PL due to Discrete Gap Levels in Some Chalcogenide Glasses- A Configurational Coordinate Diagram Illustration
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PL due to Discrete Gap Levels in Some Chalcogenide Glasses- A Configurational Coordinate Diagram Illustration N. Asha Bhat, K.S. Sangunni and K.S.R.K. Rao Department of Physics, Indian Institute of Science Bangalore –560 012, INDIA ABSTRACT Photoluminescence (PL) studies were carried out on a-Se and a few Ge20Se80-xBix and Ge20Se70+ xBixTe10 bulk glassy semiconductors at 4.2 K with Ar laser as excitation source. While a-Se and samples with lesser at% of Bi show fine structured PL with a large Stokes shift, samples with higher at% of Bi did not show any detectable PL. The investigations show at least three radiative recombination transitions. Features extracted by deconvoluting the experimental spectra show that the discrete gap levels associated with the inherent coordination defects are involved in the PL transitions. Absence of PL in samples with higher Bi at% are explained on the basis of nonradiative transition mechanisms. Overall PL mechanism involving gap levels in chalcogenide glasses is illustrated with the help of a configurational coordinate diagram. INTRODUCTION Chalcogenide glasses show many interesting properties in addition to being promising materials in applications ranging from memory devices to x-ray imaging[1] to infrared optical fiber applications[2]. One such property is the appearance of n-type conduction with the addition of Bi and Pb in Ge-Se glasses[3,4]. Unlike in crystalline semiconductors and in a-Si and related materials, explanation for p or n type conduction in chalcogenide glasses is not very straight forward. This is so mainly because of the presence of large density of inherent defects with unique properties. Among Bi and Pb doping, the former one was studied extensively and the earlier investigations have brought out two different descriptions for n-type conduction. The first one was based on the idea of phase separation at microscopic level in the form of c-Bi2Se3, a strong n-type semiconductor[5]. The second one, on the other hand, based on structural studies point out a coordination number of 3 for Bi, against c-Bi2Se3 phase separation and suggests the n-type conduction to be the consequence of reduction in inherent D+ defects[6]. Though the two contradictory ideas were addressed recently in a consistent way[7], there were no direct and systematic studies on the role of defects in bringing out n-type conduction in Bi doped glasses. Photoluminescence spectroscopy is one of the preferred techniques to study defect levels in the gap of semiconductors and ideal for chalcogenide glasses having a large number of defects in their forbidden energy gap. In the present paper we report our investigations on defects in a-Se and Bi doped Ge-Se and Ge-Se-Te glasses using PL spectroscopy. Multiple PL transitions observed in most of the samples except those with higher Bi concentrations are explained on the basis of phenomenological models and a configurational coordinate diagram representation. EXPERIMENTAL DETAILS
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Bulk glassy samples of interest were prepared by conventional melt-qu
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