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We report a comparative study of three rectifying gate metals, W, Pd, and Pt/Au, on ultrawide bandgap Al0.86Ga0.14N barrier/Al0.7Ga0.3N channel high electron mobility transistors for use in extreme temperatures. The transistors were electrically characterized from 30 to 600 °C in air. Of the three gate metals, the Pt/Au stack exhibited the smallest change in threshold voltage (0.15 V, or 9% change between the 30 and 600 °C values, and a maximum change of 42%), the highest on/off current ratio (1.5 × 106) at 600 °C, and a modest forward gate leakage current (0.39 mA/mm for a 3 V gate bias) at 600 °C. These favorable results showcase AlGaN channel high electron mobility transistors' ability to operate in extreme temperature environments. [ABSTRACT FROM AUTHOR]

The impact of dry-etch-induced defects on the electrical performance of regrown, c-plane, GaN p-n diodes where the p-GaN layer is formed by epitaxial regrowth using metal-organic, chemical-vapor deposition was investigated. Diode leakage increased significantly for etched-and-regrown diodes compared to continuously grown diodes, suggesting a defect-mediated leakage mechanism. Deep level optical spectroscopy (DLOS) techniques were used to identify energy levels and densities of defect states to understand etch-induced damage in regrown devices. DLOS results showed the creation of an emergent, mid-gap defect state at 1.90 eV below the conduction band edge for etched-and-regrown diodes. Reduction in both the reverse leakage and the concentration of the 1.90 eV mid-gap state was achieved using a wet chemical treatment on the etched surface before regrowth, suggesting that the 1.90 eV deep level contributes to increased leakage and premature breakdown but can be mitigated with proper post-etch treatments to achieve >600 V reverse breakdown operation. [ABSTRACT FROM AUTHOR]

Inherent advantages of wide bandgap materials make GaN-based devices attractive for power electronics and applications in radiation environments. Recent advances in the availability of waferscale, bulk GaN substrates have enabled the production of high quality, low defect density GaN devices, but fundamental studies of carrier transport and radiation hardness in such devices are lacking. Here, we report measurements of the hole diffusion length in low threading dislocation density (TDD), homoepitaxial n-GaN, and high TDD heteroepitaxial n-GaN Schottky diodes before and after irradiation with 2.5MeV protons at fluences of 4–6 X 10-13 protons/cm2. We also characterize the specimens before and after irradiation using electron beam-induced-current (EBIC) imaging, cathodoluminescence, deep level optical spectroscopy (DLOS), steady-state photocapacitance, and lighted capacitance-voltage (LCV) techniques. We observe a substantial reduction in the hole diffusion length following irradiation (50%–55%) and the introduction of electrically active defects which could be attributed to gallium vacancies and associated complexes (VGa-related), carbon impurities (C-related), and gallium interstitials (Gai). EBIC imaging suggests long-range migration and clustering of radiation-induced point defects over distances of ~500 nm, which suggests mobile Gai. Following irradiation, DLOS and LCV reveal the introduction of a prominent optical energy level at 1.9 eV below the conduction band edge, consistent with the introduction of Gai. [ABSTRACT FROM AUTHOR]

Understanding the impact of high-energy electron radiation on device characteristics remains critical for the expanding use of semiconductor electronics in space-borne applications and other radiation harsh environments. Here, we report on in situ measurements of high-energy electron radiation effects on the hole diffusion length in low threading dislocation density homoepitaxial bulk n-GaN Schottky diodes using electron beam induced current (EBIC) in high-voltage scanning electron microscopy mode. Despite the large interaction volume in this system, quantitative EBIC imaging is possible due to the sustained collimation of the incident electron beam. This approach enables direct measurement of electron radiation effects without having to thin the specimen. Using a combination of experimental EBIC measurements and Monte Carlo simulations of electron trajectories, we determine a hole diffusion length of 264 ± 11 nm for n-GaN. Irradiation with 200 kV electron beam with an accumulated dose of 24 × 1016 electrons cm−2 led to an approximate 35% decrease in the minority carrier diffusion length. [ABSTRACT FROM AUTHOR]

The growth temperature dependence of Si doping efficiency and deep level defect formation was investigated for n-type Al0.7Ga0.3N. It was observed that dopant compensation was greatly reduced with reduced growth temperature. Deep level optical spectroscopy and lighted capacitance-voltage were used to understand the role of acceptor-like deep level defects on doping efficiency. Deep level defects were observed at 2.34 eV, 3.56 eV, and 4.74 eV below the conduction band minimum. The latter two deep levels were identified as the major compensators because the reduction in their concentrations at reduced growth temperature correlated closely with the concomitant increase in free electron concentration. Possible mechanisms for the strong growth temperature dependence of deep level formation are considered, including thermodynamically driven compensating defect formation that can arise for a semiconductor with very large band gap energy, such as Al0.7Ga0.3N. [ABSTRACT FROM AUTHOR]

