bluuebunny/crossref_metadata_2025 huggingface dataset:
Lithium Intercalation in Oxides: EMF Related to Structure and Chemistry
Authors: West K. , Zachau-Christiansen B. , Jacobsen T. , Skaarup S.
Published: March 2011

Experimental results for a number of oxide host materials as electrodes in lithium batteries are compared in order to obtain a better understanding of the factors influencing the emf-composition relationships. These factors are divided into two main groups: Those that are consequences of the structure of the host lattice, and those who follow from the chemistry of the host material. Series of materials with the same structure, but different chemical composition (spinels and MO2(B)) are compared, as well as series of materials with the same chemical composition, LixV2O5, but different structure. These data show that the potential level is mainly determined by the host chemistry, although destabilisation of the host can also give a significant contribution. The host structure will determine the width of the composition interval as well as the inflexions on the emf-curve. The possibility for tailoring the emf by using mixtures of transition metals in the host lattice is discussed.
Insertion Reactions in Materials with Water-Containing Galleries
Authors: Huggins R. A. , Wohlfahrt-Mehrens M. , JÖrissen L.
Published: March 2011

The microscopic mechanism of the operation of the well - known “nickel” electrode involves the transport of protons through an outer layer of the Ni(OH)2 phase to the two-phase Ni(OH)2/NiOOH boundary, where the electrochemical reaction takes place. This boundary is displaced as the reaction proceeds. When it reaches the outer surface so that the NiOOH phase is in contact with the electrolyte, an additional process takes place, resulting in the rapid evolution of gaseous oxygen. Interpretation of the available thermodynamic data in terms of ternary phase equilibria shows that the equilibrium electrode potential should be more positive than that for the decomposition of water, as is experimentally observed.
Conductivity Vs Nmr Correlation Times, and Decoupled Cation Motion in Polymer-In-Salt Electrolytes
Authors: Fan Jiang , Marzke R.F. , Angeill C.A.
Published: March 2011

We report 7Li spin lattice relaxation times at 12.82 MHz over a range of temperatures for liquid electrolytes of various salt:polymer ratios, and compare the NMR correlation time obtained at the temperature of the T1 minimum with the conductivity relaxation time at the same temperature. We find that at low salt contents the two relaxation times have the same value, but beyond the “salt-inpolymer to polymer-in-salt transition” zone at the Tg maximum, the two times increasingly separate. This is taken as evidence for the onset of cation-matrix mobility decoupling, which maximizes at the pure salt extreme.
Quantitative High-Resolution Electron Microscopy of Grain Boundaries in α-Al2O3
Authors: Höche T , Kenway P. R. , Kleebe H.-J. , Rühle M.
Published: March 2011

Detailed structural characterization of a near Σ11 grain boundary in ultra-pure α-A12O3 bi-crystals was performed by means of high-resolution transmission electron microscopy (HRTEM). High-resolution imaging revealed a characteristic periodic pattern along the grain boundary. In addition to HRTEM studies, atomistic simulations based on an ionic model were used to calculate three-dimensional structure models that were compared with the experimentally obtained images of the grain boundary. The comparison between the simulated and experimental HRTEM images showed good agreement for the theoretically proposed grain-boundary structure with the lowest grain-boundary energy of 1.8 Jm−2.
Atomic Structure OF YΣ=5 (130) Symmetrical Tilt Boundary In Strontium Titanate
Authors: Ravikumar V. , Dravid Vinayak P.
Published: March 2011

The atomic structure of a pristine (undoped) boundary in strontium titanate has been investigated using transmission electron microscopy techniques. Results of electron diffraction studies indicate a pure tilt boundary with a common \001] tilt axis, and a tilt angle of 36.8°, which corresponds to a Σ-= 5 grain boundary in the Coincidence Site Lattice (CSL) notation. High Resolution Transmission Electron Microscopy (HRTEM) indicates a symmetric tilt grain boundary with a (130) type grain boundary plane. No cation non-stoichiometry or impurity segregants could be detected at the interface, within the limits of the Energy Dispersive X-ray microanalysis technique used. The grain boundary has a compact core, with negligible planenormal rigid body translation (RBT). An in-plane RBT of (1/2)d130 (˜ 0.62 A°) was identified from the high resolution electron micrographs. An empirical model of the relaxed atomic structure of the grain boundary is proposed.
Assessment Of GaInAs/GaInAsP Interdiffusion Profiles Obtained Using Stem-Edx And Hrem
Authors: Mallard R E , Long N J , Booker G R , Thrush E J , Scarrott K
Published: March 2011

We report on an investigation into the interfacial structure of undoped GaInAs/GaInAsP multiple quantum wells grown by metalorganic chemical vapour deposition (MOCVD), which exhibit a pronounced blue shift in luminescence output upon in-situ thermal annealing at 750°C. Using a recently developed composition mapping technique based on the scanning transmission electron microscope (STEM) in conjunction with energy dispersive X-ray (EDX) analysis, the constituent element concentration profiles across the interdiffused multilayer interfaces are measured with a spatial resolution of less than 2nm and a precision of better than 2–3%. The accuracy of the analysis is significantly improved by employing stoichiometric normalisation factors which compensate for systematic errors due to electron channelling. The results showed that the interdiffusion follows a highly non-linear path due to the relatively fast diffusion of the group V species compared to that of the group III species. This implies an increase in the coherency strain in the multilayer, a result which is supported by five-crystal Xray diffraction analysis of the layers. The samples have also been examined by high resolution electron microscopy (HREM) under chemically sensitive imaging conditions. The analysis of the interfacial chemical profile using HREM must be performed under analysis conditions for which a known and unique relationship between image contrast and composition occurs. This condition may not be satisfied in cases in which more than two chemical constituents interdiffuse and the diffusivities of these elements are not equal, as a range of similar lattice fringe motifs across the interface, representing different “diffusion paths”, could occur. The complementary nature of information provided by HREM and STEM provides a means of resolving this ambiguity.
Real-Time Viewing Of Dynamic Processes On Cdte Surfaces At Elevated Temperature
Authors: Smith David J. , Vogl R. , Lu Ping
Published: March 2011

