bluuebunny/crossref_metadata_2025 huggingface dataset:
High-Rate Rpecvd of a-Si:H Films by Means of a VHF Resonant Plasma Source
Authors: Blum T. , Suchaneck G. , Kuske J. , Stephan U. , Beyer W. , Kottwitz A.
Published: April 2011

High deposition rate (up to 5 nm/s) a-Si:H films suitable for recrystallization were deposited using a λ/4 helical resonator source. Refractive index, Tauc-gap, photo- and dark conductivities were measured for film characterization. The metastable behaviour was characterized by the light- induced degradation of the photoconductivity. Hydrogen content and bonding configuration were analyzed by IR absorption and mass separated thermal effusion transients, film microstructure was studied by intentionally incorporating carbon and oxygen. Most of the hydrogen is located on internal surfaces in the otherwise dense material. Differences between the deposition from our highly excited plasma and the conventional remote PECVD process are discussed.
Investigation of the Amorphous to Nanocrystalline Phase Transition at the Deposition of Silicon Films in an ECWR Plasma of Pure SiH4
Authors: Scheib M. , Schrcder B. , Oechsner H.
Published: April 2011

A novel plasma based chemical vapour deposition (PECVD) technique employing electron cyclotron wave resonance (ECWR) for plasma excitation was applied to the deposition of hydrogenated nanocrystalline silicon (nc-Si:H) films. nc-Si:H films were produced at deposition rates up to 8Å/sec (TS = 200°C) with a pure SiH4 plasma in contrast to the conventional glow discharge technique where the high hydrogen dilution usually needed leads to considerable lower deposition rates. The amorphous-to-nanocrystalline phase transition was investigated in dependence of substrate temperature, the hf-power and magnetic field mandatory for ECWR, and SiH4-flow into the plasma. With the knowledge of the plasma parameters derived from single probe measurements, and the intensities of excited plasma species detected by means of optical emission spectroscopy we can qualitatively describe the silane-plasma dissociation behaviour. The nanocrystalline phase is found to be always deposited when the dissociation degree of the SiH4 plasma is almost saturated.
Trap Dominated Hydrogen Transport in Disordered Silicon
Authors: Nickel N. H. , Jackson W. B. , Walker J.
Published: April 2011

Hydrogen transport in polycrystalline silicon was investigated by deuterium diffusion experiments. D was introduced either from a remote plasma or a solid-state source. The data can be explained by a two-level model used to explain diffusion in amorphous silicon. The energy difference between transport level and deuterium chemical potential was found to be 1.3 eV. A band of shallow levels for hydrogen trapping is located about 0.6 eV below the transport level, while deep levels are about 1.7 eV below.
Metastable Defects in a-Si:H from Bond-length Disorder
Authors: li Qiming , Biswas R.
Published: April 2011

A model of metastable defect formation via H-rebonding in hydrogenated amorphous silicon is developed where the defect density and defect formation energy are controlled by the bond-length disorder of the material. Dangling bond defects are created by H motion from SiH bonds to weak Si-Si bonds. The model predicts formation energies for thermal and light-induced defects in good agreement with experiment. The relaxation of thermal equilibrium defects is stretched exponential, with stretch parameters varying approximately linearly with temperature and relaxation times that are thermally activated- in good agreement with experiment. The annealing of light-induced defect densities also shows relaxation behavior. The model accounts for barriers of ≈ 1.5 eV for H diffusion. The energetics of the H*2 complex will also be discussed. The rms bond-length deviation is a new parameter that controls the quality of the material.
The Density-of-States Concept Versus the Experimentally Determined Distribution of Activation Energies
Authors: Adriaenssens Guy J. , Arkhipov Vladimir I.
Published: April 2011

Random fluctuations of localized state energies will result in thermal release of carriers trapped in those states at shorter times than would be observed from a stationary distribution of the same energies. An experimentally observed distribution of activation energies will hence differ from the distribution of average energies of the states involved. It will also be temperature-dependent. In a-Si:H, low-frequency fluctuations with a spectrum comparable to the one of 1/f noise, can account for the measured temperature dependence of the distribution. They also explain the apparent shift in localized-state energy under steady-state illumination.
Structural Evolution of Top-Junction a-Si:C:H:B and Mixed-Phase (Microcrystalline Si)-(a-Sil-xCx:H) p-Layers in a-Si:H n-i-p Solar Cells
Authors: Koh Joohyun , Burnham J. S. , Li Yeeheng , Liu Hongyue , Chen Ing-Shin , Pilione L. J. , Wronski C. R. , Collins R. W.
Published: April 2011

We have applied real time spectroellipsometry (RTSE) to study hydrogenated amorphous silicon (a-Si:H) solar cells fabricated in the Cr/n-i-p configuration using plasma-enhanced chemical vapor deposition (PECVD) in a single-chamber system. The microstructural evolution of the n-, i-, and p-layers of the devices has been determined, including the thicknesses of the bulk, interface, and surface roughness layers versus time. The optical properties of the individual layers, including the dielectric functions and optical gaps, have also been obtained in the same analysis. In this study, we have focused on i/p interface formation and, in particular, on the nucleation process for differently-prepared a-Si:C:H and mixed-phase μc-Si:H/a-Si1-xCx:H p-layers on the a-Si:H i-layer. From the thickness dependence of the p-layer void volume fraction, we can obtain an estimate of the thickness at which nuclei make contact to form a continuous film. For the mixed-phase p-layers, the nuclei contact thickness can be reduced by exposing the i-layer to a H2-plasma prior to p-layer deposition. We have found that for similarly-prepared p-layers this reduction in contact thickness leads to an increase in open-circuit voltage of the solar cell
Light Bias CPM Study of the Density of States in N-type Amorphous Silicon
Authors: Unold Thomas , Branz Howard M. , Vanecek Milan
Published: April 2011

