bluuebunny/crossref_metadata_2025 huggingface dataset:
Data Reduction for Radionuclide Transport Codes Used in Performance Assessments: An Example of Simplification Process
Authors: Dverstorp B. , Mendes B. , Pereira A. , Sundström B.
Published: April 2011

The input data required for transport models for use in long-term risk assessments of repositories for radioactive waste, in geological media, are intrinsic to the performance of the models. The flow parameters utilized in these models typically come from 2 or 3D hydro-geological calculations done in a prior phase of an assessment. This paper examines some of the standard simplifications introduced when hydrogeological data are reduced to ID as is often required for radionuclide transport models. Two key aspects of data reduction are the determination of average properties of fractured media between and along transport pathways. To quantify possible errors associated with these reduction procedures, two computer experiments have been done. We show that the use of effective flow parameters, representing the average properties of a set of independent transport pathways, in a 1 D radionuclide transport model can result in an underestimation of peak releases by one order of magnitude or more. This result is valid for short-lived nuclides whenever retardation is an important factor. On the other hand, averaging of spatially varying transport properties along a transport pathway may lead to unjustified conservatism. A simple calculation example using Monte Carlo technique, shows that a model that does not take into account spatially varying retardation properties along the transport pathways may overestimate peak release rates by several orders of magnitude. We conclude that more sophisticated transport models taking into account available hydrogeological information on spatial variability are needed to fully understand the potential errors associated with consequence calculations in the performance assessment
Novel and Practicable Approach to Modeling Radionuclide Transport Through Heterogenous Geological Media
Authors: Schneider J.W. , Zuidema P. , Smith P.A. , Gribi P. , Hugi M. , Niemeyer M.
Published: April 2011

This paper describes a novel approach to modelling radionuclide transport through a heterogeneous geological medium. It is assumed that a statistical description of the heterogeneity can be formulated. The approach then essentially consists of three steps: (i) sampling of a large number of potential transport pathways (15‘000 in this example), covering the ranges of properties; (ii) grouping of these pathways in a manner which reflects their importance for radionuclide transport into a small number of “legs” with appropriately averaged properties; (iii) modelling geosphere transport processes in these legs. It is demonstrated that the number of legs required to represent conservatively, but without excessive conservatism, all the transport pathways is small (5 in this example). This result is important for safety assessment, where many different parameter variations may need to be considered and where computational efficiency is often a vital issue.
Gas Threshold Pressure Tests in Deep Boreholes for Determining Two-Phase Flow Properties of the Host Rock at the Proposed L/ILW Repository, Switzerland
Authors: Senger Rainer , Marschall Paul , Lavanchy Jean-Marc
Published: April 2011

Safety assessment of potential gas release from the L/ILW requires information on two-phase flow behavior of the host rock. To determine the relevant formation properties for gas flow through the host rock, a combination of hydrotest and gas-injection sequences were designed and subsequently implemented in the Nagra field testing program at Wellenberg. The results of the design caculations indicate that the gas threshold pressure can be estimated from a gas injection test followed by a shut-in test using the log-log diagnostic plot of the gas injection sequence and the Homer plot of the recovery sequence. For a composite well-aquifer model, the gas threshold pressure of the outer zone, when estimated from the gas injection sequence, is increased, because water is displaced from the inner zone into the outer zone as gas is injected into the inner zone. This results in the overestimation the gas threshold pressure of the outer zone, and in turn, of the air-entry pressures in the corresponding capillary pressure models. The pressure response during the gas injection sequence in a composite well-aquifer model can be drastically different for different relative permeability models. For a Brooks-Corey model, the pressure indicates a distinct increase when gas flow starts into the outer zone. For a Grant model, the pressures can actually decrease during a constant-rate gas injection sequence. A decrease in interval pressure during gas injection was observed in two different field tests in the Palfris Formation at Wellenberg. In borehole WLB-SB4as, the gas injection test in interval VM16 was analyzed with the inverse code ITOUGH2 to determine the two-phase flow parameters for two different conceptual models. The simulations indicated that the particular pressure response, characterized by a sudden decrease during gas injection, could not be reproduced with a Brooks-Corey relative permeability model. However, assuming a Grant model, a sudden pressure decrease during the gas injection sequence could be simulated. The results indicate the importance in determining the hydraulic properties and the well-aquifer model from a combined analysis of hydro- and gas injection tests, in order to accurately determine the two-phase flow parameters.
The “FEBEX” Project: A Demonstration and Testbench of the Spanish Concept for Crystalline Rock Repositories
Authors: Fuentes-Cantillana J.L. , Huertas F. , Santiago J.L.
Published: April 2011

A full-scale “In situ” test is currently being conducted as part of the FEBEX project. This test is being carried out at the Grimsel Test Site in Switzerland and reproduces the “AGP-Granite” concept of the Spanish agency ENRESA for HLW repositories. Two heaters, of the same dimensions and weight as the canisters in the reference concept, have been placed in a horizontal drift of 2,28 m diameter and backfilled with a total of 115.7 t of highly compacted bentonite blocks, up to a total test length of 17 m. More than 600 sensors have been installed in the experiment, which runs basically in an automatic mode and is being remotely controlled and monitored from Spain. This paper addresses the engineering, installation and instrumentation aspects of this test. In addition, some preliminary results are shown
Synthesis of Actinide-Doped Zirconia by Plasma Calcination
Authors: Burakov B.E. , Helean K.B. , Korolev V.A. , Ewing R.C. , Anderson E.B. , Shpunt L.B. , Strykanova E.E.
Published: April 2011

