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We investigated the properties of indium-doped zinc oxide layers grown by metalorganic chemical vapor deposition on semi-insulating GaN(0001) templates. Specular and transparent films were grown with n-type carrier concentrations up to 1.82 × 1019 cm−3 as determined by Hall measurements, and all In-doped films had carrier concentrations significantly higher than that of a comparable undoped film. For low In flows, the carrier concentration increased accordingly with trimethyl-indium (TMIn) flow until a maximum carrier concentration of 1.82 × 1019 cm−3 was realized. For higher In flows, the carrier concentration decreased with increasing TMIn flow rate. Sheet resistance as low as 185 Ω/sq was achieved for the In-doped films, which is a significant decrease from that of a comparable undoped ZnO film. Our n-type doping studies show that In is an effective dopant for controlling the n-type conductivity of ZnO.

Deep level defects were characterized in Ge-doped (010) β-Ga2O3 layers grown by plasma-assisted molecular beam epitaxy (PAMBE) using deep level optical spectroscopy (DLOS) and deep level transient (thermal) spectroscopy (DLTS) applied to Ni/β-Ga2O3:Ge (010) Schottky diodes that displayed Schottky barrier heights of 1.50 eV. DLOS revealed states at EC − 2.00 eV, EC − 3.25 eV, and EC − 4.37 eV with concentrations on the order of 1016 cm−3, and a lower concentration level at EC − 1.27 eV. In contrast to these states within the middle and lower parts of the bandgap probed by DLOS, DLTS measurements revealed much lower concentrations of states within the upper bandgap region at EC − 0.1 – 0.2 eV and EC − 0.98 eV. There was no evidence of the commonly observed trap state at ∼EC − 0.82 eV that has been reported to dominate the DLTS spectrum in substrate materials synthesized by melt-based growth methods such as edge defined film fed growth (EFG) and Czochralski methods [Zhang et al., Appl. Phys. Lett. 108, 052105 (2016) and Irmscher et al., J. Appl. Phys. 110, 063720 (2011)]. This strong sensitivity of defect incorporation on crystal growth method and conditions is unsurprising, which for PAMBE-grown β-Ga2O3:Ge manifests as a relatively “clean” upper part of the bandgap. However, the states at ∼EC − 0.98 eV, EC − 2.00 eV, and EC − 4.37 eV are reminiscent of similar findings from these earlier results on EFG-grown materials, suggesting that possible common sources might also be present irrespective of growth method. [ABSTRACT FROM AUTHOR]

The dependence of electron mobility on growth conditions and threading dislocation density (TDD) was studied for n--GaN layers grown by ammonia-based molecular beam epitaxy. Electron mobility was found to strongly depend on TDD, growth temperature, and Si-doping concentration. Temperature-dependent Hall data were fit to established transport and charge-balance equations. Dislocation scattering was analyzed over a wide range of TDDs (∼2×106 cm-2 to ∼2×1010 cm-2) on GaN films grown under similar conditions. A correlation between TDD and fitted acceptor states was observed, corresponding to an acceptor state for almost every c lattice translation along each threading dislocation. Optimized GaN growth on free-standing GaN templates with a low TDD (∼2×106 cm-2) resulted in electron mobilities of 1265 cm2/Vs at 296K and 3327 cm2/Vs at 113 K. [ABSTRACT FROM AUTHOR]

Semipolar oriented laser diodes (LDs) have fundamental advantages over c-plane oriented LDs, however, the thickness and composition of InGaN waveguiding layers and AlGaN cladding are limited by the onset of stress relaxation via threading dislocation glide on the c-plane slip system. Limited area epitaxy minimizes misfit dislocation (MD) formation by preventing pre-existing TDs from entering a patterned mesa. We examine the effects of mesa height and facet evolution for LAE grown strained layers. Significant MD formation was suppressed by at least a factor of four for Al0.1Ga0.9N/GaN superlattices, enabling AlGaN-clad structures similar to those used in c-plane LDs. We then demonstrate AlGaN-clad blue (456 nm) LDs with threshold current density (Jth) of 4.5 kA/cm2 and GaN-clad true green (523 nm) LDs with Jth of 12 kA/cm2. TEM measurements of the green LD confirm reduction of MD densities with reduction of MD run length. LAE is effective for improving the optical confinement factor of semipolar LDs and offers a glimpse of the design space that may soon be available using low TD density bulk GaN substrates. [ABSTRACT FROM AUTHOR]

