Showing papers on "Gallium nitride published in 2000"

Nitride semiconductors free of electrostatic fields for efficient white light-emitting diodes

Abstract: Compact solid-state lamps based on light-emitting diodes (LEDs) are of current technological interest as an alternative to conventional light bulbs The brightest LEDs available so far emit red light and exhibit higher luminous efficiency than fluorescent lamps If this luminous efficiency could be transferred to white LEDs, power consumption would be dramatically reduced, with great economic and ecological consequences But the luminous efficiency of existing white LEDs is still very low, owing to the presence of electrostatic fields within the active layers These fields are generated by the spontaneous and piezoelectric polarization along the [0001] axis of hexagonal group-III nitrides--the commonly used materials for light generation Unfortunately, as this crystallographic orientation corresponds to the natural growth direction of these materials deposited on currently available substrates Here we demonstrate that the epitaxial growth of GaN/(Al,Ga)N on tetragonal LiAlO2 in a non-polar direction allows the fabrication of structures free of electrostatic fields, resulting in an improved quantum efficiency We expect that this approach will pave the way towards highly efficient white LEDs

Fabrication and performance of GaN electronic devices

Abstract: GaN and related materials (especially AlGaN) have recently attracted a lot of interest for applications in high power electronics capable of operation at elevated temperatures. Although the growth and processing technology for SiC, the other viable wide bandgap semiconductor material, is more mature, the AlGaInN system offers numerous advantages. These include wider bandgaps, good transport properties, the availability of heterostructures (particularly AlGaN/GaN), the experience base gained by the commercialization of GaN-based laser and light-emitting diodes and the existence of a high growth rate epitaxial method (hydride vapor phase epitaxy) for producing very thick layers or even quasi-substrates. These attributes have led to rapid progress in the realization of a broad range of GaN electronic devices, including heterostructure field effect transistors (HFETs), Schottky and p–i–n rectifiers, heterojunction bipolar transistors (HBTs), bipolar junction transistors (BJTs) and metal-oxide semiconductor field effect transistors (MOSFETs). This review focuses on the development of fabrication processes for these devices and the current state-of-the-art in device performance, for all of these structures. We also detail areas where more work is needed, such as reducing defect densities and purity of epitaxial layers, the need for substrates and improved oxides and insulators, improved p-type doping and contacts and an understanding of the basic growth mechanisms.

AlGaN/GaN metal oxide semiconductor heterostructure field effect transistor

Abstract: We report on the AlGaN/GaN metal oxide semiconductor heterostructure field effect transistor (MOS-HFET) and present the results of the comparative studies of this device and a base line AlGaN/GaN heterostructure field effect transistor (HFET). For a 5-/spl mu/ source-to-drain opening, the maximum current was close to 600 mA/mm for both devices. The gate leakage current for the MOS-HFET was more than six orders of magnitude smaller than for the HFET.

Large-Scale Catalytic Synthesis of Crystalline Gallium Nitride Nanowires

Abstract: 738 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2000 0935-9648/00/1005-0738 $ 17.50+.50/0 Adv. Mater. 2000, 12, No. 10 porosity between grains and at the grain boundaries, where continued sintering will likely result in further crystal growth until a fully dense polycrystalline body is formed. In contrast to MEEA-A and MEA-A, A-A has a small particle size with a high surface area/volume ratio and small organic substituent groups [CH3CO2] ± on the surface of the nanoparticles. The small size of the organic substituents results in a significantly smaller inter-particle distance between individual alumoxanes nanoparticles. Upon thermolysis, the organic surface of the A-alumoxane nanoparticle is pyrolyzed, leaving discrete alumina particles with a small inter-particle separation. As previously discussed, the high surface energy inherent in these alumoxanes allows for rapid crystallization; when combined with the small inter-particle distance, crystal growth within one particle results in crystallization of the adjacent particle. This aknockono or adominoo effect is somewhat akin to an epitaxial growth. The rapid sintering results in the crystallization of large groups of adjacent particles within a single body, such that the resulting crystals are macroscopic (i.e., £ 2 mm) in diameter. It may be expected that crystallization (and crystal growth) is sufficiently rapid so that defects or channels created by the outgassing of the pyrolysis products (e.g., primarily, CO2, H2O, and acetone in the case of A-alumoxane), will result in the creation of pores, and that these defects are retained in the resulting ceramic. The resulting ceramic is highly crystalline with porosity contained inside the individual crystal grains. The alumina formed from MA-alumoxane is intermediate between that formed from MEA-alumoxane and A-alumoxane. There exists both a high number of seed crystals distributed throughout a polycrystalline matrix, as well as large single crystals that are independent of the primary matrix material. It is likely that the seed crystals will form larger crystal domains upon further sintering. Carboxylate-alumoxanes are novel, environmentally benign precursors for developing alumina bodies and films with controlled porosity. The pore size and pore size distribution is influenced by the selection of the organic substituent on the nanoparticle surface, while the average pore sizes may be altered through either physical or chemical mixtures of two (or more) carboxylate-alumoxanes. This has applications especially in advanced ceramic materials in that pores located between crystal grains, and especially at the grain boundaries, encourage crack propagation which increases the opportunity for catastrophic failure. However, pores isolated inside crystal grains provide less opportunity for detrimental pore/boundary/crack interactions to occur, and may therefore provide a mechanism to increase fracture toughness.