Zirconia containing actinides in solid solution, (Zr, An)Ox, where An = U, Pu, Am, Np, Cm is considered as a starting material for the synthesis of two durable crystalline host-phases for actinide immobilization: zircon (Zr, An)SiO4 and cubic zirconia (Zr,An)O2. The use of this starting material is necessary in order to provide for total actinide incorporation into the structures of the final phases, zircon and cubic zirconia, while avoiding the presence of unincorporated actinides. Difficulties arise when the chemically inert pure oxides, ZrO2 and AnOx are used as precursor materials. Melting techniques, which are commonly used to provide solid solution of inert oxides are not acceptable for ceramic preparation, particularly the synthesis of ceramic Pufuels. This paper presents a new approach utilizing plasma calcination. Solids containing both zirconium and actinides are calcined in a plasma. This allows the use of intermediate, stabilized actinide forms, for example, co-precipitated actinide-zirconium oxalates which have a low solubility level or a solid sol-gel precursor, such as (Zr, An)SiOx.

Calcination was completed using an induction plasma in an argon gas mredium. Two types of material were used: co-precipitated oxalates of Zr and Ce with 10 wt.% Ce and a solidified sol-gel (Zr, Ce)SiOx with 10 wt.% Ce. Weakly milled material with a particle size of 0.34 mm was dropped through the plasma at a speed of 40-60 grams per hour. The final product was examined by high resolution transmission electron microscopy (HRTEM). The calcined oxalate material consisted of aggregates of globular particles (tens to hundreds of nm) of monoclinic and tetragonal (Zr,Ce)O2. The calcined sol-gel consisted of globular particles (50 to 100s nm) of monoclinic and tetragonal (or pseudo-cubic) (Zr, Ce)O2 in an amorphous silica matrix. This paper discusses the application of plasma calcination to the immobilization of excess weapons Pu and other actinides. The plasma calcination of solid materials would be used for ceramic Pu-fuel fabrication, as well as for the final disposal in geological formations.
Aspects Concerning Retrievability in the Planned Swiss Low- and Intermediate-Level Waste Repository
Authors: Nold A. L. , Fritschi M. , Kowalski E.
Published: April 2011

Swiss legislation requires the safe final disposal of radioactive wastes. Final disposal is defined by the safety authority guidelines as follows: ‘After a repository has been sealed, no further measures shall be necessary to ensure safety. The repository must be designed in such a way that it can be sealed within a few years.’ The time for repository sealing is not specified.

In Switzerland, the Wellenberg site was selected for the LIILW repository after an extensive site evaluation process and, in June 1994, the application for the general licence was submitted to the Swiss Federal Government. In addition to the federal licence, a mining concession has to be granted by the canton in a public referendum. The decisive cantonal vote in June 95 was negative (52%) and led to a temporary set-back in the project.

Among the reasons for this negative result was the proposed repository emplacement concept. It was planned to successively backfill the voids between the emplaced disposal containers in the caverns layer by layer. Retrieval of the waste would therefore involve high expenditure and effort. The concept therefore implies closing part of the repository at a very early stage. The outcome of the referendum reflected an unwillingness on the part of the population either to accept this situation or to make a decision today on final disposal. In many discussions, a stepwise implementation of the repository was requested, not only from the point of view of technical feasibility but also to allow political and legal aspects to be taken into consideration. A final decision - particularly regarding closure of the repository - could then be taken in the future, after giving the coming generation an opportunity to think over our proposed solution.

To meet this requirement, GNW/Nagra studied modified repository emplacement concepts within the framework of Swiss legislation. In the proposed new project, concrete containers are emplaced in the caverns without any backfill, allowing easy retrieval until the decision is taken to close the facility. This decision may be taken only after completion of container emplacement, allowing for further monitoring of the filled repository caverns. A decision on how to proceed is currently being prepared by a federal working group with representatives from the canton, the safety authorities and GNW/Nagra.

Studies have shown that, with an appropriate design, drainage and ventilation system, the required stability of the modified repository cavern can be achieved also under seismic loading and that the requirements of operational safety can be met. The potential long opening times of the repository caverns show some minor effects on certain repository components, such as e.g. slight carbonatisation of the container backfill. Overall, it can nevertheless be demonstrated that the long-term safety is not adversely affected.
Process Model of Gas-Related Phenomena Associated With a Proposed L/ILW Repository in Switzerland
Authors: Senger Rainer , Gribi Peter , Hugi Markus , Zuidema Piet
Published: April 2011

