Tin Sulfide Phase Exploration: Dependence of Optoelectronic Properties on Microstructural Growth and Chemical Variations in Thin Film Material

Open Access
Author:
Banai, Rona Elinor
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
April 29, 2015
Committee Members:
  • Jeffrey Brownson, Dissertation Advisor
  • Mark William Horn, Dissertation Advisor
  • Jeffrey Brownson, Committee Chair
  • Mark William Horn, Committee Chair
  • Joan Marie Redwing, Committee Member
  • Noel Christopher Giebink, Committee Member
  • Suzanne E Mohney, Committee Member
Keywords:
  • thin film
  • optical properties
  • electronic properties
  • microstructure
  • photovoltaic
  • sputtering
  • semiconductor
Abstract:
Herzenbergite tin (II) monosulfide (alpha-SnS) is of growing interest as a photovoltaic material because of its interesting optoelectronic properties and Earth abundance. It has several stable phases due to the dual valency of tin. As a layered material, alpha-SnS has the ability to form varying microstructure with differing properties. For this dissertation, films were RF sputtered from a SnS and SnS2 target to produce films with varying microstructure. Growth of high energy phases including beta-SnS and amorphous SnS2 were possible through sputtering. Films of mixed or strained phase resulted from both targets. Pure phase alpha-SnS was made by annealing amorphous SnS2 films. Microstructure was measured using grazing incidence XRD and field emission SEM. The impact of microstructure was seen for both optical and electronic properties. Films were evaluated using spectroscopic ellipsometry as well as unpolarized UV-Vis transmission and reflection measurements. Optical modeling of the films is sufficient for developing models corresponding to specific microstructure, enabling it to be an inexpensive tool for studying the material. Absorption coefficient and band gap were also derived for these films. Films deposited with the SnS target had resistivity values up to 20,000 Ohm-cm. Annealing of amorphous films deposited from the SnS$_2$ target resulted in alpha-SnS films with much lower resistivity (<50 Ohm-cm) values. This method for producing alpha-SnS offered better control of the phase, microstructure and therefore optoelectronic properties. While SnS films made from either target were typically p-type, sputtering of the SnS2 target with substrate heating resulted in n-type SnSx of a potentially new phase similar to SnS2 but with a 2:3 tin-to-sulfur ratio. Resistivity of those films typically ranged from 1 to 40 Ohm-cm. Both p- and n-type films made from the SnS2 target had high carrier concentration of 10$^17 to 10^20 cm^-3, but films had low Hall mobility such that conductivity type was not determined. Titanium, molybdenum, and aluminum contacts were tested for Ohmic and Schottky behavior using transmission line measurements. The complexity of its microstructure and flexibility in formation of varying phase and altered phase presents challenges to its use as a PV absorber.