Online video recording with a high-resolution electron microscope has been used to study real-time atomic events occurring at cadmium telluride surfaces over a range of temperatures from 27°C to 500°C. Using the profile imaging mode of observation, different types of surface activity have been documented on (001), (111) and (110) surfaces. For example, the (001) surfaces displayed reversible phase transformations between 2× 1 and 3×1 reconstructions at a transition temperature of about 200°C. The (111) surfaces exhibited sublimation by a ledge mechanism that depended upon the terminating surface: layer-by-layer removal invariably occurred for (110) surface terminations whereas bilayer removal was usually seen for terminations by (100) surfaces. Finally, the (110) surface rearranged by a hopping mechanism, but no substantial loss of material was observed.
Image Simulations of Ge Twin Boundaries.
Authors: Mckernan Stuart , Carter C. Barry
Published: March 2011

General high-angle tilt grain boundaries may be described by an arrangement of repeating structural units. Some grain-boundary defects may also be modeled by the incorporation of structural units of related boundary structures into the boundary. The simulation of these structures requires the use of prohibitively large unit cells. The possibility of modeling these boundaries by the superposition of image simulations of the individual structural units isinvestigated.
Low Energy Photoelectron Holography on Gaas
Authors: Denecke R. , Eckstein R. , Ley L. , Bocquet A. , Riley J. , Leckey R.
Published: March 2011

For GaAs (001) and (III) we have measured the photoelectron diffraction patterns at a kinetic energy of 86 eV. We applied an extended Fourier-transform algorithm to the (001) data to obtain real space images. The origin of structures in these images not representing atomic positions is investigated with the help of single scattering calculations
Imaging of Metal/Semiconductor Interface by Ballistic-Electron-Emission Microscopy (Beem)
Authors: Lee E. Y. , Turnew B. R. , Jimenez J. R. , Schowalter L. J.
Published: March 2011

Studies in ballistic-electron-emission spectroscopy (BEES) have enabled precise energy measurements of Schottky barrier heights with excellent spatial resolution and, more recently, it was shown that even scattering at the metal/semiconductor interface affects the BEES spectrum [1]. Monte Carlo simulations have been done to predict the spatial resolution of ballistic-electron-emission microscopy (BEEM) [2]. In this paper, we will discuss the experimental spatial resolution of BEEM, and we will also give some of our BEES results for Au/Si and for Au/PtSi/Si. Our experimental BEEM studies indicate that, for Au/Si, hot electron transport is diffusive rather than ballistic, because the inelastic mean free path length (∼100 nm) is much larger than the elastic mean free path length (∼10 nm). This is in agreement with existing theories and with the literature on the internal photoemission method of studying the transport. Even in this diffusive regime, the spatial resolution of BEEM is still expected to be very good, being on the order of 10 nm [2]. Our preliminary work on PtSi shows that it has an attenuation length of 4 nm, which differs significantly from that of Au.
Rem and Rheed Studies of Pb Adsorption on Si(111)
Authors: Tanishiro Yasumasa , Fukuyama Masahiko , Yagi Katsumichi
Published: March 2011

Structure changes of Si(111)-Pb surfaces due to deposition and heat treatment are studied by REM-RHEED. Surface structures observed are summarized as a phase diagram. Formation of an incommensurate layer(α) and a phase transition between incommensurate structures of α and α′ is described.
Atomic Structure of the (310) Twin in Niobium: Theoretical Predictions and Comparison with Experimental Observation
Authors: Campbell Geoffrey H. , Foiles S. M. , Rühle M. , King W. E.
Published: March 2011

High - resolution transmission electron microscopy (HREM) has been used to characterize the atomic structure of the symmetric 36.9° tilt grain boundary with [001] tilt axes forming a twin about (310) in Nb. The projected structure was imaged along two different directions in the plane of the boundary and was compared to model structures through high - resolution image simulation. The atomic structure of this Σ5 boundary was predicted with atomistic simulations using interatomic potentials derived from the Embedded Atom Method (EAM), Finnis-Sinclair (FS), and the Model Generalized Pseudopotential Theory (MGPT). The EAM and FS predicted structures with translations of the adjacent crystals which break mirror symmetry. The MGPT predicted one stable structure with mirror symmetry. The atomic structure of the (310) twin in Nb was found by HREM to be mirror symmetric. These findings indicate that the angular dependent interactions modeled in the MGPT are important for determining the grain boundary structures of bcc transition metals.
Analogue Memory Effects in Metal/ a-Si:H /Metal Thin Film Structures
Authors: Snell A.J. , Hajto J. , Rose M.J. , Osborne I.S. , Holmes A. , Owen A.E. , Gibson R.A.G.
Published: March 2011

The ac conductivities of non-volatile analogue memory states are measured in electro-formed Cr/p+/V amorphous silicon structures for a broad frequency range (from 0.1 Hz to 32 MHz). The results suggest that the memory action is associated with electronic processes.
Positron-Beam Observation of Dopant-Defect Complexes in Amorphized Silicon
Authors: Goldberg R.D. , Leung T.C. , Mitchell I.V. , Schultz Peter J.
Published: March 2011

