bluuebunny/crossref_metadata_2025_split huggingface dataset:
Density of States of Amorphous Germanium Thin Films Deposited by the Pecvd of H2-Diluted Germane
Authors: Godet C. , Chu V. , Equer B. , Bouizem Y. , Chahed L. , El Zawawi I. , Theye M. L. , Basrour S. , Bruyere J. C. , Stoquert J.P. 
Published: March 2011

ABSTRACTThe disorder in a-Ge:H thin films produced by the plasma-enhanced chemical vapor deposition (PECVD) technique is strongly reduced when the GeH4 gas is diluted at 1% in H2 and the radiofrequency power density is increased to 0.1 W.cm−2. This improvement is attributed to a better surface passivation by the hydrogen atoms during the growth. However, the poor transport properties indicate a still high defect density. The midgap defect absorption and the Urbach energy, obtained from the photothermal deflection spectra calibrated with optical data, both decrease as a function of the film thickness. The optical defect density is calibrated with EPR spin measurements. For a-Ge:H films thicker than 2 μm, obtained at a deposition temperature Ts ranging from 150 to 250°C, the Urbach tail parameter E° is lower than 50 meV and not sensitive to Ts ; the dangling bond density is around 4.107 cm−3, which is higher by a factor of 100 than in a-Si:H. Preliminary transport measurements indicate that the Fermi level density of states is larger than 1018 cm−3.eV−1.
Determination of a-Si:H Film Quality Throughfst and Sclc Techniques
Authors: Martins R. , Vieira M. , Fortunato E. , Ferreira I. , Soares F. , GuimarĀes L. 
Published: March 2011

ABSTRACTThe ambipolar diffusion length (L*) and the effective lifetime (τ*) in undoped a-Si:H films have been measured by the Flying Spot Technique (FST). This technique consists in measuring the decay range of charges in Schottky barriers and the potential induced by a laser beam with a constant motion rate. The data show that the FST measurements are quite suitable to determine separately the effective diffusion length and the effective lifetime of the carriers, taking into account the surface effect. The results also indicate that the photosensitivity is not the best parameter to characterize film quality. Indeed, films with high photosensitivity can present a poor lifetime and diffusion length that make them unsuitable to be used in optoelectronic devices. The results obtained by FST have been correlated with the Density of States (DOS) inferred by the Space Charge Limited Current (SCLC) measurements performed in the temperature range of 300–430 K and by the Constant Photocurrent Method (CPM) in forward biased Schottky diodes at room temperature. The dependence of the DOS on temperature shows that the SCLC technique is clearly influenced by the thermally activated carriers.
The Influence of Interfaces on the Gap State Distribution of Undoped a-Si:H
Authors: Schumm G. , Bauer G. H. 
Published: March 2011

ABSTRACTModulated primary photocurrent (MPC) studies on pin structures show spatial variations of the gap state distribution across the i-layer that can be correlated with Fermi level shifts by band bending towards interfaces. These results as well as reverse bias annealing effects are explained in terms of the defect pool model. It is demonstrated that MPC measurements are basically identical to TOF measurements with clear advantages in the post-transit time regime.
Strong Thickness Dependence of Photoelectronic Properties in Hydrogenated Amorphous Silicon
Authors: Yang L. , Balberg I. , Catalano A. , Bennett M. 
Published: March 2011

ABSTRACTMany of the photoelectronic properties of a-Si:H, including photoconductivity and ambipolar diffusion length, were observed to deteriorate rapidly with decreasing film thickness. Calculations show that the thickness dependence can be attributed mainly to the effect of surface recombination. In fact, any surface recombination velocity above 100 cm/s will result in an equally good fit to the experimental data. We conclude that the intrinsic photoelectronic properties of bulk a-Si:H and other related materials can be measured only on thick samples (≥1μm) and by using uniformly absorbing light (at long wavelengths).
Defect Thermodynamics, Inhomogeneity, and the Density of Gap States in Hydrogenated Amorphous Silicon
Authors: Branz Howard M. , Silver Marvin 
Published: March 2011

