X-ray Rocking Curve Full-width At Half-maximum Research Articles

Improved crystallinity and surface morphology of a-plane AlN grown on high temperature annealed AlN/sapphire template by pulsed-flow mode metal-organic vapor phase epitaxy

Preparing high quality non-polar aluminum nitride (AlN) templates is the key to improving the performance of non-polar deep-ultraviolet light-emitting diodes. In this study, we investigated the effect of buffer layer on the crystallinity and surface morphology of a-plane AlN films regrown by pulsed-flow mode (PM) metal-organic vapor phase epitaxy (MOVPE). Three buffer layers were compared including low-temperature AlN buffer layer grown by MOVPE (MO-buffer), sputtered AlN buffer layer (SP-buffer), and high-temperature annealed sputtered AlN buffer layer (HTA-buffer). It is found that the (11-20) plane x-ray rocking curve-full width at half maximum (XRC-FWHM) values of a-plane AlN films are significantly reduced after the regrowth process. Thanks to the high crystalline quality of HTA-buffer, AlN regrown on HTA-buffer exhibits the smallest (11-20) plane XRC-FWHM values of 1260/1440 arcsec along the [0001]/[1-100] direction. The mosaic tilt and basal plane stacking fault density are estimated to be 0.41° and 1.76 106 cm−1, respectively. The surface shows a uniform stripe-like pattern with the lowest root mean square value of 0.82 nm. Furthermore, the a-plane AlN epilayer regrown on HTA-buffer displays a weak in-plane stress anisotropy and high optical transmittance in the ultraviolet-visible region. Our work suggests that combining the PM regrowth with HTA-AlN buffer layer is a promising route to prepare high-quality a-plane AlN templates for efficient non-polar deep-ultraviolet light emitters.

395 nm GaN-based near-ultraviolet light-emitting diodes on Si substrates with a high wall-plug efficiency of 52.0%@350 mA.

The high-performance 395 nm GaN-based near-ultraviolet (UV) light emitting diodes (LEDs) on Si substrates have been obtained by designing an AlN buffer layer to decrease the dislocations density of the GaN layer. By adopting a multi-layer structure with a high- and low-V/III ratio alternation, a high-quality AlN buffer layer has been obtained with a small full-width at half-maximum (FWHM) for AlN(0002) X-ray rocking curve (XRC) of 648 arcsec and a small root-mean-square roughness of 0.11 nm. By applying the optimized AlN buffer layer, the high-quality GaN layer with GaN(0002) and GaN(10-12) XRC FWHM of 260 and 270 arcsec have been obtained, and the high-performance GaN-based near-UV LED wafers and chips have been fabricated accordingly. The as-fabricated near-UV LED chips exhibit a light output power of 550 mW with a forward voltage of 3.02 V at 350 mA, corresponding to a wall-plug efficiency of 52.0%. These chips with outstanding performance are of paramount importance in the application of curing, sterilization, efficient white lighting, etc.

Fabrication of AlN templates on SiC substrates by sputtering-deposition and high-temperature annealing

High-quality AlN templates fabricated by sputtering-deposition and post-deposition high-temperature annealing have great potential for deep ultraviolet light-emitting device applications. In this work, we fabricated AlN films on 6H-SiC substrates by sputtering and face-to-face annealing and characterized the structural quality of the AlN films before and after annealing. As reported in previous studies, to accomplish high-quality AlN films on SiC substrates using conventional methods, such as molecular beam epitaxy or metalorganic vapor phase epitaxy (MOVPE), it is important to grow the AlN on the SiC coherently. However, in this work, although the annealed AlN films were fully relaxed from the SiC substrates, or even had tensile strain, the AlN films indicated high crystallinity. The X-ray rocking curve full width at half maximum (XRC-FWHM) values of the 200-nm-thick annealed AlN film were 17 and 246 arcsec for the AlN (0002) and (10–12) diffraction, respectively. Though the annealed AlN film indicated rough surfaces with bunched step structures, the surface morphology was remarkably improved by MOVPE growth and clear atomic step-and-terrace structures were formed. The XRC-FWHM values of the MOVPE-grown AlN were 90 and 239 arcsec for the AlN (0002) and (10–12) diffraction, respectively.

