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We demonstrate efficient, polarized and monolithic white semipolar (20–21) InGaN light-emitting diodes (LEDs) grown on high crystal quality 4-inch (20–21) GaN/sapphire template. Materials growth by metal-organic chemical vapor deposition (MOCVD) and characterization by atom probe tomography (APT) were carried out. The fabricated regular 0.1 mm2 size LEDs show a high electrical performance with an output power of 3.9 mW at 100 mA, an emission spectrum with two peaks located at 445 nm and 565 nm, a CIE point of (0.37, 0.42) and a polarization ratio of 0.30, which make them promising candidates for backlighting in liquid crystal displays (LCDs) application. Moreover, the fabricated square phosphor-free white μLED with size ranging from 20 to 60 μm, exhibit a high 3 dB modulation bandwidth of 660 MHz in the visible light communication (VLC) system, which benefits from the shorter carrier lifetime grown on the semipolar (20–21) plane. To our best knowledge, this is the first demonstration of monolithic white semipolar μLEDs in the VLC application, which can overcome the limitation of the slow frequency response of yellow phosphors converted commercial white LEDs. These results demonstrate the huge potentials to produce high efficiency monolithic white semipolar InGaN LEDs on cost-effective large area sapphire substrates. The following figures plots the EL emission spectra for the (20–21) device with the polarizer aligned along [1–210] (x ' -direction) and [10-1-4] (y ' -direction), the 3 dB bandwidth measurements results in the VLC system for the fabricated square μLEDs with length varied from 20 to 100 μm, and the reconstructed epitaxial structure is by atom probe tomography (APT). Image 1 • The first demonstration of efficient polarized monolithic white (20-21) semipoloar LEDs grown on low-cost 4-inch sapphire substrate. • The monolithic white semipolar LEDs shows two emission peaks located at 445 nm and 565 nm, with a CIE of (0.37, 0.42). The polarization ratio is measured to be 0.30, which shows a much higher converted efficiency in the LCDs application, as compared to the unpolarized commercial c-plane polar LEDs. • The fabricated monolithic white semipolar µLEDs present a high 3dB bandwidth of 660 MHz in the visible light communication system, which can overcome the limitation of the slow response of yellow phosphor converted commercial white LEDs. [ABSTRACT FROM AUTHOR]

Due to the disparity between observed gallium nitride (GaN) growth under conditions for which literature reports normal solubility, GaN solubility in supercritical NH 3 -Na containing solutions was re-evaluated. Isothermal gravimetric experiments on polycrystalline GaN were performed in the temperature range ( T =415–650 °C) for which retrograde growth of GaN routinely occurs ( P ≈ 200 MPa, molar NH 3 :Na fill ratio =20:1). Two previously-unreported error contributions to the gravimetric determination of GaN solubility were identified: Ga-alloying of exposed Ni-containing components, and the presence of a dense, Ga-absorbing Na-rich, second phase under these conditions. Due to the inability to measure Ga-alloying of the exposed autoclave wall for each experiment, considerable scatter was introduced in the refined GaN solubility curve. No clear dependence of GaN solubility on temperature was resolvable, while most solubility values were determined to be within a band of 0.03–0.10 mol% GaN, normalized by fill NH 3 . [ABSTRACT FROM AUTHOR]

This paper discusses promising materials for use as internal, non-load bearing components as well as molybdenum-based alloys for autoclave structural components for an ammonothermal autoclave. An autoclave was constructed from the commercial titanium–zirconium–molybdenum (TZM) alloy and was found to be chemically inert and mechanically stable under acidic ammonothermal conditions. Preliminary seeded growth of GaN was demonstrated with negligible incorporation of transition metals (including molybdenum) into the grown material (<10 17 cm −3 ). Molybdenum and TZM were exposed to a basic ammonothermal environment, leading to slight degradation through formation of molybdenum nitride powders on their surface at elevated temperatures (T>560 °C). The possibility of a ‘universal’, inexpensive, liner-free ammonothermal autoclave capable of exposure to basic and acidic chemistry is demonstrated. [ABSTRACT FROM AUTHOR]

We demonstrate the growth of epitaxial (100)-oriented, rutile Sb-doped SnO 2 (ATO) and (111)-oriented, cubic Sn-doped In 2 O 3 (ITO) transparent conducting oxide (TCO) contacts on top of an InGaN/GaN(0001) light emitting diode (LED) by plasma-assisted molecular beam epitaxy (PAMBE). Both oxides form rotational domains. The in-plane epitaxial alignment of the two ITO(111) rotational domains to the GaN(0001) was: GaN [21-10]|| ITO Domain1 [‐ 211]|| ITO Domain2 [‐ 1‐12]. A growth temperature as low as 600 °C was necessary to realize a low contact resistance between ATO and the top p-GaN layer of the LED but resulted in non-optimal resistivity (3.4 × 10 − 3 Ω cm) of the ATO. The current–voltage characteristics of a processed LED, however, were comparable to that of a reference LED with a standard electron-beam evaporated ITO top contact. At short wavelengths, the optical absorption of ATO was lower than that of ITO, which is beneficial even for blue LEDs. Higher PAMBE growth temperatures resulted in lower resistive ATO but higher contact resistance to the GaN, likely by the formation of an insulating Ga 2 O 3 interface layer. The ITO contact grown by PAMBE at 600 °C showed extremely low resistivity (10 − 4 Ω cm) and high crystalline and morphological quality. These proof-of-principle results may lead to the development of epitaxial TCO contacts with low resistivity, well-defined interfaces to the p-GaN to help minimize contact losses, and enable further epitaxy on top of the TCO. [ABSTRACT FROM AUTHOR]

