Ultrawide bandgap willemite-type Zn2GeO4 epitaxial thin films
Sijun Luo, Lukas Trefflich, Susanne Selle, Ron Hildebrandt, Evgeny Krüger, Stefan Lange, Jingjing Yu, Chris Sturm, Michael Lorenz, Holger von Wenckstern, Christian Hagendorf, Thomas Höche, Marius Grundmann
Abstract
Willemite-type Zn2GeO4 is a promising ultrawide bandgap semiconductor material. To date, experimental results on growth and physical properties of epitaxial thin films of willemite-type Zn2GeO4 are not available. Here, we report the heteroepitaxial growth of (00.1)-oriented Zn2GeO4 thin films on c-plane sapphire substrates using pulsed laser deposition. The in-plane orientation relationships are [11.0] Zn2GeO4//[11.0] Al2O3 and [11¯.0] Zn2GeO4//[11¯.0] Al2O3. A 450 nm thick epitaxial film with a surface roughness of 2.5 nm deposited under 0.1 mbar oxygen partial pressure exhibits a full width at half maximum (FWHM) of rocking curve of (00.6) reflex of 0.35°. The direct bandgap is evaluated to be 4.9 ± 0.1 eV. The valence band maximum is determined to be 3.7 ± 0.1 eV below the Fermi level. Together with the density-functional theory band structure calculation, it is suggested that the O 2p orbital and Zn 3d orbital dominantly contribute to the valence band of Zn2GeO4. The steady-state photoluminescence (PL) spectra of the films under 266 nm excitation at room temperature exhibit a broad defect-related emission band centered at 2.62 eV with a FWHM of 0.55 eV. The origin of this native defect-related PL is suggested to correlate with Zn interstitials. This work advances the fundamental study on willemite-type Zn2GeO4 epitaxial thin films for potential device application.