Current-voltage (IV) characteristics of two AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) with differing densities of open-core threading dislocations (nanopipes) are analyzed. A three-diode circuit is simulated to emulate the forward-bias IV characteristics of the DUV-LEDs, but is only able to accurately model the lower leakage current, lower nanopipe density DUV-LED. It was found that current leakage through the nanopipes in these structures is rectifying, despite nanopipes being previously established as inherently n-type. Using defect-sensitive etching, the nanopipes are revealed to terminate within the p-type GaN capping layer of the DUV-LEDs. The circuit model is modified to account for another p-n junction between the n-type nanopipes and the p-type GaN, and an excellent fit to the forward-bias IV characteristics of the leaky DUVLED is achieved. [ABSTRACT FROM AUTHOR]

Electrical current transport through leakage paths in AlGaN-based deep ultraviolet (DUV) lightemitting diodes (LEDs) and their effect on LED performance are investigated. Open-core threading dislocations, or nanopipes, are found to conduct current through nominally insulating Al0.7Ga0.3N layers and limit the performance of DUV-LEDs. A defect-sensitive phosphoric acid etch reveals these open-core threading dislocations in the form of large, micron-scale hexagonal etch pits visible with optical microscopy, while closed-core screw-, edge-, and mixed-type threading dislocations are represented by smaller and more numerous nanometer-scale pits visible by atomic-force microscopy. The electrical and optical performances of DUV-LEDs fabricated on similar Si-doped Al0.7Ga0.3N templates are found to have a strong correlation to the density of these nanopipes, despite their small fraction (<0.1% in this study) of the total density of threading dislocations. [ABSTRACT FROM AUTHOR]

The carrier trapping properties and current transport behavior of Ni/n-Al0.30Ga0.70N Schottky diodes were quantitatively characterized by a combination of deep level optical spectroscopy (DLOS), thermally based deep level transient spectroscopy (DLTS), current-voltage-temperature (I-V-T), and internal photoemission (IPE) measurements. High quality Schottky diode behavior was observed with an IPE-determined barrier height of 1.66 eV and the observed temperature-independent reverse leakage current behavior was found to be consistent with field emission in reverse bias and thermionic-field emission in forward bias as the dominant transport mechanisms. The trap spectroscopy measurements revealed the presence of several bandgap states located at EC-0.9 eV (seen by both DLOS and DLTS), EC-1.5, EC-3.11, and EC-3.93 eV-all via DLOS. The EC-3.10 level, which is present in very high concentration, is found to correlate with the energy position expected for the cation vacancy in AlGaN, based on the vacuum referred binding energy model for the AlxGa1-xN alloy. The relatively shallow trap at EC-3.93/EV+0.15 eV, which is possibly Mg-related, is also present in significant concentration. The total observed trap concentration in this sample is in excess of the net doping extracted from capacitance-voltage, which will likely impact device behavior and is consistent with the observed I-V-T behavior. [ABSTRACT FROM AUTHOR]

To examine further the strain relaxation produced by inclined threading dislocations in AlGaN, a heterostructure with three AlGaN layers having successively increasing Ga contents and compressive strains was grown on an AlN template layer by metalorganic vapor-phase epitaxy. The strain state of the layers was determined by x-ray diffraction (XRD) and the dislocation microstructure was characterized with transmission electron microscopy (TEM). As the GaN mole fraction of the heterostructure increased from 0.15 to 0.48, the increased epitaxial strain produced inclined dislocations with successively greater bend angles. Using the observed bend angles, which ranged from 6.7° to 17.8°, the measured strain relaxation within each layer was modeled and found to be accounted for by threading-dislocation densities of 6–7×109/cm2, in reasonable agreement with densities determined by TEM and XRD. In addition to the influence of lattice-mismatch strain on the average bend angle, we found evidence that local strain inhomogeneities due to neighboring dislocations influence the specific bend angles of individual dislocations. This interaction with local strain fields may contribute to the large spread in the bend angles observed within each layer. A detailed TEM examination found that the initial bending of threading dislocations away from vertical often occurs at positions within <15 nm of the AlGaN/AlN heterointerface. Under the assumption that dislocation climb mediated by bulk-defect diffusion is effectively suppressed at the growth temperature, this result implies that inclination is established by processes occurring at the dynamic growth surface. We describe a mechanism where dislocation bending occurs by means of dislocation-line jogs created when surface steps overgrow vacancies that attach to threading-dislocation cores at their intersection with the growth surface. [ABSTRACT FROM AUTHOR]