We measure subgap absorption on n-type amorphous silicon using the “absolute” constant photocurrent method. We find that for typical monochromator probe beam intensities the measurement is not significantly influenced by lifetime changes. When the measurement is performed under light bias, an apparent increase in the defect absorption coefficient is observed, but no change in the photoexcitation threshold or spectral shape of the absorption band is seen. We show that this increase is likely due to a bias-light amplification of spectrally dependent lifetime changes. Our measurements suggest a larger electron capture cross section of positive valence band tail states compared to neutral dangling bonds.
Low Frequency Noise Behavior in a-Si:H Schottky Barrier Devices
Authors: Aflatooni K. , Nathan A. , Hornsey R.
Published: April 2011

We present a systematic experimental study of low frequency noise behavior in Mo/a-Si:H Schottky diodes under reverse bias operation. The noise in the Schottky diode was found to increase with increasing reverse current and with an approximate 1/f behavior at low bias voltages, yielding a Hooge parameter in the range (2 to 3) × 10−4. At high reverse voltages, due to electrical stressing and hence, bias-induced material instability, a significant deviation from the 1/f behavior was observed.
Time-Resolved Photoluminescence in a-Si1-xCx:H
Authors: Tessler Leandro R. , Cirino Lucicleide R.
Published: April 2011

The dependence of the photoluminescence (PL) decay on temperature and composition of a series of amorphous silicon-carbon alloys (a-Si1-xCx:H) with 0 <x <0.4 was studied. The samples were prepared by “low power” PECVD from SiH4/CH4 gas mixtures. The decay curves are non-exponential and can be described as lifetime distributions (LTD). For pure a-Si:H at 17 K the peak of the LTD is of the order of 5×10-4 sec. It shifts to shorter lifetimes as x or the temperature increases. Samples with x > 0.3 present in addition a faster peak with maximum at 10-8 sec. The fast peak is almost temperature independent. The slow component shifts to shorter lifetimes as the temperature increases, and vanishes at room temperature. The presence of the fast peak is interpreted as due to a change in the radiative recombination mechanism. We associate this peak to the recombination of trapped “excitons”.
The Increased Response Time in Hydrogenated Microcrystalline Silicon - A Fermi Level Effect or a Structural Effect in a Grainy Material?
Authors: Grebner S. , Popovic P. , Furlan J. , Gu Q. , Schwarz R.
Published: April 2011

The typical photocurrent decay time τR in intrinsic prepared hydrogenated microcrystalline silicon (μc-Si:H) is around lms similar to its n-doped amorphous counterpart (a-Si:H:P). Depending on the crystalline fraction Xc, the μc-Si:H films show an activation energy near to or below 0.5eV. To find out if this analogy ofτR could be due to a Fermi level shift or to the grainy structure in gc-Si:H films, we have studied the behaviour of τR in doped a-Si:H and gc-Si:H films of different XC. One-dimensional numerical simulation based on the Multiple Trapping Model (MTM) can explain this increase in terms of a Fermi level shift towards the conduction band. On the other hand, detailed measurements for temperatures from 100 to 400 K point to carrier trapping in deep states, most probably located at grain boundaries.
Correlation Between Electrical-Optical and Structural Properties of Microcrystalline Silicon N Type Films
Authors: Martins R. , Macarico A. , Ferreira I. , Nunes R. , Bicho A. , Fortunato E.
Published: April 2011

Wide band gap microcrystalline silicon films have aroused considerable interest since they combine some electro-optical advantages of amorphous and crystalline materials highly important to produce electro-optical devices such as TFTs and solar cells. In this paper we present results concerning the electro-optical characteristics of highly transparent and conductive n-type µc-Si based films. Here, emphasis is given to the production of n-type ýtc-films with optical gaps of 2.3 eV and dark conductivity's of 6.5 Scm-1
Drift Mobility Measurements in Porous Silicon
Authors: Tsybeskov L. , Peng C. , Fauchet P. M. , Gu Q. , Schiff E. A.
Published: April 2011

Modulated electroluminescence (EL) measurements performed on a series of porous silicon (PSi) diodes are presented. The maximum response time of the devices scales with the square of the PSi layer thickness and inversely with the applied forward bias voltage. These scaling results indicate that the maximum response time is a carrier transit time from which a drift mobility μ of 10−4 cm2/Vs is deduced at room temperature. Time-of-flight transport measurements on PSi are in qualitative agreement with this value for μ in addition, they identify μ as the electron mobility and show that transport is dispersive, in contrast to the interpretation of the modulated EL experiments.
Characterization of the Visible Photoluminescence from Porous a-Si:H and Porous a-Si:C:H Thin Films
Authors: Estes M. J. , Hirsch L. R. , Wichart S. , Moddel G.
Published: April 2011

