Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – In combination with or also constituting light responsive...
Reexamination Certificate
2001-01-11
2002-10-15
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
In combination with or also constituting light responsive...
C257S098000, C257S099000
Reexamination Certificate
active
06465808
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and structure for forming an electrode on a light emitting device. More particularly, the present invention relates to the method and structure for providing a plurality of ohmic contact dots formed on a light emitting device.
BACKGROUND OF THE INVENTION
In recent years, a great deal of attention has been directed to light-emitting devices utilizing gallium nitride-based III-V group semiconductors such as GaN, AlGaN, InGaN, and AlInGaN. Furthermore, a transparent sapphire substrate is usually used for such devices. Different from a conductive substrate used for the other semiconductor light-emitting device, sapphire is electrically insulated. Thus, it is not possible to mount, directly on the substrate, electrodes for supplying a predetermined current to the compound semiconductor layer causing the device to emit light. Both p-electrode and n-electrode must be formed in direct contact with the p-type compound semiconductor layer and the n-type compound semiconductor layer, respectively.
Referring to
FIG. 1
, a top view shows the conventional gallium nitride-based III-V group semiconductor light emitting device. Referring to
FIG. 2
, a cross-sectional view is taken along the line IV—IV of FIG.
1
. The light-emitting device has a structure in which a layer of an n-type GaN
20
, a layer of an n-type AlGaN
30
, an active layer
40
(which is selected by using InGaN, AlInGaN or GaN to form the double hetero-junction or quantum well structure), a layer of an p-type AlGaN
50
, and a layer of an p-type GaN
60
are all stacked on a sapphire substrate
10
.
After etching process, a portion of the n-type GaN
20
is exposed. Then, the first electrode
70
and the second electrode
80
are formed respectively on the exposed n-type GaN surface
20
and on the exposed p-type GaN surface
60
. The first electrode
70
comprises a metallic material. The metallic material that achieves preferable ohmic characteristics contains two metals of titanium formed in direct contact with the n-type GaN layer
20
, and a layer of aluminum formed on the titanium layer. In order to obtain a perfect ohmic contact, annealing the metallic material layer is required. The annealing treatment is preferably conducted at a temperature of 400 degree. C. or more.
Because the carrier concentration of the p-type GaN is only 5×10
17
/cm
3
, the second electrode
80
, which is not similar to the small area of the first electrode
70
, will cover the most part of the p-type GaN
60
exposed surface to spread the current. The second electrode
80
is formed to directly cover an entire exposed surface of the p-type GaN layer
60
for increasing the efficiency of the current spreading. But the second electrode
80
will shade the light emitting from the light emitting device. In this way, a thin second electrode
80
is formed on the p-type GaN
60
to transmit the light emitting from the light emitting device. A light transmitting electrode provided in contact with the p-type semiconductor layer is described in the U.S. Pat. No. 5,563,422. That is a gallium nitride-based III-V compound semiconductor device and method of producing the same. The second electrode
80
may be formed by any suitable metallic material. A particularly preferable metallic material contains gold and nickel. Gold and nickel are preferably formed such that a layer of nickel is formed in direct contact with the p-type GaN layer
60
, and a layer of gold is formed upon the nickel layer. The annealing treatment is preferably conducted at a temperature of 400 degree. C. or more. A metallic material used for the second electrode
80
is preferably formed such that the annealed material has a thickness of 10 angstrom to 1000 angstrom. By adjusting the thickness of the second electrode
80
in the range of 10 angstrom to 1000 angstrom, the second electrode
80
can be rendered light-transmission. Due to the thin second electrode
80
, a bonding pad
90
is contacted to the p-type GaN layer
60
. The process of forming the bonding pad
90
is to firstly form a window
95
upon the second electrode
80
exposing the p-type GaN layer
60
surface. The bonding pad
90
is then formed covering portions of the second electrode
80
and adhering on the p-type GaN layer
60
surface.
Because the second electrode
80
is formed by metallic material, the process of forming the thickness of second electrode
80
should be seriously concerned. If the thickness of the second electrode
80
is thicker than that of expectation, most of the light emitting from the light emitting device will be absorbed by the second electrode
80
causing a poor transparent efficiency. If the thickness is thinner than that of expectation, it is difficult to have a second electrode
80
with good ohmic characteristics. Furthermore, the second electrode
80
of predetermined thickness formed on the p-type GaN layer
60
, it is inevitable that a constant portion of the light emitting from the light emitting device will be absorbed by the second electrode
80
causing a low transparent efficiency of about between 60% and 80%.
Referring
FIG. 3
, a schematic diagram shows the conventional GaAs-based, InP-based, GaP-based, SiC-based or ZnSe-based light emitting device. The light emitting device includes at least an n-type substrate
96
, an n-type semiconductor layer
98
, an active layer
100
, a p-type semiconductor layer
102
, an n-electrode
104
, and a p-electrode
106
. Generally speaking, after the n-electrode
104
and the p-electrode
106
are formed, the annealing treatment is then processed. Consequently, regions of high light absorption are formed on the ohmic contact area, and cause the difficulty of fabricating a device with higher output efficiency.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide a method and structure for forming an electrode on a light emitting device. The present invention provides a brand-new method and structure to form the transparent electrode or reflective electrode on a p-type gallium nitride-based compound semiconductor. The electrode comprises a plurality of opaque ohmic contact dots formed on the p-type gallium nitride-based compound semiconductor and a transparent conductive layer (or a light reflective conductive layer) covering the p-type gallium nitride-based compound semiconductor.
It is another object of this invention to provide a method and structure for forming an electrode on a light emitting device. Comparing with the conventional electrode formed on p-type GaN-based III-V compound semiconductor, the present invention has advantages of higher light penetration and easier in process. Moreover, utilizing the present invention, the electrode is suitable for any light emitting devices. The output efficiency of the light emitting device is higher than that of a conventional light emitting device. Furthermore, this process of forming the electrode is easier than that of the conventional process.
In accordance with all aspects of this invention, this invention provides a structure for forming an electrode on a light emitting device, comprising: a semiconductor layer of a light emitting device having a first surface and a second surface, a plurality of ohmic contact dots formed on said first surface, and a conductive layer covering said ohmic contact dots and said first surface.
In accordance with the aforementioned objects of this invention, this invention provides a method for forming an electrode on a light emitting device, comprising: forming a plurality of contact dots on the surface of a semiconductor layer of a light emitting light device, carrying out an annealing treatment, forming a conductive layer covering said contact dots and said surface.
REFERENCES:
patent: 4232440 (1980-11-01), Bastek
patent: 5358880 (1994-10-01), Lebby et al.
patent: 5608287 (1997-03-01), Hung et al.
patent: 5822351 (1998-10-01), Kang
patent: 5917202 (1999-06-01), Haitz et al.
patent: 6278236 (2001-08-01), Madathil et al.
Highlink Technology Corporation
Ho Tu-Tu
Nelms David
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