bluuebunny/crossref_metadata_2025_split huggingface dataset:
Defect-Related Photoluminescence of a-Si:H
Authors: Ulber Isabell , Barth Axel , Fuhs Walther , Mell Helmut 
Published: April 2011

ABSTRACTThe defect-related photoluminescence (0.8–0.9eV) of a-Si:H is studied as a function of the defect density ND and the temperature using subgap excitation of Ex= 1.16eV. Electron bombardment, light exposure and phosphorus doping were employed to create defects. The defect density ND was infered from CPM- and PDS-spectroscopy. We conclude from the measured density-of-states distributions of doped films that the radiative process is tunneling of bandtail carriers (Majority carriers) into the defect states. The dependence of the emission intensity on ND suggests that only a subgroup of the defects acts as radiative center. We propose that the local structure of the defect determines whether it acts as radiative or nonradiative center.
On Modeling Trivalent Dangling Bonds with Bivalent Levels
Authors: Suntharalingam Vyshnavi , Branz Howard M. 
Published: April 2011

ABSTRACTWe examine the treatment of the hydrogenated Amorphous silicon (a-Si:H) dangling-bond defect used in the Analysis of microelectronic and Photonic Structures (AMPS) computer model and in other models of a-Si:H semiconductor devices. The dangling bond defect (D) is trivalent, with two correlated electronic levels in the gap. However, Modelers typically employ two uncorrelated bivalent levels to represent D; this introduces fictitious neutral dangling bond (D°) levels whenever D is charged. We find that the bivalent (AMPS) representation captures the important aspects of D physics and introduces only small errors into simulations. To reach this conclusion, we examine charge density and recombination and compare the trivalent D defect and its bivalent representation in both thermal equilibrium and non-equilibrium (illuminated) cases. The charge density is identical, implying that band bending computed by the simulation is accurate. We find errors in the recombination rate for the bivalent representation are normally less than or equal to σ0/σch, where σ° is the capture cross section of D0 and σch is the capture cross section of a charged state of D. Typically Modelers use σ0/σch of 1:10 to 1:100 yielding insignificant errors in the recombination rate with the uncorrelated bivalent representation.
Molecular Dynamics Simulation of Hydrogenated Amorphous Silicon with Tersoff Potential
Authors: Ohira Tatsuya , Inamuro Takaji , Adachi Takeshi 
Published: April 2011

ABSTRACTA Molecular dynamics method with a Many-body Tersoff-type interatomic potential has been being applied to analyses of hydrogenated Amorphous silicon (a-Si:H) thin-film growth processes. As a first step toward film growth simulations, Molecular dynamics simulations of SiH3 radical, which would be a significant precursor for the a-Si:H thin-film growth processes, and a-Si:H formation with a rapid quenching method have been performed by developing new Tersoff-type interatomic potential between Si and H in this study. Visualization of SiH3 radical dynamics by computer graphics has made it possible to observe the inversion and rotation of SiH3 radical, which had been predicted by infrared diode-laser spectroscopie measurement in other group. In addition, visualization of the a-Si:H sample has helped us to find that there are some microcavities in the sample and that there are two kinds of hydrogen in the sample, gathering closely together while lying scattered, which had been predicted in IR absorption experimental results.
Non-Radiative Recombination in a-Si:H
Authors: Schubert M. , Stachowitz R. , Saleh R. , Fuhs W. 
Published: April 2011

ABSTRACTFrequency-resolved photoluminescence spectroscopy (FRS) is used to study non-radiative recombination in a-Si:H using generation rates sufficiently small to garantee geminate recombination at low temperature. The quenching of the photoluminescence by a higher defect density ND and an increase of temperature influences the QFRS spectra differently: Whereas for increasing ND the quenching of the signal is more pronounced on the low frequency side raising temperature leads to a uniform decrease in the entire frequency range. The dependence of the lifetime distribution on ND is quantitatively explained in a model where radiative recombination competes with non-radiative tunneling into defect states.
Monte Carlo Modelling of Electrophotographic Dark Discharge
Authors: Elmer S. J. , Marshall J. M. , Van Swaaij R. A. C. M. M. , Bezemer J. , Hepburn A. R. 
Published: April 2011

