bluuebunny/crossref_metadata_2025_split huggingface dataset:
Self-assembled Ge quantum dots on SiC substrates grown by UHV-CVD
Authors: Calmes C. , Le V. , Bouchier D. , Saddow S. E. , Yam V. , Débarre D. , Laval R. , Clerc C. 
Published: April 2011

AbstractWe report our first results using a ultra high vacuum chemical vapor deposition (UHV-CVD) system to form Ge quantum dots on off-axis SiC substrates. Pure SiH4 and hydrogen-diluted GeH4 were used as gas precursors. The SiC substrates were chemically cleaned using the modified RCA process and the SiO2 layer was removed in-situ under a low SiH4 flow rate at a temperature between 1030°C and 1080°C. The Ge quantum dots were grown at a temperature of 750°C. In-situ reflection high-energy electron diffraction (RHEED) was used to monitor the surface cleaning and the Ge quantum dot growth. Ex-situ scanning electron microscope and atomic force microscopy were used to confirm the presence of Ge dots. The observed dots are smaller (350 Å width and 100 Å height) than similar Ge dots grown on Si.
SiC TO SiC WAFER BONDING
Authors: Yushin G. N. , Kvit A. V. , Collazo R. , Sitar Z. 
Published: April 2011

ABSTRACTSiC wafers with an RMS roughness of 1.5 nm were bonded in a dedicated ultrahigh vacuum bonding chamber. Successful fusion of wafers was observed at temperatures as low as 800°C under a uniaxial mechanical stress of 20 MPa. Cross-section transmission electron microscopy (XTEM) of a specimen bonded at 1100°C revealed parts of the interface where wafers were in intimate contact, while other parts contained an up to 3 nm thick amorphous carbon interlayer. The bonded SiC retained its high crystalline quality; no extended defects emanating from the interface were observed within the sampling region. Electrical measurements showed that the azimuthal orientation of the bonded couple significantly influences the electrical character of the junction.
Afterglow Thermal Oxidation of Silicon Carbide
Authors: Hoff Andrew M. , Tibrewala Arti , Saddow Stephen E. 
Published: April 2011

ABSTRACTWe report, for the first time, the growth of thermal oxides on silicon carbide at temperatures below 900 °C using a novel afterglow thermal reactor. Flowing afterglows of microwave plasmas have been used for decades to study fundamental chemical reactions and in recent years they are employed as rich atomic oxygen sources for one of the principal techniques of polymeric photoresist removal in IC plants around the globe. The advantage of afterglow generation of a high flux of reactive species is that microwave plasmas may be excited inside clean quartz tubing and further the plasma potential of such discharges is less than 10 electron volts, which is far below the sputtering threshold of most materials, lowering the chance of contamination. In our case, we have integrated an afterglow source with a vacuum furnace system. The substrates to be oxidized are placed in the hot zone of the furnace where they are exposed to the flowing afterglow at a pressure near 1 Torr. The afterglow chemistry generated from pure oxygen traveling through the source includes O2, atomic oxygen, and excited states of O2. All ions and electrons generated by the plasma source are confined to the microwave cavity limits and photons are excluded by trapping techniques. The wafers in the furnace are exposed to a rich chemistry of afterglow species that were generated by non-thermal means. The furnace temperature alone controls the chemical reaction between the afterglow species and the substrates. We present the results of oxide growth on 4H and 6H silicon carbide substrates over a range of temperature. An example result is over 200Å of film grown at 800 °C in 2 hours. Atmospheric processing in O2 at 1050 °C would require 14 hours to grow a similar thickness on SiC. Possible mechanisms and oxide properties will also be discussed.
Transport Mechanisms in Focused Ion Beam Assisted Ohmic Contacts to p-Type 6H-SiC
Authors: Iliadis Agis A. 
Published: April 2011

ABSTRACTThe current transport mechanism in non-annealed Ohmic contact metallizations on p-type 6H-SiC formed by using focused ion beam (FIB) surface-modification and direct-write metal deposition is reported, and the properties of such focused ion beam assisted non-annealed contacts are discussed. The process uses a Ga focused ion beam to modify the surface of the semiconductor with different doses, and then introduces an organometallic compound in the Ga ion beam, to effect the direct-write deposition of a metal on the modified surface. Contact resistance measurements by the transmission line method produced values in the low 10-4 Ω cm2 range for surface-modified and direct-write Pt and W non-annealed contacts, and mid 10-5 Ω cm2 range for surface-modified and pulse laser deposited TiN contacts. The current transport mechanism of these contacts was examined and found to proceed mainly by tunneling through the metal-modified-semiconductor interface layer.
Characterization of a Ceramic-Metal-Ceramic Bond: Chemical Vapor Deposited (CVD) Silicon Carbide Joined by a Silver-Based Active Brazing Alloy (ABA)
Authors: Marzik James V. , Oyama Toshi , MoberlyChan Warren J. , Croft William J. 
Published: April 2011

