bluuebunny/crossref_metadata_2025_split huggingface dataset:
Identification of the Dominant Electron Deep Trap in a-Si:H: A Critical Test of the Defect Pool vs. Defect Relaxation Pictures
Authors: Kwon Daewon , David Cohen J. 
Published: April 2011

ABSTRACTModulated photocurrent (MPC) measurements in intrinsic a-Si:H reveal a prominent band of electron deep traps with a thermal emission energy near 0.6eV. We have identified this defect band by directly comparing MPC and ESR spectra for both an intrinsic and a lightly n-type doped sample for a various metastable states such that the Fermi level, EF, ranges from less than 0.5eV to more than 0.7eV below Ec. This comparsion unambiguously demonstrates that the MPC band arises from the Do charge state of the defects (specifically, the D−&Do transition). This identification is also confirmed when the quasi-Fermi level is varied by the application of light bias even though the peak emission rate from the MPC defect band is changed by more than a factor of 100. These observations specifically rule out the possibility of large populations of charged defects in intrinsic samples predicted by proponents of the defect pool model. Instead, observed behaviors have a natural explanation in terms of a defect relaxation process.
Selfconsistent Analysis of Mobility-Lifetime Products and Subgap Absorption on Different PECVD A-SI:H Films
Authors: Jiao Lihong , Semoushikina S. , Lee Yeeheng , Wronski C. R. 
Published: April 2011

ABSTRACTThe photoconductivity and subband gap absorption measurements over a wide range of generation rate(G) have been carried out ondiluted and undiluted a-Si:H films. It is found that in these high quality films there are significant differences in the functional dependence of mobility-lifetime(μτ) products on G. In additionto the different values of subgap absorption(a) there are also distinct differences in the dependence of a on photon energy (E) as well as G. It is difficult to selfconsistently analyze the results on theundiluted film with the previously used three gaussian distribution, particularly at high generation rates. Selfconsistent analysis isobtained when the (+/0) transitions of negative charged defects andthe (0/-) transitions of positive charged defects are introducedrespectively closer to the valence and conduction bands. This new gapstate distribution is a better representation for the defectpool model and potential fluctuation model.
Photoconductivity and Density of States in “Microstructural” Amorphous Silicon
Authors: Budaguan B. G. , Aivazov A. A. , Radosel'sky A. G. , Popov A. A. 
Published: April 2011

ABSTRACTIt has been reported in previous works that using of RF 55 kHz PECVD method allows to deposit microstructural inhomogeneous a-Si:H films at high deposition rate (10–20Å/c) and with high photoconductivity The structural analysis with using of IR spectroscopy and atomic force microscopy (AFM) performed in this work have shown that these films possess a relatively regular microstructure consisting of “grains” with characteristic size of∼300–500Å. The regular microstructure of investigated films differs them from inhomogeneous a-Si:H with deteriorate electronic properties. At the same time the diffraction analysis didn't reveal the presence of microcrystals. Therefore, we denoted our films as “microstructural” a-Si:H films In this work we performed the modeling of the photoconductivity of “microstructural” a-Si:H films to analyze the density-of-states (DOS) responsible for recombination kinetics For this purpose different approaches to photoconductivity modeling have been used to simulate the experimentally measured temperature dependence of photoconductivity The comparative analysis of results of these simulations and ESR measurements have shown that recombination in high photoconductive ‘microstructural’ films is controlled by deep neutral states
Initial Growth of Microcrystalline Silicon on Atomically Flat Hetero-Substrate
Authors: Saitoh K. , Kondo M. , Matsuda A. 
Published: April 2011

ABSTRACTInitial growth of microcrystalline silicon μ-Si:H) deposited on an atomically flat GaAs (001) wafer using a RF glow-discharge decomposition of hydrogen diluted monosilane gas mixture has been studied by means of atomic force microscope (AFM), Auger electron spectroscopy (AES), and cross-sectional transmitssion electron microscopy (XTEM).It is shown that the initial growth of μc-Si:H deposited at a substrate temperature of 50∼250°C consists of four successive stages, i.e., (1) a layer-by-layer growth of a-Si:H up to d ∼5 Å, (2) island formation of a-Si:H, (3) the coalescence of the islands and the nucleation of microcrystallite at d∼ 10–40 Å depending on the growth temperature, and (4) a rapid roughening with microcrystalline growth.
Novel Plasma Control Method in PECVD for Preparing Microcrystalline Silicon
Authors: Nishimiya T. , Kondo M. , Matsuda A. 
Published: April 2011