NAGRA (National Cooperative for the Disposal of Radioactive Waste) has been investigating the feasibility for siting a low- and intermediate-level radioactive waste (L/ILW) repository in subsurface geologic environments. The design of the L/ILW repository consists of a horizontally accessed cavern system located beneath Wellenberg in the low-permeability unit of Valanginian marl. One of the concerns for the safety analysis is the effect of gas generation (primarily hydrogen) from anaerobic corrosion and degradation of waste material. The development of a free gas phase and the concomitant pressure buildup can result in the displacement of contaminated pore waters from the repository. Moreover, excess buildup of pressures in the repository can impact the structural integrity of the engineered barrier and of the surrounding host rock. For the investigation of the gas-related phenomena in a proposed L/ILW repository, a process model was developed to simulate the different performance periods of the repository which include: (1) construction/operation period of the repository resulting in desaturation of the host rock caused by pressure decline and ventilation in the cavern, and (2) post-closure period after backfilling and sealing of the caverns resulting in resaturation of the host rock and of the cavern, and in the release of waste-generated gas. Period (1) takes into account degassing of dissolved gas (methane) in the formation water due to the pressure decline around the cavern, and period (2) considers dissolution of free gas caused by the pressure increase associated with the resaturation and with the gas production in the cavern.
H Out-Diffusion and Device Performance in n-i-p Solar Cells Utilizing High Temperature Hot Wire a-Si:H i-Layers
Authors: Mahan A. H. , Reedy R. C. , Iwaniczko E. , Wang Q. , Nelson B. P. , Xu Y. , Gallagher A. C. , Branz H. M. , Crandall R. S. , Yang J. , Guha S.
Published: April 2011

Hydrogen out-diffusion from the n/i interface region plays a major role in controlling the fill factor (FF) and resultant efficiency of n-i-p a-Si:H devices, with the i-layer deposited at high substrate temperatures by the hot wire technique. Modeling calculations show that a thin, highly defective layer at this interface, perhaps caused by significant H out-diffusion and incomplete lattice reconstruction, results in sharply lower device FF's due to the large voltage dropped across this defective layer. One approach to this problem is to introduce trace dopant tailing to ‘compensate’ these defects, but the resultant cells exhibit a poor red response. A second approach involves the addition of buffer layers designed to retard this out-diffusion. We find that an increased H content, either in the n-layer or a thin intrinsic low temperature buffer layer, does not significantly retard this out-diffusion, as observed by secondary ion mass spectrometry (SIMS) H profiles on devices. All these devices have a defect-rich i-layer region near the n/i interface and a poor device efficiency. However, if this low temperature buffer layer is thick enough, the outdiffusion is minimized, yielding nearly flat H profiles and a much improved device performance. We discuss this behavior in the context of the H chemical potentials and H diffusion coefficients in the high temperature, buffer, n-, and stainless steel (SS) substrate layers. The chemical potential differences between the layers control the direction of the H flow and the respective diffusion coefficients, which depend upon many factors such as the local electronic Fermi energy and the extent of the H depletion, determine the rate. Finally, we report a 9.8% initial active area device, fabricated at 16Å/s, using the insights obtained in this study.
Material Aspects of A-Sin:H Based 2D Direct X-Ray Sensor Array
Authors: Popov I. , Doorselaer G. Van , Calster A. Van , Smet H. De , Boesman E. , Callens F.
Published: April 2011

The possibility of creating an 2D X-ray sensor array on the base of a-SiN:H thin films without switching elements and an X-ray conversion layer is presented in this report. The behavior of a-SiN:H Thin-Film Diode under X-ray irradiation, its limitations, and ways of increasing long-term stability and sensitivity of the sensor are discussed.
Process Integration of A-Si:H Schottky Diode and thin Film Transistor for Low-Energy X-Ray Imaging Applications
Authors: Park B. , Murthy R.V.R. , Sazonov A. , Nathan A. , Chamberlain S.G.
Published: April 2011

In this paper, we discuss the various design and fabrication issues related to the process integration of a molybdenum/amorphous silicon (Mo/a-Si:H) Schottky diode and a thin film transistor (TFT), for realization of an X-ray pixel intended for large area and low energy imaging applications. Here, the Schottky diode serves as the sensor and the TFT as a switching element for charge readout. Different pixel configurations are fabricated and compared taking into consideration design aspects such as leakage current, process sequence in terms of mask count, fill factor, and mechanical integrity of various layers. Preliminary X-ray response measurements, over the range (40 – 115) kVp, yield a pixel sensitivity of (1–10) million electrons for a readout sampling period of 16·7 ms.
Carbon Rich a-Si1-xCx:H Films: An Investigation On Radiative Recombination Properties
Authors: Giorgis F. , Giuliani F. , Pirri C.F. , Mandracci P. , Rava P. , Reitano R. , Calcagno L. , Musumeci P.
Published: April 2011

Amorphous silicon-carbon a-Sil-xCx:H films with x in the range 0.3-1 have been deposited by PECVD of SiH4+CH4 and SiH4+C2H2 gas mixtures. Photoluminescence characterizations have been performed, together with optical measurements. The dependence of radiative recombination properties as a function of x and as a function of damage introduced by H+-ion irradiation has been presented and correlated with the changes in the absorption spectra.
Photo and Electroluminescence of a-Si:Er:H
Authors: Tessler Leandro R. , Iñiguez Ana Carola
Published: April 2011

Trivalent erbium (Er3+) presents a characteristic intra 4f optical transition 4I13/2 → 4I15/2 at 1.54 μm when incorporated in several solid hosts. Hydrogenated amorphous silicon (a-Si:H) is a good candidate as a host for applications in optical communications and photonic integration. We have studied Er3+ photo and electroluminescence in a-Si:H prepared by co-sputtering from a silicon target partially covered with metallic erbium chunks. Since the presence of oxygen impurities enhances the luminescence intensity, we studied the influence of oxygen added to the sputtering gas on the material properties. We found that oxygen reduces the erbium incorporation into the films. We obtained samples presenting 1.54 μm photoluminescence as deposited for a wide range of erbium concentrations. Maximum room-temperature photoluminescence efficiency is obtained for samples that contain ∼ 1% [O]/[Si] concentrations. The temperature quenching is small, mainly due to the temperature dependence of the luminescence lifetime.

