bluuebunny/crossref_metadata_2025_split huggingface dataset:
Stress Adjustment and Characteristics Improvement in a 1.8GHz Range Film Bulk Acoustic Wave Resonator by Using Multi-layer Structure of ZnO/A12O3/SiO2
Authors: Takeuchi Masaki , Yamada Hajime , Kawamura Hideki , Goto Yoshihiko , Nomura Tadashi , Fujino Hiroyuki , Yoshino Yukio , Makino Takahiro , Arai Seiichi 
Published: April 2011

ABSTRACTStress adjustment and improvement of electrical characteristics in film bulk acoustic wave resonator (BAW) have been successfully carried out in 1.8GHz range by using a multi-layer structure of ZnO / Al2O3 / SiO2. The BAW resonator, designed on secondary harmonics at about 1.8GHz, has Al / ZnO / Al / Al2O3 / SiO2 structure. ZnO and SiO2 thin films have a negative and a positive temperature coefficient of sound velocity, respectively. So temperature coefficient of frequency (TCF) of the BAW resonator can be controlled by the thickness ratio of ZnO and SiO2 thin films. Since both ZnO and SiO2 have compressive stress, and Al2O3 has tensile one, the stress of the membrane is reduced by combining these thin films so that the membrane can avoid deformation. The BAW resonator, with thickness of ZnO / Al2O3 / SiO2=1.2 /0.45/1.25 microns, was designed by finite element method (FEM) simulation and fabricated. The value of quality factor (Q factor) and the TCF of the BAW resonator were realized over 1000 and −20 ppm/degree C, respectively. The Q of ZnO / Al2O3 / SiO2 structure was higher than that of ZnO / SiO2 one with keeping small TCF.
Self-aligned Split Gate Electrodes Fabricated on Suspended Carbon Nanotubes
Authors: Lee S.-B. , Robinson L.A.W. , Teo K.B.K. , Chhowalla M. , Amaratunga G.A.J. , Milne W.I. , Hasko D.G. , Ahmed H. 
Published: April 2011

ABSTRACTWe describe the fabrication of self-aligned split gate electrodes on suspended multiwalled carbon nanotube structures. A suspended multiwalled carbon nanotube structure was used as an evaporation mask for the deposition of metal electrodes resulting in the formation of discontinuous wire deposition. The metal deposits on the nanotubes are removed with lift-off due to the poor adhesion of metal to the nanotube surface. Using Al sacrificial layers, it was possible to fabricate self-aligned contact electrodes and control electrodes nanometers from the suspended carbon nanotubes with a single lithography step. It was also shown that the fabrication technique may also be used to form nano-gaped contact electrodes. The technique should prove useful for the fabrication of nano-electromechanical systems.
Whole-Wafer Optical Mapping of Defects in Insulating Silicon Carbide Wafers
Authors: Mier Millard G. , Boeckl John J. , Hill David A. , Bertrand Scott D. , Ramakrishnan Easwar , Roth Matthew D. , Balkas Cengiz , Nelson Matthew P. 
Published: April 2011

ABSTRACTPlotting defect locations in insulating SiC presents a challenge because the total number of locations on a wafer is so large. We scan the wafer with visible light at an appropriate resolution and sort out transmissions appropriate for the defects we are looking for. Under these conditions, we find that voids and micropipes reduce the pixel transmission to 0.3 to 0.5. Sorting for this transmission reduces the number of pixels of interest to a manageable number, especially with recent progress in growing lower defect SiC. Now a commercial plotting program can easily display defect locations within a circle representing the wafer boundary. We verify the defect locations by scanning electron microscope secondary electron images and scanning optical microscope visible-light images at several resolutions.
Field Enhancement Mechanisms and Electron Field Emission Properties of Ion Beam Synthesized and Modified SiC/Si Heterostructures
Authors: Tsang W. M. , Wong S. P. , Lindner J. K. N. 
Published: April 2011

ABSTRACTSiC/Si heterostructures were synthesized by high dose carbon implantation into silicon using a metal vapor vacuum arc ion source. Their electron field emission properties were studied and correlated with results from other characterization techniques including atomic force microscopy (AFM), conducting AFM, Fourier transform infrared absorption spectroscopy, x-ray diffraction and photoelectron spectroscopy. It is clearly demonstrated that there are two types of field enhancement mechanisms responsible for the improvement of the electron field emission properties of these ion beam synthesized SiC/Si heterostructures, namely, the surface morphology effect and the local electrical inhomogeneity effect. The dependence of the FE properties on the carbon implant dose and thermal annealing conditions could be understood in terms of these two field enhancement mechanisms. It is also demonstrated that improvement in the FE properties can be achieved by implanting tungsten ions into these SiC/Si heterostructures.
The Effect of Annealing on High-resistivity and Semi-insulating 4H-SiC
Authors: Smith S. R. , Evwaraye A. O. , Mitchel W. C. 
Published: April 2011

