Electronic and Optical Properties of SiGe alloys within first-principles schemes
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Electronic and Optical Properties of SiGe alloys within first-principles schemes G. Cappellini*, G. Satta*, M. Palummo**, G. Onida*** *SLACS-INFM and Dipartimento di Fisica, Universit`a di Cagliari, I-09042 Monserrato (Cagliari), Italy **INFM and Dipartimento di Fisica, II Universit`a di Roma ”Tor Vergata”, I-00133 Rome, Italy ***INFM and Dipartimento di Fisica, Universit`a di Milano, I-20133 Milano, Italy ABSTRACT We present first-principles calculated electronic and optical properties of some SiGe alloys. The ground-state, electronic excitations and optical properties have been calculated with Ge and Si atoms arranged in different ways among the sites of a diamond-type lattice. For the ground state a DFT-LDA scheme and for the electronic excitations a DFT-GW approach have been respectively used. For the optical properties the DFT-LDA-RPA scheme has been applied for alloys going in composition from Si(100%) to Ge(100%) : obtained results have been compared with existing experimental and theoretical data. For the noticeable Si(50%)Ge(50%) alloy also two-particle effects have been evaluated using the Bethe-Salpeter equation. INTRODUCTION The elemental semiconductors silicon and germanium form a continuous series of disordered alloys. A detailed comprehension of the properties of the Si-Ge alloys is a prerequisite for successful device applications. These alloys seem promising candidates as substitute material for Si in low-power and high speed semiconductor devices[1]. Interesting properties range from enhanced mobility [2] to appearance of new direct optical transitions[3].Si-Ge semiconductors are being used to create high-performance bipolar transistors and integrated circuits[4, 5], highly integrated focal-plane array infrared detectors[6], and infrared light-emitting diodes [7]. The wide range of applications of Si-Ge alloys have been presented nowadays also to a wider public [8]. In this paper, we concentrate on the bulk properties of Si-Ge alloys. We determine for these systems, with state of the art computational techniques,
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the ground state, the electronic excitations and the optical absorption, going in composition from Si(100%) to Ge(100%). Obtained results have been compared with existing experimental and theoretical data. MODEL AND STRUCTURAL PROPERTIES In the present paper we mimic SiGe alloys by a series of ordered crystals formed from Ge and Si atoms arranged in different ways among the sites of a diamond-type lattice. Since a diamond-type lattice is composed of eight interprenetating simple cubic lattices, it is convenient to work with a series of ordered crystals having in compositions Gek Si1−k where k = 0, 1/8, ..., 8/8 [9]. In this way we can simulate ordered alloys with compositions steps of 12.5%. The simulation of ordered structures permits us to takle the study of the SiGe alloys using small simulation cells, with a relative high number of symmetries, mantaining therefore the same efficiency of all the computational schemes which treat ground-state, electronic-excitation an
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