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Device application of diamonds
Thin Solid Films, 216 (1992) 134 136
134
Device application of diamonds M. W. Geis Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA 02173 (USA)
Abstract Preliminary work on the problems of obtaining device quality diamond substratcs is discussed.
Diamond is a wide band gap (5.5 eV) semiconductor that has superior properties to many of the commonly used semiconductors. A high breakdown voltage, 2 0 40 times that of silicon [1-3], and high electron and hole mobilities (about 2000cm2V J s i) [4] result in transistors with projected higher operating frequencies and higher power levels than those obtainable with silicon and GaAs. Further, diamond has an air-stable negative electron affinity [5, 6]; the energy of electrons in the conduction band is above the minimum energy of electrons in vacuum, as shown in Fig. 1. This negative electron affinity made possible diamond cold cathodes [7], which may have applications with high power, high frequency vacuum devices and flat panel displays. Such devices have not been realized because of several technical barriers. The two most significant barriers are the production of large area, device quality diamond substrates and the homoepitaxial diamond growth with impurity control. Two approaches have been taken to obtain device quality diamond substrates. The first is to grow heteroepitaxial diamond on a single-crystal substrate. Copper, nickel, cobalt, and cubic boron nitride are nearly lattice matched to diamond. Some heteroepitaxial success has been realized with nickel [8] and copper [9, 10], and the best results were obtained on cubic boron nitride substrates [11, 12]. However, no device quality substrate has been obtained to date, because either the heteroepitaxy consists of a few isolated islands of diamond on the foreign substrate, as for the case of nickel and copper, or the substrate is too small to be of use, as in the case of cubic boron nitride. The second approach uses many small, inexpensive, faceted diamonds which are crystallographically oriented by placing them in faceted, etched pits on patterned substrates. An array of such pits can be filled with these diamonds forming an array of diamond seeds [13], as in Fig. 2. By homoepitaxially growing on
0040-6090/92/$5.00
on the array of diamonds, a large area continuous diamond substrate is obtained. Although there are low angle grain boundaries of a few tenths of a degree, where the individual crystals grow together, initial electrical characterization shows that these substrates are equivalent to single-crystal substrates [14]. Substrates of a few square centimeters in area have been made using this technique and larger substrates compatible with device fabrication appear likely. The second technical barrier, of obtaining homoepitaxial diamond with impurity control, requires the addition of boron to the diamond, making it a p-type semiconductor. Because of the high ionization energy of the boron atom in diamond [4] (0.36eV) only a small fraction to the atoms contribute holes to the valence band at room temperature. This problem can be further compounded if any compensation dopant, such as nitrogen, which can trap some of the holes from the valence band, is used. This reduces the conductivity of the diamond both by lowering the concentration of holes and by decreasing the hole mobility through ionized impurity scattering. At present all the reported boron-doped diamond films have their conductivity limited by such compensating impurities. Both of these technical barriers appear to be surmountable. Homoepitaxy on seeded arrays of diamonds has already produced useful mosaic diamond substrates and it appears likely the impurity levels in homoepitaxial diamond can be controlled when more research is directed to this problem.
Acknowledgments This work was supported by S D I O - O S T through Office of Naval Research and by the Department of the Air Force.
~", 1992
ElsevierSequoia. All rights reserved
M. IV. Ge& / Device application of diamonds
135
EC 0.7 eV
VACUUM LEVEL 0.9 eV
Ec
/ 4.8 eV
3.2 eV
BORON ACCEPTORS
=0.6 eV
0
0
01'0
0
0
0
9 eV SiO2
DIAMOND
AI
<111>
p,
Ev
Fig. 1. The energy levels of aluminum, silicon dioxide and diamond for the (liD-oriented diamond-SiO2 interface. The conduction band of diamond is above the minimum energy of electrons in vacuum. Ec is the conduction band energy, E~. is the Fermi energy, and Ev is the valence band energy.
OCTAHEDRAL DIAMOND SEEDS WITH (111) FACETS
Appl iI=rt I:RE~M .ql IIRRV
g ~
f
~
Fig. 2. Schematic drawing showing faceted diamond seeds, suspended in a liquid, falling onto an etch pit patterned substrate where they become fixed and oriented in the pits.
