Amorphous Silicon Alloys Research Articles

Initiation of corrosion in metal substrates coated with plasma-deposited hydrogenated amorphous silicon alloy ceramic thin films

Amorphous hydrogenated silicon, silicon nitride, silicon–carbon and boron-doped silicon–carbon films on molybdenum and aluminum substrates were evaluated as corrosion barriers. Specimens were anodically polarized in NaCl and Na2SO4 solutions and also treated with NaCl and Na2SO4 during exposure in an environmental chamber at 85% RH at 30°C. The silicon nitride and silicon–carbon films were inert and rendered substantial protection while the silicon films showed signs of extensive deterioration. Initiation sites for localized corrosion in the silicon nitride and silicon–carbon films were found not only at film pinholes caused by spallation, but also at microcracks and in regions where visible defects could not be detected using scanning electron microscopy.

Transient photocurrent response of three-color detectors based on amorphous silicon

Color detectors based on multilayers of amorphous–silicon alloys facilitate the detection of the three fundamental components of visible light in one single pixel of a sensor array. In order to achieve sensitivity for the blue, green, and red components of light, three different bias voltages are applied to the device. By switching them sequentially the detector is read out. n-i-p-i-i-n structures with a controlled band gap and mobility–lifetime product exhibit excellent stationary properties, namely: good color separation and have dynamic behaviors above 95 dB. Besides the stationary behavior the transient response of a color detector is a further optimization criterion. The experimentally found transient photocurrent response after switching on monochromatic light at different applied bias voltages showed reasonable delay times in the range of tens of milliseconds before reaching steady state. Numerical simulations have been carried out which reproduce this characteristic behavior and facilitate a study of time dependent processes within the device, such as charge transport and storage in localized states. The delay times can be explained by the recharging of electrical defect states in the amorphous material. Consequently, the electrical potential within the device changes, which remarkably affects the carrier transport. Based on these results optimization criteria for the transient behavior of the color detectors are discussed.

Infrared vibration spectra of hydrogenated, deuterated, and tritiated amorphous silicon

This article presents infrared absorption data of amorphous silicon alloys in which the hydrogen isotopes deuterium and tritium have been substituted for hydrogen. Silicon–deuterium and silicon–tritium vibration frequencies are related to silicon–hydrogen vibration frequencies by simple mass relationships. The silicon–deuterium wagging vibration is broadened and blueshifted due to strong coupling to the amorphous silicon network vibrations.

Optimization of High Efficiency Amorphous Silicon Alloy based Triple-Junction Modules

AbstractA systematic approach has been used to scale up high efficiency 0.25cm2 active-area amorphous Si alloy based triple-junction devices to high-efficiency encapsulated modules of aperture area ~920cm2. In order to analyze the losses involved in the scale-up, intermediate aperture area, 40cm2 and 450cm2, modules have also been fabricated. The best stable active-area efficiency obtained on the small-area cells is 12.9%. The best initial efficiency of a ~920cm2 aperture area encapsulated module is 12.1%. National Renewable Energy Laboratory (NREL) has independently light soaked three of the ~920cm2 modules. They have measured a stable efficiency of 10.5% which represents a new world record. This paper presents various aspects of the large-area module work.

Amorphous Silicon Alloy Materials and Solar Cells Near the Threshold of Microcrystallinity

AbstractOne of the most effective techniques used to obtain high quality amorphous silicon alloys is the use of hydrogen dilution during film growth. The resultant material exhibits a more ordered microstructure and gives rise to high efficiency solar cells. As the hydrogen dilution increases, however, a threshold is reached, beyond which microcrystallites begin to form rapidly. In this paper, we review some of the interesting features associated with the thin film materials obtained from various hydrogen dilutions. They include the observation of linear-like objects in the TEM micrograph, a shift of the principal Si TO band in the Raman spectrum, a sharp, low temperature peak in the H2 evolution spectrum, a shift of the wagging mode in the IR spectrum, and a narrowing of the Si (111) peak in the X-ray diffraction pattern. These spectroscopic tools have allowed us to optimize deposition conditions to near the threshold of microcrystallinity and obtain desired high quality materials. Incorporation of the improved materials into device configuration has significantly enhanced the solar cell performance. Using a spectral-splitting, triple-junction configuration, the spectral response of a typical high efficiency device spans from below 350 nm to beyond 950 nm with a peak quantum efficiency exceeding 90%; the triple stack generates a photocurrent of 27 mA/cm2. This paper describes the effect of the improved materials on various solar cell structures, including a 13% active-area, stable triple-junction device.

