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Herein, aluminum nitride (AlN) powder is thermally treated via a thermocycle process for use in PVT growth.

In this study, a numerical simulation of edge-defined film-fed growth (EFG) was performed to determine the appropriate capillary conditions for Ga2O3 melt. Meniscus and capillary rise were significantly influenced by the design of the die in the EFG system. The ratio of the seed crystal and die width was >0.73 for a die width of 4.4 mm. Narrower slit width resulted in higher capillary rise with longer process time compared with wider slit width. Under conditions consistent with the simulation results, highly crystalline (100) β-Ga2O3 single crystals were successfully achieved.

Epitaxial growth using graphene (GR), weakly bonded by van der Waals force, is a subject of interest for fabricating technologically important semiconductor membranes. Such membranes can potentially offer effective cooling and dimensional scale-down for high voltage power devices and deep ultraviolet optoelectronics at a fraction of the bulk-device cost. Here, we report on a large-area β-Ga2O3 nanomembrane spontaneous-exfoliation (1 cm × 1 cm) from layers of compressive-strained epitaxial graphene (EG) grown on SiC, and demonstrated high-responsivity flexible solar-blind photodetectors. The EG was favorably influenced by lattice arrangement of SiC, and thus enabled β-Ga2O3 direct-epitaxy on the EG. The β-Ga2O3 layer was spontaneously exfoliated at the interface of GR owing to its low interfacial toughness by controlling the energy release rate through electroplated Ni layers. The use of GR templates contributes to the seamless exfoliation of the nanomembranes, and the technique is relevant to eventual nanomembrane-based integrated device technology.

Single-crystal gallium nitride (GaN) thin films were grown using a graphene mask via multiple epitaxial lateral overgrowth (multiple-ELOG). During the growth process, the graphene mask self-decomposed to enable the emergence of a GaN film with a thickness of several hundred nanometres. This is in contrast to selective area growth of GaN using an SiO2 mask leading to the well known hexagonal-pyramid shape under the same growth conditions. The multiple-ELOG GaN had a single-crystalline wurtzite structure corresponding to the crystallinity of the GaN template, which was confirmed with electron backscatter diffraction measurements. An X-ray diffraction rocking curve of the asymmetric 102 reflection showed that the FWHM for the multiple-ELOG GaN decreased to 405 from 540′′ for the underlying GaN template. From these results, the self-decomposition of the graphene mask during ELOG was experimentally proven to be affected by the GaN decomposition rather than the high-temperature/H2 growth conditions.

This study examined the microstructural gradation in Sn-doped, n-type Ga2O3 epitaxial layers grown on a two-inch sapphire substrate using horizontal hot-wall mist chemical vapor deposition (mist CVD). The results revealed that, compared to a single Ga2O3 layer grown using a conventional single-step growth, the double Ga2O3 layers grown using a two-step growth process exhibited excellent thickness uniformity, surface roughness, and crystal quality. In addition, the spatial gradient of carrier concentration in the upper layer of the double layers was significantly affected by the mist flow velocity at the surface, regardless of the dopant concentration distribution of the underlying layer. Furthermore, the electrical properties of the single Ga2O3 layer could be attributed to various scattering mechanisms, whereas the carrier mobility of the double Ga2O3 layers could be attributed to Coulomb scattering owing to the heavily doped condition. It strongly suggests the two-step-grown, lightly-Sn-doped Ga2O3 layer is feasible for high power electronic devices.

Two-step growth of κ-Ga2O3 thin films on 4H-SiC substrates was attempted with temperature-varied buffer layers via mist chemical vapor deposition. The first-step Ga2O3 buffer layers affect the phase formation and grain size variation depending on growth temperatures. In the second-step thin-film growth, the κ-Ga2O3 thin film was grown at a fixed temperature of 500 °C regardless of various buffer layers. Three zones, namely, amorphous, κ phase, and mixed phase, were categorized in the κ-Ga2O3 thin films according to the buffer growth temperature. High-quality and smooth κ-Ga2O3 thin films could be achieved through the grain growth competition and slight buffer temperature variation in the two-step growth.