136
M. W. Geis / Device application q[ diamonds
References 1 A. V. Bogdsanov, I. M. Vikulin and T. V. Bodgdoanovas, Soy. Phys--Semicond., 16 (1982) 720. 2 P. Liu, R. Yen and N. BIoembergen, IEEE J. Quantum Electron., 14 (1978) 574. 3 M. W. Geis, Proc. 1EEE, 79 (1991) 669. 4 A. T. Collins and E. C. Lightowler, Electrical properties. In J. E. Field (ed.), The Properties of Diamonds, Academic Press, New York, 1979, p. 79. 5 F. J. Himpsel, J. A. Knapp, J. A. VanVechten and D. E. Eastmen, Phys. Re~'. B, 20 (1979) 624. 6 M. W. Geis, J. A. Gregory and B. B. Pate, IEEE Trans. Electron Devices, 38 (1991) 619. 7 M. W. Geis, N. N. Efremow, J. D. Woodhouse, M. D. McAleese, M. Marchywka, D. G. Socker and J. H. Hochedez, 1EEE Electron Device Lett., 12 (1991) 456. 8 Y. Sato and M. Kamo, in R. Roy, J. T. Glass and R. Messier (eds.), Proc. New Diamond Science and Technology, 2nd Int. Conf., Materials Research Society, Pittsburgh, PA, 1991, to be published.
9 J. F. Prins, in R. Roy, J. T. Glass and R. Messier (eds.), Proc. New Diamond Science and Technology, 2nd Int. Conf., Materials Research Society, Pittsburgh, PA, 1991, to be published. l0 J. Narayan, in T. D. Moustakas, J. I. Pabkove and Y. Hamakawa (eds.), WMe Band-Gap Semiconductors, Vol. 242, Materials Research Society, Pittsburgh, PA, 1992 to be published. 11 S. Koizumi, T. Murakami and T. Inuzuka, Appl. Phys. Lett., 6 (1990) 563. 12 A. Badizian and T. Badzian, in T. M. Besmann, B. M. Gallois. and J. Warren (ed.), Proc. Con[i on Chemical Vapor Deposition q[" Refractory Metals and Ceramics Vol. 250, Materials Research Society, Pittsburgh, PA, 1992, to be published. 13 M. W. Geis, H. I. Smith, A. Argoitia, J. Angus, G.-H. M. Ma, J. T. Glass, J. Butler. C. J. Robinson and R. Pryor, Appl. Phys. Lett,, 58 (1991) 2485. 14 R. W. Pryor and M. W. Geis, in T. D. Moustakas, J. I. Pabkove, and Y. Hamakawa (eds.), Wide Band-Gap Semiconductors, Vol. 242. Materials Research Society, Pittsburgh, PA, 1992, to be published.
134
Device application of diamonds M. W. Geis Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA 02173 (USA)
Abstract Preliminary work on the problems of obtaining device quality diamond substratcs is discussed.
Diamond is a wide band gap (5.5 eV) semiconductor that has superior properties to many of the commonly used semiconductors. A high breakdown voltage, 2 0 40 times that of silicon [1-3], and high electron and hole mobilities (about 2000cm2V J s i) [4] result in transistors with projected higher operating frequencies and higher power levels than those obtainable with silicon and GaAs. Further, diamond has an air-stable negative electron affinity [5, 6]; the energy of electrons in the conduction band is above the minimum energy of electrons in vacuum, as shown in Fig. 1. This negative electron affinity made possible diamond cold cathodes [7], which may have applications with high power, high frequency vacuum devices and flat panel displays. Such devices have not been realized because of several technical barriers. The two most significant barriers are the production of large area, device quality diamond substrates and the homoepitaxial diamond growth with impurity control. Two approaches have been taken to obtain device quality diamond substrates. The first is to grow heteroepitaxial diamond on a single-crystal substrate. Copper, nickel, cobalt, and cubic boron nitride are nearly lattice matched to diamond. Some heteroepitaxial success has been realized with nickel [8] and copper [9, 10], and the best results were obtained on cubic boron nitride substrates [11, 12]. However, no device quality substrate has been obtained to date, because either the heteroepitaxy consists of a few isolated islands of diamond on the foreign substrate, as for the case of nickel and copper, or the substrate is too small to be of use, as in the case of cubic boron nitride. The second approach uses many small, inexpensive, faceted diamonds which are crystallographically oriented by placing them in faceted, etched pits on patterned substrates. An array of such pits can be filled with these diamonds forming an array of diamond seeds [13], as in Fig. 2. By homoepitaxially growing on
0040-6090/92/$5.00
on the array of diamonds, a large area continuous diamond substrate is obtained. Although there are low angle grain boundaries of a few tenths of a degree, where the individual crystals grow together, initial electrical characterization shows that these substrates are equivalent to single-crystal substrates [14]. Substrates of a few square centimeters in area have been made using this technique and larger substrates compatible with device fabrication appear likely. The second technical barrier, of obtaining homoepitaxial diamond with impurity control, requires the addition of boron to the diamond, making it a p-type semiconductor. Because of the high ionization energy of the boron atom in diamond [4] (0.36eV) only a small fraction to the atoms contribute holes to the valence band at room temperature. This problem can be further compounded if any compensation dopant, such as nitrogen, which can trap some of the holes from the valence band, is used. This reduces the conductivity of the diamond both by lowering the concentration of holes and by decreasing the hole mobility through ionized impurity scattering. At present all the reported boron-doped diamond films have their conductivity limited by such compensating impurities. Both of these technical barriers appear to be surmountable. Homoepitaxy on seeded arrays of diamonds has already produced useful mosaic diamond substrates and it appears likely the impurity levels in homoepitaxial diamond can be controlled when more research is directed to this problem.
