Active solid-state devices (e.g. – transistors – solid-state diode – With means to control surface effects – Insulating coating
Reexamination Certificate
2001-01-09
2002-03-26
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
With means to control surface effects
Insulating coating
C438S046000
Reexamination Certificate
active
06362515
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a GaN substrate which is used in a semiconductor element, and in which the defect density is low. The present invention also relates to a process for producing a GaN substrate which is used in a semiconductor element, and in which the defect density is low. The present invention further relates to a semiconductor element including a semiconductor laser device which uses a GaN substrate in which the defect density is low.
2. Description of the Related Art
S. Nakamura et al. (“Violet InGaN/GaN/AlGaN-Based Laser Diodes Operable at 50° C. with a Fundamental Transverse Mode,” Japanese Journal of Applied Physics, vol. 38 (1999) L226-L229) disclose a short-wavelength semiconductor laser device which emits laser light in the 410 nm band.
This semiconductor laser device is formed as follows. First, a GaN substrate is formed by growing a first GaN layer on a sapphire substrate, selectively growing a second GaN layer by using a SiO
2
mask, and removing the sapphire substrate. Then, an n-type GaN buffer layer, an n-type InGaN crack preventing layer, an AlGaN
-type GaN modulation-doped superlattice cladding layer, an n-type GaN optical waveguide layer, an undoped InGaN
-type InGaN multiple quantum well active layer, a p-type AlGaN carrier block layer, a p-type GaN optical waveguide layer, an AlGaN/p-type GaN modulation-doped superlattice cladding layer, and a p-type GaN contact layer are formed on the above GaN substrate. However, the defect density in the semiconductor laser device is still high, and therefore the semiconductor laser device is not reliable in the high output power range.
In addition, T. S. Zheleva et al. (“Pendeo-Epitaxy-A New Approach for Lateral Growth of Gallium Nitride Structures,” MRS Fall Meeting, Boston, 1998, Extended Abstracts G3.38) report that a flat GaN layer can be formed by utilizing lateral growth of GaN. In the reported process, a first GaN layer is formed without a mask, and then stripe regions of the GaN layer are removed until a sapphire substrate is exposed. Then, a second GaN layer is grown on the exposed sapphire substrate so that the second GaN layer is grown in the lateral directions.
Further, S. Nakamura (“Three Years of InGaN Quantum-well Lasers: Commercialization Already,” SPIE Proceedings, Vol. 3628, 1999, pp.158-168) reports that an InGaN-based multiple quantum well semiconductor laser device can be produced by using the above process proposed by T. S. Zheleva et al. However, the semiconductor laser device produced by the process is reliable only when the semiconductor laser device operates with the output power of 5 mW or less. Therefore, it is necessary to further decrease the defect density.
Furthermore, Japanese Unexamined Patent Publication, No. 10 (1998)-312971 discloses a process for preventing occurrence of a defect, such as a crack, which is caused by differences in the thermal expansion and the lattice constant between a GaN compound semiconductor layer and a sapphire substrate crystal. In the process, regions of growth are confined by a mask, facet structures of the GaN compound semiconductor layer are formed by epitaxial growth, and then the facet structures are further grown so that the mask is completely covered, and finally the surface of the grown crystal of the GaN compound semiconductor layer is planarized. However, in this process, the entire base layer on which the above GaN compound semiconductor layer is grown is formed on a substrate, and the lattice-mismatch between the base layer and the substrate is great. Therefore, the GaN compound semiconductor layer is affected by the substrate, the crystal orientations of the GaN compound semiconductor layer grown in lateral directions vary, and it is difficult to planarize the surface of the GaN compound semiconductor layer. Further, even when the above process is repeated, differences arise in the orientations of the crystal faces, and it is therefore impossible to reduce the defect density to a practical level.
Moreover, Japanese Unexamined Patent Publication, No. 11 (1999)-312825 discloses a process for realizing a low-defect region in a GaN layer formed on a GaN base layer by lateral growth, where the GaN base layer is formed on a plurality of portions of a surface of a sapphire substrate. In addition, a dielectric film is formed on the GaN base layer so as to suppress vertical growth from the GaN base layer. However, in this process, the crystal axis is likely to incline due to the mismatch between the sapphire substrate and portions of the GaN layer which are laterally grown over the sapphire substrate, or stress generated in the vicinity of the boundary between the sapphire substrate and the portions of the GaN layer. Further, as mentioned in Japanese Unexamined Patent Publication No. 11 (1999)-312825, a cavity is formed between the sapphire substrate and the laterally grown portions of the GaN layer, and the formation of the cavity is uncontrollable.
In the GaN substrate disclosed in Japanese Journal of Applied Physics, vol. 38 (1999) L226-L229, the SiO
2
film stops the dislocation which is caused by the lattice mismatch in the vicinity of the boundary between the GaN substrate and the GaN buffer layer, and extends in the thickness direction. In addition, the aforementioned second GaN layer is formed mainly by the lateral growth from a plurality of portions of the aforementioned first GaN layer which are exposed at a plurality of windows of the SiO
2
mask. However, since the laterally grown portions of the second GaN layer coalesce in central portions of a plurality of regions which are located above the remaining SiO
2
film of the SiO
2
mask, defects tend to gather in the central portions of the plurality of regions above the remaining SiO
2
film. In addition, dislocation is likely to extend in the thickness direction, and pass through the above plurality of windows, Therefore, only the above plurality of regions above the remaining SiO
2
film other than their central portions are low-defect regions of the second GaN layer. Such low-defect regions each have a width about 4 micrometers. That is, the low-defect regions are very narrow, and the semiconductor laser devices having a stripe of a 2 &mgr;m width must be formed in such narrow regions.
In addition, according to the processes disclosed in the Extended Abstracts G3.38 of the MRS Fall 1998 Meeting and the SPIE Proceedings, Vol. 3628, 1999, pp.158-168, defects also tend to gather in a plurality of regions in which laterally grown portions of the aforementioned second GaN layer coalesce, In addition, the dislocation is likely to extend in the thickness direction from the first GaN layer, which functions as a base of the growth of the second GaN layer. Therefore, the low-defect regions in the second GaN layer are very narrow, and the semiconductor laser devices having a stripe of a width of several micrometers must be formed in such narrow regions.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a GaN substrate which is used in a semiconductor element, and in which the defect density is low in a wide region.
Another object of the present invention is to provide process for producing a GaN substrate which is used in a semiconductor element, and in which the defect density is low in a wide region.
Still another object of the present invention is to provide a semiconductor element which uses a GaN substrate in which the defect density is low in a wide region.
A further object of the present invention is to provide a semiconductor laser device which uses a GaN substrate in which the defect density is low in a wide region.
(1) According to the first aspect of the present invention, there is provided a GaN substrate comprising: a substrate; a first GaN layer being formed on the substrate and including a plurality of stripe portions which form at least one first groove between adjacent ones of the plurality of stripe portions; a second GaN layer formed over the substrate and the first GaN layer; a first p
Fuji Photo Film Co. , Ltd.
Hoang Quoc
Nelms David
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