Compression glass molding is a promising technique for mass production of near net-shaped, high-precision, and low-cost optical glass elements. However, the glass molding process causes the damage of glass and the mold during demolding because of the chemical or physical adhesion of the glass to the mold. To overcome this limitation, graphite molds are used owing to their good lubrication and easy machining. However, graphite materials show rapid oxidation at high temperatures in the presence of oxygen. Therefore, in this study, a thin SiC coating layer was deposited on the graphite mold using the chemical vapor deposition (CVD) method to utilize the lubrication properties of graphite and the anti-oxidation properties of SiC. The specimen obtained using the low-temperature CVD method showed high carbon content and good lubrication properties than that obtained using the high-temperature CVD method.

The effectiveness of rear-flow-controlled mist chemical vapor deposition (mist CVD) for the growth of 2-inch α-Ga2O3 epilayers was studied. The numerical simulation indicated that the low velocity of the flow is appropriate for inducing an upward flow on the growth front without a vortex. Under the flow velocity of 0.08 m/s, α-Ga2O3 the epilayers were successfully grown on c-plane sapphire substrates. The epilayers were high-quality with full widths at half maximum of 42 arcsec and 1993 arcsec for the (0006) and (104) plane reflections, respectively. The rear-flow-controlled mist CVD was demonstrated to be effective for long-time growth. The thickness was adequately increased with increasing growth time. At the same time, corundum α-phase crystal features were distinguished. The suggested mist CVD system not only provides a cost-saving solution for Ga2O3 epilayers' growth but is also effective for retaining the uniformity of the Ga2O3 epilayers over a large area.

Silicon carbide (SiC) is an ideal material for high-power and high-performance electronic applications. Top-seeded solution growth (TSSG) is considered as a potential method for bulk growth of high-quality SiC single crystals from the liquid phase source material. The crystal growth performance, such as growth rate and uniformity, is driven by the fluid flow and constitutional flux in the solution. In this study, we numerically investigate the contribution of the external static magnetic field generated by Helmholtz coils to the fluid flow in the silicon melt. Depending on the setup of the Helmholtz coils, four static magnetic field distributions are available, namely, uniform vertical upward/downward and vertical/horizontal cusp. Based on the calculated carbon flux coming to the crystal surface, the vertical downward magnetic field proved its ability to enhance the growth rate as well as the uniformity of the grown crystal.

We report on the thorough investigation of light emitting diodes (LEDs) made of core-shell nanorods (NRs) with InGaN/GaN quantum wells (QWs) in the outer shell, which are grown on patterned substrates by metal-organic vapor phase epitaxy. The multi-bands emission of the LEDs covers nearly the whole visible region, including UV, blue, green, and orange ranges. The intensity of each emission is strongly dependent on the current density, however the LEDs demonstrate a rather low color saturation. Based on transmission electron microscopy data and comparing them with electroluminescence and photoluminescence spectra measured at different excitation powers and temperatures, we could identify the spatial origination of each of the emission bands. We show that their wavelengths and intensities are governed by different thicknesses of the QWs grown on different crystal facets of the NRs as well as corresponding polarization-induced electric fields. Also the InGaN incorporation strongly varies along the NRs, increasing at their tips and corners, which provides the red shift of emission. With increasing the current, the different QW regions are activated successively from the NR tips to the side-walls, resulting in different LED colors. Our findings can be used as a guideline to design effectively emitting multi-color NR-LEDs.

Hexagonal boron nitride (h-BN) was directly grown on sapphire substrate using alternating ammonia (NH 3 ) and triethylboron (TEB) supply (pulsed mode) in metalorganic vapor phase epitaxy. The best condition is when just enough NH 3 is supplied to fully convert the TEB within one cycle. Excess NH 3 caused islands on h-BN film surface while a lack of NH 3 does not form h-BN at all. The epitaxial relationship between grown h-BN layer and c -plane sapphire was confirmed to be [0001] h-BN ∥[0001] sapphire and [10-10] h-BN ∥[11-20] sapphire . It is known that, compared to AlN, BN requires higher V/III ratios for good crystallinity, which due to severe gas-phase reactions is difficult to achieve using continuous supply. Thus using pulsed mode the FWHM of the symmetric (0002) diffraction was almost halved and the growth rate was several times faster.

