Synthetic and structural control of thin film zinc germanium phosphide as a tunable band gap semiconductor for optoelectronic devices

Abstract

The III-V family of materials have greatly advanced optoelectronic devices including high-efficiency solar cells and LEDs. However, these applications rely on high-quality epitaxial heterostructures with low defect densities, which limits the number of semiconductor materials that can be utilized. Broadening the number of available semiconductors that can be epitaxially integrated into these technologies could enable significant efficiency improvements and a reduction in the cost of these devices. The ternary II-IV-V2 compounds have the potential to address this need, as changes to cation site disorder can be utilized to modify the bandgap without substantial changes in the lattice parameter. However, reliably controlling cation site disorder during growth and characterizing the resulting degree of disorder are outstanding challenges. Characterizing disorder is particularly challenging in films with cations of similar Z number, such as the Zn-Ge-V2 compounds, where the cations are indistinguishable by traditional X-ray diffraction techniques. In this dissertation, we use ZnGeP2 thin films to solve these challenges around the synthesis and characterization of disordered material. ZnGeP2 was chosen for this work as it is lattice matched to Si and GaP, providing a possible route to future integration in Si and III-V devices.

In this work, we show the ability to synthesize ZnGeP2 films using two different synthesis methods: hot-wire CVD and reactive combinatorial sputtering. The degree of cation disorder in the films is quantified by extracting a long-range order parameter, S, using Resonant Energy X-ray Diffraction (REXD). By conducting the XRD measurement at a resonant energy we can induce a difference between the scattering factors of Zn and Ge and distinguish between the two elements. We then observe a lowering in the absorption onset energy in the films with a decreased S value. Additionally, we identify trends in the absorption onset energy with other parameters, such as composition and film quality. Finally, we show the ability to grow high quality epitaxial ZnGeP2 films on both GaP and Si substrates using reactive combinatorial sputtering in PH3 gas. Overall, this thesis demonstrates the potential for ZnGeP2 as a tunable band gap semiconductor for integration in Si and III-V optoelectronic devices.