bluuebunny/crossref_metadata_2025 huggingface dataset:
Hydrogen Plasma Treatment of Natural and Homoepitaxial Diamond
Authors: Rawles R. E. , Gat R. , Morris W. G. , D'Evelyn M. P.
Published: April 2011

As-polished and homoepitaxial diamond (100)- and (111)-oriented single crystals, natural diamond powders of 0.12-6 μm diameter, and high-pressure/hightemperature (HPHT) diamond powders of 50-100 μm diameter were treated in a microwave hydrogen plasma under four sets of conditions. Negligible changes in the weights (±10 μg) of the as-polished and homoepitaxial diamonds were observed.Post-treatment atomic force microscopy (AFM) showed a combination of smoothing and pit formation on (100) surfaces, the details of which were sensitive to the structure of the starting surface. Asymmetric pits formed on an as-polished (111) surface, but no qualitative changes in the structure of highly defective (100) and (111) films were observed. Natural diamond powders, which were quite irregular and rough prior to treatment, became markedly smoother and well-faceted, as observed by scanning electron microscopy (SEM). The degree of faceting was sensitive to plasma power level but was independent of H2 flow rate. The size and degree of faceting appeared to be the same after plasma treatment for isolated and closelypacked particles; however, the latter fused into a quasi-continuous film. “Regrown” crystallites were observed on the surfaces of synthetic type Ib diamond particles following plasma treatment. We argue that surface diffusion is the dominant mechanism for the observed morphological changes.
Electrical Properties of Diamond for Device Applications
Authors: Fox B. A. , Hartsell M. L. , Malta D. M. , Wynands H. A. , Tessmer G. J. , Dreifus D. L.
Published: April 2011

The semiconducting properties of diamond make it a material of interest for the fabrication of active electronic devices. Device performance depends on the properties of the host diamond film and the device structure. Improvements in the electronic properties of chemical vapor deposited (CVD) diamond films and in device design are required for development of commercial diamond devices. Hall-effect and capacitance-voltage measurements have been used to assess the electronic quality of diamond films and to evaluate potential device materials and structures.
Homoepitaxial, highly oriented and polycrystalline CVD diamond films have been deposited and characterized by variable-temperature Hall-effect measurements. The highest room temperature hole mobility measured in a homoepitaxial film was 1590 cm2 /V•s where the compensation was ∼2 × 1015 cm-3. These properties are now beginning to approach those of the best type IIb natural diamonds. In a comparison of homoepitaxial, highly oriented diamond film, and polycrystalline diamond films, the room temperature hole mobility was 1470, 229 and 70 cm2/V•s, respectively. This data provides further evidence that the alignment of the grains improves the transport properties of diamond films.
Capacitance-voltage measurements have been performed on single crystal type lib diamond with metal-insulating diamond-semiconducting diamond and metal-oxi de-semiconducting diamond structures. Using a conductance versus frequency technique, the density of interface states was estimated to be ∼1012 cm-2eV-1 for both structures. Although optimization of these structures must still be performed, the ability to measure the interface trap density offers the opportunity to provide direct feedback to the device fabrication process so that the structure may be optimized.
Examples of Diamond Sensing Applications
Authors: Davidson J. L. , Kang W. P.
Published: April 2011

Diamond has attractive properties as an advanced electronic material. Its combination of high carrier mobility, electric breakdown, and thermal conductivity results in the largest calculated figures of merit for speed and power of any material. Recently, the discovery and development of what might be called sophisticated secondary effects in diamond, are opening interesting sensor applications. Boron doped diamond will change electrical resistance with strain (piezoresistance), meaning it can be used as a strain gauge on rugged electronic microsensors for pressure and acceleration sensing. This paper will present some critical issues of diamond for microelectromechanical sensing applications such as its rupture stress and edge stress of diamond diaphragms and the high temperature responses of a diamond pressure sensor. We will describe an all diamond pressure microsensor that measures pressure at more than 300°C. Also, we have observed that layered diamond films can behave as chemical sensors measuring hydrogen, oxygen and many other chemicals’ concentration. For example, a diamond-based chemical gas sensor using Pd/i-diamond/p+-diamond metal-insulator-semiconductor diode structure has been made and the behavior of a new diamond-based chemical gas sensor has been studied. Also, the creation and properties of diamond microtips as field emitters are discussed.
Diamond Sensors and Vacuummicroelectronics
Authors: Pan L. S.
Published: April 2011

This paper will cover two diverse electronic applications for which diamond devices have shown great promise. The first application is diamond radiation sensors for high radiation environments, where the competition is mainly silicon devices. These environments arise in high energy physics experiments, and tests show diamond to be superior to silicon in many ways. The second application is vacuum microelectronics, which generally refers to field emission, where the main competitor is metal and semiconductor microtip arrays. Certain diamond and diamondlike carbon materials emit electrons readily, but the physical mechanisms for this are not well understood. Negative electron affinity and other possible explanations are discussed in this paper.
Characterization of Stress and Mosaicity in Homoepitaxial Diamond Films
Authors: Alexander W. Brock , Holloway Paul H. , Doering Patrick , Linares Robert
Published: April 2011

