Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With reflector – opaque mask – or optical element integral...
Reexamination Certificate
1999-11-04
2002-12-10
Tran, Minh Loan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With reflector, opaque mask, or optical element integral...
C257S103000, C372S045013, C372S046012
Reexamination Certificate
active
06492661
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to light emitting semiconductor devices and methods of producing the same. More particularly, the present invention relates to light emitting semiconductor devices having reflection layer structure and methods of producing the same.
BACKGROUND OF THE INVENTION
Conventional surface-emitting type of light-emitting devices (LED) are manufactured by using a light-absorbing substrate.
FIG. 1
illustrates the structure of a prior art surface-emitting type LED, wherein a lower cladding layer
21
is formed on the light-absorbing substrate
20
first, and then an active layer
22
is formed on the lower cladding layer
21
, and an upper cladding layer
23
is subsequently formed on the active layer
22
, so as to form a double heterostructure. The wavelength of the light emitted from such an LED is dependent on the ratio of the composition of the active layer. The energy gap of each cladding layer is higher than that of the active layer, such that not only the carrier injection rate can be increased but also so that the light emitted from the active layer will not be absorbed by the cladding layers. Finally, a front metal electrode
24
is coated on the light-emitting surface of the LED, and a rear electrode
25
is coated on the surface of the substrate
20
opposite to the surface on which the double heterostructure is formed. Since such a vertical type LED uses a light-absorbing substrate, for example, a GaAs substrate, which can absorb the light with wavelengths from 570 nm to 650 nm, its light-emitting efficiency will be reduced. Therefore, how to overcome the light absorption problem caused by the substrate is a key to improve the light-emitting efficiency of such an LED.
To overcome the shortcoming of light absorption caused by the light-absorbing substrate and to improve the light-emitting efficiency of prior art LEDs, an alternative conventional structure, as illustrated in
FIG. 2
, has been provided by adding a current-block region
34
and a Bragg reflector layer
33
. The current-block region
34
is formed on the upper cladding layer
23
and is of the same material as that of the upper cladding layer
23
but has different doping type. The current-block region
34
can increase the current-spreading area, and thus improve the light-emnitting efficiency. The Bragg reflector layer
33
which is formed between the light-absorbing substrate
20
and the lower cladding layer
21
can reflect the light directed to the light-absorbing substrate
20
and thus increase the light-emitting efficiency. However, in such a conventional structure, a further process should be used to define the area of the current-block region
34
and double MOCVD epitaxial processes should used to form the current-block region
34
. Therefore, its manufacturing process will be complicated and the manufacturing time thereof is very long. In addition, the Bragg reflector layer
33
is manufactured by iterately stacking a pair of two different layers with different refraction indexes. The range of the reflection angle of the Bragg reflector layer depends on the difference between the refraction indexes of the two layers of the pair. However, since the material of the pair is restricted to be a compound semiconductor, the difference between the refraction indexes of the two layers of the pair is very limited, and thus merely the almost vertical incident light can be reflected by the Bragg reflector layer, while other incident light can pass the Bragg reflector layer and be absorbed by the substrate. Therefore, its effect of avoiding light from being absorbed by the substrate is very limited.
Another conventional structure, as illustrated in
FIG. 3
, has been provided, wherein the heterostructure
36
of an LED is formed on a temporal light-absorbing substrate
20
to meet the requirement of lattice match, and after the formation of the heterostructure
36
is completed, the temporal substrate
20
is removed, and then a transparent conductive substrate
35
is attached to the heterostructure
36
by using the technique of thermal wafer bonding. The transparent conductive substrate
35
will increase the current-spreading area and will not absorb the light emitted from the active layer, and thus will increase the light-emitting efficiency. In such a conventional structure, the concept of the thermal wafer bonding technique which is used to combine the heterostructure
36
with the transparent conductive substrate
35
is that the difference between the thermal expansion coefficients of two different materials will generate a single axis press, during a thermal process, which will force the generation of the binding, caused by atom-to-atom Van der Waals' force, between the heterostructure
36
and the transparent conductive substrate
35
. To achieve uniformity throughout a large area, it must generate uniform single axis press over a large area. Therefore, not only the thermal bonding machine should be specially designed, but also, that the surfaces of the transparent conductive substrate
35
and the light emitting heterostructure
36
must be of the same lattice direction so as to obtain enough bonding force and low resistance on the bonding surfaces thereof. Therefore, such a conventional method is very complicated and very difficult in manufacturing, and thus its yield rate is hard to increase.
Furthermore, a conventional gallium nitride-based light-emitting device using sapphire substrate must be manufactured as a lateral device, as illustrated in
FIG. 4
, for the reason that the sapphire substrate is insulated. Its structure comprises a sapphire substrate
40
on which a buffer layer
41
, an n type lower cladding layer
42
, an active layer
43
, a p type upper cladding layer
44
and a p type ohmic contact layer
45
are formed serially as well as a front electrode
46
and a lateral rear electrode
47
which are formed subsequently. Silicon carbide has also been used as a substrate for a conventional gallium nitride-based light-emitting device. Although silicon carbide is conductive and the light-emitting device using silicon carbide substrate can be made to have vertical electrodes, silicon carbide is hard to manufacture and the cost thereof is very high. Conventional light-emitting devices using insulated substrates cannot be made to have traditional vertical type electrodes but must be of a lateral electrode structure. Therefore, not only special wiring mechanisms and special packaging techniques are needed, but also the area of a die is increased, so that the manufacturing process thereof will become very complicated and the cost for each unit is increased.
In view of the above, the shortcomings of prior art techniques are as follows:
1. adding a current-block region needs complicated MOCVD epitaxial processes, and the Bragg reflector layer can only reflect the light with an incident angle within a specific range;
2. the process of thermal wafer bonding to achieve uniform bonding and low resistance in the bonding interface is very complicated and difficult; and
3. gallium nitride-based light-emitting devices using sapphire substrates cannot be made to have vertical type electrodes, and thus increase the cost for each unit.
SUMMARY OF THE INVENTION
An objective of the present invention is to overcome the reduction of light-emitting efficiency by adapting a light-absorbing substrate.
Another objective of the present invention is to provide a method for easily combining a substrate with a light emitting semiconductor structure so as to reduce the complexity and difficulty of the manufacturing process and greatly increase the yield rate.
A further objective of the present invention is to simplify the manufacturing process of the current-block region and provide an effective current-spreading effect to improve light-emitting efficiency.
A still further objective of the present invention is to provide a manufacturing process which can easily convert the semiconductor light-emitting device with lateral electrode structure to be the one ha
Chien Fen-Ren
Hon Schang-Jing
Lai Mu-Jen
Jenkens & Gilchrist P.C.
Tran Minh Loan
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