A contacted electroreflectance technique was used to investigate AlGaN/GaN heterostructures and their intrinsic electric field-induced properties. By studying variations in the electroreflectance with applied field, spectral features associated with the AlGaN barrier, the two-dimensional electron gas at the interface, and bulk GaN were identified. Barrier-layer composition and electric field were determined from the AlGaN Franz–Keldysh oscillations. For a high mobility heterostructure grown on SiC, measured AlGaN polarization electric field and two-dimensional electron gas density approached values predicted by a standard bandstructure model. The two-dimensional electron gas produced a broad, field-tunable first derivative electroreflectance feature. With a dielectric function calculation, we describe the line shape and relative amplitude of the two-dimensional electron gas electroreflectance feature for a wide range of electron density and applied field values. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report a theoretical study of the structural, electronic, and optical properties of hBN-AlN superlattice (SL) heterostructures using a first-principles approach based on standard and hybrid density functional theory. We consider a short-period (L < 10 nm) SL and find that its properties depend strongly on the AlN layer thickness LAlN. For L A l N ≲ 1 nm , AlN stabilizes into the hexagonal phase and the SL displays insulating behavior with type II interface band alignment and optical gaps as small as 5.2 eV. The wurtzite phase is formed for thicker AlN layers. In these cases, built-in electric fields lead to the formation of polarization compensating charges as well as two-dimensional conductive behavior for electronic transport along interfaces. We also find defect-like states localized at interfaces which are optically active in the visible range. [ABSTRACT FROM AUTHOR]

GaN p-n diodes were formed by selective area regrowth on freestanding GaN substrates using a dry etch, followed by post-etch surface treatment to reduce etch-induced defects, and subsequent regrowth into wells. Etched-and-regrown diodes with a 150 µm diameter achieved 840 V operation at 0.5 A/cm² reverse current leakage and a specific on-resistance of 1.2 mΩ·cm². Etched-andregrown diodes were compared with planar, regrown diodes without etching on the same wafer. Both types of diodes exhibited similar forward and reverse electrical characteristics, which indicate that etch-induced defectivity of the junction was sufficiently mitigated so as not to be the primary cause for leakage. An area dependence for forward and reverse leakage current density was observed, suggesting that the mesa sidewall provided a leakage path. [ABSTRACT FROM AUTHOR]

GaN p‐n diodes were formed by selective area regrowth on freestanding GaN substrates using a dry etch, followed by post‐etch surface treatment to reduce etch‐induced defects, and subsequent regrowth into wells. Etched‐and‐regrown diodes with a 150 μm diameter achieved 840 V operation at 0.5 A/cm2 reverse current leakage and a specific on‐resistance of 1.2 mΩ·cm2. Etched‐and‐regrown diodes were compared with planar, regrown diodes without etching on the same wafer. Both types of diodes exhibited similar forward and reverse electrical characteristics, which indicate that etch‐induced defectivity of the junction was sufficiently mitigated so as not to be the primary cause for leakage. An area dependence for forward and reverse leakage current density was observed, suggesting that the mesa sidewall provided a leakage path. [ABSTRACT FROM AUTHOR]

The use of metal-organic chemical vapor deposition at high temperature is investigated as a means to produce epitaxial hexagonal boron nitride (hBN) at the wafer scale. Several categories of hBN films were found to exist based upon precursor flows and deposition temperature. Low, intermediate, and high NH 3 flow regimes were found to lead to fundamentally different deposition behaviors. The low NH 3 flow regimes yielded discolored films of boron sub-nitride. The intermediate NH 3 flow regime yielded stoichiometric films that could be deposited as thick films. The high NH 3 flow regime yielded self-limited deposition with thicknesses limited to a few mono-layers. A Langmuir-Hinshelwood mechanism is proposed to explain the onset of self-limited behavior for the high NH 3 flow regime. Photoluminescence characterization determined that the intermediate and high NH 3 flow regimes could be further divided into low and high temperature behaviors with a boundary at 1500 °C. Films deposited with both high NH 3 flow and high temperature exhibited room temperature free exciton emission at 210 nm and 215.9 nm. [ABSTRACT FROM AUTHOR]

AlGaN:Si epilayers with uniform Al compositions of 60%, 70%, 80%, and 90% were grown by metal-organic vapor phase epitaxy along with a compositionally graded, unintentionally doped (UID) AlGaN epilayer with the Al composition varying linearly between 80% and 100%. The resistivity of AlGaN:Si with a uniform composition increased significantly for the Al content of 80% and greater, whereas the graded UID-AlGaN film exhibited resistivity equivalent to 60% and 70% AlGaN:Si owing to polarization-induced doping. Deep level defect studies of both types of AlGaN epilayers were performed to determine why the electronic properties of uniform-composition AlGaN:Si degraded with increased Al content, while the electronic properties of graded UID-AlGaN did not. The deep level density of uniform-composition AlGaN:Si increased monotonically and significantly with the Al mole fraction. Conversely, graded-UID AlGaN had the lowest deep level density of all the epilayers despite containing the highest Al composition. These findings indicate that Si doping is an impetus for point defect incorporation in AlGaN that becomes stronger with the increasing Al content. However, the increase in deep level density with the Al content in uniform-composition AlGaN:Si was small compared to the increase in resistivity. This implies that the primary cause for increasing resistivity in AlGaN:Si with the increasing Al mole fraction is not compensation by deep levels but rather increasing activation energy for the Si dopant. The graded UID-AlGaN films maintained low resistivity because they do not rely on thermal ionization of Si dopants. [ABSTRACT FROM AUTHOR]