InxAl1-xN layers, with 0.09≤x≤0.23, were grown on GaN on both the In-polar and N-polar orientations by metal organic chemical vapor deposition. The impact of growth conditions, including temperature and the group-III flow rates, on the surface morphology and indium mole fraction was investigated. In-polar layers had a smooth surface morphology characterized by mounds which decreased in size with increasing supersaturation during growth. Smooth N-polar InAlN was achieved through the use of vicinal sapphire substrates with misorientation angles of 3°–5° toward <1010>GaN, and a trend of an increase in step bunching with decreasing supersaturation was observed for N-polar InAlN layers. The indium incorporation increased with decreasing growth temperature and increasing growth rate for both In-polar and N-polar layers. The indium incorporation was similar for both orientations on samples which were coloaded in our reactor. [ABSTRACT FROM AUTHOR]

We describe the development of N-polar GaN-based high electron mobility transistors grown by N2 plasma-assisted molecular beam epitaxy on C-face SiC substrates. High mobility AlGaN/GaN modulation-doped two-dimensional electron gas channels were grown, and transistors with excellent dc and small-signal performance were fabricated on these wafers. Large-signal dispersion was observed, and the trap states responsible for this were identified, and layer designs to remove the dispersive effects of these traps were demonstrated. Finally, an AlGaN-cap layer was used to reduce gate leakage in these devices, and a low-dispersion high breakdown voltage device was achieved. This detailed study of dispersion and leakage in N-polar GaN-based transistors establishes a technological base for further development of field effect devices based on N-polar III-nitrides. [ABSTRACT FROM AUTHOR]

In the present study, the authors report on a modified Riber radio frequency (RF) nitrogen plasma source that provides active nitrogen fluxes more than 30 times higher than those commonly used for plasma assisted molecular beam epitaxy (PAMBE) growth of gallium nitride (GaN) and thus a significantly higher growth rate than has been previously reported. GaN films were grown using N2 gas flow rates between 5 and 25 sccm while varying the plasma source's RF forward power from 200 to 600 W. The highest growth rate, and therefore the highest active nitrogen flux, achieved was ~7.6 µm/h. For optimized growth conditions, the surfaces displayed a clear step-terrace structure with an average RMS roughness (3 × 3 µm) on the order of 1 nm. Secondary ion mass spectroscopy impurity analysis demonstrates oxygen and hydrogen incorporation of 1 × 1016 and ~5 × 1017, respectively. In addition, the authors have achieved PAMBE growth of GaN at a substrate temperature more than 150 C greater than our standard Ga rich GaN growth regime and ~100°C greater than any previously reported PAMBE growth of GaN. This growth temperature corresponds to GaN decomposition in vacuum of more than 20 nm/min; a regime previously unattainable with conventional nitrogen plasma sources. Arrhenius analysis of the decomposition rate shows that samples with a flux ratio below stoichiometry have an activation energy greater than decomposition of GaN in vacuum while samples grown at or above stoichiometry have decreased activation energy. The activation energy of decomposition for GaN in vacuum was previously determined to be ~3.1 eV. For a Ga/N flux ratio of ~1.5, this activation energy was found to be ~2.8 eV, while for a Ga/N flux ratio of ~0.5, it was found to be ~7.9 eV. [ABSTRACT FROM AUTHOR]