ABSTRACTA new hydrogenated amorphous silicon (a-Si:H) density of states (d.O.s.) in+cluding the transition levels of both neutral (T3o) and charged (T3+ and T3−) dangling-bond defects is proposed. We derive closed-form and numerical solutions for the d.o.s. from a thermodynamic equilibrium theory of defect concentrations in which material inhomogeneity is assumed to give rise to ∼1020 cm−3 of electrostatic potential fluctuations. The connection between thermodynamic transition level energy and defect formation energy implicit in this and other “defect pool” models is included explicitly in the calculation. We calculate the d.o.s. for a range of parameters and for different values of Fermi energy. We apply the calculated d.o.s. to explain and unify various experimental results in a-Si:H. In particular, we reconcile recent depletion-width-modulated ESR data with the near-perfect Curie law T-dependence of the dangling-bond spin density observed by several groups. It is seen +that the depletion results in roughly equal numbers of T3−T3–>° and T3°–>T3+ transitions despite the positive value of effective correlation energy. We also discuss possible sources of the short-to-medium range potential fluctuations in amorphous silicon.
Saturation of Metastable-Defect Density in a-Si:H
Authors: Redfield David , Bube Richard H. 
Published: March 2011

ABSTRACTThe existence of saturation (or steady state) in the density of light-induced defects in amorphous Si:H is shown to have major importance for the interpretation of the nature and origin of these defects. First, a number of characteristics of the steady-state and transient responses to light and temperature are described and contrasted. These lead to the conclusion that the saturation value is the only useful criterion of the number of defects in these materials. We then describe a new atomic model for defects, unifying both dopant-induced and light-induced defects. This model invokes foreign atoms in defects, and saturation reflects the limitation imposed by the numbers of such atoms. Many other observed properties of defects are explained by this model.
Electronic States in the Gap of a-Si from Bond Angle Variations
Authors: Davidson B. N. , Lucovsky G. 
Published: March 2011

ABSTRACTWe investigate the formation of defect states in the gap of a-Si arising from deviations from the ideal tetrahedral bond angles. The local density of states for Si atoms in disordered environments is calculated using tight-binding parameters for the cluster-Bethe lattice method. The Hamiltonian for a-Si with bond angle distortions is taken as an average over many configurations associated with a random choice of bond angles, weighted by Gaussian distributions with standard deviations between 2°.and 10°. Bond angle deviations in this range generate a density of defect states at the valence band edge that: 1) increases as the average bond angle deviation increases; and 2) is significantly larger than the density of band tail states generated at the conduction band edge. We obtain a shift of the absorption edge from the joint density of states (DOS) as a function of bond angle deviations. In addition, a calculation of the DOS for a distorted tetrahedral cluster embedded in an idealized Bethe lattice yields a threshold bond angle distortion of ±20° for the appearance of a discrete state in the gap near the valence band edge.
The First Fifty Billion Nanoseconds of Electron Photocarrier Evolution in Undoped a-Si:H
Authors: Antoniadis Homer , Schiff E.A. 
Published: March 2011

ABSTRACTThe drift of electrons generated by a 3 ns illumination impulse was studied in several undoped a-Si:H specimens by using transient photocharge measurements from 10 ns to 50 sec. The transients show four stages of evolution: band transport, deep-trapping and subsequent re-emission, and recombination. From the temperature dependence of the emission time we estimated the mean energy of the trap distribution and the attempt-to-escape frequency associated with this trap to be 0.6 eV below the conduction band and 5×1012 sec−1 respectively; this trap has been identified in previous work with the D-center found in electron spin resonance. In addition the data resolve the two orders of magnitute discrepancy between mobility-lifetime estimates from earlier charge-collection experiments and from steady-state photoconductivity.
A General Analysis of Steady State Photocarrier Grating Technique for the Determination of Ambipolar Diffusion Length
Authors: Li Yuan-Min 
Published: March 2011

ABSTRACTA general photoconductivity formula is derived for the case of a semiconductor steady state photocarrier grating (SSPG)1. It is shown that, under the condition of weak applied electric field, the ambipolar diffusion length can be determined by the SSPG technique1without the lifetime-regime restriction2,3 if the lifetime of photocarriers is known. The general formula presented here is reduced to the simple lifetime-regime formula1–3 under the condition of fast dielectric relaxation.
Achievement of More than 10% Efficiency for a Single-Junction 100cm2 a-Si Solar Cell and Development of a New-Type Module Structure
Authors: Hishikawa Yoshihiro , Ohnishi Michitoshe , Kuwano Yukinori 
Published: March 2011