The ability of positrons to probe local defect structures has been utilized to compare differences in residual damage structures in self-irradiated silicon for differing tempera¬tures of the implanted substrate. Sb-doped < 100 > silicon was self irradiated with a 600 keV beam to fluences ranging from 3 x 1014 to 1 X 1016 ions/cm2 at liquid nitrogen, room and slightly elevated (61° C) temperatures. A defect was identified near the end-of-range of the implanted ions whose concentration diminished at higher implant temperatures, but was only weakly dependent on total fluence. Comparison between Sb- and B-doped Si suggests that the defect is impurity based.
Interdiffusion At a-Ge:H/Al and a-Si:H/Al Interfaces
Authors: Jones S.J. , Swartzlander-Franz A.B. , Chen Y. , Williamson D.L.
Published: March 2011

The degree of interdiffusion at the amorphous semiconductor/bulk Al interface was studied using Auger electron spectroscopy analysis. 300-500 Å thick a-Si:H and a-Ge:H films were deposited onto high-purity Al and 5052 Al alloy substrates and subsequently annealed to various temperatures up to 500 °C for 6 hrs. The high-purity Al is used as a substrate for our small-angle x-ray scattering studies of amorphous silicon-based alloys. For all the films deposited on the pure Al, little or no interdiffusion was noted at or below anneal temperatures of 400°C. This result is contrary to those commonly found for samples produced by evaporating Al onto the previously deposited amorphous semiconductor without breaking the vacuum where interdiffusion has been noted at temperatures at or below 200°C. We suggest interdiffusion in the amorphous semiconductor/bulk Al samples is hindered by the presence of a 150-300 Å Al oxide on the Al. A large amount of interdiffusion and partial crystallization is noted in the a-Ge:H sample after an anneal of 450°C while a temperature between 450 and 500°C is required for interdiffusion and crystallization to occur in the a-Si:H sample. In the case of the a-Ge:H films deposited on the 5052 Al alloy, interdiffusion occurs after 300°C anneals due possibly to the migration of Mg and other components of the alloy into the amorphous semiconductors or structural defects in the alloy which enhance interdiffusion.
Low-Temperature Chemical-Vapor Deposition of Amorphous Semiconductors and Insulators
Authors: Matsumura Masakiyo , Sugiura Osamu
Published: March 2011

We have developed a low-temperature (plasma-free) thermal-CVD method for the purpose of eliminating the plasma-CVD method on the amorphous-silicon thin-film transistor (a-Si TFT) process. The motivation comes from the fact that, in the plasma-CVD method, there arc many shortcomings originating from inherent properties of plasma, and that only the CVD method has potentially no drawbacks among various alternative methods. High deposition temperature, the most serious technological barrier in the present a-Si and silicon- nitridc (SiN) CVD methods, has been overcome by the use of high silicon-hydridcs and high nitrogen-hydrides for source gases. Electronic properties of the CVD-produced a-Si film arc improved by post-hydrogenation. Excimer-laser annealing is the next generation technology for improving dramatically the electronic properties of the CVD-produced Si and SiN films. This paper reviews these novel technologies developed in our group aiming at the TFT process, and also the CVD-produced TFT characteristics.
Low Filament Temperature Deposition of a-Si:H by Catalytic Chemical Vapor Deposition
Authors: Brogueira P. , Grebner S. , Wang F. , schwarz R. , Chu V. , Conde J.P.
Published: March 2011

Hydrogenated amorphous silicon, a-Si:H, is deposited from silane (SiH4) and hydrogen (H2) using a tungsten wire at low filament temperatures (Tfil = 1200°C) by catalytic chemical vapor deposition.

The deposition rate increases monotonically with the depositions pressures and shows a maximum at an H2 : SiH4 flow ratio of unity. Vanishingly small deposition rates were observed for silane-only depositions and for H2-to-SiH4 flow ratios of 2.5 and above. The optoelectronic properties show complex dependence on substrate temperature (Tsub). Three intervals of Tsub with distinct optoelectronic were observed: as Tsub increases from 180 to 220°C, the optical bandgap, EgTauc increases from 1.9 to 2.4eV, the dark conductivity,σd, decreases from 10-10 to 10-15 Ω-1cm-1and the photoconductivity, σph, decreases from 10-5 to 10-10Ω-1cm-1 (region (i)). As Tsub increases from 220 to 250 °C, EgTauc decreases to 1.8eV and the photosensitivity, σph/σd decreases to ~ 1 due to an increase of both σd and σph (region (ii)). Throughout these two regions, the photoconductivity γ factor remains between 0.6 and 0.9 and the activation energy of the dark conductivity, Ea,σd, remains between 0.7 and 0.9eV. Above 250°C, the σph and σd remain approximately constant at 10-4 Ω-1cm-1 and γ decreases to below 0.5 and Ea,σd ~ 0.3eV.
Separating the Contributions of Hydrogen and Structural Relaxation to Damage Annealing in a-Si:H
Authors: Stolk P.A. , Berntsen A.J.M. , Saris F.W. , Van Der Weg W.F.
Published: March 2011

