Bulk transport properties of bismuth selenide thin films grown by magnetron sputtering approaching the two-dimensional limit

Abstract

Transport properties of topological insulator Bi2Se3 thin films are investigated with an emphasis on understanding finite-size effects as the two-dimensional limit is approached. Ultra-smooth, (000l)-orientated Bi2Se3 thin-films fabricated using radio-frequency magnetron sputtering reveal disproportionately large changes in bulk resistivity and Hall mobility below six quintuple layers (QL). We correlate such changes to the bulk (other than surface) electronic structure where a bandgap enlargement is observed below six QL in optical absorption spectroscopy measurements. This effect is strongest at the three and two quintuple layers that show the largest changes in bandgap. Temperature dependent transport measurements reveal the effect of disorder from substrate and high carrier concentration. Films on sapphire substrate demonstrate better transport properties compared to amorphous quartz. While temperature dependence of bulk resistivity is both thickness and substrate sensitive, the temperature dependence of Hall coefficient is determined only by the carrier concentration in films. Our work highlights the influence of the bulk electronic structure on the transport properties of few-layer Bi2Se3 which is of interest in the applied areas of optoelectronics, nanoelectronics, and spintronics.