ZnO and Related Compounds and Quantum Well Structures Grown by Plasma-Assisted MBE

Zinc Oxide (ZnO) and related materials have attracted considerable attention due to their potential applications in optoelectronic devices. ZnO has a direct band gap of 3.37 eV and a large exciton binding energy of 60 meV at 300 K, which makes it a candidate for more efficiency optical emitters. ZnO and related compounds have large conduction band offset, which is essential for realizing short wavelength intersubband (ISB) devices. The large longitudinal optical phonon energy of ZnO-related compounds also makes them a promising material system for terahertz quantum cascade lasers working at room temperature.;In this dissertation, I present our study on the growth and characterization of ZnO and related compounds and structures. Growth conditions for obtaining high quality ZnO epilayers and related compounds and structures were established. We observed, for the first time, ISB absorptions in ZnO/ZnMgO multiple quantum wells (MQWs), demonstrating the potential to use the material system for intersuband devices.;We started with the establishment of the general MBE growth conditions for obtaining high quality ZnO thin films on c-plane sapphire substrates. Particular emphasis was devoted to the study of the effect of MgO buffer layers and the control of surface polarity of the ZnO thin films. Our study shows that ZnO films grown on MgO buffer layers with an optimal thickness of about 2 nm have better quality compared to those grown directly on c-plane sapphire without any buffer layers. The surface polarity can also be controlled by the MgO buffer layer. With the thickness of the MgO buffer layer increased from 2 nm to more than 3 nm, the surface of the ZnO films turns from O-polar to Zn-polar. We also found that the oxygen plasma source conditions can significantly affect the quality and growth rate of ZnO. N-type doping of ZnO by Ga with electron concentration of up to the mid 1019 cm-3 has been achieved.;Alternative substrates for the growth of ZnO thin films were explored. We studied the growth of ZnO on GaAs substrate with a ZnSe buffer layer. (001), (111)A, and (111)B GaAs substrates were used. Of the three, ZnO grown on (111)B substrate has the best quality. ZnO thin films grown on (111) ZnS substrates were also studied. The common things among these substrates/buffers and the ZnO epilayers are that they are all II-VI semiconductors (ZnS, ZnSe buffter, and ZnO) and they share a common cation of Zn. Our study shows that although single crystal ZnO thin films can be grown on these substrates/buffers, the quality of the ZnO films are generally inferior to those grown on c-plane sapphire substrates.;As the first step to grow the multi-layered heterostructures, the growth conditions for obtaining high quality ZnMgO ternary alloys were studied. Our study found that ZnMgO grown in the temperature window from 400°C to 500°C has the best structural and optical quality. With the growth conditions for obtaining high quality ZnO and ZnMgO optimized, we have successfully grown ZnO/ZnMgO MQWs with high structural and optical qualities. At room temperature, clear mid-infrared ISB absorptions in ZnO-based quantum structures were observed for the first time, demonstrating the potential to use the material system for the fabrication of ISB devices.