Room temperature electroluminescence at 1.54 νm was observed in reverse biased Si/a-Si:Er:O:H/Al structures.
Annealing Effect on the Activation of 1.54 [νm Emission from Erbium in a-Si:H Matrix Prepared by DC Magnetron Sputtering
Authors: Andreev A.A. , Golubev V.G. , Medvedev A.V. , Pevtsov A.B. , Voronkov V.B.
Published: April 2011

Cumulative thermal annealing (TA) changes the photoluminescence (PL) intensity in erbium-doped a-Si:H films prepared using DC magnetron sputtering of a composite Er-Si target at substrate temperature 200°C. The intensity of erbium-related 1.54 νm PL at 77 K is enhanced about 50 times after TA at 300°C for 15 min in nitrogen atmosphere. No erbium-related PL is observed after TA at T≤500°C. The TA process is discussed in terms of a model of partial structural rearrangement in an a-Si(Er):H amorphous network.
Design of Atcd three Color Detector for Color Detection in Transient Regime
Authors: Irrera F. , Palma F. , Lemmi F. , Diotallevi M.
Published: April 2011

In this paper we present the characterization of an ATCD three color detector used in the charge integration regime, as necessary in large area matrices. In particular, we present linearity measurements of mesa insulated devices and characterization of the self-bias process occurring in the transient read-out of stacked structures. A new system architecture is introduced which considers interlaced row charge restore, and separates the charge restore process from the charge sampling process in order to reach self-bias within a short frame time.
Image Sensors in Tfa Technology - Status and Future Trends
Authors: Böhm M. , Blecher F. , Eckhardt A. , Seibel K. , Schneider B. , Sterzel J. , Benthien S. , Keller H. , Lulé T. , Rieve P. , Sommer M. , Uffel B. Van , Librecht F. , Lind R. C. , Humm L. , Efron U. , Roth E.
Published: April 2011

Image sensors in TFA (Thin Film on ASIC) technology have been successfully fabricated and tested. This paper provides a survey of TFA research results so far and outlines future perspectives. The properties of different a-Si:H b/w and color thin film detectors are evaluated, including spectral sensitivity, dark current, temperature influence and transient behavior. Furthermore several TFA prototypes and emerging concepts are presented, ranging from a simple one-transistor cell design to a locally autoadaptive sensor.
A Polysilicon Tft Parameter Extractor
Authors: Lui O.K.B. , Tam S.W.B. , Migliorato P.
Published: April 2011

The achievement of large size displays and the capability of high throughput low temperature laser re-crystallised polysilicon TFTs to provide complex circuitry has opened up new market opportunities, such as the “system on panel” architecture, where the circuitry necessary for the operation of an interactive portable computer is to be fabricated on the display substrate. In the development of polysilicon technology for these applications, device physics and modelling are expected to play a major role as they have done in the case of single crystal silicon. However, polysilicon TFTs present a much higher level of complexity than single crystal devices, due to the presence of grain boundaries and associated defects which drastically affect the electrical properties. For the first time, a polysilicon TFT parameter extractor has been developed based on data from capacitance (C-V) and conductance (I-V) measurements. The method yields the flat band voltage, the front interface state density and the bulk density-of-states (DOS) for the entire range. The method is applied to laser re-crystallised polysilicon TFTs and verified via comparison with 2-D simulation. It is found that this technique allows a much higher degree of accuracy than other methods published so far.
High Conductivity Gate Metallurgy for Tft/Lcd's
Authors: Fryer Peter M. , Colgan E. , Galligan E. , Graham W. , Horton R. , Jenkins L. , John R. , Kuo Y. , Latzko K. , Libsch F. , Lien A. , Nywening R. , Polastre R. , Rothwell M. E. , Wilson J. , Wisnieff R. , Wright S.
Published: April 2011

Amorphous silicon based thin film transistor liquid crystal displays (TFT/LCD) have become the dominant technology used for flat panel displays for notebook computers. The need for higher resolution, larger diagonal displays for both notebook and desktop applications is discussed. Calculations have shown that the use of high conductivity gate metal such as aluminum or copper, together with the implementation of improved groundrules, can significantly extend today's technology. Aluminum suffers from problems with hillock formation during PECVD processing, and copper typically has poor adhesion to glass, reaction problems with silicon and other PECVD films, and difficulties in contacting it to other metals. Approaches to solving problems with both materials are presented, and a novel reduced mask process to fabricate high resolution, high aperture ratio 10.5” SXGA (1280×1024) displays is described. The process uses copper gate metallurgy with redundancy, without the need for extra processing steps. The resulting displays have 150 dpi color resolution, an aperture ratio of over 35%, and excellent image quality, making them the first high resolution displays that are suitable for notebook applications.
Computer Simulation for Solar Cell Applications: Understanding and Design
Authors: Zhu Hong , Fonash Stephen J
Published: April 2011