ABSTRACTChemical vapor deposited (CVD) silicon carbide (SiC) ceramic material was joined to itself via an air stable, silver-based active brazing alloy (ABA). The microstructure and microchemistry of the interface was characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Results were compared to previous studies on the active alloy brazing of sintered silicon carbide using higher copper alloys.
Homoepitaxial 4H-SiC films grown by microwave plasma chemical vapor deposition
Authors: Okamoto Mitsuo , Kosugi Ryoji , Nakashima Shinichi , Fukuda Kenji , Arai Kazuo 
Published: April 2011

ABSTRACTHomoepitaxial 4H-SiC film growth has been carried out at temperatures as low as 1000°C on 4H-SiC of Si-face and C-face by microwave plasma chemical vapor deposition method. The extent of step-bunching of those films grown on C-face was low in comparison with that on Si-face, although large and irregular shaped step-bunching was occurred in both films grown on Si-face and C-face. For the first step to application for the electrical devices, the electrical properties of the μPCVD grown films was characterized by fabricating simple pn-junction structure. The obtained SiC films indicated n-type conductivity and the amount of background donor impurities of the films grown on C-face substrates were lower by one order than that on Si-face.
Aluminum and boron diffusion in 4H-SiC
Authors: Linnarsson M. K. , Janson M. S. , Schöner A. , Svensson B. G. 
Published: April 2011

ABSTRACTA brief survey is given of some recent result of boron diffusion in low doped n-type (intrinsic) and p-type 4H-SiC. Aluminum diffusion and solubility limit in 4H-SiC are also discussed. Ion implantation of boron has been performed in epitaxial material to form a diffusion source but also epitaxial 4H-SiC structures, with heavily boron or aluminum doped layers prepared by vapor phase epitaxy have been studied. Heat treatments have been made at temperatures ranging from 1100 to 2050°C for 5 minutes up to 64 h. Secondary ion mass spectrometry has been utilized for analysis. For boron diffusion in acceptor doped 4H-SiC, 4×1019 Al atoms/cm3, an activation energy of 5.3 eV has been determined and a strong dependence on Al content for the diffusion coefficient is revealed. Transient enhanced diffusion of ion-implanted boron in intrinsic 4H-SiC samples is discussed. Solubility limits of ∼1×1020 Al/cm3 (1700°C) and <1×1020 B/cm3 (1900°C) have been deduced.
Electrical Instability Suppression in 4H-SiC Power MESFETs
Authors: Tucker J. B. , Beaupre R. A. , Zhang A. P. , Garrett J. L. , Rowland L. B. , Kaminsky E. B. , Kretchmer J. W. , Vertiatchikh A. , Eastman L. F. , Allen A. F. , Edward B. 
Published: April 2011

ABSTRACTSiC has attracted great interest for high power microwave applications because of its superior intrinsic properties compared to Si and GaAs. Steady demonstrations of increasingly higher power handling capability have been achieved in recent years. However, SiC MESFETs still suffer from significant drain current degradation under RF operation or long-term DC stress. This degradation can be recovered after long periods of relaxation or immediately by illumination under UV light, which is indicative of a trapping effect. The origin of this effect has been attributed to either electron trapping at the device surface between the gate and drain or trapping at the epi-substrate interface due to the presence of electrically active contaminants in the bulk. Newly available “high purity” (non-vanadium compensated) bulk 4H semi-insulating SiC substrates were used in an effort to limit the effect of V-related deep level trapping at the substrate/epilayer interface. To investigate the effect of V on SiC MESFET performance, we compare similar devices fabricated on V compensated, and “high-purity” 4H-SiC substrates without intentional V doping. Presence or absence of V is confirmed by secondary ion mass spectrometry (SIMS) analysis. Pulsed I-V measurements as well as current- and capacitance-based deep level transient spectroscopy (DLTS) measurements were performed to assess trapping activation energy and density. An assessment of device performance and stability for each substrate type is made using RF load-pull measurements and device long-term DC bias stressing at temperature.
Influence of Interface States on High Temperature SiC Sensors and Electronics
Authors: Ghosh Ruby N. , Tobias Peter , Golding Brage 
Published: April 2011