We report on the influence of doping, temperature, porosity, and bandgap on the visible photoluminescence properties of anodically-etched porous a-Si:H and a-Si:C:H thin films. Only boron-doped, p-type a-Si:H or a-Si:C:H samples exhibited any visible photoluminescence. We see evidence of discrete defect or impurity levels in temperature-dependent luminescence measurements. Unlike in porous crystalline silicon, we see no correlation of luminescence energy with porosity. We do, though, observe a correlation of luminescence energy with bandgap of the starting a-Si:C:H films. We discuss the implications of these observations on the nature of the luminescence mechanism.
The Stability Improvement of a-Si:H Films for Photovoltaic Applications
Authors: Budaguan B. G. , Aivazov A. A. , Meytin M. N.
Published: April 2011

The origin of metastability in a-SiH films deposited at different deposition conditions has been investigated by joint analysis of light-induced defect generation kinetics (Staebler-Wronski Effect) and differential scanning calorimetry (DSC) measurements. Light-induced defect generation kinetics were measured at elevated temperatures of 80-150°C_ DSC measurements were performed in a temperature range of 20°C - 570°C. The films microstructure were studied by IR spectroscopy analysis.

The simulation of DSC curves has been used to deduce the kinetics of the low-temperature exothermic peak (LTEP) observed at 80-150° C. It was found that kinetic parameters of LTEP and SW effects has the same dependence on microstructure parameters, γ, which suggests their common nature. The analysis of a-SiH stability improvement concerning the influence of films microstructure on the kinetics of relaxation processes is presented.
Finite Element Calculations to Optimize the Design of a Stressor for Strained Induced Quantum Wires and Quantum Dots in GaAs
Authors: Pinardi K. , Jain S. C. , Maes H. E.
Published: April 2011

We have calculated the normalized stress σfxx/σ0 (σ0 is the stress in the large area stressor) in the stressor and σsxx/σ0 in the substrate for values of RE = Ef/Es (Ef is the Young's modulus of the stressor and Es, is the Young's modulus of the substrate) in the range 0.5 to 1.2. Substrate stresses for 13 stripe stressor samples are also calculated for RE = 0.9 which corresponds to an InGaAs stressor on GaAs with an In concentration of about 25%. It is found that for any given 1, the stress at a given depth increases monotonically as h increases (1 and h are the halfwidth and thickness of the stressor). The increase is rapid in the beginning for small value of h (l/h > 2). It becomes slow for 1/h < 2 and saturates at l/h = 0.5. For large i/h(l/h > 50) there are two stress wells in the substrate separated by a barrier. For l/h = 20 the two wells merge into one well with a flat bottom. As l/h decreases further the bottom curves downward, and for l/h < 2 the shape of the stress distribution curve resembles that of a parabola. The stress σsxx/σ0 decays rapidly with distance z from the interface. It is reduced to 1/3 of its value near the interface at z ˜ h. It is therefore necessary to construct the active layer close to the interface. Quality of the interface plays a dominant role in the Quantum structures fabricated in this manner. The shape and the strength of the stress well cannot be changed independently in these structures. We have suggested novel stressor designs to remove this limitation.
Electron Cyclotron Resonance Etching of SiC in SF6/O2 and NF3 /O2 Plasmas
Authors: Ren F. , Grow J. M. , Bhaskaran M. , Lee J. W. , Vartuli C. B. , Lothian J. R. , Flemish J. R.
Published: April 2011

Etching of β-SiC with electron cyclotron resonance (ECR) system was investigated. Anisotropic and smooth etching of SiC was demonstrated with SF6/O2 based discharges. The root-mean-square roughness increases from 35 nm to 56 nm for as deposit and etched sample, respectively. The addition of small amount oxygen enhanced the etch rate of SiC slightly, but further increase of oxygen content reduced the etch rate which resulted from dilution of F ion and free radical densities. NF3/O2 based discharges also showed same trends and produced anisotropicly etching. However, the smoothness is not as good as SF6/O2 based discharges.
A Study of Low-Temperature Grown Gap by Gas-Source Molecular Beam Epitaxy
Authors: Bi W. G. , Mei X. B. , Kavanagh K. L. , Tu C. W. , Stach E. A. , Hull R.
Published: April 2011

We report the effects of growth conditions on the strain and crystalline quality of lowtemperature (LT) grown GaP films by gas-source molecular beam epitaxy. At temperatures below 160 °C, poly-crystalline GaP films are always obtained, regardless of the PH3 low rate used, while at temperatures above 160 °C, the material quality is affected by the PH3 flow rate. Contrary to compressively strained LT GaAs, high-resolution X-ray rocking curve measurement indicates a tensile strain of the LT GaP films, which is considered to be due to PGa antisite defects. The strain is found to be affected by the PH3 flow rate, the growth temperature, and post-growth annealing. Contrary to LT GaAs, no P precipitates are observed in cross-sectional transmission electron microscopy.
Dry Etching of InGaP and AlInP in CH4/H2/Ar
Authors: Lee J. W. , Pearton S. J. , Santana C. J. , Lambers E. S. , Abernathy C. R. , Hobson W. S. , Ren F.
Published: April 2011

Electron Cyclotron Resonance (ECR) plasma etching with additional if-biasing produces etch rates ≥ 2,500Å/min for InGaP and AlInP in CH4/H2/Ar. These rates are an order of magnitude or much higher than for reactive ion etching conditions (RIE) carried out in the same reactor. N2 addition to CH4/H2/Ar can enhance the InGaP etch rates at low flow rates, while at higher concentrations it provides an etch-stop reaction. The InGaP and AtlnP etched under ECR conditions have somewhat rougher morphologies and different stoichiometries up to ˜200Å from the surface relative to the RIE samples.
Temperature Dependence of the Electrical Transport of Carbon Doped Gan
Authors: Geerts Wim , MacKenzie J.D. , Abernathy C.R. , Pearton S.J , Schmiedel Thomas
Published: April 2011

