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The reduction of the dislocation density in a high-temperature (HT)-Al0.17Ga0.83N epitaxial layer was achieved by a middle-temperature (MT)-intermediate layer technique, in which the HT and the MT were 1050 and 950 °C, respectively. For one set of the MT-intermediate layer, the structure was 4.5-µm-thick HT-Al0.17Ga0.83N/1-µm-thick MT-intermediate layer/100-nm-thick HT-Al0.17Ga0.83N layer/low-temperature (LT)-GaNP buffer/trenched (0001) sapphire. The full-width at half maximum values of (0002) and (1012) diffraction peaks of the X-ray diffraction for the Al0.17Ga0.83N epitaxial layer using one set of the MT-intermediate layer were improved to 359 and 486 arcsec compared with 401 and 977 arcsec for the film without the MT-intermediate layer technique, respectively. Transmission electron microscopy result showed that the dislocation density in the Al0.17Ga0.83N film using one set of MT-intermediate layer was reduced from (1–4)×1010 to 1.7×109 cm-2. The Al0.17Ga0.83N epitaxial film including two sets of MT-intermediate layers improved the most, showing a dislocation density of 9.3×108 cm-2.

With the advantages of low cost, simplicity, and scalability, a direct reaction using gallium (Ga) and ammonia (NH3) has been employed to deposit GaN film on a (0001) sapphire substrate. We find that only GaN crystallites with diameters in the range of 0.4–5 µm are sparsely deposited on a bare (0001) sapphire substrate. To increase the density of the GaN nucleation site and thus form continuous GaN film, a 50-nm-thick AlN layer [grown on the (0001) sapphire substrate by metalorganic chemical vapor deposition (MOCVD) at 1150 °C in a pulsed manner] is used as a template layer. Plan-view scanning electron microscopy (SEM) and cross-sectional SEM experimental results show that the GaN film has a specular smooth surface and that the film thickness is about 10 µm after 1 h deposition. X-ray diffraction (XRD) and cathodoluminescence (CL) experimental results indicate that the formed GaN film is of single crystals with a hexagonal structure. The full widths at half-maximum (FWHM) of the (002) and (102) X-ray rocking curves are 420 and 556 arcsec, respectively. Our experimental results demonstrate that the growth of high-quality and thick GaN film via a direct reaction is possible.

BP with the size of 50 µm to 3 mm was synthesized by the Cu flux method. The BP crystals have a zincblend structure, and the lattice constant and the cathodoluminescence peak wavelength were 4.557 A and 370 nm, respectively. GaN and AlN were grown by metalorganic chemical vapor deposition on BP. It was found that AlN grown at 1150 °C on (100)BP was grown smoothly but that grown on (111)BP had a rough surface. GaN, however, was irregularly grown on both (100) and (111)BP. It was demonstrated that AlN on (100)BP is another candidate as a substrate for a UV-light-emitting diode.

AlxGa1-xN (x ≤0.1) epilayers grown on (0001) sapphire substrates by low-pressure metalorganic chemical vapor deposition with low-temperature (LT)-GaNP buffer have been characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is found that the full width at half maximum (FWHM) values of the GaNP-buffer-based AlxGa1-xN (x ≤0.1) epilayers decrease are lower than that of the conventional GaN-buffer-based AlxGa1-xN (x ≤0.1) epilayers, but the difference becomes smaller as the Al composition increases. These results could be mainly attributed to a longer diffusion length with the GaNP buffer layer than with the GaN buffer layer.

AlN was grown by a sublimation method on 6H-SiC. We found the grown AlN layer is easily separated from the substrate when Si powder is added to the AlN source powder. The formation of AlSiN layer with the Si content of 15% at the AlN/6H-SiC interface was confirmed by energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). This AlSiN layer causes the separation of AlN.

We report a new method of increasing the output power of an ultraviolet light-emitting diode (UV-LED) based on an AlGaN/GaN single quantum well (SQW). In this method, a thin Ga droplet layer is intentionally grown before the growth of an AlGaN/GaN SQW active layer. The Ga droplet layer causes a spatial and compositional fluctuation on the SQW active layer, which induces exciton localization in the potential minima. The LEDs fabricated with the Ga droplet layer show an emission peak of 353 nm and a higher optical output power than those of the same structure but without the Ga droplet layer.

The effect of oxygen mixed in nitrogen on p-type activation in Mg-doped GaN epilayers grown by metalorganic chemical vapor deposition (MOCVD) was investigated. The samples annealed in N2/O2 (1%) ambient exhibited the best electrical properties with respect to hole concentration. SIMS data suggested that oxygen reacted with hydrogen present in the Mg-doped GaN samples during the thermal annealing process, thereby enhancing the activation of Mg acceptors.

