Boron diffusion into diamond under electric bias
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Boron diffusion into diamond under electric bias T. Sung, G. Popovici, and M. A. Prelas Nuclear Engineering Department, University of Missouri-Columbia, Columbia, Missouri 65211
R. G. Wilson Hughes Research Laboratories, Malibu, California 90265
S. K. Loyalka Particulate System Research Center, University of Missouri-Columbia, Columbia, Missouri 65211 (Received 13 May 1996; accepted 2 January 1997)
Three natural type IIa diamond crystals were used for forced diffusion of boron. The diffusion was performed under bias otherwise with the same conditions. The boron diffusion coefficient in diamond was found to be 8.4 3 10215 and 4 3 10214 cm2ys at 1000 ±C, depending on the direction of the electric field. The drift velocity of boron in diamond under 850 V at 1000 ±C was found to be about 1.2 3 1028 cmys.
Diffusion is widely used in the electronic industry for the introduction of impurities into semiconductors in order to obtain desired changes in their electric and/or optical properties. In a semiconductor lattice impurity atoms can be in neutral (usually a deep level), positively charged (ionized donors) or negatively charged (ionized acceptors) states. If an impurity atom can be ionized thermally at room temperature (at least partially), it is said to form a shallow level in the semiconductor band gap.1 An electrical field applied to a semiconductor during diffusion can influence the movement of ionized dopant atoms as well as neutral dopant atoms due to the cross terms in the Onsanger matrix. However, the effect of an applied electrical field on neutral atoms is very small and can be neglected. The maximal applied voltage is determined by the resistance of the sample. In diamond and other wide band gap semiconductors, the electrical conductivity may remain low even at high temperatures. This allows the application of relatively high voltage (hundreds of volts) to increase the drift rate of ions in the lattice. The first experiments on the forced diffusion of impurities in diamond have been performed by our group.2,3 We found that the forced diffusion of impurities was of practical importance for diamond doping, allowing for higher diffusion coefficients in comparison with conventional diffusion. In this paper the diffusion of boron under an electric field was studied. The drift velocity and the mobility of the boron atoms in the diamond lattice at the diffusion temperature were determined. J. Mater. Res., Vol. 12, No. 5, May 1997
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Impurities in materials tend to diffuse from areas of high concentration to those of low concentration. Forced diffusion is a method that uses an electric field to assist the normal diffusion process. There are two driving forces involved in this method. One is the chemical force due to concentration gradients. Conventional diffusion behavior is controlled by this force. The diffusion coefficient indicates the rate of the normal diffusion process. An electric field is the main driving f
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