Simulations of reverse breakdown behavior of GaN power diodes with junction termination extensions (JTEs) are presented. The p-type JTE is located at the edge of the main p-n-junction, and under reverse bias, the charge in the JTE causes spreading and reduction of the peak electric fields to avoid premature avalanche breakdown. To determine the available charge in the JTE, it is shown that the electric field under reverse bias causes severe band bending within the JTE and full ionization of the Mg acceptor. Therefore, all the Mg dopants contribute charge and determine the performance of the JTE. The dependence of the breakdown voltage on the JTE’s acceptor concentration and thickness is shown. When the JTE is properly designed, the simulations show improved reverse breakdown behavior and breakdown efficiencies approaching 98% of the ideal limit for planar geometry. Finally, the challenges of creating JTEs within GaN power diodes are discussed. [ABSTRACT FROM PUBLISHER]

Unintentionally doped (UID) AlGaN epilayers graded over Al compositions of 80%–90% and 80%–100% were grown by metal organic vapor phase epitaxy and were electrically characterized using contactless sheet resistance (Rsh) and capacitance-voltage (C–V) measurements. Strong electrical conductivity in the UID graded AlGaN epilayers resulted from polarization-induced doping and was verified by the low resistivity of 0.04 Ω cm for the AlGaN epilayer graded over 80%–100% Al mole fraction. A free electron concentration (n) of 4.8 × 1017 cm−3 was measured by C–V for Al compositions of 80%–100%. Average electron mobility (...) was calculated from Rsh and n data for three ranges of Al composition grading, and it was found that UID AlGaN graded from 88%–96% had ... = 509 cm2/V s. The combination of very large band gap energy, high ..., and high n for UID graded AlGaN epilayers make them attractive as a building block for high voltage power electronic devices such as Schottky diodes and field effect transistors. [ABSTRACT FROM AUTHOR]

An AlN barrier high electron mobility transistor (HEMT) based on the AlN/Al0.85Ga0.15N heterostructure was grown, fabricated, and electrically characterized, thereby extending the range of Al composition and bandgap for AlGaN channel HEMTs. An etch and regrowth procedure was implemented for source and drain contact formation. A breakdown voltage of 810V was achieved without a gate insulator or field plate. Excellent gate leakage characteristics enabled a high Ion/Ioff current ratio greater than 107 and an excellent subthreshold slope of 75 mV/decade. A large Schottky barrier height of 1.74 eV contributed to these results. The room temperature voltage-dependent 3-terminal off-state drain current was adequately modeled with Frenkel-Poole emission. [ABSTRACT FROM AUTHOR]

Vertical GaN power diodes with a bilayer edge termination (ET) are demonstrated. The GaN p-n junction is formed on a low threading dislocation defect density ( 10^4-10^5 cm ^-2 ) GaN substrate, and has a 15- \mu \textm -thick n-type drift layer with a free carrier concentration of 5\times 10^15 cm ^-3 . The ET structure is formed by $N$ implantation into the p+-GaN epilayer just outside the p-type contact to create compensating defects. The implant defect profile may be approximated by a bilayer structure consisting of a fully compensated layer near the surface, followed by a 90% compensated (p) layer near the n-type drift region. These devices exhibit avalanche breakdown as high as 2.6 kV at room temperature. Simulations show that the ET created by implantation is an effective way to laterally distribute the electric field over a large area. This increases the voltage at which impact ionization occurs and leads to the observed higher breakdown voltages. [ABSTRACT FROM AUTHOR]

We use planar metamaterial resonators to enhance by more than two orders of magnitude the near infrared second harmonic generation obtained from intersubband transitions in III-Nitride heterostructures. The improvement arises from two factors: employing an asymmetric double quantum well design and aligning the resonators' cross-polarized resonances with the intersubband transition energies. The resulting nonlinear metamaterial operates at wavelengths where single photon detection is available, and represents a different class of sources for quantum photonics related phenomena. [ABSTRACT FROM AUTHOR]

Visible- and solar-blind detection was demonstrated using Al0.45Ga0.55N/Al0.30Ga0.70N and Al0.85Ga0.25N/Al0.70Ga0.30N high electron mobility transistors (HEMTs), respectively. Peak responsivities (S) of 3.9 × 106 A/W in the saturation mode and 6.2 × 104 A/W in the pinch-off mode were observed for the visible-blind Al0.45Ga0.55N/Al0.30Ga0.70N HEMT, and a peak S of 4.9 × 104 A/W was observed for the solar-blind Al0.85Ga0.15N/Al0.70Ga0.30N HEMT in the saturation mode. Spectrally resolved photocurrent investigation indicated that sub-bandgap absorption by defect states was the primary origin of the HEMTs' photoresponse. Defect-mediated responsivity caused slow photocurrent rise and fall times, but electrical pulsing was used to improve the bandwidth at the cost of optical gain. Operating HEMTs in this dynamic mode achieved a 25 Hz bandwidth with S = 2.9 × 105 A/W in accumulation and S = 2.0 × 104 A/W in pinch-off for visible-blind detection and S = 5.1 × 103 A/W for solar-blind detection. [ABSTRACT FROM AUTHOR]