Piezoelectric coefficient of aluminum nitride and gallium nitride

Abstract: The piezoelectric coefficient d33 of aluminum nitride (AlN) and gallium nitride (GaN) thin films grown on silicon substrates by molecular beam epitaxy have been measured using a laser interferometer. X-ray diffraction reveals that the AlN and GaN films consist mainly of crystals with a hexagonal wurtzite structure. In order to grow epitaxial GaN films, an AlN film was first deposited on silicon as the buffer layer, so the d33 measurement for GaN was actually performed on GaN/AlN/Si multilayer systems. The relative permittivity and electrical resistivity of each constituent layer of the film and the potential drop across each layer were determined as a function of frequency. The potential drops were then used to calculate the piezoelectric coefficient d33 of GaN. After correcting for substrate clamping, d33 of AlN and GaN were found to be (5.1±0.1) and (3.1±0.1) pm V−1, respectively.

Free-Standing GaN Substrates by Hydride Vapor Phase Epitaxy

Abstract: Thick gallium nitride films 250–350 µm in thickness were grown on 2-inch-diameter (0001) sapphire wafers by hydride vapor phase epitaxy. The size of the free-standing GaN substrates without cracks separated from the sapphire substrates by laser processing was equal to that of the initial sapphire substrates. The origin of bowing and the broad photoluminescence (PL) spectra of GaN films was considered the difference in the residual strain between the front and bottom surfaces caused by threading dislocations.

Synthesis of GaN-carbon composite nanotubes and GaN nanorods by arc discharge in nitrogen atmosphere

Abstract: A method using an arc discharge in a nitrogen atmosphere for synthesizing large quantities of gallium nitride (GaN)–Carbon composite nanotubes and GaN nanorods is reported. The reaction is achieved by a dc arc discharge between a graphite anode filled with a mixture of GaN, graphite, and nickel powders and a graphite cathode in a nitrogen atmosphere. The GaN are presented as rodlike fillings in the composite tubes and the isolated GaN nanorods have diameters in the range of 7–45 nanometers and a length of up to 40 μm. The outer graphitic shells of the composite carbon nanotubes have thicknesses ranging from 1 to 8 nm. It was found that the use of a nitrogen atmosphere plays a crucial role for the growth of the GaN nanorods fillings and the individual GaN nanorods.

Crystalline gallium nitride and method for forming crystalline gallium nitride

Abstract: A gallium nitride growth process forms crystalline gallium nitride. The process comprises the steps of providing a source gallium nitride (15); providing mineralizer (17); providing solvent (17); providing a capsule (10); disposing the source gallium nitride, mineralizer and solvent in the capsule; sealing the capsule; disposing the capsule in a pressure cell (1); and subjecting the pressure cell to high pressure and high tempeature (HPHT) conditions for a length of time sufficient to dissolve the source gallium nitride and re-precipitate the source gallium nitride into at least one gallium nitride crystal. The invention also provides for gallium nitride crystals formed by the processes of the invention.

Lattice parameters and thermal expansion of GaN

Abstract: Neutron powder diffraction methods with Rietveld analysis are utilized to determine GaN lattice parameters from 15 to 298.1 K. Using these measurements and literature data, we calculated the thermal expansion of gallium nitride (GaN) and predicted its higher temperature thermal expansion. The results are compared with available experimental data and earlier work.

SiC and GaN bipolar power devices

Abstract: The present status of the silicon carbide and gallium nitride bipolar power semiconductor devices is reviewed. Several unipolar and bipolar figures of merit have been examined to demonstrate the potential performance gain to be obtained from silicon carbide and gallium nitride based power devices. Several conventional as well as novel device structures have been examined, some of which have already been demonstrated and others are in their early stages of development. Conventional silicon theory has often been found to be inadequate to explain the characteristics of silicon carbide. Appropriate modifications have been applied to investigate more complicated characteristics of silicon carbide devices.

