Showing papers on "Amorphous silicon published in 1984"

Melting temperature and explosive crystallization of amorphous silicon during pulsed laser irradiation

Abstract: Measurements during pulsed laser irradiation indicate that amorphous Si melts at a temperature 200 \ifmmode\pm\else\textpm\fi{} 50 K below the crystalline value. Below energy densities required to melt the amorphous layer fully, the data are interpreted in terms of an explosive crystallization. The initial liquid layer solidifies to form coarse-grained polycrystalline Si. A thin, self-propagating liquid layer travels through the remaining amorphous Si at a velocity of 10-20 m/s, producing fine-grained polycrystalline Si.

Gap states in silicon nitride

Abstract: The energy levels of defect states in amorphous silicon nitride have been calculated and the results are used to identify the nature of trap states responsible for charge trapping during transport and the charge storage leading to memory action. We argue that the Si dangling bond is the memory trap in chemical vapor deposited memory devices and is also the center in plasma‐deposited nitride responsible for hopping at low electric fields and for charge‐trapping instabilities in amorphous silicon‐silicon nitride thin‐film transistors.

Physics of amorphous silicon based alloy field‐effect transistors

Abstract: In this paper we develop a new theory to describe the characteristics of amorphous silicon based alloy field‐effect transistors. We show that the transition from below to above threshold operation occurs when the Fermi level in the accumulation region moves from the deep to tail localized states in the energy gap. The current‐voltage and capacitance‐voltage characteristics are related to the basic material parameters such as the distribution of localized states in the energy gap, band mobility, device geometry, channel doping, and series resistances. Our analysis shows that an on current in excess of 2×10−7 A/μm gate width can be obtained with a 10‐μm gate length. We also demonstrate that even in the above threshold regime the field‐effect mobility is dependent on the gate voltage. Our theory can be used to optimize the design of amorphous silicon based alloy field‐effect transistors.

Electrical properties of n-amorphous/p-crystalline silicon heterojunctions

Abstract: We have measured C‐V characteristics and temperature dependence of J‐V characteristics of undoped hydrogenated amorphous silicon (a‐Si:H) heterojunctions formed on p‐type crystalline silicon ( p c‐Si) substrates with different resistivities. It has been found that an abrupt heterojunction model is valid for a‐Si:H/p c‐Si heterojunctions, and the electron affinity of a‐Si:H has been estimated as 3.93±0.07 eV from C‐V characteristics. The forward current of all the junctions studied shows voltage and temperature dependence expressed as exp(−ΔEa f/kT) exp(AV), where ΔEa f and A are constants independent of voltage and temperature, being successfully explained by a multitunneling capture‐emission model. The reverse current is proportional to exp(−ΔEar/kT)(VD−V)1/2, where VD is the diffusion voltage and ΔEar is a constant. This current is probably limited by generation‐recombination process.

Disorder effects on deep trapping in amorphous semiconductors

Abstract: Disorder and band-tail localization are shown to influence the deep trapping processes in an amorphous semiconductor. The capture cross-section includes a term involving tunnelling from a cluster of tail states near a deep centre. Data for trapping in hydrogenated amorphous silicon is analysed to obtain the capture cross-sections and to identify the tail-state contribution.

Photoluminescence in hydrogenated amorphous silicon

Abstract: By starting with exponential band tails of localized states, it is shown that the photoluminescence of $a$-Si: Hi can be explained with no further assumptions. The spectrum and its changes with temperature, with time, and with pump intensity and energy, and the decay curves and lifetime distributions, are predicted by the model, qualitatively, and, when the calculation is feasible, quantitatively.

Kinetics of the Staebler–Wronski effect in hydrogenated amorphous silicon

Abstract: We have investigated the time and intensity dependence of the creation process for light‐induced metastable defects (Staebler–Wronski effect) in hydrogenated amorphous silicon (a‐Si:H). The observed changes in electron spin resonance spin density (dangling bonds) and photoconductivity are consistent with a model which explains the Staebler–Wronski effect as a self‐limiting process intrinsic to a‐Si:H. A possible microscopic mechanism based on the nonradiative recombination of band tail carriers is discussed.