Though several methods exist to produce bulk crystals of gallium nitride (GaN), none have been commercialized on a large scale. The sodium flux method, which involves precipitation of GaN from a sodium–gallium melt supersaturated with nitrogen, offers potentially lower cost production due to relatively mild process conditions while maintaining high crystal quality. We successfully developed a novel apparatus for conducting crystal growth of bulk GaN using the sodium flux method which has advantages with respect to prior reports. A key task was to prevent sodium loss or migration from the growth environment while permitting N 2 to access the growing crystal. We accomplished this by implementing a reflux condensing stem along with a reusable capsule containing a hermetic seal. The reflux condensing stem also enabled direct monitoring of the melt temperature, which has not been previously reported for the sodium flux method. Furthermore, we identified and utilized molybdenum and the molybdenum alloy TZM as a material capable of directly containing the corrosive sodium–gallium melt. This allowed implementation of a crucible-free system, which may improve process control and potentially lower crystal impurity levels. Nucleation and growth of parasitic GaN (“PolyGaN”) on non-seed surfaces occurred in early designs. However, the addition of carbon in later designs suppressed PolyGaN formation and allowed growth of single crystal GaN. Growth rates for the (0001) Ga face (+ c -plane) were up to 14 µm/h while X-ray omega rocking (ω-XRC) curve full width half-max values were 731″ for crystals grown using a later system design. Oxygen levels were high, >10 19 atoms/cm 3 , possibly due to reactor cleaning and handling procedures. [ABSTRACT FROM AUTHOR]

The ammonothermal method is one of the most promising candidates for large-scale bulk GaN growth due to its scalability and high crystalline quality. However, emphasis needs to be put on understanding the incorporation and effects of impurities during growth. This article discusses how impurities are incorporated in different growth zones in basic ammonothermal GaN, and how they affect the structural, electrical and optical properties of the grown crystal. The influence of growth time on the impurity incorporation is also studied. We measure the oxygen, silicon, and carbon impurity concentrations using secondary ion mass spectrometry, and measure their effect on the lattice constant by high resolution x-ray diffraction (HR-XRD). We determine the resulting free carrier concentration by spatially resolved Fourier transform infrared spectroscopy and study the optical properties by spatially resolved low-temperature photoluminescence. We find that oxygen is incorporated preferentially in different growth regions and its incorporation efficiency depends on the growth direction. The oxygen concentration varies from 6.3×10 20 cm −3 for growth on the { 11 2 ¯ 2 } planes to 2.2×10 19 cm −3 for growth on the ( 0001 ) planes, while silicon and carbon concentration variation is negligible. This results in a large variation in impurity concentration over a small length scale, which causes significant differences in the strain within the boule, as determined by HR-XRD on selected areas. The impurity concentration variation induces large differences in the free carrier concentration, and directly affects the photoluminescence intensity. [ABSTRACT FROM AUTHOR]

We report recent results of bulk GaN crystal growth using the sodium flux method in a new crucible-free growth system. We observed a (0001) Ga face (+ c -plane) growth rate >50 µm/h for growth at a N 2 overpressure of ~5 MPa and 860 °C, which is the highest crystal growth rate reported for this technique to date. Omega X-ray rocking curve (ω-XRC) measurements indicated the presence of multiple grains, though full width at half maximum (FWHM) values for individual peaks were <100 arcseconds. Oxygen impurity concentrations as measured by secondary ion mass spectroscopy (SIMS) were >10 20 atoms/cm 3 . By monitoring the nitrogen pressure decay over the course of the crystal growth, we developed an in situ method that correlates gas phase changes with precipitation of GaN from the sodium-gallium melt. Based on this analysis, the growth rate may have actually been as high as 90 µm/h, as it would suggest GaN growth ceased prior to the end of the run. We also observed gas phase behavior identified as likely characteristic of GaN polynucleation. [ABSTRACT FROM AUTHOR]

InAlN grown by plasma-assisted molecular beam epitaxy often contains a honeycomb microstructure. The honeycomb microstructure consists of 5–10 nm diameter aluminum-rich regions which are surrounded by indium-rich regions. Layers without this microstructure were previously developed for nominally lattice-matched InAlN and have been developed here for higher indium content InAlN. In this study, InAlN was grown in a nitrogen-rich environment with high indium to aluminum flux ratios at low growth temperatures. Samples were characterized by high-resolution x-ray diffraction, atomic force microscopy, high-angle annular dark-field scanning transmission electron microscopy, and atom probe tomography. Atomic force microscopy showed InAlN layers grown at temperatures below 450 °C under nitrogen-rich conditions were free of droplets. InAlN films with indium contents up to 81% were grown at temperatures between 410 and 440 °C. High-angle annular dark-field scanning transmission electron microscopy and atom probe tomography showed no evidence of honeycomb microstructure for samples with indium contents of 34% and 62%. These layers are homogeneous and follow a random alloy distribution. A growth diagram for InAlN of all indium contents is reported. [ABSTRACT FROM AUTHOR]

