Metal–Organic Chemical Vapor Deposition Growth of ZnGeN₂ Films on Sapphire

Abstract

Heterovalent ternary ZnGeN2 thin films were grown on c-, r-, and a-sapphire substrates using metalorganic chemical vapor deposition. The crystal structure of the films was identified by X-ray diffraction spectroscopy. It is consistent with orthorhombic Pna21 assuming perfect ordering of the cations. For a fully disordered cation sublattice, the X-ray diffraction spectra correspond to a wurtzitic crystal. The growth directions were determined to be along the c-axis for the films grown on c- and a-sapphire substrates, and along the orthorhombic [010] axis (wurtzite[112̅0]) for films grown on r-sapphire substrates. The Zn/Ge atomic ratios, determined by energy dispersive X-ray spectroscopy, were observed to decrease as growth temperatures were increased. Growth rates for all substrates decreased by approximately 10% with changes in growth temperatures from 600 to 710 °C. Broad photoluminescence peaks at ∼2.05 eV were observed at room temperature. These are associated with transitions involving deep level defects. Room temperature photoluminescence excitation spectra showed peaks at ∼3.4 eV associated with enhanced absorption near the band gap of the orthorhombic Pna21 phase. Broad tails in the excitation spectra observed to persist to approximately 2.8 eV may be band tailing due to disorder on the cation sublattice. The unintentionally doped films are n-type with carrier concentrations varying from 2 × 1018 cm–3 to 2 × 1019 cm–3, decreasing with the increase in growth temperature. Room temperature Hall mobilities of up to 17 cm2/V·s were obtained.