ISSN 1063-7826, Semiconductors, 2014, Vol. 48, No. 12, pp. 1545–1551. © Pleiades Publishing, Ltd., 2014. Original Russian Text © V.A. Romaka, P. Rogl, V.V. Romaka, Yu.V. Stadnyk, R.O. Korzh, V.Ya. Krayovskyy, A.M. Horyn, 2014, published in Fizika i Tekhnika Poluprovodnikov, 2014, Vol. 48, No. 12, pp. 1585–1591. 1545 1. INTRODUCTION In this study, we investigate the conduction mech- anisms in n-HfNiSn thermoelectric semiconductor heavily doped with the Ru acceptor impurity. The results obtained are used for optimizing the parame- ters of thermoelectric materials in order to obtain the maximum thermoelectric Q factor, i.e., the conversion efficiency of thermal energy to electric energy [1]. In our previous studies of n-HfNiSn [2–4], we, on the one hand, established the nature of structural defects and their charge state, calculated the elec- tronic structure, and investigated the kinetic and mag- netic characteristics and, on the other hand, deter- mined the reaction of the semiconductor to doping with Co, Rh, and Sb atoms, i.e., the causes of these characteristics. We clarified the reasons why electrons in the semiconductor are initially the majority charge carriers. It appeared that not all atoms in HfNiSn occupy the crystallographic positions characteristic of the MgAgAs structural type (space group F 3m [5]). The investigations showed that the 4a crystallographic position of Hf atoms (5d 2 6s 2 ) is occupied by Ni atoms (3d 8 4s 2 ) up to ~1% (y ≤ 0.01). Since Ni atoms contain a larger number of 3d electrons as compared with Hf atoms, the occupation of the 4a position by a Ni atom 4 generates a donor structural defect in the crystal. This is the mechanism of a priori doping of a semiconduc- tor with a donor impurity. In this case, taking into account the mechanism of structural-defect genera- tion, the formula of the compound is (Hf 1  y Ni y )NiSn (y ≤ 0.01). The doping of n-HfNiSn with Co (3d 7 4s 2 ) and Rh (4d 8 5s 1 ) atoms by the displacement of Ni atoms was also accompanied by the generation of acceptor struc- tural defects in the crystal and doping with Sb (4d 10 5s 2 5p 3 ) atoms by the displacement of Sn (4d 10 5s 2 5p 2 ) atoms was accompanied by the generation of donor defects [2–4]. In addition, it was established that doping changes the electronic structure of the semiconductor, in particular, the band gap ε g , degree of compensation, Fermi-level position ε F , and the density of states at the Fermi level n(ε F ). In this study, we investigate the doping of n-HfNiSn with the Ru impurity, including the mechanisms and conditions for the incorporation of Ru atoms in the semiconductor matrix, with the use of both experi- mental analysis of the crystal structure and calculation of the electronic structure. In particular, we study the dynamics of variations in the crystal and electronic structures and the kinetic and energy characteristics, which allow us to propose the mechanism of the ELECTRONIC PROPERTIES OF SEMICONDUCTORS Features of the Band Structure and Conduction Mechanisms in the n-HfNiSn Semiconductor Heavily Doped with Ru V. A. Romaka a, b ^, P. Rogl c , V. V. Romaka b , Yu. V. Stadnyk d , R. O. Korzh b , V. Ya. Krayovskyy b , and A. M. Horyn d a Pidstrygach Institute for Applied Problems in Mechanics and Mathematics, National Academy of Sciences of Ukraine, Lviv, 79060 Ukraine b National University Lvivska Politekhnika, Lviv, 79013 Ukraine ^e-mail: vromaka@polynet.lviv.ua c Institut für Physikalische Chemie, Universität Wien, A-1090 Wien, Austria d Ivan Franko National University of Lviv, 79005 Ukraine Submitted March 17, 2014; accepted for publication March 22, 2014 Abstract—The crystal and electronic structure and energy and kinetic properties of the n-HfNiSn semicon- ductor heavily doped with a Ru acceptor impurity are investigated in the temperature and Ru concentration ranges T = 80–400 K and ≈ 9.5 × 10 19 –5.7 × 10 20 cm –3 (x = 0–0.03), respectively. The mechanism of structural-defect generation is established, which changes the band gap and degree of compensation of the semiconductor and consists in the simultaneous concentration reduction and elimination of donor structural defects by means of the displacement of ~1% of Ni atoms from the Hf (4a) positions, the generation of accep- tor structural defects upon the substitution of Ru atoms for Ni atoms in the 4c positions, and the generation of donor defects in the form of vacancies in the Sn (4b) positions. The calculated electronic structure of HfNi 1  x Ru x Sn is consistent with the experiment. The results obtained are discussed within the Shklovsky– Efros model for a heavily doped and compensated semiconductor. DOI: 10.1134/S1063782614120203 N A Ru