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Gallium Oxide has attracted a great deal of attention for power electronics due to its large bandgap (∼4.8 eV) as well as availability of cost effective, large area, high quality substrates. In this article, we will discuss advancements in homoepitaxial and heteroepitaxial growth of β-(Al,Ga)₂O₃ and their heterostructures via different growth techniques with the emphasis on metal organic chemical vapor deposition, molecular beam epitaxy, and halide vapor phase epitaxy. We will also review various device structures demonstrated for high power applications.

Recent breakthroughs in bulk crystal growth of β-Ga₂O₃ by the edge-defined film-fed technique has led to the commercialization of large-area β-Ga₂O₃ substrates. Standard epitaxy approaches are being utilized to develop various thin-film β-Ga₂O₃ based devices including lateral transistors. This article will discuss the challenges for metal organic chemical vapor deposition (MOCVD) of β-Ga₂O₃ and the design criteria for use of this material system in power electronic device structures.

Evolution of morphology change was investigated for ZnO nanoneedle array grown by low-temperature MOCVD. Well-aligned ZnO nanoneedle array was deposited on the ZnO buffer-film/Si substrate at temperatures below 500°C. A rod-shaped ZnO nanowire in the initial growth stage changed into needle-shaped as the deposition proceeds. ZnO nanoneedle array deposited on the annealed buffer-film showed better alignment compared to that deposited on the as-grown film. XPS analysis showed that Zn 2p peak has a single binding energy state of a stoichiometric Zn–O bond while O 1 s peak has three different chemical binding states. Highly crystalline ZnO nanoneedle array showed a strong bandedge emission at 380 nm in photoluminescence measurements.

InGaN/GaN multiple quantum well (MQW) structures have been grown by metalorganic chemical vapor deposition on c-plane sapphire substrates. Inverted diamond-like surface defects (i.e., V-pits) with a density of $\sim 5\times 10^9$cm${}^{-2}$ have been observed in an eight-QW structure with Ga${}_{0.83}$In${}_{0.17}$N (3.6nm thick) and GaN (12.7nm thick) as the well- and barrier-material, respectively. The density of the surface defects has been significantly decreased by simply reducing the well-width to 2.7nm while keeping the other structural parameters nominally intact. The decrement in the well-width caused remarkable changes in depth profiles of In/Ga probed by secondary ion mass spectroscopy (SIMS) from the MQW structures, which is related to the varied effects of cratering roughness and accumulative interface roughness. The SIMS depth profiles also revealed increased In incorporations and QW-period in the QWs near the top surface for the MQW structure with larger well-width. They are most likely associated with surface defects, accumulative interface roughness, and strain evolutions along the growth direction.

Low dislocation density pseudomorphic epitaxial layers of Al_xGa_1-xN have been grown on c-face AlN substrates prepared from high quality bulk crystals. As reported previously, pseudomorphic growth yields very low dislocation density layers with atomically smooth surfaces throughout the active region of a full LED device structure. An advantage of the low dislocation density is the ability to n-type dope the high aluminum content Al_xGa_1-xN (x ~ 70%) epitaxial layers required for UVLED devices to obtain sheet resistances less than 350 Ohm/square for 0.5 μm thick layers. Here, we report on the characterization of our pseudomorphic epitaxial AlGaN layers via cathodoluminescence (CL) and on-wafer and initial packaged level characterization of fully fabricated pseudomorphic ultraviolet LEDs (PUVLEDs) with an emission wavelength between 250 - 265 nm. An additional benefit of PUVLED devices is the ability to run these devices at high input powers and current densities. Further, the aforementioned low dislocation density of the epitaxial structure results in improved device performance over previously published data. Mean output powers of greater than 4 mW were obtained on-wafer prior to thinning and roughening while output powers as high as 45 mW were achieved for packaged devices.

We have fabricated and investigated several types of GaN MOSFETs with normally-off operation. The recessed-gate GaN MOSFET is preferred for normally-off operation, because the threshold voltage (V_th) of the device can be easily controlled, but it suffers from relatively modest current drivability which must be improved by adopting appropriate device structure and/or process. Enhanced performances have been achieved in this work by combining the recessed-gate technology with additional processes, such as: the post-recess tetramethylammonium hydroxide (TMAH) treatment to remove the plasma damage, the post-deposition annealing of gate oxide to decrease the gate leakage current, the re-growth of n+ GaN layer for source/drain to improve the access resistance and V_th uniformity, the stress control technology to achieve extremely high 2-D electron-gas density (2DEG) on source/drain and decrease the series resistance, and the use of the p-GaN back-barrier to decrease the buffer leakage current. The GaN-based FinFET with very narrow fin was also investigated as a possible candidate for high performance normally-off GaN MOSFETs.

