Active solid-state devices (e.g. – transistors – solid-state diode – Specified wide band gap semiconductor material other than... – Diamond or silicon carbide
Reexamination Certificate
1999-11-30
2002-07-02
Flynn, Nathan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Specified wide band gap semiconductor material other than...
Diamond or silicon carbide
C257S761000, C257S763000, C257S768000, C257S770000
Reexamination Certificate
active
06414338
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to methods for producing n-type semiconducting diamond, particularly such methods which utilize a chemical vapor deposition (“CVD”) preparation of carbon to form a diamond structure wherein a deposition substrate is heated by a rhenium hot wire, and more particularly to semiconducting diamond materials having a uniformly distributed concentration of rhenium throughout.
An n-type semiconducting diamond is indispensable to the fabrication of semiconducting diamond devices which depend on a p-n junction for their successful operation. Several attempts have been made to dope diamond with impurities by the super-high pressure synthesis process or ion implantation but none has succeeded in synthesizing an n-type semiconducting diamond having low electrical resistivity and high electron mobility.
The present invention was developed to fill a need for a semiconducting diamond material having n-type conductivity. Semiconducting diamond has numerous attributes which are attractive for high-frequency, high-power semiconductor applications. These properties include a high electrical field breakdown voltage, elevated temperature stability, high electron and hole mobilities, high thermal conductivity, and excellent resistance to radiation.
2. Background Art
Semiconducting synthetic or natural diamonds are mostly prepared or found as p-type materials, with boron atoms being the most common impurity. M. W. Geiss. D. D. Rathman, D. J. Ehrlich. R. A. Murphy, W. T. Lindley, “High-Temperature Point-Contact Transistors and Schottky Diodes Formed on Synthetic Boron-Doped Diamond,” IEEE Electron Device Letters, Vol. EDL-8, No. 8, (August 1987), pp. 341-343, discloses the formation of point-contact transistors on synthetic boron-doped diamond. The diamond used in formation of the point-contact transistor is a diamond single crystal produced using a high-temperature-high-pressure process. H. Shiomi, Y. Nishibayashi, N. Fujimore, “Electrical Characteristics of Metal Contacts to Boron-Doped Diamond Epitaxial Films,” Japanese Journal of Applied Physics, Vol. 28, No. 5, (May 1989), pp.758-762, teaches the production of a planar field effect transistor device based on a diamond film. The device disclosed comprises a single crystal diamond substrate on which a single crystal epitaxial layer of boron doped diamond is deposited, thereby producing a p-type semiconductor layer. Titanium (Ti) source and drain contacts, as well as an aluminum (Al) gate Schottky contact are deposited on the diamond film. Type IIb diamonds are described, in A. S. Vishnevskil, A. G. Gontar, “Electrical Conductivity of Heavily Doped P-Type Diamond,” Soviet Physics-Semiconductor, Vol. 15(6), (June 1981), pp. 659-661; A. S. Vishnevskil, A. G. Gontar, “Electrical Conductivity of Synthetic Diamond Crystals,” Soviet Physics-Semiconductor, Vol. 11(1), (October 1977), pp.1186-1187; G. N. Bezrukov, L. S. Smrnimov, “Some Electrical and Optical Properties of Synthetic Semiconducting Diamonds Doped With Boron,” Soviet Physics-Semiconductor, Vol. 4(4), (October 1970), pp. 587-590; J. J. Hauser, J. R. Patel, “Hopping Conductivity in C-Implanted Amorphous Diamond, or How to Ruin a Perfectly Good Diamond,” Solid State Communications, Vol. 18, (1976), pp. 789-790; I. G. Austin, R. Wolfe, “Electrical and Optical Properties of a Semiconducting Diamond,” Proc. Phys. Soc., (1956), pp. 329-338; P. T. Wedepohl, “Electrical and Optical Properties of Type IIb Diamonds,” Proc. Phys. Soc., Vol. LXX, No. 2B, (1957), pp.177-185; A. T. Collins, A. W. S. Williams, “The Nature of the Acceptor Centre in Semiconducting Diamond,” J. Phys. C: Solid St. Phys., Vol. 4, (1971), pp. 1789-1800; and V. S. Vavilov, “Ion Implantation into Diamond,” Radiation Effects, Vol. 37, (1978), pp. 229-236.