Diamond films were grown on (100) and (110) oriented natural diamond substrates by hot filament assisted chemical vapor deposition (HFCVD) to thicknesses of 7 to 100μm. Raman spectroscopy was used to measure tensile stresses of up to ∼2GPa for some of the (110) films. The development of stress was attributed to the incorporation of impurities (Re, Mo, and H). Impurity concentrations were greater at the interface than through the film thickness. Up to∼11% H and 50ppm Re were measured in the films with secondary ion mass spectrometry (SIMS). Homoepitaxial diamond films were further characterized using a seven crystal high resolution x-ray diffraction system. This new characterization tool allowed the separation of the effects of mosaicity from those of variation in lattice parameter.
Three-Phase Combustion Modelling: Frozen Ozone, a Prototype System
Authors: Miller Martin S.
Published: April 2011

A number of efforts are currently underway in the U.S. to model the self-sustained combustion of solid energetic materials at the level of fundamental physical processes and elementary chemical reactions. Since many of these materials bum, at least at some pressures, with a liquid surface layer, these models must address phenomena in three physical phases. Most of these modeling efforts have focussed on pure RDX as a prototype. From a systems viewpoint this was a natural choice since it, or materials like it, is one of the components of modem propellants. It is argued here that, from a scientific viewpoint, RDX is already too complex with too many uncertain mechanisms and unavailable supporting data to serve this role effectively. We discuss here a number of issues related to the condensed phase and interfacial phenomena which have not been previously identified and which warrant more detailed research. In this paper frozen ozone is adopted as a more suitable prototypical 3-phase system. In this paper progress toward addressing this system is presented. The first new process to be modelled was a gas/surface reaction A detailed analysis was performed on the reaction mechanisms at play in the ozone flame and how they are affected by the heterogeneous reaction. Other new mechanisms are associated with the multicomponent nature of the liquid surface layer and will be addressed in future work.
Solid Propellant Flame Structure
Authors: Parr T. P. , Hanson-Parr D. M.
Published: April 2011

Planar Laser Induced Fluorescence (PLIF), UV/Vis Absorption, and thermocouple measurements were done for HNF, RDX, HMX, and XM39 deflagration with and without CO2 laser-support. RDX and especially HNF have very short self-deflagration flame length scales. HMX and XM39 have taller self-deflagration flames. XM39 has a marked dark zone with plateau temperature about 1400K. RDX's dark zone, present under laser supported deflagration, collapses when the external laser flux is removed. PLIF was used to measure the 2D NH, OH, and CN species profiles for these materials and OH temperature profile for RDX and HNF under non-laser supported conditions. The best spatial resolution for the RDX PLIF was about 4μm. Sandwiches of HNF and various binders were studied with PLIF and while obvious diffusion flames were present at low pressure, they are weak and are not expected to be burn rate controlling.
A Simplified Method for Determining Reactive rate Parameters for Reaction Ignition and Growth in Explosives
Authors: Miller Philip J.
Published: April 2011

A simplified method for determining the reactive rate parameters for the ignition and growth model is presented. This simplified ignition and growth (SIG) method consists of only two adjustable parameters, the ignition (I) and growth (G) rate constants. The parameters are determined by iterating these variables in DYNA2D hydrocode simulations of the failure diameter and the gap test sensitivity until the experimental values are reproduced. Examples of four widely different explosives were evaluated using the SIG model. The observed embedded gauge stress-time profiles for these explosives are compared to those calculated by the SIG equation and the results are described.
Investigation of Shock-Induced Chemical Decomposition of Sensitized Nitromethane Through Time-Resolved Raman Spectroscopy
Authors: Pangilinan G. I. , Constantinou C. P. , Gruzdkov Y. A. , Gupta Y. M.
Published: April 2011

Molecular processes associated with shock induced chemical decomposition of a mixture of nitromethane with ethylenediamine (0.1 wt%)are examined using time-resolved, Raman scattering. When shocked by stepwise loading to 14.2 GPa pressure, changes in the nitromethane vibrational modes and the spectral background characterize the onset of reaction. The CN stretch mode softens and disappears even as the NO2 and CH3 stretch modes, though modified, retain their identities. The shape and intensity of the spectral background also shows changes characteristic of reaction. Changes in the background, which are observed even at lower peak pressures of 11.4 GPa, are assigned to luminescence from reaction intermediates. The implications of these results to various molecular models of sensitization are discussed.
Detonation in Shocked Homogeneous High Explosives
Authors: Yoo C. S. , Holmes N. C. , Souers P. C.
Published: April 2011

We have studied shock-induced changes in homogeneous high explosives including nitromethane, tetranitromethane, and single crystals of pentaerythritol tetranitrate (PETN) by using fast time-resolved emission and Raman spectroscopy at a two-stage light-gas gun. The results reveal three distinct steps during which the homogeneous explosives chemically evolve to final detonation products. These are i) the initiation of shock compressed high explosives after an induction period, ii) thermal explosion of shock-compressed and/or reacting materials, and iii) a decay to a steady-state representing a transition to the detonation of uncompressed high explosives. Based on a gray-body approximation, we have obtained the CJ temperatures: 3800 K for nitromethane, 2950 K for tetranitromethane, and 4100 K for PETN. We compare the data with various thermochemical equilibrium calculations. In this paper we will also show a preliminary result of single-shot time-resolved Raman spectroscopy applied to shock-compressed nitromethane.
Detonation Synthesis of Nano-Size Materials
Authors: Forbes J. , Davis J. , Wong C.
Published: April 2011