This paper investigates the effects of ion implantation and annealing for pure (a-Si) and hydrogenated amorphous silicon (a-Si:H). The photocarrier lifetime in as-deposited a-Si:H decreases from ≥200 to 3 ps after 1 MeV Si+ implantation to doses exceeding 1014/cm2. A comparison with relaxed a-Si suggests that damage generation in a-Si:H merely arises from displacements in the silicon network. Annealing of ion-damaged a-Si:H at 200-500 °C recovers the carrier lifetime to 60-100 ps as a result of hydrogen passivation of electrical defects. However, Raman spectroscopy shows that hydrogen does not significantly enhance long-range network relaxations during annealing. This implies that thermal treatments of ion-implanted a-Si:H can not fully recover the as-deposited state.
Hydrogen Content of a-Si:H and a-Si:H,F as a Function of Chemical Annealing
Authors: Bullock J.N. , Rim K. , Wagner S.
Published: March 2011

Chemical annealing, the periodic exposure of the growing amorphous silicon surface to H radicals, has evoked much interest because of its potential for modifying the film properties during the crucial process of sub-surface restructuring. We report the results of measurements of H content in chemically annealed a-Si:H and a-Si:H,F films.

We prepared a-Si:H and a-Si:H,F in a dc-triode reactor from SiH4 and SiF4 plus H2, respectively. We chemically anneal with a H2 discharge by switching off the Si bearing gas and switching on the H2 flow after 7 s of deposition. We measure the H content with a differential pressure effusion apparatus. We also report the opto-electronic properties of these films, including the defect density after light-soaking.

The key parameter of chemical annealing is the anneal duty cycle, which we specify as the fraction of the time that the H2 discharge is on. In the silane samples, we find that the H content is independent of chemical annealing. For the fluorinated samples we observe that the H content starts out at about 5 at.% without chemical annealing, rises to 10-15 at.% at 0.5 to 0.7 annealing duty cycle, and then decays to 6-7 at.% at 0.9 duty cycle. Although not fully understood, we offer evidence that these effects are due to the physics of the glow-discharge rather than surface reactions.
High Substrate Temperature a-Si:H Grown by DC Reactive Magnetron Sputtering
Authors: Liang Y. H. , Maley N. , Abelson J. R.
Published: March 2011

We report the electronic properties, stability and microstructure of a-Si:H films grown at very high substrate temperature (Ts = 320∼425°C) by DC reactive magnetron sputtering (RMS). The partial pressures of Ar and H2 are fixed at 1.5 and 0.8 mT, respectively, during the deposition. The initial defect state density, determined by the constant photocurrent method (CPM), varies from 2∼5×l015 cm−3with H content changing from 15–10 at.% as Ts increases from 320–375°C. For 100 hrs white light exposure at lW/cm2, a heavily degraded state was obtained with mid–gap state density in the range 2-3×l016cm−3 over this Ts range. These are among the lowest values reported for intrinsic a–Si:H.
Electronic Structure and Dynamics of Defect in a-Si:H by Ab-Initio Molecular Dynamics
Authors: Orita N. , Sasaki T. , Katayama–Yoshida H.
Published: March 2011

Electronic structure and dynamics of defects in hydrogenated amorphous silicon (a-Si:H) are investigated based upon ab–initio molecular–dynamics simulations. It is shown that (i) the hydrogen–passivated dangling bond (Si-H), (ii) the positively-ionized three–centered bond (Si– H+–Si), (iii) the negatively–ionized three–coordinated dangling bond (D−) and (iv) the five- coordinated floating bond (F5) are the intrinsic defects in a–Si:H. Based upon the calculated result, we discuss the role of hydrogen and the origin of the photo–induced defect in a-Si:H.
In Situ Ellipsometric Observation of the Growth of Crystalline Silicon From Fluorinated Precursors
Authors: Akasaka Tetsuya , Araki Yuhzo , Shimizu Isamu
Published: March 2011

The growth of polycrystalline silicon thin films fabricated from fluorinated precursors SiFnHm (n+m≤3) on SiO 2 substrates was examined in detail by real time ellipsometry. The volume fraction of crystal in the film was within 50 vol.% on the average when it was grown continuously on glass. A low density amorphous layer of 300Å thick was formed in the early stage of the growth. The crystallinity, however, was improved with an increase in accumulated film thickness. The layer-by-layer technique of alternating the deposition of very thin film and the exposure to hydrogen plasma was effective on the promotion of crystallization. Optimal conditions of both the deposition and hydrogen plasma treatments were also established by in situ ellipsometry.
Structure and Stability of Microvoids in a-Si:H
Authors: Biswas R. , Kwon I.
Published: March 2011

Microvoids have been observed in a-Si:H as demonstrated by small angle X-ray scattering. We have studied the structural properties of these microvoids with molecular dynamics simulations. Using molecular dynamics simulations with classical potentials, we have created microvoids by removing Si and H atoms from a computer generated a-Si:H network. The internal surfaces of the microvoids were passivated with additional H atoms and the microvoids were fully relaxed. Microvoids over a limited range of sizes (5-90 missing atoms) were examined. We obtained a relaxed microvoid structure with no dangling bonds for a microvoid with 17 missing atoms, whereas other sizes examined produced less relaxed models with short H-H distances at the microvoid surface. The strains near the microvoid surface are described. The microvoid model was stable to local excitations on weak bonds in the vicinity of the microvoid.
The Effect of Post-Deuteration on Metastability in a-Si:H
Authors: Nickel N.H. , Jackson W.B. , Tsai C.C.
Published: March 2011

