bluuebunny/crossref_metadata_2025_split huggingface dataset:
Piezoelectric Micromachined Ultrasonic Transducers Based on PZT Films
Authors: Baborowski Jacek , Ledermann Nicolas , Muralt Paul 
Published: April 2011

ABSTRACTTest structures for piezoelectric micromachined ultrasonic transducers have been fabricated and investigated. The basic element consisted of a oxidized and platinized silicon membrane coated with a 2 μm thick (100)-textured Pb(Zr,Ti)O3 (PZT) thin film deposited by sol-gel techniques. SOI wafers have been applied to obtain a good definition of the silicon part of the membrane. Test devices have been characterized in air and in an insulating liquid.
A comparative microtribological investigation of diamond-like carbon films for applications in microsystems
Authors: Imad-Uddin Ahmed S. , Bregliozzi Giuseppe , Haefke Henry 
Published: April 2011

ABSTRACTThis study reports on the microfrictional properties under reciprocating sliding of different types of commercially available diamond-like carbon (DLC) films, prepared using low and high frequency plasma-assisted chemical vapor deposition (PACVD) and the vacuum arc method. Silicon and sapphire balls were used as counterbodies. Friction-load curves suggest that, for applied forces in the N to mN regime, two properties have a strong influence on the microfriction: First, the chemical composition plays a dominant role and second, the film roughness. With silicon against DLC, microfriction of hydrogen-free DLC films was greater than the hydrogen containing films. With sapphire counterbodies, microfriction was inversely proportional to film roughness. For all films tested, microfriction was independent of the sliding velocity. Mild wear in the form of scratches was observed only on silicon, while no wear could be detected on any of the DLC films. These results illustrate the utility of implementing microtribological testing in a coating development for microsystems.
Ion dose dependence on solid phase epitaxy of amorphous silicon carbide induced by ion implantation
Authors: Bae In-Tae , Ishimaru Manabu , Hirotsu Yoshihiko 
Published: April 2011

ABSTRACTAmorphous silicon carbides (a-SiC) fabricated by Xe+ ion implantation into 6H-SiC (0001) to fluences of 1015 and 1016/cm2 have been annealed at 850 °C for 1 hour. Transmission electron microscopy (TEM) observations revealed that the 1015 Xe+/cm2 implanted sample was completely recrystallized, while most of the a-SiC remains in the 1016 Xe+/cm2 implanted sample. Pair-distribution function analyses of both of the as-implanted samples show that the peak intensity of Si-C heteronuclear bonds is higher and the peak intensities of Si-Si and C-C homonuclear bonds are lower in the 1015 Xe+/cm2 implanted sample, indicating that the atomistic structure of the 1015 Xe+/cm2 implanted sample is more chemically ordered than that of the 1016 Xe+/cm2 implanted sample. This result suggests that more chemically ordered atomistic structure of 1015 Xe+/cm2 implanted a-SiC leads to complete recrystallization during thermal annealing.
Effects of Structural Defects on Diode Properties in 4H-SiC
Authors: Skromme B. J. , Palle K. C. , Mikhov M. K. , Meidia H. , Mahajan S. , Huang X. R. , Vetter W. M. , Dudley M. , Moore K. , Smith S. , Gehoski T. 
Published: April 2011

ABSTRACTThe influence of two types of structural defects on Schottky diodes in 4H-SiC is investigated. First, a structural transformation that takes place during oxidation or annealing of heavily n-doped wafers is studied using X-ray diffraction, high resolution TEM, secondary electron imaging (SEI), photoluminescence, and electrical measurements on Schottky diodes. Lamellae of 3C SiC with six bilayers are introduced primarily in the heavily doped substrates, and to a lesser extent in the lightly doped epilayers. Lowered Schottky barrier heights result and are spatially correlated to the 3C phase. The 3C lamellae exhibit charge contrast in SEI images where they intersect the surface. Second, we study isolated screw dislocations and show preliminary evidence that they are not the main factor controlling Schottky barrier height and ideality in Schottky diodes in lower-doped 4H-SiC wafers.
Process Induced Extended Defects in SiC Grown via Sublimation
Authors: Yakimova R. , Syväjärvi M. , Jacobson H. , Ciechonski R. R. , Vouroutzis N. , Stoemenos J. 
Published: April 2011