The detonation of explosives typically creates 100's of kbar pressures and 1000's K temperatures. These pressures and temperatures last for only a fraction of a microsecond as the products expand. Nucleation and growth of crystalline materials can occur under these conditions. Recovery of these materials is difficult but can occur in some circumstances. This paper describes the detonation synthesis facility, recovery of nano-size diamond, and plans to synthesize other nano-size materials by modifying the chemical composition of explosive compounds. The characterization of nano-size diamonds by transmission electron microscopy and electron diffraction, X-ray diffraction and Raman spectroscopy will also be reported.
Metal/Metal Exothermic Reactions Induced by Low Velocity Impact*
Authors: Woody Diana L. , Davis Jeffery J. , Miller Philip J.
Published: April 2011

This paper discusses experimental results from an effort conducted to discern the basic mechanism of reactions in porous metal/metal compositions under rapid plastic flow conditions. Small-scale impact tests were performed on various intermetallic mixtures: 3CuO + 2AI, Fe2O3 + 2AI, Ni + Al, and 5Ti + 3Si. The addition of polytetrafluoroethylene (Teflon) to the metal/metal mixtures has been demonstrated to affect the extent of the reactions. Real-time emissivity and species evolution measurements of the reacting materials were used to discern the chemical reactions occurring under rapid plastic flow conditions.
Phase Relationships Involving RDX and Common Solid Propellant Binders
Authors: Boyer E. , Brown P. W. , Kuo K. K.
Published: April 2011

The solubilities of the common propellant ingredients acetyl triethyl citrate (ATEC) and cellulose acetate butyrate (CAB) and their effects on RDX (cyclotrimethylenetrinitramine) unit cell dimensions were investigated. If there is appreciable solid solubility, solutions will form and will have enthalpies of fusion, melting temperatures, and other characteristics different from those of pure RDX. It is desirable to establish the properties of such mixtures when designing new propellant formulations. Samples were aged at an elevated temperature to speed the formation of solid solutions. A least-squares analysis of X-ray diffraction data was used to obtain the lattice parameters from which unit cell volume and solubility was deduced. Both ATEC and CAB caused an expansion of the unit cell, indicating the formation of a solid solution. The limit of solubility in the ATEC/RDX mixture appeared to be approximately 13 wt% ATEC, while the CAB/RDX limit is above 16 wt% CAB. In both mixes, the cell volume expanded linearly with increasing proportion of binder.
Overview on Energetic Polymers
Authors: Boileau Jacques
Published: April 2011

Energetic materials for missiles, gun munitions or pyrotechnic devices often are mixtures in a biphasic form, with a filler and a binder. To satisfy the user needs, an analysis of functional requirements together with constraints (safety, vulnerability, ageing, environment, disposal, price...) is useful to choose a convenient binder. From this point of view numerous synthetic energetic polymers proposed or developed as binders are reviewed with regard to their syntheses, processing, properties and possible uses. These polymers contain explosophore groups: C-NO2aliphatic or aromatic, ONO2, NNO2, NF2and N3.

Some research projects are suggested. Among them in the list of published polymers, following a NIMIC (NATO) suggestion, note the reason of a development interruption. Some dinitropolystyrene – polyvinyl nitrate mixtures or copolymers could exhibit interesting properties. For unknown reasons, some mixtures of crystalline filler- with polymeric binder, generally in a biphasic form, may also be monophasic for a same composition. What properties are modified between the both forms (e.g. combustion mechanisms, erosion, ideal character of the detonation) ? It is also interesting to pursue into a newly open route to thermo-plastic elastomers.
Characteristics of XeCl Excimer-Laser Annealed Insulator
Authors: Jang Keun-Ho , Choi Hong-Seok , Jun Jae-Hong , Yoo Jhun-Suk , Han Minkoo
Published: April 2011

The laser annealing effects on the TEOS (Tetra-Ethyl-Ortho-Silicate) oxide of MOS (AI/TEOS/n+ Silicon ) structures was investigated with different initial oxide conditions, such as breakdown field. The breakdown field increased upto the 170 mJ/cm2 with increasing laser energy density and decreased at 220 mJ/cm2. It is considered that the increase of breakdown field is originated from the restore of strains which exist mainly at the metal/oxide interface.
Hot-Wire Deposited Amorphous Silicon Thin-Film Transistors
Authors: Schropp R. E. I. , Feenstra K. F. , Werf C. H. M. Van Der , Holleman J. , Meiling H.
Published: April 2011

We present the first thin film transistors (TFTs) incorporating a low hydrogen content (5 - 9 at.-%) amorphous silicon (a-Si:H) layer deposited by the Hot-Wire Chemical Vapor Deposition (HWCVD) technique. This demonstrates the possibility of utilizing this material in devices. The deposition rate by Hot-Wire CVD is an order of magnitude higher than by Plasma Enhanced CVD. The switching ratio for TFTs based on HWCVD a-Si:H is better than 5 orders of magnitude. The field-effect mobility as determined from the saturation regime of the transfer characteristics is still quite poor. The interface with the gate dielectric needs further optimization. Current crowding effects, however, could be completely eliminated by a H2 plasma treatment of the HW-deposited intrinsic layer. In contrast to the PECVD reference device, the HWCVD device appears to be almost unsensitive to bias voltage stressing. This shows that HW-deposited material might be an approach to much more stable devices.
a-Si:H Photo Diode With Variable Spectral Sensitivity
Authors: Rieve Peter , Giehl Jürgen , Zhu Qi , Böhm Markus
Published: April 2011

