Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Compound semiconductor
Reexamination Certificate
2001-03-22
2002-10-22
Booth, Richard (Department: 2812)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Compound semiconductor
Reexamination Certificate
active
06468824
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method and structure for semiconductor devices, and more particularly to a method and structure for optical semiconductor devices.
2. Description of the Prior Art
Semiconductor devices are employed in a wide variety of electrical applications, for example, in central processors, memory devices, microwave devices, and light emitting devices.
One concern with the semiconductor devices is heat influencing on the semiconductor device. When temperature is raised by heat, reliability and life time of the semiconductor device are reduced. For optical semiconductor device, such as light emitting diode(LED), lateral laser, or planar laser, the efficiency of light emitting is also reduced by raising the temperature. An example of AlGaInP type LED, the light is emitted from electrons at an energy level of &Ggr; bandgap combined with electronic holes. When the temperature is raised, partical electrons will jump on an energy level of X bandgap and then combine with the electronic holes, which may generate heat that reduces efficiency of internal quantum and efficiency of light-emitting.
The layers of many conventional light-emitting diodes (LEDs) are grown on an optically absorbing substrate having an energy gap less than the emission energy of the active region of the LED. The substrate absorbs some of the light generated within the active region, thereby reducing the efficiency of the device. An example of a prior art AlGaInhP LED of the double heterojunction type is shown in FIG.
1
. An epitaxial layer
112
of n-doped (Al
x
Ga
1−x
)
0.5
In
0.5
P, a light extraction layer
114
of (Al
x
Ga
1−x
)
0.5
In
0.5
P and an epitaxial layer
116
of p-doped (Al
x
Ga
1−x
)
0.5
In
0.5
P are grown on an n-type substrate
110
where “x” is a percent of chemical composition. A double heterojunction structure as a region of light emitting is formed between layers
112
-
116
. An optically transparent window layer
118
of p-doped Al
x
Ga
1−x
. As or GaP is grown on the epitaxial layer
116
. The optically transparent window layer
118
enhances lateral conductibility of p-type region and further improves current spreading on the double heterojunction structure. On the other hand, the amount of “x” in the light extraction layer
114
determines wavelength of light emitting. The bandgaps of the epitaxial layers
112
,
116
, and the optically transparent window layer
118
are chosen so as to cause light to be generated in the light extraction layer
114
and to travel through the epitaxial layers
112
,
116
, and the optically transparent window layer
118
without being absorbed. However, absorption of light does occur at the GaAs substrate
110
which causes downwardly emitted or directed light to be absorbed and reduces light-emitting efficiency of the light emitting devices.
There are several techniques for resolving the light to be absorbed by the substrate. A first technique is to grow the light-emitting devices on a non-absorbing substrate. However, a problem with this technique is that acceptable lattice matching may be difficult to achieve, depending upon the lattice constant of the substrate similar to that of the LED epitaxial layers. A second technique is to grow a distributed Bragg reflector between the LED epitaxial layers and the substrate. An increase in efficiency is achieved, since the distributed Bragg reflector will reflect light that is emitted or internally reflected in the direction of the absorbing substrate. However, the improvement is limited because the distributed Bragg reflector only reflects light that is of near normal incidence. Light that differs from a normal incidence by a significant amount is not reflected and passes to the substrate, where it is absorbed.
A third technique is to grow the LED epitaxial layers on an absorbing substrate that is later removed. A transparent “substrate” is fabricated by growing a thick, optically transparent and electrically conductive epitaxial layers formed thereon. The absorbing substrate is then removed by methods of polishing, etching, or wafer lift off. The thin wafers are so thin and susceptible to fragile that a rather thick substrate is required. However, a “thick” transparent substrate requires a long growth time, limiting the manufacturing throughout of such LEDs. Moreover, the epitaxy growth spends much time and costs.
A concern with current distribution of LED is considered. Depicted in
FIG. 2
, an n-type substrate
132
is on an n-type ohmic contact
130
that contains a composition of Au/Ge. A light extraction
134
is on the n-type substrate
132
, which is a structure of single or double heterojunction, or a structure of multiple-quantum well. A p-type transparent window layer
136
is grown on the light extraction
134
. A bonding pad
138
of p-type ohmic contact generally contains a composition of Au/Be or Au/Zn. A light emitted from the light extraction layer
134
results from current travelling from the bonding pad
138
to the p-type transparent window layer
136
. However, partial current may laterally travel between the p-type transparent window layer
136
. A part of current may be ineffective because it cannot be achieved to exterior of grain when current travels upwardly and is blocked by the bonding pad
138
. A technique for resolving the problem is to grow a current blocking right below the bonding pad and whereby the current cannot directly travel downwardly to the light extraction layer
134
. For example, shown in
FIG. 3
, a current blocking
140
of n-type layer, whose conductivity is different from that of the transparent window layer
136
, is utilized to achieve the effect of current blocking. There are two current methods for fabricating the current blocking
140
. A two-step epitaxy method is to grow sequentially the current blocking layer and the light extraction layer
134
on the substrate
132
. The current blocking layer is etched to form the current blocking
140
of n-type layer and placed again into an epitaxy chamber for sequentially growing the transparent window layer
136
. However, the epitaxy chamber is susceptible to pollution that influences the properties of epitaxial layers. A second method is to utilize selectively local diffusion. However, it is difficult to control the fabrication condition.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for forming metallic substrate replacing conventional semiconductor substrate. The reliability and duration time of a semiconductor device may be enhanced by high thermal and electrical conductivity of the metallic substrate. Moreover, the metallic substrate also can enhance efficiency of light output for an optical semiconductor device.
It is another object of the present invention to provide a method for forming a mirrored or rough surface between the metallic substrate and the semiconductor layers for a light-emitting device. An acceptive mirrored surface may be a metallic surface or one caused by the differential of refractive index. The mirrored or rough surface can reorient downward lights from the light extraction layer and enable the downward lights to be far away from surface of grain so as to enhance the efficiency of light emitting.
It is yet an object of the present invention to provide a technique of metallic substrate. A current blocking layer below a light extraction layer is provided to block current and to enhance the efficiency of light emitting.
It is yet another object of the present invention to provide a method for forming a metallic layer as a temporary substrate whereby thin layers of semiconductor may be took out for other applications.
In the present invention, a method for forming a semiconductor device with a metallic substrate. The method comprises providing a semiconductor substrate. At least a semiconductor layer is formed on the semiconductor substrate. A metallic electrode layer is formed on the semiconductor layer. The metallic substrate is formed on the metallic
Chen Chien-An
Chen Nai-Chuan
Tzou Yuan-Hsin
Wu Bor-Jen
Yih Nae-Guann
Booth Richard
Powell Goldstein Frazer & Murphy LLP
Roman Angel
Uni Light Technology Inc.
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