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The electronic properties of hydrogenated nanocrystalline silicon (nc-Si:H) were studied using drive-level capacitance profiling (DLCP) to obtain defect density profiles as well as transient photocapacitance (TPC) and transient photocurrent (TPI) spectroscopies to study the spectra of defect related optical transitions. These measurements were performed on a series of n–i–p solar cell devices with intrinsic layer thickness of roughly 1 μm. The nc-Si:H intrinsic layers were deposited using RF or MVHF glow discharge with various hydrogen dilution profiles predominantly on specular stainless steel substrates (SS/n + /i nc-Si:H/p + /ITO), but also on textured back reflectors (SS/Ag/ZnO/n + /i nc-Si:H/p + /ITO) in some cases. Crystallite fractions were estimated using Raman spectroscopy. The electronic properties determined by our measurements could be correlated with variations in structural device parameters and with the degree of hydrogen dilution profiling during growth. We also found, depending on the growth conditions, that the devices exhibited markedly different behaviors after prolonged light exposure (100 h using light at 610 nm and 500 mW/cm 2 intensity). We discuss one specific microscopic mechanism that may be responsible for the light-induced changes that we have observed.

Current-voltage, admittance spectroscopy, and drive-level capacitance profiling measurements were taken on Cu(In1−xGax)(Se1−ySy)2 solar cell devices. The devices were made using two different types of absorbers. One set of absorbers was deposited via physical vapor deposition, while the other set of absorbers was made by selenization of metal precursors. Additionally, each type of absorber was completed with one of two different types of buffer treatments: a CdS layer or a cadmium partial electrolyte surface modification. The devices with the evaporated absorbers had larger values of VOC, higher carrier densities, lower densities of trapping defects, and likely shallower gap states. Results were qualitatively similar for the CdS and partial electrolyte buffers. Copyright © 2005 John Wiley & Sons, Ltd.

The deep defect distributions affecting majority carrier trapping in polycrystalline CuIn 1− x Ga x Se 2 (CIGS) have been studied using drive level capacitance profiling. This technique provides, a spatial and energetic profile of sub-band gap defect transitions in the CIGS layer of working photovoltaic devices, while remaining insensitive to surface states. The bulk response was dominated by a defect which varied between 0.1 and 0.3 eV, according to the Meyer-Neldel rule. Devices grown at reduced substrate temperatures had smaller grain sizes and additional defect response. The effect of a light-soaking treatment, using near-band gap optical excitation, was studied. Both the free carrier density and the density of deeper defects were increased by this treatment. Device quality was degraded, predominantly due to a decreased fill factor.

The density and distribution of point defects in Cu(In,Ga)Se (CIGS) layers used for solar cell applications is critical to the 2 resulting device performance.These devices are generally thought to be limited by recombination in the space-charge region of the collecting heterojunction.The situation is complicated by the presumed presence of an n-type surface layer on the CIGS absorber.Both the surface inversion and space-charge recombination processes are intimately tied to near-surface point defects. Here, we overview recent results on surface chemistry, transient photocapacitance spectroscopy (TPC) and depth-resolved cathodoluminescence (CL) for polycrystalline device layers from two laboratories, and single crystal epitaxial layers of three orientations.The results are combined with device modeling to provide a picture of the near-surface defect structures in these materials.The TPC results show deep defect levels ;0.7 and 0.9 eV above the valence band. CL shows evidence of subgap radiative recombination, which increases dramatically near the sample surfaces.The results point to a near-surface Cd-containing layer, which could be responsible for the surface carrier type inversion, a near-surface region containing an elevated defect density, possibly near the valence band edge, and deep hole traps near the conduction band.Implementation of the results in a device model provides reasonable fits to the device performances. 2003 Elsevier Science B.V. All rights reserved.

The four national amorphous silicon research teams held their 8th guidance team and technical team meetings in conjunction with the 25th Anniversary Symposium of the Institute of Energy Conversion at the University of Delaware. The teams were formed in 1992 and represent a success story of teamed photovoltaics research, integrating the efforts of industrial groups, universities and national laboratories (NIST and NREL). In this paper, we will make an attempt to summarize the accomplishments and the lessons learned during this period. © 1997 John Wiley & Sons, Ltd.