Hydrogenated amorphous silicon films were deuterated through a sequence of 1h exposures to a remote deuterium plasma at 350°C. The concentration profiles of hydrogen and deuterium were determined by SIMS at various times during the exposure sequence. The defect density in state A, after deuteration and after illumination with white light were determined using CPM measurements following each 1h exposure sequence. We find that post-deuteration does not alter the defect density in state A, change the Urbach edge, nor significantly alter metastable defect formation. Intense light soaking increases the defect density by about 5 × 10l6cm−3 independent of the total H + D concentration. These results suggest that D always enters the sample in pairs pinning the hydrogen chemical potential which supports the idea of a negative U system for hydrogen and deuterium. Despite an increase of Si-H bonds by as much as 3 × 1021cm−3, the annealed dangling bond density and the weak Si-Si bond density did not change.This suggests that the density of weak Si-Si bonds as well as the dangling bond density is determined by equilibration with strong Si-Si bonds through the interchange of H. The implications of these results for H bonding will be discussed.
Diffusion of Hydrogen in Amorphous Silicon in the Low Concentration Regime
Authors: Roth J.A. , Olson G.L. , Jacobson D.C. , Poate J.M.
Published: March 2011

We report the first measurements of hydrogen diffusion kinetics in a-Si in the regime of low H concentration (<2×l019 cm−3). The results differ substantially from the diffusion behavior typically observed in hydrogenated a-Si:H at H concentrations >1020 cm−3. The activation energy and pre-exponential factor for low-concentration H diffusion are found to be 2.70 ± 0.02 eV and 2.2 × 104 cm2s−1, respectively, and are shown to be independent of both annealing time and H concentration. It is difficult to reconcile the combination of high activation energy and large pre-exponential factor with a simple deep-trap-limited diffusion model. Consequently, an alternative mechanism for H diffusion involving the migration of dangling bonds coupled with a H bond-switching step is proposed.
Measurements of SiH3 and SiH2 Radical Densities in RF Silane Plasmas Using Laser Spectroscopic Techniques
Authors: Goto T.
Published: March 2011

In the RF (13.56 MHz) silane plasma, recently the SiH3 radical density was measured using infrared diode laser absorption spectroscopy, and the correlation between the SiH3 radical density and the growth rate of hydrogenated amorphous silicon thin film was investigated. The SiH2 radical density was also measured using modified laser induced fluorescence spectroscopy and intracavity laser absorption spectroscopy. Those measurement methods and main results are reviewed here.
Carrier Injection in a-Si:H P-I-N Devices: Hydrogen Redistribution and Defect Creation
Authors: Asensi J.M. , Andreu J. , Puigdollers J. , Bertomeu J. , Delgado J.C.
Published: March 2011

A straightforward analytical expression of the density-of-states (DOS) of a-Si:H valid in non-equilibrium steady state situation has been obtained. The model is based on a statistical-mechanical treatment of the hydrogen occupation for different defect sites. The broadening of available defect energy levels (defect pool) and the possibility of charged defects are taken into account. This leads to a new explanation of the Staebler-Wronski effect, based on the "conversion" of shallow charge centers to neutrals near the middle of the gap as a consequence of hydrogen redistribution induced by electron-hole recombination.
Study of a-Si:H Using the Solid/Electrolyte System
Authors: Goldstein Y. , Many A. , Weisz S.Z. , Penalbert J. , Munoz W. , Gomez M.
Published: March 2011

Pulsed measurements on the solid-electrolyte system, which proved very useful in the study of crystalline semiconductors, have been found to be equally effective when applied to hydrogenated amorphous Si films. Here, as well, the aSi:H/electrolyte interface is essentially blocking to current flow and, as a result, surface space-charge layers, ranging from large depletion to very strong accumulation conditions, can be induced and studied. In particular, valuable information can be gained on the density of the localized bulk states. Measurements in the depletion range under illumination yield directly the total density of occupied states in the entire energy gap. This is very useful in obtaining a quick and reliable assessment of the quality of the amorphous films. In high-grade films we find that the total density of occupied states is around 1018 cm−3. The data in the accumulation range, on the other hand, provide useful information on unoccupied states near the conduction band edge. The blocking nature of the amorphous Si/electrolyte interface is utilized also to apply a sweep-out technique for an accurate determination of μτ the product of the electron mobility and lifetime, even when this value is very low. In a rather poor-quality film, for example, we find μτ to be 5 × 10−8 cm2/V.
Effective Densities of States in Conduction and Valence Bands in a-Si
Authors: Furlan JoŽE , Smole Franc , PopoviĆ Pavle
Published: March 2011

Effective densities of states in conduction and valence band, Nc and Nv, are usually set to a fixed value of 1019 to 1020 cm−3 in all computer simulations of a-Si structures. In this contribution the densities Nc and Nv are analytically expressed for different selected extended states distributions. The derivatives d(lnNc)/dx and d(lnNv)/dx, representing specific terms in transport equations, are expressed as function of position dependent band structure. The effect of an increased disorder in heterojunction is simulated by Gaussian distribution of states in a linearly graded heterojunction region.
High Field Electron Drift in a-Si:H
Authors: Gu Qing , Schiff Eric A. , Chevrier Jean Baptiste , Equer Bernard
Published: March 2011

We have measured the electron drift mobility in a-Si:H at high electric fields (E ≤ 3.6 x 105 V%cm). The a-Si:Hpin structure was prepared at Palaiseau, and incorporated a thickp+ layer to retard high field breakdown. The drift mobility was obtained from transient photocurrent measurements from 1 ns - 1 ms following a laser pulse. Mobility increases as large as a factor of 30 were observed; at 77 K the high field mobility de¬pended exponentially upon field (exp(E/Eu), where E u= 1.1 x 105 V%cm). The same field dependence was observed in the time range 10 ns – 1 μs, indicating that the dispersion parameter change with field was negligible. This latter result appears to exclude hopping in the exponential conduction bandtail as the fundamental transport mechanism in a-Si:H above 77 K; alternate models are briefly discussed.
Sub-Surface Equilibration of Hydrogen with the a-Si:H Network Under Film Growth Conditions
Authors: An ILSIN , Li Y.M. , Wronski C.R. , Collins R. W.
Published: March 2011

