Search

Background The COVID-19 pandemic highlighted the need for early detection of viral infections in symptomatic and asymptomatic individuals to allow for timely clinical management and public health interventions. Methods Twenty healthy adults were challenged with an influenza A (H3N2) virus and prospectively monitored from 7 days before through 10 days after inoculation, using wearable electrocardiogram and physical activity sensors. This framework allowed for responses to be accurately referenced to the infection event. For each participant, we trained a semisupervised multivariable anomaly detection model on data acquired before inoculation and used it to classify the postinoculation dataset. Results Inoculation with this challenge virus was well-tolerated with an infection rate of 85%. With the model classification threshold set so that no alarms were recorded in the 170 healthy days recorded, the algorithm correctly identified 16 of 17 (94%) positive presymptomatic and asymptomatic individuals, on average 58 hours postinoculation and 23 hours before the symptom onset. Conclusions The data processing and modeling methodology show promise for the early detection of respiratory illness. The detection algorithm is compatible with data collected from smartwatches using optical techniques but needs to be validated in large heterogeneous cohorts in normal living conditions. Clinical Trials Registration. NCT04204493.

OBJECTIVE To determine the feasibility of short-term cardiovascular responses to postural change as a screening tool for mild traumatic brain injury (mTBI), using heart rate metrics that can be measured with a wearable electrocardiogram sensor. SETTING Military TBI clinic. DESIGN Data collected from active-duty service members who had sustained a medically diagnosed mTBI within the prior 72 hours and from age- and sex-matched controls. Cardiac data collected while participants performed a sequence of postural changes. MAIN MEASURES Model classification compared with clinical mTBI diagnosis. RESULTS Cardiac biomarkers of mTBI were identified and logistic regression classifiers for mTBI were developed from different subsets of biomarkers. The best model achieved 90% sensitivity and 69% specificity using data from 2 different postural changes. CONCLUSION Noninvasive measurement of cardiovascular response to postural change is a promising approach for field-deployable post-mTBI screening.

We have examined the performance of gallium nitride (GaN) high-power Schottky diodes fabricated on unintentionally doped (UID) metalorganic chemical vapor deposition (MOCVD) films grown simultaneously on four substrates ranging in threading dislocation density from 5 × 103 cm - 2 to 1010 cm - 2. The substrates were an intentionally doped and a UID freestanding hydride vapor phase epitaxy substrate, an MOCVD GaN template grown on a sapphire wafer, and a bulk GaN substrate grown via an ammonothermal method. Capacitance–voltage (C–V) results showed the carrier concentration was ∼2 × 1016 cm−3 for films grown on each of the four substrates. With that doping level, the theoretical breakdown voltage (Vb) is ∼1600 V. However, measured Vb for the devices tested on each of the four substrates fell short of this value. Also, the breakdown voltages across each of the four substrates were not substantially different. This result was especially surprising for films grown on bulk GaN substrates, because of their superior crystal quality, as determined from their x-ray rocking curve widths. Simple probability calculations showed that most of the diodes tested on the bulk substrate did not cover a single threading dislocation. Although optimization of edge-termination schemes is likely to improve Vb, we believe that point defects, not threading dislocations, are the main reason for the reduced performance of these devices.

Systematic site-specific comparisons of the densities of threading dislocations (TDs) and associated V-defects in six-period In x Ga 1− x N/GaN multi-quantum wells (MQWs) with In mole fractions of x =0.1 and 0.2 have been conducted using several electron microscopy techniques including electron channeling contrast imaging. The MQWs were grown on GaN epitaxial films deposited on either (0001) GaN substrates or on (0001)AlN/6H-SiC heterostructures. The densities of TDs and V-defects in the surfaces of the homoepitaxial GaN films and the In 0.1 Ga 0.9 N/GaN MQWs were both 5.5×10 6 cm −2 . By contrast, the V-defect densities in the In 0.2 Ga 0.8 N/GaN MQWs exceeded by two and three times the densities of TDs measured in the surfaces of the underlying GaN films grown on GaN and AlN/SiC substrates, respectively. Similar observations of V-defects not associated with threading dislocations have been previously reported to occur in InGaN MQWs grown on GaN template films on sapphire substrates and having either an initially high In mole fraction or grown at the end of a series of MQWs with an increasing In mole fraction. From a synthesis of the results of this previous work using GaN/sapphire substrates with the present results for bulk GaN substrates, it is hypothesized that increases in stress generated by the larger mismatches in lattice parameters between the GaN film and the In 0.2 Ga 0.8 N/GaN MQWs and the additional stresses generated between the GaN barriers and the wells within these MQWs generated new TDs within the MQWs, which then acted as nucleation centers for new V-defects.

A heteroepitaxial GaN-on-sapphire template growth strategy is presented that eliminates wafer bow in the final structure at both room temperature and at high crystal growth temperatures. The technology is demonstrated here on 2″ and 4″ substrate sizes with GaN epitaxy thicknesses up to 150 µm with a sub-nm, epi-ready, final surface roughness. Wafer bow vs. temperature measurements for (a) 15 µm of GaN on 4″ 650 µm thick sapphire showing 280 µm bow shift, (b) 10 µm GaN film on 2″ 430µm thick sapphire showing 120 µm bow shift, and (c) and (d), 15 µm of FLAAT GaN on 2″ and 4″ sapphire, respectively, each showing less than 30 µm of bow shift. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report the structural and electrical properties of ultrathin-barrier AlN/GaN heterostructures grown on freestanding GaN substrates by rf plasma-assisted molecular beam epitaxy. Structures with barrier thicknesses between 1.5 nm and 7.5 nm were grown and characterized. We observe that AlN/GaN structures with barriers of 3.0 nm exhibit the highest Hall mobility, approximately 1700 cm 2 /Vs. Furthermore, the Hall mobility is much diminished in heterostructures with AlN barriers thicker than 4.5 nm, coincident with the onset of strain relaxation.