Computer device transport simulation can be a very powerful tool for device physics understanding, design, and optimization. This is especially true when it is done with a PC version that can be easily run on the desktop. However, like any powerful tool, computer simulation must be used with care and appreciation. Using our newest version of our AMPS (Analysis of Microelectronic and Photonic Structures) [1,2] transport physics code, AMPS for Windows 95/NT, we discuss using solar cell simulations for increased understanding, for materials evaluation, and for device design and optimization.
Defect Distributions in Doped and Undoped A-SiGe:H Alloys
Authors: Stiebig Helmut , Siebke Frank , Carius Reinhard , Klomfaβ Josef
Published: April 2011

In this work, gap states in doped and undoped a-SiGe:H alloys are examined by numerical simulations of sub-bandgap absorption spectra measured by the constant photocurrent method and photothermal deflection spectroscopy. Deconvolution methods, neglecting the condition of charge neutrality, can be used for a rough estimate of the defect density value but not for ob- taining detailed information on the distribution of gap states in undoped samples. Our numerical analysis uses adapted occupation statistics and takes into account the condition of charge neutrality. Good agreement between measured and simulated PDS and CPM spectra is obtained. For a certain composition, i.e. a certain bandgap, the investigation of doped films yields infor- mation on the density and the position of charged defect states in the bandgap. In addition, the density of neutral defect states can be derived from a comparison of CPM and PDS spectra. The results reveal the coexistence of charged and neutral defects. In doped as well as in undoped films, charged defect states dominate the defect density. In the investigated range of compo- sitions the defect distribution of a-SiGe:H is similar to those found in a-Si:H. The width of the defect distributions does not decrease with decreasing bandgap. No evidence for a different be- havior of Si- and Ge-related defect states can be found in sub-bandgap absorption spectra.
Device Modeling of a-Si:H Alloy Solar Cells: Calibration Procedure for Determination of Model Input Parameters
Authors: Zeman M. , Van Swaaij R.A.C.M.M. , Schroten E. , Vosteen L.L.A. , Metselaar J.W.
Published: April 2011

A calibration procedure for determining the model input parameters of standard a-Si:H layers, which comprise a single junction a-Si:H solar cell, is presented. The calibration procedure consists of: i) deposition of the separate layers, ii) measurement of the material properties, iii) fitting the model parameters to match the measured properties, iv) simulation of test devices and comparison with experimental results. The inverse modeling procedure was used to extract values of the most influential model parameters by fitting the simulated material properties to the measured ones. In case of doped layers the extracted values of the characteristic energies of exponentially decaying tail states are much higher than the values reported in literature. Using the extracted values of model parameters a good agreement between the measured and calculated characteristics of a reference solar cell was reached. The presented procedure could not solve directly an important issue concerning a value of the mobility gap in a-Si:H alloys.
The Effect of Ion-Bombardment on the Formation of Voids During Deposition of a-Ge:H
Authors: Origo F. , Hammer P. , Comedi D. , Chambouleyron I.
Published: April 2011

The role of substrate ion bombardment on the structural and H bonding properties of hydrogenated amorphous germanium (a-Ge:H) films was studied by infrared (ir) spectroscopy. A Kaufman type ion source was used to produce an Ar1 beam directed towards a Ge target for a- Ge:H ion beam sputtering deposition in a H2-containing vacuum chamber. A low energy (100 eV) H2++Ar+ beam obtained from an additional ion source was allowed to impinge directly on the substrate during film growth at various beam currents.

It was found that substrate bombardment by 100 eV ions favors the formation of voids, as deduced from the increasing contribution of the surface-like Ge-H stretching mode to the ir spectrum with increasing ion current. The void density was reduced below the ir detection limit by totally removing the ion beam on the substrate while keeping all other parameters fixed. For this condition, we observe no or very small surface-like contributions to the ir spectra, irrespective of substrate temperature (25-260°C) or growth rate used. A narrowing of the infrared Ge-H stretching mode peak is observed with increasing deposition temperature, indicating a concomitant tendency towards a more ordered structure.
Hydrogen in a-Si:H Deposited by an Expanding Thermal Plasma: A Temperature, Growth Rate and Isotope Study
Authors: Kessels W.M.M. , Sanden M.C.M. Van De , Severens R.J. , Ijzendoorn L.J. Van , Schram D.C.
Published: April 2011

The hydrogen and silicon density of a-Si:H-films deposited by an expanding thermal plasma have been investigated for a wide range of substrate temperatures and growth rates by infrared absorption spectroscopy in combination with elastic recoil detection and Rutherford backscattering. The study reveals that, despite the increasing atomic hydrogen interaction and high substrate temperatures, the a-Si:H remains purely amorphous at low growth rates as concluded from Raman spectroscopy. Additionally, the infrared spectroscopy proportionality constants of the silicon-hydrogen and silicon-deuterium bondings have been recalibrated.
Large Area Deposition of Amorphous and Microcrystalline Silicon by Very High Frequency Plasma
Authors: Sansonnens L. , Howling A. A. , Hollenstein Ch.
Published: April 2011

Two aspects of VHF plasma deposition in a large area reactor are investigated: 1) Experiments and model show that voltage inhomogeneities become serious at high frequency but can be reduced by suitable RF connection configuration. These voltage inhomogeneities are the main factor limiting the excitation frequency in order to satisfy film thickness uniformity requirements; 2) the effect of the frequency on the plasma properties has been studied between 13.56 MiHz and 70 MHz. The results show that an increase of the electron density with the frequency leads to a more efficient dissociation of silane. This increase of the gas phase reactivity of the plasma is largely responsible for the higher deposition rates observed athigh frequency.

