Metastable Changes Research Articles

The Light Induced Changes in A-Si: H Materials and Solar Cells—Where We are Now

ABSTRACTThe quest for understanding and especially controlling the reversible light induced changes in a-Si:H based materials has been ongoing for the last twenty years. This has been accompanied by a corresponding large effort in minimizing their effects on more efficient a-Si:H based solar cells. Despite the complexities in both the phenomena as well as the solar cells, progress has been made in both the scientific and technological arenas. This paper briefly reviews primarily studies on the characterization and reduction of the metastable changes in materials and the correlation of these changes with those in efficient solar cells. It will discuss the impact of studies on materials as well as the continuous advances made with “engineering” of solar cell structures on their improved stabilized performance.

Transport and Recombination Channels in Undoped Microcrystalline Silicon Studied by ESR and EDMR

ABSTRACTWe present a detailed study of ESR and spin-dependent transport (EDMR) on μc-Si. We identify to different types of defects at g=2.0055(±3) and g=2.0044(±5) and study their influence on transport and recombination by stepwise annealing the samples. We find that transport is not controlled by defects if ND<1018 cm−3. For ND>1018cm−3 a dramatic decrease of the conductivity is found and we identify a hopping contribution in transport. To explain our ESR and EDMR results we propose a simple model where most defects are distributed at the surface of the columns and transport is along percolation paths. We also observe minor metastable changes of the defect density which are assigned to adsorption of atmospheric oxygen.

Metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere

Earth's deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km. This depth interval closely brackets the mantle transition zone, characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher‐pressure phases. Deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal, thermodynamic, and mechanical ferment inside slabs. Numerical simulations of reaction rates show that the olivine → spinel transformation should be kinetically hindered in old, cold slabs descending into the transition zone. Thus wedge‐shaped zones of metastable peridotite probably persist to depths of more than 600 km. Laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting. This instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser, finer‐grained phase. Hence in cold slabs, such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite. It is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs. This consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features. It explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone, where minerals in downgoing slabs should transform to spinel and ilmenite structures. The onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab. Even if a slab penetrates into the lower mantle, earthquakes should cease at depths near 700 km, because the seismogenic phase transformations in the slab are completed or can no longer occur. Substantial metastability is expected only in old, cold slabs, consistent with the observed restriction of deep earthquakes to those settings. Earthquakes should be restricted to the cold cores of slabs, as in any model in which the seismicity is temperature controlled, via the distribution of metastability. However, the geometries of recent large deep earthquakes pose a challenge for any such models. Transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics, including focal mechanisms indicating localized shear failure rather than implosive deformation, are so similar to those of shallow earthquakes. Finally, metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab. Such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and/or are detached from subducting slabs. Metastability in downgoing slabs could have considerable geodynamic significance. Metastable wedges would reduce the negative buoyancy of slabs, decrease the driving force for subduction, and influence the state of stress in slabs. Heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage, causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity. Because wedge formation should occur only for fast subducting slabs, it may act as a “parachute” and contribute to regulating plate speeds. Wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel, favoring the subsequent sinking of the slab into the lower mantle.

Hydrogen-induced defects in polycrystalline silicon

The incorporation of hydrogen in polycrystalline silicon (poly-Si) is essential to improve the materials properties which contains a high density of localized states in the grain-boundary regions. The presence of hydrogen in poly-Si gives rise to new and hitherto unexpected phenomena. In this paper, light-induced defect creation and H-induced metastable changes of the dark conductivity are reviewed. Experimental evidence is presented which demonstrates the direct participation of H in the metastable effects.

Light-induced metastable changes in defect density and photoconductivity of a-Si:H between 4.2 and 300 K

The creation kinetics of light-induced defects (LIDs) in intrinsic a-Si:H are found to be quite independent of exposure temperature, T e, between 4.2 and 300 K and exposure light intensity. The findings suggest that LID creation is a very robust process. The creation efficiency of LIDs varies by a factor 2.5 with T e and has a broad minimum between 80 and 200 K. 10 5 V/cm electric fields at 4.2 K decrease the creation efficiency by 30%. A spectral relaxation of subgap absorption due to LIDs created below 200 K is found above 200 K prior to the anneal of LIDs.

Short-laser-pulse and steady-light induced degradation of intrinsic, p-type and compensated a-Si:H

Degradation of the same hydrogenated amorphous silicon (a:Si:H) samples with short laser pulses (30 ns) and steady bandgap light (CW) reveals that the kinetics is different for pulses and CW. For pulse degradation the kinetics depends on doping. Particularly in doped samples pulses degrade the photoconductivity, σ p, much more than CW light for the same increase in light-induced defects, N D. The lack of correlation between σ p and N D suggests that other metastable structural changes besides defects are induced by bandgap light.