Impurities such as lithium which produce n-type diamond have been incorporated into previously formed diamond crystal lattices by ion implantation methods. See U.S. Pat. No. 5,792,256 to Kucherov, et al., and “Electrical Properties of Diamond Doped by Implantation of Lithium Ions,” V. S. Vavilov, E. A. Konorova, E. B. Stepanova, and E. M. Trukhan, Soviet Physics-Semiconductors, Vol. 13(6), (1979), pp.635-638. In the case of the former employs a nuclear transmutation technique to convert boron-10, previously incorporated into a carbon-12 matrix, to lithium-7 by bombardment with a high flux of thermal neutrons. The matrix is then converted to a diamond lattice by initiating a solid phase transformation in the carbon using high pressure and temperature.
The latter method produces thin layers of n-type diamond having lithium directly inserted by high energy implantation. The thickness of the layer is, therefore, limited by the energy of accelerated ions that are not infinite. Moreover, the incorporation of n-type impurities into diamond crystal lattices by ion implantation causes a severely damaged surface layer due to graphitization of the diamond which cannot be removed by annealing. Doping with lithium by this method to concentrations higher than 10
15
/cm
3
also leads to graphitization. The implanted crystal must then be heat treated to electronically activate the implanted impurity. The severely damaged layer resulting from ion implantation and the inhomogeneity and significant concentration gradients of the implanted ion across the implanted film thickness render the n-type diamond unsuitable for semiconductor applications.
Another method for producing n-type diamond involves the formation of p-type diamond thin film by adding a boron compound such as diborane to the raw material gas for microwave plasma chemical vapor deposition and then converting boron-10 to lithium-7 by neutron irradiation. However, the chemical vapor deposition method limits the depth of the semiconducting substrate to approximately 100 microns. Moreover, the material is unsuitable for semiconductor materials because of radioactive isotopes created during neutron irradiation.
Other attempts to make an n-type semiconducting diamond are described in V. S. Vavilov, E. A. Konorova, “Electric Properties of Diamond Doped by Implantation of Lithium”, Soviet Physics-Semiconductors, Vol. 13(6), p. 635 (1979); in V. S. Vavilov, E. A. Konorova, “Conductivity of Diamond Doped by Implantation of Phosphorus Ions,” Soviet Physics-Semiconductors, Vol. 9(8), (1976), pp. 962-964; in V. S. Vavilov, E. A. Konorova, “Implantation of Antimony Ions Into Diamond,” Soviet Physics-Semiconductors, Vol. 6(12), (1972), pp. 1998-2002; in Jean-Francois Morhange, “Study of Defects Introduced by Ion Implantation in Diamond,” Japanese Journal of Applied Physics, Vol. 14(4), (1975), pp. 544-548; in Tsai et al, “Diamond MESFET Using Ultrashallow RTP Boron Doping,” IEEE Electron Device Letters, Vol. 12, No. 4, (April 1991), pp. 157-159; and in Gildenblat, et al., “High-Temperature Thin-Film Diamond Field-Effect Transistor Fabricated Using a Selective Growth Method,” IEEE Electron Device Letters, Vol. 12, No. 2, (February 1991), pp. 37-39.
Finally, the art closest to the present invention is discussed in U.S. Pat. No. 5,051,785 to Beetz, Jr. et al., and U.S. Pat. No. 5,381,755 to Glesener, et al. In the former, a chemical vapor method for producing an n-type diamond semiconductor is described, wherein lithium, antimony, bismuth, arsenic, phosphorous, and scandium are used as doping ions. A chemical vapor deposition method is used to laid-down the carbon film. A methane/hydrogen gas mixture is flowed over a hot tungsten filament to heat and dissociate the carbon-containing gas. Beetz's n-type impurity, however, is introduced with the gas in order that the impurity be incorporated into the carbon lattice in situ. Beetz, Jr., et al., however, did not recognize that the hot filament itself could act as a source of n-type impurity material. The latter patent describes a method for growing a doped diamond film wherein a diamond carbon layer is deposited from a carbon containing gas mixture. As with Beetz, et al., however, Glesener, et al., does not recognize the efficacy
Evans Timothy P.
Flynn Nathan
Sandia National Laboratories
Sefer Ahmed J.
LandOfFree
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