Amorphous Silicon Alloys Research Articles

PECVD/ECR/HWCVD Multichamber System with Robotic Substrate Handling System for deposition of Thin Film Electronic Devices

The present work reports on progress in the design of modular UHV cluster tool multichamber systems. A wide range of processes has been implemented in the Deposition Process Chambers (DPC's), including rf-PECVD, vhf-PECVD, ECR-PECVD and HWCVD. A wide range of intrinsic and doped amorphous and microcrystalline silicon and silicon alloy materials have been produced and have been used in the fabrication of several types of electronic devices such as solar cells, Light Emitting Devices (LED's), Thin Film Transistors (TFT's), etc. using multichamber systems at several laboratories worldwide.

TCAD studies of novel nanoplate amorphous silicon alloy thin-film solar cells

A novel nanoplate-structured thin-film solar cell was investigated that could solve the conflict between light absorption and carrier transport in a p-type amorphous silicon carbide (a-SiC)/i-type amorphous silicon germanide (a-SiGe)/n-type amorphous silicon (a-Si) thin-film solar cell. This structure has an n-type a-Si nanoplate array on the substrate, a-SiC p-layer, and an a-SiGe i-layer which are sequentially grown along the surface of each n-type a-Si nanoplate. Under illumination by sunlight, light is absorbed along the vertical direction of the nanoplate, while the carrier transport is along the horizontal direction. The nanoplate structure may absorb most of the sunlight and provide a thinner film for the effective transport of photon-generated carriers as compared to the conventional planar structure.

Flexible, Lightweight, Amorphous Silicon Based Solar Cells on Polymer Substrate for Space and Near-Space Applications

ABSTRACTUnited Solar Ovonic has leveraged its history of making amorphous silicon solar cells on stainless steel substrates to develop amorphous silicon alloy (a-Si:H)-based solar cells and modules on ∼25 μm thick polymer substrate using high-throughput roll-to-roll deposition technology for space and near-space applications. The solar cells have a triple-junction a-Si:H/a-SiGe:H/a-SiGe:H structure deposited by conventional plasma enhanced CVD (PECVD) using roll-to-roll processing. The cells have distinct advantages in terms of high specific power (W/kg), high flexibility, ruggedness, rollability for stowage, and irradiation resistance. The large area (23.9 cm x 32.1 cm) individual cells manufactured in large quantity can be readily connected into modules and have achieved initial, 25 °C, AM0 aperture-area efficiency of 9.8% and initial specific power of 1200 W/kg. We have conducted light-soak studies and measured the temperature coefficient of the current-voltage characteristics to determine the stable values at an expected operating temperature of 60 °C. The stable total-area efficiency and specific power at 60 °C are 7.2% and 950 W/kg, respectively. In this paper, we review the challenges and progress made in development of the cells, highlight some applications, and discuss current efforts aimed at improving performance.

Preface: Phys. Status Solidi C 7/3‐4

Preface: Phys. Status Solidi C 7/3‐4

An overview of uncooled infrared sensors technology based on amorphous silicon and silicon germanium alloys

AbstractAt the present time there are commercially available large un‐cooled micro‐bolometer arrays (as large as 1024×768 pixels) for a variety of thermal imaging applications. Different thermo‐sensing materials have been employed as thermo sensing elements as Vanadium Oxide (VOx), metals, and amorphous and polycrystalline semiconductors. Those materials present good characteristics but also have some disadvantages. As a consequence none of the commercially available arrays contain optimum pixels with an optimum thermo‐sensing material. This paper reviews the development of the un‐cooled bolometer technology and the research achievements on this area, with special attention on the key factors that would lead to improve the pixels performance characteristics. The work considers the R&D of microbolometer arrays and the integration with MEMS and IC technologies. A comparative study with the state of the art and data reported in literature is presented. Finally, further directions of uncooled bolometer based in thin films materials are also discussed in this paper. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Boron doped amorphous diamond window layer deposited by filtered arc for amorphous silicon alloy p–i–n solar cells