ABSTRACTSilicon carbide based metal/oxide/semiconductor (MOS) devices are well suited for operation in chemically reactive high temperature ambients. The response of catalytic gate SiC MOS sensors to hydrogen-containing species has been assumed to be due to the formation of a dipole layer at the metal/oxide interface, which gives rise to a voltage translation of the high frequency capacitance voltage (C-V) curve. From in-situ C-V spectroscopy, performed in a controlled gaseous environment, we have discovered that high temperature (800 K) exposure to hydrogen results in (i) a flat band voltage occurring at a more negative bias than in oxygen and (ii) the transition from accumulation to inversion occurring over a relatively narrow voltage range. In oxygen, this transition is broadened indicating the creation of a large number of interface states. We interpret these results as arising from two independent phenomena – a chemically induced shift in the metal/semiconductor work function difference and the passivation/creation of charged states (DIT) at the SiO2/SiC interface. Our results are important for both chemical sensing and electronic applications. MOS capacitance gas sensors typically operate in constant capacitance mode. Since the slope of the C-V curve changes dramatically with gas exposure, we discuss how sensor-to-sensor reproducibility and device response time are influenced by the choice of operating point. For electronic applications understanding the environmentally induced changes in DIT is crucial to designing drift-free MOS devices. Our results are applicable to n-type SiC MOS devices in general, independent of the specifics of sample fabrication.
Cantilever Epitaxy of GaN on Sapphire: Further Reductions in Dislocation Density
Authors: Follstaedt D. M. , Provencio P. P. , Koleske D. D. , Mitchell C. C. , Allerman A. A. , Missert N. A. , Ashby C. I. H. 
Published: April 2011

ABSTRACTThe density of vertical threading dislocations at the surface of GaN grown on sapphire by cantilever epitaxy has been reduced with two new approaches. First, narrow mesas (<1 μm wide) were used and {11–22} facets formed over them early in growth to redirect dislocations from vertical to horizontal. Cross-sectional transmission electron microscopy was used to demonstrate this redirection and to identify optimum growth and processing conditions. Second, a GaN nuc-leation layer with delayed 3D → 2D growth transition and inherently lower threading dislocation density was adapted to cantilever epitaxy. Several techniques show that a dislocation density of only 2–3×107/cm2 was achieved by combining these two approaches. We also suggest other developments of cantilever epitaxy for reducing dislocations in heteroepitaxial systems.
Electrical, Optical, Structural, and Analytical Properties of Very Pure GaN
Authors: Look D. C. , Sizelove J. R. , Jasinski J. , Liliental-Weber Z. , Saarinen K. , Park S. S. , Han J. H. 
Published: April 2011

ABSTRACTPresent hydride vapor phase epitaxial growth of GaN on Al2O3 can produce material of very high quality, especially in regions of the crystal far from the substrate/epilayer interface. In the present study, we characterize a 248-μm-thick epilayer, which had been separated from its Al2O3 substrate and etched on top and bottom to produce flat surfaces. Temperature-dependent Hall-effect data have been fitted to give the following parameters: mobility μ(300) = 1320 cm2/V-s; μ(peak) = 12,000 cm2/V-s; carrier concentration n(300) = 6.27 × 1015 cm−3; donor concentration ND = 7.8 × 1015 cm−3; acceptor concentration NA = 1.3 × 1015 cm−3; and effective donor activation energy ED = 28.1 meV. These mobilities are the highest ever reported in GaN, and the acceptor concentration, the lowest. Positron annihilation measurements give a Ga vacancy concentration very close to NA, showing that the dominant acceptors are likely native defects. Secondary ion mass spectroscopic measurements show that ND is probably composed of the common donors O and Si, with [O] > [S1]. Transmission electron microscopy measurements yield threading dislocation densities of about 1 × 107 cm−2 on the bottom (N) face, and < 5 × 105 cm−2 on the top (Ga) face. Photoluminescence (PL) spectra show a strong donor-bound exciton (D°X) line at 3.47225 eV, and a weaker one at 3.47305 eV; each has a linewidth of about 0.4 meV. In the two-electron satellite region, a strong line appears at 3.44686 eV, and a weaker one at 3.44792 eV. If the two strong lines represent the same donor, then ED,n=1 – ED,n=2 = 25.4 meV for that donor, and the ground-state activation energy (EC – ED,n=1) is (4/3)25.4 = 33.9 meV in a hydrogenic model, and 32.7 meV in a somewhat modified model. The measured Hall-effect donor energy, 28.1 meV, is smaller than the PL donor energy, as is nearly always found in semiconductors. We show that the difference in the Hall and PL donor energies can be explained by donor-band conduction via overlapping donor excited states, and the effects of non-overlapping excited states which should be included in the n vs. T data analysis (charge balance equation).
Hollow core dislocations in Mg-doped AlGaN
Authors: Cherns D. , Wang Y. Q. , Liu R. , Ponce F. A. , Amano H. , Akasaki I. 
Published: April 2011

