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.

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.

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.

The developments in AlGaN/GaN heterojunction field effect transistors (HFETs) are beginning to allow harnessing of the great potential of this technology in high-power radio-frequency (RF) applications. However, the integration of HFET into a circuit environment requires accurate small and large signal modeling of the device operating under various biasing conditions. The conventional small signal equivalent circuit modeling methods consist of “cold” measurements for extracting parasitic elements, and on-bias measurements in determining the intrinsic device circuit elements. Also, the optimization routines are often explored directly using the “hot” measurement data to minimize errors. In this paper, an 18-element small signal equivalent circuit model for AlGaN/GaN HFET is proposed and implemented, and contrasted to various de-embedding methods. Among the methods treated, the hot-FET optimization extraction based on the parasitic capacitances obtained from cold measurements leads to the smallest error between the simulated S-parameters and the measured ones at all bias points employed, with an average error of about 5%. This hybrid extraction algorithm is strengthened by imposing constraints to avoid any nonphysical convergence. We note that the extrinsic parasitics determined by this method differ considerably from the values obtained by cold-FET measurements, which implies that the assumption on the bias independency for extrinsic parameters in the latter method might be questionable for AlGaN/GaN HFET.

GaN-based heterojunction field effect transistors (HFETs) will play major roles in the high-power, high-frequency military and commercial arenas for microwave and millimeter wave transmitters and receivers used in communications and radar devices. In fact, devices operative in the X-band (7-12.5 GHz) and beyond are already at market and boast quite impressive performances. Having improved the crystal quality now to levels where the reliability [expressed as mean time to failure (MTTF)] is claimed to exceed ten million hours, the work now needs to focus on which of the physical mechanisms responsible for degradation are the most important, and how the existing degradation accelerates subsequent degradation, ultimately resulting in device failure. Available data show that not all devices from the same wafer show similar longevity and the wide spread of activation energies reported for three-temperature extrapolation-based predictions of the lifetime are troubling. If we hope to make consistent, reliable predictions of device lifetimes, particularly when the devices are being pushed in radio-frequency (RF) operation to near their limits, more work will need to be done in characterizing the long term stability of the devices, and new physical models for the failure mechanisms will have to be developed.

This special issue addresses to a large extent the challenges and opportunities in GaN and ZnO devices and materials deemed critical to both the GaN- and ZnO-based technologies. There are twenty articles in this special issue.

ZnO is an attractive material for applications in electronics, photonics, acoustics, and sensing. In optical emitters, its high exciton binding energy (60 meV) gives ZnO an edge over other semiconductors such as GaN if reproducible and reliable p-type doping in ZnO were to be achieved, which currently remains to be the main obstacle for realization of bipolar devices. On the electronic side, ZnO holds some potential in transparent thin film transistors (TFTs) owing to its high optical transmittivity and high conductivity. Among the other promising areas of application for ZnO are acoustic wave devices, due to large electromechanical coupling in ZnO, and devices utilizing nanowires/nanorods such as biosensors and gas sensors and solar cells, since it is relatively easy to produce such forms of ZnO nanostructures, which have good charge carrier transport properties and high crystalline quality. Despite the significant progress made, there is still a number of important issues that need to be resolved before ZnO can be transitioned to commercial use, not to mention the stiff competition it is facing with GaN, which is much more mature in terms of devices. In this paper, recent progress in device applications of ZnO is discussed and a review of critical issues for realization of ZnO-based devices is given.

Light-emitting diodes (LEDs) have become quite a high-performance device of late and are revolutionizing the display and illumination sectors of our economy. Due to demands for better performance and reduced energy consumption there is a constant race towards converting every single electron hole pair in the device to photons and extracting them as well while using only the minimum required voltage. This raises the bar on GaN-based LEDs in terms of elimination of nonradiative recombination processes not just at low but just as importantly if not more at very high injection levels needed for high brightness sources, and design of heterostructures for efficient electron and hole recombination without carrier loss and voltage/photon energy conversion loss. The haunting efficiency droop observed in GaN-based LEDs at high injection levels has been attributed to nonradiative Auger recombination, but can be simply explained by electron spillover. Investigations of quantum well (QW) InGaN LED structures with different barrier heights, widths, and doping suggest that limited hole transport in the active region and the resulting electron spillover is responsible for efficiency droop at high injection levels. In this paper, highly critical, demanding, and challenging nature of high-efficiency high-brightness LEDs, in particular the basics surrounding the internal quantum efficiency of LED structures and the ongoing research/development, will be discussed.