Science and technology of amorphous silicon alloy photovoltaics

Significant advances have been made in our understanding of amorphous silicon alloy materials and solar cells that have resulted in the achievement of high cell and module efficiency. The R&D results have also been translated to production, and several manufacturers have expanded their production capacity, raising the worldwide annual capacity exceeding 25 MW. In this paper, we shall review the progress in the science and technology of amorphous silicon alloy photovoltaics and discuss the future directions.

Advances in amorphous silicon photovoltaic technology

With the advent of new multijunction thin film solar cells, amorphous silicon photovoltaic technology is undergoing a commercial revival with about 30 megawatts of annual capacity coming on-line in the next year. These new a−Si multijunction modules should exhibit stabilized conversion efficiencies on the order of 8%, and efficiencies over 10% may be obtained in the next several years. The improved performance results from the development of amorphous and microcrystalline silicon alloy films with improved optoelectronic properties and from the development of more efficient device structures. Moreover, the manufacturing costs for these multijunction modules using the new large-scale plants should be on the order of $1 per peak watt. These modules may find widespread use in solar farms, photovoltaic roofing, as well as in traditional remote applications.

Amorphous silicon alloy photovoltaic technology and applications

The last year has seen enormous advances in the science and technology of amorphous silicon alloy photovoltaic. A new world record in cell efficiency has been achieved; a large volume manufacturing plant is also operational. Innovative building-integrated products have been developed which are aesthetically pleasing and easy to install on the roof. In this paper, we give a brief summary of these developments.

Simulation and analysis of X-ray photoemission and Auger valence band spectra of hydrogenated amorphous silicon

The X-ray photoemission (XPS) and Auger electron spectra (AES) of amorphous silicon have been simulated using the semi-empirical quantum mechanical molecular modelling package, MOPAC. The calculated spectra compare well with those obtained experimentally and provide a basis for the interpretation and analysis of the features in the experimental spectra. Changes in the spectra as a result of hydrogenation of the material are also analysed. The results of this study illustrate how simulation of XPS and AES spectra can be used to obtain semi-quantitative information about the hydrogen content and the type of Si–H bonding present in amorphous silicon alloys.

A measurement set-up for photoconductivity decay experiments

Measurements of photoconductivity decay from the steady state for samples of amorphous silicon and silicon-carbon alloys were carried out using the experimental set-up with a Pockels cell as a light chopper. The developed electronic switching circuit allows measurements of decay times ranging down to microseconds. The advantage of the adopted method of light switching is that one can easily control the light intensity and photon energy. From knowledge of the photoconductivity decay time and the value of the steady state photoconductivity for a given sample, the carrier drift mobility can be calculated.

Advances in amorphous silicon alloy technology—the achievement of high-efficiency multijunction solar cells and modules

Thin-film amorphous silicon alloy technology has emerged as a strong contender for providing low-cost photovoltaic products to meet the energy needs of the 21st century. World record 14·6% initial and 13·0% stable active-area conversion efficiencies have been achieved for small-area solar cells in our laboratory using a spectral-splitting, triple-junction structure. This 0·25-cm2 device exhibited a total-area efficiency of 12·0%, as confirmed by the National Renewable Energy Laboratory. Key factors leading to this achievement will be discussed. We have also demonstrated a 10·2% stable module efficiency for a one-square-foot area. A state-of-the-art, continuous roll-to-roll production line with an annual capacity of 5 MW has been built and started production. Building-integrated photovoltaic applications are being demonstrated worldwide. Public awareness of environmental and ecological issues has provided a great opportunity for the maturing thin-film amorphous silicon alloy technology to demonstrate its viability as a major alternate energy source. © 1998 John Wiley & Sons, Ltd.

