Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Compound semiconductor
Reexamination Certificate
1998-04-14
2001-02-27
Chaudhuri, Olik (Department: 2814)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Compound semiconductor
C438S047000
Reexamination Certificate
active
06194241
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to light emitting devices having, on a substrate, semiconductor layers that are different in lattice constant, providing a light emitting layer. More particularly, the invention relates to a semiconductor light emitting device and method of manufacturing the same adapted to reduce the effects by the mismatch of lattice constant between the semiconductor layers to thereby improve light emitting characteristics.
BACKGROUND OF THE INVENTION
There is a conventional semiconductor light emitting device formed by a gallium-nitride based compound semiconductor for emitting bluish (ultraviolet to yellow) light has a structure, for example, as shown in FIG.
4
. That is, a sapphire substrate
21
has a low-temperature GaN buffer layer
22
and a high-temperature n-type GaN (cladding) layer
23
epitaxially grown thereon. On the n-type layer
23
is formed an active (light emitting) layer
24
of a material having a bandgap energy lower than that of the cladding layer so as to determine an emission-light wavelength, e.g. a compound semiconductor based on InGaN (having a variable ratio of In and Ga). A p-type (cladding) layer
25
, having a compound semiconductor sublayer
25
a
and a GaN sublayer
25
b,
is formed on the active layer
24
. A p-side electrode
28
is formed on a surface of the p-type layer
25
, while an n-type electrode
29
is formed on the n-type layer
23
exposed by partly etching away the overlying semiconductor layers. Incidentally, the n-type layer may include an AlGaN-based semiconductor sublayer on an active layer
23
side in order to enhance carrier confining effects.
As stated above, the conventional bluish-light semiconductor light emitting device employing a gallium-nitride based compound semiconductor is formed by the layers overlaid on the sapphire substrate, which layers include those of a GaN-based compound semiconductor, an AlGaN-based compound semiconductor, InGaN-based compound semiconductor, and so on. However, sapphire has a lattice constant of 4.76 angstroms, and GaN has a lattice constant of 3.18 angstroms. Al
0.1
Ga
0.9
N, for example, has a lattice constant of 3.12 angstrom, and In
0.05
Ga
0.95
N has 3.198 angstroms. Thus, these layers are different in lattice constant. These semiconductor layers, when formed overlying one another, tend to induce distortions of crystal lattice. This raises such problems that electric current is difficult to flow through the layers, and cracks are induced in the crystal reaching to the light emitting (active) layer, resulting in lower in light emitting efficiency. In particular, the n-type layer is formed thick, i.e. approximately 2-5 &mgr;m, on the sapphire substrate with a largely-different lattice constant through a thin low-temperature buffer layer. Therefore, the n-type layer tends to cumulate crystal distortions and ready to induce lattice mismatch therein.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a semiconductor light emitting device which is excellent in light emitting efficiency by reducing crystal lattice mismatch between semiconductor layers formed different in lattice constant on a substrate.
It is another object to provide a method of manufacturing a semiconductor light emitting device by which a device with excellent light emitting efficiency is obtained by reducing crystal lattice mismatch between the semiconductor layers having different lattice constant formed on a substrate.
A semiconductor light emitting device according to the present invention, comprises: a substrate; a semiconductor layered portion formed of a gallium-nitride semiconductor overlying the substrate and having an n-type layer and a p-type layer to form a light emitting layer; and a gradient layer provided at an interfacial portion between an lower layer and an upper layer of the semiconductor layered portion, wherein the gradient layer has a composition varied from a composition from the lower layer to a composition of the upper layer.
This gradient layer can be structured by a semiconductor layer having a composition continuously varied or a thin-filmed multilayer structure having compositions varied stepwise. With this structure, the semiconductor layers different in lattice constant are out of direct interface with or interfaced with each other through a thin-filmed multilayer structure. Therefore, the composition of the gradient layer has a composition and hence lattice constant varied to prevent the crystal lattice to be mismatched. In order to prevent this lattice mismatch, the gradient layer is sufficiently provided in a thickness of approximately 1-700 nm, more preferably 1-300 nm. This range of the thickness does not influence on the difference in bandgap energy between the cladding layer and the active layer, preventing light emitting efficiency from degrading.
Here, the gallium-nitride based compound semiconductor means a semiconductor formed of a compound having a group-III element Ga and a group-V element N wherein part of the group-III element Ga may be substituted by other group-III elements such as Al and In, and/or part of the group-V element N may be substituted by other group-V elements such as P and As.
The semiconductor layered portion is preferably formed by a first-conductivity type cladding layer, an active layer, and a second-conductivity type cladding layer, and the gradient layer being provided between the active layer and the first-conductivity type layer and/or between the active layer and the second-conductivity type cladding layer. This prevents the crystallinity of the active layer and its vicinity, particularly, for a doublehetero junction structure from degrading due to the difference in lattice constant.
More specifically, the first-conductivity type cladding layer may be formed of GaN, the active layer of an InGaN-based compound semiconductor, and the second-conductivity type cladding layer of AlGaN-based compound semiconductor at least on a side of the active layer. Also, on a side opposite to the active layer with respect to the second-conductivity type cladding layer, a gradient layer may be formed in the second-conductivity type to have a composition varied from a AlGaN-based compound semiconductor to GaN, and a second-conductivity type GaN layer. Further, the substrate and the first-conductivity type cladding layer may have therebetween a low-temperature buffer layer deposited at a low temperature. With such structures, the semiconductor layered portion is improved in crystallinity.
A method of manufacturing a semiconductor light emitting device according to the present invention, comprises the steps of: growing a first-conductivity type cladding layer of a gallium-nitride based compound semiconductor on a substrate by an MOCVD method; growing a gradient layer while varying a composition thereof continuously or stepwisely such that the composition gradually approaches from a composition of the first-conductivity type cladding layer to a composition of an active layer formed of a gallium-nitride based compound semiconductor having a bandgap energy lower than that of the first-conductivity type cladding layer so as to determine an emission light wavelength; forming an active layer continuously after the gradient layer reaches the composition of the active layer; and growing a second-conductivity type cladding layer on the active layer.
A further gradient layer may be formed while varying a composition thereof continuously or stepwisely such that the composition is varied from the composition of the active layer to a composition of the second-conductivity type cladding layer, the second-conductivity type cladding layer being grown on the further cladding layer.
In the MOCVD method, the gradient layer may be formed while gradually increasing a flow rate of a reacting gas being supplied over a semiconductor layer as the gradient layer being grown or gradually varying a temperature without varying the flow rate. Where using In, it is more sensitive to temperature variation than flow-rate change so tha
Itoh Norikazu
Nakata Shunji
Shakuda Yukio
Sonobe Masayuki
Tsutsui Tsuyoshi
Arent Fox Kintner & Plotkin & Kahn, PLLC
Chaudhuri Olik
Rohm & Co., Ltd.
Wille Douglas A.
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