We have investigated the N- and Ga-polar faces of bulk GaN substrates with photoluminescence (PL) and the surface photovoltage (SPV) technique using a Kelvin probe attached to an optical cryostat. Experiments were conducted in vacuum. Some of the surfaces were mechanically polished (MP), while others were epi-ready after a chemical-mechanical polish (CMP). From the SPV measurements, the band bending in a sample having both surfaces treated with the CMP method was calculated to be about 0.83 and 0.70 eV for the Ga- and N-polar surfaces, respectively. The restoration of the SPV after ceasing the UV illumination showed that the SPV from CMP-treated surfaces behaved as predicted by a thermionic model, whereas the SPV from MP-treated surfaces restored with a much faster-than-predicted rate. This result can be interpreted by the hopping of charge carriers in the highly-defective near-surface layer of the MP-treated samples. Remarkably, removing the top 700 nm defective layer by dry etching restored the quality of the electrical and optical properties of GaN. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Hexagonal AlN is a non-ferroelectric material and does not have any phase transition up to its melting point (>2000°C), which indicates the potential use of AlN for hightemperature sensing. In this work, the elastic, dielectric, and piezoelectric constants of AlN single crystals were investigated at elevated temperatures up to 1000°C by the resonance method. We used resonators of five different modes to obtain a complete set of material constants of AlN single crystals. The electrical resistivity of AlN at elevated temperature (1000°C) was found to be greater than 5 × 1010 Ω· cm. The resonance frequency of the resonators, which was mainly determined by the elastic compliances, decreased linearly with increasing temperature, and was characterized by a relatively low temperature coefficient of frequency, in the range of −20 to −36 ppm/°C. For all the investigated resonator modes, the elastic constants and the electromechanical coupling factors exhibited excellent temperature stability, with small variations over the full temperature range

An organometallic vapor phase epitaxy-based process route has been developed to achieve homoepitaxial deposition of GaN(0001) films via step-flow growth on substrates having

With its wide bandgap and associated high critical field, GaN is a suitable material for high power electronics. Our group has identified carbon incorporation into the drift region as problematic for the development of GaN power Schottky diodes. The likely source of carbon is from the trimethylgallium source during metalorganic chemical vapor deposition (MOCVD) growth. Diodes grown at low pressure (100 Torr) have a high onresistance, high turn-on voltage and display a snap-on effect. These observations are independent of MOCVD growth system where similar results were obtained for diodes with films grown at 100 Torr at multiple institutions. Similar results were also observed for diodes with films grown on sapphire or freestanding hydride vapor phase epitaxy (HVPE) GaN substrates. Adjusting growth pressure, temperature and V/III ratio can reduce carbon incorporation into the device layers. Diodes with device layers grown at a growth pressure of 500 Torr show low turn-on voltages and a low specific onresistance with a figure of merit (Vb/Ron) of ~ 261 MW/cm. One way of circumventing the carbon issue is to fabricate vertical diodes directly on freestanding HVPE substrates. Ohmic contacts on the N-polar GaN surface consist of a Ti/Al/Ni/Au metal stack annealed at a temperature of 750 oC for 30 s. Circular Schottky metal contacts consisting of a non-annealed Ni/Au metal stack are made to the Ga-polar surface. Metal diameters range from 30 – 300 microns. Carrier concentrations measured via capacitance-voltage techniques range between 10 10 cm. Shown in Figure 1, such diodes were able to achieve breakdown voltages of ~ 900 V; however, the overall figure of merit for power devices is lowered due to a high specific on-resistance. This high specific onresistance stems from the low-doped substrate. This is confirmed by fabricating front-side diodes on UID freestanding HVPE substrates, in which diodes show a reduced specific on-resistance compared to the vertical case. Ideally, GaN power Schottky diodes consist of a low-doped (< 10 cm) epitaxial film for the drift region on a highly conductive substrate, thereby permitting high breakdown voltages and reducing the overall series resistance through the substrate. This talk will focus on such vertical devices. Device layers grown by MOCVD and HVPE on multiple substrates including freestanding HVPE GaN substrates, as well as truly bulk GaN substrates grown by the ammonothermal growth technique, will be discussed. Results will be compared to lateral devices fabricated from GaN device layers grown on sapphire, as well as the previously mentioned diodes fabricated directly on low-doped freestanding HVPE GaN substrates.

Phone: (þ886) 3-4227151 ext: 25256, Fax: (þ886) 3-4252897InGaN/GaN quantum wells (QWs) grown at identical con-ditions on m-plane GaN and c-plane sapphire substrates werecharacterized by several techniques, aiming to clarify thereason for different emission wavelengths often observed insimilarLEDstructureswithm-andc-planesurfaceorientations.Cathodoluminescence (CL) spectra of m-plane QWs revealedshorter wavelength and no blueshift with increasing CL probecurrent in accordance with previous reports. Relative indiumcompositions were estimated by high-resolution X-ray diffrac-tion to be 5.1 and 13.9% for m-plane and c-plane QWs,respectively.Cross-sectionaltransmissionelectronmicroscopyimages revealed that the well widths of the m-plane QWs werenoticeably thicker than those of the c-plane QWs. The lowerindium compositions and thicker well widths of the m-planeQWs indicated that different indium incorporation anddiffusionoccurredinthestructuresgrownontheGaNsubstrate,which is attributed to its surface off-cut toward [0001] and thehigher thermal conductivity with respect to that of sapphire.

