Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With particular semiconductor material
Reexamination Certificate
1999-06-17
2001-05-15
Tran, Minh Loan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With particular semiconductor material
C257S622000, C257S627000, C257S628000, C257S631000
Reexamination Certificate
active
06232623
ABSTRACT:
RELATED APPLICATION DATA
The present application claims priority to Japanese Application No. P10-180930 filed Jun. 26, 1998, which application is incorporated herein by reference to the extent permitted by law.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a semiconductor device and its manufacturing method especially suitable for application to semiconductor lasers and light emitting diodes or electron mobility devices using nitride III-V compound semiconductors such as GaN.
2. Description of the Related Art
Heretofore, semiconductor lasers and light emitting diodes have been developed, using nitride III. V compound semiconductors made of group III elements such as Al, Ga and In and group V elements including N, as represented by GaN, as their light emitting materials in the range extending from green or blue to ultraviolet. On the other hand, although continuous oscillation at room temperatures has been realized with semiconductor lasers, there is still the need for further improvement in crystallographic property of nitride III-V compound semiconductors toward their longer lifetime. For this purpose, a technique for improving crystallographic property of GaN such as ELOG-GaN (Epitaxially Laterally Overgrown GaN) is being used and actually performing effects. However, it is also desired to further improve the basis of the technique, namely, crystallographic property of nitride III-V compound semiconductors grown on crystalline substrates.
Usually used as a substrate crystal is sapphire or SiC. Nitride III-V compound semiconductor layers are grown on such a crystalline substrate after it is processed in a growth apparatus for sufficiently smoothing, polishing and cleaning its surface. In order to manufacture a desired semiconductor device, it is necessary to make various multi-layered structures having surfaces or interface structures which are flat in the atomic level. Therefore, also for substrates, surface cleaning methods and growth methods capable of maintaining or making their surfaces flat are being used.
There has also been proposed a method which improves crystalline property of nitride III-V compound semiconductor layers by slightly inclining a crystalline substrate surface such that layers be grown thereon in a step flow mode (Japanese Patent Lai-Open Publication No. hei 7-201745).
When using a GaAs substrate or InP substrate, a technique is being used, which first grows a buffer layer on a substrate having formed on its (001) surface a (111)B-oriented diffraction grating for the purpose of reflecting guided light in the waveguide. (Japanese Patent Laid-Open Publication No. hei 8-264901).
Also known are a method for improving crystalline property by interposing an AlN buffer layer (Japanese Patent Laid-Open Publication No. hei 2-229476) or a GaN or AlGaN buffer layer (Japanese Patent Laid Open Publication No. hei 4-297023) between a sapphire substrate and a nitride III-V compound semiconductor thereon; a method for improving by nitrifying a sapphire substrate surface (Japanese Patent Laid Open Publication No. hei 5-41541). Also known is a method which sequential stacks a plurality of buffer layers different in lattice constant to reduce differences in lattice constant between the substrate and the buffer layer and between the buffer layer and an epitaxial layer thereon (Japanese Patent Laid Open Publication No. hei 9-63962).
It is generally noted that “fluctuation” in crystalline orientation exists in nitride III-V compound semiconductor crystals grown on a sapphire or SiC substrate. That is, these nitride III-V compound semiconductor crystals are “mosaic crystals”. Fluctuation in crystalline orientation is roughly classified into two components, namely, “twist” mosaic components which fluctuate in a rotational direction around an axis vertical to the substrate surface, and “tilt” mosaic components which are fluctuation in axial orientation from a vertical direction. Magnitude of these fluctuations is noted to be normally 0.01 to 0.5°. Causes of fluctuation in crystalline orientation are considered to lie in a difference in crystalline structure between the substrate and a nitride III-V compound semiconductor layer grown thereon and a large difference in lattice constant between them. It is also considered to be another factor that, crystal orientation is not accurately carried over during growth from the sapphire substrate because of relatively weak atomic coupling between stable sapphire crystal and a nitride III-V compound semiconductor crystal as compared with those inside these crystals.
Due to such mosaic crystalline property, in semiconductor light emitting devices using nitride III-V compound semiconductors, improvement in device characteristics has been prevented, by degradation in emission efficiency, and there are problems such as short lifetime regarding reliability required for the device.
OBJECT AND SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a semiconductor device and its manufacturing method capable of improving crystalline property of a nitride III-V compound semiconductor layer grown on a sapphire substrate.
The Inventor made researches to overcome the above-mentioned problems involved in conventional techniques, as summarized below.
The Inventor made researches to find optimum surface states of a sapphire substrate for improving crystalline property of nitride III-V compound semiconductors grown on the sapphire substrate. It has been believed that being smoother is more desirable for surfaces of sapphire substrates, and efforts have been made for smoothing. For example, Japanese Patent Laid-Open Publication No. hei 8-83802, for example, proposes a technique for obtaining a super-smooth substrate surface exclusively made of terrace surfaces substantially equal in crystalline orientation and having a straight, regular step site, by selecting a heating time and a heating temperature in accordance with the surface orientation upon annealing a sapphire substrate by raising the temperature to 900° C. or higher in a normal pressure atmosphere.
However, according to researches by the Inventor, it is better to positively make recesses on the surface of a sapphire substrate, in contrast to smoothing mentioned above, for the purpose of increasing crystalline property of nitride III-V compound semiconductors grown on the sapphire substrate. This is for the reason explained below.
In the process of crystal growth (including vapor-phase growth and solid-phase growth) on a smooth sapphire substrate surface, crystalline orientation of a nitride III-V compound semiconductor crystal under growth is determined exclusively by interaction between atoms of the nitride III-V compound semiconductor and atoms of the sapphire crystal interposing only one interface parallel to the substrate surface. Therefore, if the interaction at the interface is not large, fluctuation in crystalline orientation mentioned above increases. Especially, control of fluctuation in crystalline orientation along the plane (twist component) becomes weak. In contrast, along the interface having recesses, crystalline orientation can be controlled as a result of limitation of crystalline orientation by lateral constraint force applied through a slanted crystal surface not parallel to the substrate. Therefore, it is possible to establish more precise coincidence in crystalline orientation between the sapphire substrate and a nitride III-V compound semiconductor crystal grown thereon.
In a method using a step-shaped sightly angled substrate uniform in crystalline orientation, constraint of crystalline orientation is one direction. In contrast, in the case where a sapphire substrate surface has formed recesses, growth of a nitride III-V compound semiconductor on the sapphire substrate is restricted by a plurality of crystal directions and crystal surfaces. Therefore, a larger effect can be obtained regarding coincidence in crystalline orientation between the sapphire substrate and a nitride III-V compound semiconductor layer grown t
Sonnenschein Nath & Rosenthal
Sony Corporation
Tran Minh Loan
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