ABSTRACTWe have examined specimens of high-resistivity, and semi-insulating, 4H-SiC before and after thermal annealing at 1600 °C, using Optical Admittance Spectroscopy. We have found enhanced ultraviolet response in most specimens. Enhanced activation of previously undetected V impurities has also been observed. Peaks believed to be attributable to complex V-related defects were greatly reduced by annealing. The annealing was in addition to a thermal oxidation at 1150 °C for 4 hours. The purpose of the oxidation was to remove surface-related deep levels known to be present in polished SiC. Transition metal impurities in these bulk specimens were quantified by SIMS. In specimens where Ti was not detected by SIMS, no further activation of Ti centers was detected by Optical Admittance Spectroscopy.
The Effect of Channel Recess and Passivation on 4H-SiC MESFETs
Authors: Cha Ho-Young , Thomas Christopher I. , Koley Goutam , Eastman Lester F. , Spencer Michael G. 
Published: April 2011

ABSTRACTChannel-recessed 4H-SiC MESFETs were fabricated and demonstrated excellent small signal characteristics. A saturated current of 250 − 270 mA/mm at Vgs = 0 V and a maximum transconductance of 40 − 45 mS/mm were measured for channel-recessed devices with a gate length of 0.45 m. The three-terminal breakdown voltages (Vds) range from 120 V to 150 V. The Ft and Fmax of the 2 × 200 m devices were measured to be 14.5 GHz and 40 GHz, respectively. The channel recess technique results in a lower saturation current but higher breakdown voltage which makes it possible for the devices to operate at high voltages. Si3N4 passivation suppresses the instability in DC characteristics and improves CW power performance by reducing the surface effects. Less dispersion in the drain current during a power sweep was observed after passivation.
System Design Considerations for Optimizing the Benefit by Unipolar SiC Power Devices.
Authors: Rupp Roland , Zverev Ilia 
Published: April 2011

ABSTRACTFocussing on unipolar SiC power devices a variety of applications are described, where cost reduction can be a achieved on system level even for SiC device costs being several times higher than the costs of the competing Si devices. Based on the specific properties of SiC devices like Schottky diodes and JFETs it is explained with the help of these examples how this is attainable.
X-ray Spectroscopic Studies of the Bulk Electronic Structure of InGaN Alloys
Authors: McGuinness Cormac , Downes James E. , Ryan Philip , Smith Kevin E. , Doppalapudi Dharanipal , Moustakas Theodore D. 
Published: April 2011

ABSTRACTSynchrotron radiation excited soft x-ray emission and soft x-ray absorption spectroscopies are applied to the study of the electronic structure of InxGa1-xN alloys with (0 ≤ x ≤ 0.29). The elementally resolved partial density of states of the valence and conduction bands may be measured using these spectroscopies. The x-ray absorption spectra indicate that the conduction band broadens considerably with increasing indium incorporation. The band gap evolution as a function of indium content derives primarily from this broadening of the conduction band states. The emission spectra indicate that motion of the valence band makes a smaller contribution to the evolution of the band gap. This gap evolution differs from previous studies on the AlxGa1-xN alloy system, which observed a linear valence band shift through the series (0 < x < 1). Instead in the case of InxGa1-xN the valence band exhibits a significant shift between x = 0 and x = 0.1 with minimal movement thereafter. Furthermore, evidence of In 4d -N 2p and Ga 3d- N 2p hybridisation is reported. Finally, the thermal stability of an In011Ga089N film was investigated. Both emission and absorption spectra were found to have a temperature dependent shift in energy, but the overall definition of the spectra was unaltered even at annealing temperatures well beyond the growth temperature of the film.
An In-Situ TEM-Cathodoluminescence Study of Electron Beam Degradation of Luminescence from GaN and In0.1Ga0.9N Quantum Wells
Authors: Boyall Nicholas M. , Durose Ken , Watson Ian M. 
Published: April 2011