Acknowledgments This work was supported by S D I O - O S T through Office of Naval Research and by the Department of the Air Force.
~", 1992
ElsevierSequoia. All rights reserved
M. IV. Ge& / Device application of diamonds
135
EC 0.7 eV
VACUUM LEVEL 0.9 eV
Ec
/ 4.8 eV
3.2 eV
BORON ACCEPTORS
=0.6 eV
0
0
01'0
0
0
0
9 eV SiO2
DIAMOND
AI
<111>
p,
Ev
Fig. 1. The energy levels of aluminum, silicon dioxide and diamond for the (liD-oriented diamond-SiO2 interface. The conduction band of diamond is above the minimum energy of electrons in vacuum. Ec is the conduction band energy, E~. is the Fermi energy, and Ev is the valence band energy.
OCTAHEDRAL DIAMOND SEEDS WITH (111) FACETS
Appl iI=rt I:RE~M .ql IIRRV
g ~
f
~
Fig. 2. Schematic drawing showing faceted diamond seeds, suspended in a liquid, falling onto an etch pit patterned substrate where they become fixed and oriented in the pits.
136
M. W. Geis / Device application q[ diamonds
References 1 A. V. Bogdsanov, I. M. Vikulin and T. V. Bodgdoanovas, Soy. Phys--Semicond., 16 (1982) 720. 2 P. Liu, R. Yen and N. BIoembergen, IEEE J. Quantum Electron., 14 (1978) 574. 3 M. W. Geis, Proc. 1EEE, 79 (1991) 669. 4 A. T. Collins and E. C. Lightowler, Electrical properties. In J. E. Field (ed.), The Properties of Diamonds, Academic Press, New York, 1979, p. 79. 5 F. J. Himpsel, J. A. Knapp, J. A. VanVechten and D. E. Eastmen, Phys. Re~'. B, 20 (1979) 624. 6 M. W. Geis, J. A. Gregory and B. B. Pate, IEEE Trans. Electron Devices, 38 (1991) 619. 7 M. W. Geis, N. N. Efremow, J. D. Woodhouse, M. D. McAleese, M. Marchywka, D. G. Socker and J. H. Hochedez, 1EEE Electron Device Lett., 12 (1991) 456. 8 Y. Sato and M. Kamo, in R. Roy, J. T. Glass and R. Messier (eds.), Proc. New Diamond Science and Technology, 2nd Int. Conf., Materials Research Society, Pittsburgh, PA, 1991, to be published.
9 J. F. Prins, in R. Roy, J. T. Glass and R. Messier (eds.), Proc. New Diamond Science and Technology, 2nd Int. Conf., Materials Research Society, Pittsburgh, PA, 1991, to be published. l0 J. Narayan, in T. D. Moustakas, J. I. Pabkove and Y. Hamakawa (eds.), WMe Band-Gap Semiconductors, Vol. 242, Materials Research Society, Pittsburgh, PA, 1992 to be published. 11 S. Koizumi, T. Murakami and T. Inuzuka, Appl. Phys. Lett., 6 (1990) 563. 12 A. Badizian and T. Badzian, in T. M. Besmann, B. M. Gallois. and J. Warren (ed.), Proc. Con[i on Chemical Vapor Deposition q[" Refractory Metals and Ceramics Vol. 250, Materials Research Society, Pittsburgh, PA, 1992, to be published. 13 M. W. Geis, H. I. Smith, A. Argoitia, J. Angus, G.-H. M. Ma, J. T. Glass, J. Butler. C. J. Robinson and R. Pryor, Appl. Phys. Lett,, 58 (1991) 2485. 14 R. W. Pryor and M. W. Geis, in T. D. Moustakas, J. I. Pabkove, and Y. Hamakawa (eds.), Wide Band-Gap Semiconductors, Vol. 242. Materials Research Society, Pittsburgh, PA, 1992, to be published.