ABSTRACTThree possible situations of extended defect formation during sublimation growth of SiC have been investigated: a-plane growth, occurrence and effect of irregular growth interfaces, and development of elongated open volume morphological defects. While in the first case the most preferred defects are basal plane stacking faults (SFs), in other studied samples threading dislocations and high energy SFs have been revealed, these suggesting stress accumulated in the course of the growth process. Formation of morphological defects of the above character has been observed in the presence of micropipes and step flow growth mode on vicinal surfaces. Growth on atomically flat surfaces can eliminate this type of defects but the polytype uniformity is more difficult to maintain.
Extended Defects in 4H-SiC PiN Diodes
Authors: Twigg M. E. , Stahlbush R. E. , Fatemi M. , Arthur S. D. , Fedison J. B. , Tucker J. B. , Wang S. 
Published: April 2011

ABSTRACTUsing site-specific plan-view transmission electron microscopy (TEM) and lightemission imaging (LEI), we have identified SFs formed during forward biasing of 4H-SiC PiN diodes. These SFs are bounded by Shockley partial dislocations and are formed by shear strain rather than by condensation of vacancies or interstitials. Detailed analysis using TEM diffraction contrast experiments reveal SFs with leading carbon-core Shockley partial dislocations as well as with the silicon-core partial dislocations observed in plastic deformation of 4H-SiC at elevated temperatures. The leading Shockley partials are seen to relieve both tensile and compressive strain during PiN diode operation, suggesting the presence of a complex and inhomogeneous strain field in the 4H-SiC layer.
Confinement of Screw Dislocations to Predetermined Lateral Positions in (0001) 4H-SiC Epilayers Using Homoepitaxial Web Growth
Authors: Neudeck Philip G. , Spry David J. , Trunek Andrew J. , Anthony J. , Beheim Glenn M. 
Published: April 2011

ABSTRACTThis paper reports initial demonstration of a cantilevered homoepitaxial growth process that places screw dislocations at predetermined lateral positions in on-axis 4H-SiC mesa epilayers. Thin cantilevers were grown extending toward the interior of hollow pre-growth mesa shapes etched into an on-axis 4H-SiC wafer, eventually completely coalescing to form roofed cavities. Each completely coalesced cavity exhibited either: 1) a screw dislocation growth spiral located exactly where final cantilever coalescence occurred, or 2) no growth spiral. The fact that growth spirals are not observed at any other position except the central coalescence point suggests that substrate screw dislocations, initially surrounded by the hollow portion of the pre-growth mesa shape, are relocated to the final coalescence point of the webbed epilayer roof. Molten potassium hydroxide etch studies revealed that properly grown webbed cantilevers exhibited no etch pits, confirming the superior crystal quality of the cantilevers.
Contact resistivity of Al/Ti ohmic contacts on p-type ion implanted 4H- and 6H-SiC.
Authors: Nipoti Roberta , Moscatelli Francesco , Scorzoni Andrea , Poggi Antonella , Cardinali Gian Carlo , Lazar Mihai , Raynaud Christophe , Planson Dominique , Locatelli Marie-Laure , Chante Jean-Pierre 
Published: April 2011

ABSTRACTAl-Ti alloys with 72 wt% Al were employed for the realisation of ohmic contacts on 4×1019 cm-3 p-type ion implanted 4H- and 6H-SiC samples. Contact resistivity characterisations by TLM measurements were done at wafer level in the temperature range 28–290°C. Analysis of the TLM measurements took into account current crowding at the metal pads. More than half of the evaluated contact resistivity reached the minimum value detectable by the used TLM devices, that was slightly higher than 1×10-6 Ωcm2. Above this limit value, contact resistivity decreased for increasing temperature and was spread over a few decades. The maximum contact resistivity at 28°C was 2×10-4 Ωcm2, which changed to 5×10-6 Ωcm2 at 290°C. The thermal behaviour of these TLM structures featured thermionic-field emission conduction.
7 kV 4H-SiC GTO Thyristors
Authors: Campen Stephen Van , Ezis Andris , Zingaro John , Storaska Garrett , Clarke R. Chris , Temple Vic , Thompson Mark , Hansen Todd 
Published: April 2011

