Showing papers on "Amorphous silicon published in 1981"

Disorder and the Optical-Absorption Edge of Hydrogenated Amorphous Silicon

Abstract: The effect of thermal and structural disorder on the electronic structure of hydrogenated amorphous silicon is investigated by measurement of the shape of the optical absorption edge as a function of temperature and thermal evolution of hydrogen. The data are consistent with the idea that the thermal and structural disorder are additive, and suggest that the disorder, rather than the hydrogen content, is the fundamental determining factor in the optical band gap.

Evidence for Exponential Band Tails in Amorphous Silicon Hydride

Abstract: The width of the conduction- and valence-band tails in amorphous silicon hydride are inferred from time-of-flight measurements of the temperature dependence of the electron and hole drift mobilities, and a multiple-trapping model of dispersive transport.

Macroscopic theory of pulsed-laser annealing. I. Thermal transport and melting

Abstract: Pulses of radiation from ruby and Nd:YAG $Q$-switched lasers have been used recently to anneal the lattice damage caused by ion implantation of semiconductors. Other similar applications include the laser-induced diffusion of thin dopant films deposited on the surface of samples, recrystallization of doped amorphous films deposited on single-crystal substrates, and the removal of precipitates present after conventional high-temperature dopant diffusion. All of these processes can be understood in terms of models and calculations based on macroscopic diffusion equations for heat and mass transport, cast in a finite-difference form to allow for the temperature and spatial dependences of the thermal conductivity, absorption coefficient, reflectivity, and other quantities. Results of calculations on silicon with the models show that the near-surface region of a sample can melt and stay molten for times of the order of 100 nsec during which dopant diffusion in the liquid state and nonequilibrium segregation during ultrarapid recrystallization are sufficient to explain the major features of the experimental results. In this paper, a description of the model used in our heat-transport calculations is given. Results of the modeling are illustrated by a variety of calculations which should be of particular interest to experimentalists working with pulsed-laser annealing. These results include, e.g., the effects of pulse duration, shape, and energy density, the effects of assumptions made about the latent heat of amorphous silicon, the effects of substrate heating, the role played by the absorption coefficient in determining melt-front penetration, and the duration of surface melting.

Light-induced dangling bonds in hydrogenated amorphous silicon

Abstract: After intensive and long illumination of undoped a‐Si:H samples the dark ESR signal is considerably enhanced. The g value of 2.0055 and the linewidth DHpp = 6–7 G leads to the conclusion that the light‐induced defects are single dangling bonds. They disappear again by thermal anneal at about 220 °C.

Luminescence and recombination in hydrogenated amorphous silicon

Abstract: Luminescence and related investigations of recombination in hydrogenated amorphous silicon prepared by glow discharge and sputtering are described. Emphasis is given to a detailed discussion of the various competing radiative and non-radiative recombination mechanisms. The evidence of the luminescence data is compared to other measurements that involve recombination processes. The experiments demonstrate that the dominant luminescence is a tunnelling transition between band tail electrons and holes. Other luminescence transitions involving defects and impurities, and the role of phonon interactions in the transition are discussed. Experiments in which luminescence is used as a probe of the properties of a-Si:H and related materials are also discussed.

Application of amorphous silicon field effect transistors in addressable liquid crystal display panels

Abstract: It is shown that thin-film field effect transistors (FETs) made from amorphous (a-) silicon deposited by the glow-discharge technique have considerable potential as switching elements in addressable liquid crystal display panels. The fabrication of the elements and their characteristics with steady and pulsed applied potentials are discussed in some detail. Two important points are stressed: (i) a-Si device arrays can be produced by well-established photolithographic techniques, and (ii) satisfactory operation at applied voltages below 15VV is possible. Small experimental 7×5 transistor panels have been investigated and it is shown that with the present design up to 250-way multiplexing could be achieved. The reproducibility of FET characteristics is good and in tests so far no change has been observed after more than 109 switching operations.

a‐SiC:H/a‐Si:H heterojunction solar cell having more than 7.1% conversion efficiency

