Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal
Reexamination Certificate
1999-05-11
2001-06-12
Mulpuri, Savitri (Department: 2812)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
C438S046000, C438S977000
Reexamination Certificate
active
06245588
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a semiconductor device which has a light-emitting layer formed of an AlGaInP-base semiconductor material for emitting green to red portions of visible light. More specifically, this invention is concerned with a semiconductor light-emitting device which is improved in light-emitting efficiency so as to be utilized for a light source, requiring brightness of light, such as for outdoor displays, tail lamps or indicators for automobiles, besides applications for optical transmission or optical information processing.
BACKGROUND OF THE INVENTION
There is illustrated in
FIG. 7
a structure of a light-emitting device formed of a conventional AlGaInP-base semiconductor material. That is, as shown in the figure, an n-type GaAs substrate
11
has thereon a light-emitting layer forming portion
19
. The light-emitting layer forming portion
19
comprises an n-type lower clad layer
12
formed of an n-type AlGaInP-base semiconductor material on the substrate
11
, an active layer
13
formed of a non-doped AlGaInP-base semiconductor material on the lower clad layer
12
, and an upper clad layer
14
formed of a p-type AlGaInP-base semiconductor material on the active layer
13
. Furthermore, a window layer
15
is formed on the surface of the light-emitting layer forming portion
19
. These layers are sequentially formed on the substrate
11
by epitaxial growth. An upper electrode
17
is formed of gold or aluminum through a GaAs contact layer
16
, while a lower electrode
18
is formed also gold or aluminum on a back surface of the substrate
11
.
The light-emitting device of this kind is of a double hetero structure, in which the mixed crystal ratio for the light-emitting layer forming portion
19
, i.e., the clad layers
12
,
14
and the active layer
13
, is determined so as to confine carriers within the active layer
13
sandwiched between the both clad layers
12
,
14
for enhancing efficiency of emission of light. Since the light-emitting device of this type is adapted to radiate light through a top-surface side, the upper electrode
17
is provided in such a minimal size for supplying electric current that the light is less obstructed or absorbed by the upper electrode.
However, in the conventional light-emitting device formed of the AlGaInP-base semiconductor material as above, the light created by the light-emitting layer forming portion
19
having the active layer
13
sandwiched by the clad layers
12
,
14
travels, besides upwards, sideways and downward of the chip to be radiated outward. However, the semiconductor substrate
11
of GaAs is of not transparent for the light thus emitted so that the light traveling toward the semiconductor substrate is absorbed by the substrate. To this end, the light radiation available is limited only to the portion of the light emitted upward, without being obstructed by the upper electrode
17
, from the light-emitting layer forming portion
19
. Therefore, there is a problem that the efficiency of light radiation is low, and accordingly the brightness of light is also low.
On the other hand, there can be thought of using the material such as an AlGaAs-base material which is epitaxially grown over a light-emitting layer forming portion
19
for a substrate
11
. The AlGaAs-base material has a composition with a mixture crystal ratio of Al greater than 0.7 so that the longer wavelength portion of visible light than that of yellow light is less absorbed by the material. However, the AlGaAs-base material is liable to oxidize so that it is practically impossible to grow an AlGaInP-base material on the AlGaAs-base material.
SUMMARY OF THE INVENTION
It is therefore a primary object of the present invention to solve the abovestated problem and provide a semiconductor light-emitting device wherein there occurs less absorption of the light emitted toward the back surface of the substrate, thereby enhancing the efficiency of light radiation.
It is another object of the present invention to provide a method of manufacturing semiconductor light-emitting device wherein the light emitted to travel toward the substrate is utilized for light radiation.
In accordance with the present invention, there is provided a semiconductor light-emitting device comprising: a substrate formed of an AlGaAs-base semiconductor material; a light-emitting layer forming portion formed of an AlGaInP-base compound semiconductor material overlying the substrate so as to provide a light-emitting layer, the light-emitting layer forming portion having an upper-side layer; an upper electrode formed on the upper-side layer in electrical connection therewith; and a lower electrode formed on the substrate in electrical connection therewith.
Here, the AlGaInP-base material refers to a material that is expressed by a chemical formula for example of (Al
x
Ga
1-x
)
0.51
In
0.49
P, where the value x is variable between 0 and 1. Also, the AlGaAs-base material refers to a material that is expressed by a chemical formula of Al
y
Ga
1-y
As, where the value y is variable between 0.5 and 0.8.
Preferably, a window layer is provided on the upper-side layer, the window layer being formed of an AlGaAs-base compound semiconductor material to a layer thickness of greater than 30 (m. With such structure, electric currents passing through the semiconductor chip are spread almost close to side faces of the chip, widening the light radiation area.
More preferably, the lower electrode is provided on part of a back surface of the substrate. With such structure, if a highly-reflective die-pad on which the light-emitting chip is to be mounted is employed, the light reflected by the die-pad is utilized for radiation with higher efficiency.
Further preferably, the light-emitting layer forming portion is of an overlaid structure formed by a clad layer formed in a first conductivity type on the substrate, an active layer formed on the clad layer to have a composition with a lower band gap energy than that of the clad layer, and a clad layer formed in a second conductivity type on the active layer to have the same composition as the first-conductivity clad layer. With such structure, the efficiency of emission of light is further enhanced.
In accordance with the present invention, there is also provided a method of manufacturing a semiconductor light-emitting device comprising the steps of: (a) forming by overlaying on a GaAs substrate, in order, a first window layer of an AlGaAs-base compound semiconductor material, a light-emitting layer forming portion of an AlGaInP-base compound semiconductor material, and a second window layer of an AlGaAs-base compound semiconductor material; (b) forming an AlGaAs-base compound semiconductor layer on the second window layer by a liquid-phase epitaxial growth method; (c) removing away the GaAs substrate to render the epitaxially grown AlGaAs-base semiconductor layer to be a new substrate, providing a semiconductor overlaying structure; and (d) forming electrodes on opposite surfaces of the semiconductor overlaying structure.
Preferably, the liquid-phase epitaxial growth of a third window layer of AlGaAs-base material is made on the surface of the first window layer exposed by removing the GaAs substrate, which makes possible formation of a thick window layer in a brief time period. By doing so, the resulting light-emitting device has electric currents passing therethrough spread almost the entire of the semiconductor chip.
The abovestated object, other objects, features and advantages of this invention will be more apparent from the following description on examples with reference to the accompanying drawings.
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p
Arent Fox Kintner & Plotkin & Kahn, PLLC
Mulpuri Savitri
Rohm Co. Ltd
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