ABSTRACTTransmission electron microscopy has been used to investigate the core structure of threading dislocations in heavily Mg-doped (1020 cm−3) Al0.03Ga0.97N films grown on (0001) sapphire by metal-organic chemical vapour deposition. Evidence is presented that Mg segregates to edge and mixed dislocations, and that these dislocations often have open cores with diameters in the range 1–5nm. The mechanism of hollow core formation and the role of Mg are discussed.
Measurements of the Refractive Indices of MOCVD and HVPE Grown AlGaN Films Using Prism-Coupling Techniques Correlated with Spectroscopic Reflection/Transmission Analysisa
Authors: Sanford Norman A. , Robins Lawrence H. , Davydov Albert V. , Shapiro Alexander J. , Tsvetkov Denis V. , Dmitriev Vladimir A. , Spring Silver , Keller Stacia , Mishra Umesh K. , DenBaars Steven P. 
Published: April 2011

ABSTRACTWaveguide prism-coupling methods were used to measure the ordinary and extraordinary refractive indices of AlxGa1-xN films grown on sapphire substrates by HVPE and MOCVD. Several discrete wavelengths ranging from 442 nm to 1064 nm were used and the results were fit to one-term Sellmeier equations. The maximum standard uncertainty in the refractive index measurements was ± 0.005 and the maximum standard uncertainty in the self-consistent calculation for film thickness was ± 15 nm. Analysis of normal-incidence spectroscopic transmittance and reflectance measurements, correlated with the prism-coupling results, was used to determine the ordinary refractive index as a continuous function of wavelength from the band gap wavelength of each sample (between 252 nm and 364 nm) to 2500 nm. The Al compositions of the samples were determined using energy-dispersive X-ray spectroscopy analysis (EDS). HVPE grown samples had compositions x = 0.279, 0.363, 0.593, and 0.657. MOCVD samples had x = 0.00, 0.419, 0.507, 0.618, 0.660, and 0.666. The maximum standard uncertainty in the absolute EDS-determined value for x was ± 0.02.
Excitons bound to surface defects in GaN
Authors: Reshchikov M. A. , Huang D. , Morkoç H. 
Published: April 2011

ABSTRACTSharp intense peaks are sometimes detected in the low-temperature photoluminescence (PL) spectrum of undoped GaN samples in the photon energy range of 3.0 – 3.46 eV. Some of these peaks can be attributed to excitons bound to dislocations and inversion domains, whereas some others originate from the GaN surface because they can be affected essentially by surface treatment. In our samples, grown by molecular beam epitaxy on sapphire substrate, the 3.42 eV peak always disappeared after removing the surface layer by etching for a few seconds in hot phosphoric acid. Atomic force microscopy images confirmed that such light etching modifies the surface morphology, although the etched depth is negligibly small. Moreover, intensities of two other peaks (at 3.32 and 3.35 eV) were observed to depend on sample etching, as well as on the length of subsequent exposure to air. The 3.32 and 3.35 eV peaks evolved with time of UV illumination, increasing by several times and demonstrating memory effect at low temperature. We attribute the 3.42 and 3.35 eV peaks to bound excitons, whereas the 3.32 eV peak is tentatively attributed to a surface donor-acceptor pair transition.
Electron Stimulated Desorption of Deuterium from GaN (0001) Surface
Authors: Yang Y. , Lee J. , Thoms B. D. 
Published: April 2011

AbstractTemperature programmed desorption (TPD) was performed on deuterated GaN(0001) surfaces which had been exposed to various doses of 90-eV electrons. TPD of the deuterated surface without electron exposure shows a broad D2 desorption feature with a peak desorption temperature at ∼400 °C. Electron exposure results in a decrease in intensity of the desorption peak which is attributed to removal of surface deuterium by electron stimulated desorption (ESD). This removal of deuterium by ESD produces no change in the peak desorption temperature indicating that recombinative desorption is first order in deuterium coverage.
Dislocation-related deep states induced by irradiation in HVPE n-GaN
Authors: Castaldini A. , Cavallini A. , Polenta L. 
Published: April 2011