Depth-profiles in compositionally-graded amorphous silicon alloy thin films analyzed by real time spectroscopic ellipsometry

Real-time spectroscopic ellipsometry (RTSE) has been applied to characterize composition depth-profiles in compositionally-graded amorphous silicon–carbon alloy (a-Si 1− x C x :H) thin films, prepared using continuous variations in the flow ratio z( t)=[CH 4]/{[SiH 4]+[CH 4]} during r.f. plasma-enhanced chemical vapor deposition (PECVD). In order to calculate the dielectric functions of a-Si 1− x C x :H alloys for any value of x, a new parameterization of the measured dielectric functions was established using the recently-derived Tauc–Lorentz (T-L) model. This model is found to provide improved fitting of the measured dielectric functions of a-Si 1− x C x :H over the analyzed energy region of 2.2<E<4.2 eV, in comparison to previous parameterization schemes. Applying the T-L parameterization, the depth-profiles in the C-content were then analyzed using a virtual interface approximation. For several a-Si 1− x C x :H graded layers having triangular variations in the C-content (0< x<0.24) over 25–130 Å thick layers, we found very good agreement between the analyzed depth-profiles and those predicted from z( t) based on individually-deposited samples. In the depth-profile of the thinnest C-graded layer (∼25 Å), monolayer depth resolution was evident with an average compositional uncertainty of±0.009.

The Role of Charged Defects In Current Transport Through Hydrogenated Amorphous Silicon Alloys

ABSTRACTCurrent transport in metal-semiconductor-metal structures based on amorphous silicon alloys has been studied in relation to the density of dangling bond state defects. The density of defects was changed by varying alloy composition or by current stressing. We show that the change of current-voltage characteristics and activation energy with defect density and the onset of Poole-Frenkel conduction with composition require charged defects. It is found that there are more charged defects in amorphous silicon nitride (a-Si1−xNx:H) than in amorphous silicon carbide (a-Si1−xCx:H). In addition, an excess of negatively charged dangling bond defects compared to positively charged dangling bond defects is observed in a-Si1−xNx:H films. This is attributed to the presence of N4+ act as the donor states in silicon nitride. We find that the density of charged dangling bond defects can be higher than 1019cm−3.

Amorphous and Disordered Materials - the Basis of New Industries

AbstractAs in the past, materials will shape the new century. Dramatic changes are taking place in the fields of energy and information based on new synthetic materials. In energy, the generation of electricity by amorphous silicon alloy thin film photovoltaics; the storage of electricity in nickel metal hydride batteries which are the batteries of choice for electric and hybrid vehicles. In the information field, phase change memories based on a reversible amorphous to crystalline transformation are widely used as optical memories and are the choice for the new rewritable CDs and DVDs. The scientific and technological bases for these three fields that have become the enabling technologies are amorphous and disordered materials. We will discuss how disordered, multielemental, multiphase materials can throw new light upon metallic conductivity in both bulk and thin film materials. We will demonstrate new types of amorphous devices that have the ability to learn and adapt, making possible new concepts for computers.

Material Issues in the Commercialization of Amorphous Silicon Alloy Thin-Film Photovoltaic Technology

ABSTRACTTwo significant developments took place in 1997 in the field of amorphous silicon alloy photovoltaic technology. First, a world record stable cell efficiency of 13% was demonstrated using a spectral-splitting, triple-junction structure. Second, a triple-junction photovoltaic manufacturing facility of an annual capacity of 5 MW was commissioned. In order to make the transition from R&amp;D to production, critical material issues and deposition methods which ensure the lowest module cost per delivered watt needed to be evaluated. In this paper, we discuss some of these issues with special reference to the cell materials.

