The effects of slight misorientation from a singular plane on sapphire (α-Al2O3) substrates on the surface morphology and luminescence properties of MOVPE-grown GaN films have been studied. Macrostep morphology with periodic terraces (singular plane) and risers (clustered steps) has been observed for the first time on epitaxial GaN films grown on 3°–10° misoriented sapphire substrates toward both the View the MathML sourcesapphire and View the MathML sourcesapphire directions. In addition, it is found that the macrostep causes inhomogeneity of cathodoluminescence (CL) and electroluminescence (EL) patterns in Zn-doped GaN films, suggesting that the Zn-luminescence center formation depends on the growth planes of the terrace and riser.

A novel metalorganic chemical vapor deposition (MOCVD) system, which has two different flows, has been developed. One flow carries a reactant gas parallel to the substrate, and the other an inactive gas perpendicular to the substrate for the purpose of changing the direction of the reactant gas flow. The growth of a GaN film was attempted using this system, and a high quality, uniform film was obtained over a  in sapphire substrate. The carrier concentration and Hall mobility are 1×1018/cm3 and 200 cm2/V s, respectively, which are the highest for GaN films grown directly on a sapphire substrate by the MOCVD method.


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ZnO thin films were deposited on a R-plane sapphire substrate. The effects of the thermal energy and the kinetic energy of the sputtered species on the growth of ZnO thin films were investigated. By varying the substrate temperature, chamber pressure, and radio frequency power, the structure of ZnO thin films was transformed from polycrystalline to epitaxial on R-plane sapphire substrates. High quality ZnO epitaxial thin films were grown at the condition of 400 °C, 250 W, and 5 mTorr. According to reflection high energy electron diffraction and reflection electron microscopy observations, there were no double diffraction distortion and any other patterns. Its surface roughness observed by atomic force microscopy was about 27 nm.

A 1.5 GHz range low insertion loss surface acoustic wave (SAW) filter has been developed using a ZnO/sapphire substrate and IIDT-type electrodes with two external reflectors. This filter had an insertion loss of 1.3 dB, stop-band attenuation of greater than 30 dB and matching impedance of 50 Ω pure resistivity without external matching networks. This filter has the smallest insertion loss of all transverse-type GHz-range SAW filters reported previously and it is suitable for use as a RF-stage filter for future 1.5 GHz range Japanese digital cellular systems.


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Characterization of a Zn0.7Mg0.3O/ZnO heterostructure field-effect transistor (HFET) is reported. The HFET was based on a Zn0.7Mg0.3O/ZnO/Zn0.7Mg0.3O single quantum well(SQW)grown on an a-plane sapphire substrate by molecular-beam epitaxy, and was fabricated by a conventional photolithography technique combined with dry etching. Room-temperature characteristic of the HFET was a n-channel depletion type with a transconductance of 0.70mS/mm and a field-effect mobility of 140cm2/Vs, in good agreement with the electron Hall mobility in SQW of 130cm2/Vs. The on∕off ratio at VDS=3V was ∼800, which was limited by an insufficiently suppressed leakage current through the bottom Zn0.7Mg0.3O barrier.

In this paper we describe recent experimental efforts to produce high quality thick (300 μm) GaN layers on sapphire, the removal of such a layer from the sapphire substrate, and the properties of the so obtained free-standing GaN material. The growth process is described in some detail in the vertical reactor geometry used in this work. Defects like dislocations, micro-cracks and pits produced during growth are discussed, along with procedures to minimize their concentration on the growing surface. The laser lift-off technique is shown to be a feasible technology, in particular if a powerful laser with a large spot size can be used. A major problem with the free-standing material is the typically large bowing of such a wafer, due to the built in defect concentrations near the former GaN-sapphire interface. This bowing typically causes a rather large width of the XRD rocking curve of the free-standing material, while optical data confirm virtually strain free material of excellent quality at the top surface.


