Molecular beam epitaxy of three dimensional topological insulator Bi₂Se₃ thin films

Abstract

In this thesis, molecular-beam epitaxy (MBE) of three-dimensional (3D) topological insulator (TI) Bi_2 Se_3 thin films on different substrates is presented. The substrates experimented include InP(111)A, GaAs(111)A, InP(001) and GaAs(001). Multiple characterization techniques are employed to investigate the film’s structural, morphological and electrical properties. To facilitate growth of high quality epitaxial Bi_2 Se_3, thermal treatment of the substrate surfaceturnsout to be crucial for both InP(001) and InP(111). On the other hand, for high-index epitaxial Bi_2 Se_3 growth on GaAs(001), the In_2 Se_3 buffer layer has to be employed.

Twin defects in epitaxial Bi_2 Se_3 (111) thin films on hexagonal substrates have been found inevitable in the past. In this study, however, such defects are successfully suppressed on InP(111)A and GaAs(111)Asubstrates, as evidenced in electron diffraction and morphological measurements. The prerequisite for the twin-free Bi_2 Se_3 (111) epitaxy appears to be the step-flow growth mode on the purposely treated stepped substrate surfaces, where deposits incorporate in film at step edges. The lattice of InP or GaAs substrate then plays a guiding role for epitaxial Bi_2 Se_3. Twin suppression is also seen to occur for growth on vicinal and islanded InP(111)A substrate, where a high density of steps inherently exists on surface. Transport studies on such single-domain Bi2Se3epifilms show superior electronic characteristics when compared to those of twinned films grown on, e.g., Si(111). The Shubnikov–de Haas (SdH)oscillations due to bulk state Landau quantization are observed in the magnetoresistance (MR) measurements of Bi_2 Se_3films grown on InP(111)A.

So far, a majority of experimental work of 3D TIs is exclusively on the (111) surfaces, primarily due to the ease to obtain such a surface by cleavage or by growth. On the other hand, for strong topological insulator, nontrivial surface states are expected to exist on other surfaces as well, which remain to be experimentally confirmed. In this study, a high-index epitaxial Bi_2 Se_3is achieved by epitaxial growth on facetted InP(001) substrate. The latter is obtained by a cautious thermal treatment of the substrate wafer under Se flux, where the rhombohedral In_2 Se_3buffer layer forms, facilitating the growth of Bi_2 Se_3 (221) film.Such a high index Bi_2 Se_3 film is evidenced by low-energy electron diffraction (LEED), reflection high-energy electron diffraction (RHEED) and x-ray diffraction (XRD) measurements.

The unique strapped morphology on Bi_2 Se_3 (221) surface is revealed by scanning tunneling microscopy (STM). Angle-resolved photoemission spectroscopy (ARPES) measurements unambiguously show the Dirac surface states elucidating the 3D topological nature ofBi_2 Se_3. Significantly, constant energy plot shows an anisotropic Fermi surface, being ofellipticalshape, which qualitatively agrees with the theoretical calculation. Transport studies of such Bi_2 Se_3(221) films reveal the ratio of conductivities along directions parallel and transverse the van der Waals (vdW) gaps to be as high as 4.4.