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Highlights • (10 1 ¯ 0) m -plane AlGaN grown on m-plane sapphire over the entire range of composition. • The structural, morphological and optical properties were investigated. • The AlN mole fraction for m -plane layers is comparable with c -plane co-loaded layers. • The crystal-field splitting energy has a bowing parameter of about −0.15 eV. Abstract Heteroepitaxial growth of single-phase nonpolar (10 1 ¯ 0) m -plane Al x Ga 1 - x N layers on m -plane sapphire substrates was investigated by metalorganic vapour phase epitaxy. Different Al/Ga gas phase ratios were used to adjust the AlN mole fraction over the entire range of composition. All m -plane AlGaN layers show an orthorhombic distortion in the wurtzite unit cell due to anisotropic in-plane strain. This distortion decreases with increasing AlN mole fraction due to a decreased anisotropic biaxial strain. The AlN mole fraction of m -plane layers and c -plane co-loaded layers estimated by X-ray diffraction is comparable. This is consistent with their comparable energy bandgaps, which were estimated from room-temperature dielectric functions. The dependence of the energy bandgap on composition indicates a bowing parameter of about 0.9 eV. The crystal-field splitting energy is found for the m -plane layers with a bowing parameter b cf of about −0.15 eV. [ABSTRACT FROM AUTHOR]

Highlights • MOVPE-growth and high-temperature annealing of m -plane AlN on m -plane sapphire. • (10 1 ¯ 0) XRC FWHM along [0001]/[11 2 ¯ 0] decreased from 2900/1940 to 1030/800 arcsec. • The density of basal stacking faults decreased from ~ 2.8 × 10 5 to ~ 1. 5 × 10 4 cm−1. • Larger effective bandgap after annealing (6.15 eV) than as-grown layers (6.04 eV). Abstract Heteroepitaxial growth of AlN layers on (10 1 ¯ 0) m -plane sapphire substrates was investigated by metalorganic vapour phase epitaxy. Different nucleation temperatures were used to achieve single phase (10 1 ¯ 0) AlN layers. The crystallinity of the layers further increased with increasing annealing temperature and annealing time. The full-width at half maximum of the symmetric (10 1 ¯ 0) X-ray rocking curves decreased from about 2900/1940 arcsec to 1030/800 arcsec along [0001]/[11 2 ¯ 0] AlN. The density of basal stacking faults of the annealed layers was found to decrease from ∼ 2.8 × 105 to ∼ 1.5 × 104 cm−1. The annealed layers had a larger effective optical bandgap energy of ∼ 6.15 eV than ∼ 6.04 eV of the as-grown layers due to their better crystallinity and structural order. [ABSTRACT FROM AUTHOR]

• Sn-doped n-type GaN layers were grown by halide vapor phase epitaxy. • Sn precursor was formed by reacting Sn metal with HCl gas. • Sn concentration in GaN layers increased with decreasing the growth temperature. • High electron density of 2 × 1020 cm−3 was achieved. • Sn introduction promoted the surface migration of Ga adatoms. A Sn-doped n-type GaN layer with a high electron density of 2 × 1020 cm−3 and a low resistivity of 8.7 × 10−4 Ω∙cm was grown by halide vapor phase epitaxy (HVPE). Sn doping was performed through the reaction between Sn metal and HCl gas. The Sn concentration markedly increased with decreasing growth temperature and the activation energy of Sn desorption from the GaN surface was found to be 4.1 eV. Smooth surfaces were obtained by introducing the Sn precursor even though the samples were grown at a low temperature of 905 °C, suggesting that Sn atoms act as surfactants and promote the migration of Ga adatoms. Almost all the Sn atoms act as donors in GaN below the Sn concentration of 2 × 1020 cm−3. These results indicate that using the Sn donor is promising for fabricating low-resistivity n-type GaN substrates by HVPE. [ABSTRACT FROM AUTHOR]

