Title

Quantification and Influence of Cation Sublattice Disorder in Ternary Materials with Specific Application to ZnSnN2

Date of Award

12-2019

Degree Name

Doctor of Philosophy

Department

Engineering and Applied Sciences

First Advisor

Dr. Steven M. Durbin

Second Advisor

Dr. Muralidhar Ghantasala

Third Advisor

Dr. Daniel M. Litynski

Fourth Advisor

Dr. Damon Miller

Keywords

ZnSnN2, MBE, bond gap, disorder

Abstract

ZnSnN2, a member of the family of II-IV-N2 ternary heterovalent semiconductors, has recently received considerable interest as a potential earth-abundant element-based compound semiconductor for photovoltaic device applications. A major reason for the interest in ZnSnN2 stems from its constituent elements; they are relatively inexpensive, readily available in high-purity form, non-toxic, and both zinc and tin benefit from mature recycling infrastructure. Although ZnSnN2 has been synthesized using a variety of techniques, there remain many open questions regarding its fundamental properties.

Density functional theory (DFT) calculations for ZnSnN2 predict an orthorhombic lattice belonging to the Pna21 space group, with a band gap between 1.78 and 2.0 eV. These predictions are for a completely ordered cation sublattice; however, as in binary alloys such as InGaN, heterovalent ternary materials like ZnSnN2 can have varying degrees of ordering on the cation sublattice. For ZnSnN2 with a completely disordered cation sublattice, DFT calculations, using the special quasi-random structures (SQS) model to model the random distribution of Zn and Sn on the cation sublattice, predict the lattice structure will be wurtzitic and the band gap will be between 0.5 and 1.0 eV. It has previously been confirmed that it is possible to obtain both orthorhombic and hexagonal lattice structures via plasma-assisted molecular beam epitaxy (PAMBE), and that the band gaps for each structure are close to the DFT predicted values.

Up to this point, studies of disorder on the cation sublattice in this material system, and in general heterovalent ternary materials, has been largely qualitative, with research groups reporting to have obtained either the disordered wurtzitic lattice or the orthorhombic ordered lattice. This work focuses on the development of the methodology for quantifying the degree of order on cation sublattice in ternary heterovalent materials, specifically on the influence of PAMBE growth parameters on ZnSnN2 film quality and cation sublattice ordering. The quantified cation sublattice ordering is then used to develop a theory based on the Ising model that provides a quantitative relationship between the ordering of the cation sublattice the optical band gap of ternary heterovalent materials and experimentally validates this theory for ZnSnN2. Furthermore, using data extracted from the literature, the methodology of quantifying the cation sublattice ordering and the Ising model relationship between the band gap and cation ordering is shown to apply generally to all ternary semiconductors.

Access Setting

Dissertation-Abstract Only

Restricted to Campus until

12-2029

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