Stability Against Concentration Fluctuations in Compound Semiconductors
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STABILITY AGAINST CONCENTRATION FLUCTUATIONS IN COMPOUND SEMICONDUCTORS.
P.J. DESRE. Laboratoire de Thermodynamique et Physico-Chimie Mdtallurgiques, CNRS URA 29, INPG/ENSEEG, BP 75, 38402 Saint Martin d'H6res Ctdex, France.
ABSTRACT: The variation of the composition within the narrow range of off stoichiometry allowed for compounds is determinant for the population of various intrinsic point defects. As such concentration fluctuations result in inhomogeneities of defect distributions. On the basis of statistical thermodynamics, applied to concentration fluctuation analysis in the III-V compounds, a thermodynamic stability length is defined and calculated as a function of temperature for GaAs, GaSb and GaP. This thermodynamic stability length defines the scale above which full composition homogeneity is unattainable. INTRODUCTION. The control of intrinsic point defects in the native semiconductor crystal is of main importance for technical applications. It appears that a better control of the concentration and the spatial distribution of these defects may lead to an improvement in device performance. In III-V semiconductors there is a variety of basic intrinsic point defects : vacancies on both sublattices, self interstitials of the two components as well as antisite defects associated to the existence of two sublattices. Several thermodynamic approaches have been followed in order to evaluate the concentration at equilibrium of these defects in III-V compounds. Some of these studies [ 1], [2], [3], were applied to the evaluation of the deviation from stoichiometry of AsGa versus temperature leading to the solidus of the phase diagram. Due to the numerous defects which have to be considered and the corresponding numerous unknown equilibrium constants, which have to be determined from available experimental data, poor accuracy is expected in the results. The thermal equilibrium concentration of intrinsic point defects in AsGa depends on the arsenic vapour pressure [4]. In other terms, this signifies that the point defect equilibrium concentration is sensitive to the chemical potential of As in the semiconductor. This chemical potential g.tAs changes rapidly with concentration in the very narrow range of concentration corresponding to the deviation from the stoichiometry. This means that the point defects equilibrium concentration is very sensitive to the nominal concentration c (molar concentration) around the stoichiometry (c = 0.5). An accurate control of the nominal concentration during growth of the Ill-V semiconductor crystal is rather difficult. Inhomogeneities in concentration result from the process itself (segregation induced by convection during crystal growth from a liquid phase). Independently of these inhomogeneities inherent to the process itself, thermal fluctuations of concentration are excited. Such fluctuations are found generally to be small in their deviations from the average concentration ; but as the concentration range where a III-V compound is stable is very small, it is of interest to ana
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