A particulate precursor of superconductive Bi2Sr2CaCu2 oxide dispersed in an aqueous poly(vinyl alcohol) solution, was shaped into fibres. The fibres fired at a Tmax of 850 °C were very porous. Although they exhibited a Meissner on-set transition at 85 K, electrical resistance at 0.02 Acm−2 at 77 K was still not zero. Exposing the porous fibres to a temperature slightly above their melting point densified them. The treated fibres had a sharp Meissner transition and could carry critical current (Jc) as high as 331 A cm−2 at 77 K. They were also more resistant to the effect of a magnetic field than the fibre fired at 850 °C. Eighty percent of the treated fibre was found to be the superconductive 2 2 1 2 phase.

The effect of trace carbon impurities on the density of states in amorphous silicon has been studied. The deep-defect densities and mobility-gap electronic structure were characterized with electron-spin resonance, drive-level capacitance profiling, and transient photocurrent measurements. Good quantitative agreement in the defect densities deduced from these three methods has been found. This implies a ratio between charged and neutral dangling bonds of at most 2 to 1. Light-induced changes in the mobility-gap electronic structure were also investigated in these films. A small but significant increase in the density of light-induced defects was observed for samples with carbon impurities at the 1 at. % level. The time to saturation of the light-induced degradation for the carbon containing samples was also significantly increased. We discuss the interpretation of these results in terms of two possible mechanisms: either from the presence of carbon-related precursor sites, or by widening of the band gap with carbon alloying.

Defects in the band gap of CuIn1−xGaxSe2 have been characterized using transient photocapacitance spectroscopy. The measured spectra clearly show response from a band of defects centered around 0.8 eV from the valence band edge as well as an exponential distribution of band tail states. Despite Ga contents ranging from Ga/(In+Ga)=0.0 to 0.8, the defect bandwidth and its position relative to the valence band remain constant. This defect band may act as an important recombination center, contributing to the decrease in device efficiency with increasing Ga content.

Hydrogenated amorphous-silicon (a-Si:H) is grown by hot-wire chemical vapor deposition (HWCVD) at deposition rates (Rd) exceeding 140 A/s (∼0.8 μm/min). These high rates are achieved by using multiple filaments and deposition conditions different than those used to produce our standard 20 A/s material. With proper deposition parameter optimization, an AM1.5 photo-to-dark-conductivity ratio of 105 is maintained at an Rd up to 130 A/s, beyond which it decreases. In addition, the first saturated defect densities of high Rd a-Si:H films are presented. These saturated defected densities are similar to those of the best HWCVD films deposited at 5–8 A/s, and are invariant with Rd up to 130 A/s.

High-hydrogen-diluted films of hydrogenated amorphous Si (a-Si:H) 0.5 μm in thickness and optimized for solar cell efficiency and stability, are found to be partially microcrystalline (μc) if deposited directly on stainless steel (SS) substrates but are fully amorphous if a thin n layer of a-Si:H or μc-Si:H is first deposited on the SS. In these latter cases, partial microcrystallinity develops as the films are grown thicker (1.5–2.5 μm) and this is accompanied by sharp drops in solar cell open circuit voltage. For the fully amorphous films, x-ray diffraction (XRD) shows improved medium-range order compared to undiluted films and this correlates with better light stability. Capacitance profiling shows a decrease in deep defect density as growth proceeds further from the substrate, consistent with the XRD evidence of improved order for thicker films.

This report summarizes the development and evaluation of higher-bandgap absorbers in the CIS alloy system. The major effort focused on exploring suitable absorbers with significant sulfur alloying in collaboration with Shafarman's group at the Institute of Energy Conversion. Three series of samples were examined; first, a series of quaternary CuIn(SeS)2-based devices without Ga; second, a series of devices with pentenary Cu(InGa)(SeS)2 absorbers in which the Se-to-S and In-to-Ga ratios were chosen to keep the bandgap nearly constant, near 1.52 eV. Third, based on the most-promising samples in those two series, we examined a series of devices with pentenary Cu(InGa)(SeS)2 absorbers with roughly 25 at.% S/(Se+S) ratios and varying Ga fractions. We also characterized electronic properties of several wide-bandgap CuGaSe2 devices from both IEC and NREL. The electronic properties of these absorbers were examined using admittance spectroscopy, drive-level capacitance profiling, transient photocapacitance, and transient photocurrent optical spectroscopies. The sample devices whose absorbers had Ga fraction below 40 at.% and S fractions above 20 at.% but below 40% exhibited the best electronic properties and device performance.