ABSTRACTIn an electrophotographic experiment, surface voltage is measured for a-Si1-xCx:H films of different thicknesses. It is observed that the thickness normalised dark decay rate (TNDDR) at longer times is smaller for thicker than for thinner specimens. This phenomenon has also been reported for other materials. In order to get a better understanding of the dark discharge process, the monte Carlo technique is used to model electrophotographic dark discharge in materials of which a-Si1-xCx:H is typical. The present study considers every carrier within the model after each increment of a short time step. This then allows bimolecular effects and the presence of space charge to be accounted for.The study concentrates on two different discharge mechanisms and the effects they have on the TNDDR for varying specimen thicknesses. For the first of these, a negative charge is deposited on the surface of the sample and drifts through the bulk under the influence of the local internal field (Surface Injection model). In the second, electron/hole pairs are generated within the bulk with both carriers being mobile through the specimen (Bulk Generation model). The subsequent surface decay is due to the recombination of bulk generated holes and trapped surface electrons. Results from these models are compared with those found experimentally.
Exploration of Defect Relaxation Dynamics in Hydrogenated Amorphous Silicon using Temperature Switching Experiments
Authors: Gardner Adam , David Cohen J. 
Published: April 2011

ABSTRACTWe have carried out a series of charge transient measurements on a-Si:H in which we insert a double temperature step during the period when electrons are being emitted from deep defects. The behavior of this emitted defect charge is completely inconsistent with any density of states that remains static during the emission; that is, defect relaxation must be invoked. Such measurements allow us to separate the temperature dependence of relaxation from that of thermal emission. In particular, we demonstrate that the emission itself exhibits thermally activated behavior in spite of the ongoing relaxation processes.
Explanation of Light-Enhanced Annealing of Defects in Amorphous Silicon
Authors: Redfield David , Bube Richard 
Published: April 2011

ABSTRACTSeveral recent measurements have shown that annealing of metastable defects in a- Si:H can be accelerated by the presence of light. This is the opposite of the usual light-induced defect generation, and no existing rate equation explains it while maintaining the necessary symmetry of generation and recovery processes, and consistency with the stretched-exponential transients that best describe observed generation and anneal behavior. This paper shows that this light-enhanced annealing (LEA) can be explained readily by the usual rate equation leading to stretched exponentials with no other terms by allowing a variation of coefficients with temperature or light intensity. This equation then leads to good simulations of observed LEA. Interpretation of these results in terms of distributional changes is presented, and an experimental test is proposed.
A Quantitative Model for the Metastable Defect Creation in Amorphous Semiconductors by Kev-Electron Irradiation
Authors: Scholz A. , Schröder B. , Oechsner H. 
Published: April 2011

ABSTRACTThe interaction mechanisms of keV-electrons with the hydrogenated Amorphous semiconductor are briefly discussed and the differences to the metastable defect creation by photons are set out. Based on the knowlegde of the energy dissipation mechanisms of keV-electrons in the hydrogenated Amorphous semiconductor, a model for the creation of metastable defects by keV-electron irradiation is developed and its quantitative agreement with the experimental results is shown.
Hydrogen Bonding and Microvoid Stability in a-Si:H
Authors: Van Den Boogaard M. J. , Van Der Steege A. C. , Van Sark W. G. J. H. M. , Van Der Weg W. F. 
Published: April 2011

ABSTRACTWe have annealed PECVD a-Si:H films at 250, 300, and 350°C and measured the evolution of the infrared absorption spectrum. We observe that, during the initial stage of such a heat treatment, atomic hydrogen migrates from the isolated state to the clustered state. Thus diffusion of atomic hydrogen must occur around 300°C. Microvoids with internal surfaces covered with SiH bonds appear to be more stable than voids lined with SiH2 bonds and (SiH2)n polymers.
Metastable Defect Studies in Hydrogen Modulated Multilayer Amorphous Silicon
Authors: Kwon Daewon , David Cohen J. 
Published: April 2011

ABSTRACTJunction capacitance methods were employed to investigate the metastable states in a-Si:H with hydrogen modulated Multilayer sample as well as companion homogeneous samples. Each layer in the multilayer sample was grown from the same gas mixture (SiH4+Ar or SiH4+H2) as that of homogeneous samples. We observed that in state A and metastable states produced by a short time light soaking the defect density of each layer in the multilayer sample track well the defect density of the corresponding homogeneous sample. This implies that “local” aspects of metastable defect creation dominate in the earlier light soaked states, and also the stable defects in state A are due to local properties of each layer as introduced during growth. However, for longer time light soaked states and succeeding partial annealed states the defect level of the multilayer sample is higher than either of homogeneous samples. However, the spatial variations of the stable defects are finally recovered when the sample is annealed at the same temperature as the initial state A.
Hydrogen Diffusion in Undoped and B-Doped a-Si1-xCx:H
Authors: Shinar R. , Shinar J. , Subramania G. , Jia H. , Sankaranarayanan S. , Leonard M. , Dalai V. L. 
Published: April 2011