The temperature dependence of the Hall voltage and resistivity of highly carbon doped GaN were measured. From the sign of the Hall voltage, the material appears to be p-type. Charge transport takes place in an impurity band and the valence band. The effective activation energy as estimated from the maximum in the temperature versus Hall voltage relation is 10–30 meV.
Parametric Study of Compound Semiconductor Etching Utilizing Inductively Coupled Plasma Source
Authors: Constantine C. , Johnson D. , Barratt C. , Shul R. J. , Mcclellan G. B. , Briggs R. D. , Rieger D. J. , Karlicek R. F. , Lee J. W. , Pearton S. J.
Published: April 2011

Inductively Coupled Plasma (ICP) sources are extremely promising for large-area, highion density etching or deposition processes. In this review we compare results for GaAs and GaN etching with both ICP and Electron Cyclotron Resonance (ECR) sources on the same singlewafer platform. The ICP is shown to be capable of very high rates with excellent anisotropy for fabrication of GaAs vias or deep mesas in GaAs or GaN waveguide structures.
Low Interface State Density Oxide-GaAs Structures Fabricated by In-Situ Molecular Beam Epitaxy
Authors: Passlack M. , Hong M.
Published: April 2011

We have extended the spectrum of molecular-beam epitaxy (MBE) related techniques by introducing in-situ deposition of oxides. The oxide films have been deposited on clean, atomically ordered (100) GaAs wafer surfaces using molecular beams of gallium-, magnesium-, silicon-, or aluminum oxide. Among the fabricated oxide-GaAs heterostructures, Ga2O3-GaAs interfaces exhibit unique electronic properties including an interface state density Dit in the low 1010 cm−2eV−1 range and an interface recombination velocity S of 4000 cm/s. The formation of inversion layers in both n- and p-type GaAs has been clearly established. Further, thermodynamic and photochemical stability of excellent electronic interface properties of Ga2O3-GaAs structures has been demonstrated.
Self Organized Growth in Thulium Doped Gaas Using MBE
Authors: Bennett M. R. , Singer K. E. , Wright A. C. , Jafri Z. H.
Published: April 2011

The growth of epitaxial GaAs doped with the rare earth thulium (Tm) by MBE is reported for the first time. The nature of the incorporation of Tm has been studied using SIMS, RBS, TEM, PL and DCXRD. Sharp doping profiles have been observed by SIMS from samples doped in a staircase structure. Under standard GaAs growth conditions, TEM has revealed a solubility limit of ∼5×1019cm−3, above which Tm precipitates as spherical nano-particles of uniform size (1.3–1.7nm). High resolution TEM has provided no evidence for strain relaxation around these precipitates. Growth at higher substrate temperatures (≥620°C) or at As:Ga ratios close to stoichiometry, results in the formation of precipitate wires and/or bifurcated structures (“quantum trees”) aligned approximately in the direction of growth. Such behaviour is in common with that of Er in GaAs and suggests that the precipitates result from the self-organised growth of TmAs. Mechanisms for the formation of the quantum wires and trees are suggested. Initial photoluminescence spectra from these samples show two groups of narrow Tm3+ intra 4f transition lines corresponding to the 3H5 – 3H6 (∼1.01eV/1.2μm) and 3H4 – 3H6 (∼ 0.71eV/1.7μm) transitions. The spectra exhibit an excellent correlation with those obtained from samples grown by MOVPE and also those implanted with Tm by other groups.
Characterization of 4H-SiC JFETs for Use in Analog Amplifiers Capable of 723K Operation
Authors: Casady J. B. , Sheridan D. C. , Dillard W. C. , Johnson R. W.
Published: April 2011

Discrete, buried-gate 4H-SiC JFETs (W/L = 1 mm/5 μm) were packaged and characterized at temperatures ranging from 290 K to 773 K for use in a hybrid, 4H-SiC analog amplifier. A contaminated passivation oxide was found to limit high-temperature operation initially, but upon removal of the passivation layer the devices demonstrated stable operation to 773 K with adequate amplification (Av greater than 200 V/V) up to 673 K. From the 16 devices tested, a peak extrinsic saturated transconductance (gmsa) of 27.1 mS/mm was measured at 308 K, corresponding to a channel mobility of 400 cm2/V.s, excluding significant series resistance effects.
Chemical and Structural Characterization of the Ni-Ti Alloy/6H-SiC Contacts
Authors: Levit M. , Grimberg I. , Weiss B.-Z. , Eizenberg M.
Published: April 2011