We report the realization of quantum cascade (QC) light emission in the III-nitride material system, designed with effective interface grading (EIG). EIG induces a continuous transition between wells and barriers in the quantum confinement, which alters the eigenstate system and even delocalizes the states with higher energy. Fully transverse-magnetic spontaneous emission is observed from the fabricated III-nitride QC structure, with a center wavelength of ~4.9 μm and a full width at half maximum of ~110 meV, both in excellent agreement with theoretical predictions. A multi-peak photo-response spectrum is also measured from the QC structure, which again agrees well with theoretical calculations and verifies the effects of EIG. [ABSTRACT FROM AUTHOR]

Epitaxial (111) MgO films were prepared on (0001) AlxGa1-xN via molecular-beam epitaxy for x=0 to x=0.67. Valence band offsets of MgO to AlxGa1-xN were measured using X-ray photoelectron spectroscopy as 1.6560.07 eV, 1.3660.05 eV, and 1.0560.09 eV for x=0, 0.28, and 0.67, respectively. This yielded conduction band offsets of 2.75 eV, 2.39 eV, and 1.63 eV for x=0, 0.28, and 0.67, respectively. All band offsets measured between MgO and AlxGa1-xN provide a>1 eV barrier height to the semiconductor. [ABSTRACT FROM AUTHOR]

Electrical current leakage paths in AlGaN-based ultraviolet (UV) light-emitting diodes (LEDs) are identified using conductive atomic force microscopy. Open-core threading dislocations are found to conduct current through insulating Al0.7Ga0.3N layers. A defect-sensitive H3PO4 etch reveals these open-core threading dislocations as 1-2 µm wide hexagonal etch pits visible with optical microscopy. Additionally, closed-core threading dislocations are decorated with smaller and more numerous nanometer-scale pits, which are quantifiable by atomic-force microscopy. The performances of UV-LEDs fabricated on similar Si-doped Al0.7Ga0.3N templates are found to have a strong correlation to the density of these electrically conductive open-core dislocations, while the total threading dislocation densities of the UV-LEDs remain relatively unchanged. [ABSTRACT FROM AUTHOR]

High hole concentrations in AlxGa1-xN become increasingly difficult to obtain as the concentration of Al increases. It is well known in GaN and related alloys that hole concentration is directly affected by compensation and extended defects. Using electron paramagnetic resonance (EPR) spectroscopy, we studied the amount of neutral Mg in AlxGa1-xN with x = 0 to 0.28. 0.4-0.9 μm thick Mg-doped AlxGa1-xN films were grown by metal-organic chemical vapour deposition and annealed at 900 °C anneal in N2. EPR measurements indicate that the amount of neutral Mg decreased by 60% in AlxGa1-xN films for x = 0.18 and 0.28 as compared to x=0.00 and 0.08. Experiments also showed that the lower neutral Mg for higher Al compositions trend did not depend on threading dislocation densities in the range of 3-20x109 cm-2, capping the surface with 5 nm of P+ GaN, or detailed annealing conditions. Additional studies show that oxygen and carbon concentrations are insufficient to account for the decrease in neutral Mg observed in the samples. Although the study cannot isolate the cause for the decrease in neutral Mg, the results clearly demonstrate that the acceptor concentration decreases with increasing Al, providing an additional limitation to achieving high hole densities. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Layer disordering and doping compensation of an Al0.028Ga0.972N/AlN superlattice by implantation are demonstrated. The as-grown sample exhibits intersubband absorption at ~ 1.56 i m which is modified when subject to a silicon implantation. After implantation, the intersubband absorption decreases and shifts to longer wavelengths. Also, with increasing implant dose, the intersubband absorption decreases. It is shown that both layer disordering of the heterointerfaces and doping compensation from the vacancies produced during the implantation cause the changes in the intersubband absorption. Such a method is useful for removing absorption in spatially defined areas of III-nitride optoelectronic devices by, for example, creating low-loss optical waveguides monolithically that can be integrated with as-grown areas operating as electro-absorption intersubband modulators. [ABSTRACT FROM AUTHOR]