ABSTRACTA novel plasma enhanced vapor deposition (PECVD) technique employing biased wall (BW) method has been developed for the enhanced growth rate of the hydrogenated microcrystalline silicon (μc-Si:H) films. Using this method, we have achieved a growth rate of more than 6Å/sec for the formation of μc-Si-H having an average grain size of 200Å; at 350°C.
Growth of polycrystalline silicon at low temperature on hydrogenated macrocrystalline silicon(μ c-Si:H) seed layer
Authors: Park W. S. , Jang Y. H. , Takeya M. , Jong G. S. , Ohmi T. 
Published: April 2011

ABSTRACTHigh quality polycrystalline silicon (poly-Si) films were deposited on SiO2 substrate using hydrogenated microcrystalline silicon (μ c-Si:H) seed layer by very high frequency (182.5MHz) sputtering system and Chemical Vapor Deposition (CVD) system in pure Ar ambient at atemperature of 300°C. In this study, we investigated the correlation between the initial seed layer conditions and the grown layer, and studied the nucleation process. Electrical characteristics and surface roughness of the grown layer as a function of film thickness and deposition rate will be shown. Grown layers were deposited on several kinds of hydrogenated seed layers in pure Ar ambient. The resistivity and Hall mobility of the grown layer are 0.33Ω.cm and 7.5cm2/V-sec respectively for a film with thickness of 3000Å. In this measurement, we found that the resistivity is lower as deposition rate is slower, and when deposition rate is relatively high, the resistivity increases with thickness. On the contrary, in the case of low deposition rate, resistivity decreases as film thickness increases. For the deposition rate of 0.32Å/sec, the surface roughness ranges over 1.39Å-8.52Å. This is a very smooth surface compared to hydrogenated polycrystalline silicon films. The resistivity of grown layer dramatically decreases in proportion to that of the seed layer. From Reflection High Energy Electron Diffraction (RHEED) patterns, it is clearly shown that the film is crystallization of occurred in the earlier stage than that of conventional deposited film. Our results shows that the electrical characteristics of the films are affected by the conditions of the seed layer and the deposition rate. Consequently, we conclude that the initial conditions are very important for the crystal growth of the as-deposition poly-Si.
An Absorption Study of Microcrystalline Silicon Deposited by Hot-Wire CVD
Authors: Diehl F. , Herbst W. , Schröder B. , Oechsner H. 
Published: April 2011

ABSTRACTThe effect of variation of the preparation parameters filament temperature Tfil, gas pressure p and hydrogen dilution (H2/SiH4-flow ratio) on the absorption spectra of microcrystalline silicon deposited by the hot-wire technique (hw-μc-Si:H) has been studied by means of Photothermal Deflection Spectroscopy (PDS). We find an enhanced absorption of the μc-Si:H compared to crystalline silicon in the band gap (defect absorption) as well as in the interband transition region. An increase of absorption has already been reported for μc-Si:H films prepared by different techniques. In the case of hw-pc-Si:H we observe a relation between the absorption enhancement and the crystallite size. Increasing the gas pressure from 35 to 400 mTorr (Tfil=1850°C) or the filament temperature from 1750°C to 1950°C (p=100mTorr) the crystallite sizes, deduced from X-ray diffraction measuements, range from 10 to 60 nm. An alteration of the hydrogen dilution by varying the flow ratio between 2.5 and 25 does not affect the crystallite size and the optical absorption remains constant. In our opinion the enhancement cannot be described by a simple superposition of an amorphous and a crystalline absorption coefficient weighted by the volume fractions of the amorphous and crystalline phase, respectively. The possible reasons for the enhanced absorption will be discussed. The variation of the crystallite size with deposition conditions offers the possibility to control the optical absorption of μc-Si:H which is important for incorporating the material either as window layers or intrinsic layers in solar cells.
Characterization of Monolayer-Level Composition and Optical Gap Profiles in Amorphous Silicon-Carbon Alloy Bandgap-Modulated Structures
Authors: Fujiwara H. , Koh Joohyun , Wronski C. R. , Collins R. W. 
Published: April 2011

