W.C. Hsieh

Water-modulated oxidation in the growth of m-ZnO epitaxial thin film by atomic layer deposition

The effects of extra H2O-modulated oxidation are reported on the structural, optical, and electrical properties of nonpolar m-plane ZnO thin films grown on m-plane Al2O3 substrates by atomic layer deposition. Films without modulation, one modulated layer, and two modulated layers are compared. Structural properties studied using x-ray reflectivity, x-ray diffraction, and transmission electron microscopy show that all the films have a largely similar thickness and epitaxial relations with their substrates, but the rocking curves grow broader as the number of modulations increases. However, the extra layer of water modulation reduces the surface roughness drastically and also improves the electrical properties as compared to the unmodulated ZnO films. Water modulation is believed to serve as a source of atomic oxygen that promotes compensation of the pre-existing oxygen vacancies. The films tend to exhibit larger mosaicity around the a-axis as compared to that around the c-axis.

Controlling Surface Morphology and Circumventing Secondary Phase Formation in Non-polar m-GaN by Tuning Nitrogen Activity

For the development of non-polar nitrides based optoelectronic devices, high-quality films with smooth surfaces, free of defects or clusters, are critical. In this work, the mechanisms governing the topography and single phase epitaxy of non-polar m-plane gallium nitride (m-GaN) thin films are studied. The samples were grown using plasma-assisted molecular beam epitaxy on m-plane sapphire substrates. Growth of pure m-GaN thin films, concomitant with smooth surfaces is possible at low radio frequency powers and high growth temperatures as judged by the high resolution x-ray diffraction, field emission scanning electron microscopy, and atomic force microscopy measurements. Defect types and densities are quantified using transmission electron microscopy, while Raman spectroscopy was used to analyze the in-plane stress in the thin films which matches the lattice mismatch analysis. Energy dispersive spectroscopy and cathodoluminescence support a congruent growth and a dominant near band edge emission. From the analysis, a narrow growth window is discovered wherein epitaxial growth of pure m-plane GaN samples free of secondary phases with narrow rocking curves and considerable smooth surfaces are successfully demonstrated.

Effects of mid-gap defects and barrier interface reactions on tunneling behaviors of ZnO-i-Si heterojunctions

Low-leakage pin diodes based on ZnO-i-Si are realized by redox reaction of aluminum with the native oxide SiOx into AlOx and by proper selection of annealing conditions. The main sources of electric leakage was found to arise from charge carrier tunneling via mid-gap states in the semiconductors or lowered tunneling barriers. Less mid-gap states in n-ZnO and high tunneling barrier of the i-layer are key to lowering the leakage. Proper post-annealing of pin diodes effectively heal the mid-gap defects, while maintaining the integrity of the tunneling layers, thus lowering the leakage currents to reach a rectification ratio of 2400, surpassing most similarly benchmarked devices reported in literature. Excessive annealing causes some part of the i-layer to transform into to ZnAl2O4 and Al:ZnO. High Al-doping and lowered potential barrier provided by ZnAl2O4 are responsible for high leakage currents in devices so fabricated.