Hydrogen-Induced Metastability of Polycrystalline Silicon

AbstractThe presence of H in polycrystalline silicon gives rise to new and hitherto unexpected phenomena. In this paper two of the most recent observations are reviewed: (i) Hydrogen-induced metastable changes of the dark conductivity due to the formation and dissociation of an electrically active H complex and (ii) the generation of acceptor states during prolonged exposure of poly-Si to monatomic H at elevated temperatures. The observed type conversion is clearly due to the diffusion of excess H from the plasma since it does not occur during exposure to other species such as oxygen.

Light-Induced Metastable Changes in a-SiSx:H

ABSTRACTMetastable light-induced increases in the dark conductivities of a-SiSx:H alloys are explained as photo-activation of hydrogen-passivated sulfur donor sites. For a sulfur concentration (sulfur-to-silicon ratio) of 5.6 × 103 the excess dark conductivity as a function of illumination temperature is thermally activated with an activation energy of 0.72 eV. When the sulfur concentration is 3.3 × 102, the temperature dependence is very weak. This dramatic difference in the temperature dependence of the creation of increased dark conductivity is explained by a lowering of the annealing temperature for the metastable changes as the sulfur concentration increases. We discuss the influence of this new metastability on the possibility of obtaining more stable films.

Light-Induced Changes Among Fast-Relaxing Hydrogens in Plasma-Deposited Hydrogenated Amorphous Silicon

ABSTRACTLight-induced rearrangements in a-Si:H,D during and after illumination have been studied by deuteron magnetic resonance (DMR). A film sample was deposited on a 230°C Al substrate by plasma decomposition of S1H4 and D2. The high quality film contained 4 at.% H and 4 at. % D. DMR spectra of fast-relaxing components were examined at 700 msec or less after radio frequency saturation. At 30 K these showed a broad central signal (primarily nanovoid-trapped HD and D2), part of which shifted paramagnetically during illumination. No comparable shifts were shown by spectral components from Si-bonded hydrogen or from microvoid-contained dense fluid hydrogen. The observed post-illumination shifts vanished upon 150°C dark anneal and reflected light-induced metastable magnetic changes in the surroundings of hydrogen trapped in nanovoids.

Optical non-linearities and defect generation in a-Si:H thin films

The non-linear change of refractive index and absorption coefficient of a-Si:H films has been measured through the Z-scan technique using a picosecond ( τ FWHM = 1 ps) high repetition rate ( ƒ rep = 3.8−9.5 MHz ) laser. The films were prepared by the radiofrequency glow discharge technique. A reversible increase of the optical absorption under high-intensity illumination conditions has been measured, indicating an increase of dangling bond states concentration up to N r ≈ 10 19 cm −3. In order to explain the experimental results, a theoretical model has been developed which takes into account the change of absorption, due to light-induced metastable changes, during a Z-scan measurement. Non-linear changes of the refractive index as large as Δn = −0.07 have been detected.

Shape Memory: A New Concept of Explanation and of Mathematical Modelling

The shape memory effect and pseudoelasticity that are observed typically with some binary and ternary alloys are explained on the basis of interaction of the sets of different atoms of the elements constituting the alloy. The concrete discussion is limited to binary alloys only, assuming that the processes in ternary alloys are similar in principle, but substantially more complicated and therefore difficult to be modelled. The differing thermal expansions of the two elements present in a binary alloy are assumed to be the cause of the changes of energy of the interatomic forces with changing temperature. Cooling of such an alloy in austenitic state leads to an increase of this energy, then the energy is partly dissipated and the structure of the alloy transforms to a martensitic one; the metastable austenitic configuration changes into the lower energy martensitic structure. The mechanism of the transformation consists in the following variations of the interatomic distances: the distances between the differing atoms in the binary alloy (pairs with the smallest distances and with the strongest bonds) remain nearly unchanged, whereas the distances between atoms of the same el ement (pairs with larger distances and with weaker bonds) do change with martensitic transforma tion substantially, which leads to the lowering of the bond energy. It is shown that this understanding of the process agrees with a number of observed phenomena and it opens the possibility of model ling the shape memory effect and pseudoelasticity by a continuum model consisting of two con tinuous infrastructures corresponding to the two sets of atoms present in the binary alloy.

