MgZnO/ZnO HETEROSTRUCTURES FOR UV LIGHT EMITTERS AND SPINTRONIC APPLICATIONS: MATERIAL GROWTH AND DEVICE DESIGN

摘要

High quality MgZnO epitaxial layers and MgZnO/ZnO heterostructures were grown on GaN using RF plasma-assisted molecular-beam epitaxy (MBE). Optical and crystallographic properties of the Mg_xZn_(1-x)O epitaxial layers and heterostructures based on them are reported. Strong CL emission at ～377.5 nm with FWHM of ～ 17 nm was observed from thick ZnO films. Novel band-gap engineering of type-Ⅱ/type-Ⅰ hexagonal heterostructures incorporating the strong piezoelectric and spontaneous polarization fields in ZnMgO and AlGaN-based materials was investigated. Band structures of various MgZnO/AlGaN/GaN heterojunctions were simulated, revealing a strong hole confinement near n-ZnO/p-AlGaN interface with a hole sheet density up to 1.8 x 10~(13) cm~(-2). The formation of such a hole accumulation layer near the p-n junction can increase the probability of radiative recombination under forward bias. Introduction of the MgZnO/ZnO heterojunction provides an additional confinement for electrons injected into the ZnO active region at forward bias. Based on these simulations, MgZnO/ZnO/AlGaN/GaN triple-heterostructure (THS) p-n junction light emitting diodes (LEDs) were fabricated and characterized. Electrical and optical characteristics of THS LEDs are discussed. Ⅰ-Ⅴ characteristics of the LEDs show a strong rectifying behavior with a turn on voltage of ～3.2-3.5V. The series resistance of the LEDs, which is dominated by the conductance of the p-layer was observed to decrease with increasing temperatures. Optical emission with a peak intensity at ～ 390nm was observed from these structures. This is attributed to optical transitions near ZnO/AlGaN interface. Light emission from the THS LEDs was detected for temperatures up to 650K, suggesting an excitonic origin for the observed optical transitions. For applications in spin-polarized-enhanced light emitters, dilute magnetic doped ZnO films were also studied. Experimental measurements of the ferromagnetic properties of ZnO films doped with Mn by ion implantation are reported. Superconducting quantum interference device (SQUID) magnetometry and RT hysteresis loop measurements demonstrated that the ZnO:Mn films are ferromagnetic at room temperature with a normalized magnetization of about 1 emu/cm~3. These results show that ZnO is also a candidate for spintronic device applications.