We have investigated the effect of V/III ratio on the surface morphology, impurity concentration and electrical properties of m –plane ( 10 1 ¯ 0 ) Gallium Nitride (GaN) homoepitaxial layers. Four-sided pyramidal hillocks are observed on the nominally on-axis m –plane GaN films. Hillocks sizes relatively increase by increasing the V/III ratio. All facets of pyramidal hillocks exhibit well-defined step-terrace features. Secondary ion mass spectrometry depth profiles reveal that carbon impurities decrease by increasing the V/III ratio while the lowest oxygen content is found at an optimized V/III ratio of 900. Vertical Schottky barrier diodes fabricated on the m –GaN samples were characterized. Low leakage current densities of the order of 10 −10 A/cm 2 at −5 V are obtained at the optimum V/III ratio. Oxygen impurities and screw-component dislocations around hillocks are found to have more detrimental impact on the leakage current mechanism.

We successfully grew semipolar ( 10 1 3 ) and ( 10 1 5 ) GaN films on Si(001) substrates employing metal-organic chemical vapor deposition (MOCVD) by inserting a directionally sputtered AlN (DS-AlN) buffer layer. To improve the crystal quality of the orientation-controlled semipolar ( 10 1 3 ) and ( 10 1 5 ) GaN films, a two-step epitaxial lateral overgrowth (ELO) process was performed with a striped mask. According to low-temperature cathodoluminescence (LT-CL) characterization, the ELO results in a coalesced morphology and a low defect density of 8 cm −2 for both semipolar ( 10 1 3 ) and ( 10 1 5 ) GaN films. For comparing the properties of planar and ELO semipolar GaN, a rocking curve of x-ray diffraction (XRD) and low-temperature photoluminescence (LT-PL) spectra was measured. The crystal orientation of semipolar GaN films was confirmed using electron backscatter diffraction (EBSD).

Severe melt-back etching has forced the epitaxy of GaN on Si to use an AlN buffer layer for growing high-quality two-dimensional layers, despite its high resistivity. Herein, we report a metal-based pre-orienting layer (POL) for growing GaN nanostructures (NSs) to replace the traditional AlN buffer layer. Two metals, titanium (Ti) and hafnium (Hf), were evaluated as POLs. We succeeded in fabricating arrays of GaN NSs with highly preferred orientation using selective-area growth. The crystallographic phase of the POLs critically affected the evolved orientation of the crystals. Photoluminescence measurements revealed that GaN NSs with Hf-based POLs were of reasonably high quality. We believe that this result will facilitate broader III-V semiconductor applications using alternative substrates moving beyond conventional Si-based optoelectronics.

In this study, multiphysics simulations were carried out to understand the convection mechanisms of the top seeded solution growth (TSSG) of SiC. Experimental melting tests and crystal growth were conducted to verify the simulation results in the growing temperatures between 1700 and 1900 °C with rf induction heating furnace. From the solidified melt of Si–Cr solution after the melting test, the melt flow in the simulation was successfully verified. In the given experimental conditions, the electromagnetic convection was found to govern the global fluid flow, while other mechanisms including the Marangoni convection, the buoyancy convection and the centrifugal forced convection influence the fluid flow near the crystal. Based on an understanding of the fluid flow obtained with the simulations, a structural flow modifier (FM) was applied to enhance the growth rate of the SiC crystal. The growth rates of SiC with/without FM were successfully estimated from simulations showing good agreements with the experimental values. After the experimental crystal growth using FM, a remarkable enhancement in the growth rate was found in an FM configuration, which suggests a way to improve the growth rate by the TSSG method based on the efficient use of the dissolved C in the melt.

To extend the availability of nanostructure-based optoelectronic applications, vertically elongated nanorods with precisely controlled morphology are required. For group III nitrides, pulsed-mode growth has recently been reported as an effective method for growing nanorod arrays with geometric precision. Here, we demonstrated the growth of arrays of highly elongated nanorods on Si substrates by metal–organic chemical vapor deposition using a pulsed-mode approach. Unlike the thick and high (or middle)-quality GaN templates normally used, nanorod growth was performed on an ultrathin and low-quality AlN/Si platform. Using kinetically controlled growth conditions and a patterning process, exceptionally long GaN nanorods were achieved with high geometric precision. The grown nanorods showed considerably improved optical and structural properties while remaining in uniform arrays. This approach can be used with a variety of materials to obtain nanorods with high quality, high uniformity, and high aspect ratio, and it can also serve as an effective fabrication method for InAlGaN-alloyed core/shell nanostructures for optoelectronic nanodevices with ultrahigh efficiency.

