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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.

We investigated the structural and optical properties of AlInGaN/AlGaN quantum wells (QWs) and the quaternary AlInGaN epilayers as a function of V/III flux ratio (flow rate of NH3). From the photoluminescence and cathodoluminescence (CL) measurements, the emission wavelengths of the AlInGaN/AlGaN QWs linearly became blueshifted and the intensities decreased with increasing the flow rate of NH3. In the X-ray rocking curve, a pattern of the sample grown at lower NH3 flow rate gives an indication of the good crystalline quality of the films. The monochromated scanning CL image of the AlInGaN epilayers measured at around 80 K showed that more densely and uniformly distributed bright spot densities, indicating In segregation regions, increased at lower NH3 flow rate. Therefore, these results suggest that the relatively decrease of NH3 flow rate results in the increase of In composition fluctuations into the AlInGaN alloys, which generate exciton localized states, and thereby enhances the emission. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

For the first time, this paper reports a novel kind of three-flow reactor which can be developed for metal-organic chemical vapor deposition (MOCVD) system to grow GaN-based structures. Two parallel carrier gas-flow for Group-III sources and NH3 separated by a quartz plate which are also parallel to the substrate are used. The N2, which is used as a sub-flow, flows through a quartz filter perpendicular to the substrate where it meets the two parallel group III and group V source flows. This new flow arrangement enhances the mixture of group III–V sources at the contacting substrate, compared with the previous three parallel laminar gas MOCVD systems due to the perpendicular direction of the N2 sub-flow. Based on this MOCVD system with this special configuration, a mirror-like epitaxial GaN layer on sapphire substrate is obtained with an excellent uniformity. Also, the quality of the GaN layers are characterized by room temperature photoluminescence (PL) and X-ray diffraction (XRD) measurements, where the full-width at half-maximum (FWHM) of 41.12 meV for PL spectra and FWHM of about 220 arcsec for XRD rocking curve were measured, respectively. In addition, the carrier concentration and Hall mobility are found to be 2.6×1017/cm3 and 300 cm2/V s, respectively.

AlGaN growth using epitaxial lateral overgrowth (ELO) by metalorganic chemical vapor deposition on striped Ti, evaporated GaN on sapphire, has been investigated. AlGaN/AlN films growth on GaN/AlGaN superlattices (SLs) structure on the Ti masks, with various SLs growth temperature (1030, 1060 and 1090 °C) were grown. With increasing the growth temperature, AlGaN surface became flat. The AlGaN film had a cathodoluminescence peak around 345 nm. However, in secondary ion mass spectrometry (SIMS) measurement, Ti signal was detected on the top of AlGaN surface when GaN/AlGaN SLs was grown on Ti striped masks. By inserting the AlN blocking layer on SLs, Ti diffusion was stopped at the AlN layer, and the AlGaN crystalline quality was improved.

Lateral epitaxial overgrowth (LEO) of GaN by sublimation has been carried out to understand the dependence of growth conditions on the morphology and quality of the layers. Growth conditions have been optimised to have flat and smooth lateral overgrown surfaces with (1 1 2 0) stripe window opening. The changing feature from triangular stripe to rectangular stripe growth with change in growth temperature have been observed using scanning electron microscopy (SEM). Atomic force microscopy (AFM) result shows the dominance of step growth mechanism leading to two-dimensional growth in the lateral overgrown GaN layers.

Excitonic optical properties of homoepitaxial GaN layers have been studied by means of magneto-luminescence spectroscopy. Under magnetic field perpendicular to the hexagonal c-axis, Zeeman splittings and diamagnetic shifts were observed for the luminescence lines due to the radiative recombination of excitons bound to neutral donors and acceptors. The effective g-factors for both the donor- and acceptor-bound excitons were estimated to be 2.02 and 2.47, respectively. The diamagnetic shift of the donor-bound exciton was found to be smaller than that of the acceptor-bound exciton.

This paper reports for the first time the successful growth of a single GaN layer on an amorphous glass substrate using metal organic chemical vapor deposition (MOCVD) technique. The layers were characterized by room temperature photoluminescence and X-ray diffraction measurements. In addition, the lateral overgrowth of GaN on this substrate was also tried, which could provide an effective method to obtain high-quality GaN substrates in the near future. In comparison to the overgrowth on sapphire substrate, the lateral overgrowth on our substrate is independent of the mask stripe direction, which could indicate a different growth mechanism.

Bulk single crystals and selectively grown gallium nitride (GaN) have been obtained using the sublimation technique. Crystals of size about 2–3 mm in length and 0.8–1.0 mm in width have been grown successfully. Atomic force microscopy has been employed to analyze the surface morphology of the as-grown samples to understand the growth mechanism. In free standing bulk GaN single crystals, two-dimensional growth is dominated by step growth mechanism. However, in the selective growth of GaN by sublimation, spiral growth originating from screw dislocation dominates.

