Basal Plane Sapphire Research Articles

High Resolution, Two-Dimensional Image Mapping of ZnO Nanowires by Confocal MicroPhotoluminescence and MicroRaman Spectroscopy

High-quality ZnO nanowires were synthesized using both Au catalysts and ZnO seeds by chemical vapor depositionon basal plane sapphire substrates. The nanowires were hexagonal and aligned with their c-axis closely perpendicular to the sapphire substrate surface. The structural characteristics of the nanowiresgrown using the different catalysts/seeds were compared using scanning electron microscopyand X-ray diffraction. Their optical properties were assessed using microphoto-luminescence and confocal microRaman spectroscopy and compared. The nanowires exhibited a strong near band-edge related UV luminescence emission along with a defect related visible emission. The dependence of the luminescence as a function of incident excitation power and depth along the axis of the nanowires was studied. The wurtzite structure of the ZnO was confirmed from the Raman measurements. Two-dimensional mappings of the microphotoluminescence emission at different wavelengths and microRaman scattering from the nanowire samples were carried out using a confocal laser scanning microscope. This enabled the ability to correlate the near band-edge UV and visible emissions over the mapped area.

Static and dynamic indentation response of basal and prism plane sapphire

Static and dynamic indentation experiments are conducted on sapphire along the c[0 0 0 1] and a [ 1 1 2 ¯ 0 ] crystallographic axes to determine the loading rate dependence on its mechanical response. Static hardness is measured using a conventional Vickers tester while the dynamic hardness is measured using a custom test fixture based on split Hopkinson pressure bar technique with the capability to produce single indentations at strain rates of roughly 10 3 s −1. Static and dynamic hardness values as well as characteristic fracture response for both axes are compared at a range of indentation loads. The results indicate that dynamic hardness increases by 10% and 12% over static hardness for basal and prism plane sapphire, respectively. Observed crack orientations due to indentation of the a- and c-planes are consistent with the expected location of twinning or slip on the r ( 0 1 1 ¯ 2 ) , ( 1 1 ¯ 0 2 ) , ( 1 ¯ 0 1 2 ) and c ( 0 0 0 1 ) planes for both static and dynamic indentations. Prism slip is also suspected to interact with rhombohedral twinning systems to create a weakened zone oriented along the c-axis. Static indentations yielded well defined crystallographically oriented cracks with repeated directional switchbacks indicating a zone of competition between the two crack systems. Dynamic indentations, on the other hand, resulted in shorter and less jagged cracks accompanied by increased localized spalling beneath each indentation site as compared to the static indentations. This mode of fracture would suggest a shift in energy dissipation mechanism from crystal axis oriented long crack propagation under static loading to micro cracking that is independent of crystallographic orientation under higher strain rate events.

High quality, transferrable graphene grown on single crystal Cu(111) thin films on basal-plane sapphire

The current method of growing large-area graphene on polycrystalline Cu surfaces (foils or thin films) and its transfer to arbitrary substrates is technologically attractive. However, the quality of graphene can be improved significantly by growing it on single-crystal Cu surfaces. Here we show that high quality, large-area graphene can be grown on epitaxial single-crystal Cu(111) thin films on reusable basal-plane sapphire [α-Al2O3(0001)] substrates for transfer to another substrate. While enabling graphene growth on Cu single-crystal surfaces, this method has the potential to avoid the high cost and extensive damage to graphene associated with sacrificing bulk single-crystal Cu during graphene transfer.

Size and shape evolution of faceted bicrystal nanoparticles of gold on sapphire

We produced an array of Au nanoparticles on the basal plane of sapphire via solid-state dewetting of a thin film, and followed the size and shape evolution of individual particles during a 950 °C anneal in air. The particles exhibited {1 0 0} and {1 1 1} facets and shapes that tend to be far from those predicted based upon equilibrium considerations. Most of the single-crystal particles exhibited remarkable size and shape stability up to the longest annealing time, 65 h. The bicrystal particles rapidly transformed into single crystals after the shortest annealing time, 1 h. This transformation was accompanied by an apparent rotation of one of the grains and its alignment along its immobile counterpart. We interpreted both phenomena in terms of very slow mass transport along the singular {1 1 1} and {1 0 0} facets. In the case of bicrystal particles, the fast migration and escape of the boundary from the particle changed the crystallography of the facets in the grain swept by the boundary from a crystallographically singular orientation to a non-special orientation. Such surfaces exhibit high diffusivity and, hence, rapid shape evolution via curvature-driven surface diffusion. A quantitative model of this rotation was proposed and showed to confirm our qualitative description. Based on these findings, we conclude that grain-boundary migration, rather than surface diffusivity, controls the kinetics of mid- and late-term agglomeration of thin polycrystalline films of Au.