A novel two terminal thin film photo diode for color detection has been developed. The device structure which is based on standard amorphous silicon nipin multilayers exhibits three or even more linearly independent spectral sensitivity peaks and provides linearity over a wide range of illumination levels. Band gap engineering and electric field tailoring allow a precise voltage controlled shift of the collection region of photo generated carriers. The steady-state as well as the transient device characteristics have been studied in detail. Emphasis was put on optimization of the spectral sensitivity of the color diodes. Furthermore, an electronic color correction algorithm is presented which results in an improved color separation.
Optimized Three-Color Detector Based on a-SiGe:H Heterojunctions
Authors: Stiebig H. , Knipp D. , Fölsch J. , Finger F. , Wagner H.
Published: April 2011

Bandgap and μτ - (carrier mobility x lifetime) - engineered multi-color nipin detectors with three linear independent curves of the spectral response in a range from 470nm to 670nm are successfully fabricated. The spectral response of nipin structures employed as two terminal devices shifts by the applied voltage. Switching transients after changing the color sensitive voltages under different monochromatic illumination conditions are investigated and can be classified in different cases. In order to improve the understanding of the photo current transients, we have characterized the influence of different illumination conditions on the voltage dependent capacitance behavior. To verify the capacitance measurements, a SPICE model has been developed, which describes the transient behavior of the detector. The simulations are in good agreement with the experimental data and support the explanation of the transient cases after bias switching. Based on these results further optimization criteria to accelerate the transient behavior are presented.
Transient Behavior of Color Diodes
Authors: Giehl Jürgen , Zhu Qi , Rieve Peter , Böhm Markus
Published: April 2011

Amorphous silicon (a-Si:H) based n-i-p-i-n/p-i-n-i-p structures serve as two color detectors. These basic structures can be extended to obtain multi color detection. In this paper capacitance measurements in the dark and under various illumination conditions as well as photo and bias current transient measurements in the time range 10-6-10-2 s are presented. The transients have been investigated for different bias voltages and photon fluxes.

Device capacitance becomes strongly bias dependent if illumination is applied to the sensor. It is dominated by the reverse biased diode of the nipin/pinip structure. Bias and photocurrent transients are influenced by defect recharging due to injection. For high photon fluxes only defect states near the band edges contribute to the transient current. This behavior is confirmed by dispersion like transient phenomena. Under dark conditions deep defects influence the transient behavior.
A-Si:H Solar Cells Deposited Using VHF–PECVD
Authors: Sark W. G. J. H. M. Van , Bezemer J. , Heijden R. Van Der , Weg W. F. Van Der
Published: April 2011

A-Si:H p+-i-n+ solar cells have been made employing plasma enhanced chemical vapour deposition at frequencies between 30–80 MHz. Here, only the i-layer was fabricated at these very high frequencies (VHF). Both the p+- and n+-layer were made using 13.56 MHz. A previous study has shown the material quality to depend on mainly the applied rf-power, and only slightly on the frequency. It should be noted that for homogeneity reasons a certain optimized pressure is required for each frequency. There is a clear correlation between material quality and solar cell parameters. An initial efficiency of 10 % has been obtained for cells deposited at 65 MHz using a low power density, while the deposition rate still is 2–3 times higher than the one at 13.56 MHz. Light-soaking reveals stabilisation at 6 % for the best cell, which compares well to conventional 13.56 MHz cells.
Effect Of μ-Doped Compensated Material on Stability of a-Si:H Solar Cells
Authors: Caputo D. , Cesare G. De , Palma F. , Tucci M.
Published: April 2011

In this paper we focus our attention on compensated materials with μ-doping concentration in order to obtain a stable intrinsic layer with initial high photoconductivity suitable for p-i-n solar cells. Films were grown from a mixture of undiluted silane, hydrogen diluted phosphine and hydrogen diluted diborane. Values of dark conductivity around 10-11 Ω-1cm-1 and photosensitivity ratio under AM 1.5 of 6 orders of magnitude have been obtained for phosphine/diborane ratio around 102. The difference between the two dopant concentrations is in agreement with the difference in doping efficiency of the two gases found in the characterization of single μ-doped films.

We compared the degradation behavior of compensated and intrinsic materials with the same initial dark and light conductivity. After about 20 h the photoconductivity of the compensated and the intrinsic material decreased of 33% and 70%, respectively. The space of investigable deposition parameters has been limited by the stress induced by the simultaneous presence of phosphine and diborane which leads to a macroscopic, periodic and regular damage of the film.
Photoconductive Polycrystalline Silicon Films on Glass Obtained by Hot-Wire CVD
Authors: Middya A. R. , Guillet J. , Perrin J. , Bouree J. E.
Published: April 2011