ABSTRACTA deuterium secondary ion mass spectrometry (SIMS) study of hydrogen diffusion in undoped and boron-doped a-Si0.86C0.4:H deposited by an electron cyclotron resonance plasma is described. The undoped films deposited at 250°C clearly indicated deuterium-hydrogen interdiffusion at T ≥ 350°C. The dispersion parameter a of the power-law time dependent diffusion constant D = D00(ωt)−α decreased from ∼0.3 at T = 350 and 400°C to ∼0.1 at 450°C, and the activation energy for a diffusion length of 1000 Å was ∼1.0 eV. These results are discussed in relation to previous studies of a-Si:H. The diffusion in ∼0.2 and ∼0.6 at.% B-doped a-Si0.86C0.14:H sharply differs from that in B-doped a-Si:H, where an enhancement of up to ∼103 was previously observed. In doped a-Si0.86C0.14:H, the diffusion of most of the H atoms is strongly suppressed, but a small fraction undergoes fast diffusion. IR Measurements indicate that the B-doping reduces the bulk-like Si-H stretch vibration at ∼2000 cm1. Upon annealing, the Si-CHn and C-H wag modes at ∼780 and ∼1000 cm−1, resp., increase, while the 640 and ∼2000 cm1 Si-H wag and stretch modes, resp., weaken, indicating transfer of hydrogen from Si- to C-bonds, in which the H atoms are apparently deeply trapped. As in a-Si:H, the fast diffusion component is apparently due to carrier recombination-enhanced weak Si-Si bond breaking. The results suggest that B-doping also induces microvoids and enhances the rate of breaking of weak Si-C bonds, leading to enhanced trapping of H.
Growth of Amorphous Silicon Materials and Devices with Improved Stability
Authors: Dalai Vikram L. , Ping E. X. , Kaushal Sanjeev , Leonard Mark , Bhan Mohan K. , Han K. 
Published: April 2011

ABSTRACTWe discuss the growth of a-Si:H Materials and devices using a low pressure remote ECR plasma. We show that by using this plasma in an etching mode with a high H flux, we can grow high quality a-Si:H films at high temperatures (325–375 C). These films have significantly improved stability compared to standard a-Si:H films deposited using glow discharge. We can further improve the stability of these films by incorporating minute (sub ppm levels) of boron during growth. We also report on the fabrication of devices at these temperatures using this very reactive plasma. We discuss the precautions taken to obtain good devices, and discuss how the devices can be further improved.
Built-In Potentials Via Electroabsorption Measurements in a-Si:H p-i-n Solar Cells: a Critical Assessment
Authors: Wang Qi , Schiff E. A. , Hegedus Steven S. 
Published: April 2011

ABSTRACTWe have measured electroabsorption (EA) in a-Si:H p-i-n solar cells in order to estimate their built-in potentials Vbi. This Method was pioneered by the Osaka University group; in simple cases Vbi is identified with a parameter V0 obtained by a simple linear fit to the dependence of EA on external bias voltage. Using signal averaging techniques we are able to Measure V0 with a reproducibility of better than 2%. In one sample from the Institute of Energy Conversion with an a-SiC:H p+ layer we estimate Vbj = 0.97 ± 0.06 V, which is in good agreement with previous JVT Measurement. The reason that the error estimate is larger than the reproducibility error is the presence of several small effects not accounted for by the original EA theory: in particular V0 depends slightly on the laser wavelength. We attribute these effects to the small portion of Vbi dropped in the a-SiC:Hp+ layer, which has a different electroabsorption than a-Si:H. We discuss models for the wavelength dependence.
Analysis of the Physical Origin of the blue shift of the Optical Bandgap of a-Si:H based Multilayers
Authors: Bernhard Norbert , Bauer Gottfried H. 
Published: April 2011