Effect of Excitation Frequency on the Performance of Amorphous Silicon Alloy Solar Cells

ABSTRACTWe have studied amorphous silicon alloy solar cells made by using a modified-very-highfrequency glow discharge at 75 MHz with a deposition rate of ∼6 Å/s. The solar cell performance is compared with those made from conventional glow discharge at 13.56 MHz with lower deposition rates. Cells made at ∼6 Å/s with 75 MHz showed comparable stabilized efficiency to those made at ∼3 Å/s with 13.56 MHz. The best performance, however, was obtained with ∼1 Å/s, including a stabilized 9.3% a-Si alloy single-junction cell employing conventional glow discharge technique. Using 75 MHz, we have achieved 11.1% and 10.0% initial active-area efficiencies for a-Si alloy and a-SiGe alloy n i p cells, respectively. An initial efficiency of 11.0% has also been obtained in a dual bandgap double-junction structure.

Programmable 3-Dimensional Memories Based on Current Induced Conductivity in Hydrogenated Amorphous Silicon Nitride

ABSTRACTChanges of several orders of magnitude in the low field conductivity of hydrogenated amorphous silicon alloy metal-semiconductor-metal MSM devices can be obtained by current stressing. This feature is suitable for memory applications, since the device can be programmed from an unstressed state to a stressed state with a ratio greater than 104. The change in conductivity is attributed to the creation of a large concentration of silicon dangling bond states during current stressing which form a defect band with a low activation energy for current transport. In this paper, we consider the use of current induced conductivity in hydrogenated amorphous silicon nitride (a-SiNx:H) multi-layer structures and show that, in principle, highly complex three dimensional circuits could be made. In particular, the potential of this approach is illustrated by using a simple MSMSM device with two silicon rich silicon nitride layers. An array with lxi0 elements has been fabricated and programmed to store 28 bits of binary information with three outputs per input.

Substrate imposed stress-strain effect on photoluminescence in hydrogenated amorphous silicon alloys

Hydrogenated amorphous silicon films were deposited on the unstrained and strained Si substrates by an electron cyclotron resonance plasma source. The photoluminescence spectra show that emission energies are different from each other. The redshift of photoluminescence induced by the biaxial tensile stress is increased with decreasing the temperature in the range of 0.11–0.17 eV. The stress effect also enhances the quantum efficiency significantly. The pseudomorphic growth of Si on a relaxed Si0.75Ge0.25 (100) surface provides the strain energy of about 0.17 eV. These comparable results indicate that the shift of emission energy is attributed to the stress effect perturbing the polysilane structure.

Electronic effects of ion damage in hydrogenated amorphous silicon alloys

Electronic properties of silicon rich amorphous silicon alloys were investigated using diode structures and thin films on Corning 7059 glass following the implantation of 1 MeV Ge+ ions with doses up to 2.4×1015 ions/cm2. Optical absorption measurements on hydrogenated amorphous silicon (a-Si:H) and amorphous silicon nitride (a-SiNx:H) showed that the optical band gap decreases with the implanted ion dose. The hydrogen concentration was not affected by the implantation and therefore the change in optical band gap was consistent with a broadening of the band tails as confirmed by dual beam photoconductivity measurements. Annealing studies showed that recovery of the band gap could be achieved at temperatures of ≈250 °C. This recovery was almost complete following low dose ion implantation, but a residual amount of damage remained which increased with the dose before saturating. Results of electrical measurements on metal-semiconductor barriers showed a correlation between optical and electrical behavior with ion dose. We propose a model in which the barrier height and the optical band gap vary in the same way with the ion dose.

Advances in amorphous silicon alloy cell and module technology

The low material cost and proven manufacturability of amorphous silicon (a-Si) alloy solar panels make them ideally suited for low-cost terrestrial application. A major challenge for the researchers has been how to improve the light-to-electricity conversion efeiciency. Extensive R & D efforts have resulted in a significant improvement in stable cell and module efficiencies with the achievement of 12.8% active-area cell efficiency and 10.4% module efficiency using a spectral-splitting triple-band gap, triple-cell approach. Further gains in efficiency are expected through an improved understanding of plasma chemistry and growth kinetics. In this paper, we shall discuss the progress in science and technology of a-Si alloy photovoltaics with special emphasis on the opportunities and the challenges that exist.