MgZnO Alloy Films With Tunable Bandgap And The UV Photodetectors

The intense interest in shot-wavelength optoelectronic devices has promotedlarge research efforts into the wide bandgap semiconductors. PseudobinaryZnO-MgO alloy system received increasing attention in the past decade, mostlyattributed to its potential applications in a wide UV spectrum region from380nm to160nm promised by its possible tunable-bandgap (3.37eV for ZnO to7.8eV forMgO). However, the solid solution of MgxZn1-xO is of eutectic type at normalconditions, and the restricted solid solubility poses significant challenges to thebandgap engineering. This thesis focuses on understanding and suppressing phasesegregation of the MgxZn1-xO alloy films, and a technique using acompositionally-graded buffer layer and low growth temperature is developed forthe growth of single cubic MgZnO alloy films with high Zn content in aLP-MOCVD system. The optical and structural properties and thermal stability ofthe metastable alloy films and their applications in UV detectors are studied. Themain contents are as follows:[1] Single (110)-oriented cubic MgxZn1-xO alloy films with Zn content up to 55%were prepared on m-plane sapphire by using MOCVD method. The opticalbandgap can be modulated to a value of4.7eV. The epitaxial relationships have beendetermined by XRD analysis and the films present good structural quality withuniform in-plane orientation. An MSM solar-blind UV photodetector was fabricatedon the Mg0.45Zn0.55O thin film, which shows a sharp cutoff at278nm.[2] A graded quasi-homo buffer layer, composed of a MgO layer and acontinuously Zn-increased MgZnO layer, was used to grow pure cubic phaseMgxZn1-xO thin films on c-plane sapphire in a LP-MOCVD system. The Zn contentin rocksalt MgxZn1-xO is expanded to1-x~0.7and the optical bandgap of the film is4.45eV. The role of the buffer layer was discussed, and a substrate temperaturerange from350oC to400oC was found to be benefit to grow the metastableMgxZn1-xO films with high Zn content. The metastable cubic MgxZn1-xO films aresufficiently stable to be used in UV detectors, and thermal annealing experimentswere also carried out to investigate their phase-stability.[3] An MSM solar-blind UV photodetector was fabricated on a Mg0.29Zn0.71Othin film, which shows a large responsivity of27.9A/W at268nm30V bias. Anultra thin phase-transition layer is regarded to lead to improvement of themetal-semiconductor contact and more internal gain. The results provide a newchoice to realize high performance MgZnO-based solar-blind UV detectors.