Local Order of Te Impurity Atoms in Heavily Doped GaAs:Te and Accompanying Electron Localization Effect
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existence of the limit of n and an annealing behavior of n are closely related phenomena. Presented x-ray scattering results indicate an arising of complex state of local order of impurity atoms, involving the creation of impurity pairs, when the concentration of free electrons is lowered by an annealing. We draw such conclusions from the microscopic model based on Krivoglaz theory of x-ray scattering from a dilute solid solution due to fluctuations of pair-correlation function (PCF), proposed many years ago [7,8.91, but never used for an interpretation of experimental results in semiconductors, according to our knowledge. Our results for GaAs:Te support the hypothesis of a dominating role of the impurity atom pairs in limiting the free carrier concentration.
EXPERIMENTAL GaAs:Te samples came from bulk single crystals, specially designed and grown from the melt by Czochralski technique in our laboratory. The concentration of Te impurity atoms was from 1 10" cm-' to about 2 1019 cm-3, determined on the basis of the segregation phenomena [10] during crystal growth from the melt and confirmed by SIMS for selected samples. Annealings were performed in resistance furnace at temperatures from about 700 to 1200 TC. GaAs samples were sealed under vacuum in fused quartz ampoules, with a piece of elemental As added in to prevent GaAs surface from a decomposition at high temperatures. Studies of kinetics of the free electron concentration changes [1] indicated that electron concentration reach steady state as a function of annealing time at a given temperature. In our studies the annealing times from 15 min to 500 hours were applied, proper for each temperature to reach a steady value of the electron concentration. Since longer times did not cause further changes of the electron concentration, we believe that the value of the electron concentration established at each annealing 261 Mat. Res. Soc. Symp. Proc. Vol. 583 © 2000 Materials Research Society
temperature corresponds to an equilibrium concentration, at this temperature, of defects which can influence the electron concentration. Each annealing process was terminated by a fast cooling in water (quenching, rate about 50 °C/sec) to freeze the structural state of defects in GaAs:Te solid solution established at high temperature. After annealing each sample was lapped on both sides at least 20 gum and etched. Hall electron concentration was measured by van der Pauw technique. Then samples were carefully lapped and chemo-mechanically polished in NaOCI-H 20 solution to mirror-like surface, necessary for reliable performing x-ray studies. High resolution x-ray diffraction reciprocal space maps were measured using Philips MRD diffractometer, set for 440 reflection for Cu Kai line. Some of the samples were annealed many times subsequently at different temperatures and measured after each annealing. MICROSCOPIC MODEL OF X-RAY SCATTERING Here we outline the microscopic theory [7-9,11] of x-ray diffuse scattering (XDS) by a crystalline solid solution, allowing for substitutions o
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