Flip chip led apparatus

Abstract: A flip chip structure of a light-emitting device comprising a UV/blue light emitting diode (LED) is disclosed. The flip chip structure is optimized to produce unique light focusing and phosphor illumination out the bottom of the structure. The flip chip structure includes a substrate, a gallium nitride layer epitaxially grown on a top surface of the substrate, and one or more layers of lensing material deposited on a bottom surface of the substrate. The lensing material is preferably a polymer lensing material, an index matching material, or a mixture thereof. The gallium nitride layer is deposited in the form of one or more odd-sided polygons, for enhanced light extraction.

GaN Electronics For High Power, High Temperature Applications

Abstract: A brief review is given of recent progress in fabrication of high voltage GaN and AlGaN rectifiers. GaN/AlGaN heterojunction bipolar transistors and GaN metal-oxide semiconductor field effect transistors. Improvements in epitaxial layer quality and in fabrication techniques have led to significant advances in device performance.

Prospect for high brightness III–nitride electron emitter

Abstract: We describe p-type gallium nitride (GaN) as a candidate for high brightness photocathodes. Experiments utilizing photoemission spectroscopy and quantum yield measurements were performed on GaN films to characterize various cesium and oxygen activations. Quantum efficiencies of 0.1%–4% were obtained in reflection for the cesiated p-type 0.5 μm thick GaN films and 25%–50% on the 0.1 μm thick GaN films. The corresponding emission currents are 142–300 nA for 0.5 μm thick films and 0.7–1.3 μA for the 0.1 μm thick films. This results in an increase of several orders of magnitude in the emission current from the starting GaN films. Furthermore, an initial desorption measurement was performed in order to evaluate the Cs binding strength to GaN relative to GaAs. We observe Cs was bound to the GaN surface (0001_) at 700 °C and completely desorbed at 450 °C for a (100) GaAs surface. Finally, an alternate barium activation on GaN is included for preliminary comparison with the various cesium activations.

Blue Diode Lasers

Abstract: The recent achievement of compact blue‐emitting gallium nitride semiconductor lasers is likely to have far‐reaching technological and commercial effects. The lasers' short wavelengths—around 400 nm, half that of gallium arsenide‐based lasers—permit higher spatial resolution in applications such as optical storage and printing. And the high photon energy will open up new applications for these inexpensive, compact light sources. An aesthetic satisfaction with these devices stems from finally extending the existing and mature semiconductor laser technology for the near‐infrared and red to encompass the “new frontier” blue and near‐ultraviolet regions, thereby bridging the entire visible spectrum. At the same time, there are significant research opportunities arising from a plethora of poorly understood microscopic issues in the underlying material system, which include such fundamental properties as charge control, transport, and formation of optical gain for stimulated emission.

Determination of small-signal parameters of GaN-based HEMTs

Abstract: The small-signal equivalent circuit of AlGaN/GaN HEMT is discussed It is shown that an extremely high gate voltage has to be applied to correctly determine series resistances. Effects appearing at high forward gate voltages are discussed. Good agreement between measured and simulated data has been achieved.

A Study of Bisazido(dimethylamino‐propyl)gallium as a Precursor for the OMVPE of Gallium Nitride Thin Films in a Cold‐Wall Reactor System under Reduced Pressure

Abstract: The use of alternative nitrogen sources for growing GaN materials by organometallic vapor phase epitaxy (OMVPE) is being continuously investigated in the hope of achieving device-quality films under moderate conditions, in comparison to conventional methods. Employing the single molecule precursor (N 3 ) 2 Ga[(CH 2 ) 3 NMe 2 ], and using a cold-wall CVD reactor, epitaxial films of GaN, transparent in appearance and stoichiometric in composition, were deposited on c-plane sapphire, in the absence of ammonia, above 1073 K, under low pressures (between 0.080 and 100.0 mbar). Dense, amorphous, and very smooth films were grown at temperatures as low as 773 K. The influence of substrate temperature, reactor pressure, and the effect of small quantities of additional ammonia, on the growth rate and the film properties, were studied in some detail. The films were characterized by high-resolution X-ray diffraction (XRD) (e.g., full width at half maximum (FWHM) of the 0002 GaN rocking curve of 130 arcsec), X-ray reflectometry, scanning electron microscopy (SEM), atomic force microscopy (AFM) (root mean square roughness of 1.9 nm), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), Rutherford backscattering (RBS) (Ga/N = 1:1 ± 0.05), and photoluminescence (PL) measurements (band edge luminescence at 3.45 eV and FWHM of 0.22 eV at 300 K).