ABSTRACTThe effect of electron beam irradiation on the cathodoluminescence (CL) emission from In0.1Ga0.9N/GaN single quantum wells (QW) has been investigated by in-situ measurement of CL in a transmission electron microscope. Analysis of CL quenching over 600s showed that the QW luminescence decayed more quickly than the barrier emission. Both the In0.1Ga0.9N and GaN CL decay curves could be fitted to a simple recombination based model suggesting the decay was due to the introduction of non-radiative centres.
Room-Temperature Time–Resolved Photoluminescence Studies of UV Emission from GaN/AlN Quantum Wells
Authors: Furis Madalina , Chen Fei , Cartwright Alexander N. , Wu Hong , Schaff William J. 
Published: April 2011

ABSTRACTRoom temperature time-resolved photoluminescence (TRPL) studies of multiple quantum well (MQW) structures of the binaries GaN and AlN grown by molecular beam epitaxy are reported. The eventual application of these structures is for GaN intersubband IR light emitters. However, as an initial study, the structures are evaluated at UV to investigate materials parameters relevant to IR light emission. The nominally 0.9, 1.3 and 1.5 nm GaN quantum wells are clad by 6nm of AlN on top of a thick AlN buffer grown on sapphire. All samples consisted of 20 quantum wells. The observed peak energy of the emission spectrum is in excellent agreement with a model that includes the strong confinement present in these structures and the existence of the large built-in piezoelectric field and spontaneous polarization present inside the wells. Furthermore, consistent with screening of the in-well field as carriers are injected in the well, a clear blue shift of the emission is observed at short times after carrier injection. Subsequently, as the carriers recombine, the peak emission red-shifts and the screening of the field is reduced. Moreover, the observed lifetimes were energy dependent as should be expected from field dependent elongation of lifetimes due to spatial separation of the injected carriers. Specifically, the decay time at high energies can be fitted by a stretched exponential with a beta value of 0.8 which is consistent with carrier spatial separation. The lifetimes obtained from the fitting are of the order of 1ns, longer than the reported recombination lifetimes in similar GaN/AlGaN MQW's. On the low energy side of the PL feature the intensity time decay becomes exponential with lifetimes ranging from 3 to 10ns. The strong UV emission at room temperature makes these structures promising for UV emitters.
A Study of Indium Nitride Films Grown Under Conditions Resulting in Apparent Band-gaps from 0.7 eV to 2.3 eV
Authors: Butcher K. S. A. , Wintrebert-Fouquet M. , Motlan  , Shrestha S. K. , Timmers H. , Prince K. E. , Tansley T. L. 
Published: April 2011

ABSTRACTThe band-gap of indium nitride has long been believed to be about 1.9eV with slight variations due to band-tailing in polycrystalline samples and degenerate doping. Recently, other values as low as 0.7 eV have apparently been observed. We have compared samples spanning this apparent range of band-gap using secondary ion mass spectroscopy (SIMS), X-ray Photoelectron Spectroscopy (XPS) and heavy ion elastic recoil detection analysis (ERDA), in conjunction with spectral optical density measurements. Once structural inhomogeneiteies are taken into account, we show that much of the conflicting data are compatible with direct photoionisation with a threshold energy of about 1.0eV. This feature was first reported in polycrystalline indium nitride over 15 years ago and attributed to a ∣p> like defect state. We ask whether the feature may instead be a direct band-gap.
Optical Properties of Controllable Self-Assembled Lateral Nanostructures on InN, InAlN, and AlN Thin Films
Authors: Danylyuk Yuri , Romanov Dmitri , McCullen Eric , Haddad Daad , Naik Ratna , Auner Gregory 
Published: April 2011

ABSTRACTUtilizing plasma source molecular beam epitaxy (PSMBE), we have grown epitaxial InxAl1-xN films on (0001) sapphire substrates; the indium concentration, x, varied from 0 to 1. The atomic force microscopy of the films reveals characteristic surface patterns of nanometer scale. The feature size distribution is determined by the film composition and thickness. Both absorption and reflection spectra of the films have additional peaks below the fundamental absorption threshold. These peaks cannot be associated with N vacancies or any other known crystal defects and impurities. We attribute the peaks to electron confinement in the hillocks of the lateral structure by the strong electric field of piezoelectric and spontaneous polarization that is characteristic to nitride semiconductor compounds. The calculated values of the electron energy levels are in good agreement with the spectroscopic data; moreover, the electron confinement model explains the observed temperature dependence of the additional peaks. The hillock size control will allow one to control the optical and transport properties of the films.
Plane-wave pseudopotential study on mechanical and electronic properties for group III-V binary phases
Authors: Wang S. Q. , Ye H. Q. 
Published: April 2011