Ni-Ti alloy is a promising candidate for high-temperature contact metallization for SiC electronic devices. In the present study Ni-Ti alloy thin films (100 nm) of two different compositions (Ni90Ti10 and Ni50Ti50) were coevaporated on 6H-SiC substrate. Interfacial reactions, microstructure, compositional changes, and phase formation were investigated as functions of heat-treatments in the range of 400–800 °C. The study was carried out using Auger electron spectroscopy, x-ray diffraction, and analytical transmission electron microscopy. In the case of the Ni90Ti10 alloy the interaction was found to begin at 450 °C. Ni and C are the dominant diffusing species. The reaction zone is divided into three layers. In the first layer, adjacent to the SiC substrate, the presence of a Ni-rich silicide, Ni2Si, and C precipitates, was observed. The second layer is composed mainly of TiC, while the third - of Ni2 Si. In the case of the Ni50 Ti50 alloy the interaction began at 800 °C. Carbon is the dominant diffusant. The reaction zone is divided into two layers. The first, next to the substrate layer is composed of epitaxially grown TiC and the second - of Ni3Ti2Si compound. A thin (∼5 nm) amorphous discontinuous layer was found at the TiC/SiC interface. Factors controlling phase formation in the Ni-Ti/SiC system are discussed.
Dry Etch Damage In InN, InGaN and InAIN
Authors: Pearton S. J. , Lee J. W. , MacKenzie J. D. , Vartuli C. B. , Donovan S. M. , Abernathy C. R , Shul R. J. , Ren F. , Lothian J. R.
Published: April 2011

Damage introduction in III-V nitrides during dry etching can be simulated by exposingthe samples to pure Ar plasmas for study of the physical (ion-bombardment) effects.Changes in conductivity of InN, In0.5Ga0.5N and In0.5Al0.5N layers exposed to Ar plasmasunder both Electron Cyclotron Resonance and reactive ion etching conditions have beenmeasured as a function of rfpower, pressure and exposure time. The combination of highmicrowave and high rf powers produces large increases (10-_104times) in sheet resistanceof the nitrides, but conditions more typical of real etching processes (rf power < 150W) donot change the bulk electrical properties. The nitrides are more resistant to damageintroduction than other III-V semiconductors. The removal of damage-related trapsoccurs with an activation energy of ∼2.7eV. High ion currents during ECR etching canproduce substantial conductivity changes, whereas the lower currents under RIEconditions do not affect the nitrides. It is difficult to avoid preferential loss of N in thenear-surface of these materials, which leads to leakage currents in rectifying metal contactsdeposited on these surfaces.
Thermodynamic Analysis and Growth Characterization of thick GaN films grown by Chloride VPE using GaCl3/N2 and NH3/N2
Authors: Lee Heon , Yuri Masaaki , Ueda Tetsuzo , Harris James S.
Published: April 2011

Thermodynamic calculations were carried out on chloride transport vapor phase epitaxy of GaN using GaCl3/N2 and NH3/N2. At typical growth temperature and gas flow rates, both GaN formation and gas phase etching reactions of GaN are thermodynamically favored. Under thermodynamic equilibrium, most ammonia should decompose to nitrogen and hydrogen gases and gas phase etching of GaN occurs by HCI. From experimental measurements, less than 10% of the incoming ammonia decomposes and under this condition, GaN formation from GaCl3 /N2, and NH3 /N2 is thermodynamically favored. Higher V/III ratios give a larger driving force for GaN fromation. These calculations match our experimental results. Experimentally, we have optimized the growth conditions of GaN. High crystalline quality thick GaN films (10 ˜15μm) were grown on c-Al2O3. The GaN films show band edge emission dominated PL at both room temperature and 77 K. Only one set of diffraction peaks from (1012) planes with 60° spacing in the φ-scan of X-ray diffraction are observed. This indicates that the GaN films grown on c-Al2O3 are single crystalline. Typical growth rates were about 10 ˜ I5μm/hr and typical Hall mobility values of GaN films were in the range of 3 to 40 cm2/Vsec.
Stability and Reconstruction of, β-SiC (100) Surfaces
Authors: Gutierrez R. , Frauenheim Th.
Published: April 2011

We present a detailed investigation of β-SiC (100) surface reconstructions. Minimal energy configurations at both C- and Si-terminations are determined by using a density-functional based molecular-dynamics method. We provide information on the energetic order of various metastable surfaces and describe the geometrical details. In addition, we also study Si- and C-enriched configurations.
Computer Simulation of Surface Diffusion of Silicon and Carbon Adatoms on SiC(001)
Authors: Bhatti Q. A. , Moran G. J. , Matthai C. C.
Published: April 2011

We have performed molecular dynamics simulations of adatom diffusion on the SiC(001) surface and found that the barriers for carbon adatoms is less than that for silicon adatoms. The diffusion paths were also found to be temperature dependent and at high temperatures the adatom diffusion constant was found to of the order of 10−5 cm2 s−1.
Theoretical Prediction of Zinc Blende Phase GaN Avalanche Photodiode Performance Based on Numerically Calculated Electron and Hole Impact Ionization Rate Ratio
Authors: Kolnik J. , Oguzman I. H. , Brennan K. F. , Wang R. , Ruden P. P.
Published: April 2011

In this paper, we present the first calculations of the electron and hole initiated interband impact ionization rate in zinc blende phase GaN as a function of the applied electric field strength. The calculations are performed using an ensemble Monte Carlo simulator including the full details of the conduction and valence bands along with a numerically determined, wave-vector dependent interband ionization transition rate determined from an empirical pseudopotential calculation. The first four conduction bands and first three valence bands, which fully comprise the energy range of interest for device simulation, are included in the analysis. It is found that the electron and hole ionization rates are comparable over the full range of applied electric field strengths examined. Based on these calculations an avalanche photodiode, APD, made from bulk zinc blende GaN then would exhibit poor noise and bandwidth performance. It should be noted however, that the accuracy of the band structure employed and the scattering rates is presently unknown since little experimental information is available for comparison. Therefore, due to these uncertainties, it is difficult to unequivocally conclude that the ionization rates are comparable.
Tem Analysis of the Observed Phases During the Growth of Oriented Diamond on Nickel Substrates
Authors: Liu W. , Yang P. C. , Tucker D. A. , Wolden C. A. , Davis R. F. , Glass J. T. , Prater J. T. , Sitar Z.
Published: April 2011

