Showing papers on "Gallium nitride published in 2008"

GaN-Based RF Power Devices and Amplifiers

Abstract: The rapid development of the RF power electronics requires the introduction of wide bandgap material due to its potential in high output power density, high operation voltage and high input impedance GaN-based RF power devices have made substantial progresses in the last decade This paper attempts to review the latest developments of the GaN HEMT technologies, including material growth, processing technologies, device epitaxial structures and MMIC designs, to achieve the state-of-the-art microwave and millimeter-wave performance The reliability and manufacturing challenges are also discussed

GaN Photonic-Crystal Surface-Emitting Laser at Blue-Violet Wavelengths

Abstract: Shorter-wavelength surface-emitting laser sources are important for a variety of fields, including photonics, information processing, and biology. We report on the creation of a current-driven blue-violet photonic-crystal surface-emitting laser. We have developed a fabrication method, named "air holes retained over growth," in order to construct a two-dimensional gallium nitride (GaN)/air photonic-crystal structure. The resulting periodic structure has a photonic-crystal band-edge effect sufficient for the successful operation of a current-injection surface-emitting laser. This represents an important step in the development of laser sources that could be focused to a size much less than the wavelength and be integrated two-dimensionally at such short wavelengths.

Vertically Aligned p-Type Single-Crystalline GaN Nanorod Arrays on n-Type Si for Heterojunction Photovoltaic Cells

Abstract: Vertically aligned Mg-doped GaN nanorods have been epitaxially grown on n-type Si substrate to form a heterostructure for fabricating p-n heterojunction photovoltaic cells. The p-type GaN nanorod/n-Si heterojunction cell shows a well-defined rectifying behavior with a rectification ratio larger than 10(4) in dark. The cell has a high short-circuit photocurrent density of 7.6 mAlcm2 and energy conversion efficiency of 2.73% under AM 1.5G illumination at 100 mW/cm2. Moreover, the nanorod array may be used as an antireflection coating for solar cell applications to effectively reduce light loss due to reflection. This study provides an experimental demonstration for integrating one-dimensional nanostructure arrays with the substrate to directly fabricate heterojunction photovoltaic cells.

A Review on the Reliability of GaN-Based LEDs

Abstract: We review the degradation mechanisms that limit the reliability of GaN-based light-emitting diodes (LEDs). We propose a set of specific experiments, which is aimed at separately analyzing the degradation of the properties of the active layer, of the ohmic contacts and of the package/phosphor system. In particular, we show the following: 1) low-current density stress can determine the degradation of the active layer of the devices, implying modifications of the charge/deep level distribution with subsequent increase of the nonradiative recombination components; 2) high-temperature storage can significantly affect the properties of the ohmic contacts and semiconductor layer at the p-side of the devices, thus determining emission crowding and subsequent optical power decrease; and 3) high-temperature stress can significantly limit the optical properties of the package of high-power LEDs for lighting applications.

Vertical GaN-Based Trench Gate Metal Oxide Semiconductor Field-Effect Transistors on GaN Bulk Substrates

Abstract: Completely vertical trench gate metal oxide semiconductor field-effect transistors (MOSFETs) have been produced using gallium nitride (GaN) for the first time. These MOSFETs exhibited enhancement-mode operation with a threshold voltage of 3.7 V and an on-resistance of 9.3 mΩcm2. The channel mobility was estimated to be 131 cm2/(Vs) when all the resistances except for that of the channel are considered. Such structures, which satisfy the key words "vertical", "trench gate", and "MOSFET", will enable us to fabricate practical GaN-based power switching devices.

Gallium nitride electronics

Abstract: 1. Introduction.- 2. III-N Materials and the State of the Art of Devices and Circuits.- 3. Epitaxy for III-N-Based Electronic Devices.- 4. Device Process Technology.- 5. Device Characterization and Modeling.- 6. Circuit Considerations and III-N Examples.- 7. Reliability Aspects and High Temperatures Operation.- 8. Integration, Thermal Managements, and Packaging.- 9. Outlook.

Directional emission control and increased light extraction in GaN photonic crystal light emitting diodes

Abstract: Limitations in extraction efficiency of gallium nitride (GaN) photonic crystal (PhC) light emitting diodes (LEDs) are addressed by implementing an LED design using both two-dimensional PhCs in-plane and index guiding layers (IGLs) in the vertical direction. The effects of PhCs on light extraction and emission directionality from GaN LEDs are studied experimentally. Angular-resolved electroluminescence clearly shows the combined effect of controlling the vertical mode profile with the IGLs and tailoring the emission profile with the periodicity of the PhC lattice. Increases in directional emission as high as 3.5 times are achieved by taking advantage of this directionality and guided mode control.