Abstract: A clear valency electron controllability with substitutional impurity doping in the hydrogenated amorphous silicon carbide has been found. The amorphous silicon carbide is produced by the plasma decomposition of [SiH4(1−X)+CH4(X)] with the dopant gas of a B2H6 or PH3 system. Electrical and optical properties of doped amorphous SiC films are briefly demonstrated. Utilizing this a‐SiC:H as a wide‐band‐gap window material, we have developed a new type of a‐SiC:H/a‐Si:H heterojunction solar cell. A typical cell performance is Voc = 0.887 V, Jsc = 12.33 mA/cm2, fill factor = 0.653, and the conversion efficiency of 7.14% under AM‐1 illumination.

Stability of n‐i‐p amorphous silicon solar cells

Abstract: Unencapsulated, amorphous silicon indium tin oxide/n‐i‐p/stainless‐steel solar cells were tested for stability. All cells have excellent shelf life. Changes occur during exposure to light, but can be controlled by the deposition conditions of the amorphous silicon. The changes are due to trapping and recombination of optically generated carriers in the i layer, and are reversibly annealed out above 175 °C. Preliminary life tests on two relatively stable cells showed a small initial drop to 5%, followed by a weak logarithmic decay that predicts only ∼20% further decrease in efficiency after 20 years in sunlight. Work is continuing on improving the efficiency and stability of these cells.

Effects of inert gas dilution of silane on plasma‐deposited a‐Si:H films

Abstract: Electrical, optical, and structural characterization of hydrogenated amorphous silicon films plasma‐deposited from mixtures of SiH4 with different inert‐gas diluents reveals substantial differences in a number of properties. A general trend of increasing defect density with atomic weight of the inert gas is observed. Of specific interest to device applications is the observation that high deposition rates can be achieved concurrently with low defect densities when helium is used as a deluent.

Picosecond photoconductivity in radiation‐damaged silicon‐on‐sapphire films

Abstract: Radiation damage caused by ion implantation is used to control the carrier lifetime in silicon‐on‐sapphire (SOS) films. Photoconductivity measurements show the relaxation time changes by several orders of magnitude and can be as short as 8 ps. The carrier mobility is found to be at least an order of magnitude higher than amorphous silicon materials with similar relaxation times. A photodetector is described that demonstrates the high‐speed capability of these high‐defect‐density films.

A model for the electronic transport in hydrogenated amorphous silicon

Abstract: We present a model for the steady-state transport in glow-discharge (g.d.)-deposited silicon that can account in detail for the ‘activated mobility’ observed from a comparison of thermopower and co...

On light‐induced effect in amorphous hydrogenated silicon

Abstract: Amorphous silicon films prepared by a discharge of 10% SiH4‐90% H2 mixture are shown to have properties comparable to those prepared from 100% SiH4. These films are found to be quite stable against prolonged light exposure.

Amorphous silicon-silicon nitride thin-film transistors

Abstract: A thin‐film field‐effect transistor has been fabricated using glow‐discharge amorphous silicon as the semiconductor and silicon nitride as the insulator. The transistor operates in the electron (n type) accumulation mode and by changing the gate potential from zero to only 3 V a change in the source‐drain conductance of greater than four orders of magnitude is obtained. The results imply upper limits to the density of gap states in amorphous silicon and interface states at the amorphous silicon‐silicon nitride interface of 3×1016 cm−3 eV−1 and 5×1011 cm−2 eV−1, respectively.

Surface Damage on Si Substrates Caused by Reactive Sputter Etching

Abstract: Surface damage on silicon substrates caused by RSE has been investigated. Defects are classified into four categories in order of destruction, namely, precipitates surrounded by elastic strain field, surface roughness pattern, polycrystalline and amorphous silicon, and no defects, based on the results of TEM and RHEED observation. In particular, precipitates which reach a depth more than 500 A were found to cause OSF. The surface damage consists of contamination (C, F, O) layer, C and defect mixed layer, and defect layer in order from the top. The degree of these defects and contaminations expand with increasing power density and etching duration. RSE conditions, where no defects are formed, were determined, e.g., within 1 minute at 0.4 W cm-2.