Method of manufacturing photovoltaic device

Abstract: Mutually spaced apart TCO film portions are formed on a glass substrate. An amorphous silicon film of PIN junction type is formed to cover these TCO film portions, and an aluminum film is formed thereon. The aluminum film in the vicinity of the gap between adjacent TCO film portions is removed by means of a laser beam, and the amorphous silicon film is removed by reactive plasma etching using the aluminum film as a mask to thereby expose portions of the TCO film. An aluminum-titanium film is formed over the aluminum film and the exposed portions of the TCO film. The aluminum-titanium film is separated by means of a laser beam to form a plurality of photoelectric converting regions. The photoelectric converting regions thus formed on the glass substrate are connected in series.

Pulsed laser melting of amorphous silicon layers

Abstract: We have investigated microstructural changes in self‐implanted and arsenic‐ion‐implanted amorphous silicon layers as a function of energy density after pulsed ruby laser irradiation, using cross‐section transmission electron microscopy and Rutherford backscattering. In specimens irradiated with energy densities less than that required to cause complete annealing, we have identified two distinct regions; the first one consisting of fine polycrystals and the second one consisting of large polycrystals. The changes in thickness of these two regions as a function of pulse energy density are described. Concomitant changes in arsenic concentration profiles are consistent with diffusion in liquid silicon. From the profile broadening in the large polycrystalline region, the crystal growth velocity was estimated to be 4–6 ms−1.

Pulsed-laser melting of amorphous silicon: Time-resolved measurements and model calculations

Abstract: It is demonstrated that the thermal conductivity of self-ion--implanted, amorphized silicon is an order of magnitude less than that of crystalline silicon and is by far the dominant parameter determining the dynamical response of the ion-implanted silicon system to pulsed-laser radiation; the latent heat and melting temperature of amorphous silicon are relatively unimportant. Transmission electron microscopy indicates that bulk nucleation occurs in the highly undercooled liquid phase; a model simulating this effect is presented.

Photoconductivity and recombination in amorphous silicon alloys

Abstract: We present results of a new model to realistically describe the steady‐state photoconductivity in amorphous silicon alloys. The room temperature photoconductivity is very dependent on the position of the dark Fermi level and the sensitization is a consequence of both a change in the recombination path and dopant created gap states. We also demonstrate the relationship between the power dependence of photoconductivity and dark Fermi level position and show that as a result of space charge neutrality, this dependence can be related to a characteristic energy slope of the density of states only in the absence of injected charge or dopants. Moreover, in agreement with recent experimental data, we show that our model predicts a power dependence of less than 0.5 for high intensity illumination on n‐type amorphous silicon. Finally we examine the temperature dependence of photoconductivity and find good agreement between our theory and experimental results.

Atomic structure of ion implantation damage and process of amorphization in semiconductors

Abstract: Atomic structure of ion implantation damage and the process of amorphization in silicon have been investigated using high‐resolution electron microscopy techniques. The specific damage energy density for crystalline to amorphous transition has been determined to be 6.0×1023 eV/cm3 or 12 eV/atom at 4 K with no annealing. The amorphous regions are produced when the damage energy deposited by the ions exceeds this critical value. Since the damage energy deposited by the ions is a strong function of ion implantation and substrate variables, the formation of amorphous regions and the process of amorphization are strong functions of these variables. The details of atomic structures of amorphous silicon containing microcrystallites and that of amorphous‐crystalline interfaces are presented. The calculations of the mean‐free path between collisions and the energy deposited per atom are found to be consistent with experimental observations on amorphization of silicon. Some results on the projected ranges of low‐en...

Sunlight-into-energy conversion apparatus

Abstract: An apparatus in which amorphous silicon solar cells are formed on a heat collecting plate. The solar cells are formed by using a superstrate or a substrate. Both a light-permeable superstrate and a metallic substrate are available for use. If the light-permeable superstrate is adopted, metallic electrodes, formed on the side of the heat collecting plate as lower electrodes of the solar cells, are attached to the heat collecting plate through electrically insulating adhesives, provided that the upper surface of the superstrate is exposed against the incident sunlight. The light-permeable superstrate is made of a heat absorbing material transmitting light having a wavelength range which is absorbed by the amorphous silicon layer of the solar cells while absorbing light having a wavelength range which is transmitted through the amorphous silicon layer thereby to convert the light into thermal energy. Alternatively, a heat absorbing layer made of a material having the above-mentioned property may be provided on a transparent superstrate. On the other hand, if the metallic substrate is used, the substrate is also available for the heat collecting plate, or is attached to the heat collecting plate while said heat absorbing layer may be provided on transparent electrodes formed on the upper side of the substrate as upper electrodes of the solar cells.

