Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With particular dopant material
Reexamination Certificate
2000-09-01
2003-02-11
Jackson, Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With particular dopant material
C257S097000, C257S103000
Reexamination Certificate
active
06518602
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nitride compound semiconductor light emitting device produced on a nitride compound semiconductor substrate and a method for producing the same.
2. Description of the Related Art
In the prior art, nitride compound semiconductors have been used in or studied for use in light emitting devices and high power devices, utilizing their advantageous characteristics.
For example, a nitride compound semiconductor light emitting device can technically be capable of emitting light of a wide range of wavelengths, e.g., from violet to orange, by appropriately adjusting the composition thereof. In recent years, blue light emitting diodes and green light emitting diodes have been put into practical use utilizing the advantageous characteristics of nitride compound semiconductors. As semiconductor laser devices, blue-violet semiconductor laser devices have also been developed in the art.
When producing a nitride compound semiconductor film, sapphire, SiC, spinel, Si, GaAs, GaN, or the like, may be used as a substrate. Where sapphire, for example, is used as a substrate, a GaN or AlN buffer layer is formed at a low temperature of 500° C. to 600° C. prior to the epitaxial growth of a GaN film. Thereafter, the substrate is heated to a high temperature of 1000° C. to 1100° C. and a nitride compound semiconductor film is epitaxially grown. It is known in the art that in this way, it is possible to obtain a structurally and electrically desirable crystal also having a good surface condition. It is also known in the art that where SiC is used as a substrate, it is desirable to use a thin AlN film as a buffer layer at a growth temperature at which an epitaxial growth process is performed.
However, where a substrate other than a nitride compound semiconductor substrate, e.g., a GaN substrate, is used, a large amount of defects (e.g., dislocations) may be introduced into the produced nitride compound semiconductor due to differences in thermal expansion coefficient and in lattice constant between the substrate and the nitride compound semiconductor film grown thereon. The total density of such defects may be as high as about 1×10
9
cm
−1
to 1×10
7
cm
−2
. Dislocations of such a density are known to trap the carriers which control the electrical conduction of the nitride compound semiconductor substrate, thereby deteriorating the electrical characteristics of the produced film. Such dislocations are also known to shorten the operating lifetime of a laser device which uses a high level current.
In order to reduce the resulting lattice defects and to improve the electrical characteristics, various methods have been tried in the art, including a hydride vapor phase epitaxy (H-VPE), a high pressure synthesis method, a sublimation method, and the like, to form a thick film of a nitride compound semiconductor, e.g., GaN, having a thickness of about 20 &mgr;m or more, which can be used as a nitride compound semiconductor thick film substrate.
By using such a nitride compound semiconductor thick film substrate, it is possible to reduce the density of defects reaching the substrate surface and to obtain a light emitting device having desirable characteristics.
However, even with such a nitride compound semiconductor thick film having a thickness over 20 &mgr;m on which a nitride compound semiconductor film is epitaxially grown (hereinafter, referred to also as a “nitride compound semiconductor substrate”), edge dislocations which extend in a direction perpendicular to the C axis are not completely eliminated and an amount of dislocations of about 1×10
6
cm
−2
or more still exists. It has been found that such dislocations, even though the amount thereof is reduced by an order of magnitude as compared with those resulting when using other types of substrates, adversely affect the emission intensity and the operating lifetime of a light emitting device such as a laser diode (hereinafter, referred to also as a “laser device”) to which a high density current is injected.
A nitride compound semiconductor substrate doped with no impurity exhibits a high electrical resistance. Such an electrical resistance has to be reduced by doping with an impurity. However, a number of problems arise when a certain amount of impurity is injected during growth of a GaN thick film by using an N-VPE method, or the like, as in the prior art. For example, when a nitride compound semiconductor substrate which has been produced by injecting a certain amount of high concentration impurity thereinto is used in a nitride compound semiconductor laser device, the threshold voltage is reduced, but the thermal current density increases on the other hand. This is due to a mutual diffusion which occurs through dislocations in the crystal between an impurity doped into the nitride compound semiconductor substrate and an impurity doped into a film which is epitaxially grown on the substrate as a part of the light emitting device structure. Thus, a current barrier is partially formed at the interface between the nitride compound semiconductor substrate and the epitaxially grown film. This gives rise to adverse influences, e.g., an increase in the driving voltage of the light emitting device and a reduction in the operating lifetime of the light emitting device.
Moreover, regarding the surface morphology of the nitride compound semiconductor substrate doped with a high concentration of an impurity, such a nitride compound semiconductor substrate has a substantial surface roughness as compared with that of nitride compound semiconductor substrates. Therefore, although a laser device produced on such a substrate has a reduced threshold voltage, the threshold current density tends to increase due to scattering of propagated light caused by the substantial surface roughness.
In order to provide a light emitting device having improved electrical characteristics and a desirable operating lifetime, it has been desired to produce a nitride compound semiconductor substrate having a substrate surface (hereinafter, referred to also as a “growth surface”) on which a nitride compound semiconductor film is epitaxially grown with a reduced dislocation density and a desirable electrical contact between the substrate and the epitaxially grown film.
SUMMARY OF THE INVENTION
In order to solve these problems, it is important to reduce the defect density of the nitride compound semiconductor substrate and to appropriately control the electrical contact between the growth surface of the nitride compound semiconductor substrate and the epitaxially grown film on the nitride compound semiconductor substrate.
According to one aspect of this invention, there is provided a nitride compound semiconductor light emitting device, including: a nitride compound semiconductor substrate; and alight emitting device section including a nitride compound semiconductor provided on the nitride compound semiconductor substrate. The nitride compound semiconductor substrate contains a group VII element as an impurity.
In one embodiment of the invention, the nitride compound semiconductor substrate contains as its main components nitride and gallium.
Thus, a group VII element having a large ion radius is introduced into a crystal of another element having a small ion radius (e.g., nitrogen, gallium, or aluminum) which forms the nitride compound semiconductor substrate, thereby stopping the propagation of displacements to the surface of the crystal. As a result, the dislocation density in the surface of the nitride compound semiconductor substrate is reduced. The use of such a substrate increases the emission intensity and the operating lifetime of the light emitting device epitaxially grown on the nitride compound semiconductor substrate.
In one embodiment of the invention, the light emitting device section includes: a nitride compound layer of a first conductivity type; a cladding layer of the first conductivity type provided on the nitride compound layer of the first
Ishida Masaya
Taneya Mototaka
Tsuda Yuhzoh
Yuasa Takayuki
Jackson Jerome
Morrison & Foerster / LLP
Sharp Kabushiki Kaisha
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