Highlights • Low impurities concentration in GaN grown by MOVPE using quartz free reactor. • N-polar GaN atomic steps with double height. • Different off-cut angle N-polar GaN for homo-epitaxial growth. Abstract This study examines the effect of (0 0 0 −1) GaN substrate misorientation on the residual impurities and surface morphology of N-polar GaN grown by metalorganic vapor-phase epitaxy. Carbon, silicon, and oxygen concentrations decreased with increasing GaN substrate misorientation angle, with the lowest impurity concentration achieved for a misorientation angle of 2° toward the m-axis, with 6 × 1015 cm−3 carbon, 6 × 1015 cm−3 silicon, and 4 × 1017 cm−3 oxygen atoms. The oxygen concentration was measured at a depth of 0.5 μm below the wafer surface, and the oxygen concentration decreased with increasing thickness. The incorporation of carbon and oxygen revealed a strong dependence on the misorientation angle. The step distance height of the steps parallel to the [1 1 −2 0] direction (or perpendicular to the [1 −1 0 0] m-direction) was confirmed to be a double-height layer step. This phenomenon indicated that m-direction steps are stable for N-polar growth in GaN. In cases of large misorientation toward the m-axis in of the GaN substrate it was difficult to control the misorientation perpendicular to the nominal direction leading to a-axis direction by wafer bowing at wafer manufacturing. Therefore, step-bunching was generated for each symmetric m-axis due to an increase in the compound's off-angle, thus causing the surface roughness to become large. [ABSTRACT FROM AUTHOR]

Thermal annealing at high temperatures of nonpolar (1 0 1 ¯ 0) m -plane AlN layers directly sputtered on m -plane sapphire was investigated. The crystallinity of the layers increased with increasing annealing temperature. The full-width at half maximum of the symmetric (1 0 1 ¯ 0) X-ray rocking curves along [0 0 0 1]/[1 1 2 ¯ 0 ] AlN decreased from about 3.5/2.0 ° to 0.24/0.19 ° . The density of basal stacking faults of the annealed layers was found to decrease from ∼ 1 × 10 5 to ∼ 5 × 10 3  cm −1 . The annealed layers had a larger optical bandgap energy than the as-sputtered layers due to their better crystallinity and structural order. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A-plane GaN was attempted to be grown on (11-20) 4H-SiC bulk substrate without using a traditional thick buffer layer. By inducing TMAl treatment before the GaN growth step and using both a low pressure and V/III ratio, the interlayer thickness of the a-plane GaN/SiC was reduced to 1.7 + 0.5 nm. The ultrathin interlayer was observed to be either AlN or AlGaN with a low Ga composition. This study is aimed to contribute to the understanding of GaN growth on the sidewalls of c-plane trench structure SiC. The ultrathin growth technique is also hoped to be applied to the fabrication of GaN-based vertical structure nonpolar optical or electrical devices. [ABSTRACT FROM AUTHOR]

• A surface kinetic model of HVPE-GaN (0001) layers is developed using BCF theory. • The hillock density is controlled by the supersaturation and/or the off-cut angle. • The critical off-cut angle is controlled by the supersaturation. The effects of halide vapor phase epitaxial (HVPE) growth conditions such as input V/III ratio and substrate off-cut angle on the surface morphology of n-type GaN layers grown on GaN (0001) freestanding substrates were investigated to develop a model for growing smooth surfaces. The spiral hillock density increased with increasing input V/III ratio and/or decreasing off-cut angle. The critical off-cut angle between the spiral growth and the step-flow growth depended on the vapor supersaturation calculated by thermodynamic analysis. To understand the transition of the growth mode, we proposed a Burton–Cabrera–Frank-theory-based model considering the effect of spiral growth, which was utilized to explain the obtained experimental results. The developed growth model can be effective for predicting the HVPE growth mode between the spiral growth and the step-flow growth. [ABSTRACT FROM AUTHOR]

Abstract: Gallium nitride (GaN) films have been grown by using our newly developed Radical-Enhanced Metalorganic Chemical Vapor Deposition (REMOCVD) system. This system has three features: (1) application of very high frequency (60MHz) power in order to increase the plasma density, (2) introduction of H2 gas together with N2 gas in the plasma discharge region to generate not only nitrogen radicals but also active NH x molecules, and (3) radical supply under remote plasma arrangement with suppression of charged ions and photons by employing a Faraday cage. Using this new system, we have studied the effect of the trimethylgallium (TMG) source flow rate and of the plasma generation power on the GaN crystal quality by using scanning electron microscopy (SEM) and double crystal X-ray diffraction (XRD). We found that this REMOCVD allowed the growth of epitaxial GaN films of the wurtzite structure of (0001) orientation on sapphire substrates with a high growth rate of 0.42μm/h at a low temperature of 800°C. The present REMOCVD is a promising method for GaN growth at relatively low temperature and without using costly ammonia gas. [Copyright &y& Elsevier]

