Investigation on the growth and characterization of Tin Sulfide 2D thin films

Abstract

Two-dimensional (2D) materials have recently attracted increasing amount of research attention, because of their exotic characteristics of asymmetric connections in the bulk. They involve strong intra-layer covalent bonding and weak interlayer interactions consisting mainly of van der Waals forces. Consequently, there is almost no dangling bond at the interfaces and on the surfaces. In addition, strain is also nearly absent between the layers and at the interfaces between such materials. These features make 2D materials promising for modern electronic and optoelectronic devices with outstanding performances. In this thesis, the van der Waals epitaxial growth of 2D SnS thin films on 2D mica substrates and 2D templates such as a SnSe buffer layer, a Bi2Se3 conducting thin layer, had been investigated. In addition, preliminary devices based on these materials were fabricated to demonstrate their applications. Additionally, efforts on building all-2D van der Waals (vdW) heterostructures have also been made. The van der Waals epitaxial growth of 2D materials was carried out using mica as substrates, which also possesses 2D layered structure exhibiting flexible, transparent, and insulating properties at low price. Molecular Beam Epitaxial (MBE) grown SnS thin films on mica substrates with excellent film quality were achieved, reflected by the narrow full width at half maximum (FWHM) of 0.1° in X-ray diffraction rocking curve. Analysis of this all-2D-layer stacking with the help of interface model was developed, to better understand the van der Waals epitaxial growth mechanism and facilitate further improvement of the film quality. Utilization of SnSe buffer layer significantly modified the growth mechanism by effectively aligning the lateral growth orientations, thereby enhancing the electrical property with lowered defect density within the films. To fundamentally reduce the native point defects caused by the tin deficiency in the films, a simultaneous tin compensation source was adopted. Apart from the great enhancement in the electrical property, the crystal structure also bore a resemblance to that with SnSe buffer layer. The FWHM of Sn-compensated SnS film exhibited a record low value between 0.07° and 0.04°, implying an extremely high film quality. Additionally, an about 67% increase in Hall mobility was accomplished in the film. The Sn-compensated SnS/GaN:Si heterojunction devices were fabricated to demonstrate the positive effect of Sn compensation, as well as the feasibility of SnS as optoelectronic applications. The SnS 2D thin films with high quality and tunable properties hold great potential for optoelectronic devices with better performances.