In order to improve the conversion efficiency of amorphous silicon (a-Si:H) alloy p–i–n solar cells, the original p-a-Si:H window layer is substituted by the boron-doped amorphous diamond (a-D:B) films deposited using filtered cathodic vacuum arc technology. The microstructural, optical and electrical properties as functions of the boron concentrations in the films were, respectively, evaluated by an X-ray photoemission spectroscopy, an ultraviolet–visible spectrometer and a semiconductor parameter analyzer. The photovoltaic parameters of the solar cell modules were also detected as functions of boron concentration. It has been shown that the conductive a-D:B films could be obtained and still remained a wide optical gap. The p–i–n structural amorphous silicon solar cell using the a-D:B window layer increased the conversion efficiency by a roughly 10% relative improvement compared to the conventional amorphous silicon solar cell because of the enhancement of short wavelength response.

The US Department of Energy's Working Group on Photoelectrochemical Hydrogen Production: Promoting Technology-Enabling Breakthroughs in Semiconductor Materials Research

AbstractPhotoelectrochemical (PEC) hydrogen production, using sunlight to split water, is an important enabling technology for a future “Green” economy which will rely, in part, on hydrogen as an energy currency. The traditional semiconductor-based PEC material systems studied to date, however, have been unable to meet all the performance, durability and cost requirements for practical hydrogen production. Technology-enabling breakthroughs are needed in the development of new, advanced materials systems, and toward this end, the U.S. Department of Energy’s Working Group on PEC Hydrogen Production is bringing together experts in analysis, theory, synthesis and characterization from the academic, industry and national-laboratory research sectors. Key Working Group activities, as described in this paper, include performing techno-economic analyses of large-scale PEC production systems and establishing standardized testing and screening protocols for candidate PEC materials systems. In addition, a number of Working Group “Task Forces” are focused on advancing critical PEC materials theory, synthesis and characterization capabilities for application in the research and development of broad-ranging materials systems of promise, including complex metal-oxide and -nitride compounds, amorphous silicon alloys, III-V semiconductors and the copper chalcopyrites. The current status of Working Group activities and progress is summarized.

Inverse approach to atomistic modeling: Applications to a-Si:H and g-GeSe2

We discuss an inverse approach for atomistic modeling of glassy materials. The focus is on structural modeling and electronic properties of hydrogenated amorphous silicon and glassy GeSe2 alloy. The work is based upon a new approach ‘experimentally constrained molecular relaxation (ECMR)’. Unlike conventional approaches (such as molecular dynamics (MD) and Monte Carlo simulations(MC), where a potential function is specified and the system evolves either deterministically (MD) or stochastically (MC), we develop a novel scheme to model structural configurations using experimental data in association with density functional calculations. We have applied this approach to model hydrogenated amorphous silicon and glassy GeSe2. The electronic and structural properties of these models are compared with experimental data and models obtained from conventional molecular dynamics simulation.

Photoinduced absorption in B-doped hydrogenated amorphous silicon alloys applied to all-optical modulators

All-optical modulators have been fabricated, based on the infrared photoinduced absorption produced within an optical waveguide upon visible light illumination. The modulation data are analyzed by means of simulation software based on a numerical mode solver. It is found that the modulation depth increases for pump illumination energy closer to the energy gap of the guiding material, while illumination at varying intensity shows a sublinear dependence of the photoinduced absorption. The results are discussed in terms of occupation statistics of gap states. It is shown that the major contribution to the photoinduced signal derives from the modulation of the occupation of tail states under illumination. Modeling of the phenomenon allows one to predict the behavior of a given device and opens the way to practical applications.

Investigation of the emitter band gap widening of heterojunction solar cells by use of hydrogenated amorphous carbon silicon alloys

The hydrogenated amorphous carbon-silicon alloys [a-SixC1−x(n):Hy] and [a-Six(n):Hy] layers were investigated in order to prove the feasibility to widen the optical band gap in emitters of the heterojunction solar cells. The alloys were fabricated by decomposition of silane (SiH4), phosphine (PH3), methane (CH4), and hydrogen (H2), using a plasma enhanced chemical vapor deposition. Particularly, we focused on the incorporation of hydrogen and carbon within the resulting [a-SixC1−x(n):Hy] and [a-Six(n):Hy] films, which later form the emitter. The corresponding local vibrational modes of Si−Hx, C−H, and the corresponding network have been analyzed by μ-Raman spectroscopy. The addition of carbon degrades the photoelectronic properties in the emitter layer. This deterioration can be minimized by H dilution. The resulting optical band gap EG as well as the thickness of the emitter were determined by spectroscopic ellipsometry. It was confirmed that the band gap EG can be tailored by using an appropriate gas mixture during the decomposition. Furthermore, we analyzed the I−V characteristics of the prepared heterojunction solar cells. A trade-off between the electrical defects density and the optical losses induced an improvement of the I−V characteristics with increasing carbon and hydrogen concentration in the feedstock during the deposition.

