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Thick film growth of nonpolar a-plane GaN was carried out on r-plane sapphire by direct synthesis method for a substrate of homoepitaxy of a-plane GaN. A 30 μm thick a-plane GaN film was obtained at growth temperature at 1050 °C with a low-temperature buffer layer deposited at 700 °C. The surface was smooth but has some pits. Without a buffer layer, 20 μm thick a-plane GaN with much fewer pits was obtained. The best FWHM of XRD peak in ω-mode obtained was less than 700 arcsec. This film can be used as a substrate for a-plane III-nitrides growth.

We studied the growth technique for the dislocation reduction in a-plane GaN grown by metal organic chemical vapour deposition (MOCVD) using AlInN buffer layer, high temperature atomic layer epitaxy, and trenched r-sapphire technique. By using these techniques, the crystal quality was much improved. We also fabricated blue light emitting diodes (LEDs) on a-plane GaN film over r-sapphire substrate and on a-plane bulk GaN substrate. The electroluminescence (EL) characteristics of the LED samples were examined, and we found that the EL near field pattern from homo-epitaxially grown a-GaN based LED was spatially uniform, although the pattern from the LED on r-sapphire substrate was not uniform. The output power at the wavelength of 430nm was 0.72mW at the 20mA injection current for the sample on a-plane bulk GaN. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigated the effect of a middle temperature intermediate layer (MTIL) of AlGaN and A1N/AlGaN super lattice structure (SLS) on the crystal quality of AlGaN epilayers grown by low-pressure metalorganic chemical vapor deposition (LP-MOCVD). MTIL-Al 0-17 Ga 0.83 N layers could enhance the lateral growth area at 950 °C, and the dislocations made dislocation loop. The full-width at half-maximum from (1012) X-ray rocking curve peak of HT-Al 0.17 Ga 0.83 N epilayers grown on MTIL decreased from 1220 to 525 arcsec. The dislocation density evaluated by atomic force microscopy density decreased from 1.0 x 10 10 to 1.3 x 10 9 cm -2 , respectively. These results reveal that MTIL technique is very useful to further improve the quality of AlGaN-based films, and to fabricate the high-efficiency ultraviolet light emitting diode (UV-LED).

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.

A technique to grow low-dislocation-density AlGaN films is presented in this paper. The Al 0.17 Ga 0.83 N films using middle temperature (MT)-intermediate layer technique grown by metalorganic chemical vapor deposition on c-plane patterned sapphire were confirmed. The AlGaN films have been investigated by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy techniques. MT-intermediate layer technique can effectively reduce the dislocation density in the AlGaN films.

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)

Bulk growth by direct synthesis method was carried out. Thick GaN films with smooth surfaces were obtained on metalorganic chemical vapor deposition (MOCVD) GaN films. Surface became worse when bare sapphire was used as a substrate. A buffer layer was introduced and a GaN film with the thickness of about 70 μ m was obtained on sapphire in 1-hour growth. The crystallinity of the grown layer on sapphire with buffer is comparable to that on a MOCVD film.

p-GaN were grown on a nano-patterned n-GaN fabricated by nanoimprint lithography (NIL). Originality of this paper is to grow the p-GaN layer on a nano-patterned GaN. The regrowth mechanism of p-GaN on nano-processing n-GaN was identified. The carrier concentrations by capacitance-voltage characteristics of GaN p-n junction are measured, and the high in 1018 cm-3 is measured in the nano-processed sample, while it is in the middle of 1017 cm-3 in the un-processed one. The reason comes from the dangling bonds on the tilted planes from the c-axis. Therefore, a GaN p-n nano-structured sample is not a good idea, but the n-n or p-p nano-structure is a very promising, because it can improve the crystal quality by the nano-processing (© 2010 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.

In the present article we report on the selective fabrication of InGaN nanostructures on Si coated GaN/sapphire substrates using the focused ion beam (FIB)/metalorganic chemical vapor deposition (MOCVD) process. InGaN quantum dots and InGaN quantum wires have been fabricated. The process combines window openings in the Si mask layer with localized highly energetic Ga+ FIB irradiation, subsequent photo-assisted wet chemical etching in a solution of KOH:H2O2 (3:1 by mole) and finally the growth of InGaN/GaN nanostructures using MOCVD. This technique proved to be efficient in realizing practically damage-free etching, hence preventing the deterioration of the nanostructure’s crystal quality. The density, size, and positions of the nanostructures could be well designed and controlled using the above process. Structural characterization by transmission electron microscopy, atomic force microscopy observations, and optical investigation by cathodoluminescence were carried out.

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 investigated the relationship between the dislocation densities and the electro-luminescence (EL) peak intensities in AlGaN, which was grown on GaN- or AlGaN-on sapphire, by low-pressure metalorganic chemical vapor deposition. Crystallographic properties of the AlGaN epilayer has been investigated by a high-resolution X-ray diffraction method. We showed that EL intensity was strongly dependent on the full-width at half maximum (FWHM) from (102) AlGaN diffraction peaks for GaN- or AlGaN- based LEDs at omega mode. This result indicated the reduction of edge and mixed dislocation densities led to improvement of the EL characteristics. Although the GaN-based LEDs absorb UV-light, the EL intensity was higher than that AlGaN-based LEDs. Comparing at the same FWHM value of AlGaN (102) peaks, EL intensity of AlGaN-based LEDs were several times higher than that of GaN-based LEDs. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

The inversion domain (ID) formed in the Al 0.17 Ga 0.83 N layer grown by metalorganic chemical vapor deposition on c-plane patterned sapphire was confirmed by transmission electron microscopy. The IDs were found to be generated at the sapphire/AlGaN interface. Most of IDs vanish in the grown layer but some IDs propagate to the surface. Contriving the growth process remarkably reduced the density of IDs.

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.

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.

Recombination dynamics of excitonic transitions in GaN homoepitaxial layers has been studied by means of time-resolved luminescence spectroscopy. An excitation-density-dependent transition of the dominant radiative recombination process at 3.471 eV from donor-bound excitons to biexcitons was clearly observed in the temporal behavior. In addition, double-exponential decay of biexciton luminescence was observed, which is one of the characteristics of biexciton luminescence at high excitation densities.

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.

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.

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.

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 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.

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.

The selective overgrowth method has been used to grow bulk-like GaN layers by sublimation method. Si and SiO2, which have a different evaporation rate, thermal conductivity, and thermal expansion coefficient, have been chosen as mask materials. The effect due to the reduction of dislocation density with different mask materials has been discussed. The lateral growth rates strongly depend on the direction of the mask stripe. For the stripe windows aligned in GaN〈1100〉 direction, the lateral growth rate is approximately four times higher than with stripe direction in GaN〈1120〉. The microstructure of selectively regrown GaN has been investigated by transmission electron microscopy, scanning electron microscopy, and cathodoluminescence to understand the lateral growth mechanisms in sublimation. The threading dislocations in the region of laterally regrown GaN are extended in two different ways. First, the threading dislocations are perpendicularly propagated into the top surface in the window region. In thi...