The thermal enhancement of tetraplatin (racemic, d-trans and l-trans isomers) and carboplatin was studied as a function of temperature in vitro in JM, a human acute lymphoblastic leukaemia cell line. Exponentially growing JM cells were exposed to tetraplatin (0-10 micrograms/ml) or carboplatin (0-45 micrograms/ml) for 1 h at 37-43 degrees C in 1 degree C increments. Graphic analysis demonstrated a breakpoint for the onset of thermal enhancement for tetraplatin at 40 degrees C. Thermal enhancement was maximal at 42 degrees C with no significant increase at 43 degrees C. The tetraplatin thermal enhancement ratio (TER) was 2.7 at 42 degrees C. The TERs for d and l isomers at 41.8 degrees C were not significantly different. The relationship of TER to temperature for carboplatin closely paralleled that of tetraplatin. The implications of these results are discussed in the context of tetraplatin's unique properties.

Voltage-pulse stimulated capacitance transient measurements on a series a-Si:H/a-Si, Ge:H devices disclose a large density of hole traps at or very close to the heterojunction interface. The transient signal magnitude is independent of temperature or applied bias, ruling out charge polarization effects or a defect creation process caused by the filling pulse. While the areal density of such hole traps is considerable (within a factor of 2 of 1011 cm−2 for all samples) these traps do not appear to behave as recombination centers. Also, the treatment of the a-Si:H/a-Si,Ge:H interface during growth can significantly alter the concentration of these traps.

A major effort during this subcontract period has been to evaluate the microcrystalline Si material under development at United Solar Ovonics Corporation (USOC). This material is actually a hydrogenated nanocrystalline form of Si and it will be denoted in this report as nc-Si:H. Second, we continued our studies of the BP Solar high-growth samples. Third, we evaluated amorphous silicon-germanium alloys produced by the hot-wire chemical vapor deposition growth process. This method holds some potential for higher deposition rate Ge alloy materials with good electronic properties. In addition to these three major focus areas, we examined a couple of amorphous germanium (a-Ge:H) samples produced by the ECR method at Iowa State University. Our studies of the electron cyclotron resonance a-Ge:H indicated that the Iowa State a Ge:H material had quite superior electronic properties, both in terms of the drive-level capacitance profiling deduced defect densities, and the transient photocapacitance deduced Urbach energies. Also, we characterized several United Solar a Si:H samples deposited very close to the microcrystalline phase transition. These samples exhibited good electronic properties, with midgap defect densities slightly less than 1 x 1016 cm-3 in the fully light-degraded state.

We report the deposition of improved hydrogenated amorphous silicon germanium (a-SiGe:H) films by the hot wire CVD (HWCVD) technique using a tantalum filament operating at a low temperature. We gauge the material quality of the a-SiGe:H films by comparing infrared, small angle X-ray scattering (SAXS), photocapacitance and conductivity measurements to earlier results, and fabricate single junction n-i-p solar cell devices using these i-layers.

The electronic properties of hydrogenated nanocrystalline silicon (nc-Si:H) were studied using junction capacitance methods. Drive-level capacitance profiling (DLCP) measurements revealed significant differences for nc-Si:H layers deposited under constant hydrogen dilution compared to those deposited using hydrogen profiling, with lower DLCP densities in the latter case. Transient photocapacitance (TPC) measurements revealed the mixed-phase nature of these materials. It disclosed spectra that appeared quite microcrystalline-like at lower temperatures, but more similar to a-Si:H at higher temperatures where the minority carrier collection is higher in the nanocrystalline component of these samples. This then suppresses the TPC signal from this component compared to the a-Si:H component. In contrast, because transient photocurrent signals are enhanced by the additional minority carrier collection, those spectra appear microcrystalline like at all temperatures. We also investigated the effects of light-induced degradation in these devices. This caused a dramatic decrease in hole collection, similar to that caused by reducing the measurement temperature of the samples. However, the light exposure did not appear to increase the deep defect density (dangling bonds).

