Search

Rediscovered in the last decade, zinc oxide (ZnO) shows a great potential for many optoelectronics and to some extent microelectronics applications. However, a clear majority of effort expended in this fast developing field has been limited to heteroepitaxial structures grown on foreign substrates with lattice-parameter and thermal-expansion mismatch with ZnO which is detrimental. Recognizing the importance, the effort has shifted to include developing technologies capable of producing freestanding ZnO wafers in large-scale for ZnO based device applications, which is the subject matter of this manuscript. Three competing approaches - hydrothermal method, melt growth (modifications of the well known Bridgman technique), and seeded vapor transport growth - have now reached or are approaching commercial viability. In this article, we discuss the progress, outstanding problems, and prospects of these growth methods employed for commercial manufacturing of ZnO wafers.

Both GaN- and ZnO-based diluted semiconductors (DMSs) have recently attracted considerable interest fueled by theoretical predictions of ferromagnetic ordering in these materials above room temperature, making them especially attractive for spintronics. The intense experimental research that followed has revealed, however, a great deal of controversy. The local structure and magnetic behavior of GaN- and ZnO-based DMSs were found to depend strongly on a preparation technique and growth conditions for the materials of the same nominal composition and the reported results varied considerably from group to group. This problem highlighted clearly the lack of theoretical understanding of physical mechanisms underlying ferromagnetisms in these materials and the inadequacy of standard characterization techniques used to probe structural and magnetic properties of the DMSs. In this paper, we report on the recent progress in the theoretical and experimental studies of ZnO- and GaN-based DMSs and make special impact on critical discussion of experimental methods employed for investigation of their magnetic and optical properties.

GaN-based optoelectronic and electronic devices such as light-emitting diodes (LEDs), laser, and heterojunction field-effect transistors (HFETs) typically use material grown on foreign substrates such as sapphire, Si, and SiC. However, thermal and lattice mismatch present prevent attainment of quality films deemed necessary by ever increasing demand on device performance. In fact in LEDs intended for solid state lighting, internal quantum efficiencies near 100% might be needed, and further these high efficiencies would have to be retained at very high injection current levels. On the electronic device side, high radio-frequency (RF) power, particularly high-power switching devices, push the material to its limits. Consequently, as has been the case for other successful semiconductor materials systems, native substrates must be developed for the GaN family. In this paper, various approaches such as high-pressure nitrogen solution (HPNS), ammonothermal, and Na flux methods, and an intermediary technique called the hydride vapor phase epitaxy (HVPE; to a lesser extent as there is a review devoted to this technique in this issue) along with their strengths and challenges are discussed.

ZnO has gained considerable interest recently as a promising material for a variety of applications. To a large extent, the renewed interest in ZnO is fuelled by its wide direct band gap (3.3 eV at room temperature) and large exciton binding energy (60 meV) making this material, when alloyed with, e.g., Cd and Mg, especially attractive for light emitters in the blue/ultraviolet (UV) spectral region. Unfortunately, as with other wide-gap semiconductors, ZnO suffers from the doping asymmetry problem, in that the n-type conductivity can be obtained rather easily, but p-type doping proved to be a formidable challenge. This doping asymmetry problem (also dubbed as the p-type problem in ZnO) is preventing applications of ZnO in light-emitting diodes and potential laser diodes. In this paper, we provide a critical review of the current experimental efforts focused on achieving p-type ZnO and discuss the proposed approaches which could possibly be used to overcome the p-type problem.