Basal Plane Sapphire Substrates Research Articles

Crystallization of Copper Films on Sapphire Substrate for Large-Area Single-Crystal Graphene Growth.

Chemical vapor deposition synthesis of graphene on polycrystalline copper substrates from methane is a promising technique for industrial production and application. However, the quality of grown graphene can be improved by using single-crystal copper (111). In this paper, we propose to synthesize graphene on epitaxial single-crystal Cu film deposited and recrystallized on a basal-plane sapphire substrate. The effect of film thickness, temperature, and time of annealing on the size of copper grains and their orientation is demonstrated. Under optimized conditions, the copper grains with the (111) orientation and a record size of several millimeters are obtained, and the single-crystal graphene is grown over their entire area. The high quality of synthesized graphene has been confirmed by Raman spectroscopy, scanning electron microscopy, and the sheet resistance measurements by the four point probe method.

Comparative Spectroscopic Study of Aluminum Nitride Grown by MOCVD in H2 and N2 Reaction Environment

We report a comparative spectroscopic study on the thin films of epitaxial aluminum nitride (AlN) on basal plane sapphire (Al2O3) substrates grown in hydrogen (H2) and nitrogen (N2) gas reaction environments. AlN films of similar thicknesses (~3.0 µm) were grown by metal-organic chemical vapor deposition (MOCVD) for comparison. The impact of the gas environment on the AlN epilayers was characterized using high-resolution X-ray diffraction (HR-XRD), X-ray photoelectron spectroscopy (XPS), Raman scattering (RS), secondary ion mass spectroscopy (SIMS), cathodoluminescence (CL), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The study showed that AlN layers grown in a N2 environment have 50% less stress (~0.5 GPa) and similar total dislocation densities (~109/cm2) as compared to the films grown in a H2 environment. On the other hand, AlN films grown in a H2 gas environment have about 33% lesser carbon and 41% lesser oxygen impurities than films grown in a N2 growth environment. The possible mechanisms that influenced the structural quality and impurity incorporation for two different gas environments to grow AlN epilayers in the MOCVD system on sapphire substrates were discussed.