Optical Phonons in Amorphous Silicon Oxides. II. Calculation of Phonon Spectra and Interpretation of the IR Transmission of SiOx

Abstract: Basing on the theoretical investigations of Part I of this work a simple model for the calculation of the density of vibrational modes and of the IR response function of the geometrically and chemically disordered material SiOx (0 < x ≦ 2) is developed. This model leads for the first time to a theoretical interpretation of LO—TO splittings and of chemical shifts of the minima of IR transmission spectra of thin SiOx films prepared by reactive sputtering.

A novel structure, high conversion efficiency p-SiC/graded p-SiC/i-Si/n-Si/metal substrate-type amorphous silicon solar cell

Abstract: A novel structure, high conversion efficiency amorphous silicon (a‐Si)/metal substrate‐type solar cell has been developed. The new structure, deduced from the conventional pin junction by the use of a gradual compositional grading p‐type a‐SiC:H layer between an ultrathin (∼20 A) wide optical band gap (∼2.4 eV) p‐type a‐SiC:H layer and the i layer, exhibits markedly enhanced open‐circuit voltage (Voc) and short‐circuit current density (Isc) over the conventional a‐Si pin/substrate‐type solar cell. Especially, the collection efficiency in the newly developed structure was found to be remarkably increased at short wavelengths. The experimentally observed improvement in the blue response is due to the reduction in effective interface recombination combined with the enhanced window effect. An energy conversion efficiency of 8.40% under air mass (AM) 1 (100 mW/cm2) illumination has been obtained in the first trial of a cell fabricated by the rf glow discharge decomposition of pure silane (SiH4).

Effect of silane dilution on intrinsic stress in glow discharge hydrogenated amorphous silicon films

Abstract: Measurements of the intrinsic stress in hydrogenated amorphous silicon (a‐Si : H) films grown by rf glow discharge decomposition of silane diluted to varying degrees in argon are presented. Films are found to grow under exceedingly high compressive stress. Low values of macroscopic film density and low stress values are found to correlate with high growth rate. An abrupt drop in stress occurs between 2 and 3% silane at precisely the point where columnar growth morphology appears. No corresponding abrupt change is noted in density, growth rate, or plasma species concentrations as determined by optical emissioin spectroscopy. Finally a model of diffusive incorporation of hydrogen or some gaseous impurity during growth into the bulk of the film behind the growing interface is proposed to explain the results.

Electrophotographic sensitive body

Abstract: PURPOSE:To provide high sensitivity at a short wevelength and high moisture resistance by forming further an amorphous silicon (a-Si:F) layer laminated to a specific film thickness on a conductive substrate. CONSTITUTION:Amorphous silicon (a-Si:H) contg. hydrogen is laminated to a prescribed thickness on a conductive substrate and gaseous fluorine having high purity is ionized in a vacuum and the ions are implanted to the a-Si:H layer to form the a-Si:F layer having a prescribed thickness. Since F has higher affinity to Si than H, the F implanted into the a-Si:H layer is bonded with Si to form an Si-F bond. The a-Si:F film having a high grade is thus formed and the electrophotographic sensitive body coated with the a-Si:F film having high moisture resistance is obte. The film thickness of the a-Si:F layer is preferably in a 0.1- 5mum range to obtain desired photosensitivity. Formation of the uniform film is difficult at <=0.1mum and the photosensitivity is deteriorated on the contrary if the thickness exceeds 5mum.

Infrared Absorption in a-Si: H: First Observation of Gaseous Molecular H 2 and Si-H Overtone

Abstract: We report the first observations of collision-induced infrared absorption of the fundamental vibrational band of molecular hydrogen incorporated in microvoids in heavily hydrogenated amorphous silicon. These results provide the first direct evidence of high-pressure gas (\ensuremath{\sim} 2000 atm) occluded in $a$-Si: H. We also report the observation of the overtone absorption spectrum of the Si-H bond in $a$-Si: H, thus providing useful information regarding anharmonic potential well describing the Si-H bond in bulk amorphous silicon.