ABSTRACTHigh Power asymmetric SiC GTOs (Gate Turn-Off Thyristors) were fabricated on n-type 4H-SiC substrates with multiple epi-layers and were tested to investigate breakdown voltage, maximum current density, switching characteristics, and temperature dependences.Comparison of breakdown voltages achieved by GTOs with Guard Ring edge termination and GTOs with a proprietary Junction Termination Extension (JTE) fabricated on the same wafer are made. Individual GTOs with a nominal area of 4 mm2 (2 mm × 2 mm) utilizing the JTE were tested in forward bias and found to support over 6 kV at leakage currents of less than 5 μA. In addition, GTOs with a nominal area of 0.25 mm2 (0.5 mm × 0.5 mm) utilizing the JTE were found to support over 7 kV. These blocking voltages are the highest reported for GTOs in SiC. [1–5] Wafer maps of the breakdown voltages for older and newer wafers suggest an improvement of the material. The defect density for the newer wafer is estimated to be greater than the micropipe density.The on-state characteristics are equally impressive, with 4 mm2 GTOs carrying > 1000 A/cm2. The GTO has a forward voltage drop of 3.66 V at a current density of 300 A/cm2 at room temperature and a forward voltage drop of 3.1 V at 300 A/cm2 at 224 °C. This indicates that on-state losses should not be excessive, even at high current densities and high temperatures.
Distinct Magnesium Incorporation Behavior in Laterally Grown AlGaN
Authors: Liu R. , Bell A. , Ponce F. A. , Cherns D. , Amano H. , Akasaki I. 
Published: April 2011

ABSTRACTDifferent magnesium incorporation behavior has been observed in heavily Mg-doped AlGaN epitaxial layers. The films were grown by metal-organic vapor phase epitaxy involving a lateral overgrowth technique on patterned sapphire substrates. TEM observations show that direct growth on sapphire exhibits pyramidal defects, while lateral overgrowth is homogeneous and free of structural defects. The orientation of the growth front significantly influences the microstructure, and the {0001} growth facet appears to be essential for the formation of the pyramidal defects. In addition, cylindrical and funnel-shaped nanopipes have been observed at dislocations with an edge component. The relationship between Mg segregation and these defects is discussed, and formation mechanisms are proposed taking into consideration the orientation of the growth front.
Lateral growth of AlxGa1–xN and GaN on SiC substrates patterned by photo-electrochemical etching
Authors: Rossów U. , Riedel N. , Hitzel F. , Riedl T. , Hangleiter A. 
Published: April 2011

AbstractThe large defect densities in heteroepitaxially grown group-III-nitride layers on sapphire or SiC cannot be tolerated in applications such as lasers. We report here on a defect reduction by overgrowth of patterned n-6H-SiC(0001)surfaces.First, we formed mesa structures in the windows of metal masks and then after removal of the masks layers of AlxGa1–xN and GaN were grown by low-pressure MOVPE under conditions of high lateral growth rates. We demonstrate that layers and layered structures can be grown with smooth surfaces and reduced defect densities.
Blue Luminescence in Undoped and Zn-doped GaN
Authors: Reshchikov M. A. , Morkoç H. , Molnar R. J. , Tsvetkov D. , Dmitriev V. 
Published: April 2011

ABSTRACTA broad band with a maximum at about 2.9 eV (blue band) is widely observed in the photoluminescence (PL) and cathodoluminescence (CL) spectra of unintentionally doped GaN grown by metalorganic chemical vapor deposition (MOCVD) or by hydride vapor phase epitaxy (HVPE). In some samples this band exhibits fine structure attributed to electron-phonon coupling, in others it appears featureless. Different defect origin and recombination mechanisms responsible for the blue band have been suggested in the past. The situation is complicated by the fact that bands similar in shape and position were observed also in Zn- and Mg-doped GaN, as well as in undoped GaN after dry or wet etching. We investigated PL in HVPE-grown undoped, Si- and Zn-doped GaN layers in wide temperature and excitation intensity ranges. We have found that the shape, temperature and excitation intensity dependencies of the blue band in undoped GaN are almost identical to those in GaN lightly doped with Zn. Moreover, in both undoped and Zn-doped samples we observed a distinctive set of peaks related to an exciton bound to the Zn acceptor. Although the exact structure of the Zn acceptor is still unknown, our experimental results unambiguously demonstrate that Zn impurity is responsible for the blue band in unintentionally doped GaN. The results of transient PL study are also consistent with the above attribution.
Self-Induced Photon Absorption by Screening of the Electric Fields in Nitride-based Quantum Wells
Authors: Kalliakos S. , Lefebvre P. , Taliercio T. , Gil B. 
Published: April 2011

ABSTRACTWe have calculated the change of interband absorption spectra of a quantum well based on hexagonal group-III nitride semiconductors under photo-injection of high densities of electron-hole pairs. The screening of internal electric fields by such optical excitation is known to blue-shift and reinforce the ground-state optical transition. Due to the large values of densities of states and of internal fields, we predict novel properties that rather concern optical absorption via transitions between excited states. The absorption coefficient can be strongly enhanced by the optical excitation itself, in this particular spectral region, yielding the possibility for self-induced absorption properties. In other words, if sufficiently intense, an excitation laser can increase the absorption coefficient of the system at its own wavelength, thus providing a strong nonlinear optical response. Finally, we briefly discuss the potential application of these optical phenomena.
Ohmic and Rectifying Contacts to n and p-type GaN Films
Authors: Hall H. P. , Awaah M. A. , Kumah A. , Das K. , Semendy F. 
Published: April 2011