In this study we characterize hydrogen diffusion and reaction processes in the near-surface (top 200 Å) of a-Si:H that lead to network equilibration under standard conditions of plasma-enhanced chemical vapor deposition (PECVD). Real time spectroscopic ellipsometry (SE) is used to provide continuous kinetic information on the near-surface conversion of Si-Si to Si-H bonds during exposure of in situ-prepared films at 250°C to filament-generated atomic H. We have found that for optimum PECVD a-Si:H, the formation of additional Si-H bonds is limited by the capture of H at trapping sites, and the rapid diffusion process (D>10-14 cm2/s) by which H reaches the site is not detected optically. Deep trapping occurs at a rate of ∼10 3 s-1 under our filament conditions, estimated to generate ∼1020 cm-3 mobile H in the near-surface of the film. Finally, more than 1021 cm-3 additional H atoms are trapped with emission rates <2×10-7 s-1, suggesting trap depths >2.0 eV. Shallower traps are also detected at lower concentration.
Steady-State Photocarrier Grating Technique Applied to a-Si:H Thin Film Transistors
Authors: WANG F. , REISSNER M. , FISCHER T. , GREBNER S. , SCHWARZ R.
Published: March 2011

The transport properties of a-Si:H based thin film transistors have been characterized by the ambipolar diffusion length (Lamb) which was measured by the steady-state photocarrier grating technique (SSPG). The results show that with increasing positive gate voltage (Vg) the photoconductivity increases, but Lamb decreases. The above results can be consistently explained in terms of the shift of the Fermi level towards the conduction band edge with increasing V g. On the other hand, from the analysis of the SSPG results it is found that the factor γS, which is related to the interface recombination velocity, increases with V g.This indicates that when narrowing the conductive channel the recombination of the carriers at the interface increases.
Doping Efficiency of N-Type a-Si:H Doped with a Liquid Organic Source
Authors: Gaughan K. , Lin ZHAOHUI , Viner J.M. , Taylor P.C. , Mathur P.C.
Published: March 2011

N-type amorphous silicon films were grown using a mixture of silane and tertiarybutylphosphine (TBP-C4H11P) vapor in a plasma enhanced chemical vapor deposition system. The concentration of TBP in silane was varied from 0 to 3% by volume. As expected, at low doping levels, the photoluminescence (PL) intensity associated with both the band-tail recombination (peak at 1.3 eV) and deep-defect recombination (peak at 0.8 eV) decreased as the impurity concentration increased, but for TBP concentrations > 0.1% the PL intensity increased again. For moderate doping levels the activation energy for conductivity leveled off at ∼ 0.2 eV. For concentrations of TBP > 0.1% the activation energy for dark conductivity increased. A shift in the optical gap was observed for the highest impurity concentrations due to the incorporation of carbon from the TBP. These results are interpreted as a pronounced decrease in the doping efficiency for heavily doped films (> 0.1%) perhaps influenced by the increased carbon concentration.
An Approach to High Quality a-Ge:H by VHF Deposition
Authors: Zedlitz Ralf , Heintze Moritz , Bauer Gottfried H.
Published: March 2011

Amorphous hydrogenated germanium a-Ge:H was deposited by very high frequency glow discharge (VHF-GD) at frequencies between 25 and 220MHz, low pressure (2.5Pa) and high deposition rates (≤4Á /s) on both electrodes of a parallel plate reactor. The films are comparable to material deposited on the powered electrode of a conventional RF-GD. Mobility-lifetime products ηµτ for photogenerated charge carriers around 10-7cm2/V and a Fermi level position at EC-EF≈400meV indicate good opto-electronic properties. The IR spectra show that the samples are free of oxydization and incorporate slightly more voids than the RF material. Measurements of ion energy Ei and flux Фi in the substrate plane show that “hard&” preparation conditions are obtained comparable to the deposition on the powered electrode of an RF-GD.
Comparison of Current and Light Induced Defects in a-Si:H
Authors: Street R.A. , Jackson W.B. , Hack M.
Published: March 2011

Metastable defect creation by illumination and by a forward current in p-i-n devices are compared using CPM and reverse current measurements of the defect density. The data show that the same defects are formed by the two mechanisms, but with different spatial profiles. Numerical modelling shows how the spatial profile influences the reverse bias current.
Degradation of Amorphous Silicon Based Photoconductors by Corona Discharge
Authors: Swaaij R.A.C.M.M.VAN , Willems W.P.M. , Bezemer J. , Lokker H.J.P. , Van Der Weg W.F.
Published: March 2011

Thin three-layered a-Si:H photoconductive structures have been studied for application in high resolution electrophotography. The resolution of prints is determined by the a-Si:C:H top layer. It is observed that the resolution of prints decreases from 100 to 30 lines/mm after about 25 charge cycles of corona discharge. The effect of corona charging on the surface properties of a-Si:C:H films with different carbon content is studied. Before and after corona exposure Fourier transform infrared absorption and lateral dark conductivity were measured. We observe that modes around 3300 cm-1, ascribed to H2O and -OH related modes, arise in combination with an increase of the lateral dark conductivity. This effect is not dependent on the film thickness, indicating that only the surface is affected by the corona discharge. The increase of the dark conductivity is interpreted as due to band bending at the surface, induced by the adsorption of H2O or the growth of a thin Si-OH layer at the surface.
Dependence of Steady-State Defect Density in Hydrogenated Amorphous Silicon on Carrier Generation Rate Studied Over a Wide Range
Authors: Hata Nobuhiro , Ganguly Gautam , Matsuda Akihisa
Published: March 2011

