Robust topological surface states of Bi₂Se₃ thin films on amorphous SiO₂/Si substrate and a large ambipolar gating effect

The recent emergence of topological insulators (TI) has spurred intensive efforts to grow TI thin films on various substrates. However, little is known about how robust the topological surface states (TSS) are against disorders and other detrimental effects originating from the substrates. Here, we report the observation of a well-defined TSS on Bi₂Se₃ films grown on amorphous SiO₂ (a-SiO₂) substrates and a large gating effect on these films using the underneath doped-Si substrate as the back gate. The films on a-SiO₂ were composed of c-axis ordered but random in-plane domains. However, despite the in-plane randomness induced by the amorphous substrate, the transport properties of these films were superior to those of similar films grown on single-crystalline Si(111) substrates, which are structurally better matched but chemically reactive with the films. This work sheds light on the importance of chemical compatibility, compared to lattice matching, for the growth of TI thin films, and also demonstrates that the technologically important and gatable a-SiO₂/Si substrate is a promising platform for TI films.