Sodium reduction is efficacious for primary prevention of hypertension, but the feasibility of achieving this effect is unclear. The objective of the paper is detailed analyses of adherence to and effects of the sodium reduction intervention among overweight adults in the Trials of Hypertension Prevention, Phase II. Sodium reduction (comprehensive education and counselling about how to reduce sodium intake) was tested vs no dietary intervention (usual care) for 36-48 months. A total of 956 white and 203 black adults, ages 30-54 years, with diastolic blood pressure 83-89 mmHg, systolic blood pressure (SBP)

This report describes our experimental studies which have been concentrated in roughly five areas. Specifically: (1) We have examined a?Si:H grown very close to the microcrystalline phase boundary, so-called''edge material,'' to help understand why such material is more stable with respect to light-induced degradation; (2) We have also studied the electronic properties, and degradation characteristics of mixed phase material that is mostly a?Si:H, but which contains a significant microcrystalline component; (3) We have examined the electronic properties of high deposition rate material. These studies have included both moderately high deposition rate material (up to 6/s) produced by the PECVD growth method, and extremely high deposition rate material (up to 130/s) produced by the HWCVD growth method. (4) We have examined series of a-Si,Ge:H alloys from several sources. In one extensive series of studies we examined low Ge fraction alloys in an attempt to learn more about the fundamentals of degradation in general. In a couple other studies we evaluated the properties of a-Si,Ge:H alloys produced by methods we had not previously examined. (5) Finally, for three different types of samples we compared basic material properties with companion cell performance data. This was carried out in each case on series of samples for which one or more specific deposition parameters were varied systematically.

An overriding theme of the work described in this report has been the effect of partial crystallinity, or the approach to partial crystallinity, on the electronic properties of a-Si:H. This includes, of course, how degradation or the relative stability of these films is affected by the approach to, or onset of, microcrystallinity. The authors first discussed the results on a set of samples produced by dc reactive magnetron sputtering, obtained in collaboration with John Abelson's group at the University of Illinois, for which they demonstrated the existence of a small, but significant, microcrystalline component. For these films, the degradation kinetics was found to be quite unusual; however, it could be well accounted for by a model that postulated two phases of degrading material. One was a-Si:H host material of good quality and the other was a more defective component associated with boundary regions near the microcrystallites. The sub-band-gap photocapacitance measurements on these films also indicated the existence of a distinct feature (a ``shoulder'' with a threshold near 1.1 eV) that signaled the presence of the microcrystalline phase. The second set of samples investigated were produced by Uni-Solar, deposited under conditions of high hydrogen dilution, very close to but just below the microcrystalline phase boundary. Here they found that the defect density following light-induced degradation decreased as the film thickness increased. Corroborating their findings with X-ray diffraction results obtained by Don Williamson on sets of similar films, the authors concluded that the films were becoming more ordered and less defective just prior to the onset of a detectable microcrystalline component. Furthermore, they found that at almost exactly the conditions that Williamson found XRD evidence for the onset of microcrystallinity, they found the appearance of the distinctive ``shoulder'' in the sub-band-gap photocapacitance spectra. Third, they investigated two sets of samples where the deposition rate had been varied to include samples grown at moderate to high rates. In one set of samples, produced at ETL, samples deposited under H{sub 2} dilution were found to exhibit extremely low deep defect densities and narrow Urbach tails, indicating films of exceptional quality. The photocapacitance spectra for these films were found to contain evidence for a small degree of microcrystallinity. In another set of samples, produced at UniSolar, they found evidence for increasing defect densities plus somewhat larger Urbach energies for the films deposited at higher rates. This is consistent with the fact that the photovoltaic device performance is significantly poorer for the higher deposition rate material. Finally, they discussed the general issue of deep defect densities in the a-Si,Ge:H alloys. They again demonstrated how well the deep defect densities in such samples from several sources could be fit using the spontaneous bond-breaking model of Martin Stutzmann. This implies that such state-of-the-art alloy films have been optimized in a quantifiable sense. They also found that the increase in deep defect density with small amounts of P and B dopants could also be reproduced reasonably well by modifying the spontaneous bond-breaking model to include the extra energy terms associated with charged defects.

