Evolution of the THz emission signal as a function of the interface quality between Fe and large-area topological insulator Sb2Te3 on Si

Abstract

Topological insulators (TI) are gaining attention from a technological point of view due to their foreseen highly efficient capability to control adjacent magnetic media through spin to charge (S2C) conversion [1]. However, in order to make a decisive step toward technology transfer, it is necessary to develop fabrication methods suitable to guarantee their large-scale production.We developed a Metal Organic Chemical Vapour Deposition technique (MOCVD) process to grow epitaxial-quality antimony telluride (Sb2Te3) topological insulator (TI) on 4” Si(111) substrates [2]. When compared to granular-Sb2Te3 grown on SiO2 [3], the epitaxial-Sb2Te3 on top of Si(111), shows improved magnetoconductance (MC) performances, especially upon proper annealing, as described in [2]. Figure 1 displays the MC signal emerging from the epitaxial (annealed) Sb2Te3 on Si(111), at 5.5 K. Clearly, the MC displays a non-parabolic shape, which we attribute to weak antilocalization. In the framework of the Hikami-Larkin-Nagaoka (HLN) model [3], we extract the parameters α (being connected to the number of conducting channels), and coherence length (lφ), as depicted in the inset of Figure 1. The values of α and lφ for the annealed sample are 0.3 and 55 nm, respectively, thus indicating a 2D-type of conduction due to the existence of topologically-protected surface states (TSS). The as-deposited sample shows a much smaller MC signal, and the extracted α and lφ are comparable with those previously obtained for granular-Sb2Te3 on SiO2 [3].Following the MC studies, both the as-deposited and epitaxial Sb2Te3 samples have been covered with evaporated 57Fe(5nm) and subsequently in situ capped with Au(5nm) to prevent Fe oxidation. To investigate the potential role of the Sb2Te3 preparation on S2C efficiency, terahertz (THz) emission spectroscopy has been conducted on both samples. The Fe layer is saturated by an external in-plane magnetic field. The multilayer system is excited using femtosecond laser pulses, which launch a longitudinal spin-polarized electron current. Through S2C conversion, the spin current is converted into a transverse charge current concomitantly ejecting a THz pulse which we record in the time domain via electro-optic (EO) detection (see Figure 2) [4,5]. Our results show a considerable impact of Sb2Te3 annealing on the THz signal amplitude suggesting a S2C conversion contribution from the Fe/Sb2Te3 interface.Conversion-electron Mössbauer spectroscopy (CEMS) has been conducted on the same samples studied by THz emission spectroscopy. The Fe/as-deposited-Sb2Te3 spectrum resembles those previously reported for the Fe/granular-Sb2Te3 system [6], and CEMS reports that a large fraction of Fe atoms (≤60%) coordinates paramagnetically with Te in a “FeTe” type of bonding. Surprisingly, this fraction collapses to ≤10% at the interface with annealed (epitaxial) Sb2Te3, with a CEM-spectrum mostly dominated by pure Fe, reflecting a remarkable improvement of the chemical-structural-magnetic quality of the Fe/annealed- Sb2Te3 interface. We propose the suppression of the “FeTe”-type of bonding at the interface as being the main source for the enhancement of the S2C conversion as observed by THz emission spectroscopy.To conclude, the annealed epitaxial Sb2Te3 develops TSS-connected conduction as shown by MC (Figure 1), which turns out in an enhancement of the THz signal in Fe/Sb2Te3 heterostructures as due to S2C conversion (Figure 2). By means of CEMS, the evolution of the S2C efficiency with the Sb2Te3 annealing is attributed to a remarkable suppression of intermixing occurring at the Fe/Sb2Te3 interface, when Fe is deposited in direct contact on top of annealed and epitaxial Sb2Te3. **