Transient photoluminescence (PL) characteristics and localization phenomena in InGaN/GaN core-shell nanorods (NRs) were investigated from 6 K up to 285 K. The NRs exhibit three well-defined PL bands in the near-UV, blue, and green range ascribed to the emission of quantum well (QW) areas situated at the (1.00) sidewalls, (10.1) top facets, and (00.1) tip, respectively. At low temperature, time-resolved PL shows a fast decay time of about 0.5 ns for the semi- and non-polar QWs, while the polar QWs exhibit at least a twice-longer time. Rapid delocalization of carriers above 50 K indicates shallow potential fluctuations in the QWs. At room temperature, the characteristic fast PL decay time of the three QW bands stabilizes around 300 ps. The slow decaying PL components have different characteristic decay times that are explained by additional localization at basal stacking faults (BSFs), taking into account the quantum confined Stark effect. In addition, narrow excitonic luminescence lines are observed in the BSF-enriched polar QWs, providing direct evidence of the impact of the BSF/QW crossings on the optical properties of the NRs. A PL rise time of about 100 ps does not show any deviation between bands. These findings are suggestive of similar transport mechanisms in temperature equilibrium without inter-facet transport between different QWs. We believe that predictable transient characteristics can play a key role in creating uniform NR ensembles for device applications.

The growth of GaN 3-D microstructures is investigated by SAG-HVPE. Capitalizing on the properties of this kinetically-controlled process, the main experimental parameters and physical mechanisms that control the shaping of 3D GaN prisms and pyramids in SAG-HVPE are highlighted. Growth experiments performed on N-polar AlN/Si(100) and Ga-polar GaN/Si(111) substrates also provide insight into how to switch from a pyramid to a prismatic shape for a given substrate polarity. The aspect ratio of GaN rods could be tuned by playing with the HCl partial pressure additionally introduced during growth. The influence of both mass transport and surface kinetics is discussed, as the crystal growth rate varies with increasing surface area as time goes by. Ammonia treatment prior to the growth, aimed at blocking the r planes thanks to H2 passivation, is proposed to tune the morphology of the GaN rods. Raman spectroscopy performed on individual GaN rods shows no relevant strain field and no structural differences between the rods and state-of-the-art bulk GaN.

Highly ordered, position-controlled gallium nitride (GaN) nanowire based multiple-quantum-wells (MQWs) core–shell architecture arrays are synthesized by metalorganic chemical vapor deposition (MOCVD). We investigate the possibility of using GaN nanowire arrays as a basal template for the growth of In x Ga 1− x N/GaN MQWs. The MQWs on three different crystal facets ( c -, m -, and semipolar -plane) of GaN nanowire exhibit dissimilar structural properties. The structural characteristics of InGaN/GaN core–shell arrays are inspected by cross-sectional high-resolution transmission electron microscopy (HR-TEM). We also investigate the optical properties of MQW core–shell structure nanoarrays. The luminescent characteristics of InGaN/GaN core–shell structure arrays are determined by photoluminescence (PL) and cathodoluminescence (CL) measurements. The monochromatic CL images clearly show the light emission behavior of InGaN/GaN MQW coaxial structure. Two distinguishable light emission peaks were observed in the GaN nanowire based core–shell structure. The characteristic of light emission mainly depends on the properties of MQWs, which are generated from different crystal facets of GaN. In addition, the light emission intensity shows different behaviors depending on the area of the GaN nanowire m -plane. The results of this study suggest that GaN nanowire arrays can be used as a good alternative basal template for next-generation light-emitting diodes (LEDs).