By systematically changing growth parameters, the growth of β-(AlxGa1-x)2O3/Ga2O3 (010) heterostructures by plasma-assisted molecular beam epitaxy was optimized. Through variation of the Al flux under O-rich conditions at 600 °C, β-(AlxGa1-x)2O3 (010) layers spanning ~10% to ~18% Al2O3 were grown directly on β-Ga2O3 (010) substrates. Nominal β-(AlxGa1-x)2O3 (010) compositions were determined through Al:Ga flux ratios. With x = ~0.18, the β-(AlxGa1-x)2O3 (020) layer peak in a high-resolution x-ray diffraction (HRXRD) ω-2θ scan was barely discernible, and Pendellösung fringes were not visible. This indicated that the phase stability limit of Al2O3 in β-Ga2O3 (010) at 600 °C was less than ~18%. The substrate temperature was then varied for a series of β-(Al~0.15Ga~0.85)2O3 (010) layers, and the smoothest layer was grown at 650 °C. The phase stability limit of Al2O3 in β-Ga2O3 (010) appeared to increase with growth temperature, as the β-(AlxGa1-x)2O3 (020) layer peak with x = ~0.18 was easily distinguishable by HRXRD in a sample grown at 650 °C. Cross-sectional transmission electron microscopy (TEM) indicated that β-(Al~0.15Ga~0.85)2O3 (010) layers (14.4% Al2O3 by energy dispersive x-ray spectroscopy) grown at 650 °C were homogeneous. β-(Al~0.20Ga~0.80)2O3 (010) layers, however, displayed a phase transition. TEM images of a β-(Al~0.15Ga~0.85)2O3/Ga2O3 (010) superlattice grown at 650 °C showed abrupt layer interfaces and high alloy homogeneity. [ABSTRACT FROM AUTHOR]

We have examined the effect of an indium surfactant on the growth of p-type GaN by ammonia-based molecular beam epitaxy. p-type GaN was grown at temperatures ranging from 700 to 780 °C with and without an indium surfactant. The Mg concentration in all films in this study was 4.5-6 × 1019 cm-3 as measured by secondary ion mass spectroscopy. All p-type GaN films grown with an indium surfactant had higher p-type conductivities and higher hole concentrations than similar films grown without an indium surfactant. The lowest p-type GaN room temperature resistivity was 0.59 Ω-cm, and the highest room temperature carrier concentration was 1.6 × 1018 cm-3. Fits of the temperature-dependent carrier concentration data showed a one to two order of magnitude lower unintentional compensating defect concentration in samples grown with the indium surfactant. Samples grown at higher temperature had a lower active acceptor concentration. Improvements in band-edge luminescence were seen by cathodoluminescence for samples grown with the indium surfactant, confirming the trends seen in the Hall data. [ABSTRACT FROM AUTHOR]

In2O3 is important because it has been widely used as a transparent contact material and an active gas sensor material. To understand and utilize its intrinsic physics as a semiconductor, it is necessary to have In2O3 with a high material quality. In this article, single-crystalline (001)-oriented In2O3 thin films were grown on yttria-stabilized zirconia (001) substrate, and a group theory analysis and transmission electron microscopy (TEM) experiments were conducted to investigate the defects within the In2O3 film. Owing to the reduced symmetry of the bixbyite structure (space group Ia3) in comparison with the fluorite template (space group Fm3m), the formation of antiphase domains and 90°rotation domains in the In2O3 thin films is anticipated. This prediction is confirmed experimentally by TEMand high-angle annular dark-field scanning transmission electron microscopy images. The size of the enclosed domains ranges from 50 to 300 nm, and the major domain boundaries are along the (110), (110), (010) and (100) planes. The rotation domains are related by a fourfold rotation operation along the(001)directions, which will cause the permutation of the axes of the bixbyite structure. [ABSTRACT FROM AUTHOR]

This paper reviews the progress of N-polar (000 1) GaN high frequency electronics that aims at addressing the device scaling challenges faced by GaN high electron mobility transistors (HEMTs) for radio-frequency and mixed-signal applications. Device quality (Al, In, Ga)N materials for N-polar heterostructures are developed using molecular beam epitaxy and metalorganic chemical vapor deposition. The principles of polarization engineering for designing N-polar HEMT structures will be outlined. The performance, scaling behavior and challenges of microwave power devices as well as highly-scaled depletion- and enhancement-mode devices employing advanced technologies including self-aligned processes, n+ (In,Ga)N ohmic contact regrowth and high aspect ratio T-gates will be discussed. Recent research results on integrating N-polar GaN with Si for prospective novel applications will also be summarized. [ABSTRACT FROM AUTHOR]