The improvement in light extraction efficiency from reflective scattering structures in AlGaN ultraviolet light-emitting diodes (UVLEDs) emitting at ~270 nm is shown to be influenced by optical polarization. Three UVLEDs with different reflective scattering structures are investigated and compared to standard UVLEDs without scattering structures. The optical polarization and therefore the direction of light propagation within the various UVLEDs are altered by changes in the quantum well (QW) thickness. The improvement in light extraction efficiency of the UVLEDs with reflective scattering structures increases, compared to the UVLEDs without scattering structures, as the fraction of emitted light propagating parallel to the QW plane increases. Additionally, the light extraction efficiency increases as the average distance to the reflective scattering structures decreases. [ABSTRACT FROM AUTHOR]

Gd2O3 films were prepared on (0001)-oriented AlxGa1-xN (0 ≤ x ≤ 0.67) thin film substrates via reactive molecular-beam epitaxy. X-ray diffraction revealed that these films possessed the cubic bixbyite structure regardless of substrate composition and were all 111-oriented with in-plane rotations to account for the symmetry difference between the oxide film and nitride epilayer. Valence band offsets were characterized by X-ray photoelectron spectroscopy and were determined to be 0.41±0.02 eV, 0.17±0.02 eV, and 0.06±0.03 eV at the Gd2O3/AlxGa1-xN interfaces for x=0, 0.28, and 0.67, respectively. [ABSTRACT FROM AUTHOR]

The influence of deep levels defects located in highly resistive GaN:C buffers on the on-resistance (RON) and threshold voltage (Vth) of AlGaN/GaN high electron mobility transistors (HEMTs) power devices was studied by a combined photocapacitance deep level optical spectroscopy (C-DLOS) and photoconductance deep level optical spectroscopy (G-DLOS) methodology as a function of electrical stress. Two carbon-related deep levels at 1.8 and 2.85 eV below the conduction band energy minimum were identified from C-DLOS measurements under the gate electrode. It was found that buffer-related defects under the gate shifted Vth positively by approximately 10%, corresponding to a net areal density of occupied defects of 8 × 1012 cm-2. The effect of on-state drain stress and off-state gate stress on buffer deep level occupancy and RON was also investigated via G-DLOS. It was found that the same carbon-related deep levels observed under the gate were also active in the access region. Off-state gate stress produced significantly more trapping and degradation of RON (~140%) compared to on-state drain stress (~75%). Greater sensitivity of RON to gate stress was explained by a more sharply peaked lateral distribution of occupied deep levels between the gate and drain compared to drain stress. The overall greater sensitivity of RON compared to Vth to buffer defects suggests that electron trapping is significantly greater in the access region compared to under the gate, likely due to the larger electric fields in the latter region. [ABSTRACT FROM AUTHOR]

Ultra-violet (UV) light emitting diodes operating at 339 nm using transparent interband tunnel junctions are reported. Tunneling-based ultraviolet light emitting diodes were grown by plasma-assisted molecular beam epitaxy on 30% Al-content AlGaN layers. A low tunnel junction voltage drop is obtained through the use of compositionally graded n and p-type layers in the tunnel junction, which enhance hole density and tunneling rates. The transparent tunnel junction-based UV LED reported here show a low voltage drop of 5.55 V at 20 A/cm2 and an on-wafer external quantum efficiency of 1.02% at 80 A/cm2. [ABSTRACT FROM AUTHOR]

The effects of point defects in AlxGa1-xN (x = 0-0.33) films with similar threading dislocation densities were investigated. The epitaxial layers were grown under conditions applicable to laser-diode quantum wells, barriers and waveguide layers. The evolution of deep level defect energy and density were quantitatively tracked versus x using deep level optical spectroscopy. Three defect levels were observed, whose defect density increased with x. The energy level of a defect suspected to be related to the group-III vacancy appeared to track the vacuum level, and a near-valence band defect level deepened with respect to the valence band maximum. [ABSTRACT FROM AUTHOR]

Abstract: Metalorganic vapor phase epitaxy was used to grow 15μm of Al0.26Ga0.74N on GaN that was patterned with trenches 10μm wide and 1μm deep. The top of the AlGaN showed 4-μm-wide areas on either side of the trench centerline that had low threading dislocation densities, measured to be less than ∼1.5×108 cm−2. Cross-sectional transmission electron microscopy showed that in the early stages of growth, AlGaN grew at an angle from the corners of the trench and eventually coalesced over the center. These laterally propagating growth sections overgrew the vertical growth in the trench bottom, with the result that low dislocation-density areas formed at the top of the AlGaN. Detailed examination showed that the vertical dislocations from the trench bottom were bent by the angled growth toward the center of the trench where they annihilated with other dislocations, allowing the low dislocation-density areas to form above. Elemental analysis showed that the angled growth sections had slightly lower Al content. The low dislocation-density areas are sufficiently wide to permit optically emitting devices to be grown. [Copyright &y& Elsevier]