ABSTRACTOver the past few years we have applied real time spectroscopie ellipsometry (RTSE) to characterize the structural, compositional, and optical gap profiles in continuously-graded amorphous silicon-carbon alloy films (a-Si1-xCx:H). Most recently, we have extended the RTSE methods to their monolayer sensitivity and resolution limits. In this study, continuous triangular variations in the carbon content × (0.02≤x≤0.24) within ∼25 to 130 Å thick graded layers were introduced at the i/p interfaces of the n-i-p solar cell structures using continuous variations in the flow ratio z=[CH4]/{[SiH4]+[CH4]} during rf plasma-enhanced chemical vapor deposition (PECVD). A virtual interface approximation has been applied to interpret the RTSE data collected during the growth of the graded interface layers. This analysis yields C-content depth-profiles with monolayer-level resolution and a compositional uncertainty of ±0.004. Even compositional gradients in which x changes by >0.2 within a few monolayers’ thickness are readily characterized. Lastly, a continuous increase in open circuit voltage with increasing graded interface layer thickness, saturating at ΔVoc=0.1 V after 100 Å, is observed in the n-i-p solar cells with graded layers. These results demonstrate the importance of the RTSE analysis in assessing bandgap engineered device designs.
Deposition Conditions for Large Area PECVD of Amorphous Silicon
Authors: Kuske J. , Stephan U. , Nowak W. , Röhlecke S. , Kottwitz A. 
Published: April 2011

ABSTRACTThe production of amorphous silicon devices usually requires large area, high-deposition-rate plasma reactors. Non-uniformity of the film thickness at high power and deposition rate is found to be an important factor for large area deposition.Increasing the radio frequency from the conventional 13.56 MHz up to VHF has demonstrated advantages for the deposition of a-Si:H films, including higher deposition rates and lower particle generation. The use of VHF for large area deposition leads to the generation of standing waves and evanescent waveguide modes at the electrode surface and on the power feeding lines. Thereby increasing the non-uniformity of the film thickness. The uniformity of the film thickness for an excitation frequency strongly depends on the deposition parameters e.g. pressure, input power, silane flow and the value of load impedances. With increasing exciting frequencies the range of deposition parameters for obtaining uniform films narrows.Subsequently it is shown that for a large-area plasma-box reactor (500 × 600 mm2 plate size) with a double-sided RF electrode, the non-uniformity of the film decreases due to a homoge-neization of the electrode voltage distribution by using multiple power supplies and load impedances on the end of the RF electrode. The uniformity errors decrease from ±20% to ±2.4% (27.12MHz) and from ±40% to ±5.9% (54.24MHz). Experimental results of the film uniformity will be discussed in dependence on excitation frequencies and the deposition parameters.
Role of The Gas Flow Parameters on The Uniformity of Films Produced by PECVD Technique
Authors: Martins R. , Maçarico A. , Ferreira I. , Fortunato E. 
Published: April 2011

ABSTRACTThe aim of this work is to present an analytical model able to interpret the experimental data of the dependence of film's uniformity on the discharge pressure, gas flow and temperature used during the production of thin films by the plasma enhancement chemical vapour deposition technique, under optimised electrode's geometry and electric field distribution. To do so, the gas flow is considered to be quasi-incompressible and inviscous leading to the establishment of the electro-fluid-mechanics equations able to interpret the film's uniformity over the substrate area, when the discharge process takes place in the low power regime.
a-Si:H and a-SiGe:H Alloys Fabricated Close to Powder Regime of RF PECVD
Authors: Middya A. R. , Suktihazra  , Ray Swati , Longeaud C. , Kleider J. P. 
Published: April 2011

ABSTRACTWe have been observing that a-Si:H and a-SiGe:H films deposited under high chamber pressure or close to powder regime (500 to 2000 mT for a-Si:H and 200 to 1000 mT for a-SiGe:H) show many new features : the mobility lifetime product is 10 to 100 times higher and the density of states above Fermi level of a-Si:H and a-SiGe:H ([Ge] < 0.20) is lower than that of standard samples. This enhancement in photoconductivity can neither be attributed to autodoping nor to creation of sensitization centers in the samples. Hydrogen bonding of these films, is mostly monohydride and the density of microstructural (polyhydrides and microvoids) defects is low. Evidence for the presence of nanocrystals is noted. Though the films seems amorphous by all conventional techniques, they crystallize easily under low laser intensity. Kinetics of light induced degradation is very fast (less than 20 hrs), for a-Si:H and the value of photoconductivity in the light soaked state is comparable to that of the standard samples in the annealed state. The process gas utilization efficiency in this regime is also higher than in the low power and pressure regime.
Molecular Dynamics Investigations of Boron Doping in a-Si:H
Authors: Fedders Peter A. , Drabold D. A. 
Published: April 2011