Structure and interdiffusion of epitaxial ZnO∕ZnMgO nanolayered thin films

Interdiffusion was studied in Zn1−xMgxO:Al∕ZnO expitaxial superlattices with periods of 15 and 25nm grown on basal-plane sapphire by dc reactive sputtering. Interdiffusion coefficients were determined by analyzing low angle x-ray reflectivity peak intensity decreases during isothermal annealing, using analytical interdiffusion expressions. The results were corroborated with diffusion data obtained from secondary ion mass spectrometry compositional depth profiles. An initial fast diffusion stage was observed for, e.g., the first few hours at 500°C, followed by slower diffusion at longer times.

Preferentially oriented ZnO thin films on basal plane sapphire substrates derived from polymeric precursors

Abstract Preferentially oriented zinc oxide (ZnO) thin films on basal plane sapphire have been deposited by spin coating polymeric precursors synthesized via Pechini process. The polymeric precursors were synthesized using citric acid as chelating agent and ethylene glycol as reaction medium. The effects of annealing (from 673 K to 1273 K) on surface morphology, crystallinity, and optical properties were examined. XRD results illustrate polycrystalline ZnO thin films with preferred orientation along (0 0 2) plane that increases with an increase in annealing temperature. The textured ZnO films were obtained on annealing for 10 min at 1173 K without the formation of interfacial spinels. Surface morphology of these films changed from spherical to highly faceted grains. Transmission spectra reveal that all films are >70% transparent except for the sample annealed at 1273 K, which has 45% optical transparency in the visible region. Room temperature photoluminescence spectra revealed broad ultraviolet emission centered at 380 nm without deep-level emission, suggesting high quality ZnO thin films.

Pulsed lateral epitaxial overgrowth of aluminum nitride on sapphire substrates

The authors report on pulsed lateral epitaxial overgrowth of aluminum nitride films on basal plane sapphire substrates. This approach, at temperatures in excess of 1150°C, enhanced the adatom migration, thereby significantly increasing the lateral growth rates. This enabled a full coalescence in wing regions as wide as 4–10μm. Atomic force microscopy and cross-section transmission electron microscopy were used to establish the reduction of threading dislocations in the lateral growth. Cross-sectional monochromatic cathodoluminescence and photoluminescence measurements confirmed the improved optical properties of the laterally overgrown aluminum nitride films.

Growth of AlN films and their characterization

AbstractSingle crystal AlN layers have been produced by migration enhanced metal organic chemical vapor deposition (MEMOCVD), hydride vapor phase epitaxy (HVPE) and their combination. The growth was carried out on 2” basal plane sapphire substrates. In MEMOCVD, the duration and waveforms of precursors were varied to achieve better surface mobility and thus better atomic incorporation. It resulted in superior layer quality templates with the narrowest (002) X‐ray rocking curve full width half maximum (FWHM). Such high quality AlN templates were used as seeds for subsequent HVPE growth. Thick films with thickness ranging from 1‐25 µm have been grown by HVPE with growth rates as high as 200 µm/min, highest ever reported. Films grown by the two methods have been extensively characterized by Nomarski microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), high‐resolution X‐ray diffractometry (HRXRD), and photoluminescence (PL). (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Crystal chemistry of the epitaxy of cristobalite (SiO 2) on basal plane sapphire

Epitaxial thin films of α-cristobalite were synthesized on c-plane sapphire using the chemical solution deposition method. The films grow with (1 0 1) orientation at temperatures as low as 800 °C and crystallize only in the presence of Na, either by intentional doping or contamination. X-ray diffraction reveals an in-plane orientation of [ 1 ¯ 0 1 ] cris | | [ 1 0 1 ¯ 0 ] sapp , with three cristobalite in-plane variants which correspond to the three-fold symmetry of the sapphire. The use of crystal chemistry concepts enabled the accurate prediction of the in-plane orientation between the two structures prior to their experimental determination. To that end, we propose that the interface consists of silicon-centered tetrahedrons that properly coordinate all neighboring cations and provide charge balance between the two structures.