ABSTRACTThis paper deals with defects induced by proton irradiation in n-GaN. The samples were HVPE grown, irradiated with 24 GeV protons. DLTS was performed on both as-grown and irradiated samples with planar and normal collector configurations to evidence the dislocation effect. Two electron traps were identified in the as-grown material by both diode structures: trap EC1 (EC – 0.19eV) and trap EC2 (EC – 0.25eV), EC2 being the dominant one. Irradiation consistently affects the pre-existing levels in such a way so as their appearance strongly depends on the diode structure, hence on the region probed by DLTS.This contribution focuses on the actual existence of two deep traps EC1 and EC2 emitting in GaN at low temperature and on the different nature of the deep levels associated to EC1 and EC2. Their filling kinetics was studied since we supposed that EC2 would be associated to extended defects, as already reported in literature. Indeed, the site density of EC2 logarithmically depends on the filling pulse width, demonstrating that this trap is definitely associated to extended defects. The trap EC1, on the contrary, exhibits the filling kinetics peculiar of point defects.
Surface Passivation of AlGaN terminated and GaN Terminated HEMT Structures Studied by XPS
Authors: Gila B. P. , Lambers E. , Luo B. , Onstine A. H. , Allums K. K. , Abernathy C. R. , Ren F. , Pearton S. J. 
Published: April 2011

ABSTRACTSamples of both AlGaN and GaN terminated HEMT structures were studied using x-ray photo spectroscopy (XPS). It was found that the XPS spectra of both AlGaN and native oxide surfaces were shifted by a surface charge of 0.5 to 1.0 eV. The samples were then oxidized using an UV-ozone treatment for 25 minutes at room temperature. The ozone oxide XPS spectra of the AlGaN terminated surface was found to have the same 0.5 to 1.0 eV shift while the ozone oxide XPS spectra of the GaN terminated surface was found to have a 5.0 eV shift, indicating that the native GaN oxide is more insulating. Processed HEMT devices using both surface terminations were given the same UV-ozone treatment followed by a 10nm MBE grown Sc2O3 film for passivation. The GaN terminated HEMT structures showed on average a 10% increase in channel current (from gate-lag measurements, pulse mode) over the AlGaN terminated HEMT structures.
Stable Ohmic Contacts on GaAs and GaN Devices for High Temperatures
Authors: Piotrowska A. , Kaminska E. , Barcz A. , Golaszewska K. , Wrzesinska H. , Piotrowski T. T. , Dynowska E. , Jakiela R. 
Published: April 2011

ABSTRACTWe have studied thermal stability of Nb and NbN contacts to GaAs and GaN by x-ray diffraction and SIMS, and demonstrated their excellent behaviour under high temperature annealing. GaAs/Nb and GaAs/NbN contacts are stable up to 800°C and 900°C, respectively while GaN/NbN and GaN/Nb/NbN remain stable up to 1000°C.
Contacts to High Aluminum Fraction p-type Aluminum Gallium Nitride
Authors: Hull Brett A. , Mohney Suzanne E. , Chowdhury Uttiya , Dupuis Russell D. , Gotthold David , Birkhahn Ronald , Pophristic Milan 
Published: April 2011

ABSTRACTGold, palladium, platinum or nickel ohmic contacts on Mg doped p-type AlxGa1-xN with x = 0.4 and x = 0.45 have been examined. The Au contact provided the lowest contact resistivity with pc = 1.8 (± 1.1) x 10−3 Ωcm2, but only following annealing at 850°C. For the Pd, Au, and Pt contacts annealed at greater than 700°C, a rapid degradation in the current-voltage curves was observed upon testing. The degradation was induced by exposure to sub-bandgap light and was reversed with a mild anneal at 500°C. Possible mechanisms for the degradation are discussed.
Properties of Surface States on GaN and Related Compounds and Their Passivation by Dielectric Films
Authors: Hasegawa Hideki , Hashizume Tamotsu 
Published: April 2011

ABSTRACTThis paper reviews the authors′ recent efforts to clarify the properties of electronic states near surfaces of GaN and AlGaN by using variousin-situandex-situcharacterization techniques, including UHV contact-less C-V, photoluminescence surface state spectroscopy (PLS3), cathode luminescence in-depth spectroscopy (CLIS),and gateless FET techniques that have been developed by the authors’ group.As a result, a model including a U-shaped surface state continuum, having a particular charge neutrality level, combined with frequent appearance of near-surface N-vacancy related deep donor states having a discrete level at Ec - 0.37eV is proposed as a unified model that can explain large gate leakage currents and current collapse in AlGaN/GaN HFETs. Hydrogen plasma treatment and SiO2deposition increase N-vacancy related deep donors. Reasonably good surface passivation can be achieved by ECR-plasma SiNx films and by ECR-plasma oxidized Al2O3films both combined with ECR N2plasma treatment.