Impurity Conduction in N-Type 4H-SiC
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ABSTRACT Impurity conduction ( or hopping conduction ) has been observed in the more heavily n-type 4H-SiC samples by both temperature dependent resistivity measurements and thermal admittance spectroscopy. The measured activation energies8 3 for hopping were 4-5 meV and 2.3-3.0 meV respectively. No evidence of hopping conduction was seen by either method in the sample where ND-NA < 1018 cm- 3 . The thermal admittance spectrum of the lightly n-type sample showed the two nitrogen levels at 53 and 100 meV.
INTRODUCTION
The conductivity in a semiconductor can in general be expressed by[ 1
(Y(T) =
/]e-e'/kT + a 2 e-e2/kT
+ _3e-E3 /kT
( 1)
where -1 , E2 and E3 are activation energies for conduction in different regimes. The activation energy el is necessary to excite an electron from a donor in to the conduction band. The activation energy c3 is required for jumping from an occupied donor site to an unoccupied donor site without an excursion into the conduction band. When the doping concentration in an n-type semiconductor is high enough, there is an overlap of the electron wave-functions of neighboring sites. As the temperature is reduced, the conduction electrons are frozen on to the donors. Because of compensation and the overlap of the electron wave-functions, there can be movement of charge carriers from occupied donor sites to unoccupied donor sites. This movement constitutes the hopping or impurity conduction. The impurity conduction dominates the conductivity at low temperatures even though the mobility of the electrons moving in the impurity levels is small. As the impurity concentration increases, the temperature at which impurity conduction becomes experimentally observable shifts to higher temperature. Impurity conduction has been observed in many semiconductors ( such as germanium, silicon, silicon carbide, cadmium sulfide, and indium antimonide) at low temperatures[ 2-7 ]. Temperature dependent Hall effect and resistivity measurements were the main tools used in the investigations. The anomalies observed in the electrical measurements were adequately explained by the two band model introduced by Hung[ 8].
637
Mat. Res. Soc. Symp. Proc. Vol. 423 0 1996 Materials Research Society
Busch and Labhart [ 9 ] observed impurity conduction in silicon carbide in 1946. But more recently [ 10, 11] impurity conduction has been observed in n-type 4H-SiC. In this paper, we shall present the details of temperature dependent resistivity measurements of n-type 4H-SiC samples and of thermal admittance spectroscopy( TAS ). The activation energy E3 obtained from the resistivity measurements is compared with that obtained from TAS. A satisfactory agreement between the techniques is found. The samples studied were n-type 4H-SiC grown by the physical vapor transport method. The samples are labeled W1, W2 and W3. Temperature dependent resistivity measurements were made using van der Pauw[ 12] structures with either Ni, Ti, or Al contacts. Schottky diodes for the thermal admittance spectroscopy were fabricated as describe
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