Transmission electron microscopy (TEM) was used to investigate the interfacial microstructures and phases involved in the nucleation and growth of the oriented diamond on Ni substrates by a multi-step growth process. A molten surface layer is formed during the process, which appears to be critical for both promotion of the diamond nucleation and suppression of graphite formation. Cross-section TEM analysis revealed that a polycrystalline nickel carbide interfacial structure exists between the diamond particles and the single crystal Ni substrate. X-ray diffraction analysis (XRD) identified the carbide phase as Ni4C. It is suggested that the Ni4C is formed in the molten layer and stabilizes sp3C precursor for diamond nucleation.
Structural Defects and Their Relationship to Nucleation of Gan Thin Films
Authors: Gian Weida , Skowronski Marek , Rohrer Greg S.
Published: April 2011

Microstructure and extended defects in α-GaN films grown by organometallic vapor phase epitaxy on sapphire substrates using low temperature AIN (or GaN) buffer layers have been studied using transmission electron microscopy. The types and distribution of extended defects were correlated with the film growth mode and the layer nucleation mechanism which was characterized by scanning force microscopy. The nature of the extended defects was directly related to the initial three-dimensional growth. It was found that inhomogeneous nucleation leads to a grain-like structure in the buffer; the GaN films then have a columnar structure with a high density of straight edge dislocations at grain boundaries which are less likely to be suppressed by common annihilation mechanisms. Layer-by-layer growth proceeds in many individual islands which is evidenced by the observation of hexagonal growth hillocks. Each growth hillock has an open-core screw dislocation at its center which emits monolayer-height spiral steps.
Electron Scattering by Native Defects in Ill-V Nitrides and their Alloys
Authors: Hsu L. , Walukiewicz W.
Published: April 2011

We have calculated the electron mobilities in GaN and InN taking into consideration scattering by short range potentials, in addition to all standard scattering mechanisms. These potentials are produced by the native defects which are responsible for the high electron concentrations in nominally undoped nitrides. Comparison of the calculated mobilities with experimental data shows that scattering by short range potentials is the dominant mechanism limiting the electron mobilities in unintentionally doped nitrides with large electron concentrations. In the case of AlxGal1−xN alloys, the reduction in the electron concentration due to the upward shift of the conduction band relative to the native defect level can account for the experimentally measured mobilities. Resonant scattering is shown to be important when the defect and Fermi levels are close in energy.
The Relationship Between Micropipes and Screw Dislocations in Pvt Grown 6H-Sic
Authors: Giocondi Jennifer , Rohrer Gregory S. , Skowronski Marek , Balakrishna V. , Augustine G. , Hobgood H. M. , Hopkins R. H.
Published: April 2011

The growth surface of a 6H-SiC boule, grown by physical vapor transport, was examined using scanning force microscopy. The dimensions of surface/micropipe intersections and screw dislocation Burgers vectors have been determined from topographic data. All micropipes are positioned along the lines of super screw dislocations with a Burgers vectors of at least 4 times the c-axis repeat distance (15.2 Å). Perfect c-axis screw dislocations with Burgers vectors of only 15.2 Å are stable and do not have open cores. Measurements show that micropipe core radii, determined indirectly from the width of the craters formed at the surface/micropipe intersections, increase with the square of the dislocation Burgers vector.
Characterization of Defect Structures in 3C-SiC Single Crystals Using Synchrotron White Beam X-ray Topography
Authors: Huang W. , Dudley M. , Fazi C.
Published: April 2011

Defect structures in (111) 3C-SiC single crystals, grown using the Baikov technique, have been studied using Synchrotron White Beam X-ray Topography (SWBXT). The major types of defects include complex growth sector boundary structures, double positioning twins, stacking faults on { 111 } planes, inclusions and dislocations (including growth dislocations and partial dislocations bounding stacking faults). Detailed stacking fault and double positioning twin configurations are determined using a combination of Nomarski interference microscopy, SEM and white beam x-ray topography in both transmission and reflection geometries. Possible defect generation phenomena are discussed.
Thermal And Optical Admittance Spectroscopy Studies Of Defects In 15r-Sic
Authors: Smith S. R. , Evwaraye A. O. , Mitchel W. C.
Published: April 2011

Nitrogen is the common n-type dopant of the various polytypes of silicon carbide. The nitrogen levels in 4H-SiC (at EC-53 meV and EC-100 meV) and in 6H-SiC (at EC-89 meV, EC- 100 meV, and EC-125 meV) have been studied in detail by temperature dependent Hall effect measurements, electron spin resonance (ESR), and thermal admittance spectroscopy. Until now, such detailed studies of the nitrogen levels in 15R-SiC have not been carried out.

