| MgxZn1-xO ternary alloy, as an important member of II-VI wide bandgap semiconductors, offers many advantages, including continuously tunable band gaps (3.37-7.8 eV) and tailored physicochemical properties, which makes it suitable for developing various short-wavelength optoelectronic devices by constructing MgZnO-based heterostructures, such as wavelength-tunable ultraviolet light emitting (laser) diodes, and solar-blind ultraviolet photodetectors.In this work, MgxZn1-xO films of different compositions were grown on Al2O3 (1120) substrates by MOCVD technique. The effects of Mg concentrations on the structural and optical properties of MgZnO films were investigated by energy-dispersive X-ray analysis (EDX), X-ray diffraction (XRD), and optical absorption and photoluminescence (PL) measurements. The experimental results reveal that for Mg concentration x<0.38, MgxZn1-xO films exist in hexagonal wurtzite structure. However, when x exceeds 0.38, the separation between hexagonal and cubic phase occurs. The as-grown MgxZn1-xO films have a wide and tunable band gap from 3.3 to 4.0 eV, which leads to the remarkable blueshift of optical absorption and near-band-edge (NBE) emissions.Thermal stability of hexagonal MgxZn1-xO films with high Mg concentration x=0.53 were especially studied by rapid thermal annealing the films in O2 ambient and in the temperature range of 200-1000 oC. It is found that the Mg0.53Zn0.47O film keeps hexagonal structure until 500°C, and cubic phase MgZnO begins to form at the temperature higher 600°C. Phase segregation leads to the decrease of Mg concentration in hexagonal MgZnO phase. As a result, the optical absorption and NBE emissions shift toward lower energy. Moreover, it was observed in temperature dependent PL studies of Mg0.53Zn0.47O film that NBE emission peaks show an anomalous blueshift with increasing temperature. This phenomenon can be understood in terms of the carrier localization effect caused by alloy potential fluctuations.
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