ABSTRACTElectrical contacts to both n and p-type GaN films have been investigated using electron-beam evaporated and sputtered films of metals such as Al, Au, Cr, Cu, Ni, Pt, and Ti. Films deposited by electron-beam evaporation for the n-type films with doping levels of 1 × 1018/cm3 and lower showed rectifying characteristics with all the metals studied with the exception of Al. Aluminum contact diodes were ohmic in the as-deposited state. The Pt rectifying contact was near-ideal with an ideality factor close to 1.0. Ideality factors for the other metals were much greater than 1. This deviation from thermionic behavior was interpreted as space charge limited current conduction in the presence of deep-level states. Sputtered films showed very similar characteristics to electron-beam deposited films, with the exception of Ti. The Ti contact was ohmic in the as-deposited state. Non-linear Cu contacts to n-type films became ohmic on annealing. However, for p-type films, Ar ion sputter-cleaning prior to metal deposition by sputtering created ohmic contacts with Cu and Pt. Low resistance ohmic contacts were achieved by ion implantation and anneal of Si in n-type and Mg in p-type films, prior to metallization. The implant parameters and anneal temperatures are currently being optimized.
Epitaxial Growth of AlN on 6H-SiC (1120) by Molecular-Beam Epitaxy and Effect of Low-Temperature Buffer Layer
Authors: Onojima N. , Suda J. , Matsunami H. 
Published: April 2011

ABSTRACTAluminum nitride (AlN) has been grown on 6H-silicon carbide (SiC) substrates with the non-polar (1120) face using rf plasma-assisted molecular-beam epitaxy (rf-MBE). Reflection high-energy electron diffraction (RHEED) revealed that AlN and 6H-SiC (1120) had an exact epitaxial relationship, i.e., [1120]AlN|[1120]SiC and [0001]AlN∥[0001]SiC. From the result of microscopic Raman scattering spectroscopy, the stacking structure of the AlN epitaxial layer was suggested to be a 2H structure, not a 6H structure. A directly grown AlN layer and layer with AlN low-temperature (LT) buffer layer were investigated based on atomic force microscopy (AFM) and X-ray diffraction (XRD).
Study on Chemical Treatment and High Temperature Nitridation of Sapphire for III-Nitride Heteroepitaxial Growth
Authors: Dwikusuma F. , Saulys D. , Kuech T. F. 
Published: April 2011

ABSTRACTWe have systematically studied the effects of wet chemical etching and high temperature nitridation on the resulting sapphire surface morphology and chemical transformation. The etching of c-plane sapphire substrates using H2SO4, H3PO4, and a 3:1 H2SO4 :H3PO4 mixture as a function of temperature and etching time was studied and compared with H2 etching at 1100°C and air-annealing at 1400°C. The surface nitridation using NH3 and N2 at 1100°C was studied as a function of NH3 concentration, nitridation time, and surface pretreatment. Atomic force microscopy and x-ray photoelectron spectroscopy were used to study the surface morphology and chemical composition. The detailed surface morphology after chemical etching was a function of the chemical composition and the specific time and temperatures. The smoothest, pit-free sapphire surface was obtained by etching in pure H2SO4 at 300°C for 30 min. Sulfuric acid etching at higher temperatures or for longer periods generated an insoluble mixture of Al2(SO4)3 and Al2(SO4)3·17H2O crystalline deposits on the surface. Phosphoric acid and the 3:1 H2SO4:H3PO4 mixture etched the sapphire preferentially at defect sites and resulted in pits formation on the surface. The high temperature sapphire nitridation resulted in nitrogen incorporation into the surface. The nitrogen content of nitridation layer depends on NH3 concentration, nitridation time, and surface pretreatment. The nitrogen contents of sapphire treated with H2SO4 and 3:1 H2SO4:H3PO4 are about the same as the as-received sapphire. While the nitrogen content of the air-annealed sapphire is ∼1.6 times higher then the nitrogen content of the as-received sapphire.
Screw Dislocations in MBE GaN Layers Grown on Top of HVPE Layers: Are They Different?
Authors: Liliental-Weber Z. , Zakharov D. , Jasinski J. , Washburn J. , O'Keefe M. A. , Morkoc H. 
Published: April 2011