Abstract: AlN bulk single crystals were grown by spontaneous nucleation on the side wall of a hot pressed BN crucible, using both aluminum metal and AlN powder as source materials in an open system. Optimization of reactor pressure, temperature profile, and crucible design yielded AlN crystals with an a-plane platelet morphology. The crystals had low (112¯0) ω rocking curve full-width at half-maximum values of 17–60arcsecs indicative of their high crystalline quality. Further, the addition of 1% indium by weight to the source material was found to significantly influence growth. When Al metal was used as a source material, the addition of indium to the source material resulted in an increase in the average crystal size by roughly an order of magnitude. When AlN powder was used as a source material, the indium addition resulted in a decrease in the total number of nucleation sites. The results show that the addition of indium is a promising method for improving the physical vapor transport growth of AlN. [Copyright &y& Elsevier]

We report on the development of AlGaN/GaN high-electron mobility transistors (HEMTs) grown on SiC using plasma-assisted molecular beam epitaxy (MBE). In this work, we show that performance comparable to state-of-the-art AlGaN/GaN HEMTs can he achieved using MBE-grown material. Buffer leakage was an important limiting factor for these devices. The use of either carbon-doped buffers, or low Al/N ratio in the nucleation layer growth were effective in reducing buffer leakage. Studies varying the thickness and concentration of the carbon doping were carried out to determine the effect of different carbon doping profiles on the insulating and dispersive properties of buffers. On devices without field plates, at 4 GHz an output power density of 12 W/mm was obtained with a power-added efficiency (PAE) of 46 % and gain of 14 dB. 15.6 W/mm with PAE of 56 % was obtained from these devices after field-plating. Two-tone linearity measurements of these devices were also earned out. At a C/13 level of 30 dBc, the devices measured had an output power of 1.9 W/mm with a PAE of 53 %. The effect of the Al/N ratio in the AlN nucleation layer on buffer leakage was studied. N-rich conditions yielded highly insulating GaN buffers without carbon doping. At 4 GHz, devices without field plates delivered 4.8 W/mm with a PAE of 62 %. At a higher drain bias (50 V), 8.1 W/mm with a PAE of 38 % was achieved. [ABSTRACT FROM AUTHOR]

The authors have applied positron annihilation spectroscopy to study the effect of different growth conditions on vacancy formation in In- and N-polar InN grown by plasma-assisted molecular beam epitaxy. The results suggest that the structural quality of the material and limited diffusion of surface adatoms during growth dictate the In vacancy formation in low electron-density undoped epitaxial InN, while growth conditions and thermodynamics have a less important role, contrary to what is observed in, e.g., GaN. Furthermore, the results imply that in high quality InN, the electron mobility is likely limited not by ionized point defect scattering, but rather by threading dislocations. [ABSTRACT FROM AUTHOR]

Ultralow Ohmic contact resistance and a self-aligned device structure are necessary to reduce the effect of parasitic elements and obtain higher ft and fmax in high electron mobility transistors (HEMTs). N-polar (0001) GaN HEMTs, offer a natural advantage over Ga-polar HEMTs, in terms of contact resistance since the contact is not made through a high band gap material [Al(Ga)N]. In this work, we extend the advantage by making use of polarization induced three-dimensional electron-gas through regrowth of graded InGaN and thin InN cap in the contact regions by plasma (molecular beam epitaxy), to obtain an ultralow Ohmic contact resistance of 27 Ω μm to a GaN 2DEG. [ABSTRACT FROM AUTHOR]