Thick AlN Templates By MOCVD for the Thermal Management of III-N Electronics

Ultra-wide bandgap semiconductors have attracted much interest over the last decade. These materials have strong chemical bonding that led to high temperature tolerance, high voltage and power handling capabilities as described by the Baliga figure of merit. Al-rich III-nitride materials are expected to have the best performance over the industry standard Si, and newer options such as SiC and GaN. Continuous improvements dictate the miniaturization of electronic devices, leading to increasingly higher power densities in smaller footprints, leading to heating that causes system failure. Thus, the thermal management is the limiting task for continuous performance scaling. In past we have reported on deep ultraviolet light emitting diodes (LEDs) and AlxGa1-xN (x>0.4) channel Heterojunction Field-Effect Transistors (HFETs) on 2-3 µm thick MOCVD grown high quality AlN/sapphire templates. For template thicknesses over 5 µm either sapphire or AlN patterning is needed to avoid stress related cracking. In this research, we for the first-time report on the direct MOCVD growth and characterization of crack-free, low-impurity AlN templates in excess of 16 μm on basal plane sapphire substrates without doing any external processing. Increasing the thickness improves thermal management and enables an easier sapphire substrate’s laser liftoff for increased DUV light extraction. It would also be used as a heat spreader in power electronics to minimize the operating temperature to maximize power output to industrial loads such as electric vehicles, and large turbines in manufacturing and transportation.All growths were carried out in custom MOCVD reactors with a fast-metalorganic switching manifold using a modified epitaxy procedure in which the Al- and N-precursors (Trimethyl Aluminum and NH3) are pulsed for increased surface mobility. At the initial pulsed growth stage, the growth-temperature and precursor-flow rates are adjusted to yield air-pocketed (voids) rough AlN layers. Most of these high defect materials are located near the AlN/sapphire interface. As the AlN thickness increases in the subsequent growths, the layers turned out smooth and the number of defects is significantly reduced due to dislocation bending/annihilation and cracking is avoided due to strain relief from the voids at the interface. All template growths were carried out at temperatures ~ 1200 - 1300 °C and 40 torr.From X-ray diffraction (XRD), Reciprocal space map (RSM), and Transmission Electron Microscopy (TEM) analysis of these thick AlN templates, we established that they were fully relaxed. Cross-section TEM data clearly shows the presence of voids at ~ 0.4 – 0.9 μm from the AlN/sapphire interface. There is a high density of dislocations present near the void region. Above this region, at least one order of magnitude lower dislocations is observed at the AlN smooth layer region. From the room-temperature Cathodoluminescence (CL) data we see an increase in the band edge emission and a decrease in the defect related long-wavelength signal with increasing AlN template thicknesses. The CL spectra were measured in the edge emission pump geometry on cleaved bars due to the transverse-magnetic polarized nature of the band-edge emission. The monochromatic CL imaging data also confirmed a highly defective region at the AlN/sapphire interface which is also the origin of the long wavelength (below bandgap) CL emission. The Atomic Force Microscopy (AFM) surface scans of the templates showed the surface roughness ~ 0.15 nm to 0.25 nm (for a 5 µm x 5 µm scan). The (102) off axis X-ray linewidths for all these samples ranged from 280 to 330 arc-secs. XRD and Raman experiments confirmed a residual compressive strain in AlN templates compared to the bulk sample. The measured stress ranges from -0.6 GPa to -1.2 GPa for all the templates, reasonable agreement with previous reports. We have found no obvious thickness dependence of the stresses measured from both XRD and Raman, an indication of preserving similar stresses in thick and thin templates.The thermal conductivity of our high-quality 16 µm thick template and the bulk AlN are measured at different temperatures by Time-domain Thermoreflectance (TDTR) technique. The thermal conductivity of the thick AlN at room temperature is 320 Wm-1K-1, or ~10% higher than that of the bulk AlN (~285 W/m-K). At low temperatures (120 K), it exceeds that of bulk AlN by more than 40%. In summary, these templates showed thermal conductivity as high or better than high-cost bulk AlN substrates, demonstrating for the first time that sapphire-based power electronics could be effectively managed thermally. Figure 1

(Invited) Recent Advances in III-Nitride Devices Using Ultrawide Bandgap AlxGa1-Xn Active Layers

III-Nitride devices with high-Al AlxGa1-xN (x>0.4) active layers have the potential of addressing several key application areas. For these high Al-compositions the direct bandgap leads to a cut-off wavelength<290 nm which makes the material ideal for solar blind detectors. Similarly, AlxGa1-xN (x>0.4) multiple quantum wells (MQWs) are used in the active region of the deep ultraviolet (DUV) light emitting diodes. More recently, to take advantage of the higher breakdown field for high voltage devices, several researchers including us have explored Heterojunction Field-Effect Transistors (HFETs) and metal-oxide-semiconductor (MOS) HFETs with high-Al AlxGa1-xN (x>0.4) channel layers. Currently nearly all the (UWBG) AlGaN devices are fabricated on 3-5 µm thick MOCVD grown high quality AlN/sapphire templates. This choice is dictated by the deep UV optical transparency, the easy availability and the low-cost of sapphire. Increasing the AlN template thickness to larger than 5 µm leads to stress related cracking. The low thermal conductivity of sapphire and this AlN growth thickness limitation leads to device performance limitations especially for high power operation. To avoid this, recently using a pulsed MOCVD approach, we have for the first time succeeded in growing 16 µm thick, crack-free, low-defect AlN template layers on basal plane sapphire substrates. These were then used to deposit epitaxial structures for the AlGaN channel HFET/MOSHFETs. In this presentation we will present studies to establish the role that the high conductivity thick AlN buffers play in improving the devices’ electrical and thermal performance. Recently we have also for the first time conducted optical waveguiding studies at UVC wavelengths. For these studies we fabricated monolithically integrated UVC LEDs, detectors and waveguides on an AlN/sapphire template platform. Optical attenuation coefficient as low as 4.8 cm-1 was measured for Al0.5Ga0.5N waveguide with an AlN clad layer. Details of our integrated device fabrication and optical losses measurements scheme will also be discussed. Acknowledgement: This work was partially supported by Defense Advanced Research Projects Agency, Army Research Office and Office of Naval Research (Monitors: Dr. M. Gerhold. Dr. Paul Maki and Dr. Lynn Petersen).