The choice of the excitation frequency for a given application is a compromise between the gain of higher deposition rate and the frequency limit imposed by homogeneity requirements.
Dust Particle Diagnostics in Rf Plasma Deposition of Silicon and Silicon Oxide Films (Invited)
Authors: Hollenstein Ch. , Howling A. A. , Courteille C. , Dorier J.-L. , Sansonnens L. , Magni D. , Müller H.
Published: April 2011

Particle contamination formed in reactive plasmas imposes an upper limit on the rate for particle-free deposition. Conversely, these plasmas could be exploited to produce nanometric clusters and particles for various applications. Infrared absorption spectroscopy has been applied to analyse the chemical composition of suspended particles. Mass spectrometry was also used to investigate cluster formation in these deposition plasmas. In pure silane plasmas, a random model reproduces the measured mass spectra, whereas the rich plasma chemistry in silane/oxygen mixtures shows a remarkable tendency to produce silasesquioxane anions.
Metastability of Phosphorus- or Boron Doped a-Si:H Films
Authors: St'ahel P. , Cabarrocas P. Roca i , Sladek P. , Theye M.L.
Published: April 2011

The effects of light-soaking on either phosphorus- or boron-doped a-Si:H films were studied as functions of the doping level and the temperature. In the case of boron-doped films, the most important effect is the improvement of the conductivity during light-soaking, which is related to the activation of boron. On the contrary, phosphorus-doped films present a remarkable stability, although lightly phosphorus-doped ones show a decrease of their conductivity by five orders of magnitude when light-soaking is performed below 40 °C. This effect is attributed to the formation of P-H complexes which are stable at low temperature only. Our results suggest that in both types of doped a-Si:H films the creation of metastable defects is a second order effect with respect to the activation or passivation of dopants, which results from their interaction with hydrogen.
The Role of Charged Defects In Current Transport Through Hydrogenated Amorphous Silicon Alloys
Authors: Lau S.P. , Shannon J.M. , Sealy B.J. , Marshall J.M.
Published: April 2011

Current transport in metal-semiconductor-metal structures based on amorphous silicon alloys has been studied in relation to the density of dangling bond state defects. The density of defects was changed by varying alloy composition or by current stressing. We show that the change of current-voltage characteristics and activation energy with defect density and the onset of Poole-Frenkel conduction with composition require charged defects. It is found that there are more charged defects in amorphous silicon nitride (a-Si1−xNx:H) than in amorphous silicon carbide (a-Si1−xCx:H). In addition, an excess of negatively charged dangling bond defects compared to positively charged dangling bond defects is observed in a-Si1−xNx:H films. This is attributed to the presence of N4+ act as the donor states in silicon nitride. We find that the density of charged dangling bond defects can be higher than 1019cm−3.
Light-Induced Change of Si-H Bond Absorption in Hydrogenated Amorphous Silicon
Authors: Yue Guozhen , Chen Liangfan , Wang Qi , Iwaniczko Eugene , Kong Guanglin , Baugh Jonathan , Wu Yue , Han Daxing
Published: April 2011

Device-quality a-Si:H films were prepared by glow discharge CVD with pure or H-diluted silane as well as by hot-wire CVD. The hydrogen content was varied from ∼2 to 15 at. %. The Si-H bond absorption and its light-soaking-induced changes were studied by IR and differential IR absorption spectroscopes. The results indicate that the more stable sample exhibits an increase of the absorption at wave number ∼2000 cm−1, and the less stable one exhibits a decrease at ∼2040 cm−1and an increase at ∼1880 cm−1.
Hydrogen Collision Model of The Staebler-Wronski Effect: Microscopics and Kinetics
Authors: Branz Howard M.
Published: April 2011

A new microscopic and kinetic model of light-induced metastability in hydrogenated amorphous silicon (a-Si:H) is described. Recombination and trapping of photoinduced carriers excite hydrogen from deep Si-H bonds into a mobile configuration, leaving a dangling bond (DB) defect at the site of excitation. Normally, mobile H are recaptured at DB defects and no metastability or net DB production results. However, when two mobile H collide, they form a metastable two-hydrogen complex and leave two spatially-uncorrelated Staebler-Wronski DBs. Thermal and light-induced annealing occur when mobile H are excited from the metastable two-H complex; they diffuse and are recaptured to DBs. The microscopic model is entirely compatible with electron-spin-resonance results showing neither DB-DB nor DB-H spatial correlation of the light-induced DBs. The model leads to new differential equations describing the evolution of the mobile H and DB densities. These equation equations explain the observed room-temperature Ndb∼G2/3t1/3 dependence of DB creation upon the electron-hole pair creation rate (G) and time. The model also accounts for both t1/3-kinetics at 4.2K and t1/2-kinetics under laser-pulse soaking. Neither of these results can be explained within the prevailing electron-hole pair recombination model.
Pulsed Esr Studies on Microcrystalline Silicon
Authors: Malten C. , Finger F. , Müller J. , Yamasaki S.
Published: April 2011