Lely-grown 15R samples were used in these studies. The net carrier concentrations (ND-NA), determined by room temperature CV measurements, ranged from 1 × 1018 to 3 × 1018 cm−3. The nitrogen levels in 15R-SiC were studied using thermal admittance spectroscopy. Optical admittance spectroscopy (OAdS) was used to study the deeper defects in this polytype. It was found that optical transitions to the conduction band were inhibited in the heavily doped material.
Local-Field and Exchange-Correlation Effects in Optical Spectra of Wide-Band–Gap Semiconductors
Authors: Gavrilenko V. I. , Bechstedt F.
Published: April 2011

The density-functional theory with ab initio pseudopotentials has been used to study the linear optical response of semiconductors. We present results for optical spectra where the effects of the macroscopic local-field and microscopic exchange-correlation interaction are included beyond diagonal and random-phase approximation. Quasiparticle corrections to the single-particle energies have been added in the polarization function. Numerical calculations are performed for the column-IV materials Si, SiC, and diamond as model substances.
Surface Photovoltage Effects in Photoemission from Diamond Surfaces
Authors: Bandis C. , Pate B. B.
Published: April 2011

Photovoltaic effects in ultraviolet photoemission spectroscopy of the in-situ “rehydrogenated”, and reconstructed (111) diamond surfaces are evaluated. We show that photovoltaic charging effects during photoemission studies of the in-situ “re-hydrogenated” (111)-(l×l):H diamond surface are significant at temperatures as high as room temperature. In contrast, experiments on the reconstructed (111 )-(2×l) diamond surface find no photovoltaic charging which suggests that the surface exhibits relatively high conductivity (effectively grounded surface). These results demonstrate that extra care should be taken in determining the Fermi level pinning position relative to the bands at diamond surfaces and interfaces. The assumption that the UPS photoelectron spectra reflect the equilibrium energy band arrangement should be experimentally confirmed in each case, especially when wide band gap materials are involved.
A Novel Device Structure for High Voltage, High Performance Amorphous Silicon Thin-Film Transistors
Authors: Miri A. M. , Gudem P. S. , Chamberlain S. G. , Nathan A.
Published: April 2011

Conventional high voltage thin-film transistors (HVTFTs) suffer from performance limitations such as low on-current, Vx, shift and large curvature in the linear region of the output characteristics. These limitations are associated with the highly resistive dead region in conventional HVTFT structures. In this paper, we present a novel TFT structure which has a high on-current, improved output characteristics in the linear region, and no Vx shift. The higher on-current and significant improvement in output characteristics allows faster switching. Elimination of the Vx shift leads to more reliable circuit operation. The new structure is based on the conventional low voltage TFT (LVTFT) structure except that it does not suffer from low-voltage breakdown. The low-voltage breakdown of the gate nitride in conventional LVTFTs is perceived to be due to spiking of the drain metallization into the underlying layers which creates regions of very high electric field. In our novel structure, a higher breakdown is achieved by locating the metal contacts away from the gate edge while keeping the necessary drain to gate overlap through a heavily doped microcrystalline layer. Therefore, the new TFT extends the same performance as LVTFTs to high voltage operation. Furthermore, this structure also enhances the yield and reliability by minimizing the common faults in TFTs such as short circuits between gate, source and drain.
A novel technique for in-situ monitoring of crystallinity and temperature during rapid thermal annealing of thin Si/Si-Ge films on quartz/glass
Authors: Subramanian V. , Degertekin F. L. , Dankoski P. , Khuri-Yakub B. T. , Saraswat K. C.
Published: April 2011

A novel technique is presented to simultaneously measure temperature and crystallinity insitu during the rapid thermal annealing of thin Si / SiGe films on transparent substrates for active matrix liquid crystal display applications. The technique uses acoustic waves to monitor temperature, by measuring changes in velocity with temperature. The technique enables accurate tracking of crystalline phase transitions along with temperature, since it is independent of emissivity. This provides a methodology for closed-loop control and end-point detection. The experiments on thin amorphous Si on Quartz demonstrate temperature repeatability of 2%. Also, the technique proved sensitive enough to detect the onset of nucleation, as evidenced by TEM.
Properties and Stability of Bismuth Doped Tin Oxide Thin Films Deposited on Various Types of Glass Substrates
Authors: Foglietti V. , Alessandro A. D′ , Galbato A. , Alessandri A. , Beccherelli R. , Campoli F. , Petrocco S. , Wnek M. , Maltese P.
Published: April 2011

This work reports on deposition of transparent semi-insulating tin oxide thin films deposited by spray-pyrolysis over large area glass substrates. The precursors used are based on diluted chloride solutions. Bi-doped high resistivity tin oxide thin films are required in the fabrication process of an analog grey scale ferroelectric liquid crystal matrix display. Various parameters have been optimized in order to achieve good reproducibility and uniformity of the electrical and optical properties. Deposition temperature and the properties of the glass substrates have been found as the most critical variables which affect the deposition rate and the quality of the deposited films. The different substrates used, borosilicates and soda-lime glasses, lead to different electrical properties. A further investigation on the stability of the film properties versus different kind of aging procedures have been conducted. Preliminary results of the operation of the complete display making use of our films are shown.
Emission Characteristics of the Mo-Coated Silicon Tips
Authors: Park Heung-Woo , Ju Byeong-Kwon , Jung Jae-Hoon , Lee Yun-Hi , Park Jung-Ho , Chung In-Jae , Hascard M. R. , Oh Myung-Hwan
Published: April 2011

Uniform and reproducible silicon tip arrays were fabricated using the reactive ion etching followed by the re-oxidation sharpening. Molybdenum was then coated on the some of the silicon tip array. Current-voltage characteristics and current fluctuations were measured in the high vacuum environment. Field emission currents were proved by the Fowler-Nordheim plot studies.
Properties of Diamond-Like Carbon for Thin Film Microcathodes for Field Emission Displays
Authors: Robertson J , Milne W I
Published: April 2011