ABSTRACTA total area conversion efficiency of 10.2% has been achieved for a 1Ocm×1Ocm integrated-type single-junction amorphous silicon (a-Si) solar cell submodule. It is the highest conversion efficiency ever reported for an a-Si solar cell with an area of 100cm2, including multi-junction cells. The effective area conversion efficiency is as high as 11.3%. The high efficiency is obtained by improving the quality of the i-layer and the p/i buffer layer, as well as by utilizing a highly textured, high-quality transparent electrode. The quality of the i-layer plays a dominant role in the performance of a-Si solar cells, especially in high efficiency cells. Techniques that control the properties of the high-quality a-Si films for the i-layer are described. Electric conductivity, ESR spin density and the Raman spectra of high-quality a-Si:H films are investigated as well as their thickness-dependence and substrate-dependence.A Through-Hole Contact (THC) integrated-type submodule has been developed as a new-type a-Si solar cell module structure. Numerical simulations on the output power of the structure show that the output power can be significantly improved by the THC structure.
Effect of Bias on Recovery Rates of Stressed Amorphous Silicon Mis Structures
Authors: Jackson W. B. , Hack M. 
Published: March 2011

ABSTRACTThe effect of Fermi level position on the annealing of defects created by electron accumulation is investigated. Results indicate that holes accelerate the annealing of defects created by electrons confirming the theoretical understanding of the proximity compensated layer. Capacitance-voltage curves indicate that hole accumulation tends to create defect states located closer to the conduction band than electron accumulation–a result in agreement with energetics of defect formation.
Field Profile Tailoring in a-Si:H Radiation Detectors
Authors: Fujieda I. , Cho G. , Drewery J. , Jing T. , Kaplan S. N. , Perez-Mendez V. , Qureshi S. , Wildermuth D. , Street R. A. 
Published: March 2011

ABSTRACTThe capability of tailoring the field profile in reverse-biased a-Si:H diodes by doping and/or manipulating electrode shapes opens a way to many interesting device structures. Charge collection in a-Si:H radiation detectors is improved for high LET particle detection by inserting thin doped layers into the i-layer of the usual p-i-n diode. This buried p-i-n structure enables us to apply higher reverse-bias and the electric field is enhanced in the mid i-layer. Field profiles of the new structures are calculated and the improved charge collection process is discussed. Also discussed is the possibility of field profile tailoring by utilizing the fixed space charges in i-layers and/or manipulating electrode shapes of the reverse-biased p-i-n diodes.
The Role of Carbon in Amorphous Silicon Nip Photodiode Sensors
Authors: Weisfield R. L. , Tsai C. C. 
Published: March 2011

ABSTRACTWe have been investigating the dark current and quantum efficiency of nip and pin photodiodes for document scanning applications. Alloying the top doped layer with carbon or making it microcrystalline was found to enhance the spectral response without causing significant deleterious effects to dark current. We also observed lower dark currents in both pin and nip(μc-Si:C:H) sensors compared to nip(a-Si:C:H) sensors, which we attribute to a more graded p-i interface in the former devices.
Effects of Buffer Layers on Performance of Amorphous Silicon Image Sensors
Authors: Miyagaki S. , Ri S. , Takasaki K. 
Published: March 2011

ABSTRACTThis paper reports the effects of a-SiC:H buffer layers on a-Si:H photodiode properties, especially the dark current and decay image lag. The photodiode has an ITO/p-type a-SiC:H/a-Si:H/i-type a-SiC:H/metal structure.Using two heterojunctions–p-SiC:H/a-Si:H at the surface contact and a-Si:H/i-SiC:H at the back contact–significantly decreases the dark current. The p-SiC:H layer blocks electron injection, and the i-SiC:H layer suppresses the reaction between the a-Si:H layer and metal electrode. The use of both a-SiC:H layers may increase decay image lag. Excess boron doping to the p-SiC:H layer induces boron diffusion into the a-Si:H photo-electric conversion layer, which increases trap levels in the a-Si:H layer. The i-SiC:H layer, because of its high resistivity, decreases the electric field applied to the a-Si:H photo-electric conversion layer.
Optimizing the amount of boron in the p-SiC:H layer and the resistance of the i-SiC:H layer enabled us to produce the characteristics required for high-definition television (HDTV) image sensors–a dark current on the order of 10−10 A/cm2 and a 1% level decay image lag.
A-Si:H Deposited by Direct Photo-Cvd Using a Microwave-Excited Xe Lamp
Authors: Matsunami Hiroyuki , Shirafuji Tatsuru , Fuyuki Takashi , Yoshimoto Masahiro 
Published: March 2011