ABSTRACTThe rather low doping efficiency of B in a-Si:H is almost always explained by the argument that almost all of the B is incorporated into three-fold coordinated sites and that B is inert or non-doping in this configuration. Using ab initio molecular dynamics, we have studied the energetics and doping (electronic structure) consequences of B incorporation into a-Si:H both with and without H passivation. Our results suggest that the conventional view is in error and that the low doping efficiency is primarily due to H passivation. These results are consistent with the low doping efficiency of B as well as NMR studies on the large electric field gradients experienced by the B atoms and on NMR double resonance studies of B-H neighboring distances.
The Problem of the Top Cell for the Micromorph Tandem
Authors: Platz Rainer , Meier J. , Fischer D. , Dubail S. , Shah A. 
Published: April 2011

ABSTRACTA detailed study of the influence of different materials for the amorphous top cell on the stabilized efficiency of amorphous silicon/microcrystalline silicon (”micromorph”) tandem cells is presented. The authors investigate different amorphous i-layer materials which are applied in cells with varying thicknesses. It is shown that it is preferable to optimize the top cell in order to obtain a high current rather than a high voltage.A simple optical and electrical model is presented which allows one to predict the optimum optical bandgap of the i-layer material of the top cell and to determine its optimum thickness for maximum stabilized efficiency. By means of this model it is shown that the optimum top cell for a total cell current of 26 mA/cm2 should contain an about 2500 Å thick i-layer with an optical gap Eo4 ≃ 1.8 eV. The model furthermore shows that it is desirable to obtain slightly bottom-limited conditions after degradation in order to maximize the output power.A stabilized efficiency of 10.7 % for a micromorph tandem cell has been confirmed by an independent measurement (FhG-ISE). Another such cell yields a stabilized efficiency of 11.2 % as measured in our laboratory.
The Light Induced Changes in A-Si: H Materials and Solar Cells—Where We are Now
Authors: Wronski C. R. 
Published: April 2011

ABSTRACTThe quest for understanding and especially controlling the reversible light induced changes in a-Si:H based materials has been ongoing for the last twenty years. This has been accompanied by a corresponding large effort in minimizing their effects on more efficient a-Si:H based solar cells. Despite the complexities in both the phenomena as well as the solar cells, progress has been made in both the scientific and technological arenas. This paper briefly reviews primarily studies on the characterization and reduction of the metastable changes in materials and the correlation of these changes with those in efficient solar cells. It will discuss the impact of studies on materials as well as the continuous advances made with “engineering” of solar cell structures on their improved stabilized performance.
Structural and Optical Properties of a-Si:H/μc-Si:H:B Junctions in the a-Si:H-Based n-i-P Solar Cell Configuration
Authors: Kohi Joohyun , Fujiwara H. , Wronski C. R. , Collins R. W. 
Published: April 2011

ABSTRACTWe have extended previous real time spectroscopie ellipsometry (RTSE) capabilities in order to investigate the effects of H2-plasma treatment of i-type hydrogenated amorphous silicon (a-Si:H) on the deposition of the overlying p-type microcrystalline silicon (μc-Si:H:B)) in the formation of an n-i-p solar cell structure. In this study, we compare in detail the nucleation and growth of p-layers by plasma-enhanced chemical vapor deposition (PECVD) from SiH4 highly diluted in H2 on the surfaces of untreated and H2-plasma treated a-Si:H i-layers. We find that for intended single-phase μc-Si:H:B p-layer PECVD under optimum conditions on an untreated i-layer surface, a wide gap (∼2.0 eV Taue gap) amorphous layer nucleates and grows in the first ∼150 Å. This layer develops uniformly to a bulk thickness of ∼150 Å, but gradually acquires a crystalline structure for thicknesses greater than the desired p-layer thickness (200 Å). In contrast, for p-layer PECVD under identical conditions on the H2-plasma treated i-layer, high-density crystalline nuclei form immediately. This conclusion is drawn on the basis of the unique optical properties of the bulk p-layer that develops on the surface of the H2-plasma treated i-layer. Specifically, an absorption onset near ∼2.5 eV is observed for a 48 Å fully-coalesced p-layer, as measured by RTSE at 200°C. For this μc-Si:H:B p-layer, the optical gap decreases by ∼0.15 eV with increasing thickness from 50 to 200 Å. This effect is attributed to a reduction in the quantum confinement energy with an increase in the average crystallite size in the film.
Internal Electric Field Profile in Thin Film Hydrogenated Amorphous Silicon Diodes Studied by the Transient-Null-Current Method
Authors: Han Daxing , Yeh Chenan , Wang Keda , Qiwang  
Published: April 2011