Measurements of the steady-state defect density (Nst) in hydrogenated amorphous silicon under illumination of pulse-laser light, as well as of continuous light, were carried out; and the dependence of Nst on the effective rate of carrier generation (G) is presented. The values of G ranged from 8 x 1021 to 2.4 × 1023 cm-3 s-1, while the illumination temperature was kept at 30 °C or at 105 °C. The results showed trends of Nst increasing with G similarly to the trends in the literature, but covered a higher and wider G range, and fitted a defect model which assumes a limited number of possible defect states.
On the Role of the Staebler-WronSki Susceptibility in Hydrogenated Amorphous Silicon
Authors: Caputo D. , Cesare G. De , Irrera G. , Masini G. , Palma F. , Rossi M. C. , Conte G. , Nobile G. , Terzini E.
Published: March 2011

Photoconductivity decay during monochromatic illumination has been measured on an ensemble of a-Si:H films deposited at different substrate temperatures. Degradation behaviour has been modelled within the framework of the bond-breaking model (dN/dt = Csw np). Simmons and Taylor recombination kinetics has been assumed, taking into account the divalent nature of dangling bonds and their three possible conditions of occupancy. The Staebler-Wronski susceptibility (Csw) has been extracted through a fitting procedure. As a result, a correlation between the obtained Csw and the measured electronic, optical and structural properties of as deposited a-Si:H films can be inferred.
Dependence of a-Si:H Degradation on i-Layer Thickness and Photon Energy
Authors: Paes H. , Achete C. , Losch W.
Published: March 2011

Metastable light-induced changes in hydrogenated amoiphous silicon (a-Si:H) have been monitored by stationary single photocarrier charge collection in p-i-n diodes under reverse bias. The influence of the following parameters has been systematically investigated: i-layer thickness, applied voltage and illumination time and energy. A strong dependence of cell degradation on i-layer thickness is observed. In the case of degraded diodes superlinear current vs. voltage (Iph αVm) in the low voltage regime were observed. These results can be related to space-charge-limited photocurrents. The results from variation of photon energy and single and/or double carrier injection indicate that recombination rather than single-carrier trapping plays a key role in metastable defect creation.
Elecrically Detected Magnetic Resonance in A-SI:H Pin Cells
Authors: Lips Klaus , Fuhs Walther
Published: March 2011

We report on a detailed study of EDMR in pin-type solar cells. Like in films the signals are dominated by the contribution of the e-db resonance. It is found that the spectra depend on the applied bias and photon energy of the exciting light. The data suggest that the dark current is controlled by recombination in the bulk of the i-layer. The sign of the signal depends sensitively on the sign of the internal field. At high forward bias and illumination recombination at the pi-interface plays an important role. Degradation by both light exposure and high forward current results predominantly from an increase of the bulk defect density.
Relaxation of Electron Beam-Induced Metastable Defects in a-Si:II
Authors: Grimbergen M. , Mcconville R. , Redfield D. , Bube R.H.
Published: March 2011

Relaxation of the metastable defect density in undoped amorphous silicon is observed after keV electron irradiation. The time constant for relaxation has an activation energy close to 1 eV, similar to that for light-induced defects. Relaxation appears to follow two or more stages. A large initial density relaxes rapidly, followed by slower relaxation more characteristic of light-induced defects. Separation of these components allows for a better comparison of e-beam and light-induced saturation defect density.
Optimization of Optoelectronic Properties of a-SiC:H Films
Authors: Demichelis F. , Crovini G. , Pirri C.F. , Tresso E. , Galloni R. , Rizzoli R. , Summonte C. , Zignani F. , Amato G. , Rava P. , Madan A.
Published: March 2011

Amorphous silicon carbide films have been deposited by PECVD in SiH4+CH4+H2 mixtures at different hydrogen dilutions. The optoelecuonic properties of the films have been measured by transmittance-reflectance spectroscopy, photothermal deflection spectroscopy and photo and dark electrical conductivity. Structural properties have been obtained by FTIR spectroscopy. It was found that high hydrogen dilution leads to materials of improved quality, p-i-n device structures have been deposited with intrinsic layers at different hydrogen dilution levels.
Carbon-Rich a-SiC:H Alloys as Dielectric Materials: Injection Mechanisms in Mim-Type Structures
Authors: Vincenzoni R. , Masini G. , Leo G. , Guattari G. , Galluzzi F.
Published: March 2011

Electronic properties of C-rich a-Si1 -xCx:H alloys are investigated through the characterization of MIM-type structures, by means of current-voltage measurements carried out at different temperatures. These measurements provide some new insight in both the dielectric behavior of the material and the physics of the metal/a-Si1-xCx:H junction.
Electron Microscopy Studies of Undoped and Phosphorus Doped Si:H and Si,C:H Films
Authors: Chen Y.L. , Bentley J. , Wang C. , Lucovsky G. , Maher D.M.
Published: March 2011