Two-pulse-echo field-sweep spectra of boron-doped samples show in addition to dangling bonds (DB) a very broad feature of approximately 500 G width. This feature is the sum of several lines whose relative intensities change with doping and temperature. The total intensity increases with doping. The spin-lattice relaxation time TI of the resonance at g=1.998 referred to as conduction electrons (CE) has been studied for undoped and P-doped microcrystalline silicon. The discrepancy between T1 values previously reported has been resolved and the values for CE are at least two orders of magnitude smaller than those of the DB. In undoped samples cross-relaxation between the CE and DB spin systems might explain the similar T1 values obtained for CE and DB in inversion recovery measurements.
The Electronic Structure, Metastability and Transport Properties of Optimized Amorphous Silicon-Germanium Alloys
Authors: Chen Chih-Chiang , Lubianiker Yoram , Cohen J. David , Yang Jeffrey C. , Guha Subhendu , Wickboldt Paul , Paul William
Published: April 2011

We have carried out a comprehensive study of the electronic properties of two series of optimized a-Si1−xGex:H alloys fabricated at United Solar Systems Corporation (“Uni-Solar”) and Harvard University, encompassing the composition range 0.2 ≤ × ≤ 1.0. Both series of samples exhibit deep defect densities that obey quite accurately the spontaneous bond-breaking model proposed by M. Stutzmann which, by considering how the defect formation energy varies with the position of Fermi energy, we have been able to extend to doped samples.

We have also extended our studies to include measurements of ambipolar diffusion lengths and the effects of light-induced degradation, and thus have been able to demonstrate a direct relation between these transport properties and the measured defect levels before and after degradation.
Deep Defect Relaxation in Hydrogenated Amorphous Silicon: New Experimental Evidence and Implications
Authors: Cohen J. David , Zhong Fan , Kwon Daewon , Chen C.-C.
Published: April 2011

We review modulated photocurrent experiments which indicate that thermal emission rate for Do defects in intrinsic samples varies in response to changes in the Fermi-level or quasi-Fermi position. This apparent shift in energy threshold is confirmed using time resolved sub-band-gap spectroscopy. We also demonstrate that such a variation of emission rate with changes in the Fermi-level position, if present within the depletion region near a barrier junction, is consistent with the details of the temperature dependence of the junction capacitance in intrinsic samples.
Possible Origin of Large Response Times and Ambipolar Diffusion Lengths in Hot-Wire-Cvd Silicon Films
Authors: Schwarz R. , Múrias T. , Conde J.P. , Brogueira P. , Chu V.
Published: April 2011

We measured the response time zR and the ambipolar diffusion length Lamb in amorphous (a-Si:H) and microcrystalline silicon films (μ-Si:H) prepared by hot-wire chemical vapor deposition (HW-CVD). The response times in the amorphous and microcrystalline HW films were larger by factors of 100 and 1000, respectively, than in standard PE-CVD a-Si:H films (1-2 μs). The ambipolar diffusion length of the HW-CVD films was about twice as large as in conventional glow-discharge films. Strong doping of microcrystalline HW films with trimethylboron (TMB) led to a reduction of the response time. The results hint to a positive effect of the compact microstructure of HW-CVD films. We suggest the dark conductivity activation energy, Eact, and response time, τR, to be used as suitable parameters to describe optoelectronic film properties.
Influence of the H2 Dilution And Filament Temperature on the Properties of P Doped Silicon Carbide Thin Films Produced by Hot-Wire Technique
Authors: Ferreira I. , Águas H. , Mendes L. , Fernandes F. , Fortunato E. , Martins R.
Published: April 2011

This work deals with the role of hydrogen dilution and filament temperature on the morphology, structure and electrical properties of nanocrystalline boron doped silicon carbide thin films produced by hot-wire technique. The structural and morphological data obtained by XRD, SEM and micro-Raman show that for filament temperatures and hydrogen dilutions above 2100°C and 90%, respectively, the surface morphology of the films is granular with a needle shape, while for lower filament temperatures and hydrogen dilutions the surface morphology gets honeycomb like. The SIMS analysis reveals that films produced with filament temperatures of about 2200°C and hydrogen dilution of 99% present a higher hydrogen and carbon incorporation than the films produced at lower temperatures and hydrogen dilutions. These results agree with the electrical and optical characteristics recorded that show that the films produced exhibit optical gaps in the range from 1.8 to 2 eV and transverse conductivities ranging from 10−1S/cm to 10−3 S/cm, consistent with the degree of films crystallinity and carbon incorporation recorded.
Deposition, Defect and Weak Bond Formation Processes in a-Si:H
Authors: Robertson J , Powell M J
Published: April 2011

The growth of a-Si:H and the resulting weak bond and defect formation mechanism is analysed in terms of the adsorbed Sill3 model of growth. It is found that this model describes the surface processes well, but it needs further development to correctly describe the temperature dependence of the formation of defects and weak bonds, since the surface defect density decreases monotonically with temperature and does not show a minimum near 250C. We show that the experimentally observed increase in hydrogen content, weak bond and defect density at lower deposition temperatures can be accounted for by a hydrogen evolution reaction from H2* sites.
Microcrystalline Silicon n-i-p Solar Cells Deposited Entirely by the Hot-Wire Chemical Vapor Deposition Technique
Authors: Wang Qi , Iwaniczko Eugene , Mahan A. H. , Williamson D. L.
Published: April 2011

