Semiconductor devices

Details Semiconductor devices Semiconductor devices

2001-07-02

2002-10-01

Davie, James (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

Reexamination Certificate

active

06459712

ABSTRACT:

TECHNICAL FIELD
The present invention relates to the crystal growth of a III-V compound semiconductor composed of at least one of what is called the group-III elements including B, Al, Ga, In and Tl and at least one of what is called the group-V elements including N, P, As, Sb and Bi, or more in particular to crystal growth techniques desirable for forming the crystal of a structure having the hexagonal symmetry structure or the crystal of a III-V compound (hereinafter referred to as “the nitride semiconductor”) required to contain N (nitrogen) as a group-V element.
Also, the present invention relates to a semiconductor device composed of the crystal having a structure of the hexagonal system or a nitride-semiconductor, and to semiconductor light-emitting diodes and semiconductor laser devices suitable for emitting the light with wavelengths up to the ultra=violet ray or suitable as a light source for optical information processings or a light source for optical measurement equipments.
BACKGROUND ART
In recent years, various reports on the diodes and laser devices for emitting a light in the wavelength of blue region using GaInNIGaN/AlGa materials have been published in Appl. Phys. Lett., Vol. 64, March 1944, pp. 1687-1689 (Article 1); Appl. Phys. Lett., Vol. 67, September 1995, pp. 1868-1870 (Article 2); Jpn. J. Appl. Phys., Vol. 34-7A, July 1995, pp. L797-L799 (Article 3); Jpn. J. Appl. Phys., Vol. 34-10B, October 1995, pp. L1332-L1335 (Article 4); and Jpn. J. Appl. Phys., Vol. 34-11B, November 1995, pp. L1517-L1519 (Article 5).
What is shared by the semiconductor devices disclosed in Articles 1 to 5 is that a buffer layer composed of the above-mentioned nitride semiconductor is formed on a sapphire (Al
2
O
3
) substrate, and a nitride semiconductor layer is grown on the buffer layer. Such a structure is disclosed in JP-A-4-297023 (and JP-A-7-312350 constituting a divisional application thereof, and a corresponding U.S. application patent No. 5,290,393) and JP-A-4-321280. According to the teaching of JP-A-4-297023, a polycrystalline layer is produced by forming a first nitride semiconductor layer made of Ga
x
Al
1-x
N (0≦x≦1) on a sapphire layer at 300 to 700° C. lower than the melting points of these materials. When a second nitride semiconductor layer is grown on this polycrystalline layer at a temperature of 1000 to 1050° C., the second nitride semiconductor layer is epitaxially grown with the grains (crystal grains) constituting the first nitride semiconductor layer as nuclei. As a result, an epitaxial film of a nitride semiconductor having fine surface morphology can be formed on the sapphire substrate. A proposal thus has been made to utilize the above-mentioned polycrystalline layer as a buffer layer for forming a semiconductor device.
The reason why a semiconductor device composed of the above-mentioned nitride semiconductor is formed on a sapphire substrate is, as disclosed in JP-A-6-101587, that the crystal structure of sapphire, unlike that of GaAs or the like (having a cubic symmetry structure of zinc-blende type), has a hexagonal closed-packing structure (also called hexagonal zinc sulfide or wurtzite structure). According to this publication, however, the difference in lattice constant between GaN and sapphire is as large as about 14%. The nitride semiconductor layer formed on the sapphire substrate, therefore, develops lattice defects such as dislocations, so that the non-saturated bonding caused in the nitride semiconductor layer forms a doner level or absorbs elements of impurities constituting donors. The resulting problem is that this nitride semiconductor layer assumes N type and the life time of the carriers injected into this nitride semiconductor layer is shortened. This publication, in order to solve this problem, discloses a technique which employs a substrate made of MgAl
2
O
4
having a cubic symmetry spinel crystal structure or MgO having a NaCl-type crystal structure and fabricates nitride semiconductor layers on the substrate by matching the lattice constants between them. A semiconductor laser using this technique is reported in Appl. Phys. Lett., Vol. 68, April 1996, pp. 2105-2107 (Article 6).
The above-mentioned conventional techniques teach the possibility of realizing a semiconductor device made of what is called a nitride semiconductor required to contain N (nitrogen) as a III-V chemical compound semiconductor or a group-V element having a crystal structure of the hexagonal symmetry structure. Nevertheless, sufficient data (for example, the continuously operating time of a laser device) are not available to guarantee the practicability of such a semiconductor device. Especially, the supplementary trials conducted by the inventors show that the density of defects developed in the nitride semiconductor layer is as high as 10
11
cm
−2
, and the inventors judged that a laser device capable of being operated continuously for at least 1000 hours cannot be realized under the above-mentioned conditions.
In recent years, NIKKEI ELECTRONICS, Dec. 4, 1995 issue, No. 650, pp. 7 (Article 7) has reported that as a result of a joint research made between Cree Research, Inc. and North America Phillips, it was found that the use of SiC crystal as a substrate reduces the lattice defect density of the nitride semiconductor layer formed on the substrate to as low as 10
8
cm
−2
and thus can realize blue laser diodes higher in brightness than the conventional devices. According to Article 7, however, the defect density of the nitride semiconductor layer is insufficient to lengthen the life time of the laser diode, and the reduction in the defect density (10
4
cm
−2
at present) of the crystal of the SiC substrate is indispensable for reducing the defect density of the nitride semiconductor layer. In constructing a semiconductor laser by forming a nitride semiconductor layer on a SiC substrate, therefore, an improved quality of the SiC substrate as well as the growth of a nitride semiconductor layer is indispensable for lengthening the life time, and the development cost is expected to increase.
Also, the articles and publications introduced above refer to the configuration of a nitride material used for an optical active layer or an optical waveguide layer but not to the shape of the active layer or the waveguide for controlling the transverse mode of the semiconductor laser. Thus, none of the articles and publications contain the description of a method of reducing the crystal defect density suitable for the waveguide structure mentioned above or, especially, a method of reducing the optical loss in the neighborhood of the active layer of the waveguide.
DISCLOSURE OF INVENTION
A first object of the present invention is to realize a crystal growth technique for forming a semiconductor layer having a very low defect density made of a III-V compound semiconductor having a crystal structure of the hexagonal symmetry structure or made of what is called a nitride semiconductor required to contain N (nitrogen) as a group-V element. “A very low defect density” indicates a defect density on the order of 10
7
-cm
−2
or less which is difficult to attain by the above-mentioned technique using a SiC substrate. This crystal growth technique not only includes a technique for reducing the defect density of a fabricated semiconductor layer uniformly as a whole but also is aimed at examining what is called the selective crystal growth for reducing the defect density only in the desired region.
A second object of the present invention is to lengthen the life time of the semiconductor device fabricated using the above-mentioned crystal growth technique or to improve the life or mobility of the carriers involved in the operation of the semiconductor device to a sufficient value for practical application of the semiconductor device. Especially with the light-emitting diode or the laser diode, the second object of the invention is to define a configuration of an optical waveguide suitable for geometrically controlling or reducing the optica

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