In this paper we demonstrate ultraviolet (UV) light emitting diodes (LEDs) grown on metamorphic AlGaN buffers on freestanding GaN ( 20 2 ¯ 1 ) substrates by ammonia assisted molecular beam epitaxy (MBE). Misfit and related threading dislocations were confined to the stress relaxed, compositionally graded buffer layers, and single quantum well devices emitting at 355, 310 and 274 nm were grown on top of the graded buffers. The devices showed excellent structural and electrical ( I – V ) characteristics. [ABSTRACT FROM AUTHOR]

Abstract: The onset of plastic relaxation via misfit dislocation (MD) formation in In x Ga1−x N layers grown by metal-organic chemical vapor deposition on the ( ) semipolar plane of GaN substrates is investigated using high-resolution X-ray diffraction, transmission electron microscopy and cathodoluminescence. The results of critical thickness calculations for MD formation as a function of In x Ga1−x N alloy composition x are compared with experimental observations. MD generation is observed initially as a result of slip on the (0001) slip plane, and subsequently as a result of additional slip on inclined m-planes, which eventually leads to an increase in threading dislocation density. [Copyright &y& Elsevier]

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: InGaN single quantum wells (SQWs) grown on m-plane bulk GaN substrates show significant differences in peak emission wavelength when grown on substrates oriented nominally on-axis compared to substrates with small intentional misorientations (miscuts) towards the orthogonal −c-direction or a-direction. SQWs on substrates intentionally miscut toward the a-direction emit longer wavelengths than those with miscuts towards the −c-direction in a variety of identical growth conditions, while SQWs on nominally on-axis m-plane with pyramidal hillocks features display emission characteristics of both. These preliminary co-loaded growth studies may provide insight into broad or anomalous wavelength emission observed on nonpolar GaN-based visible light emitters and suggest opportunities for improving LED and laser diode device performance on this naturally occurring crystal plane. [Copyright &y& Elsevier]

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: Light-emitting diodes (LEDs) fabricated from gallium nitride (GaN) have led to the realization of high-efficiency white solid-state lighting. Currently, GaN white LEDs exhibit luminous efficacy greater than 150lmW−1, and external quantum efficiencies higher than 60%. This has enabled LEDs to compete with traditional lighting technologies, such as incandescent and compact fluorescent (CFL) lighting. Further improvements in materials quality and cost reduction are necessary for widespread adoption of LEDs for lighting. A review of the unique polarization anisotropy in GaN is included for the different crystal orientations. The emphasis on nonpolar and semipolar LEDs highlights high-power violet and blue emitters, and we consider the effects of indium incorporation and well width. Semipolar GaN materials have enabled the development of high-efficiency LEDs in the blue region and recent achievements of green laser diodes at 520nm. [Copyright &y& Elsevier]

Abstract: Al x In1−x N films with 0.14

Abstract: The magnitude of the low-frequency 1/f noise in GaN/AlGaN HEMTs grown under Ga-rich, N-rich, and NH3-rich conditions varies in response to hot-electron stress. Density-functional-theory (DFT) calculations show that the Ga vacancies that are responsible for the positive shift in pinch-off voltage due to electrical stress in Ga-rich and N-rich devices do not contribute significantly to the observed changes in 1/f noise with electrical stress. The N anti-sites that cause negative shifts in pinch-off voltage in ammonia-rich devices can cause an increase in the noise magnitude after stress. DFT calculations also show that singly hydrogenated and dehydrogenated Ga–N divacancies also can contribute to the noise before and after stress, respectively. A decrease in noise magnitude is also observed in some devices after stress. [Copyright &y& Elsevier]

Abstract: Microcrystalline GaN was synthesized in supercritical ammonia with acidic, neutral, and basic mineralizers. For both acidic and neutral mineralizers, a mixture of wurtzite GaN and zinc-blende GaN was obtained, whereas wurtzite GaN was preferentially synthesized with basic mineralizers. Gibbs free energy for wurtzite GaN and zinc-blende GaN was calculated to investigate phase stability of GaN. The free energy of formation for wurtzite GaN was smaller than that for zinc-blende GaN by 720 J/mol (8meV) at 600K and 1310 J/mol (14meV) at 800K. Given this small energy difference, the selection of phase in ammonothermal synthesis turned to depend more on kinetics than on thermodynamics. Water-dissolving powder which was occasionally synthesized in basic conditions suggested that the reaction path in basic conditions is different from that in acidic and neutral conditions. The difference in the reaction path is thought to affect the phase selection. [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]