Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With particular semiconductor material
Reexamination Certificate
2000-05-11
2002-04-23
Tran, Minh Loan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With particular semiconductor material
C257S614000
Reexamination Certificate
active
06376866
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor light emitting device and a method for producing the same, and more particularly, a semiconductor light emitting device comprising a gallium nitride type compound semiconductor for emission of blue light and a method for producing the same.
Such a gallium nitride type compound semiconductor is (1) a semiconductor comprising a compound of Ga of group III element and N of group V element, or (2) a semiconductor which comprises a GaN compound in which a part of Ga is substituted by other group III elements such as Al or In and/or a part of N is substituted by other group V elements such as P or As.
The semiconductor light emitting devices include light emitting diodes (hereinafter referred to as “LED”) having pn junctions or double heterojunctions such as pin junctions, super-luminescent diodes (hereinafter referred to as “SLD”), and semiconductor laser diodes (hereinafter referred to as “LD”).
Although conventional blue LEDs are lower in luminance than red or green ones and disadvantageous for practical use, they have been improved by using gallium nitride type compound semiconductor and more specifically, doping an amount of Mg thus forming a p-type semiconductor layer with a low resistance and are now available for new applications.
A conventional gallium nitride LED has a structure shown in FIG.
7
. It is fabricated by applying gaseous forms of metal organic compounds such as trimethylgallium (TMG) and ammonia (NH
3
) together with a carrier of H
2
gas to a single-crystal substrate
51
of sapphire (Al
2
O
3
) at a low temperature of 400° C. to 700° C. using a metal organic chemical vapor deposition (MOCVD) method to form a low-temperature buffer layer
54
of, approximately 0.01 to 0.2 micrometer thick, comprising GaN, and applying the gaseous forms of the same materials at a high temperature of 700° C. to 1200° C. to form a high-temperature buffer layer
55
, approximately 2 to 5 micrometers thick, comprising n-type GaN which is identical in the chemical composition to the layer
54
.
A gaseous form of trimethylaluminum (TMA) is then added to the prescribed materials to deposit an n-type cladding layer
56
of, approximately 0.1 to 0.3 micrometer thick, comprising Al
x
Ga
1−x
N (where 0<x<1) for creating a double heterojunction. Those n-type layers are prepared by depending on the fact that gallium nitride type compound semiconductor materials can be made type without addition of any n-type impurities or by simultaneous application of SiH
4
gas.
Then, the same materials including a less amount of Al and a more amount of In than in the cladding layers are deposited to form an active layer
57
which is comprising, for example, Ga
y
In
1−y
N (where 0<y≦1) and lower in band gap energy than the cladding layers.
Also, a p-type impurity of Mg or Zn in the form of a metal organic compound gas of eg. bis(cyclopentadienyl)magnesium (CP
2
Mg) or dimethylzinc (DMZn) is added to the same gaseous materials as of the n-type cladding layers in a reaction tube to form a p-type cladding layer
58
comprising p-type Al
x
Ga
1−x
N.
Furthermore, the same gaseous materials are applied for vapor deposition of a p-type GaN cap layer
59
.
Whole surfaces of growth layers of the semiconductor material is then coated with a protective layer of eg. SiO
2
and the like and annealed for approximately 20 to 60 minutes at a temperature ranging from 400° C. to 800° C., allowing both the p-type cap layer
59
and the p-type cladding layer
58
to be activated. After the protective layer is removed, a resist pattern is applied for assigning n-type electrodes. When the semiconductor layers are subjected to dry etching by chlorine plasma atmosphere, desired regions of the n-type GaN high-temperature buffer layer
55
are exposed as shown in FIG.
7
. Finally, two electrodes
61
and
60
are formed by sputtering of a metal film such as Au or Al. The semiconductor layers are then diced to LED chips,
As understood, a conventional semiconductor light emitting device using the gallium nitride type compound semiconductor material has at back side a sapphire substrate made of an insulating material. For forming electrodes on the back side, it is hence needed to use etching or other complicated processing method
Although the sapphire substrate withstands a high temperature and is easily bonded to any type of crystal surface, the sapphire is very different from the gallium nitride semiconductor material in lattice constant, 4.758 (sapphire substrate) angstrom to 3.189 (gallium nitride type semiconductor crystal) angstrom, and also, in coefficient of thermal expansion. The difference in lattice constant may result in crystal defect or dislocation in the buffer layer stacked on the sapphire substrate as denoted by A in FIG.
8
. If the crystal defect propagates to the single-crystal gallium nitride type compound semiconductor layers which are stacked on the buffer layer and act as operating layers, operating region is declined and also optical characteristics of the semiconductor layers degrade
In addition, the sapphire substrate is hardly cleft and it is thus not easy to produce semiconductor light emitting device chips by cleaving above-mentioned structure of the semiconductor layers. It is said that the conventional semiconductor layer structure described above is not appropriated for producing particular devices such as semiconductor laser devices in which two opposite sides are required to be mirror surfaces which are parallel with each other at high accuracy. It is also hard to process the sapphire substrate which may thus be processed with much difficulty.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved semiconductor light emitting device and a method for producing the same wherein the above disadvantages are eliminated and the generation of undesirable artifacts including the crystal defect and dislocation which may result from mismatch in lattice constant or thermal expansion coefficient are minimized.
It is a further object of the present invention to provide a semiconductor light emitting device and a method for producing the same, the semiconductor light emitting device having multilayer structure wherein processing like as separating wafers to chips easily by cleaving, for example, is easily performed. Consequently, gallium nitride type compound semiconductor according to the present invention enables to obtain mirror surfaces as end surfaces by cleaving for a semiconductor light emitting device which needs, like semiconductor laser, two mirror surfaces which are parallel with each other as end surfaces of the device.
A semiconductor light emitting device according to the first aspect of the present invention in order to achieve the object comprises a single-crystal silicon substrate, an insulating layer formed on the single-crystal silicon substrate, and gallium nitride type compound semiconductor layers provided on the insulating layer.
It is preferable to employ a single-crystal silicon substrate of which (111) crystal plane is a principal plane, since an insulating layer of which lattice matching with gallium nitride type compound semiconductor layer at an interface between the gallium nitride type compound semiconductor substrate and the insulating layer is appropriate can be obtained.
The gallium nitride type compound semiconductor layers may be a plurality layers including a p-type layer and an n-type layer and an active layer for emission of light This structure is preferable so as to provide the light emitting device The gallium nitride type compound semiconductor layers comprises a buffer layer, a lower cladding layer, an active layer, an upper cladding layer, and a cap layer.
The buffer layers are made of n-type GaN, the lower cladding layer is made of n-type Al
x
Ga
1−x
N (0<x<1), the active layer is made of Ga
n
In
1−n
N (0<n≦1), the upper cladding layer is made of p-type Al
x
Ga
1−x
N (0<x<1), and the cap la
Arent Fox Kintner Plotkin & Kahn
Rohm & Co., Ltd.
Tran Minh Loan
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