The work carried out by the University of Oregon Under this subcontract focused on the characterization and evaluation of low-gap (a-Si,Ge:H) alloy materials and on issues related to overall stability in the mid-gap (a-SiH) materials. First, researchers characterized an extensive series of Uni-Solar a-Si,Ge:H samples using drive-level capacitance profiling and the analysis of sub-band-gap photocapacitance and photocurrent spectra. Thus, several bands of deep defect transitions were identified. Researchers were able to verify that charged defects are responsible for the different observed defect bands in device-quality a-Si,Ge:H alloy material. Second, they reported results of their measurements on a-Si,Ge:H alloy ''cathodic'' samples produced at Harvard University; these samples were found to exhibit significantly lower defect densities in the high Ge composition range (>50at.% Ge) than alloy samples produced either by conventional glow discharge of photo-chemical vapor deposition. Third, they performed voltage pulse stimulated capacitance transient measurements on a-Si:H/a-Si,Ge:H heterostructure samples to look for carrier trapping states that might be associated with this interface; they found there was a clear signature of trapped hole emission extending over long times associated specifically with the interface itself in concentrations of roughly 10{sup 11} cm{sup -2}. Fourth, researchers reported the results on several hot-wire a-Si:H samples produced with varying hydrogen levels. Their studies indicate that hot-wire-produced a-Si:H, with H levels between 2-5at.%, should lead to mid-gap devices with superior properties. Finally, they discussed some results on glow-discharge material, as well electron-cyclotron-resonance-deposited a-Si:H grown under hydrogen dilution conditions, and confirmed that, in terms of deep-defect creation, such films exhibited improved stability compared to conventional glow-discharge material.

We review the progress of the teamed amorphous silicon research activities sponsored by NREL Thin-Film Partnership and by EPRI. We summarize the technical progress made since the last published report and attempt to assess the operational efficiency of the teams and the value of teaming.

We have applied the Drive-Level Capacitance Profiling (DLCP) method to n-i-p a-Si:H diodes to characterize the mid-gap defect densities in the i layer. Our results show that there are no significant changes in the drive-level densities, Ndl, in n-i-p diodes and p+-i-m as well as n-i-m Schottky diodes, which indicates that DLCP can directly provide reliable energy distribution and spatial distribution of the mid-gap defects in the n-i-p device. We have found that the ratio of Ndl to ND*, the ionized defect density determined by hole onset measurement, is changed with the deposition conditions; it is 3 for standard samples, 2 for helium diluted samples and 6 for hydrogen diluted samples. These results indicate that there may be different defect distributions in these materials, which suggest the ratio of charged (D-) density to the neutral defect (D0) density may be altered when growth conditions are varied.

Significant progress has been made in the area of multi-junction solar cells and stabilized efficiencies of 11.2% and 10.2% have been reported for small-area cells and large-area panels, respectively. The authors have performed voltage-pulse stimulated capacitance transient measurements along with other junction capacitance measurements on a-Si:H/a-Si,Ge:H heterostructures deposited on p-type crystalline silicon using the glow discharge technique. For a filling pulse that puts the c-Si/a-Si:H junction in forward bias, the capacitance transients consist of two components--a fast component corresponding to electron emission and a slow component corresponding to hole emission. For a fixed starting reverse bias, they have found that the density of trapped holes is proportional to the product of the filling pulse voltage and the filling pulse width and reaches a saturation value at a certain value of the product. These hole traps appear to reside at or very close to the a-Si:H/a-Si,Ge:H interface. The estimated trap concentration is around 10{sup 11}/cm{sup 2} and is independent of temperature and applied bias, ruling out the creation of the traps during the filling pulse. The authors also report results on samples in which a-Si:H/a-Si,Ge:H interface has been modified during growth to alter the concentration of the hole traps.

Etanercept, a soluble tumour necrosis factor α (TNFα) receptor fusion protein, may be combined with methotrexate (MTX) in severe rheumatoid arthritis (RA).1 Etanercept is generally given to patients treated with MTX who have an inadequate response. However, MTX could be introduced to patients who have already been treated with etanercept and who initially had a good response but subsequently a secondary failure. It was decided to evaluate, in an open prospective study, the clinical, biological, and functional benefit of the secondary addition of MTX in patients with RA with an inadequate response to etanercept alone. A cohort of 93 patients with active and severe RA was observed for a mean duration of 8 months (range 1–20). The group comprised 75 women, 18 men, with an average age of 49.5 years (range 17–75), and positive rheumatoid factor …

The work performed for this contract continued investigations of the origins of metastable effectS in a-Si:H through three kinds of studies: (1) the effect of carbon impurities in a-Si:H samples at low concentrations using drive-level capacitance profiling measurements on samples whose carbon content was intentionally modulated spatially during growth, (2) the characterization of metastable states in n-type doped a Si:H samples caused by quench cooling and by light-soaking with partial annealing, and (3) the use of depletion-width-modulated ESR spectroscopy together with junction capacitance spectroscopy to investigate deep defect states for various metastable states of a 10- and an 80-Vppm PH{sub 3}-doped a-Si:H sample.