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High-quality AlGaN/GaN undoped single heterostructures (SH) with different Al contents have been grown on sapphire substrates. The magnetotransport investigation was performed on these samples at a low temperature. The observation of Shubnikov–de Hass oscillations in the magnetic fields below 3 T and the integer quantum Hall effect confirmed the existence of the two-dimensional electron gas (2DEG) at the AlGaN/GaN interface. The Al0.18Ga0.82N/GaN SH shows a Hall mobility of 10300 cm²/V s at a carrier sheet density of 6.19×1012/cm² measured at 1.5 K. To the best of our knowledge, this is the highest carrier mobility ever measured in GaN-based semiconductors grown on sapphire substrates. The Al composition dependence of the mobility and carrier sheet density were also investigated. Based on the piezoelectric field effect, the Al composition dependence of the 2DEG sheet density was calculated, which agreed well with the experimental result. The negative magnetoresistance with parabolic magnetic-field dependence in the low magnetic field was also observed in the sample with the highest 2DEG sheet density.

A high light-extraction efficiency was demonstrated in the flip-chip light-emitting diode (FCLED) with a textured sapphire substrate. The bottom side of a sapphire substrate was patterned using a dry etching process to increase the light-extraction efficiency. Light output power measurements indicated that the scattering of photons emitted in the active layer was considerably enhanced at the textured sapphire substrate resulting in an increase in the probability of escaping from the FCLED. The light-output power of the FCLED was increased by 40.2% for a 0.4-mm deep FCLED with a periodic distance of 13-mm mesh-type texture on the bottom side of the sapphire substrate.


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The dependence of polar direction of GaN film on growth conditions has been investigated by changing either the group-V/group-III ratio (V/III ratio) in supplying the source gas or the deposition rate. GaN films were deposited on a nitrided sapphire by two-step metalorganic chemical vapor deposition. The surface morphology changed from flat hexagonal to pyramidal hexagonal facet with the increase of V/III ratio. However, the polar direction of GaN on an optimized buffer layer of 20 nm thickness was N-face (-c) polarity, independent of both the V/III ratio and the deposition rate.

The polarity of the GaN epitaxtial layer can be determined by that of an interface (nitrided sapphire, annealed buffer layer or GaN substrate) at the deposition of GaN epitaxial layer. The higher V/III ratio enhanced the nucleation density, and reduced the size of hexagonal facets. The nuclei, forming the favorable hexagonal facets of wurtzite GaN, should grow laterally along the directions to cover a room among the facets until coalescence. After coalescence, -c GaN growth on a flat hexagonal facet results in a pyramidal hexagonal facet. The growth mode for -c GaN has been discussed with respect to surface structure and migration length of adsorbing precursors, in comparison with Ga-face (+c) GaN.


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Metalorganic vapor phase epitaxial (MOVPE) growth of GaN on nanopatterned AGOG sapphire substrates was performed, and characteristics of the light-emitting diode (LED) devices grown on patterned sapphire and planar substrates were compared. The nanopatterned sapphire substrates were fabricated by a novel process (AGOG) whereby aluminum nanomesas were epitaxially converted into crystalline Al2O3 via a two-stage annealing process. The GaN template grown on the nanopatterned sapphire substrate was done via an abbreviated growth mode, where a 15-nm thick, low-temperature GaN buffer layer was used, without the use of an etch-back and recovery process during the epitaxy.

InGaN quantum wells (QWs) LEDs were grown on the GaN template on the nanopatterned sapphire, employing the abbreviated growth mode. The optimized InGaN QW LEDs grown on the patterned AGOG sapphire substrate exhibited a 24% improvement in output power as compared to LEDs on GaN templates grown using the conventional method. The increase in output power of the LEDs is attributed to improved internal quantum efficiency of the LEDs.


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We have performed a detailed investigation of the photoluminescence pressure dependence of heteroepitaxial GaN thin films on sapphire substrates. A comparison between as grown GaN on sapphire and free-standing GaN membranes, created using a laser assisted substrate liftoff process, revealed that the presence of the sapphire substrate leads to an energy gap pressure coefficient reduction of approximately 5%. This result agrees with the numerical simulations presented in this article. We established that the linear pressure coefficient of free-standing GaN is 41.4±0.2 meV/GPa, and that the deformation potential of the energy gap is -9.36±0.04 eV. Our results also suggest a new, lower value of the pressure derivative for the bulk modulus of GaN (B′=3.5).