Abstract: A novel activation process for Mg-implanted GaN was demonstrated. As opposed to the conventional thermal annealing process, an H2/NH3 alternating supply annealing process achieved better optical activation, stronger near-ultraviolet luminescence and weaker yellow luminescence in the photoluminescence spectroscopy. After this process, small hexagonal hillocks were observed on the surface, which indicated that crystal regrowth was induced by this process, consisting of decomposition of GaN by H2 supplies and re-crystallization by NH3 supplies. It was revealed that the implanted Mg could easily be located at the activation site by means of crystal regrowth by this process. [Copyright &y& Elsevier]

Abstract: c-Axis-oriented wurtzite (0001) AlN layers are grown on a (111) diamond substrate by metal-organic vapor phase epitaxy. The microstructure and growth mechanism of the AlN layer are examined using X-ray diffractometry and transmission electron microscopy. At the initial stage of AlN growth, AlN particles with a two-domain structure are nucleated on the (111) diamond surface. The orientation is either 〈112¯0〉AlN ∥ [11¯0] diamond [named the AlNI domain] or 〈101¯0〉AlN ∥ [11¯0] diamond with a rotation angle of 30° from AlNI around the c-axis [named the AlNII domain]. The initial AlN particles are predominantly composed of AlNII. Subsequently, the AlNII grains incorporate the quite-low-density AlNI grains upon further growth. Finally, a continuous AlN layer dominated by the AlNII domain is obtained on the diamond. The growth dynamics is significantly different from that of the AlN layer grown on (111) Si. The growth mechanism is understood by considering the periodic atomic arrangement of AlN and (111) diamond. We conclude that AlNII on (111) diamond has a geometrically appropriate epitaxial relationship that reduces the stress energy of the AlN lattice. [Copyright &y& Elsevier]

Abstract: The selective regrowth of GaN during sidewall-seeded epitaxial lateral overgrowth was performed. In addition to adjusting the V/III ratio, control of offset angle of the sidewall was found to be effective for realizing one-sidewall-seeded a-plane (112¯0) GaN on r-plane (11¯02) sapphire. The number of coalescence regions on the grooves was reduced, and threading-dislocation and stacking-fault densities as low as 106–107 cm−2 and 103–104 cm−1, respectively, were successfully realized. [Copyright &y& Elsevier]

Abstract: A grooved Al0.25Ga0.75N underlying layer on an AlN-coated sapphire substrate was used to grow crack free and low dislocation density Al0.25Ga0.75N to successfully realize high-performance UV A light emitters. A light-emitting diode grown on a grooved AlGaN underlying layer exhibited an output power of 12mW at a DC current of 50mA for a peak emission wavelength of 345nm with an external quantum efficiency of 6.7%, which is the highest to date in this wavelength region. We also fabricated UV A laser diodes with an emission wavelength of 356nm at a pulsed injection current of 414mA. [Copyright &y& Elsevier]

Abstract: We grew thick Ga1− x In x N epitaxial films on GaN templates having different crystal orientations. Reciprocal space mapping of asymmetrical X-ray diffraction showed that 1.1-μm-thick Ga0.95In0.05N could be coherently grown on m-plane GaN, while those grown on a-plane and c-plane GaN showed partial relaxation. A 700-nm-thick Ga0.95In0.05N film with a threading dislocation density of approximately 1×108 cm−2 can be successfully grown on a GaN template using a grooved underlying layer. The mechanism of strain relaxation in c-, a- and m-plane Ga1− x In x N films is discussed. [Copyright &y& Elsevier]