AlGaN/GaN/Be:GaN heterostructures have been grown by rf-plasma molecular beam epitaxy on freestanding semi-insulating GaN substrates, employing unintentionally-doped (UID) GaN buffer layers with thicknesses, d(sub UID), varying between 50 nm and 500 nm. We have found that the heterostructures with UID buffers thicker than 200 nm exhibit much improved Hall properties and inter-device isolation current compared to heterostructures with d(sub UID) < 200 nm. The output conductance of devices fabricated on these heterostructures increases as d(sub UID) decreases below 200 nm, and devices with gate lengths of 240 nm and 1 micron exhibited no significant difference in output conductance. Evidence of buffer trapping is observed in devices for which d(sub UID) smaller or equal to 100 nm. The observed effects are tentatively explained by the presence of parallel conduction paths in samples with non-optimized UID buffer thickness.

InAlN/GaN heterostructure field-effect transistors (HFETs) have been grown and fabricated on Fe-doped semi-insulating c-plane GaN substrates. The problematic parasitic leakage caused by interface charge between the epitaxial layers and the GaN substrate as well as any adverse effect of the substrate surface damage caused by the mechanical chemical polish employed on the substrates has been circumvented by using a combination of inductively coupled plasma dry etching and in situ H2 etching. As a result, the current leakage for 100 μm separation mesa-to-mesa was reduced down to 3×10−9 A/mm at 10 V voltage bias for a 320 μm mesa pad width normal to the current flow direction and the corresponding GaN buffer resistivity was about 3.5×108 Ω cm. Owing to the good thermal conductivity of GaN substrates, the HFETs exhibit much less current degradation, compared to those on a sapphire substrate, at high drain biases. Likewise, the dc and pulsed I-V characteristics were reasonably similar, suggestive of negligible ...

We report the growth and structural characteristics of green and yellow (529-576 nm) GaInN/GaN light emitting diodes (LEDs) grown on two types of c-plane substrates - bulk GaN and sapphire. In this longer wavelength range, depending on the substrate, we find different strain relaxation mechanisms within the GaInN/GaN quantum well (QW) region. In optimized epitaxy, structures on sapphire that contain a low density of threading dislocations (TDs) within the n-GaN show virtually no generation of additional misfit dislocations (MDs) (

We report on the growth of Al0.25Ga0.75N/GaN heterostructures grown on low dislocation density vicinal surfaces of semi-insulating c-axis GaN substrates. Atomic force microscopy (AFM), photoluminescence (PL), cathodoluminescence (CL), high-resolution x-ray diffraction (HRXRD), secondary-ion mass spectroscopy (SIMS), Hall effect, and Raman spectroscopy have been used to assess structural and electrical properties as a function of substrate offcut. Bulk GaN substrates with vicinal offcut between 0.5° and 1.4° are optimal with respect to surface roughness and dopant incorporation. AFM, PL, and CL show decreasing Mg incorporation with increasing offcut angle. Raman spectroscopy, used to analyze biaxial strain, confirms essentially strain-free heterostructure growth on vicinal substrates with offcut angles between 0.5° and 1.4° off [0001] toward $$ [1\overline{1} 00] $$ . Aluminum (Al) incorporation in the Al x Ga1−x N barrier assessed by Raman vibration is in excellent agreement with trends found by HRXRD.

The effect of m -plane GaN substrate miscut on the growth of InGaN/GaN quantum wells (QWs) was investigated. It was found that the miscut toward [0 0 0 1] c + -axis resulted in an increase of In incorporation efficiency and in a green-shift of the QW emission, while the miscut toward [1 1 2 0] a -axis resulted in even higher In compositions but it also led to an increased epitaxial surface roughness and deterioration of the QW structures. The results indicated that miscut toward a -axis is undesirable while miscut toward c + -axis is beneficial for achieving longer wavelength emission in QWs grown on m -plane GaN substrates.

GaInN/GaN multiple quantum well light-emitting diode structures in polar c -axis and non-polar m -axis growth have been compared in terms of luminescence properties. Grown under identical conditions, under low excitation density the c -axis structure has a luminescence maximum at 558 nm while the m -axis structure shows a maximum at 488 nm and shows superluminescence at 485 nm under high photoexcitation density. Under the same conditions, on increasing the excitation power, the peak intensity increases 40 fold in the m -axis structure without any variation of the emission wavelength. In similar but separately grown c -axis structures without a p-side, luminescence shifts from 555 nm at low excitation density to superluminescence at 485 nm under high excitation. The coincidence, of the superluminescence wavelength in the polar structure with the stable peak wavelength in the non-polar one, suggests that the wavelength shift in the polar structure is due to its piezoelectric polarization. The absence of such effects in the m -axis-grown structure therefore suggests a stronger dipole matrix element, potentially enabling higher quantum efficiencies and suitability for high efficiency light-emitting diode and laser diode designs in the green spectral region.

Thick c- plane (0 0 0 1)-oriented native GaN boules that have been produced by hydride vapor phase epitaxy and non-polar native m- plane (1 1 0 0) and a- plane (1 1 2 0) GaN substrates have been sliced from these crystals using a multiwire saw. An optimized polishing procedure was used to achieve a smooth epi-ready surface morphology on the finished substrates, with an RMS roughness of 0.43 nm. The non-polar substrates had two types of structural characteristics in appearance : one group was uniform, transparent and nearly colorless; while the second group had regions of varying coloration resulting from transecting V -shape pitting defects in the bulk GaN crystal. These regions had different cathodoluminescence properties but similar dislocation densities of 6 cm −2 . The native non-polar GaN substrates had orders of magnitude lower defect densities, including stacking faults, in comparison to heteroepitaxially grown quasi-substrates in non-polar direction on foreign substrates and subsequently delaminated. The structural characteristics demonstrated the current state-of-the-art in non-polar GaN substrate quality and additionally point at remaining improvement opportunities in substrate size and uniformity.

