ZnO Spintronics and Nanowire Devices

摘要

ZnO is a very promising material for spinlronics applications, with many groups reporting room temperature ferromagnetism in films doped with transition metals during growth or by ion implantation. In films doped with Mn during PLD, we find an inverse correlation between magnetization and electron density as controlled by Sn doping. The saturation magnetization and coercivity of the implanted single-phase films were both strong functions of the initial anneal temperature, suggesting that carrier concentration alone cannot account for the magnetic properties of ZnO:Mn and factors such as crystalline quality and residual defects play a role. Plausible mechanisms for the ferromagnetism include the bound magnetic polaron model or exchange is mediated by carriers in a spin-split impurity band derived from extended donor orbitals. We will also review progress in ZnO nanowires. The large surface area of nanorods makes them attractive for gas and chemical sensing, and the ability to control their nucleation sites makes them candidates for micro-lasers or memory arrays. Single ZnO nanowire depletion-mode metal-oxide semiconductor field effect transistors exhibit good saturation behavior, threshold voltage of ～-3V and a maximum transconductance of 0.3 mS/mm .Under UV illumination, the drain-source current increased by approximately a factor of 5 and the maximum transconductance was ～5 mS/mm. The channel mobility is estimated to be ～3 cm~2 /V. s, comparable to that for thin film ZnO enhancement mode MOSFETs and the on/off ratio was ～25 in the dark and ～125 under UV illumination. Pt Schottky diodes exhibit excellent ideality factors of 1.1 at 25 ℃ , very low reverse currents and a strong photoresponse, with only a minor component with long decay times thought to originate from surface states. In the temperature range from 25-150℃, the resistivity of nanorods treated in H_2 at 400℃ prior to measurement showed an activation energy of 0.089 eV and was insensitive to the ambient used . By contrast, the conductivity of nanorods not treated in H_2 was sensitive to trace concentrations of gases in the measurement ambient even at room temperature, demonstrating their potential as gas sensors. We have also made sensitive pH sensors using single ZnO nanowires.