The temperature dependence of threshold current It h in vertical-cavity surface-emitting lasers (VCSELs) can be approximated by the equation Ith (T) = α + β(T - Tmin)², where Tmin is the temperature of lowest Ith, α and β are parameters, and temperature is T. We compare the temperature dependence of threshold current in VCSELs with GaAs, InGaAs, and strain compensated InGaAs-GaAsP quantum wells. From our analysis we find the coefficient/3 is related to the gain properties of the quantum well, and is shown to serve as a benchmark for the VCSEL temperature sensitivity. The incorporation of strain-compensated high-barrier GaAsP layers in the active region of 980-nm VCSELs is demonstrated to reduce the threshold dependence on temperature. [ABSTRACT FROM AUTHOR]

We have observed a large coupling between the photonic mode of a microcavity and Stark-localized excitons from a superlattice embedded in it. The coupling is significantly stronger than in comparable microcavities that contain quantum wells solely at the antinodes. The large density of exciton states in a superlattice and their large binding energy, once they are confined by an electric field, explain this enhancement. [Copyright &y& Elsevier]

Localized lattice distortions in GaN substrates can serve as nucleation sites for epitaxial macro-steps and macro-terraces. These detrimental macro-scale features give rise to optically hazy homoepitaxial GaN surfaces. After nucleating, these macro-features grow laterally along the surface and coalesce, leading to significant coverage of the wafer surface. Dot-core GaN substrates consisting of a periodic array of cores were used as a defect-engineered system, where dislocations are intentionally concentrated at the cores. The high density of threading dislocations at the cores induced localized lattice distortions. These distortions are associated predominantly with lattice tilt on the order of hundreds of arcsec across ∼0.5 mm laterally along the wafer surface. The resulting macro-features that nucleated at these localized distorted sites were made up of macro-terraces with lengths ranging ∼30–∼150 μm and macro-step heights ranging ∼200–∼400 nm. Another source of localized distortion was threading screw dislocations or GaN nanopipes that resulted in spiral growth and hillock formation. Based on x-ray topography and optical microscopy measurements, we speculate that the coalescence of hillocks evolves into macro-terraces and macro-steps. While previous studies focused on the substrate miscut as a means to control macro-feature formation, we show that localized lattice tilt from defects is another important contributor to macro-feature formation. [ABSTRACT FROM AUTHOR]

The dependence of (Al)GaN/AlGaN multiquantum well (MQWs) optical efficiency and defect incorporation on the growth temperature (Tg) of the optically active region was investigated. Marked increase in MQW photoluminescence (PL) intensity was observed for increasing Tg. Correspondingly, increasing Tg also significantly reduced point defect incorporation under QW growth conditions, as determined by deep level optical spectroscopy. It is suggested that enhanced MQW PL with increasing Tg resulted from improved nonradiative lifetime through reduced nonradiative defect density in the MQW region. [ABSTRACT FROM AUTHOR]

Impurity-induced layer disordering is demonstrated in Al0.1Ga0.9N/AlN superlattices grown by metal-organic vapor phase epitaxy. During growth at temperatures as low as 885 °C and under post growth annealing at 1000 °C in N2 the heterointerfaces of Si-doped (Si concentration >8×1019 cm-3) superlattices exhibit layer disordering (intermixing) while the unintentionally doped superlattices remain stable. Shifts in the intersubband energy transitions and scanning transmission electron microscope images showing changes in the layer abruptness are used to verify layer disordering due to Si diffusion in Al0.1Ga0.9N/AlN superlattices. [ABSTRACT FROM AUTHOR]

We develop a reciprocal-space model that describes the (hkl) dependence of the broadened Bragg peakwidths produced by x-ray diffraction from a dislocated epilayer. We compare the model to experiments and find that it accurately describes the peakwidths of 16 different Bragg reflections in the [010] zone of both GaN and AlN heterolayers. Using lattice-distortion parameters determined by fitting the model to selected reflections, we estimate threading-dislocation densities for seven different GaN and AlGaN samples and find improved agreement with transmission electron microscopy measurements. [ABSTRACT FROM AUTHOR]

Ultraviolet light-emitting diodes (LEDs) have been grown by metalorganic vapor phase epitaxy using AlN nucleation layers and thick n-type Al0.48Ga0.52N current spreading layers. The active region is composed of three Al0.36Ga0.64N quantum wells with Al0.48Ga0.52N barriers for emission at 290 nm. Devices were designed as bottom emitters and flip-chip bonded to thermally conductive submounts using an interdigitated contact geometry. The ratio of quantum well emission to 330 nm sub-band gap emission is as high as 125:1 for these LEDs. Output power as high as 1.34 mW at 300 mA under direct current operation has been demonstrated with a forward voltage of 9.4 V. A peak external quantum efficiency of 0.18% has been measured at an operating current of 55 mA. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

