New approach to thermally stimulated transients: Experimental evidence for ZnSe: Al crystals

Abstract

Thermal stimulation is often used as method of characterizing electronic processes in solid insulators. Thermally stimulated conductivity or luminescence in particular are more closely investigated in this study. The conventional method for interpreting these assisted relaxations is to start from the differential equations describing a detailed balance relative to a three-level scheme. We show how the deduced formula refers to a restricted model and approximation; the necessary critical feedback on the starting hypothesis is not often found in experimental work. In the particular case of thermoluminescence it will be demonstrated that most of the time the equation $J_{\mathrm{TSL}}=-\frac{\mathrm{dm}}{\mathrm{dt}}$ does not apply. We propose a new approach to these transients: We show that the exact expression for a thermal stimulation parameter does not involve an algebraic function of temperature but rather a "functional" consideration of the "history" of the experiment. Such a general expression for thermally stimulated conductivity (TSC) and luminescence (TSL) leads to the elimination of the density $n_c$ of electrons in the conduction band, the most difficult parameter to evaluate under transient conditions. When both phenomena (TSC and TSL) are observed in the same experiment under fractional-emptying evolution, a method is proposed to analyze the ratio $R=\frac{J_{\mathrm{TSC}}}{J_{\mathrm{TSL}}}$. This leads to two pieces of information. The first is related to the electronic properties of the material and is expressed by a conventional function $f\left(T\right)\mathrm{}$; the second describes the evolution of the density of available recombination centers $m$. It is shown in the particular case of ZnSe: Al samples how this can be worked out. Taking a constant mobility for electrons in this material, we deduce from $f\left(T\right)\mathrm{}$ the temperature behavior of the recombination capture cross section. This is in fair agreement with a tunnel effect through a repulsive Coulomb barrier. From the evolution of $m$ it is also deduced that a strong hole emission from the radiative recombination centers takes place at low temperature. This emission is quantitatively evaluated and shown to be in accordance with a phenomenon of continuous thermal quenching as can be seen from dc measurements of photoconductivity and luminescence. The good experimental agreement of the data encourages further investigations of the "functional" expression of thermal stimulation and also emphasizes the necessity for a more critical use of conventional formulations.