Production of ultrafine alpha alumina powders and fabrication of fine grained strong ceramics

Hydrous alumina powders, pure, seeded with alpha alumina, containing ammonium nitrate and containing both ammonium nitrate and seeds, were prepared by hydroxide precipitation. Their crystallization and sintering behaviour were investigated and mechanical properties of the ceramics were tested. Pure hydrous alumina transformed to alpha alumina crystals, with a size of ca. 200 nm, at 1200°C, after undergoing the usual metastable phase changes during heat-treatment. The powder needed to be sintered at 1600°C to achieve a high density. The ceramic had an average grain size of ca. 9 μm. Seeding lowered the transformation temperature to ca. 1120°C and caused the transformation to begin at ca. 600°C. The material could be sintered at 1500°C and had a grain size of 2 μm. The nitrate, predominantly present as ammonium nitrate, lowered the transformation temperature to ca. 1150°C and altered the proportion of the intermediate phases. However, the materials still had to be sintered at 1500°C to achieve >97% density. When both seed particles and nitrate ions were present the material almost completely transformed at 950°C to uniform crystals of alpha alumina with a size 99% theoretical density at 1450°C. The final ceramic had a uniformly grained (<1.0 μm) microstructure and exhibited strength up to 800 M Pa.

Pressure-induced instability in the transition temperature of Tl 2Ba 2CuO 6+δ single crystals

A pressure-induced time dependence of the superconducting transition temperature has been observed and carefully examined in Tl 2Ba 2CuO 6+δ single crystals. The results strongly suggesta structure with two metastable states, one of which is more stable at ambient pressure while the other is more stable above ∼6 kbar. Assuming that the excess interstitial oxygen O(4) is responsible for the metastable change, consistent with the path dependence of T c in pressure-temperature space discovered by Sieburger and Schilling, the activation energy between the two metastable states of O(4) is estimated to ∼1 eV. A possible mechanism of the two states is proposed.

Hydrogen-induced metastable changes in the electrical conductivity of polycrystalline silicon.

Measurements of the dark electrical conductivity ${\mathrm{\ensuremath{\sigma}}}_{\mathit{D}}$ performed on hydrogenated polycrystalline silicon (poly-Si:H) reveal a cooling-rate dependent metastable increase of ${\mathrm{\ensuremath{\sigma}}}_{\mathit{D}}$ below 268 K. This nonequilibrium state relaxes slowly and the time to reach equilibrium is thermally activated with ${\mathit{E}}_{\mathrm{\ensuremath{\tau}}}$\ensuremath{\simeq}0.74 eV. Since thermal quenching does not affect unhydrogenated specimens, the observed metastable changes are clearly due to the formation and dissociation of an electrically active hydrogen complex. We propose that this complex consists of an isolated H atom at the bond-center site of a prestrained Si-Si bond at a grain boundary.

Light-induced changes of the non-Gaussian 1/f noise statistics in doped hydrogenated amorphous silicon

Abstract The influence of metastable photo-induced conductance changes (the Staebler-Wronski effect) on the 1/f noise power spectra of co-planar current fluctuations in n-type doped hydrogenated amorphous silicon (a-Si:H) is studied. The average magnitude and spectral slope of the 1/f noise are not altered significantly due to illumination, when measured with identical d.c. currents passing through the film before and after light exposure. The conductance noise in the annealed state is strongly non-Gaussian, which is reflected in large noise power correlation coefficients between different octaves, a noise power distribution which is much broader than the expected x 2 distribution and a 1/f-like second spectrum. Following light exposure, the correlation coefficients decrease, the noise power distribution is well described by a x 2 distribution and the second spectrum becomes frequency independent. Annealing the a-Si:H film to 450K restores the non-Gaussian statistics of the 1/f noise. The non-Gaussian noi...

Light-induced metastable changes in amorphous silicon nitride

Abstract Light-induced metastable increases in subgap absorption have been studied in semi-insulating amorphous silicon nitride as a function of illumination temperature between 100 and 300 K. These metastable absorption changes can be partially bleached by subgap light. The metastable increase in subgap absorption is related to neutralization of charged dangling-bond defects. Thermal relaxation of the metastable state is modelled by phonon-assisted hopping of carriers between neutral defects. We explain the persistent photoconductivity of amorphous silicon/silicon nitride multilayers with metastable hole trapping by defects in the nitride layers. We explore the origin of the charge state of dangling-bond defects in silicon nitride and conclude that the origin is more likely potential fluctuations than a negative effective correlation energy. The similarity of light-induced metastable changes in other materials suggests an origin that is common to semi-insulating amorphous alloys.