Effect of SiO2 hexagonal pattern on the crystal and optical properties of epitaxial lateral overgrown semipolar (11-22) GaN film

We studied the optical and crystal properties of semipolar (11-22) hexagonal epitaxial lateral overgrown (HELO) GaN films with different SiO2 hexagonal pattern widths (6–15μm). With increasing SiO2 hexagonal pattern width, it was difficult to achieve coalescence and planarization of the semipolar HELO-GaN film. Furthermore, the surface root mean square roughness of the semipolar (11-22) HELO-GaN film increased with the increasing SiO2 hexagonal pattern width. With the increasing SiO2 hexagon pattern width, the full width at half maximum (FWHM) of the X-ray rocking curve (XRC) toward [1-100] decreased from 1474 to 721arcsec, but the FWHM of XRCs toward [11-2-3] decreased to 9μm, and then, increased with the SiO2 pattern width (>12μm). However, the PL intensity of the semipolar HELO-GaN film increased with the SiO2 hexagonal pattern width. From the findings of this study, we believe that the optical and crystal properties of the semipolar (11-22) HELO-GaN film would be significantly affected by the SiO2 hexagonal pattern width because of the friction between the GaN and SiO2 pattern during the coalescence and planarization step.

Study of crystallographic tilt and surface morphology on epitaxial lateral overgrown semipolar (11-22) GaN

The authors investigated surface morphologies and crystal properties of semipolar (11-22) epitaxial lateral overgrown (ELO) GaN films with the different SiO2 pattern thicknesses (100–240 nm) and widths (6–12 μm). As increasing SiO2 pattern width, the full width at half maximum (FWHM) of x-ray rocking curve (XRC) decreased, and then increased with the SiO2 pattern width (>10 μm). Regardless of SiO2 pattern width, XRC FWHMs of ELO-GaN films with 240 nm-thick SiO2 patterns were much higher than those with 100 nm-thick SiO2 patterns. Moreover, surface morphology of semipolar (11-22) ELO-GaN film was deteriorated with increasing SiO2 pattern width and thickness. From findings of this study, the authors suggested that surface morphology and crystal properties of semipolar (11-22) ELO-GaN film would be significantly affected by SiO2 pattern width and thickness due to the friction between GaN and SiO2 pattern.

Facet-controlled three-step growth of high-quality GaN on sapphire substrates by mass-production-type metalorganic vapor phase epitaxy

Three-step growth using facet-controlled GaN (FC-GaN) technique was demonstrated to successfully grow high-quality GaN on sapphire substrates by mass-production-type metalorganic vapor phase epitaxy. The FC-GaN consists of an island-like structure with intentionally formed inclined { 1 0 1 ¯ 1 } facets. Growth time, temperature, pressure and carrier gas were employed as parameters to control island and facet formation of the FC-GaN. The dependence of full width at half maximum (FWHM) of X-ray rocking curve (XRC) on growth time and temperature of FC-GaN was investigated. FWHM of XRC has a minimum peak versus growth temperature of FC-GaN. Cross-sectional transparent electron microscopy analysis revealed that dislocations were bent where the { 1 0 1 ¯ 1 } facets formed, and the threading dislocation (TD) density at GaN surface was reduced. Correlation between FC-GaN morphology and TD reduction efficiency was discussed.

Effect of seed annealing on ZnSe single-crystals grown by vertical sublimation method

High-quality ZnSe single-crystals were grown on a ZnSe(1 1 1)B seed by vertical sublimation method. The seeds were 0.7 mm thick and 9.7 mm in diameter. We found that annealing for 10 h before growth caused the seeds to become spherical and their diameter decreased to 5 mm, without any decrease in volume. In addition, the crystallographic quality of the grown crystals strongly depended on the seed annealing time. Crystal characterization was carried out using high-resolution X-ray diffraction (HRXRD) and X-ray topography (XRT). When the annealing time was 1 h, the full-width at half-maximum (FWHM) of the X-ray rocking curve (XRC) was wider than 40 arcsec and the XRT intensity was not uniform. In contrast, when the annealing time exceeded 5 h, the FWHM of the XRC was narrower than 10 arcsec and the XRT intensity was uniform. The optimal FWHM of XRC was 6.7 arcsec and occurred when the annealing time was 10 h.

Molecular beam epitaxy growth of Be-chalcogenides and fabrication of ZnSe/ZnMgBeSe laser structures

We have carried out detailed investigation on the growth of Be-chalcogenides alloys and evaluated the device characteristics of optically pumped ZnSe/ZnMgBeSe SQW. A lattice-matched ZnMgBeSe ( E g=2.975 eV) layer shows a narrower X-ray diffraction (XRD) peak, the FWHM (full-width at half-maximum) value of which is 21 arcsec. The variation of FWHM of X-ray rocking curve (XRC) due to lattice misfit is investigated for ZnMgBeSe quaternaries with various lattice misfits extending from compressive-to-tensile strain, which shows that the FWHM value under compressive strain increases more steeply with lattice misfit than that under tensile strain. We report on the optically pumped lasing of a ZnSe/ZnMgBeSe single quantum well at 444 nm with a low threshold of 16 kW/cm 2 at room temperature and high-temperature lasing up to 473 K. The characteristic temperature is estimated to be 166 K from the exponential relation between threshold and temperature.