ABSTRACTThe physical origin of the blue shift of the optical Taue bandgap of compositional multilayers of a-Si:H and its alloys with Ge, C or N is analysed. It is shown that for multilayers with bandgap differences up to 0.7 eV between well and barrier Material, the effect can be understood completely classical in terms of the effective medium approximation, when the mean composition of the multilayer films is not kept constant. For the variation of both the electronic well and barrier widths a shift of the Taue bandgap is theoretically expected. It is of the same order of magnitude as the effect which is experimentally observed. Interfacial transition layers can account for a blue shift also for higher bandgap differences, as usually the case in a-Si:H/a-SiNx:H Multilayers, and when the mean composition of the multilayers is kept constant.
Mobility-Lifetime Products in Glow Discharge and RF Sputter Deposited A-Si:H
Authors: Farias M. H. , Roche A. , Weisz S. Z. , Jia H. , Shinar J. , Lubianiker Y. , Balberg I. 
Published: April 2011

ABSTRACTA comparative study of the deposition temperature (Ts) dependence of the Mobility-lifetime (μτ) products of the charge carriers in glow-discharge and rf sputter-deposited a-Si:H is described and discussed. The Ts-dependence of the μτ's the majority carrier light-intensity exponents of the two types of films are strikingly similar. These observations lead to the conclusion that the structure of the recombination levels as well as the recombination processes are in accord with the “defect pool” Model, in contrast to previous suggestions. The differences between the two types of films thus appear to be limited to the differences in the concentrations of dangling bonds.
The Growth, Steady State and Decay of the Photocarrier Population at low Temperatures
Authors: Tran Minh Q. , Stradins P. , Fritzsche H. 
Published: April 2011

ABSTRACTWe used the transient infrared photoconductivity to measure the rise and fall of the non-equilibrium photocarrier concentration n at helium temperatures and the conditions for recombination. The growth rate of n was found to be only a few percent of the electron-hole pair generation rate G in agreement with LESR data of Street and Biegelsen. The saturation value of n is about 1017cm−3 and nearly independent of G. After turning off the generation light, the residual n requires an hour to decrease by a factor 2 but it can be made to recombine in seconds by infrared light.
The Time-of-Flight Technique Applied to Amorphous Silicon Pin Solar Cells
Authors: Brummack Hanna , Bernhard N. , Eberhardt K. , Schubert M. B. 
Published: April 2011

ABSTRACTFast transient photocurrent measurements have been performed on a-Si:H based pin diodes with thicknesses down to 0.5 μm. On a thick sample drift mobilities derived in either the small-signal or the space-charge limited current mode are compared. On real solar cell devices in SCL voltage (integral) Mode the electron lifetimes τ are determined from the extraction time of the photo-generated charge in dependence on the degradation state of the sample. A clear and reproducible correlation between lifetime and degradation state has experimentally been established. Lifetimes τ, for which an intensity dependence has been observed in the SCL voltage Mode, are compared to the μτ-product from a small-signal charge collection Measurement. The influence of the real absorption profile on the measurements is discussed.
Hydrogenated Amorphous Silicon Germanium Alloy for Stable Solar Cells
Authors: Terakawa A. , Shima M. , Sayama K. , Tarui H. , Nishiwaki H. , Tsuda S. 
Published: April 2011

ABSTRACTThe film properties and solar cell performance of a-SiGe:H samples with the same optical gap and different combinations of hydrogen content (CH) and germanium content (CGe) have been compared. The optimum composition for the initial properties, such as the tail characteristic energy, defect density and conversion efficiency of the solar cell, was determined, and the differences could be explained by the difference in H bonding configuration. The degradation ratio of the conversion efficiency becomes larger in higher CH samples. This suggests that hydrogen or Si-H2 participates in light-induced degradation. As a result, the optimum CH for an efficient solar cell is believed to shift to the lower CH region after light soaking. Based on these findings, the stabilized conversion efficiency of 3.3% under red light (γ>650nm) for an a-SiGe:H single-junction solar cell (1cm2) and 10.6% under lsun light for an a-Si/a-SiGe double-junction stacked solar cell (1cm2) have been achieved. The degradation ratio is only 8.6% for the double-junction solar cell.
Hole Drift Mobility in a-Si1−xCx:H
Authors: Gu Qing , Wang Qi , Schiff E. A. , Li Yuañ-Min , Malone Charles T. 
Published: April 2011

ABSTRACTWe present a summary of our recent measurements on hole drift mobilities in hydro-genated amorphous silicon-carbon alloys (a-Si1−xCx:H). Increasing the bandgap has a vastly smaller effect for the hole mobility than for electrons. In conjunction with previous drift Mobility Measurements in a-Si1−xCx:H and a-Si1−xGex:H, these hole measurements complete a simple pattern for the effects of bandgap modification on drift Mobilities: electron mobilities decline as the bandgap is increased beyond 1.72 eV or decreased below 1.72 eV, but hole mobilities are relatively unaffected.
Electronic and Optical Properties of a-Si1−xCx:H Films Produced From Admixtures of Silane and Ditertiarybutylsilane
Authors: Gaughan K. , Viner J.M. , Taylor P.C. 
Published: April 2011

