Ultrawide bandgap willemite-type Zn2GeO4 epitaxial thin films

• Published in: Applied physics letters 2023-01, Vol.122 (3)
• Main Authors: Luo, Sijun, Trefflich, Lukas, Selle, Susanne, Hildebrandt, Ron, Krüger, Evgeny, Lange, Stefan, Yu, Jingjing, Sturm, Chris, Lorenz, Michael, Wenckstern, Holger von, Hagendorf, Christian, Höche, Thomas, Grundmann, Marius
• Format: Article
• Language: English
• Subjects: Aluminum oxide; Applied physics; Defects; Density functional theory; Epitaxial growth; Interstitials; Magnetism; Orientation relationships; Partial pressure; Photoluminescence; Physical properties; Pulsed laser deposition; Pulsed lasers; Room temperature; Sapphire; Semiconductor materials; Substrates; Surface roughness; Thin films; Valence band; Wide bandgap semiconductors; Zinc silicates
• ISSN: 0003-6951; 1077-3118
• DOI: 10.1063/5.0130946

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 [ 1 1 ¯ . 0] Zn2GeO4//[ 1 1 ¯ . 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.