Investigation and impact of oxygen plasma compositions on cubic ZnMgO grown by Molecular Beam Epitaxy

ZnMgO thin films were grown by Molecular Beam Epitaxy on closely-lattice-matched MgO substrates with a Radio-Frequency (RF) generated oxygen plasma. The impact on the cubic ZnMgO of oxygen flow rate and applied RF power was investigated under a high vacuum condition (1E-6 Torr). Optical Emission Spectroscopy identified active species in the plasma including O, O + , and O þ 2. The emission intensity of these species was compared at oxygen flow rates ranging from 0.5 sccm to 2.5 sccm and applied RF powers from 150 W to 500 W. Plasma composition at operating conditions was determined by correlating changes in optical emission to changes in relative concentration of active species in the plasma. Atomic Force Microscopy and profilometry characterized changes in surface roughness and growth rate of produced films. Independently increasing flow rate and RF power increased growth rate while simultaneously decreasing roughness. Growth rate enhancement of 80% was achieved with an improvement in surface roughness from 7.5 nm to 3.3 nm in a 1 lm square. Optimal conditions are discussed for ZnMgO thin film epitaxy. ZnMgO thin films attract considerable attention due to the ability to tune optical absorption from the near ultraviolet (NUV) to the deep ultraviolet (DUV) by varying metallic composition [1,2]. DUV optoelectronics can access the solar blind spectral region, enabling commercial applications ranging from combustion monitors and DUV detectors to UV sterilization equipment. While it is well known that the binaries ZnO and MgO have incompatible crystal structures, Zn-rich ZnMgO (wurtzite B4) and Mg-rich ZnMgO (rocksalt B1) are regularly produced with good crystallinity and smooth surface morphology at different ends of the compositional scale [3,4]. Adding Mg to the ZnO lattice blueshifts the ternary bandgap, while redshifting is accomplished by adding Zn to MgO [5,6]. While wurtzite-ZnMgO has received significant attention for its functionality in the NUV spectral range [7,8], cubic-ZnMgO has recently also begun to receive attention from several research groups. c-ZnMgO possesses many intriguing characteristics worthy of investigation and exploitation by optoelectronic devices and is readily grown on commercially available MgO substrates, and maintains good crystallinity with epilayer roughnesses at the ang-strom level [1]. Additionally, c-ZnMgO is inversion symmetric, enabling isotropic conduction of charge carriers and avoiding the polarization fields present in wurtzite phase quantum-confined structures. Devices with internal polarization fields like those present in wurtzite can suffer from reduced electron–hole overlap similar to the Quantum Confined Stark Effect [9,10]. While epitaxial …