Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure
Reexamination Certificate
2000-02-08
2004-12-28
Pham, Long (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
C257S013000, C257S082000, C257S085000, C257S094000, C257S101000, C257S103000
Reexamination Certificate
active
06835956
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to a device provided with a nitride semiconductor (In
x
Al
y
Ga
1−x−y
N, 0≦x, 0≦y, x+y≦1) including light emitting diode (LED), laser diode (LD), or other electronic devices and power devices. Particularly, the present invention relates to a prevention of a small cracking in nitride semiconductor layers, which occurs in the nitride semiconductor device using a GaN substrate.
BACKGROUND OF THE INVENTION
Blue LEDs using nitride semiconductors have already been provided for practical use. Recently, it becomes possible to provide a practical blue laser diode made of nitride semiconductor by using a GaN substrate.
The inventors have disclosed a nitride semiconductor laser diode using a GaN substrate in, for example, Japanese Journal of Applied Physics. Vol.37(1998) pp.L309-L312. The GaN substrate can be formed, for example, by the following method: A GaN layer is formed on a sapphire substrate and a protective film made of SiO
2
is formed partially on the surface of the GaN film. Then, GaN is grown again on the GaN film and the sapphire substrate is removed. The secondly-formed GaN layer grows mainly in a lateral direction, so that a proceeding of dislocations is prevented. By using this method, a GaN substrate having low dislocation density can be obtained. The nitride semiconductor laser device made with such a low dislocation-density GaN substrate showed continuous-wave oscillation and can be operated continuously for more than ten thousand hours.
DISCLOSURE OF THE INVENTION
The nitride semiconductor laser diode with lifetime of more than ten thousand hours can be applied for practical use. However, in some applications, much longer lifetime is desired. The inventors examined the nitride semiconductor laser device obtained by the above-described method and found that extremely small cracks tend to occur in the nitride semiconductor layers grown on the GaN substrate, particularly in the n-type GaN contact layer which is grown directly on the GaN substrate. The crack is too small to observe by a typical optical microscope, however, it can be observed by a fluorescence microscope. It is a surprising fact that small cracks tend to occur in the GaN layer which is directly grown on the same-composition GaN substrate. It is supposed that the occurrence of small cracks is a specific phenomenon for the GaN substrate which is manufactured by the lateral-growth method. However, it is also supposed that when a thin-film GaN is grown on a thick GaN substrate, small cracks occur for an unknown reason. In any case, it is probable that the small cracks cause an increase of thresholds and a deterioration of lifetime of the laser device. The small cracks may also cause a decrease in reliability for other nitride semiconductor devices, as well as in the laser device.
Therefore, the object of the present invention is to reduce extremely small cracks in the nitride semiconductor layers and to extend a lifetime of the nitride semiconductor device using a GaN substrate, thus improving a reliability of the nitride semiconductor device. For this purpose, the nitride semiconductor device of the present invention is characterized in that, among device-forming layers (=nitride semiconductor layers) formed on the GaN substrate, the device-forming layer which is directly grown on the Gan substrate is provided with compressive strain to reduce the small cracks.
The compressive strain may be achieved by forming a device-forming layer having a smaller coefficient of thermal expansion than that of GaN directly on the GaN substrate. The device-forming layer directly grown on the GaN substrate is preferably Al
a
Ga
1−a
N, (0<a≦1). Because Al
a
Ga
1−a
N, (0<a≦1) has a smaller coefficient of thermal expansion than that of GaN and can be grown on the GaN substrate as a good crystalline.
The device structure constructed by the device-forming layers preferably comprises an n-type cladding layer containing Al, an active layer containing InGaN and a p-type cladding layer containing Al. Employing this structure together with the cracks-reducing structure, a good-characteristics device is provided.
The device-forming layer directly grown on the GaN substrate, for example Al
a
Ga
1−a
N layer, may play various kinds of rolls according to the device structure. For instance, the layer may be an buffer layer for preventing small cracks, or an n-contact layer. When the whole GaN substrate is electrically conductive, the layer may be an n-clad layer.
The GaN substrate is preferably manufactured by using the lateral-growth method. By using the laterally grown GaN substrate, not only the occurrence of the small cracks but also a propagation of dislocations is prevented. Thus, a nitride semiconductor device having good characteristics is provided.
The manufacturing method of the nitride semiconductor element of the present invention comprises the steps of:
(a) forming a first nitride semiconductor layer on a auxiliary substrate made of different material from nitride semiconductor, for example sapphire or SiC;
(b) forming a stripe-shaped or island-shaped periodical concave-convex structure on said first nitride semiconductor layer;
(c) forming a single-crystal GaN layer on said first nitride semiconductor layer to make a GaN substrate; and
(d) forming a second nitride semiconductor layer on said GaN substrate, the second nitride semiconductor layer having a coefficient of thermal expansion smaller than that of GaN.
Further, the auxiliary substrate may be removed from the GaN substrate after forming the single-crystal GaN layer.
According to the present invention, a thermal expansion coefficient of the nitride layer contacting on the GaN substrate is preferably smaller than that of GaN so as to provide the compressive strain in the nitride semiconductor layer. The compressive strain prevents formation of small cracks in the nitride semiconductor layers. The reasons why this effect is obtained can be described as follows: For example, when coefficients of thermal expansion of Si, GaN and sapphire are &egr;
1
, &egr;
2
, &egr;
3
, respectively, the relation of &egr;
1
<&egr;
2
<&egr;
3
stands up. When GaN is grown on the SiC substrate, cracks are liable to occur in the GaN layer. In this case, the relation of coefficients of thermal expansion is &egr;
1
<&egr;
2
and a tensile strain is laid in the in-plane direction on the GaN layer grown on the SiC substrate. On the other hand, when GaN is grown on the sapphire substrate, cracks are not liable to occur in the GaN layer. In this case, the relation of coefficients of thermal expansion is &egr;
2
<&egr;
3
and a compressive strain is laid in the in-plane direction on the GaN layer grown on the sapphire substrate. In short, the liability of cracks to occur depends on whether the strain laid on the layer is a tensile strain or a compressive strain. When the coefficient of thermal expansion of the layer grown on the substrate is smaller than that of the substrate, a compressive strain is laid on the layer and formation of cracks can be prevented.
When GaN is grown on the GaN substrate, neither tensile strain nor compressive strain must be laid on the grown GaN layer. However, small cracks tend to occur in the grown GaN. It is supposed that, when a nitride semiconductor layer is grown on a GaN substrate, small cracks occurs in the nitride semiconductor layer if the thermal expansion coefficient of the layer is equal or greater than that of GaN, and that the formation of the cracks is suppressed if the thermal expansion coefficient of the layer is smaller than that of GaN and compressive strain is laid on the layer.
In this specification, the term “GaN substrate” refers to a substrate having a low-dislocation-density single-crystal GaN layer on its surface. The GaN substrate may be composed only of a single-crystal GaN layer, or it may be composed of an auxiliary substrate made of different material from nitride semiconductor such as sapphire or SiC and a low-
Nagahama Shinichi
Nakamura Shuji
Louie Wai-Sing
Nixon & Vanderhye PC
Pham Long
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