The microstructure of undoped and phosphorus doped Si:H and Si,C:H films was analyzed by selected-area diffraction, conical dark-field imaging, energy-dispersive x-ray spectroscopy and electron energy-loss spectroscopy in transmission electron microscopes. Thin films were synthesized by remote plasma-enhanced chemical vapor deposition and characterized in terms of degree of crystallinity. The distribution of phosphorus in the Si:H and Si,C:H films, and carbon in the Si,C:H films was evaluated. The results indicate that i) the microstructure of a film may be two phase, consisting of silicon microcrystallites in an amorphous matrix, ii) phosphorus doping as well as the presence of carbon influences the degree of crystallinity by reducing the average size and volume fraction of microcrystallites, iii) the presence of carbon and phosphorus doping completely suppresses the crystalline phase, iv) phosphorus is distributed at approximately the same concentration in both the crystalline and amorphous phases of diphasic films, and v) carbon is detected in the amorphous phase of the Si,C:H films.
Thin A-SiOxHyAlloy Films Showing Visible Luminescence Prepared by DC-Magnetron Sputtering with Water Vapor as Oxygen Source
Authors: Zacharias M. , Garke B. , Panckow A. , Freistedt H. , DrÜSedau T.
Published: March 2011

Thin films of non stochiometric hydrogenated amorphous silicon oxide (a-SiOxHy.) with x=0.09...1.14, y=0.06...0.20 were prepared by dc-magnetron sputtering from a crystalline silicon target within an argon-hydrogen-water vapor atmosphere. The film properties were investigated by infrared and optical spectroscopy, Rutherford backscattering spectroscopy (RBS) and elastic recoil detection (ERD), luminescence and activated conductivity. Absorption bands related to either silicon-hydrogen or silicon-oxygen vibration modes were observed at 650, 800, 870 and 1000cm-1. Above a water pressure of 0.06Pa, the oxygen content saturates at about 50 at %. The static refractive index n ∞ the optical Tauc gap ETand the logarithmic dark conductivity at room temperature show a linear dependence on the films' oxygen content. Films prepared under certain conditions show visible photoluminescence.
Material Control for High-Efficiency Amorphous Silicon Solar Cells
Authors: Hishikawa Yoshihiro , Sasaki Manabu , Tsuge Sadaji , Okamoto Shingo , Tsuda Shinya
Published: March 2011

A total-area conversion efficiency of 12.0% (100cm2) has been achieved for a single-junction a-Si solar cell. The film deposition rate (Rd) plays an essential role in controlling the optical and electrical properties of "device-quality" a-Si:H for high-efficiency solar cells. The properties of conventional "device-quality" a-Si:H films deposited from 100% SiH4 are primarily determined by the balance between Ts and Rd. A lower or higher deposition rate results in a-Si:H with a narrower or wider bandgap, respectively. This enables the properties of a-Si:H to be controlled independent of Ts. The controllable range of a-Si:H properties can be widened by effectively utilizing factors such as ion bombardment. For example, a high dilution of SiH4 with H2 ( H2 SiH4 = 10 or more) or an H2 plasma treatment after deposition results in a-Si:H with a very wide bandgap and low defect density, which cannot be achieved by using 100% SiH4- Controlling the properties of a-Si:H by applying vibrational / rotational energy is also investigated.
Fabrication of High Quality Poly-Si From Fluorinated Precursors
Authors: Ishihara Shin-Ichi , He Deyan , Akasaka Tetsuya , Araki Yuzoh , Shimizu Isamu
Published: March 2011

Poly-Si thin films with grains 100–200 nm in dia. showing a highly ordered texture were grown from fluorinated precursors, SiFnHm (n+m=3), on a glass substrate at 300–400 °C with the aid of atomic hydrogen. According to the in situ observation by ellipsometry, the reconstruction was undergone in a solid phase stimulated by impinging atomic hydrogens within a thin layer of about 10 nm thick owing to the strong chemical interaction of the pair of H and F in Si-network. Both H and F were released efficiently from the network to the levels of 2 × 1020 cm−3 and (2−5) × 1019 cm−3, respectively. Dangling bonds were also efficiently passivated down to 4 × 1016 cm−3 with hydrogens diffused through the network. P-doped films showing electrical conductivity of 10−2 S/cm (300 °K) with the activation energy of 0.24 eV was obtained by alternately repeating the deposition of thin layer and the treatment with atomic hydrogens.
The Simulation of a-Si:H Junction Capacitance Measurements
Authors: Shapiro Finley R. , Das Artjit
Published: March 2011

Junction capacitance measurements have been used by many researchers to study the density of states in the mobility gap of hydrogenated amorphous silicon. However, the data analysis methods used for these studies are based on approximate analytic models which may not always be appropriate. In order to understand better the experimental method and the models, we have performed simulations using a numerical simulator which can calculate the complete time-dependent response of an amorphous semiconductor device. The current in a device is simulated as a function of time when a small sinusoidal voltage applied in addition to a DC bias voltage. The out-of-phase and in-phase components of the sinusoidal part of the current are used to calculate the capacitance and series resistance, just as they measured in an experiment. The results of simulated experiments at different temperatures are shown.
Modeling of a-Si:H Solar Cells on V-Grooved Glass Substrates
Authors: Tao G. , Zeman M. , Metselaar J.W.
Published: March 2011

We present the results of a series of numerical study of a-Si:H solar cells on different V- grooved substrates. The light absorption profiles were calculated by taking into account the V-groove inner reflection and the reflection at the glass/air interface. These profiles were used for modeling the performance of solar cells with the 1-D Amorphous Semiconductor Analysis (ASA) computer package, which has been developed in our laboratory. The J-V characteristics under AMI.5 and the external quantum efficiency axe simulated for V- grooves with different tilt-angles (0 degree for flat substrate). Results show that effective light trapping occurs for V-grooves with tilt-angle larger than 21 degrees.