Effects In Amorphous Silicon Research Articles

Reversible Efficiency Variation of Tandem Amorphous/Microcrystalline Si Photovoltaic Modules in Outdoor Operation

The Staebler-Wronski effect in amorphous silicon based photovoltaic devices is responsible for degradation of their power conversion efficiency, within approximately the first one thousand hours of light soaking. Several experimental studies led to highlight the performance instability phenomena for the mentioned devices, underling that recovery and improvement of such performance are observable, by subjecting such devices (both of single-junction and tandem types) to DC reverse bias stresses under illumination, or to operation in the Maximum Power Point (MPP) under variable conditions of temperature and illumination. In this work, we present and discuss the results of novel recent outdoor tests on stabilized specimens (i.e., exposed to 1000 h extended light soaking, before our tests) of tandem amorphous/microcrystalline Si (a-Si/µc-Si) photovoltaic (PV) minimodules operating in their MPP, by analyzing the causes of the performance instability effects, systematically observed on a daily scale. During the mentioned tests, we have monitored the solar cell operating temperature and the incident solar spectrum at various times in different days to verify the effect of cell temperature and solar spectrum changes on the cell performances. The experimental results show a clear correlation between performance improvements of the photovoltaic modules and their thermal history during the outdoor tests, proving the interplay between defect build-up at a lower temperature and defect annealing at a higher temperature, taking place in the solar cells operated in MPP during conventional outdoor operation.

Gamma radiation effects in amorphous silicon and silicon nitride photonic devices.

Understanding radiation damage is of significant importance for devices operating in radiation-harsh environments. In this Letter, we present a systematic study on gamma radiation effects in amorphous silicon and silicon nitride guided wave devices. It is found that gamma radiation increases the waveguide modal effective indices by as much as 4×10-3 in amorphous silicon and 5×10-4 in silicon nitride at 10 Mrad dose. This Letter further reveals that surface oxidation and radiation-induced densification account for the observed index change.

Periodic molybdenum disc array for light trapping in amorphous silicon layer

We demonstrate the light trapping effect in amorphous silicon (a-Si:H) layer by inserting a layer of periodic molybdenum disc array (MDA) between the a-Si:H layer and the quartz substrate, which forms a three-layer structure of Si/MDA/SiO2. The MDA layer was fabricated by a new cost-effective method based on nano-imprint technology. Further light absorption enhancement was realized through altering the topography of MDA by annealing it at 700°C. The mechanism of light absorption enhancement in a-Si:H interfaced with MDA was analyzed, and the electric field distribution and light absorption curve of the different layers in the Si/MDA structure under light illumination of different wavelengths were simulated by employing numerical finite difference time domain (FDTD) solutions.

Modelling the light induced metastable effects in amorphous silicon

AbstractWe present results of computer simulations of the light induced degradation of amorphous silicon solar cells. It is now well established that when amorphous silicon is illuminated the density of dangling bond states increases. Dangling bond states produce amphoteric electronic mid‐gap states which act as efficient charge trapping and recombination centres. The increase in dangling bond states causes a decrease in the performance of amorphous silicon solar cells. To show this effect, a modelling approach has been developed which uses the density of localised states with exponentially increasing band‐tails and dangling bond defect states distribution chosen according to the defect pool model. The calculation of the evolution of dangling bond state density during illumination has been achieved through a dynamic scaling relation derived from a defect creation model. The approach considers the amphoteric nature of the dangling bond state and thus accounts for the contributions of the different charge states of the dangling bond during the degradation process. This paper attempts to describe the simulation approach which calculates the defect density as a function of energy, position in the solar cell and illumination time. In excellent agreement with other workers, our simulation results show that the increase in the density of neutral dangling bond states during illumination is higher than of the charged states. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Digital optical switch based on amorphous silicon waveguide

In this paper, the performance of a digital optical switch, operating at the infrared communications wavelength of 1550 nm and based on the thermo-optic effect in amorphous silicon, are investigated. We prove that the strong thermo-optic effect of amorphous silicon, combined with the possibility of realising micrometric integrated structures, allow the design of promising integrated switches. The device, designed for low-cost photonic applications, could be easily integrated in silicon optoelectronic circuits.

