Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With particular semiconductor material
Reexamination Certificate
2003-02-07
2004-11-23
Tran, Minhloan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With particular semiconductor material
C257S088000, C257S091000, C257S098000, C257S099000
Reexamination Certificate
active
06822270
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to semiconductor light emitting devices, and more particularly to a semiconductor light emitting device having as a light emitting layer a gallium nitride base compound semiconductor layer expressed by a general formula of In
x
Ga
y
Al
z
N that is provided on a substrate having a smaller coefficient of thermal expansion than GaN, with an intermediate layer interposed therebetween.
2. Description of the Background Art
Of the nitride semiconductor material systems employing GaN, InN, AlN and their mixed crystal semiconductors, a semiconductor light emitting device using In
x
Ga
1-x
N crystal as a light emitting layer has conventionally been fabricated employing a sapphire substrate as its substrate primarily.
When a Si substrate is applied to the material system as the substrate, it will be possible to fabricate a less expensive semiconductor light emitting device, because the Si substrate is less expensive than the sapphire substrate and the one having a large area is commercially available.
Here, as an attempt to crystal grow a nitride semiconductor film on the Si substrate, providing a BAlGaInN base single-layer or multi-layer structure as an intermediate layer to fabricate a nitride base semiconductor light emitting device has been disclosed in Japanese Patent Laying-Open Nos. 5-343741 and 2000-277441.
Further, the following publication 1 describes a way of fabricating a nitride base semiconductor light emitting device by stacking an AlN layer and an Al
0.27
Ga
0.73
N layer one another for use as an intermediate layer.
Publication 1: M. Adachi et al., “Fabrication of Light Emitting Diodes with GaInN Multi-Quantum Wells on Si(111) Substrate by MOCVD”, Proc. Int. Workshop on Nitride Semiconductors, IPAP Conf Series 1, pp. 868-871.
For the combination technique for performing lattice alignment, however, adequate studies have yet to be made. Based on the results of the inventors' studies, when a substrate such as a Si substrate having a smaller coefficient of thermal expansion than a nitride semiconductor film is employed, it would be difficult to grow a nitride semiconductor film of good quality and less dislocation by simply providing such an intermediate layer as described in the above publication. A light emitting layer fabricated on the film would suffer considerable dislocation, hindering implementation of a high-luminance light emitting device.
Further, when a nitride base semiconductor device is fabricated on a Si substrate, cracks would occur due to the difference in coefficient of thermal expansion when the fabricated film is cooled to room temperature. Thus, it has been found that it is important to employ hard AlN to reduce occurrence of such cracks.
In other words, when a substrate having a lattice constant different from that of and a coefficient of thermal expansion smaller than that of a nitride semiconductor film is being employed, it is necessary to grow an AlGaInN layer containing a large amount of AlN exhibiting high degrees of c-axis orientation and hardness. This AlGaInN intermediate layer, however, has a low lattice constant, due to AlN contained in such a large amount, and would apply large compressive strain to a GaInN light emitting layer constituting the light emitting device structural portion, thereby deteriorating its crystallinity and degrading the luminous efficiency.
For example, in the structure described in the above publication
1
, an intermediate layer
102
is formed of an AlN layer
102
a
of a thickness of 120 nm (a-axis lattice constant: 0.3112 nm) and an Al
0.27
Ga
0.73
N layer
102
b
of a thickness of 380 nm (a-axis lattice constant: 0.3168 nm) stacked one another on a Si substrate
101
, as shown in
FIGS. 5A and 5B
. On the intermediate layer
102
, a GaN layer
103
(a-axis lattice constant: 0.3189 nm) and a GaInN light emitting layer
106
are formed.
The lattice constant described herein is simply an a-axis lattice constant of a bulk, i.e., one theoretically calculated using the Vegard's Law, because an actual lattice constant would change according to deformation such as strain, thereby introducing discrepancies to the values.
FIGS. 5A and 5B
respectively show a schematic cross section of the configuration of the semiconductor light emitting device described in the above publication
1
and the a-axis lattice constants of the respective layers in the bulk states.
As such, the lattice constant of AlGaInN base intermediate layer
102
can be increased by lowering the content of Al or increasing the content of Ga or In therein gradually or stepwise. The AlGaInN intermediate layer
102
of multi-layer structure thus permits lattice alignment from Si substrate
101
to GaN layer
103
. Such an lattice alignment effect, however, is insufficient with only the multi-layer AlGaInN intermediate layer
102
. Dislocation is obvious on this intermediate layer
102
, making it difficult to grow GaN layer
103
of good quality. As such, when a light emitting layer
106
is formed on GaN layer
103
and a voltage is applied thereto, an unproductive leakage current not contributing to the emission of light emitting layer
106
would increase, hindering implementation of a high-luminance semiconductor light emitting device.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a long-life and high-luminance nitride base semiconductor light emitting device, when a substrate such as a Si substrate having a smaller coefficient of thermal expansion than a nitride semiconductor film is employed, by suppressing occurrence of cracks and ensuring good crystallinity of the nitride semiconductor film.
The semiconductor light emitting device of the present invention is a semiconductor light emitting device having a gallium nitride base compound semiconductor layer expressed by a general formula of In
x
Ga
y
Al
z
N (x+y+z=1, 0≦x≦1, 0≦y≦1, 0≦z≦1), characterized in that it includes one intermediate layer between a first GaN layer and a light emitting layer and that the one intermediate layer has a lattice constant that is closer to a lattice constant of the light emitting layer than a lattice constant of the first GaN layer.
According to the semiconductor light emitting device of the present invention, provision of the one intermediate layer having a lattice constant closer to that of the light emitting layer than that of the first GaN layer permits sufficient lattice alignment, and thus effectively reduces strain applied to the light emitting layer. A first GaN layer of high quality, suppressed with occurrence of dislocation, can be obtained. Accordingly, it is possible to obtain a long-life and high-luminance semiconductor light emitting device.
Preferably, the semiconductor light emitting device described above is further provided with a substrate having a smaller coefficient of thermal expansion than GaN, and another intermediate layer formed between the substrate and the first GaN layer. The another intermediate layer has a lattice constant that is closer to the lattice constant of the first GaN layer than a lattice constant of the substrate.
The another intermediate layer permits lattice alignment between the substrate and the first GaN layer. Accordingly, it is possible to obtain a first GaN layer of high quality with occurrence of dislocation being suppressed.
Preferably, in the semiconductor light emitting device described above, the another intermediate layer includes an Al
a
Ga
b
In
1-a-b
N layer (0≦a≦1, 0≦d≦1, a+b≦1).
Including the hard AlN layer in the another intermediate layer of Al
a
Ga
b
In
1-a-b
N layer prevents occurrence of cracks due to the difference in coefficient of thermal expansion.
Preferably, in the semiconductor light emitting device described above, the one intermediate layer includes an IN
c
Ga
d
Al
1-c-d
N layer (0<c≦1, 0≦d≦1, c+d≦1).
This allows the lattice constant of the one intermediate layer to come closer to that of the light emitting
Aioi Akio
Koide Norikatsu
Nishino Takeshi
Morrison & Foerster / LLP
Sharp Kabushiki Kaisha
Tran Minhloan
Tran Tan
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