Recent advances in ZnO materials and devices

Wurtzitic ZnO is a wide-bandgap (3.437 eV at 2 K) semiconductor which has many applications, such as piezoelectric transducers, varistors, phosphors, and transparent conducting films. Most of these applications require only polycrystalline material; however, recent successes in producing large-area single crystals have opened up the possibility of producing blue and UV light emitters, and high-temperature, high-power transistors. The main advantages of ZnO as a light emitter are its large exciton binding energy (60 meV), and the existence of well-developed bulk and epitaxial growth processes; for electronic applications, its attractiveness lies in having high breakdown strength and high saturation velocity. Optical UV lasing, at both low and high temperatures, has already been demonstrated, although efficient electrical lasing must await the further development of good, p-type material. ZnO is also much more resistant to radiation damage than are other common semiconductor materials, such as Si, GaAs, CdS, and even GaN; thus, it should be useful for space applications.