With edge-emitting GaN-based lasers in commercial systems, attention is shifting to more demanding and rewarding emitters. These encompass microcavity (MC)-based vertical cavity surface-emitting lasers (VCSELs) and polariton lasers. The impetus centers on applications such as high-speed/high-resolution laser printing/scanning technology, lighting, and new types of coherent but nearly thresholdless optical sources. Room-temperature operations of GaN-based VCSELs by electrical injection have been recently reported, and the research on GaN-based VCSELs is segueing into new opportunities such as polariton-based lasers. While still in its infancy, polariton lasing in GaN-based MCs at room temperature has been observed. Observation of spontaneous emission buildup in polariton lasing emission is attributed to a Bose-Einstein condensate of cavity polaritons. However, the realization of a polariton laser by electrical injection is still being pursued. In this paper, we discuss the recent progress in wide-bandgap semiconductor-based VCSELs and GaN-/ZnO-based polariton lasers.

Wide bandgap GaN has long been sought for its applications to blue and UV emitters and high temperature/high power electronic devices. Recent introduction of commercial blue and blue-green LED's have led to a plethora of activity in all three continents into the heterostructures based on GaN and its alloys with AlN and InN. In this review, the status and future prospects of emerging wide bandgap gallium nitride semiconductor devices are discussed. Recent successes in p-doping of GaN and its alloys with InN and AlN, and in n-doping with much reduced background concentrations have paved the way for the design, fabrication, and characterization of devices such as MESFET's, MISFET's, HBT's, LED's, and optically pumped lasers. We discuss the electrical properties of these devices and their drawbacks followed by future prospects. After a short elucidation of materials characteristics of the nitrides, we explore their electrical transport properties in detail. Recent progress in processing such as formation of low-resistance ohmic contacts and etching is also presented. The promising features of quarternaries and double heterostructures in relation to possible current injection lasers, LED's, and photodetectors are also elaborated on.>

Recent developments in strained layer epitaxial systems are reviewed. Their interest stems primarily from the additional degree of freedom that strained layers provide in the design of heterostructures and devices, which has led to device structures that can be tailored to a particular application with, in many cases, performances that are and out of reach with lattice-matched systems alone. With the advent of SiGe alloys, the concept of strained layers was extended to include the elemental semiconductors as well. Si/SiGe heterojunctions have provided a way to study quantum phenomena and explore heterojunction bipolar transistors (HBTs) and field-effect transistors (FETs). In optoelectronics, quantum well lasers with strained InGaAs active layers provide considerable reduced threshold current density. With coherently strained active layers based on GaAs, lasers with longevities superior to those with lattice-matched channels have been obtained.>

Recently developed modulation-doped field-effect transistors (MODFETs) now hold the record for high-speed logic. In this device structure only the larger bandgap (Al,Ga)As layer is doped with donors and the GaAs layer is left undoped. Electrons minimize their energy by diffusing out of the (Al,Ga)As into the lower potential GaAs where they form a two-dimensional electron gas near the heterointerface. Since the electrons and donors are spatially separated, ionized impurity scattering is avoided making it possible to obtain extremely high electron mobilities. Even at electron densities of 1019cm-3or ∼ 1 × 1012cm-2per interface, transport properties of these heterostructures are comparable to pure bulk GaAs. Modulation-doped FETs using this heterojunction system offer many advantages among which are a small gate to conducting channel separation (∼ 300 Å) leading to extremely high transconductances, a large current-carrying capability (∼ 500 mA/mm per interface), a small source resistance, and a small saturation voltage. The benefits improve quite substantially at 77 K where the transconductance increases to ∼ 500 mS/mm. Improved device performance has been obtained in both high-speed and digital applications. Modulation-doped FETs used as low-noise amplifiers exhibit noise figures as low as 0.4 dB with 14-dB gain at 10 GHz at 77 K and a noise temperature of 3.5 K at 3.3 GHz with the MODFET cooled to 15 K. The rate of increase in noise temperature is about 1 K/GHz. Quarter micrometer devices have exhibited a current-gain cutoff frequency of 70 GHz. When used as inverters in logic circuits, propagation delays of 12 ps and under 10 ps have been obtained at 300 and 77 K, respectively. Delay times as low as 5 ps are quite possible at 77 K. Static RAMs with 4-kbit complexity exhibited access times of about 2 ns. These results are far superior to any other three-terminal device which is the primary reason why numerous university, industrial, and government laboratories in the U.S., Europe, and Japan have sizable MODFET programs.