We report the fabrication of near-vertically elongated GaN nanorods on quartz substrates. To control the preferred orientation and length of individual GaN nanorods, we combined molecular beam epitaxy (MBE) with pulsed-mode metal–organic chemical vapor deposition (MOCVD). The MBE-grown buffer layer was composed of GaN nanograins exhibiting an ordered surface and preferred orientation along the surface normal direction. Position-controlled growth of the GaN nanorods was achieved by selective-area growth using MOCVD. Simultaneously, the GaN nanorods were elongated by the pulsed-mode growth. The microstructural and optical properties of both GaN nanorods and InGaN/GaN core–shell nanorods were then investigated. The nanorods were highly crystalline and the core–shell structures exhibited optical emission properties, indicating the feasibility of fabricating III-nitride nano-optoelectronic devices on amorphous substrates.

Thick InGaN alloys with high In content are essential for emerging InGaN photovoltaic applications. By applying a compositionally graded structure, various thicknesses of InGaN layers were grown using metal organic chemical vapor deposition. We could obtain pseudomorphic and highly strained layer even upto 100 nm of InGaN film as confirmed by X-ray reciprocal space mapping and Raman spectra. To probe the validity of those InGaN layers for solar cell applications, optical transmittance measurements were carried out and absorption properties were compared.

In this paper, we demonstrate a scalable process for the precise position-controlled selective growth of GaN nanowire arrays by metalorganic chemical vapor deposition (MOCVD) using a pulsed-mode growth technique. The location, orientation, length, and diameter of each GaN nanowire are controlled via pulsed-mode growth parameters such as growth temperature and precursor injection and interruption durations. The diameter and length of each GaN nanowire are in the ranges of more than 240 nm and 250–1250 nm, respectively, with different vertical-to-lateral aspect ratios that depend on the growth temperature. Also, it is found that a higher growth temperature helps increase the vertical growth rate and reduces the lateral growth rate of GaN nanowire arrays. Furthermore, in the case of longer TMGa injection duration, the Ga-rich region allows the higher lateral growth rate of GaN nanostructures, which leads to a transition in the morphology from nanowires to a thin film, while in the case of longer NH3 injection duration, the surface morphology changes from nanowires to pyramidal structures. In addition, the surface structure can also be controlled by varying the precursor interruption duration. Finally, we report and discuss a growth model for GaN nanowire arrays under pulsed-mode MOCVD growth.

We report the selective-area growth of a gallium nitride (GaN)-nanorod-based InGaN/GaN multiple-quantum-well (MQW) core-shell structure embedded in a three-dimensional (3D) light-emitting diode (LED) grown by metalorganic chemical vapor deposition (MOCVD) and its optical analysis. High-resolution transmission electron microscopy (HR-TEM) observation revealed the high quality of the GaN nanorods and the position dependence of the structural properties of the InGaN/GaN MQWs on multiple facets. The excitation and temperature dependences of photoluminescence (PL) revealed the m-plane emission behaviors of the InGaN/GaN core-shell nanorods. The electroluminescence (EL) of the InGaN/GaN core-shell-nanorod-embedded 3D LED changed color from green to blue with increasing injection current. This phenomenon was mainly due to the energy gradient and deep localization of the indium in the selectively grown InGaN/GaN core-shell MQWs on the 3D architecture.

We investigated the effect of polarization of III-nitrides upon the energy band structures and the circuit voltage (VOC) in nitride solar cells. We found that the strong spontaneous polarization will adversely reduce the open circuit voltage (VOC) in p-i-n nitride solar cells. On the contrary, the piezoelectric polarization in InGaN/GaN MQW solar cells does not play a significant role in reducing the open circuit voltage. As an alternative to p-i-n nitride solar cells, based on theoretical energy band structures we propose hybrid InGaN/ZnO solar cells which do not have an adverse influence on the reduction of the VOC. We fabricated hybrid p-GaN/n-ZnO solar cells and demonstrated for the first time the photovoltaic effects.

( 11 2 ¯ 2 ) semipolar GaN thin films were grown on intentionally miscut m-plane sapphire substrates using metal organic chemical vapor deposition. We investigated the material and electrical characteristics by changing the miscut angle from −1° to +1°. While the coexistence of ( 11 2 ¯ 2 ) surface and inclined { 10 1 ¯ 1 } surfaces was observed on GaN films on the on-axis m-plane sapphire substrates, { 10 1 ¯ 1 } surfaces were dominant on the GaN films on the +1° miscut sapphire substrates. As the miscut angle was changed from −1° to +1°, the crystallinity of the GaN films and the electroluminescence intensity of the LEDs were significantly improved.