Photoenhanced wet oxidation of gallium nitride

Abstract: We investigate the photo-oxidation process and the corresponding passivation effects on the optical properties of unintentionally doped n-type gallium nitride (GaN). When illuminated with a 253.7 nm mercury line source, oxidation of GaN is found to take place in aqueous phosphorus acid solutions with pH values ranging from 3 to 4. At room temperature, the photo-oxidation process is found reaction-rate limited and has a peak value of 224 nm/h at pH=3.5. Compared with the as-grown GaN layers, threefold enhancement in the photocurrent and photoluminescence response are observed on the oxidized GaN surfaces. These results are attributed to the surface passivation effects due to the deep ultraviolet-enhanced wet oxidation on GaN.

Band gap engineering in amorphous AlxGa1−xN: Experiment and ab initio calculations

Abstract: Amorphous alloys of aluminum nitride and gallium nitride deposited at 100 K at compositions ranging from pure AlN to pure GaN with optical band gaps which vary linearly with composition from 3.27 eV (a-GaN) to 5.95 eV (a-AlN) have been synthesized. Ab initio molecular dynamics calculations for these alloys reproduce the band gap versus composition data and give specific information on the electronic localization of the band tail states. There are no midgap states in amorphous AlxGa1−xN alloys. The calculated models have mixed four-fold and three-fold coordination and have no wrong (homopolar nuclear) bonds, demonstrating the strong ionicity in amorphous AlxGa1−xN alloys. It has been found that the valence band tail states are mostly localized on the three-fold coordinated N sites while the conduction band tail states are mostly localized on the three-fold coordinated Ga or Al sites.

Scanning second-harmonic/third-harmonic generation microscopy of gallium nitride

Abstract: Scanning second-harmonic generation and third-harmonic generation microscopy of a gallium nitride (GaN) sample was demonstrated using a femtosecond Cr:forsterite laser. Taking advantage of the electric-field enhanced second-harmonic generation effect and bandtail state resonance effect, the obtained second-harmonic and third-harmonic generation microscopic images revealed the piezoelectric field and bandtail state distributions in a GaN sample.

Simulation and optimization of 420-nm InGaN/GaN laser diodes

Abstract: Using self-consistent laser simulation, we analyze the performance of nitride Fabry-Perot laser diodes grown on sapphire. The active region contains three 4 nm InGaN quantum wells. It is sandwiched between GaN separate confinement layers and superlattice AlGaN/GaN cladding layers. AlGaN is used as an electron barrier layer. Pulsed lasing is measured near 420 nm wavelength and at temperatures up to 120 o C. Advanced laser simulation is applied to link microscopic device physics to measurable device performance. Our two-dimensional laser model considers carrier drift and diffusion including thermionic emission at hetero -boundaries. The local optical gain is calculated from the wurtzite band structure employing a non Lorentzian line broadening model. All material parameters used in the model are evaluated based on recent literature values as well as our own experimental data. Simulation results are in good agreement with measurements. Multi-lateral mode lasing is calculated with a high order vertical mode. The carrier distribution among quantum wells is found to be strongly non-uniform leading to a parasitic (absorbing) quantum well. The influence of defect recombination, vertical carrier leakage and lateral current spreading is investigated. The reduction of such carrier losses is important to achieve lower threshold currents and less self-heating. Several device optimization options are proposed. Elimination of the parasitic quantum well is shown to substantially enhance the device performance.

Methods of fabricating gallium nitride layers on textured silicon substrates, and gallium nitride semiconductor structures fabricated thereby

Abstract: A gallium nitride semiconductor layer is fabricated by exposing (111) crystallographic planes in a face of a (100) silicon substrate, and growing hexagonal gallium nitride on the (111) crystallographic planes that are exposed. Thus, a (100) silicon substrate, which is widely used for fabricating conventional microelectronic devices such as bipolar and field effect transistors, may be used to fabricate gallium nitride semiconductor layers thereon. The (111) crystallographic planes may be exposed in the face of the (100) silicon substrate by wet-etching the face of the (100) silicon substrate. More specifically, the face of the (100) silicon substrate may be dipped in KOH for a short period of time, such as about ten seconds or less, to expose the (111) crystallographic planes therein. The face of the (100) silicon substrate may be unmasked when dipped in KOH, to thereby expose randomly spaced apart (111) crystallographic planes in the face of the (100) silicon substrate. Alternatively, the face of the (100) silicon substrate may be masked prior to dipping in the KOH, to thereby expose a periodic or nonrandom pattern of (111) crystallographic planes therein.