Depletion-width-modulated (DWM) ESR spectroscopy and junction capacitance techniques were employed to investigate deep-defect states in various metastable states of 10 and 80 Vppm PH3-doped a-Si:H. From the joint application of these methods, we enumerate the change in total spin and charge associated with emission of carriers from states within the mobility gap resulting from a modulated bias voltage. We conclude that the magnitude of the modulated spin-to-charge ratio is nearly unity, consistent with earlier similar investigations of n-type a-Si:H, where the D-/D0 transition was shown to dominate. Furthermore, we present results of a previously unreported “quadrature” DWM-ESR signal of large magnitude under a wide range of experimental conditions. Similar related effects are also reported from spin- and capacitance-transient studies. These results clearly demonstrate that the changes in spin significandy lag corresponding changes in charge.

Capacitance vs. temperature, deep-level transient spectroscopy (DLTS), and transient photocapacitance spectroscopy have been used to investigate the amorphous-crystalline silicon interface region of a device made of hydrogenated amorphous silicon deposited on a lightly doped n-type crystalline silicon.By comparing our results between substrates with and without oxide contamination with those in a earlier study, we have been able to correlate the effects of substrate preparation on the density of interface states.

INTRODUCTION 403 DEFINITION AND EXTENT OF REVIEW 404 HISTORICAL PERSPECTIVE 404 Discovery of Bound Auxins 404 THE NATURALLY OCCURRING BOUND AUXINS 406 Chemical Forms 406 The esters of Zea mays 406 Other naturally occurring conjugates ....... 407 Sites of Synthesis of Bound Auxins 409 Enzymatic Synthesis 409 Enzymatic Hydrolysis 411 Function of Naturally Occurring Auxin Conjugates 413 Storage of lAA 413 Protection of auxin against peroxidative attack ......... 413 Transport of indole auxins within the plant ....... 414 Hormonal homeostasis 41:; CONJUGATES FORMED FROM APPLIED AUXINS AND GROWTH REGULATORS 416 Chemical Forms 416 Enzymatic Synthesis 418 Enzymatic HydrolYSiS 418 Control of Conjugate Formation 419 Applications 419 CHEMICAL SYNTHESIS OF IAA CONJUGATES 420 CONCLUSIONS 421

Recent results obtained by applying two methods, drive-level capacitance profiling and transient photocapacitance spectroscopy, to the study of deep defects in undoped hydrogenated amorphous silicon (a-Si:H) are reviewed. From the first method, we illustrate how it is possible to obtain the density, energy distribution, electronic occupation and spatial distribution of light-induced metastable deep defects for a set of hydrogenated amorphous silicon samples with well-characterized impurity levels. These results strongly suggest an extrinsic carbon-related component to the degree of instability in some films. We also illustrate how a detailed comparison between transient photocapacitance and photocurrent spectra provides specific information about gap state dynamics and carrier-transport processes in a-Si:H films and, in particular, metastable changes in those properties.

From junction-capacitance measurements we have determined the density, energy distribution, and electronic occupation of light-induced metastable defects near the midgap for a set of nearly intrinsic hydrogenated-amorphous-silicon samples with well-characterized impurity levels. In particular, we have inferred the changes in both neutral and negative dangling bonds for a series of isochronal anneals between the light-soaked state and full-dark-annealed state of each sample. Our results indicate that the dominant defect-creation mechanism is not Si--Si bond breaking, and also indicate the existence of at least two quasi-independent metastable defect-creation processes.

We have applied transient photocapacitance and transient junction photocurrent measurements to the study of undoped hydrogenated amorphous silicon (a‐Si:H) films, and find that the electronic optical transition from the dominant deep defect is very similar in energy to the D−→D0+e optical transition identified in n‐type doped a:Si:H films. In addition, we have observed a competing hole thermal transition, and we have obtained estimates of its thermal emission rate and of the thermal gap. We have used the difference between photocapacitance and photocurrent in the valence‐band tail region to determine the quantity (μτ)hNT at different temperatures. Finally, we have observed that light‐induced metastable defects have a hole capture cross section significantly larger than that of the intrinsic defects.