GaN nucleation layer (NL) decomposition was measured using optical reflectance over a wide range of pressure P, temperature T, and H[SUB2]/NH[SUB3] gas mixture. The GaN NLs show measurable decomposition above 800°C and do not significantly roughen until above 960°C. The NL decomposition rates increase with increasing P, increasing T, and decreasing NH[SUB3] flow. An activation energy E[SUBA] of 2.68 eV was measured (from 820 to 960°C) for NL decomposition and an E[SUBA] of 2.62 eV was measured (from 900 to 1075°C) for decomposition of thick, high-T bulk GaN films. Depending on P, the pre-exponential factor A[SUB0] was four to nine times larger for NL decomposition compared to bulk GaN decomposition. The E[SUBA] (measured for both NL and bulk GaN decomposition in mixed H[SUB2] and NH[SUB3] flows is similar to the E[SUBA] for Ga desorption, suggesting that the rate-limiting step for both NL and bulk GaN decomposition is Ga desorption. [ABSTRACT FROM AUTHOR]

Ultraviolet light emitting diodes (LEDs) have been grown using metalorganic vapor phase epitaxy, while monitoring the 550 nm reflected light intensity. During nucleation of GaN on sapphire, the transition from three-dimensional (3D) grain growth to two-dimensional (2D) coalesced growth was intentionally delayed in time by lowering the NH[sub 3] flow during the initial high temperature growth. Initially, when the reflectance signal is near zero, the GaN film is rough and composed of partly coalesced 3D grains. Eventually, the reflected light intensity recovers as the 2D morphology evolves. For 380 nm LEDs grown on 3D nucleation layers, we observe increased light output. For LEDs fabricated on GaN films with a longer recovery time an output power of 1.3 mW at 20 mA current was achieved. [ABSTRACT FROM AUTHOR]

A contacted electroreflectance technique was used to characterize the electronic properties of AlGaN/GaN heterostructures and field-effect transistors. By studying variations in the electroreflectance with applied electric field, spectral features associated with the AlGaN barrier, the two-dimensional electron gas at the interface, and bulk GaN were observed. The AlGaN barrier displayed Franz-Keldysh oscillations which provided estimates of barrier-layer composition and electric field. The electron gas Fermi energy was determined from analysis of the broad two-dimensional electron gas feature. [ABSTRACT FROM AUTHOR]

Examines the nitrogen vibrational mode spectra, Hall Modalities and minority carrier diffusion lengths for InGaAsN grown by molecular beam epitaxy (MBE). Formation of In-N bonding; Presence of electron diffusion in the MBE material; Limitation of lateral carrier transport by large scale material inhomogeneities.

Describes the design, growth by metalorganic chemical vapor deposition, and processing of an In[sub 0.07]Ga[sub 0.93]As[sub 0.02] solar cell, with 1.0 eV band gap, lattice matched to gallium arsenide. Hole diffusion length; Solar cell internal quantum efficiencies; Limitation of solar cell performance by doping and nitrogen incorporation from InGaAsN.

Describes a uniform radial flow epitaxy-grown indium gallium arsenide/indium phosphide heterostructure using Raman spectroscopy. Preparation of epitaxial films above and below the critical thickness; Analysis of Raman spectra from strained and relaxed films; Formulation of a formula for layer stress evaluation.

Evaluates indium arsenic antimonide (InAsSb)/InAs phosphide strained-layer superlattices (SLS) as active regions for midinfrared lasers and light emitting diodes. Growth of SLS by metal organic chemical vapor deposition; Performance of InAsSb lasers in threshold power and characteristic temperature; Achievement of light-heavy hole splittings from SLS band structure calculations.

Examines the growth of n- and p-type Al(As)Sb epitaxial films by metalorganic chemical vapor deposition. Use of diethylzinc or tetraethyltin for doping the epitaxial films; Comparison between the C and O levels of doped and undoped AlSb; Application of AlSb and AlAs[sub x]Sb[sub 1-x] in optoelectronic devices.

Ultrawide bandgap Al0.7Ga0.3N MESFETs with refractory Tungsten Schottky and Ohmic contacts are studied in 300–675 K environments. Variable-temperature dc electrical transport reveals large ON-state drain current densities for an AlGaN device: 209 mA/mm at 300 K and 156 mA/mm at 675 K in the ON-state (25% reduction). Drain and gate currents are only weakly temperature-dependent, suggesting potential for engineering temperature invariant operation. The ON-/ OFF-ratio is limited by OFF-state leakage through the gate, which is attributed to damage from sputter deposition. Future work using refractory metals with larger work functions that are deposited by electron beam deposition is proposed. [ABSTRACT FROM AUTHOR]

The realisation of a GaN high voltage vertical p–n diode operating at >3.9 kV breakdown with a specific on-resistance <0.9 mΩ cm² is reported. Diodes achieved a forward current of 1 A for on-wafer, DC measurements, corresponding to a current density >1.4 kA/cm². An effective critical electric field of 3.9 MV/cm was estimated for the devices from analysis of the forward and reverse current–voltage characteristics. This suggests that the fundamental limit to the GaN critical electric field is significantly greater than previously believed. [ABSTRACT FROM AUTHOR]