Electrical properties, Fe concentration, and deep centers in semi-insulating Fe-doped GaN substrates grown by hydride vapor phase epitaxy (HVPE) were characterized by temperature-dependent Hall-effect measurements, secondary ion mass spectroscopy, and thermally stimulated current (TSC) spectroscopy. Five adjacent samples from a low-[Fe] wafer displayed very high resistivity, dominated by a center at 0.94 eV. At least six traps were observed in the samples by TSC, with trap B (0.56-0.60 eV) being dominant. A metastable trap A1 at ∼0.82 eV appeared after white-light illumination at 300 K. A sample from a high-[Fe] wafer displayed a lower resistivity, dominated by a center at 0.58 eV. The largest TSC peak in this sample was trap A1, although trap B also appeared. These TSC traps are compared with deep-level-transient-spectroscopy traps reported in conductive epitaxial and bulk HVPE-GaN. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermal oxides on the Ga-face of low defect density bulk gallium nitride (GaN) were controllably produced under varying conditions and subsequently analyzed. The thermal oxidation was performed in a dry oxygen atmosphere at different temperatures and different oxidation times. Each oxide layer was identified as the monoclinic β-Ga2O3 by a θ–2θ X-ray diffraction (XRD) scan. Complementary Auger electron spectroscopy (AES) and high-resolution X-ray photoelectron spectroscopy (XPS) were also performed to study the chemical and electronic states of the oxide. The surfaces of the as-grown oxide and the GaN surfaces (after oxide removal) were studied using scanning electron microscopy (SEM). The roughness of both surfaces was found to increase with increasing oxidation temperature. Schottky diodes were fabricated on the GaN surfaces after oxide removal to further examine the surface quality. An excess reverse leakage current was found for Schottky diodes fabricated on GaN surfaces that were oxidized at 950 °C or 1000 °C, indicating possible surface decomposition at these two temperatures. The characteristics of wet and dry etching of the gallium oxide layer were also investigated. Finally, metal–oxide–semiconductor (MOS) capacitors were fabricated using thermally oxidized bulk GaN substrates. Current–voltage (I–V) measurements showed a low reverse current of 1.5 pA at −10 V. The oxide breakdown field was determined to be 0.65 MV/cm. Capacitance–voltage (C–V) measurements displayed a deep depletion feature, and the interface trap density near the conduction band was estimated to be in the low 1011 eV−1 cm−2 range.

Large gallium nitride (GaN) crystals were grown using a hydride vapor phase epitaxy (HVPE) technique and were processed into substrates for device applications. Polishing procedures were developed for GaN substrates to produce surfaces prepared for epitaxial growth. Surface preparation of (0001) and (0001) substrates was examined, along with preparation of (1 120) and (1100) non-polar surfaces. For all surfaces, chemical mechanical polishing (CMP) resulted in an average root mean square (RMS) surface roughness on a 5 μm x 5 μm scanning probe microscope (SPM) image of

We report on the structural and electrical properties of AlGaN/GaN heterostructures grown by molecular beam epitaxy on low-dislocation-density, free standing GaN substrates grown by hydride vapor phase epitaxy. Structural characterization by atomic force microscopy, transmission electron microscopy, and X-ray diffractometry reveal a smooth surface morphology, coherent interfaces, an absence of dislocations generated in the epitaxial layers, and narrow X-ray peaks. Hall measurements indicate room temperature electron mobilities of 1750 cm 2 /V s at sheet densities of 1.1×10 13 cm −2 . High electron mobility transistors exhibit excellent electrical characteristics, including output power densities of 4.8 W/mm at 10 GHz, off-state breakdown voltages of up to 200 V, and extrinsic cut-off frequencies of 36 GHz on devices with 0.45-μm gate lengths.

Nonpolar a- and m-plane bulk GaN sliced from boules : structural and optical characteristics

We investigate the role of substrate temperature and gallium flux on the DC and microwave properties of AlGaN/GaN high electron mobility transistors grown by molecular-beam epitaxy on free standing, hydride vapor phase epitaxy grown GaN substrates. The free-standing substrates have threading dislocation densities below 10 7 cm −2 . We find that AlGaN/GaN heterostructures with excellent properties may be grown within a wide range of substrate temperatures and fluxes. Electron Hall mobilities above 1700 cm 2 /V s and sheet resistances below 370 Ω/□ are typical. We are able to obtain high saturated drain currents with low gate leakage. Off-state breakdown voltages as high as 200 V with low drain and gate leakage currents have been measured. Further, we have measured microwave output power densities above 5 W/mm at 4 GHz with a power-added efficiency of 46% and an associated gain of 13.4 dB. We attribute improved electrical properties in these devices to the reduced threading dislocation density compared to those grown on non-native substrates.

The performance of high power transistor devices is intimately connected to the substrate thermal conductivity. In this study, the relationship between thermal conductivity and dislocation density is examined using the 3 omega technique and free standing HVPE GaN substrates. Dislocation density is measured using imaging cathodoluminescence. In a low dislocation density regime below 105 cm−2, the thermal conductivity appears to plateau out near 230 W/K m and can be altered by the presence of isotopic defects and point defects. For high dislocation densities the thermal conductivity is severely degraded due to phonon scattering from dislocations. These results are applied to the design of homoepitaxially and heteroepitaxially grown HEMT devices and the efficiency of heat extraction and the influence of lateral heat spreading on device performance are compared.