Absence of enhanced stability in fully deuterated amorphous silicon thin-film transistors

The stability of fully deuterated amorphous silicon (a-Si:D) thin-film transistors is compared with their hydrogenated equivalent (a-Si:H) in terms of gate bias stress. The amorphous silicon channel and silicon nitride gate insulator layers were deposited by radio-frequency plasma-enhanced chemical-vapor deposition. The use of SiD4 rather than SiH4 for the deposition of a-Si:D changes the physical properties of the plasma given the same conditions of rf power, pressure, and gas flow rates. Consequently, a higher gas pressure is required to produce a-Si:D at the same growth rate and with similar bulk properties as a-Si:H. It is shown that a-Si:H and a-Si:D deposited at the same growth rate have very similar structural properties. Therefore transistors deposited at the same growth rate may be more sensibly compared to determine the effect of replacing H with D in amorphous silicon without significantly changing the silicon continuous random network. Using this criterion for comparison, no detectable difference is observed between hydrogenated and deuterated transistors in terms of stability under the application of a gate bias. The experimental results rule out the possibility of a giant isotopic effect in amorphous silicon. Furthermore, this result supports the idea that the rate-limiting step for dangling-bond defect creation in amorphous silicon is the breaking of a weak Si–Si bond, rather than breaking of a Si–H bond.

The Geometry of Amorphous Silicon Effect on Metal Induced Lateral Crystallization Rate

ABSTRACTIn this study, we observed that the MILC behavior changed when the amorphous silicon active pattern width was changed abruptly and explain that phenomena with novel MILC mechanism model. The 10nm thick Ni layers were deposited on glass substrate that has various amorphous silicon patterns on it. Then we annealed the sample at 550 °C with RTA (rapid thermal annealing) machine and measured the crystallized length with optical microscope. The MILC rate was reduced dramatically and stopped for several hours (incubation time). After the incubation time, the MILC started again and the incubation time increased as the amorphous silicon pattern width difference getting larger. We can explain these phenomena with the tensile stress that was caused by volume shrinkage due to the phase transform from amorphous silicon to crystalline silicon.

Digital optical switch based on amorphous silicon waveguide

The performances of a digital optical switch, operating at the infrared communications wavelength of 1550 nm and based on the thermo-optic effect in amorphous silicon, are investigated for the first time. We prove that the strong thermo-optic effect of amorphous silicon, combined with the possibility of realizing micrometric integrated structures, allows the design of promising integrated switches. The device, designed for low-cost photonic applications, could be easily integrated in silicon optoelectronic circuits.

Phase Transformations Induced by Arsenic Implants into Silicon

ABSTRACTMicro-Raman spectroscopic investigations of arsenic-implanted silicon show lines characteristic of silicon crystallites even at implant doses above the amorphization threshold. The intensity and frequency of occurrence of the lines increase with the implanted dose. Polarization/orientation Raman studies indicate the crystallites are silicon in the hexagonal phase (Si-IV) and silicon in the diamond phase (Si-I). The latter are oriented differently than the substrate silicon. Monte Carlo simulations of the arsenic ion energy loss and published molecular dynamics studies suggest that each arsenic ion deposits sufficient energy to locally melt the silicon lattice. This is taken as the basis of the present attempt to explain the origin of the crystallites. A one-dimensional numerical model is developed to determine the time scale for the liquid silicon to solidify. The effect of amorphous silicon on the solidification is also investigated.

Phase Transformations Induced by Arsenic Implants into Silicon

AbstractMicro-Raman spectroscopic investigations of arsenic-implanted silicon show lines characteristic of silicon crystallites even at implant doses above the amorphization threshold. The intensity and frequency of occurrence of the lines increase with the implanted dose. Polarization/orientation Raman studies indicate the crystallites are silicon in the hexagonal phase (Si-IV) and silicon in the diamond phase (Si-I). The latter are oriented differently than the substrate silicon. Monte Carlo simulations of the arsenic ion energy loss and published molecular dynamics studies suggest that each arsenic ion deposits sufficient energy to locally melt the silicon lattice. This is taken as the basis of the present attempt to explain the origin of the crystallites. A one-dimensional numerical model is developed to determine the time scale for the liquid silicon to solidify. The effect of amorphous silicon on the solidification is also investigated.

Transient photoconductivity, density of tail states and doping effect in amorphous silicon

The Transient Photoconductivity (TPC) is studied in the pre-recombination time range on the basis of a Multiple Trapping (MT) model which uses a transient occupation function to account for simultaneous interactions of all the tail states with the conduction band. A direct inversion method for the extraction of the Density Of States (DOS) from the TPC data in amorphous semiconductors is derived. Application of this transient spectroscopy to n-type a-Si:H results in a very narrow band tail (tail width of 14 meV) starting around 0.16 eV below the mobility edge and leading to low DOS. This finding is in favour of the defect pool concept, and involves some doping effects at the donor level.