We describe a series of microcrystalline (μc) silicon n-i-p solar cell devices fabricated entirely by the hot-wire chemical vapor deposition technique. These devices are deposited on flat stainless-steel at a substrate temperature below 250°C, and are evaluated using solar-cell performance and quantum-efficiency (QE) measurements. We explore the effect of crystallite size, as examined by X-ray diffraction, by varying the hydrogen-to-silane ratio from 5 to 40, while keeping the μc-n and the μc-p layers the same. We find a significant blue shift of the QE peak and an enhancement of red response compared with a standard a-Si:H solar cell. The blue shift increases with increasing hydrogen-to-silane ratio. We attribute this shift to the i-layer becoming more n-type with increasing hydrogen dilution. We also use a hydrogen gas purifier and find a large improvement in device performance.
Effect of Filament Bias on the Properties of Amorphous and Nanocrystalline Silicon from Hot-Wire Chemical Vapor Deposition
Authors: Wanka H.N. , Brüggemann R. , Köhler C. , Zrinscak I. , Schubert M.B.
Published: April 2011

At the high temperatures during hot-wire assisted chemical vapor deposition, ther- mal emission of electrons from the filament occurs. We studied the effect of filament bias, and thus the filament-to-substrate current, on the structural, electronic and optical properties of amorphous and nanocrystalline silicon deposited by this method. The current drawn by the substrate can be varied by many orders of magnitude as thermally emitted electrons are increasingly collected with applied bias voltage. The crystallinity of the nanocrystalline samples is not affected by the bias voltage. The defect density in amorphous silicon is affected by the electron bombardment at high bias voltage only, for which we also find a significant reduction in the mobility-lifetime product from steady-state photoconductivity.
Direct Nucleation and Selective Growth of Nuclei for High Crystallinity Poly-Sige Thin Films on Sio2 Substrates
Authors: Hanna Jun-ichi , Shiota Kunihiro , Yamamoto Masaji
Published: April 2011

We have proposed a new technique for high crystallinity polycrystalline SiGe thin films with homogeneous crystallinity. The key technologies, i.e., direct nuclei formation on the substrate surface and selective growth of the resulting nuclei to gains, were established irrespective of film composition in a reactive thermal CVD with GeF4 and Si2H6. High crystallinity poly-Si0.05Ge0.95 thin films could be obtained on SiO2 substrates at 375°C and poly-Si0.95Ge0.05 thin films at 450°C by simply tuning the growth pressure for selective growth of the nuclei after creating the nuclei on the substrates. We demonstrated that not only the crystallinity but also the electrical properties of the resulting films were improved very much compared with those of continuously grown films.
High Quality Hydrogenated Amorphous Silicon Films with Significantly Improved Stability
Authors: Sheng Shuran , Liao Xianbo , Ma Zhixun , Yue Guozhen , Wang Yongqian , Kong Guanglin
Published: April 2011

High quality hydrogenated amorphous silicon (a-Si:H) films have been prepared by a simple “uninterrupted growth/annealing” plasma enhanced chemical vapor deposition (PECVD) technique, combined with a subtle boron-compensated doping. These a-Si:H films possess a high photosensitivity over 106, and exhibit no degradation in photoconductivity and a low light-induced defect density after prolonged illumination. The central idea is to control the growth conditions adjacent to the critical point of phase transition from amorphous to crystalline state, and yet to locate the Fermi level close to the midgap. Our results show that the improved stability and photosensitivity of a-Si:H films prepared by this method can be mainly attributed to the formation of a more robust network structure and reduction in the precursors density of light-induced metastable defects.
Reflective Infrared Spectra of PECVD Thin Films on Glass Substrate
Authors: He Shusheng
Published: April 2011

In order to use a reflective FTIR measurement to evaluate thin films deposited on glass substrate, we have to consider several important factors, such as surface polarization, phase shift, and interference from different interfaces. PECVD thin films, oxide, nitride and a-Si:H deposited on top of the glass substrate are discussed by the reflective FTIR measurement.
Substrate Surface Morphology and Growth Evolution of Low Temperature Silicon Nitride on Transparent Plastics
Authors: Smith L. L. , Arthur C. B. , Yang C. S. , Parsons G. N.
Published: April 2011

Surface morphology and nucleation are important for forming electronic thin films on plastic substrates. We have used atomic force microscopy (AFM) with an oscillating cantilever to examine various transparent substrates, including 7059 glass, polyethylene terephthalate (PET), indium tin oxide (ITO)-coated PET, and polycarbonate, and studied the effect of deposition processes on these surfaces. Studies include: 1) differences in dielectric nucleation on plastics and glass; 2) dielectric growth evolution; and 3) the effect of plasma exposure on surface roughness. The surface morphology of the different substrate materials varied significantly with the PET showing the largest roughness values; all the polymers were more rough than typical 7059 glass (rms <0.5 nm). Furthermore, the initial nucleation on both polymers were different than on glass. Very thin layers (<30Å nominal thickness) of plasma deposited silicon nitride films on polycarbonate and glass produced flat, smooth surfaces, with rms roughness of ≈0.5 nm. On the PET substrates, the nitride growth began with broad, thin deposits and developed a coarse, large-grained structure as growth progressed. With exposure to a NH3 + N2 plasma, the glass and PET surfaces showed detectable changes in surface roughness, which are discussed below. These results demonstrate that plasma-deposited insulators nucleate and grow differently on plastic substrates than on glass, which has implications for electronic devices fabricated directly on plastics.