Growth and characterization of beryllium-based II–VI compounds

We report on the growth and characterization of beryllium–chalcogenide layers prepared on GaAs (100) by molecular beam epitaxy. Be- and Te-terminated BeTe surfaces show (4×1) and (2×1) reconstructions, respectively. The stability of each surface is investigated by reflection high energy electron diffraction as a function of substrate temperature. The dependence of growth rate of BeTe on growth temperature and Be cell temperature is investigated. The best full width at half maximum (FWHM) of a (400) x-ray rocking curve of BeTe is 78 arcsec. The dependence of the ZnBeSe energy gap on Be composition is obtained by four-crystal x-ray diffraction (XRD) and low temperature photoluminescence measurements. The energy gap of Zn1−xBexSe varies as Eg=0.0107x+2.790 (eV) for small Be composition (x<0.25) at 77 K. Lattice-matched ZnBeSe (Eg=2.82 eV) and ZnMgBeSe (Eg=2.975 eV) layers show narrower XRD peaks, the FWHM values of which are 64 and 21 arcsec, respectively. The variation of FWHM of x-ray rocking curve due to lattice misfit is investigated for ZnMgBeSe quaternaries with various lattice misfits extending from compressive to tensile strain. The FWHM value under compressive strain increases more steeply with lattice misfit than that under tensile strain.

Growth and characterization of (111) and (001) oriented MgO films on (001) GaAs

The effects of substrate preparation on the structure and orientation of MgO films grown on (001) GaAs using pulsed laser deposition has been investigated. Textured MgO films displaying a (111)MgO∥(001)GaAs orientation relation with x-ray rocking curve full width at half maximum (FWHM) values as low as 1.8° were obtained in cases where the native GaAs surface oxide was only partially desorbed prior to growth. Reflection high-energy electron diffraction, transmission electron microscopy (TEM), and x-ray pole figure analysis of these films reveals a preferential orientation within the plane of the substrate: [11̄0]MgO∥[11̄0]GaAs and [112̄]MgO∥[110]GaAs. An interfacial layer (∼5 nm thick) was observed in high resolution TEM analysis, and was attributed to a remnant native GaAs oxide layer. Complete desorption of the native GaAs oxide at ∼600 °C in vacuum prior to MgO growth led to significant surface roughening due to Langmuir evaporation, and resulted in randomly oriented polycrystalline MgO films. Growth of MgO on Sb-passivated GaAs substrates, which provided smooth, reconstructed surfaces when heated to 350 °C in vacuum, resulted in cube-on-cube oriented films [i.e., (001)MgO∥(001)GaAs,[100]MgO∥[100]GaAs] with x-ray rocking curve FWHM values as low as 0.47°. TEM analysis of the cube-on-cube oriented films revealed evidence of localized strain fields at the MgO/GaAs interface, indicating the presence of misfit dislocations in the MgO layer.

Correlation between mercuric iodide detector performance and crystalline perfection

X-ray, neutron and gamma ray diffraction rocking curves; X-ray topography; microhardness; and optical microscopic measurements have been performed directly on several mercuric iodide (Hgl2) nuclear radiation detectors fabricated from single crystals grown from the vapor phase. Two types of detectors were measured: spectrometer types (grades A and B), which had resolutions of 5–10% for the 662 keV photopeak of 137Cs, or radiation counters (grades C and D), where the spectral resolution ranged from 11% to no resolution. A good correlation has been found between the detector grade and the full width at half maximum (FWHM) of both the X- and gamma ray rocking curves (i.e., the higher the detector grade (A or B), the narrower the FWHM of the diffraction peak). X-ray topography also correlated with well both the FWHM of the diffraction X-ray rocking curve and the detector grade. The uniformity of the microhardness of the HgI2 detectors was found to be proportional to the nuclear performance of the detector. The better spectrometer-grade detectors were softer and much more uniform in microhardness than the most inferior detectors. The better detectors were also found to have much smoother surfaces than the poorer detectors, as observed by optical microscopy studies.