Subpicosecond hot carrier cooling in amorphous silicon

Measurements of the cooling rate of hot carriers in amorphous silicon are made with a two-pump, one-probe technique. Pump photons at 2 eV create free carriers and pump photons at 1.42 eV heat the carriers up to 1.2 eV/pair. The experiment is simulated with a rate-equation model describing the energy transfer between a population of hot carriers and the lattice. An energy transfer rate proportional to the temperature difference is found to be consistent with the experimental data. An energy transfer rate independent of the temperature difference is inconsistent with the data. This contrasts with the situation in crystalline silicon and GaAs. The measured cooling time, 0.2 ps, is sufficient to explain the absence of avalanche effects in amorphous silicon at fields below 106 V/cm.

Magnetoresistance and Hall effect in amorphous silicon–tantalum alloys near the metal–insulator transition

Magnetoresistance (MR) and Hall effect (HE) measurements in hydrogenated (H) and unhydrogenated (UH) samples of silicon–tantalum alloys with compositions situated on both sides of and near the metal–insulator transition (MIT) are presented. The measurements at room temperature on both H and UH samples have shown a small positive MR of about 0.003 at B=1 T. The insulating samples gave a negative MR about 10 times greater than the metallic samples. For the measurements at low temperatures (4.2 and 1.7 K, respectively) on some metallic samples the MR was positive, and larger at the lower temperature. HE measurements have shown positive carriers. From these measurements the density of carriers and the Hall mobility have been evaluated. The Si–Ta system is discussed in comparison with the Si–Ni system. Based on the obtained results a model of the band structure is proposed, which explains the anomalous HE seen in amorphous metal–semiconductor systems.

Cooling of Hot Electrons in Amorphous Silicon

ABSTRACTMeasurements of the cooling rate of hot carriers in amorphous silicon are made with a two-pump, one-probe technique. The experiment is simulated with a rate-equation model describing the energy transfer between a population of hot carriers and the lattice. An energy transfer rate proportional to the temperature difference is found to be consistent with the experimental data while an energy transfer independent of the temperature difference is not. This contrasts with the situation in crystalline silicon. The measured cooling rates are sufficient to explain the difficulty in observing avalanche effects in amorphous silicon.

Quantative analysis of photo-induced effect in amorphous silicon by laser desorption time of flight mass spectroscopy

Quantative analysis of photo-induced effect in amorphous silicon by laser desorption time of flight mass spectroscopy

Electrons, holes, and the hall effect in amorphous silicon

The double sign anomaly in hydrogenated amorphous silicon, observed by LeComber et.al. [1] in 1977, has remained puzzling ever since. Recently we attacked this problem from two different directions using the equation-of-motion method. Calculations reproduce the double sign anomaly and were rationalised in terms of the behaviour of the spectral function. In a new type of calculation the motion of a ‘wave packet’ was studied in an applied field. Behaviour characteristics of positive and negative effective masses was found in the same ranges of energy for crystalline and a-Si. A perturbation theory is given for reconciling these results.

In search of a normal Hall effect in amorphous silicon

For twenty years it has been believed that the double sign anomaly in Hall measurements is always observed in amorphous silicon. Theory does not exclude the possibility of finding the expected sign in Hall measurement (normal Hall effect). Here, we described how this material was found.

Quantum efficiencies greater than unity: A computer study of a photogating effect in amorphous silicon p-i-n devices

We report a new effect, which we call photogating, that can be present in amorphous Si-based p-i-n structures under certain illumination conditions. This phenomenon feeds photogenerated carriers created by a bias light beam of one wavelength into the current response to a second probe light beam of a different wavelength. We demonstrate that this phenomena can lead to measured quantum efficiencies greater than unity. Our computer simulations show that this new effect owes its origins to trapped charge and to the condition that there is a low field region in the front of the structure in the presence of certain bias light.

Molecular-dynamics simulations of the stability of amorphous silicon.

We have performed molecular-dynamics simulations as a first step towards understanding the Staebler-Wronski effect in amorphous silicon, utilizing classical silicon potentials and computer-generated amorphous silicon structures. A localized hot spot is used to model the nonradiative transfer of photoexcited carrier energy to the lattice. This leads to structural degradation in a-Si networks in which weak Si-Si bonds are broken and dangling bonds are produced, requiring a threshold energy of about 0.8--1.0 eV. Thermal annealing of an a-Si model with coordination defects is also described.

The sign of the Hall effect in amorphous silicon

Abstract It is shown that the anomaly in the sign of the Hall coefficient in a-Si: H can be understood if the disorder responsible for the mobility edge is caused by the presence of stretched bonds.