E. Strods¹, M. Zubkins¹, V. Vibornijs¹, A. Sarakovskis¹, R. Ignatans¹, L. Bikse¹, R. Nedzinskas¹, M. Ottosson², A. Hallén³, J. Purans¹, A. Azens^4
1University of Latvia, Riga, Latvia
²Ångström Laboratory, Uppsala, Sweden
³KTH Royal Institute of Technology, Kista-Stockholm, Sweden
⁴AGL Technologies SIA, Riga, Latvia
Thin films of Ga₂O₃ and ZnGa₂O₄ are of technological interest due to their applications in wide bandgap electronic and optoelectronic devices. In this study, we report on the deposition of amorphous and crystalline thin films (d ̅ = 260 nm) of Ga₂O₃ and ZnGa₂O₄ by reactive pulsed direct current magnetron sputtering from a liquid gallium target onto fused (f-) quartz and c plane (c-) sapphire substrates, where the temperature of the substrate is varied from room temperature (RT) to 800°C. Non-stoichiometric ZnGa₂O₄ thin films, covering a wide range of Ga:Zn atomic ratios (≈ 0.3 – 5.7), were deposited by co-sputtering solid Zn traget next to liquid Ga target. The composition was controlled by varying the sputtering power of the Zn target and monitoring the process with plasma optical emission spectroscopy. Composition analysis shows no traces of impurities and a slight oxygen deficiency in the films. The static deposition rate of Ga₂O₃ (up to 37 nm/min at RT on f-quartz and 5 nm/min at 800 °C on c-sapphire) is two to five times higher than the rates reported in the literature for radio frequency sputtering. When deposited onto unheated substrates, the films are X-ray amorphous. Well-defined X-ray diffraction peaks of β-Ga₂O₃ begin to appear at a substrate temperature of 500°C, and ZnGa₂O₄ peaks at 300°C. Electron microscopy images reveal a dense and void-free microstructure. The thin films are highly transparent in the visible light range (≈ 84%) and the optical band-gap varies between approximately 3.9 eV and 5.1 eV, depending on the amount of Zn in the composition.