Mg-doped GaN films have been successfully prepared on Si(111) substrate by metal-organic chemical vapor deposition (MOCVD). Upon rapid thermal annealing (RTA) treatment, the films showed p-type conductivity with a carrier density of 7.84 cm^-3, a mobility of 5.54 cm²V^-1s^-1, and a resistivity of about 0.144 Ωcm, which were much better than that of the films without rapid thermal annealing (RTA) treatment. It was found that the surface morphology and crystal quality of the obtained p-type GaN films were greatly improved by RTA treatment, while the residual stress and dislocations in these films were decreased.

Titanium dioxide (TiO₂) thin films are prepared on glass substrates by metal-organic chemical vapor deposition (MOCVD) method using the different organic solvents, e.g. ethanol or cyclohexane, in nitrogen carrier gas with different their concentrations. We reported the effects of the each organic solvent in the carrier gas for MOCVD method on the morphology of the composed particles, the thickness, the surface roughness, and the transparency of the prepared TiO₂ thin films with changing the deposition temperature. The morphology of the particles which compose the thin films, and the surface roughness of the prepared TiO₂ thin films are observed and measured by atomic force microscope (AFM) and field emission scanning electron microscopy (FE-SEM). The film thickness is measured by fluorescent X-ray spectroscopy (XRF), and the transparency of the films is confirmed by UV-vis spectroscopy. The transparencies of the TiO₂ thin films differ depending on the MOCVD conditions, e.g. the organic solvents species, the concentration of the container in the carrier gas and the deposition temperatures. We have found that the species and the concentration of the organic solvents in the carrier gas are very important factor to prepare homogeneous TiO₂ thin films by MOCVD method; cyclohexane inhibits to aggregate of the TiO₂ nanoparticles on the glass substrate during the MOCVD process.

In this paper, Na-doped ZnO thin films were grown by MOCVD using cyclopentadienylsodium (CpNa) as the sodium source. The effects of substrate temperature and sodium source temperature on the properties of Na-doped ZnO thin films were studied. SIMS measurement verified that higher substrate temperature would result in higher Na doping content in ZnO:Na thin films. SEM and Hall measurements revealed that both surface morphology and electrical properties of ZnO:Na thin films showed nonlinear variation with substrate temperature and sodium source temperature. When the substrate temperature or the sodium source temperature was increased, the surface morphology of ZnO:Na thin films turned to sheet structures tilted in all directions first and then back to perpendicular hexagonal structure dominant morphology, and the hole concentration increased first and then decreased. This nonlinear variation trend was ascribed to be related to the variation of doping content and impurity position in ZnO:Na thin films, especially the Na impurity position.

Gallium Oxide has attracted a great deal of attention for power electronics due to its large bandgap (~ 4.8 eV) as well as availability of cost effective, large area, high quality substrates. In this article, we will discuss advancements in homoepitaxial and heteroepitaxial growth of β-(Al,Ga)₂O₃ and their heterostructures via different growth techniques with the emphasis on metal organic chemical vapor deposition, molecular beam epitaxy, and halide vapor phase epitaxy. We will also review various device structures demonstrated for high power applications.

Wide-bandgap dilute magnetic semiconductors (DMS), such as transition-metal doped ZnO and GaN, have gained attention for use in spintronic devices because of predictions and experimental reports of room temperature ferromagnetism which may enable their use in spintronic devices. However, there has been some debate over the source of ferromagnetism in these materials. This paper focuses on the high quality growth of wide bandgap DMS, and the characterization of Zn_1-xMn_xO produced by melt-growth techniques and Ga_1-xMn_xN grown by metal organic chemical vapor deposition (MOCVD). High resolution X-ray diffraction results revealed no second phases in either the ZnO crystals or the GaN films. Undoped as-grown, bulk crystals of Zn_1-xMn_xO and Zn_1-xCo_xO crystals are shown to be paramagnetic at all temperatures. In contrast, the Ga_1-xMn_xN films showed ferromagnetic behavior at room temperature under optimum growth conditions. Experimental identification of the Mn ion charge state and the presence of bands in the bandgap of GaN are investigated by optical spectroscopy and electron spin paramagnetic resonance (EPR). It is shown that the broadening of states in the Mn 3d shell scaled with Mn concentration, and that optical transitions due to this band correlated with the strong ferromagnetism in these samples. However, this band disappeared with an increase in free electron concentration provided by either annealing or doping. Raman studies of Ga_1-xMn_xN revealed two predominant Mn-related modes featured with increasing concentration, a broad disorder related structure at 300cm^-1 and a sharper peak at 669cm^-1. This works show that the development of practical ferromagnetic wide bandgap DMS materials for spintronic applications will require both the lattice site introduction of Mn as well as careful control of the background defect concentration to optimize these materials.