The surface roughness of In2O3(001) films is a roadblock to potential semiconductor applications of this material. Using plasma-assisted molecular beam epitaxy we found that In2O3(001) films grow rough by the formation of {111} facets and In2O3(111) films grow smooth without facetting due to the conventionally used (oxygen-rich) conditions. This behavior indicates that the (111) surface is thermodynamically prefered over the (001) surface. We demonstrate that under indium-rich growth conditions these thermodynamics are changed allowing In2O3(001) films to grow smoothly without facetting. Surface indium plays a key role by acting as an auto surfactant that lowers the surface free energy difference between the (001) and the (111) surface. [ABSTRACT FROM AUTHOR]

In this letter, we report on the microwave power and efficiency performance of AIGaN/GaN high-electron mobility transistors (HEMTs) grown by ammonia molecular beam epitaxy (ammonia-MBE) on SiC substrates. At 4 GHz, an output power density of 11.1 W/mm with an associated power-added efficiency (PAE) of 63% was measured at Vds = 48 V on passivated devices. At 10 GHz, an output power density of 11.2 W/mm with a PAE of 58% was achieved for Vds = 48 V. These results are the highest reported power performance for AIGaN/GaN HEMTs grown by ammonia-MBE and the first reported for ammonia-MBE on SiC substrates. [ABSTRACT FROM AUTHOR]

We demonstrate a technique to reduce the extended defect densities in a-plane GaN deposited on r-plane sapphire. The SiO2 lateral epitaxial overgrowth mask consisted of ⟨1100㟩GaN stripes. Both the mask and GaN were etched through the mask openings and the lateral growth was initiated from the etched c-plane GaN sidewalls, and the material was grown over the mask regions until a smooth coalesced film was achieved. Threading dislocation densities in the range of 106–107 cm-2 were realized throughout the film surface. The on-axis and off-axis full width at half maximum value and surface roughness were 0.082°, 0.114°, and 0.622 nm, respectively. [ABSTRACT FROM AUTHOR]

The effects of NH3 flow, group III flux, and substrate growth temperature on indium incorporation and surface morphology have been investigated for bulk InGaN films grown by ammonia molecular beam epitaxy. The incorporation of unintentional impurity elements (H, C, O) in InGaN films was studied as a function of growth temperature for growth on polar (0001) GaN on sapphire templates, nonpolar [formula] bulk GaN, and semipolar [formula], [formula] bulk GaN substrates. Enhanced indium incorporation was observed on both [formula] and [formula] surfaces relative to c-plane, while reduced indium incorporation was observed on [formula] for co-loaded conditions. Indium incorporation was observed to increase with decreasing growth temperature for all planes, while being relatively unaffected by the group III flux rates for a 1:1 Ga:In ratio. Indium incorporation was found to increase at the expense of a decreased growth rate for higher ammonia flows; however, smooth surface morphology was consistently observed for growth on semipolar orientations. Increased concentrations of oxygen and hydrogen were observed on semipolar and nonpolar orientations with a clear trend of increased hydrogen incorporation with indium content. [ABSTRACT FROM AUTHOR]

Corrosion of process equipment during ammonothermal synthesis at high temperature and pressure is still one of the big limitations of this process because of reduced crystal quality and safety issues. In this paper we present preliminary ammonothermal corrosion experiments with two Ni-base superalloys (Inconel®Alloy 718 and Haynes®282®Alloy) and two newly developed Co-base superalloys (CoWAlloy1&2) in a supercritical NH 3 /NH 4 Cl-solution (24 h/550 °C/105 MPa). After the experiment, the samples were analyzed by their mass change per initial surface area (specific weight change). Additionally, the morphological and elemental changes of the corrosion layers were evaluated. None of the investigated alloys developed a passivation layer and the corrosion layers were very fragile. In agreement with previous studies, the Ni-base alloys showed intergranular attack with severe general corrosive degeneration. Despite their high potential for high-pressure at high-temperature applications and in contrast to a ternary Co-Al-W-alloy, the investigated multinary Co-base alloys were strongly attacked, too. However, CoWAlloy2 presented the lowest specific weight change of the investigated alloys. Thus, it shows the best potential for application in acidic environment of the investigated alloys. Additionally, CoWAlloy2 proved good stability in a supercritical NH 3 /Na-solution (24 h/550 °C/93 MPa). [ABSTRACT FROM AUTHOR]