AlN/GaN heterostructures with 1700-cm2/V·s Hall mobility have been grown by molecular beam epitaxy on freestanding GaN substrates. Submicrometer gate-length (LG) metal-oxide-semiconductor (MOS) high-electron-mobility transistors (HEMTs) fabricated from this material show excellent dc and RF performance. LG = 100 nm devices exhibited a drain current density of 1.5 A/mm, current gain cutoff frequency fT of 165 GHz, a maximum frequency of oscillation fmax of 171 GHz, and intrinsic average electron velocity ve of 1.5 ×107 cm/s. The 40-GHz load-pull measurements of LG = 140 nm devices showed 1-W/mm output power, with a 4.6-dB gain and 17% power-added efficiency. GaN substrates provide a way of achieving high mobility, high ve, and high RF performance in AlN/GaN transistors.

Maskless pendeo-epitaxy involves the lateral and vertical growth of cantilevered "wings" of material from the sidewalls of unmasked etched forms. Gallium Nitride films grown at 1020°C via metalorganic vapor phase epitaxy on GaN / AlN /6 H - SiC (0001) substrates previously etched to form [Formula: see text]-oriented stripes exhibited similar vertical [0001] and lateral [Formula: see text] growth rates, as shown by cross-sectional scanning electron microscopy. Increasing the temperature increased the growth rate in the latter direction due to the higher thermal stability of the [Formula: see text] surface. The [Formula: see text] surface was atomically smooth under all growth conditions with a root mean square (RMS)=0.17 nm. High resolution X-ray diffraction and atomic force microscopy of the pendeo-epitaxial films confirmed transmission electron microscopy results regarding the significant reduction in dislocation density in the wings. This result is important for the properties of both optoelectronic and microelectronic devices fabricated in III-Nitride structures. Measurement of strain indicated that the wing material is crystallographically relaxed as evidenced by the increase in the c-axis lattice parameter and the upward shift of the E2 Raman line frequency. A strong D°X peak at 3.466 eV was also measured in the wing material. However, tilting of the wings of ≤0.15° occurred due to the tensile stresses in the stripes induced by the mismatch in the coefficients of thermal expansion between the GaN and the underlying substrate.

Boules and large wafers of the III-Nitrides of AlN, GaN and InN having a low density of dislocations are not available. As such, essentially all nitride films and device structures are grown on either sapphire or silicon carbide substrates containing a previously deposited buffer layer of GaN, AlN or AlGaN. These films grow via complex thermodynamically- and kinetically-controlled mechanisms and contain significant residual stresses and densities of defects that affect the properties of all optoelectronic and microelectronic devices produced in this materials system. It is the purpose of this paper to describe the challenges presented by the growth of these heteroepitaxial films and some of the recent results of research to understand the complex relationships between film growth, and stress and defect generation as well as the reduction in these mechanical and microstructural problems. Films of GaN grow on AlN/SiC substrates via the formation of a 1–1.5 nm thick wetting layer and the subsequent growth and coalescence of islands. These films are biaxially stressed. Increasing their thickness causes a gradual change in their average strain and their local strain from compression to tension due to the mismatch in lattice parameters between GaN and AlN and the mismatch in the coefficients of thermal expansion between GaN and SiC. A portion of the compressive stress is relieved within the first 20 nm due to the formation of misfit dislocations. Copious threading dislocations are also generated. Additional microstructural problems are presented by the SiC substrates that contain domains with varying size and tilt that are mimicked in the GaN films and that mask most variations in the FWHM of their X-ray rocking curves. Reduction in both the residual stresses and the dislocation density has been achieved via lateral overgrowth techniques.

Silicon carbide wafers contain domains with varying sizes and degrees of tilt. The present research has shown that this microstructure is mimicked in GaN films deposited on AlN-containing buffer layers and that it masks most variations in the FWHM of the X-ray rocking curves of the former. The shape and FWHM in the GaN curves are determined both by domain tilting and dislocation broadening; the latter was dominant in areas of reduced tilt. Analyses of the on- and off-axis X-ray data acquired from these regions of lower tilt revealed the marked effect of the higher density of edge dislocations on broadening. This effect decreased with increasing GaN thickness due to dislocation annihilation. The densities of edge dislocations in GaN films deposited at 1010°C on pitted, less pitted and very smooth AlN layers of the same thickness grown at 1010°C, 1130°C and 1220°C, respectively, were lowest and highest in those films grown on the last two respective layers. Additional studies showed that GaN films grow on Al 0.2 Ga 0.8 N layers via step-flow and possess a lower edge dislocation density than films grown via the Stranski–Krastanov mode on AlN because of the reduced misfit and the absence of boundaries between coalesced islands.

The surface morphologies of InGaN films grown at 780 °C by metalorganic vapor phase epitaxy were determined using atomic force microscopy. A qualitative model was developed to explain the observed instabilities in the step morphology of these films, namely, the formation of hillocks and v-defects that give rise to surface roughening. V-defects, observed at a surface density greater than 2×10 8 /cm 2 , are a result of interactions between moving surface steps, cores of screw-type dislocations, and two-dimensional islands of atoms that form on the terraces during growth at high surface undercooling. A delay in the formation of v-defects in InGaN to a nominal thickness of 10 nm was observed and associated with the ammonia partial pressure and the interactions between steps associated with hillock islands and cores of screw-type dislocations. Hillock formation was attributed to a transition in the thermodynamic mode of film growth, as three-dimensional islands nucleated on the cores of screw-type dislocations at a density of 2×10 8 /cm 2 . Explanations for the foregoing observations are based on growth model theory previously developed by Burton, Cabrera and Frank and on changes in the surface kinetics with temperature, In composition, and gas phase composition.

