Thickness and growth-condition dependence of \emph{in-situ} mobility and carrier density of epitaxial thin-film Bi$_2$Se$_3$

TL;DR

This study investigates how film thickness and growth conditions affect the in-situ electrical properties of epitaxial Bi$_2$Se$_3$, revealing interface doping and the influence of crystalline quality on mobility.

Contribution

It provides direct in-situ measurements of carrier density and mobility during growth, highlighting the roles of interface doping and growth temperature in Bi$_2$Se$_3$ films.

Findings

High interface n-doping at conduction onset

Mobility varies with growth temperature and crystalline quality

Doping is primarily interfacial and not due to bulk dopants

Abstract

Bismuth selenide Bi$_2$Se$_3$ was grown by molecular beam epitaxy while carrier density and mobility were measured directly \emph{in situ} as a function of film thickness. Carrier density shows high interface n-doping (1.5 x 10$^{13}$ cm$^{-2}$) at the onset of film conduction, and bulk dopant density of $\sim$5 x 10$^{18}$ cm$^{-3}$, roughly independent of growth temperature profile. Mobility depends more strongly on the growth temperature and is related to the crystalline quality of the samples quantified by \emph{ex-situ} AFM measurements. These results indicate that Bi$_2$Se$_3$ as prepared by widely employed parameters is \emph{n}-doped before exposure to atmosphere, the doping is largely interfacial in origin, and dopants are not the limiting disorder in present Bi$_2$Se$_3$ films.