ZnO and ZnCdO metal organic vapor phase epitaxy: epitaxy, defects and band gap engineering

Abstract

Zinc oxide (ZnO) and its ternary alloys have high potential to compete with III-V nitrides for optoelectronic applications. Furthermore, oxide semiconductors receive considerable attention due to their low cost of fabrication, chemical robustness and high thermal conductance. The goal of this work was two fold: (i) to explore manufacturing route of ZnO and ZnCdO films using metal organic vapor phase epitaxy (MOVPE) in vector flow epitaxy mode and (ii) to master structural/optical properties of these films for preparing such as components in electronics, optoelectronics and solar energy conversion. The starting point was to study the influence of basic synthesis parameters on the structural and luminescence properties of pure ZnO films on c-axis oriented sapphire substrates. The samples were synthesized using previously unexplored for ZnO vector flow epitaxy mode of MOVPE employing systematic variations of fundamental synthesis parameters such as temperature, pressure, II/VI molar ratio, total carrier gas flow ratio, susceptor rotation rate, etc. It was concluded that the growth temperature affects the precursor pyrolysis and in these terms pre-determines the actual II/VI molar ratio available at the reaction zone. Concurrently, direct II/VI molar ratio variations by supplying different amount of precursors influences the properties too, for example, changing intrinsic defect balance in the films. Variations of other parameters like chamber pressure, total gas flow rate and susceptor rotation rate resulted in minor deviations in the growth, uniformity and properties. Further, exploring lower-cost substrates, ZnO films have also been successfully fabricated on Si(111) substrates by using AlN buffer layers. The process resulted in ZnO/AlN/Si heterostructures, where ZnO films were grown epitaxially on AlN buffers of different thicknesses and on Si(111) by so called domain-matching epitaxy. An optimal thickness of the AlN buffer was determined, resulting in nearly in-plane strain free ZnO films. Such films exhibited excellent crystalline quality and extremely bright excitonic emissions. The control of point defects in the crystal is essential for realization of any device, and we have specifically investigated the changes in the defect balance as a function of synthesis parameters in our films. By manipulating the growth temperature, we could achieve either Zn-lean or O-lean conditions. Positron annihilation spectroscopy and photoluminescence were employed to study point defects in such films. A range of vacancy complexes was identified from signal variations going consistently with variations in the synthesis conditions. Specifically, a synthesis temperature window has been determined allowing to control the concentration balance of zinc vacancies (VZn). Finally, manufacturing routes of wurtzite ZnCdO alloys were explored utilizing the knowledge obtained in the process of mastering VZn-enriched material. The alloys exhibited mixed wurtzite, zincblende and rocksalt phases for Cd contents > 7 % also demonstrating general decrease in excitonic luminescence. The phase separation is interpreted in terms of corresponding changes in charge distribution and reduced stacking fault energy. A narrow Cd content region (< 2%) was attributed to the wurtzite single phase equilibrium. The band gap of ZnCdO thin was found to decrease with increasing Cd concentration consistently with literature. In present work, the band gap of ZnCdO was tuned from 3.4 eV to 2.3 eV by changing Cd content up to 60 %, providing an excellent opportunity for band gap engineering in novel optoelectronic applications.