ABSTRACTThe result of first-principles density functional calculations of the bulk modulus and related structural and electronic properties of the total 25 group III-V binary phases with zinc-blende and wurtzite structures are presented. The behavior of energy band structure variation under high pressures is also studied. It is found that the bulk modulus is more sensitive to the local atom configuration than the lattice structure. The crystallographic geometry plays an important role in the electronic property of these phases.
Transport Studies on Two-subband-populated AlGaN/GaN Heterostructures
Authors: Hang D. R. , Huang C. F. , Chen Y. F. , Shen B. 
Published: April 2011

ABSTRACTWe report an investigation of electronic properties of two-dimensional electron gas (2DEG) confined at AlGaN/GaN heterostructures by magnetotransport measurements. The second-subband population is manifested by the multi-frequency in the Shubnikov-de Haas (SdH) oscillations. The modulated patterns of SdH oscillations which are due to the two-subband occupancy can be drastically enhanced by employing the microwave modulation technique. This unique advantage enables us to provide direct experimental evidence that the 2DEG in the second subband has a higher mobility than that in the first subband in the modulation-doped Al0.22Ga0.78N/GaN heterostructures by means of microwave-modulated magnetotransport measurements. The carrier concentrations and 2DEG Fermi energy for each subband were determined. It was found that the second-subband population ratio increases with spacer thickness up to 5 nm, while the subband separation decreases.
A structural analysis of the Pd/GaN ohmic contact annealing behavior
Authors: Kim C. C. , Ruterana P. , Je J. H. 
Published: April 2011

AbstractFor ohmic contact on p GaN, palladium is one of the best candidates showing ohmic characteristics already without annealing. To be realized in devices, it is necessary to know the behavior of the ohmic contacts at accelerated conditions, especially for high temperatures and power. We report on the structural evolution of palladium layers (30 nm) deposited on GaN (0001) by electron beam evaporation without intentional annealing. They were next cut into various pieces which were individually submitted to rapid thermal annealing at 400, 500, 600, 700 and 800°C for 10 sec. We investigate the differences in the microstructure and the location of interfacial phases and their relationships as determined by X-ray diffraction and transmission electron microscopy, we then suggest the formation mechanism based on the relationship. It is shown that the interface is disrupted at annealing above 600°C and by 800°C only very small patches of Pd are still present, however they area completely imbedded in a matrix of intermetallic phases (gallides) formed by the reaction with GaN.
Nitrogen Gas-Cluster Ion Beam – A New Nitrogen Source for GaN Growth
Authors: Shao Y. , Chen T. C. , Fenner D. B. , Moustakas T. D. , Chu George 
Published: April 2011

ABSTRACTIn this paper we report sapphire nitridation and GaN film growth by using ionized nitrogen clusters as a nitrogen source. The clusters, typically 2000–3000 molecules/charge, are generated by a prototype GCIB source. The clusters are accelerated to 10–25 kV and disintegrate upon impact with the substrate surface where they react with Ga atoms to form GaN. The efficiency of this novel nitrogen source was tested by studying the nitridation of (0001) sapphire substrates at relatively low temperatures of 200 - 400°C. The effect of exposure of the substrate to the nitrogen cluster-ion beam was examined by XPS, RHEED and AFM. It was found that the amount of retained surface nitrogen increases nonlinearly with increasing beam energy. There exists a threshold energy, ∼ 20 kV, above which nitrogen retention is significantly enhanced.GaN films were grown with such nitrogen clusters heteroepitaxially on sapphire/AlN-buffer (MBE grown) and homoepitaxially on thick GaN on sapphire (HVPE grown).TEM cross-section images indicate that the heteroepitaxial GaN films had defect density similar to that of MBE and MOCVD grown films. The homoepitaxially-grown GaN films were found to replicate the GaN templates and show strong cathodoluminescence (CL) emission at 363 nm with FWHM of 9 nm. Furthermore, the spectra show no evidence of yellow band emission.
Growth and Characterization of non-polar (11–20) GaN and AlGaN/GaN MQWs on R-plane (10–12) sapphire
Authors: Iyer Sandeep , Smith David J. , Bhattacharyya A. , Ludwig K. , Moustakas T. D. 
Published: April 2011

