Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Compound semiconductor
Reexamination Certificate
2000-06-20
2001-12-11
Elms, Richard (Department: 2824)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Compound semiconductor
C438S046000, C257S094000
Reexamination Certificate
active
06329216
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a semiconductor light emitting device which is improved in semiconductor crystalline quality to have an enhanced light emitting efficiency, for use as a light source requiring high brightness, such as for outdoor displays and vehicular tail lamps and direction indicators, or a light source requiring high brightness but low-power consumption, such as for back-lights of battery-driven portable units such as handy telephones and indicator lamps, besides for optical communications or optical information processing.
There is a conventional semiconductor light emitting device having a light emitting layer forming portion formed by using an AlGaInP-based compound semiconductor for emitting visible light, as disclosed, for example, in Japanese Laying-open Patent Publication No. H4-212479. This known semiconductor light emitting device is structured as shown in FIG.
4
. In
FIG. 4
, the device includes an n-GaAs semiconductor substrate
21
. On the semiconductor substrate
21
are epitaxially grown, in order, an n-type cladding layer
22
of an n-type AlGaInP-based compound semiconductor, an active layer
23
of a non-doped AlGaInP-based semiconductor material in a composition having a bandgap energy lower than that of the cladding layer, and a p-type cladding layer
24
of a p-type AlGaInP-based compound semiconductor, thereby forming a doublehetero structure providing a light emitting layer forming portion
29
. Further, a p-type window layer (current diffusion layer)
25
is epitaxially grown of a GaAs-based semiconductor material on a surface of the light emitting layer forming portion
29
. On the window layer
25
a p-side electrode
27
is formed through a p-type GaAs contact layer
26
, while an n-side electrode
28
is formed on a backside of the semiconductor substrate
21
. These electrodes
27
,
28
are formed of an Au—Ge—Ni alloy or the like.
The light emitting device of this structure is arranged to confine carriers within the active layer
23
sandwiched between the respective cladding layers
22
,
24
, for emitting light. Accordingly, the cladding layers
22
,
24
and the window layer
25
are doped with an impurity to an appropriate carrier concentration. The p-type layers
24
,
25
,
26
are doped with an impurity such as Zn, Mg or Be. Meanwhile, the window layer
25
has to be formed of a material having a higher bandgap energy than the bandgap energy of the emission-light wavelength, in order not to absorb the light emitted by the active layer. Thus, the AlGaAs-based compound semiconductor material is used for the window layer
25
. Even where the AlGaAs-based compound semiconductor is employed, if it is low in Al mixed-crystal ratio, the bandgap energy is decreased to absorb a certain amount of the light emitted by the light emitting layer forming portion. Accordingly, the compound semiconductor in practical use has an Al mixed-crystal ratio increased to approximately 0.7-0.8.
In the conventional semiconductor light emitting device shown in
FIG. 4
, the GaAs substrate and the AlGaInP-based compound semiconductor are controlled in lattice-match by adjusting the mixed crystal ratio between (AlGa) and In. Also, the AlGaAs-based compound semiconductor, if doped with Zn or Mg, becomes mismatching in lattice to the AlGaInP-based compound semiconductor. In such a case, a process for further matching is necessarily performed, resulting in poor film crystallinity. On the other hand, it is not preferred for the cladding layer to increase the carrier concentration, in view of the effect of carrier confinement within the active layer and the suppression against diffusion of an impurity from the cladding layer into the active layer. In contrast to this, it is preferred that the window layer be given a carrier concentration as high as possible. However, if the window layer is doped to a high concentration with a p-type impurity (2-valence atom with respect to a III-V compound semiconductor) such as Zn, Mg or Be, the film crystallinity worsens as stated before. This might cause a phenomenon such as cracks or chip fracture in the semiconductor layer due to internal strains or distortions, as can be observed by a reliability test with applying currents at low temperatures. This raises a problem that the device is lowered in light emitting efficiency or tendency to be damaged.
Meanwhile, the emission light is transmitted toward the front of the semiconductor light emitting device, even where a window layer the device is formed of an AlGaInP with an increased Al mixed-crystal ratio on the surface thereof, similarly to the semiconductor light emitting device as shown in FIG.
4
. However, there is absorption or blockage of the emission light by the front electrode as well as the GaAs contact layer for improving ohmic-contact characteristics between the electrode and the semiconductor layer. To cope with this, the top electrode and the contact layer is removed away with a required minimum area left, thus exposing the window layer at a top surface. This surface, in some cases, is covered by a package resin or the like. However, the resin and the semiconductor are poorly matched, giving rise to intrusion of moisture or water content through an interface therebetween. Since in such a case the semiconductor material having an increased Al mixed-crystal ratio appears in the interface, the Al content reacts with the intruded moisture, causing corrosion or oxidation. The corrosion or oxidation, if proceeds deep inside the semiconductor layers, possibly deteriorating the crystalline structure of the light emitting layer forming portion. Thus, there is a problem that the device might be degraded in light emitting efficiency as well as reliability.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a semiconductor light emitting device which is free of degradation in crystallinity due to p-type impurity doping, thereby provide a high light emitting efficiency and brightness without encountering device degradation or damage.
It is another object of the present invention to provide a reliable semiconductor light emitting device which is arranged to prevent the semiconductor layers from being corroded at a surface by intrusion of moisture or water content which corrosion possibly advances deep into an inside of the semiconductor layers to cause deterioration in characteristics of the semiconductor light emitting device.
It is a further object of the present invention to provide a method of manufacturing a semiconductor light emitting device capable of prevent the crystallinity from worsening caused by p-type impurity doping.
The present inventors have eagerly studied in order to prevent internal stresses from occurring to thereby lower the light emitting efficiency and the reliability as encountered upon forming a window layer using the above-stated AlGaAs-based compound semiconductor. As a result, it was found that, when forming a p-type window layer, it is possible to utilize as a dopant a metal contained in a reacting gas in an MOCVD process, wherein the carrier concentration can be controlled by controlling the temperature during epitaxially growing semiconductor layers. It was further found that if the window layer is formed at a carrier concentration of 5×10
18
-3×10
19
cm
−3
, the resulting semiconductor light emitting device is reduced in internal stresses thus providing high brightness and reliability.
A semiconductor light emitting device according to the present invention, comprises: a substrate; a light emitting layer forming portion formed on the substrate and having an n-type layer and a p-type layer to provide a light emitting layer; and a window layer formed on a surface side of the light emitting layer forming portion; wherein the light emitting layer forming portion is formed of AlGaInP-based compound semiconductor, and the window layer is formed of Al
y
Ga
1−y
As (0.6≦y≦0.8) auto-doped in a carrier concentration of 5×10
18
-3×10
19
cm
&minus
Matsumoto Yukio
Nakata Shunji
Shakuda Yukio
Arent Fox Kintner & Plotkin & Kahn, PLLC
Elms Richard
Rohm & Co., Ltd.
Wilson Christian D.
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