AlN/GaN Insulated-Gate HEMTs With 2.3 A/mm Output Current and 480 mS/mm Transconductance

Abstract: High-electron mobility transistors (HEMTs) based on ultrathin AIN/GaN heterostructures with a 3.5-nm AlN barrier and a 3-nm Al2O3 gate dielectric have been investigated. Owing to the optimized AIN/GaN interface, very high carrier mobility (~1400 cm2/V ldr s) and high 2-D electron-gas density (~2.7times1013/cm2) resulted in a record low sheet resistance (~165 Omega/sq). The resultant HEMTs showed a maximum dc output current density of ~2.3 A/mm and a peak extrinsic transconductance gm,ext~480 mS/mm (corresponding to gm,int~1 S/mm). An fT/fmax of 52/60 GHz was measured on 0.25times60 mum2 gate HEMTs. With further improvements of the ohmic contacts, the gate dielectric, and the lowering of the buffer leakage, the presented results suggest that, by using AIN/GaN heterojunctions, it may be possible to push the performance of nitride HEMTs to current, power, and speed levels that are currently unachievable in AlGaN/GaN technology.

TEM Observation of Crack- and Pit-Shaped Defects in Electrically Degraded GaN HEMTs

Abstract: AlGaN/GaN high-electron mobility transistors stressed under dc bias at various channel temperatures were studied using transmission electron microscopy for evidence of physical damage. Stressed devices consistently developed crack- and pit-shaped defects in the AlGaN/GaN crystal material under the drain-side edge of the gate, whereas side-by-side as-processed unstressed devices did not show these features. Furthermore, the amount of physical damage was found to correlate to the amount of electrical degradation as measured by the change in IDmax from before and after stress. The formation of these defects is consistent with the theory of damage from the inverse piezoelectric effect.

High-Efficiency Continuous-Wave Operation of Blue-Green Laser Diodes Based on Nonpolar m-Plane Gallium Nitride

Abstract: We demonstrated nonpolar m-plane GaN-based blue-green laser diodes (LDs) under continuous-wave (cw) operation with a lasing wavelength of 481 nm. A maximum output power of more than 20 mW was achieved, for which the threshold current and the corresponding threshold current density (Jth) were 61 mA and 6.1 kA/cm2, respectively. The value of Jth and the electroluminescence peak wavelength shift until lasing did not change with lasing wavelength in the range from 459 to 481 nm, though the reflectivities of the cavity facets were fixed for each LD. In addition, the slope efficiency increased with increasing lasing wavelength, from 0.37 W/A at 459 nm to 0.49 W/A at 481 nm. This is the remarkable advantages of nonpolar GaN-based material compared to c-plane material for the realization of green LDs.

Steady-state and time-resolved photoluminescence from relaxed and strained GaN nanowires grown by catalyst-free molecular-beam epitaxy

Abstract: We report steady-state and time-resolved photoluminescence (TRPL) measurements on individual GaN nanowires (6–20 μm in length, 30–940 nm in diameter) grown by a nitrogen-plasma-assisted, catalyst-free molecular-beam epitaxy on Si(111) and dispersed onto fused quartz substrates. Induced tensile strain for nanowires bonded to fused silica and compressive strain for nanowires coated with atomic-layer-deposition alumina led to redshifts and blueshifts of the dominant steady-state PL emission peak, respectively. Unperturbed nanowires exhibited spectra associated with high-quality, strain-free material. The TRPL lifetimes, which were similar for both relaxed and strained nanowires of similar size, ranged from 200 ps to over 2 ns, compared well with those of low-defect bulk GaN, and depended linearly on nanowire diameter. The diameter-dependent lifetimes yielded a room-temperature surface recombination velocity S of 9×103 cm/s for our silicon-doped GaN nanowires.

Barrier-Layer Scaling of InAlN/GaN HEMTs

Abstract: We discuss the characteristics of high-electron mobility transistors with barrier thicknesses between 33 and 3 nm, which are grown on sapphire substrates by metal-organic chemical vapor deposition. The maximum drain current (at VG = 2.0 V) decreased with decreasing barrier thickness due to the gate forward drive limitation and residual surface-depletion effect. Full pinchoff and low leakage are observed. Even with 3-nm ultrathin barrier, the heterostructure and contacts are thermally highly stable (up to 1000degC).