Charge transfer doping in amorphous semiconductor superlattices

Abstract: Charge transfer doping of amorphous silicon by amorphous silicon nitride is demonstrated in amorphous semiconductor superlattice structures. The transfer‐doped material has lower gap‐state density and higher photoconductivity than conventional substitutionally doped material.

Electronic states at the hydrogenated amorphous silicon/silcon nitride interface

Abstract: Electronic states at the hydrogenated amorphous silicon (a‐Si:H)/silicon nitride interface are observed to depend strongly on the order of deposition. When the nitride is on top of the a‐Si:H, there is an interface charge of ∼5×1011 electrons/cm2 and the interface Fermi energy is 0.25 eV from the conduction band. The bottom nitride is also in electron accumulation but with a much smaller space charge. Our results are in marked disagreement with recent data.

Electronic and optical properties of glow-discharge amorphous silicon-carbon alloys

Abstract: Measurements are reported on the optical gap, dark conductivity and photoconductivity of hydrogenated amorphous silicon-carbon films (a-SiC : H) prepared from the glow-discharge decomposition of si...

Bandgap and resistivity of amorphous semiconductor superlattices

Abstract: Measurements of the electrical resistivity and optical bandgap of amorphous semiconductor superlattices consisting of alternating layers of amorphous silicon with amorphous silicon carbide and amorphous silicon nitride are compared with the predictions of a one dimensional quantum-well model.

Characteristics of amorphous silicon staggered-electrode thin-film transistors

Abstract: Amorphous silicon staggered‐electrode thin‐film transistors (TFT’s) can show current crowding near the origin in the output characteristics. The degree of current crowding is governed by the voltage dependence of the current flowing from the n+ contact to the conducting channel. This current is a space‐charge‐limited current whose magnitude depends on the bulk density of states in the undoped intrinsic layer. For a 0.5‐μm‐thick i layer, calculations predict negligible current crowding for N(E) 3×1016 cm−3 eV−1. Experimental results are consistent with N(E) in the range 1016 cm−3 eV−1–2×1016 cm−3 eV−1. This is lower than the value derived from the transfer characteristic of the TFT (∼1017 cm−3 eV−1), which is evidence for an inhomogeneous distribution of deep gap states through the 0.5‐μm film of α‐Si:H.

Electrophotographic sensitive body

Abstract: PURPOSE:To obtain an electrophotographic sensitive body improved in dark decay characteristics and acceptance potential retentivity by forming a heat- resistant polymer injection blocking layer and a specified amorphous silicon layer on a conductive substrate. CONSTITUTION:A 0.5-15mum thick heat-resistant polymer injection blocking layer 7 is formed by coating a conductive substrate 1 with a polymer compd. selected from polyimide, polyimide-amide, polyester-imide, and polyester-imide-amide. An amorphous silicon layer 3 contg. >=50wt% silicon is formed on the layer 7. As a result, since the layer 7 has mobility lower than 10 cm /V.sec of both positive and negative electrostatic charge carriers, the obtained electrophotographic sensitive body is small in dark decay of surface positive charge and chargeable to high potential.

Effect of Substrate Temperature on Properties of Glow-Discharged Hydrogenated Amorphous Silicon

Abstract: The substrate temperature dependence of the transport and photoelectric properties of glow-discharged hydrogenated amorphous silicon films has been studied in connection with the morphological heterogeneity in the films. The electron drift mobility at room temperature determined by the time-of-flight method increases exponentially as the substrate temperature is raised, and is possibly associated with the formation of a percolation path through the growth of small quasi-crystalline zones. In contrast with the exponential increase in the electron mobility, the lifetime, or the deep-level trapping time, of electrons shows a maximum at a substrate temperature of 200°C, in parallel with the ESR spin density and tail-to-tail luminescence intensity.

Optical absorption enhancement in amorphous silicon deposited on rough substrate

Abstract: A thin film semiconductor device with enhanced optical absorption properties and a method for producing it. The device comprises a substrate having at least one sandblasted surface and a thin film of semiconductor material deposited on the sandblasted surface.