Investigation on the growth of SnS thin films and the fabrication of SnS heterojunctions

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Investigation on the growth of SnS thin films and the fabrication of SnS heterojunctions

 

Author: Wang, Wei
Title: Investigation on the growth of SnS thin films and the fabrication of SnS heterojunctions
Degree: Ph.D.
Year: 2015
Subject: Thin films.
Thin film devices.
Optoelectronic devices -- Design and construction.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Electronic and Information Engineering
Pages: xix, 188 pages : color illustrations
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2828519
URI: http://theses.lib.polyu.edu.hk/handle/200/8272
Abstract: Tin monosulfide (SnS), possessing both direct (1.3eV) and indirect (1.1eV) band gap, as a two-dimensional compound material, have been investigated for the fabrication of optoelectronic devices. To improve the quality of SnS thin films, a novel van der Waals epitaxial growth using Molecular Beam Epitaxy (MBE) technique was introduced. The crystallinity of SnS thin films along the growth orientation [010] were substantially improved by introducing a graphene buffer layer between SnS and atomic flat substrates, which were confirmed by both rocking curves of X-ray diffraction (XRD) and images of Secondary Electron Microscopy (SEM).Based on our Cambridge Serial Total Energy Package (CASTEP) simulation results, Cu was a good p-type dopant for SnS, which were confirmed by systemic studies of Cu doped SnS thin films by the coevaporation of Cu metal and SnS compound. Hole concentration could precisely be controlled from ~1×10¹⁶cm⁻³ to ~6×1017cm⁻³ by carefully adjusting the temperature of Cu K-cell. Maximum hole mobility of 81cm²/Vs was observed at hole concentration of 1.2×1017cm⁻³. For a better understanding of the crystallinity improvements of SnS, Vienna Ab-initio Simulation Package (VASP) simulation was introduced to investigate the growth mechanism of SnS on both three-dimensional (3D) substrate (GaAs) and two-dimensional (2D) substrate (graphene). A SnS/GaAs(100) interface of high density of chemical bonds and serious lattice distortion was observed while the simulation results on SnS/graphene model exhibited an abrupt interface with little to no strain and a distance of 3.22A between atomic layers of SnS and graphene. Meanwhile two kinds of preferred lateral configurations of SnS/graphene with a periodicity of 60o were discovered. The results of Transmission Electron Microscopy (TEM) and XRD phi scan were in excellent agreement with the simulation results.High-resolution TEM (HRTEM) cross-sectional view image of SnS/GaAs interface showed an amorphous layer of ~5nm thick, where randomly oriented SnS clusters were observed, indicating serious interactions between GaAs and SnS. Meanwhile, abrupt SnS/graphene interface with atomic layer distance of 2.95A was observed by HRTEM.A total 36 lateral orientations of SnS on graphene were observed from XRD phi scan for SnS (160) reflection, implying a high density of grain boundaries. Direct observation of SnS grain boundaries of relative 60o and 30o orientations were observed in the top view images of HRTEM.To investigate the trap density inside SnS films,low-frequency noise measurements were conducted on SnS resistive devices.Based on the thermal activation model for low-frequency excessive noise, the Hooge parameter obtained at 260K and 10Hz was found to be 0.68, which was much larger than the value obtained on common electronic devices, ~2×10-3, indicating a rather high trap density inside the SnS material. SnS based heterojunctions were fabricated by directly depositing of SnS thin films on n-type substrates, Gallium Nitride (GaN) and Aluminium Zinc Oxide (AZO),in MBE system.To investigate the band alignment at the interface of heterojunctions, results of X-ray photoelectron spectroscopy (XPS) were studied.A type II heterojunction between SnS and AZO were identified,which was consistent to the reports in literature.For SnS/GaN devices, an electron barrier results from the large CBO (+0.5eV) was detected, which blocked the flowing of photo-generated electrons from SnS to GaN. On the contrary, the flow of photo-generated holes from GaN to SnS was not affected.In addition, the Responsivity spectra of SnS/GaN devices clearly demonstrated a strong photo-sensitivity in the UV range. Thus the SnS/GaN devices are feasible for the application of UV detector.

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