Growth, fabrication, and characterization of InGaN solar cells

Abstract: The InGaN alloy system offers a unique opportunity to develop high efficiency multi-junction solar cells. In this study, single junction solar cells made of Inx Ga1–xN are successfully developed, with x = 0, 0.2, and 0.3. The materials are grown on sapphire substrates by MBE, consisting of a Si-doped InGaN layer, an intrinsic layer and an Mg-doped InGaN layer on the top. The I –V curves indicate that the cell made of all-GaN has low series resistance (0.12 Ω cm2) and insignificant parasitic leakage. Contact resistances of p and n contacts are 2.9 × 10–2 Ω cm2 and 2.0 × 10–3 Ω cm2, respectively. Upon illumination by a 200 mW/cm2, 325 nm laser, Voc is measured at 2.5 V with a fill factor of 61%. Clear photo-responses are also observed in both InGaN cells with 0.2 and 0.3 Indium content when illuminated by outdoor sunlight. But it is difficult to determine the solar performance due to the large leakage current, which may be caused by the material defects. A thicker buffer layer or GaN template can be applied to the future growth process to reduce the defect density of InGaN films. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Buffer optimization for crack-free GaN epitaxial layers grown on Si(1 1 1) substrate by MOCVD

Abstract: We report the growth of GaN films on the Si(1?1?1) substrate by metalorganic chemical vapour phase deposition (MOCVD). Different buffer layers were used to investigate their effects on the structural and optical properties of GaN layers. A series of GaN layers were grown on Si(1?1?1) with different buffer layers and buffer thicknesses and were characterized by Nomarski microscopy, atomic force microscopy, high-resolution x-ray diffraction (XRD) and photoluminescence (PL) measurements. We first discuss the optimization of the LT-AlN/HT-AlN/Si(1?1?1) templates and then the optimization of the graded AlGaN intermediate layers. In order to prevent stress relaxation, step-graded AlGaN layers were introduced along with a crack-free GaN layer of thickness exceeding 2.6??m. The XRD and PL measurements results confirmed that a wurtzite GaN was successfully grown. The resulting GaN film surfaces were flat, mirror-like and crack-free. The mosaic structure in the GaN layers was investigated. With a combination of Williamson?Hall measurements and the fitting of twist angles, it was found that the buffer thickness determines the lateral coherence length, vertical coherence length, as well as the tilt and twist of the mosaic blocks in GaN films. The PL spectra at 8?K show that a strong band edge photoluminescence of GaN on Si (1?1?1) emits light at an energy of 3.449?eV with a full width at half maximum (FWHM) of approximately 16?meV. At room temperature, the peak position and FWHM of this emission become 3.390?eV and 58?meV, respectively. The origin of this peak was attributed to the neutral donor bound exciton. It was found that the optimized total thickness of the AlN and graded AlGaN layers played a very important role in the improvement of quality and in turn reduced the cracks during the growth of GaN/Si(1?1?1) epitaxial layers.

Optical properties of Si- and Mg-doped gallium nitride nanowires grown by plasma-assisted molecular beam epitaxy

Abstract: The optical properties of GaN nanowires grown by catalyst free plasma-assisted molecular beam epitaxy on Si (111) are investigated by photoluminescence (PL) spectroscopy. The influence of the Si- and Mg-flux as well as the III-V ratio during growth on the PL properties is discussed. The Mg concentration as determined by secondary ion mass spectroscopy ranges from 5×1018 to 1×1020 cm−3. Raman scattering reveals that the nanowires are strain-free, irrespective of Si- or Mg-doping. The near band-edge emission of undoped or slightly Si-doped material is dominated by the narrow D0X recombination at 3.4715 eV with a full width at half maximum of 1.5 meV at 4 K. For high Si-fluxes, a blueshift of the D0X peak by 1 meV is found, which is attributed to band-filling effects. For moderate Mg-fluxes the acceptor-bound exciton recombination was detected at 3.4665 eV. Point defects due to the N-rich growth conditions are discussed as the origin of the emission band at 3.45 eV. Recombination at coalescence boundaries were identified as the origin of an emission band at 3.21 eV. The luminescence properties below 3.27 eV in highly Mg-doped samples are shown to be affected by the presence of cubic inclusions in the otherwise wurtzite nanowires.