Reactive ion etching (RIE) of p-GaN and p-Al x Ga1-x N has been investigated using Cl2/CH4/Ar plasmas in the conventional RIE technique. It has been found that variation of CH4 percentage in gas mixtures leads to changes in the etch rate of both p-GaN and p-Al0.15Ga0.85N. The maximum etch rate for p-GaN has been found to be 257 nm/min in Cl2/Ar plasma containing 2.5% CH4 and the value is 170 nm/min for p-Al0.15Ga0.85N in Cl2/Ar plasma containing 10% CH4. With increasing rf power, the etch rates for p-GaN and p-Al0.15Ga0.85N reach as high as 433 nm/min and 255 nm/min respectively. Under optimum conditions of gas composition, radiofrequency (rf) power, and temperature, an anisotropic and smooth etch profile is obtained. The etched surface exhibits small roughness.

Reactive ion etching (RIE) of GaN and Al x Ga1-x N has been investigated using Cl2, CH4 and Ar gas mixtures. It has been found that a variation in the CH4 percentage in the gas mixture leads to a change in the etch rate of both GaN and Al0.85Ga0.15N with a etch selectivity of 4.2:1 for GaN over Al x Ga1-x N for an optimum value of 2.5% of CH4. This gas composition also yields an anisotropic and smooth etching profile. An etch rate of 257 nm/min of GaN has been obtained which is the highest value ever reported with the RIE technique.

Plan-view transmission electron microscopy (TEM) and cathodoluminescence (CL) images were taken for the same sample at exactly the same location in n-type GaN grown on sapphire substrate by metalorganic chemical vapor deposition (MOCVD). There was a clear one to one correspondence between the dark spots observed in CL images and the dislocations in TEM foils, indicating that the dislocations are non-radiative recombination centers. The hole diffusion length in n-type GaN was estimated to be neighboring 50 nm by comparing the diameters of the dark spots in thick samples used for CL and samples that were thinned for TEM observation. The efficiency of light emission is high as long as the minority carrier diffusion length is shorter than the dislocation spacing.

Homoepitaxial growth of GaN on bulk GaN prepared by the sublimation method was performed by metalorganic chemical vapor deposition (MOCVD). Two kinds of bulk substrates were used for homoepitaxial growth: (i) a polished bulk GaN C-face, selectively grown on MOCVD-GaN on a sapphire (0001) substrate which was partly covered by SiO2 (“quasi-bulk” GaN) and (ii) a bulk GaN spontaneously nucleated on the source powder used in the sublimation method (“free-standing” bulk GaN). A smooth surface and strong CL band-edge emission from the homoepitaxial layer were obtained when proper surface pretreatment of the substrate, such as H3PO4 etching, RIE and photoassisted wet chemical etching (PAW), was adapted before loading into the MOCVD reactor. The CL intensity of the band-edge emission was stronger and the CL image more uniform for homoepitaxial GaN compared to that of heteroepitaxial GaN on sapphire. The dislocation density of the homoepitaxial layer was also decreased after proper surface treatment of the substrate.

Crystal growth of cubic silicon carbide (3C-SiC) on α-SiC (6H- and 15R-SiC) substrates was carried out by chemical vapor deposition. 3C-SiC (111) can be epitaxially grown on 6H- and 15R-SiC (0001) substrates. Due to the peculiar stacking sequence of α-SiC, double positioning boundaries (DPBs) appear in the 3C-SiC (111) layers. The layer on 15R-SiC has far fewer DPBs than that on 6H-SiC. Successive etching of a thick grown layer and successive observation of a growing surface revealed that the DPBs decreased anisotropically as crystal growth proceeded. Facets formed along DPBs were analyzed by atomic force microscopy. The angles between the facets and the grown surface (111) varied with the crystallographic orientation of DPBs. DPBs may decrease due to the lateral growth from the facets. The difference in the velocities of the anisotropic decrease in DPBs was discussed on the basis of the number of dangling bonds on the facets.

A GaN film with nominal thickness of 0.6 µ m was homoepitaxially deposited on a sublimation-grown GaN single-crystal substrate. An electron-transparent area of 10 µ m×10 µ m prepared by focused Ga ion beam machining was examined by transmission electron microscopy. In addition to the dislocations on the basal plane of the wurtzite unit cell, another type of dislocation, near pure edge in character and aligned close to the c-axis direction, was found in the GaN substrate crystal. Neither type of dislocation threaded to the top surface. Dislocation-free growth was achieved in the GaN homoepitaxial film.