We have characterized a bulk GaN grown by sublimation technique by means of the high resolution X-ray diffraction measurements, the secondary ion mass spectrometry measurements, the Raman spectroscopy measurements and the resistivity measurements. The results indicated that a bulk GaN was of relatively high purity with a very high crystal perfection.

Transmission and reflection measurements in the frequency range (700–2000 cm−1) have been carried out on high-quality bulk GaN grown by the sublimation technique. The transmission spectra are polarization dependent and show combination bands arising from anharmonic forces. The combination absorption is much stronger for E∥c polarization than that for E⊥c polarization. A high value of 350 cm−1 for the absorption coefficient in the combination band can be explained by the strong anharmonic forces in GaN.

A GaN film grown on sapphire substrate by metalorganic chemical vapor deposition have been investigated by the plan-view TEM and CL. Direct evidence of dislocation being a non-radiative recombination center, have been provided. A bulk GaN grown by the sublimation method and a homoepitaxial GaN grown by hydride vapor phase epitaxy have also been investigated by TEM, the x-ray diffraction and PL. The results confirm that it is not the dislocations but the point defects that are responsible for the yellow luminescence of the grown GaN. It was found, in a cross- section TEM image of a GaN/Al 2 O 3 film, that there are many precipitates gathered around a mixed dislocation. The precipitates might be formed by the segregation of point defects around dislocations. The precipitates might be formed by the segregation of point defects around dislocations. Since most of the point defects in GaN seems to segregate around the dislocations, dislocations can reduce the local concentration of the point defects in the no dislocation region. In this case, the optical property of GaN might be improved by the existence of the dislocation.

The growth of bulk GaN by sublimation method and a homoepitaxial growth by MOCVD are reported. A photo-pumped stimulated emission is obtained from a homoepitaxial layer. The source powder used as a source in the sublimation method is investigated in detail, and it is shown that the powder contains many kinds of compounds consisting mainly of gallium, nitrogen and hydrogen. Growth nucleation control is performed by partly covering an MOCVD-GaN or a scratched-sapphire (0001) by Si02. Hexagonal columns of the size of about 200 μm in diameter and about 200 μm in height are selectively and uniformly grown at the window sites. This technique enables the device processing of crystallites and also helps to increase crystal size by increasing growth time, since it prevents the nucleation of new crystallites which work as sink of the growing species.

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 of GaN has been carried out in two different plasma chemistries using 10Cl2/10Ar and 1CH4/9Cl2/10Ar at various applied radio frequency (rf) power. We have addressed the postetch surface characteristics of GaN, and the effect of CH4 addition to chlorine plasma and the variation of rf power are discussed. Surface contamination, roughness, and stoichiometry have been determined from the results of secondary ion mass spectrometry. It is observed that in the case of plasma with 5% CH4, above 125 W, contamination of carbon is minimal and roughness is similar to that of the as-grown sample. At rf power less than 125 W, the Ga/N ratio of the etched surface is almost unity, whereas at rf power more than 125 W, it is more than unity. When the GaN surface is etched with only chlorine plasma, the surface is Ga excess at all rf power applied. The intensity of the main band-to-band photoluminescence peak is found to decrease when 10Cl2/10Ar is used, and the intensity was a minimum at 200 W. When 5%...

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.

The photoluminescence (PL) spectra of 3C-SiC epilayers grown on 15R-SiC and on 3C-SiC were measured. The PL spectra show strong exciton-related peaks and weak impurity-related peaks. The epilayers are of high quality and have a low density of impurities. Peaks due to excitons bound to neutral nitrogen showed very little shift induced by strain. Peaks considered to be due to free exciton recombination were observed and analyzed. Defect-related bands, usually observed for 3C-SiC grown on Si, were not observed. Although weak defect-related peaks were still observed, the epilayers have a lower density of defects than those on Si.

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.

Nucleation of GaN crystallites by sublimation method on sapphire (0001), scratched sapphire (0001), SiO2, Si (111), and metalorganic chemical vapor deposition(MOCVD)-GaN/sapphire (0001) was investigated. The density of the nuclei on SiO2 and Si (111) was estimated to be 6 × 103/cm2. On the other hand, a continuous film rather than discrete crystallites was obtained on both the MOCVD-GaN and the scratched sapphire. Growth nucleation control was performed by partly covering the MOCVD-GaN or scratched sapphire (0001) with SiO2. As a result, hexagonal columns about 200 µm in diameter and 200 µm high were selectively and uniformly grown at the window sites. The diameter of each hexagonal column was much larger than the width of each window. This method can be used for device processing utilizing the crystallites.