Cleaved facet (ga,al,in)n edge-emitting laser diodes grown on semipolar {11-2n} bulk gallium nitride substrates

Abstract: A III-nitride edge-emitting laser diode is formed on a surface of a III-nitride substrate having a semipolar orientation, wherein the III-nitride substrate is cleaved by creating a cleavage line along a direction substantially perpendicular to a nonpolar orientation of the III-nitride substrate, and then applying force along the cleavage line to create one or more cleaved facets of the III-nitride substrate, wherein the cleaved facets have an m-plane or a-plane orientation.

Integration of ZnO microcrystals with tailored dimensions forming light emitting diodes and UV photovoltaic cells.

Abstract: This article reports a new integration approach to produce arrays of ZnO microcrystals for optoelectronic and photovoltaic applications. Demonstrated applications are n-ZnO/p-GaN heterojunction LEDs and photovoltaic cells. The integration process uses an oxygen plasma treatment in combination with a photoresist pattern on magnesium doped GaN substrates to define a narrow sub-100 nm width nucleation region. Nucleation is followed by lateral epitaxial overgrowth producing single crystal disks of ZnO with desired size over 2 in. wafers. The process provides control over the dimensions (<1% STD) and the location (0.7% STD pitch variation) of the ZnO crystals. The quality of the patterned ZnO is high; the commonly observed defect related emission in the electroluminescence spectra is completely suppressed, and a single near-band-edge UV peak is observed.

Stress Reduction and Enhanced Extraction Efficiency of GaN-Based LED Grown on Cone-Shape-Patterned Sapphire

Abstract: High-quality InGaN-GaN film was grown on a cone-shape-patterned sapphire substrate (CSPSS) by using metal-oganic chemical vapor deposition. The growth mode of GaN on CSPSS was similar to that of the epitaxial lateral overgrowth (ELOG), because the growth, in the initial stage, proceeds only on flat basal sapphire substrate and there is no preferential growth plane on the cone region. An analysis of X-ray diffraction showed a shorter lattice constant of 5.1877 Aring along the c-axis for the GaN thin films grown on CSPSS, compared to 5.1913 A for the samples grown on a conventional sapphire substrate (CSS). This is because the ELOG-like mode of the GaN layer over the cone-shaped region results in less lattice mismatch and incoherency between the GaN layer and the sapphire substrate. The output power of a sideview light-emitting diode (LED) grown on CSPSS was estimated to be 7.3 mW at a forward current of 20 mA, which is improved by 34% compared to that of an LED grown on CSS. The significant enhancement in output power is attributed to both the increase of the extraction efficiency, resulted from the increase in photon escaping probability due to enhanced light scattering at the CSPSS, and the improvement of the crystal quality due to the reduction of dislocation.

Unusually low thermal conductivity of gallium nitride nanowires

Abstract: We report measurements of thermal conductivity κ on individual gallium nitride nanowires (GaN NWs) with diameters ranging from 97to181nm grown by thermal chemical vapor deposition. We observed unexpectedly small κ values, in the range of 13–19W∕mK at 300K, with very weak diameter dependence. We also observe unusual power law κ∼Tn behavior with n=1.8 at low temperature. Electron-energy-loss-spectroscopy measurements indicate Si and O concentrations in the ranges of 0.1–1 and 0.01–0.1at.%, respectively. Based on extensive numerical calculations, we conclude that both the unexpectedly low κ and the T1.8 dependence are caused by unusually large mass-difference scattering, primarily from Si impurities. Our analysis also suggests that mass-difference scattering rates are significantly enhanced by the reduced phonon group velocity in nanoscale systems. Planar defects running the length of the NW, previously characterized in detail, may also play a role in limiting the phonon mean free path.

A new small-signal modeling and extraction method in AlGaN/GaN HEMTs

Abstract: In this paper, a new 20-element small-signal equivalent circuit for GaN HEMTs is proposed and correspondingly, a direct extraction method is developed. Compared with the 16-element conventional GaAs-based HEMT small-signal model (SSM), two parasitic distributed inter-electrode extrinsic capacitances and two additional feedback intrinsic resistances are considered. The new modeling approach for GaN HEMTs is verified by comparing the simulated small-signal S-parameter with the measured data over wide frequency and bias ranges.