Surface pits in the form of v-shaped defects and resulting surface roughening, previously associated solely with InGaN films, were observed and investigated using atomic force microscopy on GaN films grown at 780°C via metalorganic vapor phase epitaxy on conventionally and pendeo-epitaxially deposited GaN thin film templates. The density of the v-shaped defects was similar to the density of threading dislocations of ∼3×10 9 cm −2 (that originate from the heteroepitaxial interface between the GaN template layer and the SiC substrate). Moreover, the v-defect density was diminished with decreases in the dislocation density via increases in the template layer thickness or the use of pendeo-epitaxial seed layers. A concomitant reduction in the full-width half-maxima of the X-ray rocking curves was also observed. A qualitative model is presented that describes the formation of v-shaped defects as a result of interactions between the movement of surface steps, screw-type dislocation cores, and clusters of atoms on the terraces that form under conditions of high surface undercooling.

Gallium nitride (GaN) films were grown on GaN(0001)/AlN/6H–SiC composite substrates at 700–780 °C by supersonic jet epitaxy using triethylgallium (TEG) and NH3. TEG was seeded in He and N2 supersonic free jets to obtain kinetic energies of ∼2.1 and ∼0.5 eV, respectively, and NH3 was supplied from a variable leak valve. Higher TEG beam intensities (by about a factor of 5) were obtained by seeding in He. In situ reflection high-energy electron diffraction indicated a transition from three-dimensional to two-dimensional (2D) growth between 730 and 750 °C for films grown using TEG seeded in He and a constant NH3/TEG flux ratio. Ex situ atomic force microscopy of films grown at 730 and 750 °C revealed smooth surfaces comprised of quasi-2D islands with irregular perimeters. Cross-sectional transmission electron microscopy evidenced that the film grown at 750 °C was homoepitaxial α-GaN with a high density of planar lattice defects. Secondary ion mass spectrometry detected high residual carbon concentrations in t...

Reflected light optical microscopy using a Nomarski prism and a differential interference contrast filter have been employed in concert to achieve a technique that provides an accurate color reference for thickness during the dimpling and ion milling of transparent transmission electron microscopy samples of 6H-SiC(0 0 0 1) wafers. The samples had thin films of AlN, GaN, and Au deposited on the SiC substrate. A sequence of variously colored primary and secondary interference bands was observed when the SiC was thinner than 20 μm using an optical microscope. The color bands were correlated with the TEM sample thickness as measured via scanning electron microscopy. The interference contrast was used to provide an indication of the dimpling rate, the ion milling rate, and also the most probable location of perforation, which are useful to reduce sample breakage. The application of pressure during the initial cross-sectional preparation reduced the separation of the two halves of the sample sandwich and resulted in increased shielding of the film surface from ion milling damage.

Pendeo-epitaxy employs lateral growth from etched seed forms to achieve a marked reduction in dislocation density in a material. In this research, high-resolution X-ray diffraction and atomic force microscopy of GaN stripes and the laterally grown wings confirmed transmission electron microscopy results regarding the reduction in dislocations in the latter regions. Micro-Raman and X-ray diffraction measurements showed the wings to be tilted /spl les/0.15/spl deg/ due to tensile stresses in the stripes induced primarily by the mismatch in the coefficients of thermal expansion between the GaN stripe and the SiC substrate. A strong, low-temperature D/spl deg/X peak at /spl ap/3.466 eV with a FWHM of /spl les/300 /spl mu/eV was measured in the wing material by micro-photoluminescence. Films grown at 1020/spl deg/C exhibited similar vertical [0001] and lateral [112~0] growth rates. Increasing the growth temperature increased the latter due to the higher thermal stability of the (112~0) GaN and initiated growth of spiral hillocks on the (0001) surface of the stripes. The latter were due to adatom diffusion to heterogeneous steps previously nucleated at the intersections of pure screw or mixed dislocations. The (112~0) surface was atomically smooth under all growth conditions with a root mean square roughness value of 0.17 nm.

Metal-organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) are the principal techniques for the growth and n-type (Si) and p-type (Mg) doping of III-nitride thin films on sapphire and silicon carbide substrates as well as previously grown GaN films. Lateral and pendeoepitaxy via MOVPE reduce significantly the dislocation density and residual strain in GaN and AlGaN films. However tilt and coalescence boundaries are produced in the laterally growing material. Very high electron mobilities in the nitrides have been realized in radio-frequency plasma-assisted MBE GaN films and in two-dimensional electron gases in the AlGaN/GaN system grown on MOVPE-derived GaN substrates at the crossover from the intermediate growth regime to the droplet regime. State-of-the-art Mg doping profiles and transport properties have been achieved in MBE-derived p-type GaN. The Mg-memory effect, and heterogeneous growth, substrate uniformity, and flux control are significant challenges for MOVPE and MBE, respectively. Photoluminescence (PL) of MOVPE-derived unintentionally doped (UID) heteroepitaxial GaN films show sharp lines near 3.478 eV due to recombination processes associated with the annihilation of free-excitons (FEs) and excitons bound to a neutral shallow donor (D/spl deg/X).

High-resolution x-ray diffraction (XRD) and atomic force microscopy (AFM) of pendeo-epitaxial (PE) GaN films confirmed transmission electron microscopy (TEM) results regarding the reduction in dislocations in the wings. Wing tilt ≤0.15° was due to tensile stresses in the stripes induced by thermal expansion mismatch between the GaN and the SiC substrate. A strong D°X peak at ≈3.466 eV (full-width half-maximum (FWHM) ≤ 300 µeV) was measured in the wing material. Films grown at 1020°C exhibited similar vertical [0001] and lateral [1120] growth rates. Increasing the temperature increased the latter due to the higher thermal stability of the GaN(1120). The (1130) surface was atomically smooth under all growth conditions with a root mean square (RMS) = 0.17 nm.