Textured polycrystalline silicon films with columnar structure have been deposited on glass at low temperature (400–550°C) and high deposition rate (10 to 15 Å/s) by hot-wire chemical vapour deposition using SiH4-H2 gases. The homogeneity of the deposited layer is ± 5% on a 8 cm diameter. As deposited films have a poor photoconductivity. However hydrogen confinement in the films during the deposition or after the deposition is found to be the key for obtaining g.tc/poly-Si with a significant diffusion length. Eventually reasonable values of the mobility lifetime product (> 10−7 cm2/V) are obtained by in situ hydrogen passivation of poly-Si films after deposition. Efficient shifting of the Fermi level is achieved by in situ B or P doping. The incorporation of boron in poly-Si network strongly influences the morphology and the crystalline structure. Undoped films have a Hall mobility of 14 ± 5 cm2/V.s which decreases versus the carrier concentration.
The Effect of Hydrogen Dilution on the Hot-Wire Deposition of Microcrystalline Silicon
Authors: Wanka H. N. , Zedlitz R. , Heintze M. , Schubert M. B.
Published: April 2011

The growth of amorphous (a-Si:H) and microcrystalline (pc-Si) silicon by hot-wire chemical vapor deposition (HWCVD) has been studied by combining in-situ ellipsometry, atomic force microscopy (AFM), and Raman spectroscopy. Generally a dense nucleation layer is formed during a-Si:H HWCVD, containing nuclei about 0.8 nm high and 10 to 20 nm in diameter. The surface roughness gradually increases with film thickness and settles at a root mean square (RMS) value of 1.6 nm at about 200 nm thickness. For hydrogen dilution at gas flow ratios x=[H2]/[SiH4] of 15 to 120 microcrystalline material was obtained. The grain size and nucleation layer, however, are strongly dependent on x. Low H2 dilution enhances the formation of an amorphous-like interface layer from which the μc-Si:H growth eventually starts. Increasing x promotes the etching of amorphous regions and the surface diffusion of precursors, resulting in larger nuclei. X = 30 yields extended μc-Si nuclei (30 nm height, 90 nm diameter) and a pronounced increase in surface roughness for thicker films, but suppresses the formation of the amorphous-like nucleation layer. A further increase in x remarkably lowers the growth rate, but smoother surfaces at comparable film thickness and larger lateral dimensions of the grains occur. This is interpreted as incipient etching of the crystallites.
On the Way Towards High Efficiency Thin Film Silicon Solar Cells by the “Micromorph” Concept
Authors: Meier J. , Torres P. , Platz R. , Dubail S. , Kroll U. , Selvan J. A. Anna , Vaucher N. Pellaton , Hof Ch. , Fischer D. , Keppner H. , Shah A. , Ufert K. -D. , Giannoulès P. , Koehler J
Published: April 2011

Recently the authors have demonstrated that compensated or “midgap” intrinsic hydrogenated microcrystalline silicon (μc-Si:H), as deposited by the Very High Frequency Glow Discharge (VHF-GD) technique, can be used as active layer in p-i-n solar cells. Compared to amorphous silicon (a-Si:H), μc-Si:H was found to have a significantly lower energy bandgap of around 1 eV. The combination of both materials (two absorbers with different gap energies) leads to a “real” tandem cell structure, which was called the “micromorph” cell. Micromorph cells can make better use of the sun's spectrum in contrast to conventional double-stacked a-Si:H / a-Si:H tandems.

The present study will show that the compensation technique (involving boron “microdoping”) used sofar for obtaining midgap μc-Si:H can be replaced by the application of a gas purifier. The use of this gas purifier has a beneficial influence on the transport properties of undoped intrinsic μc-Si:H. By this procedure, increased cell efficiencies in both, single microcrystalline silicon p-i-n as well as micromorph cells could be obtained. In the first case 7.7 % stable, and in the second case 13.1% initial efficiency could be achieved under AM1.5 conditions. Preliminary light-soaking experiments performed on the tandem cells indicate that microcrystalline silicon could contribute to an enhancement of the stable efficiency performance. Micromorph cell manufacturing is fully compatible to a-Si:H technology; however, its deposition rate is still too low. With further increase of the rate, a similar cost reduction potential like in a-Si:H technology can be extrapolated.
Effects of Deposition Power and Temperature on the Properties of Heavily Doped Microcrystalline Silicon Films
Authors: Miri A. M. , Chamberlain S. G. , Nathan A.
Published: April 2011

We studied the effect of RF deposition power and temperature on the electrical and structural properties of plasma enhanced chemical vapor deposited (PECVD), heavily doped microcrystalline silicon films (n+μC-Si:H). The film growth process was found to be CVDlike at low powers and PVD-like (Physical Vapor Deposition) at high powers. We show that the film properties strongly depend on the nature of the growth process. We observed that at low temperatures the microcrystalline formation is mainly governed by the presence of hydrogen. This can be improved by increasing the substrate temperature. However, a further increase in substrate temperature tends to reduce hydrogen incorporation into the film and hence decreases the role of hydrogen in the formation of microcrystallites. Resistivities as low as 0.1 Ω.cm were achieved for 500Å thin layers deposited at a relatively low temperature of 220°C and power density of 40mW/cm2.
On the Effect of Substrate Temperature on a-Si:H Deposition Using an Expanding Thermal Plasma
Authors: Severens R. J. , Sanden M. C. M. Van De , Verhoeven H. J. M. , Bastiaanssen J. , Schram D. C.
Published: April 2011

Fast (7 nm/s) deposition of amorphous hydrogenated silicon with a midgap density of states less than 1016 cm-3 and an Urbach energy of 50 meV has been achieved using a remote argon/hydrogen plasma. The plasma is generated in a dc thermal arc (0.5 bar, 5 kW) and expands into a low pressure chamber (20 Pa) thus creating a plasma jet with a typical flow velocity of 103 m/s. Pure silane is injected into the jet immediately after the nozzle, in a typical flow mixture of Ar:H2:SiH4=55:10:10 scc/s. As the electron temperature in the recombining plasma is low (typ. 0.3 eV), silane radicals are thought to be produced mainly by hydrogen abstraction.