ABSTRACTWe investigated the optical and electronic properties of amorphous silicon carbide (a-Si1−xCx:H) films produced by plasma enhanced chemical vapor deposition from admixtures of silane and ditertiarybutylsilane [SiH2 (C4H9) 2 or DTBS] using photothermal deflection spectroscopy, electrical conductivity and its temperature dependence as well as photoconductivity. These a-Si1−xCx:H films exhibit low Urbach energies and high photoconductivities similar to films produced with other carbon feedstock sources. We also present our results for hydrogen diluted a-Si1−xCx:H films using DTBS as the carbon feedstock source.
Ambipolar Diffusion Lengths, Lamb, and Steady-State Photoconductivity, σph, in B2H6 Doped Μc-Si
Authors: Cho S.M. , He S.S. , Lucovsky G. 
Published: April 2011

ABSTRACTWe have compared the photo-transport properties of thin films of B-compensated Μc-Si and device-grade a-Si:H prepared by remote plasma-enhanced chemical-vapor deposition (PECVD). The steady state photocarrier grating technique was used to determine the ambipolar diffusion lengths, Lamb. The conductivity of the μc-Si:B thin films changed from n-type to p-type as the B2H6 fraction in the B2H6/SiH4 source gas mixture was increased. The steady state photoconductivity, σph, decreased as B2H6 was initially introduced and the material changed from n-type to intrinsic; σph then increased as the material converted to p-type. Lamb displayed a complementary behavior with a maximum value at the approximate compensation point between the n-type and p-type conductivity regions. The steady-state photoconductivity and Lamb did not show any significant photo-induced degradations under intense illumination. Finally, the photoconductivity and Lamb displayed a strong dependence on sample thickness which is associated with Fermi level pinning at the surface.
Process-Property Relationships For a-Si1-xCx:H Deposition: Excursions in Parameter Space Guided by Real Time Spectroellipsometry
Authors: Lu Yiwei , Kim Sangbo , Gunes Mehmet , Lee Yeeheng , Wronski C.R. , Collins R.W. 
Published: April 2011

ABSTRACTReal time spectroellipsometry (RTSE) has been applied to study the growth of a-Si1-xCx:H alloys (x∼0.1; Eg=1.90–2.00 eV) for applications as i- and p-type layers in wide band gap solar cells. Two important material parameters, the optical gap and the relative bond-packing density (or void volume fraction), can be estimated from RTSE data collected during the growth of a sequence of layers onto the same substrate using different plasma-enhanced CVD conditions. In this way, large regions of parameter space have been scanned expeditiously, and an improved understanding of the effects of H2-dilution, substrate temperature (Ts), plasma power, gas pressure, and gas flow on the film properties has been obtained.
Amorphous Silicon Alloy Photovoltaic Technology - from R&D to Production
Authors: Guha S. , Yang J. , Banerjee A. , Glatfelter T. , Hoffman K. , Ovshinsky S.R. , Izu M. , Ovshinsky H. C. , Deng X. 
Published: April 2011

ABSTRACTThe key requirements for photovoltaic modules to be accepted for large-scale terrestrial applications are (i) low material cost, (ii) high efficiency with good stability, (iii) low manufacturing cost with good yield and (iv) environmental safety. Thin films of amorphous silicon alloy are inexpensive; the products are also environmentally benign. The challenge has been to improve the stable efficiency of these modules and transfer the R&D results into production. Using a Multijunction, Multi-bandgap approach to capture the solar spectrum more efficiently, we have developed one-square-foot modules with initial efficiency of 11.8%. After 1000 h of one-sun light soaking, a stable efficiency of 10.2% was obtained. Both the efficiency values were confirmed by National Renewable Energy Laboratory. The technology has been transferred to production using an automated roll-to-roll process in which different layers of the cell structure are deposited in a continuous manner onto stainless steel rolls, 14” wide and half a mile long. The rolls are next processed into modules of different sizes. This inexpensive manufacturing process produces high efficiency modules with subcell yields greater than 99%. The key features of the technology transfer and future scope for improvement are discussed.