Radiation-emitting semiconductor element has a semiconductor body formed by a stack of different semiconductor layers based on gallium nitride

Abstract: Radiation-emitting semiconductor element has a semiconductor body formed by a stack of different semiconductor layers based on gallium nitride, and first and second main surfaces (3, 4). A part of the radiation produced (5) is decoupled through the first main surface and the second main surface has a reflector (6). The stack of different semiconductor layers is produced by applying an intermediate layer (9) on a substrate (8), applying a number of different gallium nitride layers on the intermediate layer, removing the substrate including the intermediate layer, and applying the reflector to the second main surface of the semiconductor body. Preferred Features: The substrate is made of silicon and the intermediate layer is made of silicon carbide. The intermediate layer is connected to the substrate by wafer bonding.

Ion implanted dopants in GaN and AlN: Lattice sites, annealing behavior, and defect recovery

Abstract: The recovery of structural defects in gallium nitride (GaN) and aluminum nitride (AlN) after implantation of 111In+ and 89Sr+ in the dose range (0.1–3) 1013 cm−2 and ion energies of 60–400 keV has been investigated as a function of annealing temperature with emission channeling (EC) and perturbed γγ angular correlation spectroscopy. The implanted In and Sr atoms occupied substitutional sites in heavily perturbed surroundings of point defects after room temperature implantation. No amorphization of the lattice structure was observed. The point defects could be partly removed after annealing to 1473 K for 10–30 min. Lattice site occupation of implanted light alkalis, 24Na+ in GaN and AlN as well as 8Li+ in AlN, were also determined by EC as a function of implantation and annealing temperature. These atoms occupied mainly interstitial sites at room temperature. Lithium diffusion and the occupation of substitutional sites was observed in GaN and AlN at implantation temperatures above 700 K. A lattice site cha...

Treatment method of film quality for the manufacture of substrates

Abstract: A method of fabricating substrates, e.g., bulk wafers, silicon on insulator wafers, silicon on saphire, optoelectronic substrates. The method includes providing a substrate (e.g., silicon, gallium arsenide, gallium nitride, quartz). The substrate has a film characterized by a non-uniform surface, which includes a plurality of defects. At least some of the defects are of a size ranging from about 100 Angstroms and greater. The method also includes applying a combination of a deposition species for deposition of a deposition material and an etching species for etching etchable material. The combination of the deposition species and the etching species contact the non-uniform surface in a thermal setting to reduce a level of non-uniformity of the non-uniform surface by filling a portion of the defects to smooth the film of material. The smoothed film of material is substantially free from the defects and is characterized by a surface roughness of a predetermined value.

In-situ Ion Channeling Study of Gallium Disorder and Gold Profiles in Au-implanted GaN

Abstract: Disorder accumulation and annealing behavior on the Ga sublattice in gallium nitride (GaN) implanted with 1.0 MeV Au2+ (60° off surface normal) at 180 or 300 K have been studied using in situ Rutherford backscattering spectrometry in a 〈0001〉-channeling geometry. Complete amorphization in GaN is attained at 6.0 and 20 Au2+/nm2 for irradiation at 180 and 300 K, respectively. A saturation in the Ga disorder at and behind the damage peak was observed at intermediate ion fluences at both 180 and 300 K. No measurable thermal recovery was found at 300 K for the full range of damage produced at 180 K. However, distinct epitaxial regrowth in the bulk and Ga reordering at surface occurred after annealing at 870 K. The implanted Au readily diffuses into the highly damaged regions at elevated temperatures, and the redistribution of the Au atoms in the implanted GaN varies with the damage profiles. A double-peak Au profile developed with the maxima located in the amorphous surface region and near the Au mean projecte...

Growth method of gallium nitride film

Abstract: A method of growing a high quality gallium nitride (GaN) film at a high growth rate, which is used in homoepitaxy blue laser diodes or electronic devices. In the GaN film growth method, a bare sapphire substrate is nitridated in a reactor, and then sequentially exposed to a surface pretreatment with a gas mixture of ammonia (NH 3 ) and hydrochloric acid (HCl), and an additional nitridation. The obtained GaN film has a mirror surface with slight roughness.