Abstract: Ultraviolet light-emitting diodes (UV-LEDs) with a peak wavelength of 335nm were fabricated on AlN/sapphire templates. As templates for the fabrication of UV-LEDs, planar AlN and epitaxial laterally overgrown (ELO) AlN on sapphire (0001) substrates were compared. The output power of UV-LEDs grown on ELO-AlN was 27 times higher than that of UV-LEDs on the planar AlN template. [Copyright &y& Elsevier]

Abstract: We investigated impact of high-temperature growth by metal-organic vapor phase epitaxy on crystallographic quality of AlN on 6H-SiC (0001)Si substrate. With increasing growth temperature (T g), growth rate became high and residual impurities decreased. For T g=1600°C, AlN with atomically flat surface and low dislocation density were reproducibly obtained under the high growth rate at 6μm/h. The narrowest distribution of tilt and twist were 76 and 360arcsec, respectively, and the average dislocation density was measured to be 7×108 cm−2. Most of dislocations were of edge-type, generated by the misfit between the AlN and the SiC substrate. For increases of T g up to 1600°C, the number of dislocations at AlN/SiC interface decreased, and many of the dislocations were annihilated by the formation of a loop structure. [Copyright &y& Elsevier]

Abstract: Epitaxial lateral overgrowth (ELO) of high-quality AlN layers on trench-patterned AlN underlying layers has been demonstrated by high-temperature metal-organic vapor phase epitaxy. Under optimized conditions, a high growth rate and an atomically flat surface of thick ELO-AlN without cracks were obtained. Fully coalesced ELO-AlN epilayers were obtained on AlN templates having trenches. AlN grown above the groove had low dislocation density due to filtration caused by the grooves. In addition, most of the dislocations threading from the AlN template were annihilated by the formation of a loop structure during growth with increasing thickness of AlN. The average dislocation density of the ELO-AlN was calculated to be 4×107 cm−2. [Copyright &y& Elsevier]

Abstract: To realize high-performance UV-light-emitting diodes (UV-LEDs), thick, crack-free and high-crystalline-quality AlGaN films with a low threading dislocation density and a device structure without any absorbing layer such as GaN are essential. Crack-free and low-dislocation-density AlGaN is developed using facet-controlled technology, and does not contain any GaN layer. As a result, the density of the threading dislocations in the overgrown AlGaN is as low as 5×107cm-2 over the entire wafer. [Copyright &y& Elsevier]

We proposed a heteroepitaxial lateral overgrowth technique using periodically grooved substrates to prepare a GaN single crystal with low dislocation density. By this technique, GaN single crystals have been grown without a selective growth mask or GaN single crystal seeds. Moreover, the AlGaN single crystal, which is important for optoelectronic devices in the UV region, can also be grown using this technique. The reduction of the threading dislocation density in AlXGa1-XN single crystals grown on periodically grooved substrates was confirmed by transmission electron microscopy. [Copyright &y& Elsevier]

Phase separation of AlGaN is found to occur during growth on periodically trenched GaN, and it sometimes causes crack formation. The rate of separation can be suppressed by decreasing NH3 flow rate in the gas phase. We can optimize the growth condition for AlGaN growth, thereby fabricating UV-LED with an emission wavelength of 352 nm and output power of 0.6 mW at 50 mA. [Copyright &y& Elsevier]

Relaxation processes of misfit-induced stress in GaN/AlyGa1-yN and AlxGa1-xN/AlyGa1-yN structures (x>y) were studied. In case of GaN on relaxed AlyGa1-yN, relaxation of compressive-stress occurs in a very early stage of the GaN growth, while in case of AlyGa1-yN on AlxGa1-xN (x>y), tensile-stress relaxation does not occur until the catastrophic relaxation by fracture formation. Surface roughness of the GaN grown on AlyGa1-yN can be suppressed by doping with Mg. [Copyright &y& Elsevier]

We proposed the heteroepitaxial lateral overgrowth technique using grooved sapphire (0 0 0 1) and (1 1 2¯ 0) SiC(0 0 0 1)Si and Si(1 1 1) substrates to prepare AlxGa1−xN single crystalline films with low dislocation density. Straight grooves of these substrates were aligned parallel to the 〈1 1¯0 0〉 or 〈1 1 2¯ 0〉 direction of AlxGa1−xN. In this technique, neither a selective growth mask nor patterned GaN single crystal film was used. Dislocation densities lower than about 106 cm−2 were ascertained via plan-view TEM. This technique is promising for growing low-dislocation-density nitride-based semiconductors. [Copyright &y& Elsevier]