ABSTRACTThe majority of GaN films and related devices have been grown along the polar [0001] direction, and epitaxial growth along non-polar directions has received much less attention. In this paper we report the study of material properties of GaN and AlGaN/GaN multiple quantum wells (MQWs) deposited on R-plane (10–12) sapphire substrates using RF plasma-assisted molecular beam epitaxy (MBE). In this growth direction, III-Nitrides grow along the non-polar [11–20] direction, with thec-axis in the plane of growth. Various nucleation steps such as surface nitridation, as well as GaN and AlN buffer layers were investigated. Our results indicate that surface nitridation of R-plane sapphire is an undesirable nucleation step, contrary to what has been observed in the case of (0001) sapphire. The AlN buffer layer leads to well-oriented films along the [11–20] direction with many threading defects and faceted surface morphology whereas the GaN buffer leads to the formation of mis-oriented domains close to the buffer region. However, these domains are overgrown and the films have smoother surface morphology with fewer threading defects. These structural findings are supported by photoluminescence and Hall effect measurements done on the same films. Photoluminescence (PL) measurements of (11–20) AlGaN/GaN MQWs show much higher intensity than for similar structures grown on the C-plane sapphire, consistent with the absence of internal fields in the non-polar direction.
Correlation Between the AlN Buffer Layer Thickness and the GaN Polarity in GaN/AlN/Si(111) Grown by MBE
Authors: Sanchez A. M. , Ruterana P. , Vennegues P. , Semond F. , Pacheco F. J. , Molina S. I. , Garcia R. , Sanchez-Garcia M. A. , Calleja E. 
Published: April 2011

ABSTRACTIn this work it is shown that thin AlN buffer layers cause N-polarity GaN epilayers, with a high inversion domains density. When the AlN thickness increases, the polarity of the epilayer changes to Ga. The use of a low temperature AlN nucleation layer leads to a flat AlN/Si(111) interface. This contributes to decrease the inversion domains density in the overgrown GaN epilayer with a Ga polarity.
Theoretical analysis of <0001> tilt grain boundaries in GaN at the atomic scale
Authors: Chen Jun , Ruterana Pierre , Nouet Gérard 
Published: April 2011

ABSTRACTEpitaxial layers of GaN contain a very high density of threading dislocations. In the first stage of growth they may form low and high angle grain boundaries. Energetic calculations of <0001> tilt grain boundaries have been performed with the Stillinger-Weber potential modified to take into account the wrong bonds Ga-Ga and N-N. The variation of the energy has been calculated as a function of the rotation angle. Two minima exist with special atomic structures based on a limited number of structural units. They are used to describe the other misorientations in terms of dislocation cores.
Ion Beam Study of Early Stages of Growth of GaN films on Sapphire
Authors: Trifan Eugen M. , Ingram David C. 
Published: April 2011

ABSTRACTAn innovative approach for in-situ characterization has been used in this work to investigate the composition, growth mode, morphology and crystalline ordering of the early stages of growth of GaN films grown on sapphire by MOCVD for substrate temperatures in the range of 450°C to 1050°C. We have performed in-situ characterization by Rutherford Backscattering Spectroscopy (RBS), Ion Channeling, X-ray Photoelectron Spectroscopy (XPS), and Low Energy Electron Diffraction. Ex-situ the films have been characterized by Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD) and thickness profilometry. The films have been grown in an in-house designed and build MOCVD reactor that is attached by UHV lines to the analysis facilities. RBS analysis indicated that the films have the correct stoichiometry, have variable thickness and for low substrate temperature completely cover the substrate while for temperatures 850°C and higher islands are formed that may cover as few as 5 percent of the substrate. From Ion Channeling and LEED we have determined the crystallographic phase to be wurtzite. The crystalline quality increases with higher deposition temperature and with thickness. The films are epitaxialy grown with the <0001> crystallographic axis and planes of the GaN films aligned with the sapphire within 0.2 degrees.
Comparisons of Gallium Nitride and Indium Nitride Properties after CF4 / Argon Reactive Ion Etching
Authors: Wintrebert-Fouquet Marie , Scott K. , Butcher A. , Lam Simon K H 
Published: April 2011

ABSTRACTWe present a comparative study of the effects of low power reactive ion etching (RIE) on GaN and InN. This new, highly chemical, dry etching, using CF4 and Ar, has been developed for thin nitride films grown at low temperature in our laboratories. GaN films were grown by remote plasma enhanced-laser induced chemical vapor deposition and InN films were grown by radio-frequency RF reactive sputtering. Commercial GaN samples were also examined. Optical and electrical characteristics of the films are reported before and after removing 100 to 200 nm of the film surface by RIE. We have previously shown that the GaN films, although polycrystalline after growth, may be re-crystallized below the growth temperature. Removal of the surface oxide has been found to be imperative since a polycrystalline residue remains on the surface after re-crystallization.