Positron annihilation study of HVPE grown thick GaN layers

AbstractSingle‐crystalline GaN layers with a thickness up to 330 µm were grown by hydride vapor phase epitaxy on basal plane sapphire at gallium stable conditions in a bottom‐fed vertical reactor at atmospheric pressure. Positron annihilation spectroscopy experiments were implemented in order to identify native point defects in the as‐grown non‐intentionally doped n‐type GaN. Comparatively low concentrations of Ga vacancy related defects in the order of 1016 to 1017 cm–3 were extracted from the positron annihilation spectroscopy data. The Ga vacancy defect concentration was related to the intensity of the yellow photoluminescence band centered at 2.2 eV. The influence of the growth rate on the Ga vacancy related defect concentration was investigated. A trend of decreasing of the defect concentration with increasing of layer thickness is observed, which correlates with improving crystalline quality with the thickness. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Orientation Effects on the High-Temperature Morphological Evolution of Pore Channels in Sapphire

Arrays of semi-infinite and of controlled-aspect-ratio pore channels, both of controlled orientation, were introduced into undoped basal-plane sapphire substrates, using microfabrication techniques, ion-beam etching, and hot pressing. The breakup of these channels via Rayleigh instabilities during a series of annealings at a temperature of 1700°C was monitored. In all cases, the channels broke up with a characteristic wavelength (λ) that was much larger than that expected for a material with isotropic surface energy, which reflected stabilization effects that were due to anisotropy of the surface energy. The break-up wavelength also was very dependent on orientation: channels that were oriented along the [11¯00] and [112¯0] directions yielded the smallest and largest pore spacings (λ-values), respectively, which is in qualitative agreement with prior observations. The critical (minimum) aspect ratio for the breakup of finite-length channels into multiple pores also is dependent upon channel orientation, and the trend mirrors that observed for semi-infinite channels. The pattern of channel evolution suggested two-fold rotational symmetry within the basal plane, where, because of the nature of the experiment, apparent six-fold symmetry is expected. Several factors that may contribute to or cause an apparent or real loss of symmetry have been discussed.

Growth of ZnO films by MOCVD in high magnetic field

Metalorganic chemical vapor deposition of zinc oxide films on sapphire basal plane surface was performed in a high magnetic field of 10 T with diethyl zinc and water vapor as starting materials. Reducing in growth rate, particle size and suppression of formation of leaflet-like structure were observed as the effects of applying magnetic field parallel to gas stream. These effects can be understood by assuming a model where the mobility of the adsorbed atoms on the growing crystals is reduced by the Lorentz force in high magnetic field.

Influence of alloy composition and interlayer thickness on twist and tilt mosaic in AlxGa1−xN/AlN/GaN heterostructures

High-resolution x-ray diffraction, in surface symmetric, skew symmetric, and grazing incidence in-plane diffraction geometries, has been used to investigate the effect of an AlN interlayer between micron thick GaN and AlxGa1−xN layers grown by metalorganic vapor phase epitaxy on basal plane sapphire. No change is found in the tilt mosaic (threading screw dislocation density) with thickness or Al fraction x of the upper layer. A linear increase in the twist mosaic (threading edge dislocation density) was observed as a function of interlayer thickness and x. For all samples the twist mosaic of the AlGaN was significantly greater, by at least a factor of two, than that of the GaN layer. With increasing interlayer thickness the in-plane lattice parameter of the AlGaN decreased. The results are explained in terms of extra threading edge dislocations resulting from relaxation at the GaN/AlN interface.

Sputter-deposited metal contacts for n-type GaN

Sputter-deposited Au, Pt, Cr, Ni and Cu contacts for n-type GaN films were studied using current–voltage (I–V) and capacitance–voltage (C–V) measurements. These films were grown by molecular beam epitaxy (MBE), heteroepitaxially on the basal plane of sapphire. The contacts were non-ideally rectifying in nature. Assuming that the non-ideality was due to effects of series resistance and recombination current, a computer curve fitting procedure was employed that enabled the separation of these effects from the thermionic emission current, thereby permitting the calculation of the barrier height. An analysis of the results indicates that the barrier heights for metal contacts on GaN are determined by the difference between the metal and the semiconductor electronegativities and substantially influenced by metal induced gap states (MIGS)/sputtering damage induced surface states (SDISS). The concentration of metal induced gap states/sputtering induced damage states was determined to be approximately 2.7 × 1013 states cm−2 eV−1.

GaN based laser diodes – epitaxial growth and device fabrication

Four selected material issues of group-III nitride layer structures grown by metalorganic vapor phase epitaxy and molecular beam epitaxy on basal plane sapphire are reviewed. 1) The decomposition of GaN under the impact of hydrogen gas was measured and described thermodynamically. 2) The nucleation and coalescence of GaN islands on a low-temperature nucleation layer was identified to be the key process for the formation of edge-type threading dislocations and heterogeneous stress. 3) The doping with magnesium was associated with pyramidal defects which formed upon a critical layer thickness and led to self-compensation of the acceptors. 4) The relaxation of plain tensile stress via cracks in bulk layers and superlattices based on AlGaN was investigated. The described studies allowed for a material optimization which finally led to the successful demonstration of a GaN based laser diode under pulsed current injection. The operation of the device is discussed in terms of the perfection of the crystallographically wet etched mirror facets and the lateral current spreading in p-type AlGaN cladding layers. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High-quality AlGaN layers over pulsed atomic-layer epitaxially grown AlN templates for deep ultraviolet light-emitting diodes

