Observation of a charge delocalization from Se vacancies inBi2Se3: A positron annihilation study of native defects

Abstract

By means of positron annihilation lifetime spectroscopy, we have investigated the native defects present in Bi 2 Se 3 , which belongs to the family of topological insulators. We experimentally demonstrate that selenium vacancy defects (V Se1 ) are present in Bi 2 Se 3 as-grown samples, and that their charge is delocalized as temperature increases. At least from 100 K up to room temperature both V 0Se1 and V + Se1 charge states coexist. The observed charge delocalization determines the contribution of V Se1 defects to the n-type conductivity of Bi 2 Se 3 . These findings are supported by theoretical calculations, which show that vacancies of nonequivalent Se1 and Se2 selenium atoms are clearly differentiated by positron annihilation lifetime spectroscopy, enabling us to directly detect and quantify the most favorable type of selenium vacancy. In addition to open-volume defects, experimental data indicate the presence of defects that act as shallow traps, suggesting that more than one type of native defects coexist in Bi 2 Se 3 . As will be discussed, the presence of a dislocation density around 10 10 cm −2 could be the source of the detected shallow traps. Understanding the one-dimensional defects and the origin of the charge delocalization that leads Bi 2 Se 3 to be an n-type semiconductor will help in the development of high-quality topological insulators based on this material.

Photoexcited Free Carrier Dynamics in Bi2Se3, (Bi0.75In0.25)2Se3, and (Bi0.50In0.50)2Se3: From Topological to Band Insulator

“…In a TI state, it is difficult to disentangle the contribution of the SS states to room temperature electrical conductivity using conventional transport measurements. Most samples are n-doped due to the presence of Se vacancies and antisite defects, , and the resulting bulk carrier conduction can overwhelm the SS contribution. The bulk carrier density in Bi 2 Se 3 depends on growth kinetics, and can vary from very low values of ∼10 17 cm –3 to over 10 19 cm –3 .…”

Interstitial defects in the van der Waals gap of Bi2Se3

“…Refining the Se occupancies suggest the presence of Bi on these Se sites, Bi Se , as the occupancy refinement consistently gives an occupancy >1 for these positions. We speculate that an even higher Bi concentration is present on these sites, as several examples in the literature indicate the existence of Se vacancies, especially on the outer Se layers of the quintuple block (Dai et al, 2016;Scanlon et al, 2012;Unzueta et al, 2016;Devidas et al, 2014;Alpichshev et al, 2012;Mann et al, 2013;West et al, 2012;Wang et al, 2013;Xue et al, 2013). Vacancies could not be taken into account, because an unknown amount of vacancies on the already mixed Bi Se1 /Se1 sites leaves too many degrees of freedom (an excess of Bi can be compensated by more vacancies).…”

“…1(a)] (Mishra et al, 1997;Pé rez Vicente et al, 1999). However, different kinds of native defects are observed or theoretically predicted for Bi 2 Se 3 : Se and Bi vacancies (Dai et al, 2016;Scanlon et al, 2012;Unzueta et al, 2016;Devidas et al, 2014;Alpichshev et al, 2012;Mann et al, 2013;West et al, 2012;Wang et al, 2013;Xue et al, 2013), substituted atoms (Dai et al, 2016;Scanlon et al, 2012;Mann et al, 2013;West et al, 2012;Wang et al, 2013;Xue et al, 2013;Urazhdin et al, 2002;Huang et al, 2012), interstitial Se atoms (Dai et al, 2016), interstitial Bi atoms (Tumelero et al, 2016;Jia et al, 2014;Urazhdin et al, 2004) and Bi 2 layers in the VdW gap (Huang et al, 2012) (Fig. 1).…”

Disorder-induced splitting in quasiparticle interference of Bi2Te3 Dirac electrons

“…These calculations neglected however the actual form of the scattering potential. In Bi 2 Se 3 , for instance, Se-vacancies are native surface defects 30 that are charged 31 and induce the charge density located in vertices of the equilateral triangle of a lateral size ∼ 15-20 Å 32 , that is comparable with the wavelengths 2π/|q| of considered Friedel oscillations. The effects related to the finite quasiparticle lifetime were not considered yet.…”