ABSTRACTWe demonstrate that the internal field of a thin a-Si:H pin solar cells can be measured using the transient-null-current method. This method was previously developed to measure the internal field profile in a-Si alloy Schottky barrier. The internal electric field profile was determined by measuring the forward-bias voltages that tune the transient photocurrents generated by a pulsed laser at a various wavelengths to zero. We adopt the same technique to a-Si:H p-i-n solar cells. In the case of p-i-n structure, we need to consider both space charge contributed by photogenerated carriers and carrier recombination which disturb the internal field. We use two critical conditions to minimize these effects. (1) To limit the contribution of photocarriers to space-charge distribution, the total charge collected is less than 10−10 C per pulse, and a repetition rate 1 Hz is used to ensure that the diode remains close to its equilibrium state. (2) The measuring time window is about 1 – 6 μs following the displacement current. Typically the RC constant of diode is < 1 μs and the rise time of the forward-bias recombination current is 6.0 × μs. We apply the signal average to process the forward-bias voltage. The error is within ± 0.05 V. With this technique we can study the effect of variety of structure design or processing on the device performance.
Conditions for Collection Efficiencies Greater than one Hundred Percent
Authors: Brüggemann R. , Zollondz J. H. , Main C. , Gao W. 
Published: April 2011

ABSTRACTAn account is given for the conditions under which the collection efficiency in hydrogen ated amorphous silicon pin-diodes increases to values larger than 100 %. By specific bias illumination through the p-side bias generated photocarriers are collected under certain probe beam conditions of the collection efficiency measurement, leading to apparent large collection efficiencies. By numerical modelling we investigated the influence of the diode thickness, bias photon flux and probe absorption coefficient as well as applied voltage for possible sensor applications which may utilise this optical amplifying principle. The alternative with bias light through the n-side and probe light through the p-side is also explored. Collection efficiency values determined by the photogating of bias generated holes become only slightly larger than 100 % in contrast to the electron case where values in excess of 3000 % are presented.
Solar Cells from Mixed SiCi2H2 and SiH4 Plasmas
Authors: Payne A. M. , Crone B. K. , Wagner S. 
Published: April 2011

ABSTRACTWe have deposited and analyzed solar cells from mixed SiCI2H2/SiH4 plasmas. Intrinsic and doped films were also grown to explain the cell results. The SiCI2H2 cells have lower fill factor and short circuit current density than the SiH4 cells due to the higher defect densities and lower photoconductivities of SiCI2H2/SiH4 material. Cell results showed low VQC if the n-layer was in contact with an i-layer deposited from SiCI2H2/SiH4. A pure silane buffer layer between the i- and n-layers was needed to attain the standard open circuit voltage. The reduced Voc results from a reduced conductivity of phosphorus-doped SiCI2H2/SiH4 material. However, comparison of doped films showed higher conductivities for boron-doped material grown from the SiCI2H2/SiH4 mixture than from pure silane.
Spin-Dependent Transport in Si Thin-Film Transistors
Authors: Kawachi G. , Graeff C F. O. , Brandt M. S. , Stutzmann M. 
Published: April 2011

ABSTRACTDefects and carrier transport processes in silicon based thin-film transistors (TFTs) are investigated by spin-dependent transport (SDT). The resonance signal arising from less than 106 defects in the hydrogenated amorphous silicon (a-Si:H) TFT is detected with a sufficient signal-to-noise ratio. The leakage current mechanism in a-Si:H under high source-drain fields is identified by SDT as electron hopping via defect states located at the interface between undoped a-Si:H and the passivation silicon nitride layer. At temperatures below 100K, spin-dependent hopping of electrons in conduction band tail states is observed. The change of the dominant transport path from extended states conduction to variable range hopping with decreasing temperature is confirmed. SDT measurements on polycrystalline silicon (poly-Si) TFTs having silicon nitride and silicon dioxide as the gate dielectric films reveal differences in the defect structure in these devices. The overall results demonstrate that SDT is a powerful method to probe paramagnetic defects and carrier transport in TFTs.