In this paper, we report the pulsed atomic-layer epitaxy (PALE) of ultrahigh-quality AlN epilayers over basal-plane sapphire substrates and their use as templates to grow high-quality AlGaN layers with Al content ranging from 0.3 to 1. Symmetric/asymmetric x-ray diffraction (XRD) and room-temperature (RT) photoluminescence (PL) measurements were used to establish the high-structural and optical quality. The XRD (002) and (114) rocking-curve full-width at half-maximum (FWHM) values of the PALE-grown AlN epilayers were less than 60 arcsec and 250 arcsec, respectively. Using these ultrahigh-quality layers as templates, Si-doped AlGaN layers with a large Al content from 30% to 100% were grown and used for milliwatt power sub-280-nm, deepultraviolet (UV) light-emitting diodes (LEDs).

Pulsed atomic-layer epitaxy of ultrahigh-quality AlxGa1−xN structures for deep ultraviolet emissions below 230 nm

In this letter, we report the pulsed atomic-layer epitaxy of ultrahigh-quality AlN epilayers and AlN/Al0.85Ga0.15N multiple quantum wells (MQWs) on basal plane sapphire substrates. Symmetric and asymmetric x-ray diffraction (XRD) measurements and room-temperature (RT) photoluminescence (PL) were used to establish the ultrahigh structural and optical quality. Strong band-edge RT PL at 208 and 228 nm was obtained from the AlN epilayers and the AlN/Al0.85Ga0.15N MQWs. These data clearly establish their suitability for sub-250-nm deep UV emitters.

Textured ZnO Thin Films on (0001) Sapphire Produced by Chemical Solution Deposition

Highly textured zinc oxide films were produced on basal plane sapphire substrates by chemical solution deposition. Films with oriented growth were achieved by spin coating a 0.75 M precursor solution of zinc acetate dihydrate and ethanolamine in 2-methoxyethanol, heated at 300 °C/10 min, then at 500 °C/5 h, and finally at 850 °C/12 h. Films were characterized with x-ray diffraction (XRD) and scanning and transmission electron microscopy. The films exhibited only the (0002) ZnO line in XRD diagrams, proving a very well-developed out-of-plane texture. At temperatures above 700 °C the ZnAl2O4 spinel was observed, which formed as a reaction layer between sapphire and ZnO. Few specimens produced both in-plane and out-of-plane oriented growth of ZnO on basal plane sapphire. It was hypothesized that the substrate miscut, uncontrolled for the current experiments, could be the cause of the infrequent growth of epitaxial films.

Low-dislocation-density Al xGa 1− xN single crystals grown on grooved substrates

We propose a new approach to the direct growth of low-dislocation-density Al x Ga 1− x N single crystal films on highly mismatched substrates. Four different substrates, basal-plane sapphire (0001) and (11 2 ̄ 0), 6H–SiC(0001) Si and Si(111), were used. The surface of the substrates was patterned with periodic grooves. In this technique, neither a selective growth mask nor a patterned GaN single crystal film were used. Low-dislocation-density areas were observed above the trench area and at the boundaries of the terrace and trench areas. The dislocation density was reduced to mid 10 6 cm −2 which is lower than that of the films grown directly on the planar substrates by at least two orders of magnitude.

X-ray determination of strain in ion implanted GaN

The out-of-plane c, and in-plane a, lattice parameters of wurtzite gallium nitride (GaN) films, grown on the [0 0 0 1] basal plane of sapphire have been determined and the impact of ion implantation having dose between 5×10 13 and 5×10 15 cm −2 investigated. The thickness of the GaN layers was in the 1–3.5 μm range. The overall effect of the (0 0 0 1) GaN growth on (0 0 0 1) sapphire is biaxial compression in wurtzite α-GaN. Earlier X-ray studies have indicated that the films of GaN grow either purely in α-GaN phase or in α-GaN phase with a cubic β-GaN component. In contrast, our high-resolution X-ray diffraction (XRD) measurement revealed two isostructural polymorphs of α-GaN phases having different lattice parameters. Influence of ion implantation is to increase the values of lattice parameters a and c and could be rationalized in terms of an increase in the defects. At doses above 5×10 15 cm −2, XRD analysis indicates the existence of an amorphous layer preventing the determination of a and c accurately.