ABSTRACTTransmission Electron Microscopy was applied to study HVPE template and MBE over-layers in plan-view and cross-section. It was observed that screw dislocations in the HVPE layers are decorated by small voids arranged along the screw axis. However, no voids were observed along screw dislocations in MBE overlayers grown with excess Ga, despite the fact that Ga droplets were observed on the layer surface as well as imbedded in the layer. By applying a direct reconstruction of the phase and amplitude of the scattered electron wave from a focal series of high-resolution images, the core structures of screw dislocations in both materials have been studied and show that all screw dislocations have filled cores. Dislocation cores in MBE samples grown Ga-rich and N-rich show no substantial differences and no stoichiometric change compared to the matrix. However, in HVPE materials, single atomic columns show substantial differences in intensities and indicate the possibility of Ga presence. These Ga-rich cores might be responsible for the attraction impurities forming voids in their close vicinity.
Low Temperature Laser-Assisted Gas Phase Reactivity of TMGa with NH3 and Oxygen-Containing Compounds (H2O, CH3OH, O(CH3)2) in Constrained Pulsed Expansions
Authors: Demchuk Alexander , Lynch Michael , Simpson Steven , Koplitz Brent 
Published: April 2011

ABSTRACTThe present work reports on the study of III-V gas phase reactivity in constrained gas pulse expansions of trimethylgallium (TMGa) and oxygen derivative compounds (H2O, CH3OH, O(CH3)2) with and without ammonia. The precursors are introduced separately into a high vacuum chamber via a multipulsed gas nozzle assembly. The gas mixtures are then exposed to a UV pulse from an ArF excimer laser (λ=193 nm) and the products are mass analyzed with a quadrupole mass spectrometer. The efficient laser-assisted growth of Ga-O-containing clusters in the form of [(CH3)2GaOR] x, where R is H or CH3, has been revealed. Different behavior can be seen in the reaction of TMG and the oxygen species depending on the presence of H atoms bonded to the oxygen. Significant influence of NH3 on cluster formation and oxygen incorporation is demonstrated.
Morphology and surface reconstructions of m-plane GaN
Authors: Lee C. D. , Feenstra R. M. , Northrup J. E. , Lymperakis L. , Neugebauer J. 
Published: April 2011

ABSTRACTM-plane GaN(1100) is grown by plasma assisted molecular beam epitaxy on ZnO(1100) substrates. A low-temperature GaN buffer layer is found to be necessary to obtain good structural quality of the films. Well oriented (1100) GaN films are obtained, with a slate like surface morphology. On the GaN(1100) surfaces, reconstructions with symmetry of c(2×2) and approximate “4×5” are found under N- and Ga-rich conditions, respectively. We propose a model for Ga-rich conditions with the “4×5” structure consisting of ≥ 2 monolayers of Ga terminating the GaN surface.
Lattice Relaxation of AlN Buffer on Surface-Treated SiC in Molecular-Beam Epitaxy for Growth of High-Quality GaN
Authors: Suda Jun , Miura Kouhei , Honaga Misako , Onojima Norio , Nishi Yusuke , Matsunami Hiroyuki 
Published: April 2011

ABSTRACTThe effects of SiC surface treatment on the lattice relaxation of AlN buffer layers and the crystalline quality of GaN layers grown on the buffer layers were studied. AlN buffer layers and GaN main layers were grown by plasma-assisted molecular-beam epitaxy on on-axis 6H-SiC (0001)Si substrates. High-temperature HCl-gas etching resulted in an atomically flat SiC surface with (√3×√3)R30° surface reconstruction, while HCl-gas etching followed by HF chemical treatment resulted in an atomically flat surface with (1×1) structure. The AlN layer grown on the (1×1) surface showed slower lattice relaxation. GaN grown on the AlN buffer layer exhibited a (0002) X-ray rocking curve of 70 arcsec and 107 cm−2 of screw-type dislocation density, which was superior than that of GaN grown on (√3×√3)R30° surface.
In as a Surfactant for the Growth of AlGaN/GaN Heterostructures by Plasma Assisted MBE
Authors: Monroy E. , Gogneau N. , Bellet-Amalnc E. , Enjalbert F. , Barjon J. , Jalabert D. , Brault J. , Dang Le Si , Daudin B. 
Published: April 2011

ABSTRACTIn this paper, we study the surfactant capability of In for the growth of AlGaN/GaN heterostructures by plasma-assisted molecular beam epitaxy. Growth conditions were determined to have a self-regulated 1×1 In adlayer on AlxGa1-xN (0001). The presence of this In film favors two dimensional growth of AlGaN under stoichiometric conditions, and inhibits the formation of metal droplets on the surface. The quality of these layers was assessed by high resolution X-ray diffraction, atomic force microscopy and photoluminescence.