Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – In combination with or also constituting light responsive...
Reexamination Certificate
1999-04-22
2002-08-13
Whitehead, Jr., Carl (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
In combination with or also constituting light responsive...
C257S638000, C257S257000, C257S079000, C257S081000, C257S084000
Reexamination Certificate
active
06433365
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an ultra thin type epitaxial wafer and a light emitting diode.
2. Description of the Related Art
A light emitting diode is a device which directly converts electric energy into light where a forward current is applied to a p-n junction of semiconductor. III-V group compound semiconductors are frequently used as materials of light emitting diodes because they have a band gap corresponding to a wavelength of infrared to ultraviolet light. Among these, gallium phosphide (GaP) light emitting diode devices emitting red to green light., and gallium arsenide (GaAs) light emitting diode devices emitting infra-red to yellow light are widely used.
FIG. 10
shows an exemplary structure of conventional GaP light emitting diode. In this structure, an n-type GaP epitaxial layer
13
(also referred to simply as “buffer layer” hereinafter) having a thickness of 100 &mgr;m and a carrier concentration of 8×10
16
cm
−3
is grown on an n-type GaP single crystal substrate
12
having a thickness of 280 &mgr;m and a carrier concentration of 1×10
17
cm
−3
, and a light emitting region composed of an n-type GaP epitaxial layer
14
having a thickness of 20 &mgr;m and a carrier concentration of 6×10
16
cm
−3
, p-n junction
15
, and p-type GaP epitaxial layer
16
having a thickness of 20 &mgr;m and a carrier concentration of 1×10
17
cm
−3
stacked in this order is provided thereon.
By forming n-electrodes
11
and a p-electrode
17
are formed on the above structure, a GaP light emitting diode
20
having a total thickness of 420 &mgr;m of the compound semiconductor single crystal substrate
12
and the epitaxial layers
13
-
16
can be obtained.
The thickness of conventionally used buffer layer
13
is 100 &mgr;m at the utmost. This is because the buffer layer
13
having a thickness of about 100 &mgr;m can afford sufficient light emission intensity as light emission intensity conventionally required.
This buffer layer
13
is required to obtain high light emitting intensity partly for the following reason. That is, as for currently available materials for the GaP single crystal substrate
12
, dislocations are generated at a high density, i.e., around 5×10
4
cm
−2
to 1×10
5
cm
−2
, and their purity is also low. Therefore, the epitaxial layer may have bad quality at a portion near the GaP single crystal substrate
12
grown at an early stage of the growth, and such an epitaxial layer as it is cannot afford high light emitting intensity. Accordingly, the buffer layer
13
is provided as a layer for buffering between the epitaxial layers
14
-
16
, which constitute the light emitting region, and the GaP single crystal substrate
12
.
Another reason why the buffer layer
13
is required arises from the fact that the light transmissivity of the GaP single crystal substrate
12
is inferior to that of the buffer layer
13
, which is an epitaxial layer.
FIG. 11
represents light transmissivity of the GaP single crystal substrate
12
and the buffer layer
13
plotted to the. wavelength of incident light. For example, about 5% of difference in the light transmissivity is observed around 570 nm, which corresponds to a wavelength of yellow green light emission commonly used in GaP light emitting diodes.
Therefore, a thinner compound semiconductor single crystal substrate (also referred to simply as “single crystal substrate”) has been used to suppress the degradation of the light transmissivity. As a means for obtaining a thinner single crystal substrate, a method utilizing a thin single crystal substrate for epitaxial growth can be mentioned first. However, according to such a method, more single crystal substrates are broken during the epitaxial growth process, as the single crystal substrates become thinner, and therefore the productivity is markedly decreased.
Judging from the current state of the art, the best way for obtaining a thinner single crystal substrate is scraping the main back surface of single crystal substrate by, for example, lapping after the epitaxial growth. However, when a single crystal substrate is scraped by lapping or the like to an extent that the remaining thickness of the substrate becomes 10 &mgr;m or less, a part of the single crystal substrate may drop out so that the epitaxial layer is exposed, because of bad precision of the lapping process, nonuniform growth of the epitaxial layer or the like.
The conventionally used single crystal substrate
2
has a carrier concentration not less than 1×10
17
cm
−3
, and allows ohmic contact with the electrode
1
. On the other hand, in the production of light emitting diodes, the carrier concentration of the epitaxial layer which is provided directly on the single crystal substrate has conventionally been made smaller than 1×10
17
cm
−3
in order to realize high luminous intensity.
Therefore, if a part of the main back surface of the single crystal substrate drops out, and the epitaxial layer is exposed, an ohmic electrode may not be formed at the exposed area of the epitaxial layer, and problems concerning optoelectronic characteristics may be arisen, for example, the forward voltage may become high. Accordingly, the conventional single crystal substrate could not be made thinner than 10 &mgr;m as the remaining thickness.
Further, with the recent increasing demand for smaller and thinner electronic products and the like, it also becomes necessary to make light emitting diodes thinner. In the conventionally used light emitting diodes, the total thickness of the compound semiconductor single crystal substrate and the epitaxial layer was typically 250 &mgr;m to 450 &mgr;m.
SUMMARY OF THE INVENTION
The present invention has been accomplished in view of the problems of the prior art mentioned above, and its object is to provide an ultra thin type light emitting diode where generation of ohmic electrode failure is suppressed, and a epitaxial wafer for the foregoing light emitting diode.
In order to achieve the aforementioned object, the present invention provide an epitaxial wafer comprising an epitaxial layer formed on a main surface of a compound semiconductor single crystal substrate, characterized in that the epitaxial layer on the main surface is exposed in a back surface of the compound semiconductor single crystal substrate, and an exposed portion of the epitaxial layer has a carrier concentration of 1×10
17 cm
−3
to 2×10
18
cm
−3
.
An epitaxial wafer where the epitaxial layer on the main surface. is exposed in the back surface of the compound semiconductor single crystal substrate, and an exposed portion of the epitaxial layer has a carrier concentration of 1×10
17
cm
−3
to 2×10
19
cm
−3
as defined above can suppress the generation of ohmic electrode failure, because such an epitaxial wafer does not cause problems that an ohmic electrode cannot be formed at the exposed portion, the forward current becomes high and the like, even when the main back surface of the compound semiconductor single crystal substrate was scraped by lapping or the like to such an extent that the epitaxial layer on the main surface is exposed.
In the above epitaxial wafer, the carrier concentration of the epitaxial layer exposed in the back surface of the compound semiconductor single crystal substrate is preferably from 1×10
17
cm
−3
to 2×10
18
cm
−3
within a range of at least 5 &mgr;m from the main surface of the compound semiconductor single crystal substrate along the epitaxial layer growing direction.
If the carrier concentration of the epitaxial layer exposed in the back surface of the compound semiconductor single crystal substrate is from 1×10
17
cm
−3
to 2×10
18
cm
−3
within a range of at least 5 &mgr;m from the main surface of the compound semiconductor single crystal substrate along the epitaxial layer growing direction, a sufficient carrier concentra
Higuchi Susumu
Takahashi Toru
Jr. Carl Whitehead
Rose Kiesha
Shin-Etsu Handotai & Co., Ltd.
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