We have observed a reversible photoinduced modification of the bulk density of gap states in a‐Si:H associated with the Staebler–Wronski effect. A detailed numerical analysis of diode admittance and deep level transient spectroscopy measurements shows that the changes in these properties after illumination can be explained by a lowering of the bulk Fermi level and an increase in the density of states below midgap.

We have developed a new method of capacitance profiling which is greatly superior to standard profiling techniques for materials like amorphous silicon that contain high densities of deep gap states. This technique, which makes use of the variation of the junction capacitance with the alternating voltage amplitude, is largely immune to the effects of surface states and is readily interpreted in terms of a simple integral over the density of states, g(E). This method is applicable to both doped and semi‐insulating undoped samples.

We have employed junction photocapacitance and thermal transient capacitance measurements in n‐type doped hydrogenated amorphous silicon and have identified, within each of a series of samples, the optical transitions: D−→D0+e and D0→D++e, and the thermal transitions: D−→D0+e and D+→D0+h, where D−, D0, and D+ denote the three charge states of Si dangling bond defect. We have also correlated the optical and thermal transitions associated with the valence bandtail states. Lattice relaxation energies are found to be less than 0.1 eV for the dangling bond transitions, but as large as 0.5–0.6 eV for the valence bandtail states. We also determined a value of Ueff for the D−/D0 splitting of 0.24±0.07 eV.

Cigarette smoking histories were compared to carboxyhemoglobin and serum thiocyanate concentrations obtained from 426 smokers and 191 non-smokers. The mean levels of both carboxyhemoglobin and serum thiocyanate wefe significantly higher among cigarette smokers and correlated with number of cigarettes smoked per day. The specificity of both procedures was 81 per cent, and serum thiocyanate had a higher sensitivity (93 per cent vs. 83 per cent), making it potentially more suitable for use as an index of cigarette smoking.

We have carried out a detailed study of the competition between optical and thermal transitions from mobility gap states in hydrogenated amorphous silicon films. By employing transient photocapacitance and capacitance DLTS techniques on n-type doped films, we have observed the dangling bond optical transitions:D− → D0 + e and D0 → D+ + e, and the thermal transitions: D− → D0 + e and D0 → D0 + h. From the transition energies we have determined that the degree of lattice relaxation for dangling bonds does not exceed 0.1 eV and that the value Ueff of the D−/D0 splitting is 0.3 ± 0.1 eV. Voltage pulse photocapacitance studies on undoped films revealed the existence of the optical transition D−/D0 + e. In addition, in all films we have observed a photocapacitance signal due to optical transitions from the valence band tail states (over the energy range 1.4 to 1.9 eV) at temperatures as high as 440 K, in spite of the expectation that such shallow photoexcited holes would immediately be thermally emitted yielding no photocapacitance signal. We discuss possible explanations for this result by considering two different trapping models.

A novel a-Si:H/c-Si Schottky diode heterostructure device premits the study of the capacitive response to relatively low temperatures (25 K) and also allows fast pulse filling capture measurements of electrons from the c-Si substrate into a-Si:H defect states. These latter measurements, as well as capacitance vs. temperature measurements on these diodes, indicate a nearly zero conduction band offset (50±50 meV). We also have observed trapping of holes at the a-Si:H/c-Si valence band discontinuity ΔEv. A clear threshold for the subsequent optical release of these trapped holes by sub-bandgap light yields a value of ΔEv = 0.58±0.02 eV. Finally, photocapacitance spectra along with thermally stimulated capacitance (TSCAP) measurements indicate an anomalously large (l×1018/cm3) Gaussian-shaped defect band located a Ec - 0.88 eV with a FWHM of 0.46 eV. Model calculations of the high temperature capacitance-voltage dependence indicate these defects lie predominately within 350 Å of the a-Si:H/c-Si interface.

