PHYSICAL REVIEW B 103, 085203 (2021) As-doped SnSe single crystals: Ambivalent doping and interaction with intrinsic defects K. Cermak Sraitrova , 1 J. Cizek , 2 V. Holy, 3, 4 J. Kasparova , 1 T. Plechacek , 1 V. Kucek, 1 J. Navratil , 1 A. Krejcova, 1 and C. Drasar 1 , * 1 University of Pardubice, Faculty of Chemical Technology, Studentska 573, 53210 Pardubice, Czech Republic 2 Faculty of Mathematics and Physics, Charles University, V Holesovickach 2, 18000 Praha 8, Czech Republic 3 Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 121 16 Praha 2, Czech Republic 4 Masaryk University, Department of Condensed Matter Physics, Kotlarska 2, 61137 Brno, Czech Republic (Received 5 February 2020; revised 12 November 2020; accepted 25 January 2021; published 9 February 2021) We performed ambivalent doping study on single crystals of two sets, SnSe 1−x As x and Sn 1−x As x Se, with the aim to explore the interaction of doping species with intrinsic defects. We found that As atoms substitute preferentially for Se atoms in both sets forming the extrinsic substitutional point defect As Se . In the first set, As lowers the concentration of Sn vacancies, V Sn , by an order of magnitude compared to undoped stoichiometric SnSe crystal. The remaining Sn vacancies are preferentially coordinated with As atoms. Importantly, a very low concentration of As led to healing process of hosting structure in terms of intrinsic point defects and eventual SnSe 2 inclusions. This is reflected in an increase of the Hall mobility and drop of the Hall concentration. In the second set, the concentration of Sn vacancies markedly increases upon doping in contrast to the first set. Additionally, the coordination of Sn vacancies by As atoms is less evident due to the high concentration of vacancies. The substitutional defect As Se is a deep-level defect that produces no free carriers at room temperature. Moreover, the coupling of V Sn to As Se defects increases their activation energy. This results in an unprecedentedly low Hall concentration in SnSe which stays below 10 16 cm –3 for x = 0.0075. The present study indicates that doping of SnSe is a rather complex process that generally includes a strong interaction of doping atoms with the hosting structure. On the other hand, such doping allows adjustment of the type and concentration of defects. The present study reveals a general tendency of point defects to clustering, which modifies the properties of point defects markedly. DOI: 10.1103/PhysRevB.103.085203 I. INTRODUCTION The use of thermoelectric (TE) materials is one of the methods for processing waste heat and generating green elec- tricity. Thus, TE materials represent a very attractive and important research area for future technologies. The efficiency of these materials is given by the formula ZT = S 2 σ T /κ . ZT is a so-called figure of merit that is dimensionless, and its upper limit is not determined. This figure of merit consists of the Seebeck coefficient S, electrical conductivity σ , absolute temperature T, and thermal conductivity κ [1]. These parame- ters are interconnected, e.g., through the carrier concentration and band-structure parameters. Materials with ZT  1 are considered promising for prac- tical applications as TE materials. Unfortunately, these values are usually reached at temperatures above ∼700 K [2–4]. Most TE materials become physically and chemically unsta- ble at these temperatures, and their properties change upon thermal cycling. Thus, shifting the maxima of ZT towards lower temperatures is a common strategy. One of the very promising materials that have appeared during the last few years for these applications is SnSe [5]. This material has been studied in both single-crystalline [5] * cestmir.drasar@upce.cz and polycrystalline forms [6]. While SnSe single crystals have excellent TE properties at elevated temperatures [5], polycrys- talline SnSe yields mediocre [6]. According to Ref. [7], this difficulty might be partially eliminated by removing the tin oxide on the surface of this material. SnSe undergoes a displacive phase transition, and at ∼810 K, this material transforms from the α phase (Pnma) to the β phase (Cmcm)[8,9]. The α phase is characterized by a complicated orthorhombic structure that is one of the prerequisites for its significantly low thermal conductivity [10,11]. The main problem in both doped and undoped SnSe is its stability at elevated temperatures, which is even more serious in polycrystalline SnSe [12]. Additionally, the long- term stability and stability upon cycling of this material are problematic. As we showed in our previous work [13], the temperature and kinetics of the preparation and measurements play a significant role in this material. Many elements and compounds have been studied as po- tential dopants for improving the TE properties of SnSe, with mixed results. The majority of doping studies deal with p-type doping. To date, dopants such as Ag [12,14], alkali metals (e.g., Refs. [15–20]), Zn [21], Cu [22], Tl [23], and Cd [24] have been studied. According to band studies [25–27], n-type SnSe should have superior TE properties in comparison with p-type SnSe. However, it seems to be difficult to prepare n-type SnSe. Only a few studies deal with n-type doping, e.g., 2469-9950/2021/103(8)/085203(12) 085203-1 ©2021 American Physical Society