We report on the fabrication of highly integrated semipolar {101} GaN micro-pyramid light-emitting diode (LED) arrays on a c-plane sapphire substrate obtained via confined selective area growth (SAG). The cathodoluminescence (CL) revealed that the micro-pyramid arrays were under a severe local strain and potential fluctuations were observed depending on the location of the pyramid facets. The emissive region in the micro-pyramid facets was also found to be different in the monochromatically captured CL images. Furthermore, the electroluminescence (EL) of the micro-pyramid LED arrays had a palpable emissive spectra for high indium composition compared to c-plane LEDs, and optical polarization switching was also observed.

The recent development of dielectric microsphere lithography has been able to open up new means of performing simple and easy patterning on the semiconductor surfaces. Here, we report uniform and high-density arrays of microspheres using a solution-based spin-coating method. The arrays of microspheres were used for etching mask to form the arrays of III-nitride microrods. By regrowing GaN layer on the microrod structures, high-quality GaN layer was achieved in terms of surface morphology as well as XRD characterization. To apply the advantages such as improved crystal quality and light extraction enhancement, light-emitting diodes (LEDs) were grown and then fabricated. The regrown LEDs with microspheres showed much improved optical output power and forward voltage characteristics in the same current injection. Therefore, we believe that this approach is quite useful for the development of high efficiency LEDs for future lighting.

Injecting current with a uniform carrier concentration is important for applications with three-dimensional architectures such as vertical power devices or displays. In III-nitride nanostructures, dopants not only incorporate differently depending on the surface orientation but can also seriously affect the kinetic equilibrium shapes of the nanorods. Herein, we report selective-area growth of doped GaN nanorods grown by pulsed-mode metalorganic chemical vapor deposition. Two dopants, Si and Mg, were employed as donor and acceptor atoms, respectively, for a mono-doping approach. Furthermore, a mixed flow of Si and Mg was supplied for a co-doping approach. We compared the morphological effects and growth rates of each doped GaN nanorod array. Then, we proposed appropriate growth mechanisms for the doped GaN nanorods on the basis of our structural characterizations. These results might extend the morphological functionality of GaN nanorods by including doping and may also provide an appropriate foundation for the design of nanostructure-based electronic or photonic devices.

We performed detailed studies of the effect of polarization on III-nitride solar cells. Spontaneous and piezoelectric polarizations were assessed to determine their impacts upon the open circuit voltages (VOC) in p–i(InGaN)–n and multi-quantum well (MQW) solar cells. We found that the spontaneous polarization in Ga-polar p–i–n solar cells strongly modifies energy band structures and corresponding electric fields in a way that degrades VOC compared to non-polar p–i–n structures. In contrast, we found that piezoelectric polarization in Ga-polar MQW structures does not have a large influence on VOC compared to non-polar MQW structures. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Pristine graphene and a graphene interlayer inserted between indium tin oxide (ITO) and p-GaN have been analyzed and compared with ITO, which is a typical current spreading layer in lateral GaN LEDs. Beyond a certain current injection, the pristine graphene current spreading layer (CSL) malfunctioned due to Joule heat that originated from the high sheet resistance and low work function of the CSL. However, by combining the graphene and the ITO to improve the sheet resistance, it was found to be possible to solve the malfunctioning phenomenon. Moreover, the light output power of an LED with a graphene interlayer was stronger than that of an LED using ITO or graphene CSL. We were able to identify that the improvement originated from the enhanced current spreading by inspecting the contact and conducting the simulation.