Material quality in terms of refractive index, conductivity, microstructure parameter and optical bandgap was found to increase monotonously with substrate temperature, even up to 350 °C; for practically all low growth rate deposition schemes an optimum around 250 °C is observed. It will be argued that this behavior is consistent with a simple kinetic model involving physisorption and hopping, growth on dangling bonds and thermal desorption of hydrogen.
Phase Separation by Rapid Thermal Annealing in Si-Sub-Oxides Andnitrides Formed by Plasma CVD
Authors: Banerjee A. , Lucovsky G.
Published: April 2011

Hydrogenated silicon suboxide and subnitride films were deposited by a remote plasma enhanced chemical-vapor deposition (RPECVD) process. Rapid thermal annealing (RTA) of these alloys eliminated all of the bonded hydrogen and formed a two phase system. The microstructure of the annealed films showed silicon crystallites (c-Si) surrounded by amorphous layers. The amorphous layers were identified as SiO2 and Si3N4 in the annealed suboxides and subnitrides, respectively. The as-deposited films showed band edge photoluminescence (PL); however, no PL was observed from the annealed films. This indicates that there was no significant suboxide or subnitride bonding in the amorphous layers at the metallurgically-sharp c-Si-SiO2 and c-Si-Si3N4 interfaces, respectively.
In situ Ellipsometric Studies of the growth of a-Si:H films Prepared by the Hot wire Deposition
Authors: Bauer S. , Dusane R. O. , Biehl R. , Schroder B.
Published: April 2011

In situ ellipsometric studies have been performed during the nucleation and growth of hydrogenated amorphous silicon (a-Si:H) films prepared by the hot wire chemical vapour deposition (HWCVD) method in order to understand the growth mechanism of these films. For a comparison with films deposited by plasma enhanced chemical vapour deposition (PECVD), the hot wire deposition was carried out under similar conditions and reactor geometry as for the PECVD process. It is observed from the kinetic ellipsometry measurements that low filament temperature (TFil) and low gas pressure favour the growth of more dense films, but at lower deposition rates. Moreover, for a given set of conditions an increase in substrate temperature (Ts) leads to a higher final value of the film density with a different growth behaviour in the initial stage. Thus, the filament temperature in the hot wire method seems to have a similar effect on the film density as the rf power has in the PECVD process, which has been observed earlier. Film density and surface roughness obtained from spectroscopic ellipsometry using a tetrahedron model which takes into account the effect of hydrogen on the dielectric function, is used to get information about the film microstructure. A correlation between this microstructure, the growth behaviour and the electronic properties as the defect density or the ambipolar diffusion length in the films is also reported.
Advances in the Characterization of Compositionally-graded Layers in Amorphous Semiconductor Solar Cells by Real Time Spectroellipsometry
Authors: Collins R. W. , Kim Sangbo , Koh Joohyun , Burnham J. S. , Jiao Lihong , Chen Ing-Shin , Wronski C. R.
Published: April 2011

We have developed a real time spectroellipsometry data analysis procedure that allows us to characterize compositionally- graded amorphous semiconductor alloy thin films prepared by plasma-enhanced chemical vapor deposition (PECVD). As an example, we have applied the analysis to obtain the depth-profile of the optical gap and alloy composition with ≤15 Å resolution for a hydrogenated amorphous silicon-carbon alloy (a-Si1−xCx:H) film prepared by continuously varying the gas flow ratio z=[CH4]/{[CH4]+[SiH4]} in the PECVD process. The graded layer has been incorporated at the p/i interface of widegap a-Si1−xCx:H (x∼0.05) p-i-n solar cells, and consistent improvements in open-circuit voltage have been demonstrated. The importance of the graded-layer characterization is the ability to relate improvements in device performance directly to the physical properties of the interface layer, rather to the deposition parameters with which they were prepared.
Intermediate Layers in the A-Si:H Growth Processes
Authors: Aivazov A. A. , Bodyagin N. V. , Vikhrov S. P.
Published: April 2011

The intermediate layers (IL) exist at the boundary between liquid (plasma, gas) and solid phases during production processes of a-Si:H. The IL properties differ from the properties of the boundary regions anomaly. The substance in IL is in a strong nonequilibrium state of the bifurcation or the cascade of bifurcations. The processes in the IL determine the properties and the structure of the material being produced.
Optical Absorption in Amorphous Silicon
Authors: O'Leary S. K. , Zukotynski S. , Perz J. M. , Sidhu L. S.
Published: April 2011

The role that disorder plays in shaping the form of the optical absorption spectrum of hydrogenated amorphous silicon is investigated. Disorder leads to a redistribution of states, which both reduces the ‘Tauc’ gap and broadens the absorption tail. The observed relationship between the ‘Tauc’ gap and the breadth of the absorption tail is thus explained.
Temperature Dependence of the Photoconductivity and the Near Absence of Light-Induced Defects in a-SixGe1-x:H
Authors: Fritzsche H. , Stradins P. , Belomoin G.
Published: April 2011

