Oxide materials are becoming one of the most important subjects in materials science. In particular, zinc oxide (ZnO) and related compounds form a novel class of semiconductors which possess unique properties in terms of crystallography, crystal growth, optical properties, electrical properties, and magnetic properties. These unique properties make the material very important in optoelectronics, microelectronics and spintronics.
Although ZnO has been studied for more than three decades, it is only recently that it has been exploited for the fabrication of novel electronic and optical devices due, in particular, to the breakthrough and advance in thin film crystal growth techniques such as molecular beam epitaxy (MBE). ZnO-based heterostructures and nanostructures have important applications in solid state lighting, optical storage, high-speed, high-temperature and high-power electronics, bio and environmental sensors, and energy generating and saving devices.
ZnO has comparable band gap and electronic properties to another widely studied wide bandgap semiconductor, GaN. Compared to GaN, however, ZnO has several major advantages. First, large area ZnO substrates (up to 3” in diameter) are commercially available, making the homoepitaxial growth of low defect density ZnO possible. The second advantage is that the wet etching of ZnO is much easier than that of GaN, making the fabrication process of ZnO-based devices much easier than that of GaN-based devices. The large exciton binding energy (60 meV) of ZnO may lead to lasing action based on exciton recombination even above room temperature.
To facilitate the study of ZnO-based materials and devices, very recently, a ZnO MBE system has been established at City College by Prof. Aidong Shen (Department of Electrical Engineering). The MBE system is a modified DCA 350 system with an oxygen plasma source (Oxford Applied Research). In addition to the plasma source, there are six solid source effusion cells. The manipulator (substrate heater) has been modified to be oxygen compatible. Water cooling is introduced to extend the lifetime of the substrate heater. With the MBE system we are able to prepare various single crystal thin films and nanostructures based on ZnO and related compounds such as ZnMgO, ZnCdO, and transition metals doped ZnO. Some of the proposed research directions are: (1) UV/visible light emitting devices (lasers and LED); (2) Solar blind UV detectors; (3) short wavelength intersubband devices for optical communications (quantum cascade lasers and optical switches); (4) room-temperature ferromagnetic semiconductors for spintronics applications.