Diamond-like carbon is a strong candidate for field emission microcathodes for field emission displays because of its low electron affinity and chemical inertness. The field emission properties of various types of diamond-like carbon such as a-C:H and ta-C are reviewed in the framework of a bonding model of their affinity.
Study on the Diamond Field Emitter Fabricated by Transfer Mold Technique
Authors: Ju Byeong-Kwon , Kim Seong-Jin , Jung Jae-Hoon , Lee Yun-Hi , Park Beom Soo , Baik Young-Joon , Lim Sung-Kyoo , Oh Myung Hwan
Published: April 2011

Strip-shaped diamond-tip field emitter array was fabricated by using the transfer mold technique. The sharp turn-on characteristic was observed from the current-voltage measurement of the fabricated diamond-tip field emitter array. The turn-on characteristic of the diamond-tip field emitter array was compared with that of a flat diamond film. High emission current density was obtained from the diamond-tip field emitter array. The threshold voltage of the diamond-tip field emitter array was lower than that of a flat diamond film.
Characterization of Low-Voltage Phosphor Screens for Fed Applications
Authors: Jeon B. S. , Kang S. W. , Yoo J. S. , Lee J. D.
Published: April 2011

A phosphor screen which operates at low excitation energy must show good emission efficiency and sustain stabilities against high vacuum and high current density operation. In this study, low voltage phosphors suitable for FED applications were experimentally addressed and screened onto ITO-glass by the electrophoretic deposition which offers several advantages, including easy control of film thickness and high packing density. Process variables such as deposition rate, salt concentration, etc., in electrophoretic deposition method were optimized to achieve high quality of the screens. Correlations of process variables with particle size, surface morphology and luminescent properties of phosphors will be presented. Particularly, much attention was put on the optimization of phosphor screens for the maximal CL intensities and adhesion of particles over substrate when excited by field emitter arrays.
a-Si:H TFTs Patterned Using Laser-Printed Toner
Authors: Glesková Helena , Wagner S. , Shen D. S.
Published: April 2011

We fabricated top-gate amorphous silicon thin-film transistors (a-Si:H TFTs) on alkali-free glass foil, for the first time using laser-printed toner for the patterning of each layer. The toner for the first mask level was applied by feeding the glass foil through a laser printer, and for the following mask levels from patterns laser-printed on transfer paper. The transistors have off currents from ˜ 10-12 A to ˜ 10-11 A and on-off current ratios of ˜ 106. Thus we have demonstrated a technology for the patterning of TFT circuits by printing.
The Processing of LC Thermosets in Orienting External Fields
Authors: Körner Hilmar , Shiota Atsushi , Ober Christopher K.
Published: April 2011

Liquid crystals may possess macroscopically aligned structural, electrical and optical properties when oriented in external fields which leads to a wide range of applications including optical filters, displays, and data storage devices. Combining the properties of thermosets [1, 2] with the properties of liquid crystals (LC) is a logical step towards creating new materials. The development of liquid crystalline thermosets (LCT) has been motivated by their potential use in structural applications and the need for more adaptable polymeric materials with tunable strength and stiffness as well as tailored mechanical anisotropy. It is known that mechanical properties of conventional thermosets are determined by many factors including curing agent, filler, degree of cure, crosslink density, glass transition temperature, and accelerator. In LCT's all of these factors will ultimately be controlled by the both the molecular organization and orientation of the LC phase.
Deuterium NMR Spectroscopy of Solvents Adsorbed onto the Surface of Mesoporous Silica
Authors: Attard George S. , Edgar Mark , Emsley James W. , Göltner Christine G.
Published: April 2011

A series of simple deuterium NMR experiments are used to investigate the surfaces of differently structured silicas by studying the sorption of deuteriated solvents. Two mesoporous silicas (MCM-41 and H1-silica), which display hexagonally arranged channels of uniform diameter are studied, as well as amorphous column silica. The deuterium NMR lineshape strongly depends on the anisotropic environment of the adsorbed deuteriated species (D2O, benzene-d6, pyridine-d5). This allows differentiation between probe molecules adsorbed to the internal surface (anisotropic) and to the external surface (isotropic, non-ordered surface of particles). Solvents adsorbed within the channels of the silica nanostructure retain a degree of orientational order while moving on the surface, thus producing a Pake pattern. In contrast, solvents adsorbed on the outside of the ordered channels and on domain boundaries retain no orientational order while moving on the surface, therefore giving rise to a singlet in the deuterium NMR spectrum. This phenomenon allows to judge and compare the qualities of hexagonal ceramic nanostructures. The results show that H1-silica exhibits a significantly higher degree of order, due to a larger domain size, than MCM-41.
Low Molar Mass Glassy Chiral Nematic Oligomer With Large Polarization Bandwidth
Authors: Krishnamurthy S. , Kalmanash M. , Sethna V.
Published: April 2011

A novel low molar mass glassy chiral nematic oligomer has been developed that exhibits a large selective reflection bandwidth (>100 nm). This has immense potential in applications requiring the polarization of light over a broad spectral bandwidth, corresponding to the range of human vision, with minimal absorption and high efficiency. Definite advantages are observed over other low molar mass and polymeric chiral nematic materials reported in the literature that typically have narrower polarization bandwidths requiring more complex structures to cover the visible spectrum. The glass transition and clearing temperatures of this material are well separated, that in turn facilitates device processing. Devices up to 4"×5" have been successfully fabricated. Their optical properties are discussed.