Temperature-Dependent Characterization of AlGaN/GaN HEMTs: Thermal and Source/Drain Resistances

Abstract: This paper shows the application of simple dc techniques to the temperature-dependent characterization of AlGaN/ GaN HEMTs in terms of the following: 1) thermal resistance and 2) ohmic series resistance (at low drain bias). Despite their simplicity, these measurement techniques are shown to give valuable information about the device behavior over a wide range of ambient/channel temperatures. The experimental results are validated by comparison with independent measurements and numerical simulations.

Enhanced Output Power of GaN-Based LEDs With Nano-Patterned Sapphire Substrates

Abstract: GaN-based light-emitting diodes (LEDs) with an emitting wavelength of 450 nm were grown on nano-patterned sapphire substrates (NPSS) fabricated by nanosphere lithography. The crystalline quality of the epitaxial film could be improved by using the NPSS technique. The output power of LED grown on NPSS was 1.3 and 1.11 times higher than those of LEDs grown on conventional and patterned sapphire substrates at the injection current of 20 mA, respectively. The enhancement in output power could be contributed to the efficiently scattering by NPSS. But some voids formed at the GaN/NPSS interface cause a thermal dissipation problem of NPSS LED operated at high injection current.

First Observation of Bias Oscillations in GaN Gunn Diodes on GaN Substrate

Abstract: In this paper, we report on the bias oscillation of GaN-based Gunn diodes realized on a n+-GaN substrate. Different contact materials, ambient gases, and pulsewidths were used and compared with regard to device stability. A wide negative- differential-resistance (NDR) region was measured for electrical- field values E larger than a threshold field Eth of 150 kV/cm. Electrical fields much higher than the threshold value did not lead to any electromigration effects or discharging problems from the contacts. The drift velocity derived from the current-voltage characteristics, diode geometry, and doping concentration in the active layer was estimated to be 1.9 times 107 cm/s. Bias oscillations were obtained for the GaN Gunn diodes in the presence of a series inductance.

Solar-blind MSM-photodetectors based on AlxGa1−xN/GaN heterostructures grown by MOCVD

Abstract: Solar-blind MSM-photodetectors based on the AlGaN/GaN heterostructutes grown by MOCVD technology were fabricated and investigated. Directly on the MSM-diode we have measured a Schottky barrier height of 1.1 eV for Ni and 1.4 eV for Mo contacts on AlGaN. Effect of different buffer layers on the detector performances has been demonstrated. Detectors exhibit low dark currents and high sensitivity within the range of 250–290 nm. High-speed response of MSM-detectors is analyzed. Effect of optical excitation level on detector performance is discussed. At low excitation level the detector speed of response is limited by parasitic capacitance of interdigitated diode structure and by the transit time of the photogenerated carriers. At high excitation level the detector speed of response is limited by the field screening caused by the space-charge of the holes. The impulse response of AlGaN/GaN MSM-detector is compared favorably with GaAs MSM-device.

MESFETs Made From Individual GaN Nanowires

Abstract: In this paper, we demonstrate novel MESFETs based on individual GaN nanowires. The Pt/Au Schottky gates exhibited excellent two-terminal Schottky diode rectification behavior. The average effective Schottky barrier height was 0.87 eV, with an average ideality factor of 1.6. In addition, the Schottky gates efficiently modulated the conduction of the nanowires. The threshold gate voltages required for complete pinch off were as small as -2.6 V, and transconductances exceeded 1.4 muS. Subthreshold swings approaching 60 mV/decade and on/off current ratios of up to 5times108 were achieved. These results show that the Schottky gate has the potential to significantly improve the performance of GaN nanowire field-effect devices.

Micro-Raman thermometry in the presence of complex stresses in GaN devices

Abstract: Raman thermometry is often utilized to measure temperature in gallium nitride (GaN) electronics. However, the accuracy of the technique is subject to errors arising from stresses which develop during device operation as a result of both thermoelastic and inverse piezoelectric effects. To assess the implications of these stresses on Raman thermometry, we investigate the use of the Stokes peak position, linewidth, and Stokes to anti-Stokes intensity ratio to estimate the temperature of GaN devices during operation. Our results indicate that only temperature measurements obtained from the intensity ratio method are independent of these stresses. Measurements using the linewidth, meanwhile, were found to correspond well with those obtained from the intensity ratio through the use of a reference condition which accounted for the stress dependency of this spectral component. These results were then compared to a three dimensional finite element model which yielded a correlation to within 5% between the computat...