Roughened surfaces of light-emitting diodes (LEDs) provide substantial improvement in light extraction efficiency. By using the laser-lift-off technique followed by an anisotropic etching process to roughen the surface, an n-side-up GaN-based LED with a hexagonal “conelike” surface has been fabricated. The enhancement of the LED output power depends on the surface conditions. The output power of an optimally roughened surface LED shows a twofold to threefold increase compared to that of an LED before surface roughening.


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ZnO thin films were deposited on sapphire substrates with various thicknesses using a pulsed-laser deposition (PLD) technique in order to investigate the structural and optical properties of the films. The deposition conditions were optimized for UV emission property. The structural and optical properties were characterized with XRD and photoluminescence (PL). The increase in ZnO film thickness results in the improvement of the structural and optical properties. This enhancement could be due to the decrease of strain at the interface between ZnO film and sapphire substrate by the increase of film thickness.

Nitrogen-doped ZnO layers were grown on sapphire substrates by radical source molecular beam epitaxy by simultaneously introducing O2 and N2 via a RF radical source. Reflection high-energy electron diffraction and X-ray diffraction measurements revealed that high N2/O2 flow ratios induced growth twins into the ZnO layer. A nitrogen-doped ZnO fabricated using a N2/O2 flow ratio of 10% was found to have a chemical nitrogen concentration of 1×1019 cm−3. However, type conversion from n-type to p-type did not occur while large nitrogen incorporation was observed to induce extended defects.


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High-quality GaN film with a smooth surface, free from cracks, can be grown on a sapphire substrate by metalorganic vapour phase epitaxy (MOVPE) using a thin AlN buffer layer. The most essential role of the buffer layer is found to be (i) the supply of the nucleation sites with the same crystal orientation as the substrate, and (ii) the promotion of the lateral growth of GaN due to the decrease in interfacial free energy between the substrate and the epitaxial GaN film.

GaN and Ga1−xAlxN (0 < x ≦ 0.4) films grown by MOVPE on sapphire substrate are found to consist of many mosaic crystallites with various orientations. By preceding deposition of a thin AIN buffer layer, the microscopic fluctuation in crystallite orientation can be considerably reduced and the crystalline quality of the film is remarkably improved. Both the thickness and the deposition temperature of the AIN layer are found to be optimal as a buffer layer to convey the information of the substrate such as the crystallographic orientation and to relax the strain in this heteroepitaxial growth. The essential role of the AIN buffer layer is thought to be the supply of nucleation centers having the same orientation as the substrate and the promotion of lateral growth of the film due to the decrease in interfacial free energy between the film and the substrate.


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Recently, the U.S. Army Research Laboratory (ARL) has focused increased attention on the development of transparent armor material systems for a variety of applications. Future combat and non-combat environments will require lightweight, threat adjustable, multifunctional, and affordable armor. Current glass/polycarbonate technologies are not expected to meet the increased requirements. Results over the past few years indicate that the use of transparent crystalline ceramics greatly improve the performance of a system. These results coupled with recent processing and manufacturing advances have revitalized the interest in using transparent ceramics for armor systems. The materials currently under investigation at ARL are magnesium aluminate spinel (MgAl2O4), aluminum oxynitride spinel (AlON), single crystal sapphire (Al2O3), glasses, and glass-ceramics. The polymers under investigation are polycarbonate (PC) and polyurethane (PU). An overview of current ARL efforts in these areas, including the motivation for using transparent ceramics, the requirements, the potential applications, and the ongoing processing research will be reviewed.

Temperature and stress dependence of rhombohedral twinning behavior of sapphire single crystals was investigated. Specimens were deformed in compression along the c-axis with an Instron universal testing machine below 1100 °C in order to avoid basal twinning and dislocation slip. Rhombohedral twinning formation can be explained by nucleation of a double kink in the twinning dislocation, while twin growth can be treated as a twinning dislocation movement which is driven by atomic diffusion at elevated temperatures and by mechanical slip at relatively low temperatures. The activation enthalpy and activation volume of rhombohedral twinning dislocation and also the activation energy of twinning dislocation movement were measured using Smidt's empirical equation and Arrhenius' equation, respectively.


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