Electric Field Enhancement of Electron Emission from DX Centers and Consequences
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ELECTRIC FIELD ENHANCEMENT OF ELECTRON EMISSION FROM DX CENTERS AND CONSEQUENCES J.C. BOURGOIN*,
M. ZAZOUI*, S.L. FENG*, H.J. von BARDELEBEN*,
S.
ALAYA**
AND
H. MAAREF** *Groupe tional
de Physique de la
des Solides de l'Ecole Normale Supdrieure, Centre Na-
Recherche Scientifique*,
Tour 23, 2 place Jussieu, 75251 Paris Cedex 05, France **Facult6 des Sciences, Universit6 de Monastir, Monastir, Tunisie.
ABSTRACT We present data which show that electron emission from the DX center is sensitive
to
the
Poole-Frenkel
effect.
We
demonstrate that this result
implies that the DX center is an L effective-mass state of the donor impurity accompanied by a small lattice relaxation. We show that all the observations so far obtained on this center are in agreement with this model.
INTRODUCTION Recent magnetic susceptibility experiments [1] demonstrate that the ground state of the DX center is occupied by a single electron. Here we show that the enhancement of the emission rate of this electron by an electric field, when studied in proper conditions, is caused by a purely Poole Frenkel effect. This result implies that the DX center corresponds to a single donor state. This donor state cannot be associated with either the r band or the X band since these states are well characterized in, respectively, direct and indirect band gap CaAIAs alloys. The DX center is therefore the effective-mass state associated with the L band : its level is 200 meV deep whatever the donor impurity [2] because it is submitted to a shallowdeep instability induced by intervalley mixing [3]. We shall therefore examine the main properties of the DX center and discuss how they can be understood in such a model.
ELECTRIC FIELD ENHANCEMENT OF ELECTRON EMISSION The effect of the electric field on the emission rate of an electron from the DX center has already been studied [4,5] using Deep Level Transient Spectroscopy (DLTS). The data cannot be interpreted easily because the capacitance transients associated with electron emission from the DX center are usually non exponentials, and thus small changes such as those due to a perturbation by an electric field, even if they can be detected, cannot be mea-
"*laboratoire associ6
A l'Universit6 Paris VII.
Mat. Res. Soc. Symp. Proc. Vol. 163. ©1990 Materials Research Society
748
sured correctly. The main reason for such non exponential behavior of the transient is that the concentration of the DX center is not small enough compared to the free carrier concentration [6], but there are several other causes such as strain effects, alloying effects, etc. In order to perform quantitative measurements of this effect, we have used very short filling pulses so that the concentration of the filled center remains small compared to n. In addition we have used a liquid phase epitaxy material where the n-type doped layer is grown on a thick p-type layer whose alloy composition is continuously variable. We have selected Te as the n-type impurity for which the DLTS associated spectra have a shape identical to
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