Image and color recognition using amorphous silicon p–i–n photodiodes

Large area hydrogenated amorphous silicon single and stacked p–i–n structures with low conductivity doped layers are proposed as monochrome and color image sensors. The layers of the structures are based on amorphous silicon alloys (a-Si x C 1− x :H). The current–voltage characteristics and the spectral sensitivity under different bias conditions are analyzed. The output characteristics are evaluated under different read-out voltages and scanner wavelengths. To extract information on image shape, intensity and color, a modulated light beam scans the sensor active area at three appropriate bias voltages and the photoresponse in each scanning position (“sub-pixel”) is recorded. The investigation of the sensor output under different scanner wavelengths and varying electrical bias reveals that the response can be tuned, thus enabling color separation. The operation of the sensor is exemplified and supported by a numerical simulation.

Large-area Deposition of Amorphous Silicon Alloys Using a Roll-to-roll Operation

AbstractLarge-area deposition of thin-film amorphous silicon alloy triple-junction solar cells on lightweight and flexible stainless steel substrate is described. The proprietary roll-to-roll operation enables continuous depositions of sophisticated multi-layer structures. The deposition methods include sputtering and plasma-enhanced chemical vapor depositions. Spectrumsplitting triple-junction solar cell design, manufacturing processes, and product applications are presented.

Drift-mobility measurements and mobility edges in disordered silicons

Published electron and hole drift-mobility measurements in hydrogenated amorphous silicon(a-Si:H), amorphous silicon alloys (a-SiGe:H and a-SiC:H), and microcrystalline silicon(μc-Si:H) are analysed in terms of the exponential bandtail trapping model. Athree-parameter model was employed using an exponential bandtail widthΔE, the band mobilityμ0, and the attempt-to-escapefrequency ν. Low-temperature measurements indicate a value aroundμ0 = 1 cm2 V−1 s−1 for both the conduction and valence bands over the entire range of materials.High temperature-measurements for electrons in a-Si:H suggest a larger value of7 cm2 V−1 s−1. These properties and those of the frequencyν are discussed as possible attributes of a mobility edge.

In Situ XRD and Electrochemical Study of the Reaction of Lithium with Amorphous Silicon

Silicon is a very promising candidate to replace graphite as the anode in Li-ion batteries because of its very high theoretical capacity. It has not yet made its way into commercial cells because of severe problems with the charge and discharge cycling of the material. It seems that amorphous silicon and amorphous silicon-containing alloys exhibit much improved cycling performance. Therefore, it is desirable to fully understand the reaction of Li with a-Si. To this end, an in situ X-ray diffraction study of the reaction of lithium with a-Si has been performed. The results confirm that a new crystalline phase is formed below 30 mV vs. as first reported by Obrovac and Christensen in an article published in Electrochemical and Solid-State Letters. However, the crystalline phase only forms for films of a-Si above a critical thickness of about 2 μm. © 2004 The Electrochemical Society. All rights reserved.

Characterization of microstructure in amorphous and microcrystalline Si and related alloys by effusion of implanted helium

Effusion measurements of implanted helium are used for characterization of microstructure in amorphous and microcrystalline silicon and related alloy films. The results support the usefulness of the method and suggest the presence of a strongly varying void-related microstructure which depends, e.g., on the deposition method, the substrate temperature, the annealing state, doping and alloying. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Kinetics of optically excited charge carriers and defects in hydrogenated amorphous silicon and related alloys