Толстые слои alpha-Ga-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=- на сапфировых подложках, полученные методом хлоридной эпитаксии

This paper reports on epitaxial film growth and characterization of α-Ga2O3, a novel wide bandgap semiconducting material. The films were deposited by halide vapour phase epitaxy on basal plane sapphire substrates. The films were from 0.5 μm to over 10 μm in thickness, the latter being the record value by now. Structural and optical properties of the specimens were studied. All specimens were structurally uniform, single phase, and had a corundum-like r3c structure similar to that of sapphire substrate. It was found that the full width at half maximum for the (0006) α-Ga2O3 reflection varies with layer thickness and approaches 240 arcsec for the thickest layer. Both thin and thick layers were transparent in the visible and UV spectral range up to the absorption edge at 5.2 eV.

Electrical properties, structural properties, and deep trap spectra of thin α-Ga2O3 films grown by halide vapor phase epitaxy on basal plane sapphire substrates

Undoped epitaxial films of α-Ga2O3 were grown on basal plane sapphire substrates by halide vapor phase epitaxy (HVPE) in three different modes: standard HVPE, HVPE with constant flow of Ga and pulsed supply of O2 (O2-control growth regime), and with constant flow of O2 and pulsed delivery of Ga (Ga-control growth fashion). The best crystalline quality as judged by x-ray symmetric and asymmetric reflection half-widths and by atomic force microscopy morphology profiling was obtained with the O2-control deposition, and these results appear to be the best so far reported for α-Ga2O3 films. All grown α-Ga2O3 epilayers were high-resistivity n-type, with the Fermi level pinned near Ec − 1 eV deep traps. Photoinduced current transient spectra also showed the existence in standard HVPE samples and samples grown under the O2-control pulsed growth conditions of deep hole traps with levels near Ev + 1.4 eV whose density was suppressed in the Ga-control pulsed HVPE samples. The levels of the dominant deep traps in these α-Ga2O3 samples are close to the position of dominant electron and hole traps in well documented β-Ga2O3 crystals and films.

High Electron Mobility Transistors With Al0.65Ga0.35N Channel Layers on Thick AlN/Sapphire Templates

We report Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.85</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.15</sub> N/Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.65</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.35</sub> N high-electron mobility transistors on low-defect AlN buffer layers over basal plane sapphire substrates. A new epilayer design consisting of a doped barrier layer yielded sheet resistivity values as low as 1800 Ω/□. Devices with the source-drain spacing of 5.5 μm and a gate length of 1.8 μm exhibited peak drain-currents as high as 250 mA/mm at a gate bias of +4 V. We also show that the 3-μm thick low-defect AlN buffer layers over sapphire to provide sufficient thermal conduction which enables stable device operation up to at least 40 V at 250 mA/mm with no current droop.

A theoretical and experimental evaluation of III–nitride solar-blind UV photocathode**Project supported by the National Natural Science Foundation of China (Grant No. 10974015) and the National Defense Pre-Research Foundation of China (Grant No. 9140C380502150C38002).

We have developed a superior solar-blind ultraviolet (UV) photocathode with an AlGaN photocathode () in semi-transparent mode, and assessed spectra radiant sensitivity related to practical use. Before being grown over a basal plane sapphire substrate by low-pressure metal organic chemical vapor deposition (MOCVD), a reasonable design was made to the photocathode epitaxy structure, focusing on the AlGaN: Mg active layer, then followed by a comprehensive analysis of the structural and optical characterization. The spectra radiant sensitivity is peaked of 41.395 mA/W at wavelength 257 nm and then decreases by about 3 to 4 decades at 400 nm demonstrating the ability of this photocathode for solar-blind application prospects.

