at a temperature of
about 430 to 530.degree. C. for 1 minute. After that, a third
electrode layer was deposited on the second electrode layer using
Ag inside an e-beam evaporator. The resultant structure where the
third electrode layer is deposited was annealed in an O.sub.2 or N
atmosphere under the same conditions as when the second electrode
layer was annealed. As a result, the reflective electrode was
completed.
[0067] The foregoing method of forming the reflective electrode can
be applied to manufacture the light emitting devices shown in FIG.
3.
[0068] The reflective electrode of the present invention obtains
low contact resistance, high reflectance, improved electrical
conductivity, and an excellent I-V characteristic.
[0069] Also, the compound semiconductor light emitting device
including the foregoing reflective electrode requires a low
operating voltage and exhibits improved optical output and I-V
characteristic. This compound semiconductor light emitting device
reduces power dissipation, thus greatly improving luminous
efficiency.
[0070] Further, the reflective electrode of the present invention
can be applied to light emitting devices, such as LEDs and LDs.
[0071] While the present invention has been particularly shown and
described with reference to exemplary embodiments thereof, it will
be understood by those of ordinary skill in the art that various
changes in form and details may be made therein without departing
from the spirit and scope of the present invention as defined by
the following claims.
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