Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With particular semiconductor material
Reexamination Certificate
2000-09-06
2002-08-06
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With particular semiconductor material
C257S079000, C257S615000
Reexamination Certificate
active
06429465
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a nitride semiconductor device for use in photoelectronic devices in blue and ultraviolet light regions and electronic devices operating at high temperature and high power, as well as the method of manufacturing the same.
2. Description of Related Art
Nitride series semiconductors having wide band gaps such as AlN, AlGaN, GaN, GaInN and InN have been noted as materials to be applied for photoelectronic devices in blue and ultraviolet light regions and electronic devices operating under severe circumstances at high temperature and high power, and a blue light emitting diode using GaN as a main material has been attained. At present, a molecular beam epitaxy (MBE) method and an organic metal chemical vapor deposition method (MOCVD) method have been adopted mainly for growing thin films of nitride series semiconductors. The MOCVD method is adapted for transporting starting materials in a gas phase and growing them by chemical reaction on a substrate, by which an ultra thin film can be formed and mixed crystal ratio can be controlled easily by controlling the flow rate. Further, since it is possible to grow crystals at high uniformity for a large area in principle, it is an industrially important method.
However, it requires a high temperature for decomposing ammonia mainly used as a nitrogen material in the MOCVD method and it requires a substrate temperature of 900 to 1200° C. in order to manufacture GaN crystals at high quality with a practical growing rate. Because of the high substrate temperature, it is difficult for lamination with a semiconductor which is broken at high temperature such as GaAs or GaP, which restricts the kind of substrates to be used.
Further, as the substrate for use in nitride semiconductor devices, sapphire substrates greatly different in the lattice constant have been used generally at present because bulk crystals are not available. However, due to the difference of the lattice constant, when a nitride series semiconductor is grown directly on a sapphire substrate, crystals of good quality usable for semiconductor devices can not be grown. In view of the above, a two stage growing method is used at present, in which an AlN film or a GaN film is grown at a low temperature as a buffer layer and, after elevating to a high temperature, crystals are grown subsequently. For the practical quality, it requires half width of 1.5 to 9 arcmin (0.025-0.15 degrees) in an X-ray rocking curve reflecting the fluctuation in the growing direction for crystals obtained by optimizing the buffer layer as reported by Nakamura, et al. It is considered that crystal nuclei is formed and enlarged by elevating the temperature of the buffer layer thus grown at a low temperature, thereby enabling to grow crystals during high temperature growth. Accordingly, it is difficult for the growth under the condition other than the high temperature growing condition in which crystallization proceeds effectively from amorphous or crystallite by thermal energy.
For the direct growing to a sapphire substrate, Tokuda, et al (Shingaku Giho: Technical Report of IEICE ED95-120, CPM 95-88 (1995-11) p25) reported that 13.2 arcmin (0.22 degrees) of half width for an X-ray rocking curve was obtained at a substrate temperature of 700 to 800° C. by a plasma assisted MOCVD method, but this half width of the X-ray rocking curve is not sufficient in view of the quality for use in semiconductor devices. According to their report, the ratio between the group VA element (group No. 15, according to the revised nomenclature in the inorganic chemistry of IUPAC in 1989) and the group IIIA element (group No. 13 according to the revised nomenclature in the inorganic chemistry of IUPAC in 1989), (atom number ratio: VA group element/group IIIA element) is (35700/1) or less and the high frequency power is 150 W. Further, it has been reported that degradation occurs particularly at 700° C. or lower under the conditions and crystals are grown as polycrystals. Accordingly, it has been difficult to directly grow a GaN film of high quality at a low temperature of 700° C. or lower on a sapphire substrate.
On the other hand, for fabricating an electronic device, it is necessary to laminate and grow on an electrode but high substrate temperature destroys an electrode material or causes change in the state of the boundary due to thermal reaction between the electrode material and the nitride series semiconductor to deteriorate electrical characteristics as the electronic device. In view of the above, since a high substrate temperature restricts the degree of freedom for the selection of semiconductors and electrodes laminated upon fabricating a device structure, it has been demanded for a low temperature growing method.
By the way, in converting the GaN semiconductor into a p type which is essential for the device fabrication, since an optimal method at present includes a thermal annealing method in a nitrogen atmosphere and annealing is conducted at a temperature of 600° C. or higher, low temperature growing causes a problem in the activation of Mg. In usual MOCVD method, a great amount of hydrogen and ammonia are used and Mg as a dopant for p-type conversion forms a composite compound with hydrogen and becomes inactive. In view of the problems, the plasma assisted MOCVD method can provide a low temperature growing method, as well as enables crystal growth even in a state of no substantial presence of hydrogen in the reaction system, so that this is an effective method capable of suppressing the inactivation of Mg.
However, most of nitride semiconductors manufactured by plasma assisted MOCVD method at a substrate temperature of 600° C. or lower are amorphous, polycrystals, mixed crystals of hexagonal system and cubic system and polycrystals in which plural orientation of crystals are present together, and half width in an XRD rocking curve of a specimen fabricated at 500° C. or lower shows a value greater by one digit or more compared with that of the high temperature MOCVD method (Materials Science Forum Vols. 264-286 (1998) 1205, Tokuda, et al, Technical Report of IEICE ED95-120, CPM95-88 (1995) 25, Jpn. J. Appl. Phys. Vol. 37 (1998) L294). Low crystallinity in the nitride semiconductor involves a problem of trapping of photo-excited or injected carriers in the semiconductor to an impurity energy level or extinction or deactivation, making it impossible for the use as light emitting/receiving devices.
SUMMARY OF THE INVENTION
For overcoming the foregoing problems in the prior art, the present invention intends to provide a nitride semiconductor device in which a nitride series compound semiconductor of high quality and excellent in crystallinity is grown directly on a substrate.
Further, the present invention intends to provide a method of manufacturing a nitride semiconductor device which can manufacture the nitride semiconductor device at a low substrate temperature.
As a result of the earnest study for overcoming the problem in the prior art, the present inventors have found that a nitride series compound semiconductor crystal can be grown directly on a substrate with no provision of a buffer layer and nitride series compound semiconductor crystals of high quality can be grown at a low temperature by making the ratio between the material of the element belonging to the group IIIA and the nitrogen material to be supplied to the substrate different from that of the prior art and by the energy possessed by superfluous excited nitrogen, and nitride series compound semiconductor crystals of high quality can be grown at a low temperature, and have accomplished the present invention.
The subject described above can be solved in accordance with the followings.
<1> A nitride semiconductor device in which a nitride series compound semiconductor having at least an element belonging to the group IIIA and nitrogen is grown directly on a substrate, X-ray diffraction peaks of the nitride series compound semiconductor only include the peaks fro
Iwanaga Takeshi
Suzuki Seiji
Suzuki Toshihiko
Torigoe Nobuyuki
Yagi Shigeru
Fuji 'Xerox Co., Ltd.
Oliff & Berridg,e PLC
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