Photoluminescence study of optical transitions in spinel zinc gallium oxide thin films

Spinel ZnGa₂O₄ is an ultra-wide bandgap material that can have a great potential for deep ultraviolet (UV) photonics and other applications. In this work, zinc gallium oxide (ZnGaO) samples with Zn composition ranging from 0.0 to 48.0 at% were grown in a plasma-assisted molecular beam epitaxy system. The change of crystal structure from beta to spinel was determined using reciprocal space mapping in x-ray diffraction. When Zn composition is at 0.0, 0.9, 3.4, and above 7.3 at%, the crystal structure exhibits beta phase, mixture phase, weak spinel phase, and strong spinel phase, respectively. Comprehensive photoluminescence (PL) of the samples were carried out using an ArF laser excitation, and PL peak deconvolution was performed to understand the optical transitions and energy levels within the forbidden gap. For spinel ZnGaO samples, five deconvoluted peaks were observed, revealing the energy levels of three oxygen vacancies, self-trapped holes binding energy, and acceptor levels.