ABSTRACTA-Si:H films were deposited by direct decomposition of Si2H6 using vacuum ultra-violet light (147nm) emitted from a microwave-excited Xe resonance lamp. The substrate temperature was varied between 200 and 300 °C . The hydrogen content in the films is estimated to be comparable or lower than that in high-quality glow discharge (GD) films based on results from infrared absorption measurements and hydrogen effusion measurements. Hydrogen atoms are incorporated in the form of Si-H bonds (2000 cm−1 mode), and Si-Hn bonds of the 2090 mode In a unit volume was one or two order less than that of Si-H bonds (2000 cm−1 mode). In spite of the lower hydrogen content, the defect density was lower than that of GD films. The mobility gap of the photo-CVD film was closer to the optical gap. For both n- and p-type doping, the dark conductivity increased with the dopant gas composition monotonously, and reached the order of 10−3 S/cm.
Post Initial Light Induced Degradation Stability and Performance of Amorphous Silicon Modules
Authors: Berry W. B. , Hahn M.J. , Mrig L. 
Published: March 2011

ABSTRACTData is presented on the field test of amorphous silicon photovoltaic modules. The modules have been obtained by SERI under the field test and performance evaluation program to track the changes in performance following the continuing development of improvements in their manufacture. The data have been gathered for up to five years. Procurement time intervals are used to distinguish between the sets of modules under test. The results show that more recent technogies have a more rapid initial loss of performance but that stability sets in at an earlier time than that observed for the earlier modules. Data are presented to show efficiency and fill factor degradation from both the date of deployment and a date selected for post initial degradation(pi). Performance models are presented and compared, including models which account for annual variations in insolation and temperature and, therefore, generation of light induced defects and annealing.
¼c Silicon Thin Films Deposited by Remote Plasma Enhanced Chemical Vapor Deposition Process
Authors: Wang C. , Parsons G. N. , Kim S. S. , Buehler E. C. , Nemanich R. J. , Lucovsky G 
Published: March 2011

ABSTRACTIn an earlier study, we deposited ¼c-Si thin films by reactive magnetron sputtering (RMS). Here we extend our studies to the deposition of both undoped and high conductivity N-type and P-type ¼c-Si thin films by a remote PECVD. We show that ¼c-Si films can be deposited by bringing hydrogen, H2, into the source gas mixtures. The H2 could introduced by either upstream in a He/H2 mixture and directly plasma excited, or downstream, and be remotely excited along with the silane, SiH4, feed gas. The degree of crystallinity is shown to depend on the hydrogen dilution, the substrate temperature and the substrate material.
Improved Blue Response of Amorphous Silicon Alloy Solar Cells
Authors: Banerjee A. , Guha S. 
Published: March 2011

ABSTRACTA two-layer MgF2/ITO antireflection (AR) coating has been used to reduce the reflection losses from the surface of a hydrogenated amorphous silicon alloy solar cell. This has resulted in a higher efficiency device primarily due to an improved blue response. The relative thicknesses of the MgF2and ITO layers have been tailored to give the highest overall quantum efficiency (Q) values, which are higher than that obtained with a single-layer antireflection coating. Typically, the 0 value at 400 nm (Q400) has been increased from 0.58 to 0.68 for a single a:SiH cell. Incorporation of the double-layer AR coating in conjunction with μc-SiC p-layer has yielded Q400value of 0.77. The total current density obtained by adding the individual contribution of the component cells of a dual bandgap triple amorphous silicon alloy solar cell has been increased from 21.90 to 23.27 mA/cm2using the double-layer AR coating.
Reduction of Internal Stress of a-Si:H Films by in Situ Measurements of Optical Emission Intensity from SiH4 Plasma
Authors: Tamahashi K. , Wakagi M. , Ishikawa F. , Kaneko T. , Tamura K. , Satoh A. , Hanazono M. 
Published: March 2011

