Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With particular semiconductor material
Reexamination Certificate
1998-12-02
2001-05-22
Tran, Minh Loan (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With particular semiconductor material
C257S086000, C257S094000, C257S096000
Reexamination Certificate
active
06236067
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light emitting device for emitting visible light using a compound semiconductor material of AlGaInP group or AlGaAs group, or more in particular to semiconductor light emitting device capable of preventing the reduction in luminance which otherwise might be caused by the distortion due to the difference of lattice constant between the semiconductor layers in a multilayer structure.
2. Description of the Prior Art
The conventional semiconductor light emitting device for emitting visible light uses a compound semiconductor material of AlGaInP group, for example, for a emitting layer forming portion, and has a structure as shown in FIG.
4
. Specifically, in
FIG. 4
, an n-type clad layer
22
made of a semiconductor material of n-type AlGaInP group, an active layer
23
of a non-doped semiconductor material of AlGaInP group having a composition lower in band gap energy than the clad layer, and a p-type clad layer
24
made of a semiconductor material of p-type AlGaInP group are formed by epitaxial growth on a semiconductor substrate
21
of n-type GaAs, thereby forming a emitting layer forming portion
29
of a double hetero-junction structure. Further, a p-type current diffusion layer
25
made of GaP is sequentially formed by epitaxial growth on the surface of the emitting layer forming portion
29
. Further, a p-side electrode
27
of Au—Ge—Ni alloy or the like is formed on the front surface of the p-type current diffusion layer
25
, and an n-side electrode
28
of Au—Ge—Ni alloy or the like is formed on the reverse side of the semiconductor substrate
21
.
In the light emitting device of this structure, the light from the front surface of the semiconductor multilayers, i.e. the light from the side of the p-side electrode
27
is utilized, and the p-side electrode
27
which blocks the light is formed as small as possible in an area. In order to emit light by containing carriers in the active layer
23
sandwitched between the clad layers
22
,
24
, on the other hand, the current desirably flows distributed over the entire active layer. For this purpose, the current diffusion layer
25
is formed as thick as about 10 to 70 &mgr;m to expand the current over the entire chip. This current diffusion layer
25
desirably not only diffuses the current but also fails to absorb the light emitted by the active layer
23
, and is made of GaP which is large in band gap energy. Also, the p-type layer
24
, the only function of which is to contain the carriers, is normally formed to the thickness of about 0.5 &mgr;m not to absorb light.
In the conventional semiconductor light emitting device having the structure shown in
FIG. 4
, the current diffusion layer, which diffuses the current, is required to be formed as thick as possible. However, the lattice constant of the current diffusion layer of GaP and that of the compound semiconductor layer of AlGaInP group or AlGaAs group used for the emitting layer forming portion are considerably different from each other, and therefore a distortion is liable to be caused by the lattice mismatch. Specifically, although the matching can be secured between the GaAs layer of the substrate and the emitting layer forming portion for forming an emitting layer, the matching between the emitting layer forming portion and the GaP layer is difficult to secure. The present inventors have vigorously studied a method of improving the luminance of the semiconductor light emitting device of this type. As a result, the inventors have discovered that the luminance is extremely reduced if the distortion due to the lattice mismatch affects the active layer constituting the emitting layer.
SUMMARY OF THE INVENTION
The object of the present invention which has been developed based on the knowledge described above is to provide a semiconductor light emitting device comprising a semiconductor multilayer portion forming an emitting layer made of a compound semiconductor of AlGaInP group or AlGaAs group, and a current diffusion layer of GaP formed on the semiconductor multilayer portion forming the emitting layer, the semiconductor light emitting device having such a structure that the distortion due to the lattice mismatch has no effect on the emitting layer and a high luminance is obtained.
A semiconductor light emitting device according to the present invention comprises a substrate made of GaAs, an emitting layer forming portion constituting a multilayer including an n-type layer, an active layer and a p-type layer made of a compound semiconductor of AlGaInP group or AlGaAs group formed on the substrate, and a current diffusion layer of GaP formed on the front surface of the emitting layer forming portion, wherein the p-type layer or the n-type layer between the active layer and the current diffusion layer is formed to the thickness of not less than about 2 &mgr;m.
The compound semiconductor of AlGaInP group is a material which is expressed as (A
1
x
Ga
1-x
)
0. 51
In
0.49
P, where x assumes various values between 0 and 1. The crystal mixing ratio of 0.51 and 0.49 between Al
x
Ga
1-x
and In indicates the ratio at which the material of AlGaInP group is matched in lattice with the semiconductor substrate of GaAs or the like on which it is laid. The compound semiconductor of AlGaAs group indicates a compound in which the ratio between Al and Ga with the total of unity can be changed variously.
With this structure, a distortion which may be caused by the difference in lattice constant between the compound semiconductor of AlGaInP group and GaP is absorbed by a thick clad layer and can be prevented from affecting the active layer, thereby making it possible to emit light with high efficiency. Even when light is absorbed into the clad layer, the emitting efficiency is improved even more, thus improving the luminance as a whole.
The current diffusion layer is formed to the thickness of not more than 10 &mgr;m so that the effect of the distortion due to the current diffusion layer on the emitting layer forming portion is considerably reduced, thereby further contributing to a higher luminance of the device.
The current diffusion layer is formed with a carrier concentration of 2×10
18
to 2×10
19
cm
−3
or a metal layer sufficiently thin to transmit the light is formed on the surface of the current diffusion layer. In this way, the current is liable to expand and therefore the expansion of the current due to a thinner current diffusion layer is desirably guaranteed.
Specifically, according to one aspect of the invention, there is provided a semiconductor light emitting device comprising an n-type GaAs substrate, an n-type clad layer made of a compound semiconductor of n-type AlGaInP group formed on the substrate, an active layer of a compound semiconductor of AlGaInP group smaller in band gap energy than the clad layer, a p-type clad layer formed of a p-type compound semiconductor of AlGaInP group larger in band gap energy than the active layer, and a current diffusion layer made of p-type GaP formed on the p-type clad layer, wherein the p-type clad layer is formed to the thickness of not less than about 2 &mgr;m.
According to another aspect of the invention, there is provided a semiconductor light emitting device comprising a substrate of GaAs, an emitting layer forming portion for forming an emitting layer constituting a multilayer including an n-type layer, an active layer and a p-type layer of a compound semiconductor of AlGaInP group or AlGaAs group formed on the substrate, and a current diffusion layer of GaP formed on the front surface of the emitting layer forming portion, where the current diffusion layer is formed to the thickness of about 3 to 7 &mgr;m.
With this structure, the current diffusion layer having a lattice constant different from that of the emitting layer forming portion is so thin that the lattice distortion due to the difference in lattice constant between the emitting layer forming portion and the current diffusion layer is absorbed also into
Matsumoto Yukio
Nakata Shunji
Shakuda Yukio
Arent Fox Kintner & Plotkin & Kahn, PLLC
Rohm & Co., Ltd.
Tran Minh Loan
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