Developmental significance of epigenetic impositions on the plant genome: A paragenetic function for chromosomes

AbstractDevelopmental and physiological factors can impose heritable metastable changes on the plant genome, a fact that was established by maize geneticists during the 1950s and 1960s, largely through the efforts of R. Alexander Brink and Barbara McClintock. This paper describes a transgenic reporter system that monitors genomic impositions as changes in morphogenetically‐determined flower color patterns. The observations reported here on the metastable properties of plant transgenes illustrate the proposals of Brink and McClintock that chromosomal impositions occur during normal development as ordered sequences of events which contribute to the elaboration of complex developmental patterns. The relationship between this process and some recent findings about the control of gene expression in transgenic plants is also discussed. © 1994 Wiley‐Liss, Inc.

Light Controlled Photoluminescence Relaxation in Porous Silicon

ABSTRACTWe report results of photoluminescence (PL), FTIR and ESR investigations on nanoporous silicon (PS) where a reversible PL intensity relaxation effect in the chemically oxidised material is observed. To be activated the effect needs, however, additional preparation steps including light irradiation and ageing in ambient atmosphere. After illumination with visible light, the PL intensity is remarkably diminished. However it recovers in the dark within the time scale of minutes to hours under ambient atmosphere at room temperature. This cycle can be repeated several times. We show that the variation of the PL intensity is anticorrelated to an ESR signal attributed to silicon dangling bonds. From the IR spectrum, however, no significant change of the pore surface chemical structure can be observed during a cycle. Therefore we conclude that the variation of the PL intensity is rather controlled by a metastable change in the number of dangling bond centers than by modification of the surface chemistry in the porous silicon system.

Metastable Defect Kinetics in Hydrogen Passivated Polycrystalline Silicon

ABSTRACTThe temperature dependence of the dark conductivity, σD, of unhydrogenated and hydrogen passivated polycrystalline silicon (poly-Si) films was Measured. While σD of unhydrogenated poly-Si did not exhibit any influence of thermal treatment prior to the measurement, striking effects were observed in hydrogenated poly-Si films. Below 268 K a cooling-rate dependent metastable change of σD is observed. The dark conductivity increases by more than 8 orders of magnitude. This frozen-in state is metastable: Annealing and a slow cool restore the temperature dependence of the relaxed state. The time and temperature dependence of the relaxation reveal that this process is thermally activated with 0.74 eV. The lack of the quenching metastability in unhydrogenated poly-Si is direct evidence that the metastable changes in σD are due to the formation and dissociation of an electrically active hydrogen complex, in the grain-boundary regions.

Kinematics and Thermodynamics Across a Propagating Non-Stoichiometric Oxidation Phase Front in Spent Fuel Grains

Spent fuel from power reactors contains mixtures, alloy subsets, and compounds of elements; but the aggregate atomic densities in spent fuel are dominated by uranium and oxygen atoms. With the exception of some UO2 fuels with burnable poisons (primarily gadolinia in BWR rods), the other elements with significant atomic densities in spent fuel evolve during reactor operation from neutron reactions and fission plus fission decay events. Due to nuclear decay processes, the intrinsic chemical composition and activity of spent fuel will continue to evolve after it is removed from reactors as its radioactivity decays over time. During the time interval when the radioactivity levels are significant, which is the time interval relevant for design and for performance assessment of a geological repository, it is important to develop an understanding and to develop models that describe potential chemical responses in spent fuel and its potential degradational impacts on repository design and performance. One such potential impact is the oxidation response of spent fuel. The oxidation of spent fuel results in an initial phase change of the UO2 lattice to a U4O9 lattice, and the next phase change is probably to U3O8 although it has not been observed yet at low temperatures (&lt;200°C). The U4O9 lattice is non-stoichiometric with a oxygen to uranium weight ratio (O/U) at ~ 2.4. Preliminary indications are that the UO2 has a O/U of ~ 2.4 at the time just before it transforms into the U4O9 phase.[1,2] Also, in the oxygen weight gain versus time response, a plateau appears as the O/U approaches ~ 2.4. Part of this plateau response is due to geometrical effects of a U4O9 phase change front propagating into UO2 grain volumes. However, the plateau time response may be indicative of a metastable phase change delay kinetics or a diffusional related delay time until the oxygen density can attain a critical value to satisfy the stoichiometry and energy conditions for phase changes. In a previous paper and as a first step, a kinematic and thermodynamic ayalysis was developed to model spatially homogeneous oxidation phase transitions.[3] However, the experimental data clearly show a front of U4O9 lattice structure propagating into grains of the UO2 lattice structure. To describe this spatially inhomogeneous oxidation phase transition, as well as the expected U3O8 phase transition from the U4O9 lattice, lattice models are developed and spatially discontinuous kinematic and energetic expressions are derived. The approach will use concepts from statistical mechanics, discontinuum mechanics, and non-equilibrium thermodynamics. In addition, analytical techniques from shock wave analysis will be used to derive the surface discontinuity energetic expressions across the propagating oxidation phase front.