ABSTRACTA parameter which determine internal stress of hydrogenated amorphous silicon (a-Si:H) prepared by plasma CVD method has been investigated to prevent a peeling off or crack forming problem. It is clarified that the internal stress changes from tension to compression as a function of supplied rf power density during deposition process. Supplied rf power density is closely connected to H o and SiH* optical emission intensity ratio (H α /SiH* ) of CVD plasma. Ve find that the internal stress can be reduced remarkably by controlling this ratio to 0.3. This H α/SiH* parameter has a superiority that the internal stress can be in situ controlled.
Hydrogen Dilution of Silane: Correlation Between the Structure and Optical Band Gap in Gd a-Si:H Films
Authors: Meiling H. , Van Den Boogaard M. J. , Schropp R.E.I. , Bezemer J. , Van der Weg W.F. 
Published: March 2011

ABSTRACTThe influence of diluting SiH4 with H2 on the optical and structural properties of a-Si:H has been investigated. The major effects of the dilution are a pronounced decrease of the amount of (Si-H2)n-chains, an increase of the macroscopic density, and a decrease of the total amount of incorporated hydrogen in the films. Different models for the structure as a function of the hydrogen concentration are deduced. We also present new data on the dependence of the optical band gap Eg on the hydrogen concentration and density of the films. An important result from this work is that Eg appears to correlate with the distance between the silicon atoms, rather than with the overall hydrogen concentration.
Systematic Errors in the Analysis of the Infrared Transmission Data Of Hydrogenated Amorphous Silicon
Authors: Maley N. , Szafranek I. 
Published: March 2011

ABSTRACTThe validity of the Brodsky, Cardona and Cuomo (BCC) [1] and the Connell and Lewis (CL) [2] methods to analyze infrared transmission data of hydrogenated amorphous silicon (a-Si:H) was examined using computer simulations. Transmission spectra for a-Si:H films 0-5¼m thick and containing up to 30 atomic% hydrogen were simulated assuming coherent reflections in the film and incoherent reflections in the c-Si substrate. Analysis of the simulated data for the 640cm−1 Si-H wagging mode shows that the BCC and CL techniques systematically overestimate the absorption coefficeint, α, and hence hydrogen content, CH, when the film thickness, d, is less than ∼l¼m. The error is nearly independent of CH and is as large as 80% in the limit d→0. On this basis, previously reported experimental evidence for the dependence of CH on d is shown to be an analysis artifact. A simple method to correct the hydrogen content determined by the BCC or CL analysis using only the film thickness is presented.
Transmission Electron Microscopy of Excimer Laser Crystallized Amorphous Si Thin Films
Authors: Anderson G. B. , Bachrach R. Z. , Winer K. , Boyce J. B. , Ponce F. A. , Johnson R. I. , Ready S. E. 
Published: March 2011

ABSTRACTThe results of a transmission electron microscopy (TEM) study of excimer laser crystallized a-Si:H thin films are described in this paper. High resolution TEM has shown the crystallization threshold of 10−2 P a-Si:H, grown by PECVD, to be 85 mj/cm2. TEM reveals that PECVD a-Si:H films crystallize in a stratified structure at laser energy densities greater than 120 mj/cm2. This stratified structure consists of a polycrystalline Si region near the surface, a microcrystalline region below the polycrystalline region and uncrystallized a-Si near the substrate. The depth of the crystallized region increases as the laser energy density increases. Spherical voids are also present in the film in the microcrystalline region. These voids are formed by H2 evolving from the film during crystallization. TEM also reveals that thermal annealing a 50 nm film of a-Si:H followed by laser crystallization produces 100 nm Si grains with low defect density.
A New Technique for Determining Midgap States and Hole Localization in a-Si:H Devices
Authors: Dalal Vikarm L. , Knox Ralph D. 
Published: March 2011

ABSTRACTWe describe a technique for measuring localization of holes in mid-gap states in high quality a-Si:H devices. The localization of holes is determined by measuring quantum efficiency of a-Si:H devices as a function of reverse bias voltage and wavelength of light. It is shown that the QE of localized holes increases significantly upon application of high electric fields, whereas the QE of de-localized holes does not show such a behavior. The voltage-induced increase in QE is explained using a Frenkel-Poole tunneling model. It is also shown that the density of mid-gap states (states in which holes are localized) increases significantly upon light soaking, and that a major consequence of this increase in mid-gap density is a decrease in the electric field in the device. The decrease in electric field is experimentally estimated by fitting the increased current due to tunneling to the expression for Frenkel-Poole tunneling.