GaN micro- (or nano-) column structures have been studied in an attempt to enhance the LED performance by improving the light extraction and reducing the strain due to the large lattice mismatch between GaN and InGaN. Nowadays columnar LEDs are drawing attention as a candidate for a multicolor emission source as illustrated in Fig. 1. Though numerous studies have focused on selective area growth, the top-down approach has often been the subject of study since it can realize a columnar structure more easily than when using the bottom-up method. However, predominant issue in the top-down approach is dry-etch damage resulting from the inductively coupled plasma (ICP) and reactive ion etching (RIE) systems. According to the previous reports the damage usually results in the increase in the sheet-resistance of GaN, along with the decrease in the reverse breakdown voltage and the reductions in the Schottky barrier height in the diodes formed on GaN. The roughened sidewall after etching also exhibited higher series resistance in the device and deteriorates the electrical characteristics. KOH treatment has been found to improve the electrical characteristics through the removal of the damaged region and providing the vertical profile in the sidewall with a smooth surface. Moreover, the size control of GaN micro- (or nano-) columns also becomes possible by adopting the KOH treatment.

We demonstrate a cost-effective top-down approach for fabricating InGaN/GaN nanorod arrays using a wet treatment process in a KOH solution. The average diameter of the as-etched nanorods was effectively reduced from 420 nm to 180 nm. The spatial strain distribution was then investigated by measuring the high-resolution cathodoluminescence directly on top of the nanorods. The smaller nanorods showed a higher internal quantum efficiency and lower potential fluctuation, which can subsequently be exploited for high-efficiency photonic devices. (C) 2013 Optical Society of America

We report on the material and optical properties of core–shell InGaN layers grown on GaN nanorod arrays. The core–shell InGaN layers were well grown on polarization-reduced surfaces such as semipolar pyramids and nonpolar sidewalls. In addition, to compensate the biaxial strain between GaN and InGaN layers, we grew interlayers underneath a thick InGaN layer. Here, the interlayers were composed of multiple superlattice structures. We could observe that the indium composition of core–shell InGaN structures increased with the number of interlayers. This indicates that the absorption energy band of InGaN alloys can be better matched to the spectral irradiance of the solar spectrum in nature. We also implemented a simulation of Ga-polar and nonpolar InGaN-based solar cells based on the indium composition obtained from the experiments. The result showed that nonpolar InGaN solar cells had a much higher efficiency than Ga-polar InGaN solar cells with the same thickness of the absorption layer.

Microscale platelet-shaped GaN grains were grown on amorphous substrates by a combined epitaxial growth method of molecular beam epitaxy (MBE) and metal–organic chemical vapor deposition (MOCVD). First, MBE GaN was grown on an amorphous substrate as a pre-orienting layer and its structural properties were investigated. Second, MOCVD grown GaN samples using the different growth techniques of planar and selective area growth (SAG) were comparatively investigated by transmission electron microscopy (TEM), cathodoluminescence (CL), and photoluminescence (PL). In MOCVD planar GaN, strong bound exciton peaks dominated despite the high density of the threading dislocations (TDs). In MOCVD SAG GaN, on the other hand, TDs were clearly reduced with bending, but basal stacking fault (BSF) PL peaks were observed at 3.42 eV. The combined epitaxial method not only provides a deep understanding of the growth behavior but also suggests an alternative approach for the growth of GaN on amorphous substances.

In this paper, we discuss the influence of parameters such as type of carrier gas and NH3/HCl flow ratio on the growth of vertical GaN microstructures by selective-area growth (SAG) hydride vapor phase epitaxy (HVPE). On various strain-induced templates such as GaN/sapphire, GaN/Si, and AlN/Si, regular arrays of Ga-polar GaN microrods were properly achieved by adjusting the growth parameters. The photoluminescence and micro-Raman measurements reveal not only the crystal quality of the GaN microrods but also strain distribution. These results will give insight into the control of the morphology of GaN microrods in terms of the strain induced from templates in SAG-HVPE. The precisely controlled arrays of GaN microrods can be used for next-generation light-emitting diodes (LEDs) by realizing InGaN/GaN multi–quantum wells (MQWs) with a radial structure.

We report a simple and low-cost fabrication of antireflection subwavelength grating (SWG) structures on GaAs using the lenslike shape transfer by a holographic lithography and a reflowed photoresist mask. The use of an additional thermal reflow process enhances the close packing of two-dimensional SWGs with a conical shape. The aspect ratio of conical SWG was also controlled easily by adjusting the rf power during the dry etch process. The fabricated SWGs exhibited low reflection properties over a wide spectral range, in agreement with the calculated values using by a rigorous coupled-wave analysis simulation.