Materials Science and Engineering B54 (1998) 161 – 167 Study on momentum density in narrow-gap mixed III–V alloys by positron annihilation under pressure N. Bouarissa a, *, M. Certier b , N. Amrane c , H. Aourag c a Physics department, Uniersity of Setif, Setif 19000, Algeria b Laboratoire de Spectroscopie Optique des Mate ´riaux, Uniersite ´ de Metz, Metz F -57070, France c Computational Materials Science Laboratory, Uniersity of Sidi -bel -Abbes, Sidi -bel -Abbes 22000, Algeria Received 19 November 1997; accepted 17 February 1998 Abstract The pressure dependence of the electron – positron momentum density in InAs x Sb 1 -x was studied. The computational technique used here is based on the independent particle model coupled with the use of the electron pseudo-wave functions. The variation of the momentum density versus pressure is weak and shows that InAs x Sb 1 -x exhibits a more metallic character. The observation that the calculated total positron annihilation rate increases with pressure has been explained in terms of increased positron penetration into the ion cores. The obtained results have an essential role in the determination of the electronic band structure of narrow gap mixed III–V alloys under pressure. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Momentum density; Positron annihilation; Pressure 1. Introduction Narrow-gap semiconductors [1 – 3] possess a number of special properties, they can be regarded as being intermediate between covalently bonded solids and metals. Several of these materials have found applica- tion in industry because of their useful electrical and optical properties. Due to the small gap, these materials absorb infrared radiation and have found application as infrared photodetectors, infrared diode lasers, and in non-linear optics. In recent years, efforts have been made to provide a deeper understanding of electronic properties of semiconductor alloys, with a view to improving the tunability of band structure parameters of semiconductor devices [4]. lnAs x Sb l -x is one of the III–V alloy systems, which has been of great interest because of the wide range of its physical properties [4–9]. The electronic momentum densities are determined by a class of methods employing three types of sensitive microprobes, one of them is the positron. The behavior of positrons in condensed matter has been the subject of intense experimental and theoretical investigation during the last decades, and the use of positrons as a probe of electronic structure is very convenient for the study of materials [10]. In a typical experiment, positrons are injected into the material under study either directly from a radioactive source or as a mononergetic beam. Once injected, the positrons scatter elastically and inelastically off ion cores and valence electrons, and within a picosecond reach energies of about 1 eV followed by a slower approach towards thermal equilibrium. On thermalization, the positrons diffuse until they either annihilate or are trapped at open volume defects, where they subsequently annihi- late. The angular correlation of the two  -rays resulting from the most probable decay process can be measured. This gives information on the momentum density distri- bution of the annihilating positron – electron pair that can be used to study the electronic structure[11]. Narrow-gap semiconductors are sensitive to pressure effect, a feature which make them strong candidates for sensors of many kinds. The InAs x Sb 1 -x semiconductor alloys are easily compressed and significant changes in their electron densities can be produced. On the other hand, applied pressure leads to greater positron pene- tration into the region of the ion cores. On the basis of these effects it may be expected that the total annihila- tion rate will be changed under pressure. * Corresponding author. fax: +213 5 693362. 0921-5107/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S09 21- 5 1 07(98)00 1 5 4- 8