Investigations of the origins of surface roughening in GaN(0 0 0 1) have resulted in the development of a growth process route having an optimum temperature of 1020°C and a film thickness beyond 2.5 μm which results in films with the smoothest surface morphology. Atomic force microscopy (AFM) revealed uncoalesced GaN islands and hillocks for non-optimum temperatures below and above 1020°C, respectively. Uncoalesced islands were a result of insufficient lateral growth. Hillocks were a result of the rotation of heterogeneous steps formed at pure screw or mixed dislocations which terminated on the (0 0 0 1) surface. Growth of the latter features was controlled kinetically by temperature through adatom diffusion. The 10 6 cm −2 density of hillocks was reduced through growth on thick GaN templates and regions of pendeo-epitaxy (PE) overgrowth with lower pure screw or mixed dislocations. Smooth PE surfaces were obtained at temperatures that reduced the lateral to vertical growth rate but also retarded hillock growth that originated in the stripe regions. The (1 1 2 0) PE sidewall surface was atomically smooth, with a root mean square roughness value of 0.17 nm, which was the noise limited resolution of the AFM measurements.

Gallium nitride crystals grown via vapor-phase transport processes that incorporate ammonia as the only source of nitrogen below atmospheric pressures experience significant surface roughening and the eventual cessation of growth. Investigations of these phenomena in this research, and in the context of the discovery of a non-ceasing process route to larger GaN crystals, showed that the RMS values of the surface roughness of single crystal GaN (0 0 0 1) films exposed to pure ammonia flowing at 60 sccm for 2 h at 1130°C increased from the as-received value of 3.7–6.8 nm, 21.4 and 32.6 nm at 100, 430 and 760 Torr, respectively. Quadrupole mass spectrometry revealed that the concentrations of H 2 and N 2 measurably increased at pressures above 400 Torr. The primary reason for the increased roughness above 430 Torr was the enhanced etching of GaN via reaction with atomic and molecular hydrogen derived from the dissociation of the ammonia. At lower pressures, the decomposition of the GaN via the formation and evaporation of N 2 and Ga increased in importance relative to etching for enhancing surface roughness. Dilution with nitrogen reduced the amount of hydrogen generated from the dissociation of the ammonia. The GaN surface annealed at 1130°C and 430 Torr in ammonia diluted with 33 vol% N 2 maintained the smoothest surface with a nominal RMS value of 10.4 nm. Mixtures with higher and lower percentages of N 2 showed enhanced roughness under the same conditions. Use of this optimum gas mixture also allowed the seeded growth of a 1.5×1.5×2.0 mm 3 GaN crystal and a 2.3×1.8×0.3 mm 3 thick platelet with neither observable decomposition nor cessation of the growth over periods of 36 and 48 h, respectively.

Schottky contacts of Pt(111) and Au(111) were deposited on chemical-vapor-cleaned, n-type GaN(0001) thin films. The growth mode of the deposition, as determined by x-ray photoelectron spectroscopy analysis, followed the two-dimensional Frank–van der Merwe growth model. The resulting as-deposited metal films were monocrystalline and epitaxial with a (111)//(0002) relationship with the GaN. Selected samples were annealed for three minutes at 400 °C, 600 °C or 800 °C. The rectifying behavior of both contacts degraded at 400 °C; they became ohmic after annealing at 600 °C (Au) or 800 °C (Pt). High-resolution transmission electron micrographs revealed reactions at the metal/GaN interfaces for the higher temperature samples. X-ray diffraction results revealed an unidentified phase in the Pt sample annealed at 800 °C. A decrease in the room temperature in-plane (111) lattice constant for both metals, ranging from −0.1% to −0.5%, was observed as the annealing temperature was increased from 400 to 800 °C. This plastic deformation was caused by tensile stresses along the [111] direction that exceeded the yield strength as a result of the large differences in the coefficients of thermal expansion between the metal contacts and the GaN film.

Vertical Schottky rectifiers were fabricated on two GaN wafers synthesised by hydride vapor phase epitaxy (HVPE) process: n- epilayer on n+ bulk and n- bulk. Full backside ohmic contact and circular Schottky contacts were deposited on the N face and Ga face of the substrate, respectively. The devices showed excellent electrical characteristics. The device fabricated on n- epilayer on n+ bulk substrate showed a breakdown voltage of 348 V, a specific on-state resistance of 4.2 mΩ·cm2 and a leakage current density of 9.2×10-6 A/cm2 at -90 V. The device fabricated on n- bulk substrate showed a breakdown voltage of 600 V, a specific on-state resistance of 1.3 mΩ·cm2 and a leakage current density of 8.1×10-8 A/cm2 at -100 V. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The economic and energy efficiency promise of solid state lighting drive a continuous need to reduce the cost and improve the efficiency of visible LEDs. Current III-N LED manufacturing is dominated by non-native substrate approaches which result in strained epitaxial films, high dislocation density in the buffer and device active regions, and large epiwafer bow, which together present major limitations in terms of LED device yield, performance, and cost. Large area sapphire substrates (6”, 8”, and beyond) are becoming available and present cost reduction potential yet they are even more vulnerable to strain and bow related issues. We present a novel approach to realizing a zero-bow relativelythick GaN on sapphire template that can be made at essentially any diameter and which bodes well for improved device yield, performance, and cost. The elimination of bow is achieved through a simple cost-effective stress balancing technique involving backside deposition. The result is a large area GaN template which is flat at all temperatures and which has 10× lower dislocation density than current GaN buffers. We report on the properties of these templates and provide device data for LED structures grown thereon.