• P-type GaN layers with Mg concentrations of 8.0 × 1018–8.3 × 1019 cm−3 are fabricated by HVPE. • The electrical properties and structural defects of p-type GaN layers are characterized. • Pyramidal inversion domains are observed in the heavily Mg-doped GaN layers grown by HVPE. • Mg atoms accumulate in pyramidal inversion domain. The electrical properties and structural defects of p-type GaN layers with Mg concentrations from 8.0 × 1018 to 8.3 × 1019 cm−3 grown by halide vapor phase epitaxy (HVPE) were investigated. In all samples, p-type conduction was confirmed at room temperature. The hole concentration at room temperature decreased in a heavily Mg-doped sample. By analyzing the results of Hall-effect measurements at various temperatures, the acceptor concentration decreased in a heavily Mg-doped sample, whereas the compensating donor concentration increased. These results affect the decrease in the hole concentration. The hole mobility decreased with increasing acceptor concentration. In the heavily Mg-doped sample, pyramidal inversion domains (PIDs) were formed. The size of each PID in an HVPE-grown sample is in good agreement with that Mg-doped GaN layers grown by metalorganic vapor phase epitaxy (MOVPE). Thus, the formation mechanism of PIDs in HVPE-grown samples is possibly the same as that in MOVPE-grown samples. Energy-dispersive X-ray spectroscopy shows that Mg atoms accumulate in PIDs, which suggests that Mg atoms in PIDs are electrically inactive, inhibiting the increase in the acceptor concentration. These results are useful guidelines for fabricating p-type GaN layers with higher hole concentrations by HVPE. [ABSTRACT FROM AUTHOR]

Abstract: We succeeded in growing high-crystalline-quality thick (101¯1¯) Ga0.92In0.08N films on a grooved (101¯1¯) GaN/(101¯2¯) 4H-SiC underlying layer. We also fabricated GaInN/GaN multiple quantum wells (MQWs) with a peak wavelength of 580nm on a high-crystalline-quality thick GaInN film. The photoluminescence intensity of the MQWs is about six times higher than that of MQWs grown on planar GaN and twice as high as that of MQWs grown on a GaN underlying layer having the same grooved structure. [Copyright &y& Elsevier]

Abstract: We investigated the electrical properties of Mg-doped a-plane Ga1− x In x N (0

Abstract: We succeeded in growing almost completely relaxed, high-quality, thick GaInN films on an m-plane GaN template with grooves along the 〈0001〉 direction, using lateral-growth technology. Reciprocal space mapping of asymmetrical X-ray diffraction confirmed almost complete relaxation. By transmission electroscopic characterization, the growth of GaInN film with a threading dislocation density of approximately 1×108 cm−2 was confirmed. [Copyright &y& Elsevier]

• Lattice bow generated by HVPE layers on free-standing GaN wafers was studied. • Samples include HVPE homoepitaxial layers on ammonothermal and HVPE GaN wafers. • Bow increased on all samples but on the ammonothermal GaN wafers was minimal. • Lattice bow increase was related to elongation of dislocations via climb. Lattice bow generated by 40 µm thick HVPE homoepitaxial layers on commercial free-standing, ammonothermal and HVPE GaN wafers was studied. While a change in lattice bow was measured for all wafers, the additional bow on the ammonothermal GaN wafers was minimal. The main driving force for the observed increase in the lattice bow for HVPE wafers was related to stress in the films generated by the elongation of dislocations via climb and generation of new dislocations at the homoepitaxial interface. Lattice bow is a crucial wafer parameter as it determines the variation of the offcut across the surface. If an offcut variation of 0.1° is allowed for desired control surface morphology, composition of alloys, and uniformity of doping on this surface, the measured bow on the two HVPE GaN wafers and one ammonothermal GaN wafer limits their uniformity-diameter to ~0.5″, 1″ and >4″, respectively. [ABSTRACT FROM AUTHOR]