The photoconductivities ap of glow discharge deposited a-Ge:H and amorphous Si-Ge alloys prepared in different laboratories were measured between 4.2K and 300K and compared with σp(T) of intrinsic and compensated a-Si:H films. The alloys as well as a-Ge:H do not exhibit thermal quenching of σp(T) at elevated temperatures which suggests that the valence and conduction band tails have similar widths. Remarkable is the near absence of light-induced metastable defects in the alloys as well as in a-Ge:H even after prolonged exposures at low temperatures.
Effects of Light Induced Degradation on the Distribution of Deep Defects in Hydrogenated Amorphous Silicon-Germanium Alloys.
Authors: Chen C. C. , Zhong F. , Cohen J. D.
Published: April 2011

We have characterized the defect state structure in a series of device quality glow discharge produced a-Si,Ge:H alloys with Ge content ranging from 30 at.% to 50 at.% using capacitance profiling, modulated photocurrent, transient junction photocurrent and photocapacitance measurements. As previously reported, these methods have allowed us to identify two types thermally induced defect transitions plus two types of optical transitions from deep defects. In the current study we have examined the changes in these defects, along with the changes in the hole mobility-time products, that result from prolonged light exposure. By comparing these changes in the annealed state and light soaked state of the same sample, we attempt to correlate the changes in defects with the hole mobility-time product. In general, although all of the defect bands are found to increase after light soaking, the relative factor is found to be different for the various defect transitions within the same sample. We also try to identify a defect bands may be acting as a “safe electron trap”, enhancing the lifetime of the minority carriers. We propose that the observed decrease of this defect band relative to the midgap defect band with light soaking could be a significant factor in determining the degradation of these a-Si, Ge:H alloys in the device performance.
Relaxation Measurements of the Persistent Photoconductivity in Sulfur-Doped a-Si:H.
Authors: Quicker D. , Kakalios J.
Published: April 2011

The slow relaxation of the persistent photoconductivity (PPC) effect in sulfur-doped hydrogenated amorphous silicon (a-Si:H) has been measured as a function of temperature and illumination time. The relaxation is found to be thermally activated, with an activation energy which varies with sulfur concentration, while illuminating the film for a longer time leads to a longer relaxation time. A correlation is observed between changes of the photoconductivity during illumination and the magnitude of the PPC effect following illumination. These effects are also observed in compensated a-Si:H, suggesting that the mechanism for the PPC effect is the same in both sulfur-doped a-Si:H and compensated a-Si:H. The presence of donor and compensating acceptor states in sulfur-doped a-Si:H could arise from valence alternation pair sulfur atom defects.
The Effect of Hydrogen Dilution Near The P/I Interface Region of A-Si:H P-I-N Solar Cells
Authors: Yeh C. N. , Han Daxing , Wang Qi , Xu Y. Q.
Published: April 2011

It is known that the a-Si:H solar cell with H-diluted i-layer (p-iH-n) exhibits a better stability under AMI light exposure than the non-diluted solar cell (p-i-n). Recently, Wang et al. found that the a-Si:H solar cell when inserting a thin H-diluted i-layer between the p- and the i-layer (p-iH-i-n) shows the same stability as a p-iH-n solar cell after 600 hours light soaking. It was suggested that the stabilization of a H-diluted a-Si:H solar cell is through controlling its p/i interface. In this work, we report the electroluminescence (EL) measurements of a-Si:H solar cells with p-iH-n, p-iH-i-n and p-i-n structures made in identical conditions except the difference in H-dilution processing. EL spectra in a wide range of temperature indicate that the defect-band emission can be controlled through a thin H-diluted i-layer near the p/i interface region, but the main-band emission depends more on the diluted i-layer thickness. According to the temperature dependence of the EL efficiency, the valence band-tail width of the active layer in the p-iH-i-n cell is estimated to be about 13% broader than that in the p-iH-n cell. From the total EL intensity as a function of the forward current density, the recombination processes at room temperature are suggested to be a monomolecular type for p-iH-wn, p-iH-i-n, and p-i-n structures.
Calorimetric Investigation of Relaxation Processes in Disordered Semiconductors
Authors: Budaguan B. G. , Aivazov A. A. , Sazonov A. Yu.
Published: April 2011

The comparative study of relaxation processes in amorphous hydrogenated (a- Si:H) and porous silicon (PS) by use of differential scanning calorimetry (DSC) measurements is presented. Films of a-Si:H were deposited by RF glow discharge of two gas mixtures (10% SiH4+ 90% H2) and (5% SiH4 + 95% He). PS films have been prepared by electrochemical etching of 1Ω-cm (p- PS) and of 0.01Ω-cm (p+ PS) ptype Si substrates. The DSC traces were recorded during the heating of samples at a constant rate of 10°C/min from 20 to 5700C in an Ar atmosphere. All investigated samples present a low temperature exothermic effect with a maxima within 120-2800C. At higher temperatures (T>3000C) a second exothermic effect is observed for a-Si:H films prepared from both hydrogen and helium diluted silane mixtures while two endothermic effects are observed for PS samples. Analysis of the low temperature exothermic effects has been performed, and focussed on the relaxation of weak Si-Si bonds which are the features of both amorphous hydrogenated and porous silicon. It was shown that the endothermic effect connected with hydrogen effusion from PS at higher temperatures is compensated by exothermic structural rearrangement in the case of a-Si:H.
1/f Noise and Thermal Equilibration Effects in Hot Wire Deposited Amorphous Silicon
Authors: Khera G. M. , Kakalios J. , Wang Q. , Iwaniczko E.
Published: April 2011