Improvements to the growth of nitride crystals in ammonothermal growth environments can be achieved through improved autoclave designs, purity, and use of in situ monitoring techniques. Given the limited data available on the stability of materials in supercritical ammonia solutions, this study intends to broaden the known dataset by experimentally investigating the mechanical and chemical stability of 35 bulk metals, 2 bulk metalloids, and 17 bulk ceramics while identifying suitable materials for future in-depth corrosion studies. This was performed by exposing each material to three different supercritical ammonia solutions in nickel-chromium superalloy autoclaves held at an external wall temperature of 575 °C for 4–12 days. The solutions were formed by initially filling the autoclave with pure ammonia (NH 3 ), ammonia and sodium (NH 3 -Na), or ammonia and ammonium chloride (NH 3 -Cl) to achieve total system pressures of 100–250 MPa. Zirconia, silicon carbide, tungsten carbide, molybdenum and its alloys, tungsten and its alloys, and a cobalt-tungsten-aluminum alloy (Co 80 W 10.6 Al 9.4 ) appeared stable in all three environments. Vanadium, niobium, and tantalum appeared chemically stable in all environments, though these samples embrittled and gained weight. Oxides containing high concentrations of aluminum, silicon, and/or zirconium were most stable in NH 3 -Cl while magnesium oxide was not stable in any environment. Silicon nitride corroded in NH 3 -Na, yet appeared stable in NH 3 and NH 3 -Cl. Silver and a platinum-ruthenium alloy (Pt 91 Ru 9 ) appeared to be the only noble metals stable in NH 3 -Na while all of the noble metals investigated, except for silver, appeared to be stable in NH 3 -Cl. Copper and its alloys were not stable in any environment except for Constantan (Cu 53 Ni 47 ) in NH 3 . Exposed thermocouples of type K and N remained functional post-run in NH 3 and NH 3 -Na. Iron and nickel and their respective alloys appeared stable in NH 3 and NH 3 -Na but not in NH 3 -Cl. Most other metals and metalloids investigated in this study were not stable in any of the three solutions and lost weight. Results in this study agree with literature data, where available. [ABSTRACT FROM AUTHOR]

Plasma-assisted molecular beam epitaxial (PAMBE) growth of gallium nitride (GaN) was explored with a novel modification of a commercially available nitrogen plasma source. The modified nitrogen plasma source enabled a dramatic increase in the flux of active nitrogen and thus a significantly higher growth rate than has been previously reported. GaN films were grown using N2 gas flow rates between 1 and 8sccm while varying the plasma source's RF forward power from 200 to 600W. The highest growth rate, and therefore the highest active nitrogen flux achieved was ~2.65μm/h. For optimized growth conditions the surfaces displayed a clear step-terrace structure with an average RMS roughness (3µm×3µm) on the order of 1nm. Secondary ion mass spectroscopy (SIMS) impurity analysis demonstrates oxygen and hydrogen incorporation of 1×1016 and 5×1016 respectively, comparable to the metal organic chemical vapor deposition (MOCVD) grown template layer. Initial un-optimized electron mobility measurements of 1µm thick GaN layers have shown a peak mobility of ~705cm2/Vs for an electron concentration of ~3.5×1016 cm−3. A revised universal growth diagram is proposed allowing the rapid determination of the metal flux needed to grow in a specific growth regime for any and all active nitrogen fluxes available. [ABSTRACT FROM AUTHOR]

Abstract: In this paper we demonstrate work on developing the kinetic Wulff plots and quasi-equilibrium crystal shapes of GaN by hydride vapor phase epitaxy to understand the stable polar, semipolar, and nonpolar planes that emerge naturally from the GaN crystal. High quality bulk m-plane GaN substrates were masked with circular openings to perform selective area growth studies. Growths were performed by hydride vapor phase epitaxy over a range of temperatures, pressures and carrier gases. The quasi-equilibrium crystal shapes were shown to have clear m-plane {} facets and a sharp and flat () N-face or c− face. The (0001) Ga-face or c+ face became faceted with {} planes emerging with reduced pressures and temperatures. Based on the stable facets, kinetic Wulff plots were constructed. [Copyright &y& Elsevier]