Growth, doping, and characterization of ZnO nanowire arrays

Zinc oxide (ZnO) nanowire (NW) arrays were grown by chemical vapor deposition using the carbothermal reduction of ZnO powder at different pressures from 0.13 to 1.0 atm on basal plane sapphire substrates. The ZnO NWs were oriented in their [0001] direction. Lower growth pressures led to generally longer and smaller diameter wires. A model relating the length and diameter of the NWs was used to interpret the growth mechanism of these ZnO NWs as a function of pressure as the combination of adatom diffusion along the NW sidewalls and direct impingement growth on the NW tip. Al-doped ZnO NWs were synthesized by introducing Al power into the source material, resulting in an Al mole fraction up to 1.8 at. % in the NWs and a concurrent reduction in NW resistivity. Raman spectroscopy revealed slight lattice distortion to the ZnO crystal lattice, while room temperature photoluminescence showed an increase in the near band edge emission concurrently with a reduction in the green emission. The near band edge emission was also blue shifted in a manner consistent with the Burstein–Moss effect in degenerated semiconductor materials.

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.

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 &amp; 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.

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.

Lateral epitaxial overgrowth of GaN on sapphire and silicon substrates for ultraviolet photodetector applications

Lateral epitaxial overgrowth of GaN thin films was conducted by low-pressure metalorganic chemical vapor deposition on basal plane sapphire and (111) silicon substrates. The films were characterized through X-ray diffraction, photoluminescence, scanning electron microscopy, atomic force microscopy and deep level transient spectroscopy. Schottky metal–semiconductor–metal ultraviolet photodetectors were fabricated on LEO grown GaN films for the first time. The spectral responsivity, its dependence on optical excitation power and bias voltage, and the device time decay properties were characterized. The orientation of the interdigitated fingers with respect to the LEO stripes was investigated.

Characterization of flicker noise in GaN-based MODFET's at low drain bias

We report systematic characterizations of flicker noise in GaN based MODFET's. The devices were fabricated by MBE on (0001) basal plane sapphire substrates with an 800 /spl Aring/ AlN buffer layer grown at 800/spl deg/C. Flicker noise was measured across the channel of the devices from room temperature to about 130 K. The voltage noise power spectra, S/sub V/(f), were found to be proportional to 1/f/sup /spl gamma//. The frequency exponent, /spl gamma/, of S/sub V/(f) exhibits systematic dependencies on the device temperature as well as the gate bias, V/sub G/. The variation of the noise power spectra as a function of the drain voltage, V/sub D/, and the gate bias, V/sub G/, were studied in detail and were found to vary as V/sub D//sup 2//(V/sub G/-V/sub T/)/sup /spl beta// where /spl beta/ changes with temperature from about 2.1 at room temperature to about 0.9 at 130 K. Analyses of the data show that the noise originated from the thermal activation of carriers to the localized states at the AlGaN/GaN heterointerface in the channel area. The data suggested that the trapping and detrapping of carriers did not lead to fluctuations in the carrier concentration as postulated in the McWhorter's model. However, more work is needed to determine if surface mobility fluctuations played a key role in the 1/f noise.

High-performance GaN p-n junction photodetectors for solar ultraviolet applications

The fabrication and characterization of ultraviolet photodetectors based on GaN p-n junctions is reported. The devices are grown by metalorganic vapour phase epitaxy on basal-plane sapphire substrates. These detectors are visible-blind with a sharp wavelength cut-off at 360 nm. The photocurrent is linear with incident power from up to , with a responsivity of at 360 nm. The device time response is dominated by the effective resistance-capacitance time constant, and a 105 ns response is estimated for very low load resistances. A comparison with the response of GaN photoconductor detectors is also presented. The application of these high-performance photodetectors for solar ultraviolet monitoring is described.