Measurements of the conductance fluctuations and thermal equilibration of the dark conductivity of a series of undoped hydrogenated amorphous silicon thin films synthesized by Hot-Wire Chemical Vapor Deposition (HWCVD), with hydrogen contents varying from less than one to twelve atomic percent are reported. The spectral density of the conductance fluctuations varies inversely with frequency f and is dependent upon hydrogen concentration; the 1/f noise statistics are non- Gaussian, indicating correlated fluctuators as is observed in PECVD a-Si:H. These results indicate that aspects of electronic transport and defect dynamics in HWCVD films are similar to those in PECVD a-Si:H films.
New Evidence for Deep Defect Relaxation in Hydrogenated Amorphous Silicon from Junction Capacitance Methods
Authors: Cohen J. David , Gardner Adam D. , Kwon Daewon
Published: April 2011

We have carried out capacitance transient measurements on lightly PH3 doped a-Si:H films incorporating both ohmic and blocking contacts. We find that the previously reported anomalous variation of emission time with filling pulse duration is completely independent of whether one uses an n+ c-Si substrate (providing a reasonably ohmic back contact) or a p+ c-Si substrate (giving a blocking back contact). Quite similar behavior is observed on a sample which includes an n+ a-Si:H layer at the back contact, exhibiting a variation of more than 2 orders of magnitude change in emission time when the filling pulse duration is decreased from Is to 100μs. Computer simulations are used to demonstrate that for a fixed (non-relaxing) deep state distribution, a filling problem in general cannot account for the observed experimental behavior. Finally, we report measurements that demonstrate that the filling of defects too close to the barrier interface can create difficulties observing the relaxation effects when an n+ a-Si:H contacting layer is used.
Photoconductivity of a-Si:H as a Function of Doping, Temperature and Photocarrier Generation Rates Between 1013 and 1028cm-3s-1
Authors: Stradins P. , Fritzsche H. , Kopidakis N. , Tzanetakis P.
Published: April 2011

The steady state photoconductivity up of n-type, p-type and intrinsic a-Si:H has been studied up to photocarrier generation rates of G=5×1027cm-3s-1. In the 20ppm B2H6/SiH4 p-type sample photoconduction switches from holes at low G to electrons at high G. The electron photoconduction at high G is increased by n-type and decreased by p-type doping. This is explained by the charge state of the dominant electron recombination centers. The σp(G) curves of doped and intrinsic a-Si:H merge at the highest G used.
Hall-Effect and Sign Properties in Hydrogenated Amorphous and Disordered Crystalline Silicon
Authors: Nebel C. E. , Rother M. , Summonte C. , Heintze M. , Stutzmann M.
Published: April 2011

Hall experiments on a series of microcrystalline, microcrystalline-amorphous, amorphous and crystalline silicon samples with varying defect densities are presented and discussed. Normal Hall effect signatures on boron and phosphorus doped hydrogenated amorphous silicon are detected. We interpret these results to be due to a small volume fraction of nanocrystalline Si, which falls below the detection limits of Raman experiments. Hydrogenated amorphous silicon, prepared under conditions far away from microcrystalline growth, shows the known double sign anomaly, Sign reversals in c-Si, where the disorder is increased by Si implantation up to very high levels, could not be detected.
High Deposition Rate a-Si:H for the Flat Panel Display Industry
Authors: Hautala J. , Saleh Z. , Westendorp J. F. M. , Meiling H. , Sherman S. , Wagner S.
Published: April 2011

High deposition rates and good quality electrical properties and thickness uniformities over large areas are required for all three films (SiNx, a-Si:H and n+a-Si:H) composing the thin film transistors (TFTs) for the AMLCD industry, while maintaining high tool up-time and low particle formation. Generally these conditions have been achieved for most single-panel multichamber PECVD systems; however, it has become increasingly apparent that a compromise is drawn between the TFT mobility and the deposition rate of the a-Si:H layer. Thus it becomes essential to clearly assess the industry requirements for both deposition rates as well as TFT performance for the different device structures used for AMLCDs, and to discover and control these underlying material properties.

The TEL VHF (40/60 MHz) PECVD system produces high quality, low defect density a- Si:H at deposition rates exceeding 1500 Å/min when analyzed by FTIR, CPM, photo and dark conductivity. Even though the low deposition rate a-Si:H exhibits very similar bulk properties, higher mobility TFTs are produced with a-Si:H deposited at lower RF power. Having both a high ion flux and low ion energy in the SiH4 discharge are likely the most critical conditions for controlling the a-Si:H quality and thus the TFT mobility. Increasing the RF frequency enhances both of these effects, as well as provides a higher deposition rate for a given power density and a higher power threshold for particle/powder formation. For these reasons it is likely a 40/60 MHz plasma will produce better performing TFTs for a given deposition rate when compared with a conventional 13.56 MIHz system. Other process conditions such as diluting the SiH4 in H2 or Ar also seem to play an important role in the optoelectronic properties of the a-Si:H film and ultimately the TFT performance.