Abstract: Al x In1−x N films with 0.14

Abstract: The seeded growth of GaN by the ammonothermal method is updated. By using a polycrystalline GaN nutrient together with higher concentration of NaNH2, we have attained a long-term ammonothermal growth to 90 days. The as-grown crystal showed an improved structural quality to the previously reported GaN bulk crystals grown by the seeded ammonothermal growth, demonstrating the feasibility of the ammonothermal method for a high-quality bulk GaN growth. The threading dislocation density estimated by plan-view transmission electron microscopy (TEM) observations was <1×106 cm−2 for the Ga-face and 1×107 cm−2 for the N-face. There was no dislocation generation observed at the interface on the Ga-face although a few dislocations were generated at the interface on the N-face. The chemical etching together with X-ray diffraction measurements revealed macroscopic grains in the grown crystals. [Copyright &y& Elsevier]

Abstract: Growth of A1N on patterned sapphire substrates was studied. Deep grooves were made on sapphire substrates by a laser-induced backside wet etching technique. KrF excimer laser was irradiated at an interface between sapphire and phenolphthalein/N-methyl-2-pyrrolidone solution, which acts as a super-heating liquid. An etch rate of sapphire was as high as 6μm/min for laser power of 1.0J/cm2. The A1N layer was grown on the patterned sapphire substrate by chemical vapor reaction process at 1190°C for 1h. On the inclined sidewall of the etched grooves, A1N grew with its c-axis perpendicular to the sidewall. This produced m-plane surfaces which were inclined around 20° to the basal plane of sapphire. A threading dislocation density of m-plane A1N domains was 8×108cm-2. [Copyright &y& Elsevier]

Abstract: The growth of AlN thin films on (0001) sapphire by metal-organic chemical vapor-phase epitaxy has been investigated to serve as the template layer of deep-ultraviolet light-emitting diodes. Delayed coalescence of AlN films resulted in a low threading dislocation density of 5×109cm-2 for a 2.1μm thick film compared to partially coalesced 1.05μm films exhibiting a threading dislocation density of 1.1×1010cm-2. The reduction in dislocation density results from a decrease in the number of edge character threading dislocations, while the screw character density remains virtually unchanged, as determined by X-ray rocking curve measurements of the AlN off-axis and on-axis (0002) reflections, respectively. Cross-sectional transmission electron microscopy indicates the presence of a number of inversion domains in the AlN films. Electrically injected, 317nm AlxGa1-xN LEDs were successfully fabricated, with a maximum output power of 17.7μW tested on wafer at 160mA DC. Using a 248nm KrF excimer laser, a 2.8μm thick AlN film was debonded from the sapphire substrate using a laser pulse power of 1J/cm2. However, substrate debonding of an AlxGa1-xN/AlN LED layer could not be achieved. [Copyright &y& Elsevier]

Abstract: Ammonothermal growth of GaN was studied through three types of experiments: synthesis of powder GaN from metallic Ga with various kinds of mineralizers, a weight loss measurement of polycrystalline GaN, and growth of GaN via fluid transport. Hexagonal (wurtzite) GaN was exclusively synthesized with basic mineralizers, whereas neutral and acidic mineralizers resulted in a mixture of hexagonal and cubic (zinc blende) phases, confirming that basic mineralizers are suitable for growing hexagonal GaN. Through a dissolution study of GaN in basic supercritical ammonia, it was discovered that the solubility of GaN has negative temperature dependence. Based on these results, the Ga nutrient was placed in a lower temperature zone and GaN was precipitated on single crystalline GaN seeds. Seeds placed in a higher-temperature zone showed more weight gain than those in a lower-temperature zone. Spontaneously nucleated precipitates on the GaN seeds showed clear crystallographic facets. The maximum size obtained in 24-h growth was about m. Deposition of planar GaN on seed surfaces was also confirmed. [Copyright &y& Elsevier]