Electron spin resonance and optically induced electron spin resonance measurements on hydrogenated amorphous silicon and germanium are useful probes of charge carriers in localized electronic states near the edges of the conduction and valence bands (band-tail states) and of defects, such as silicon and germanium dangling bonds. We review recent results on the recombination kinetics of charge carriers trapped in band-tail states. We also review recent results on the kinetics of metastable optically induced germanium dangling bonds in hydrogenated amorphous germanium. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Near infra-red photoluminescence of Nd 3+ in hydrogenated amorphous silicon sub-nitrides a-SiN x:H〈Nd〉

Neodymium-doped hydrogenated amorphous silicon sub-nitrides a-SiN x :H〈Nd〉 thin films were deposited by rf-sputtering using a Si target partially covered by metallic Nd chips and Ar+N 2+H 2 sputtering gas. Characteristic Nd 3+ near infra-red (NIR) photoluminescence (PL) was detected between 10 and 300 K with peaks at ∼935, ∼1090 and ∼1390 nm, corresponding to the intra-4f transitions 4 F 3/2→ 4 I 9/2 , 4 F 3/2→ 4 I 11/2 and 4 F 3/2→ 4 I 13/2 , respectively. Measurements using different excitation wavelengths indicate that the Nd 3+ excitation occurs through the a-SiN x :H matrix. Varying the nitrogen content x from 0 to nearly 1.3 increases the matrix bandgap. The PL efficiency is maximum when the bandgap corresponds to twice the 4 F 3/2→ 4 I 9/2 transition, indicating a defect-related energy transfer mechanism. The temperature quenching can be as low as less than a factor 3 between 10 and 300 K for 2.8 eV gap samples. Thermal annealing can enhance the PL intensity by a factor 10. Neodymium concentrations above ∼3×10 20 atoms/cm 3 slightly reduce the PL intensity probably due to excess of inactive defect centers. Along with erbium-doped amorphous silicon alloys, a-SiN x :H〈Nd〉 can be used in the development of photonic devices in the future.

Amorphous silicon based photovoltaics—from earth to the “final frontier”

We highlight the advances made in amorphous silicon alloy photovoltaic technology leading to large-scale commercial deployment. The paper discusses multijunction devices made on lightweight flexible substrates; various aspects of attaining high efficiency devices are described. The eminent role of the roll-to-roll continuous deposition technique in propelling the technology to global market is elucidated. The logical emergence of this technology as a lightweight solar-power generator for extraterrestrial application is discussed. Results of high specific power under space conditions are presented. The future of the technology in terms of both device efficiency and product efficacy are given.

Roll-to-roll manufacturing of amorphous silicon alloy solar cells with in situ cell performance diagnostics

In order to meet the price target necessary for widespread use of solar cell products, Energy Conversion Devices, Inc., ECD, has developed and commercialized a continuous roll-to-roll manufacturing technology for the production of a-Si alloy solar cells. Since the early 1980s, we have advanced this technology from a small-scale pilot machine to a large-scale production machine. In 2002, ECD commissioned a 30 MW per year machine for United Solar Systems Corp. in Auburn Hills, Michigan. The RF PECVD a-Si alloy solar cell processor, designed and built by ECD, deposits triple-junction solar cell materials consisting of nine layers of a-Si alloys in a continuous roll-to-roll operation simultaneously on six coils of 130 μm thick, 0.36 m wide, 2.6 km long stainless-steel substrate at 1 cm/s. In order to minimize production losses due to undetected deviations of production conditions and carry on a continuous program of device optimization, we have developed and are incorporating in situ cell performance diagnostic systems.

Femtosecond Far-Infrared Studies of Carrier Dynamics in Hydrogenated Amorphous Silicon and Silicon-Germanium Alloys

AbstractWe present femtosecond time-resolved studies of the photoexcited carrier response in the far-infrared spectral range in PECVD a-Si:H and a-SiGe:H thin films. The experiments are carried out using an optical pump / terahertz (THz) probe technique, in which a femtosecond pump pulse excites carriers in the sample and a time-delayed probe pulse measures the resulting change in the far-infrared optical properties as a function of time delay following the excitation. These measurements are sensitive to carrier processes at low energy, corresponding to a range of approximately 1 - 10 meV, a key energy scale in these materials. We find that the observed photoexcited carrier dynamics are consistent with trapping of carriers into band tail states on a picosecond time scale.