Measurement of strain in GaN films grown via pendeo-epitaxy (PE) indicates that the overgrowth, or wing, material is crystallographically relaxed. An increase of ≈0.02% in the c-axis lattice parameter of the wing material was measured via high-resolution X-ray diffraction (HRXRD); additional evidence for this increase was indicated by an upward shift of the E2 Raman line frequency. Atomic force microscopy studies revealed a reduction in the density of mixed-type dislocations in the wing. A reduction in screw-type dislocations in the wings with respect to the stripes is indicated by a reduction in HRXRD rocking curve FWHM of the (0002) reflections from 646 to 354 arcsec. The off-axis FWHM of the wing area was 126 arcsec compared to 296 arcsec for the stripe indicating a reduction in the edge-type dislocations as well. Pendeo-epitaxy growth of wings off the (110) surface of a GaN stripe produced a material that is crystallographically relaxed, contains fewer defects compared to the stripe and is atomically smooth on the (110) surface.

Single-crystal wurtzitic GaN(0001) films have been grown via conventional methods on high-temperature AlN(0001) buffer layers previously deposited on 3C-SiC(111)/Si(111) substrates using metal organic vapor phase epitaxy (MOVPE). Formation of the 3C-SiC transition layer employed a carburization step and the subsequent deposition of epitaxial 3C-SiC(111) on the Si(111) surface using atmospheric pressure chemical vapor deposition (APCVD) for both processes. Similar films, except with significantly reduced dislocation densities, have been grown via pendeo-epitaxy (PE) from the (1120) sidewalls of silicon nitride masked, raised, rectangular, and [1100] oriented GaN stripes etched from films conventionally grown on similarly prepared, Si-based, multilayer substrates. The FWHM of the (0002) X-ray diffraction peak of the conventionally grown GaN was 1443 arcsec. The FWHM of the photoluminescence (PL) spectra for the near band-edge emission on these films was 19 meV. Tilting in the coalesced PE-grown GaN epilayers of 0.2 was confined to the areas of lateral overgrowth over the masks; no tilting was observed in the material suspended above the trenches. The strong, low-temperature PL band-edge peak at 3.456eV with an FWHM of 17meV in the PE films was comparable to that observed in PE GaN films grown on AlN/6H-SiC(0001) substrates.

Monocrystalline GaN and Al x Ga 1− x N films have been grown via the pendeo-epitaxy (PE) 1 technique with and without Si 3 N 4 masks on GaN/AlN/6H-SiC(0 0 0 1) and GaN(0 0 0 1)/AlN(0 0 0 1)/3C-SiC(1 1 1)/Si(1 1 1) substrates using organometallic vapor phase deposition. Scanning and transmission electron microscopies were used to evaluate the external microstructures and the distribution of dislocations, respectively. The dislocation densities in the PE grown films were reduced at least five orders of magnitude relative to the initial GaN seed layers. Tilting to 0.2° in the portion of the coalesced GaN epilayers grown over the silicon nitride masks was observed via X-ray diffraction. Neither tilting nor low angle boundaries were observed within areas of coalescence in the material grown on substrates without the masks. The strong, low-temperature PL band-edge peak at 3.45 eV with a FWHM of 17 meV was comparable to that observed in PE GaN films grown on 6H-SiC(0 0 0 1). The band-edge in the GaN grown on AlN(0 0 0 1)/SiC(1 1 1)Si(1 1 1) substrates was shifted to a lower energy by 10 meV, indicative of a greater tensile stress.

We report the development of non-polar light emitting diodes (LEDs) covering the emission spectra from 480 to 520 nm (dominant wavelength), i.e. from cyan to the green region. The devices are obtained via GaInN-based homoepitaxy on non-polar m -plane GaN bulk substrate. Owing to the absence of piezoelectric polarization, these LEDs exhibit stable emission color with a wavelength shift less than 3 nm when the drive current density is changed from 0.1–30 A/cm2. However, we observe a decreasing electroluminescence efficiency as the emission wavelengths increases from cyan to green. We tentatively attribute this to a higher density of defects in the longer wavelength structures. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

M-plane non-polar bulk GaN substrates were regrown by hydride vapour phase epitaxy (HVPE), aiming to enlarge their lateral size. The non-polar GaN substrates were synthesized by growing thick GaN boules by HVPE along the c-axis, and wafering transversely to expose non-polar (m- or a-plane) surfaces. Non-polar GaN substrates obtained in this manner contains lower defect density comparing to those produced by heteroepitaxial growth. After the regrowth, substantial lateral expansion in [0001], [1120] and [1120] was observed. Threading dislocation densities were estimated by cathodoluminescence measured at the cross-section areas. The dislocation densities in the substrate and the regrown areas were approximately 5×106 cm–2 and 1×106 cm–2, respectively, indicating a comparable structural quality achieved by the regrowth. Non-polar GaN substrates prepared by this process provide a promising method to fabricate devices for next generation. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

AlxGa1-xN (x=0.05) ultraviolet (UV) avalanche photodiodes grown on a GaN substrate are reported. The epitaxial structure was grown by metal-organic chemical vapor deposition on a free-standing bulk GaN substrate having low dislocation density. The growth conditions for AlxGa1-xN epitaxial layers on GaN substrates were optimized to achieve improved crystalline and structural quality. With UV illumination at lambda~250 nm, devices with mesa diameters of ~30 mum achieve stable maximum optical gains of ~50 at a reverse bias voltage of ~87 V.