Yeast strains carrying a functional MAL locus are inducible for the co-ordinate synthesis of both maltase and maltose permease when grown in the presence of maltose. Whether the maltose permease is encoded by a gene at the MAL loci has remained unclear due to the lack of mutants in this function. To isolate mutants defective in maltose transport, a positive selection strategy was employed in which a number of Mal(-) mutants were obtained. Among these one Mal- mutant was isolated which had normal levels of wild-type maltase in cell free extracts. This isolate, designated MGT1, has a defect in maltose transport (malT1), detected by its markedly lower uptake of [(14)C]maltose, and by its growth on media containing 10% but not 2% maltose. Since the Km of maltose uptake is altered 10-fold in this mutant and the Vmax remains unchanged, it is suggested that the mutation alters the structure of the maltose permease involved in transport of the disaccharide into the cell rather than its regulation.A genetic analysis of the malT1 mutation shows that it is in a gene allelic to one at the MAL1 locus. Transformation of this mutant to the Mal(+) phenotype using a chimeric yeast/E. coli shuttle plasmid containing a subcloned fragment of the MAL6 locus suggests that the presence of a functional analogue of the gene encoding the maltose transport function is an integral part of the MAL6 locus as well.

The effects of light saturation on the properties of undoped a-Si:H films were studied by a new capacitance profiling technique which can be used to directly determine changes in the dangling bond density of states near midgap. Coplanar conductivity and capacitance vs. temperature measurements save the changes in activation energies for electrical conductivity. These studies indicate that, while substantial increases in the dangling bond densities are observed for most samples, the detailed behavior of the light induced changes in these films are inconsistent with the creation of such defects by breaking weak valence band tail states.

Amorphous-crystalline heterostructures composed of sub-micron thick undoped amorphous hydrogenated silicon deposited on lightly doped n-type crystalline silicon substrates have been studied by capacitance techniques. Capacitance vs. temperature scans along with model calculations on these samples are used to investigate the amorphous-crystalline silicon interface region. We deduce the existence of an anomalously defective a-Si:H region extending roughly 1000 Angstroms from the growth interface. We also deduce considerable fluctuations in the interface potential which indicate lateral variations of ±20% in the defect distritribution over a 1000 Angstrom scale.

A full numerical analysis of the nonequilibrium response of a Schottky‐barrier space charge region for an arbitrary density of states, g (E), has been developed. We apply these methods to our measurements of Deep Level Transient Spectroscopy (DLTS) in n‐type a–Si:H. This gives a quantitative picture of g (E) extending, in some cases, over 70% of the mobility gap. This analysis gives qualitative agreement to our previous interpretation of similar data but produces more reliable quantitative results and, in addition, discloses features in g (E) not apparant from our simpler approach. In particular it identifies the correct value of the thermal emission energy gap (near 2.0 eV).

We have studied the light induced instability problem in hydrogenated amorphous silicon using junction capacitance techniques. These techniques are used to examine specific changes in the density of gap states, and occupation of gap states, for undoped a-Si:H samples after light saturation and for a series of partial anneal “states” which culminate in the original dark annealed state (state A). We find that the observed changes in the metastable occupied and unoccupied defects contradict the Si-Si bond breaking model and indicate at least two defect creation processes. In several samples we also find clear evidence that the metastable defect distribution near midgap has a slightly different energy distribution than the stable deep state (dangling bond) distribution. At the same time, these results seem to be qualitatively consistent with many aspects of recent ESR and optical absorption studies of metastable defect creation. We discuss these findings in terms of alternative possible microscopic models for metastable effects in a-Si:H.

A large number of different measurements which are sensitive to gap states in hydrogenated amorphous silicon have been analyzed over the past few years in an attempt to determine the generic form of the density of states function within the mobility gap of this material. These results tend to support one or the other of two quite different functions: a) the field-effect density of states, or b) the DLTS density of states. In this paper we review the evidence for the density of states function determined for a-Si:H by deep-level transient spectroscopy (DLTS). We argue that the strong and growing support for the DLTS picture is evidence that the two general forms for the density of states relate to different spatial regions of the specimen, e.g. interface vs. bulk properties.

This report contains results of Phase II of research to investigate metastable defect creation processes in hydrogenated amorphous silicon. The study relied primarily on junction capacitance methods to monitor the changes in deep defect distribution. Results obtained applying capacitance profiling methods to the study of undoped samples are summarized and indicate a significant role of extrinsic impurities in the degree of light-induced degradation that occurred. Photocapacitance transient measurements were taken to investigate changes in undoped a-Si:H. Studies were also begun of metastable changes in n-type doped a-Si:H samples by using both the drive-level capacitance profiling technique and transient photocapacitance methods, and results are described. 22 refs., 10 figs., 1 tab.