Optical and Electrical Characterization of ZnSnN2 Thin-Films

This dissertation focuses on a new thin-film semiconductor material for optoelectronic applications such as solar energy harvesting and solid state lighting. The work represents a detailed exploration of the optical and electrical properties of epitaxial thin films of ZnSnN_2 (ZSN), a member of the II-IV nitride family of compounds in which cation ordering was predicted as a novel route to tuning the relevant optoelectronic characteristics. It also represents the first attempt at quantum confinement in a ZSN isocompositional heterostructure using cation order/disorder as a control parameter. The increasing demand for alternatives to fossil fuels and the scarcity of certain metals commonly used in LED lighting, together highlight the need for earth-abundant optoelectronic materials. This has led to the development of the II-IV-V family of semiconductors--a wide variety of materials that enable access to the VIS/IR portions of the electromagnetic spectrum. As a member of this family, ZSN is composed entirely of earth-abundant elements and early theoretical investigations of it predict compatibility with solar cell applications, with band gaps ranging between 1.0 eV and 2.5 eV, depending on the degree of cation ordering. The methodology for synthesizing ZSN with specific properties was developed in collaboration with a group at Western Michigan University, resulting in the first fully ordered, orthorhombic ZSN films. We present original results on the optical and electrical characterization of ZSN thin-films grown on (111) yttria-stabilized zirconia (YSZ) via plasma-assisted molecular beam epitaxy (MBE). A key finding is that the band-gap of the material is correlated with cation ordering across a large portion of the infrared and visible spectrum. The degree of ordering was quantified by an order parameter, defined as S = r_alpha + r_beta - 1, where r_alpha and r_beta represent the fractions of Zn and Sn cation sites occupied by Zn and Sn atoms, respectively. Values of S from 0.05 to as high as 0.92 were achieved in this work, as measured using in-situ Reflection High-Energy Electron Diffraction (RHEED) and ex-situ X-Ray Diffraction (XRD). Optical characterization included Photoluminescence (PL), Cathodoluminescence (CL) and Diffuse Reflectance Spectroscopy (DRS). The CL results are the first evidence of light emission from single-crystal ZSN thin-films. Another innovative aspect of our work was to modify conventional DRS to enhance optical absorption contrast from our thin-film samples, namely ``waveguided diffuse reflectance spectroscopy'' (WDRS), providing vital data on how the band-gap depends on the degree of cation ordering. Electrical measurements were carried out to explore fundamental properties relevant to device fabrication--specifically mobility and carrier concentration. Hall measurements reveal high carrier concentrations ranging from 3.37x10^20 cm^-3 to 2.08x10^21 cm^-3, while measured mobilities ranged from 1.27 cm^2/Vs to 32.5 cm^2/Vs. These results suggest that the density of carrier-generating defects, and perhaps a degenerate population, may be responsible for the comparatively low mobilities, resulting from an increased scattering cross-section and short mean-free-paths (10-100 nm). Further refinement of the growth parameters should help to reduce uncompensated charge carrier concentrations, found to be mainly n-type in our samples. The results, the first of their kind in many cases, highlight the importance of continuing the development of ZSN as a promising earth-abundant alternative to existing III-V and II-VI compounds. In particular the correlation between heterovalent cation ordering and materials characteristics of importance for potential optoelectronic applications is shown to be especially fruitful for future studies.PhDApplied PhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/155209/1/jpmathis_1.pd