Calculations of the electronic properties of hydrogenated silicon

Abstract: We have used the coherent potential approximation to calculate the electronic densities of states for a model of hydrogenated amorphous silicon. The results demonstrate the restoration and widening of the band gap with increasing hydrogen content. In the valence band, excellent agreement with photoemission experiments is obtained. In the conduction band Si-H antibonding states are predicted that can be inferred from photoconductivity measurements.

New features of the temperature dependence of photoconductivity in plasma‐deposited hydrogenated amorphous silicon alloys

Abstract: The temperature and flux dependences of photoconductivity have been investigated for plasma‐deposited hydrogenated amorphous silicon alloys produced under a variety of processing conditions. In undoped films, new features such as thermal quenching and supralinearity are observed. Such behavior is critically dependent on the position of the Fermi level, and is not observed in alloys doped by the addition to the plasma of PH3, B2H6, O2+N2 mixtures, or air. Interpretation of the data is based on a model of competing recombination centers.

Influence of disorder on the electronic structure of amorphous silicon

Abstract: We have examined the electronic structure of amorphous silicon using a tight-binding scheme with all first- and second-neighbor couplings in a continuous random network. Matrix elements and deformation potentials were taken from the crystalline band structure. The effect of bond-length and bond-angle variations is relatively small and contributes to the narrow tails at the band edges. The effect of dihedral-angle disorder was examined keeping only the nearest-neighbor interactions in the Hamiltonian. The dihedral-angle disorder was found to be important at the valence-band edge and responsible for the observed features near the top of the valence band. Topological disorder was found to have important consequences in the bulk of the bands as well as at the conduction-band edge. Apart from the effects of the bond-length and bond-angle disorder, the states at the band edges are confined within regions closely approaching the crystalline structure locally, where they have the same form as in the crystal, but do not extend through the entire structure.

Deep electron traps in hydrogenated amorphous silicon

Abstract: Deep-level-transient-spectroscopy measurements on both Schottky barrier and $p\ensuremath{-}i\ensuremath{-}n$ $a$-Si:H solar cell structures reveal deep electron traps. The unique feature of these traps, whose activation energy varies between 0.5 and 1.5 eV, depending on the sample, is that electron capture is thermally activated. This activation energy is close to the trap energy. These traps are also associated with metastable changes in solar cells.

Electrodeposition of Silicon from Solutions of Silicon Halides in Aprotic Solvents

Abstract: Amorphous silicon has been electrodeposited from nonaqueous baths using as the silicon source. A typical bath composition was in propylene carbonate containing 0.1M tetrabutyl ammonium chloride as the supporting electrolyte. Deposits were made potentiostatically at around −2.5Vvs. Pt reference at temperatures 35°–145°C under an argon atmosphere. A variety of materials including Pt, Ti, Ti‐6Al‐4V alloy, n‐Si, and indium‐tin oxide coated fused silica were used for the substrate. The as‐deposited silicon contains some hydrogen bonded as or . The quality and hydrogen content of the deposits are controllable by selecting the proper bath composition and operating temperature. The electrodeposition process offers an inexpensive route for producing α‐Si films for possible solar cell applications.

Internal stresses in amorphous silicon films depositied by cylindrical magnetron sputtering using Ne, Ar, Kr, Xe, and Ar+H2

Abstract: Previous investigations of the thin metal films deposited using cylindrical‐post magnetron sputtering sources have identified working gas transition pressures below which the internal stresses are compressive and above which they are tensile. The transition pressures increase with the atomic mass of the target material (Mt) relative to that of the working gas (Mg). This paper reports the occurrence of stress transitions in amorphous silicon films formed by dc cylindrical magnetron sputtering from a P‐doped Si target using Ne, Ar, Kr, Xe, and Ar+H2 as working gases. The compressive stresses were comparable to those seen in metal films (∠1 GN/m2). However, the transition pressures and entrapped gas contents were greater than for metal films with comparable Mt/Mg. Experiments with special shields were conducted to investigate the mechanism of gas entrapment. The occurrence of large compressive stresses and entrapped gas concentrations for films such as these, where Mt/Mg<1, suggests that mechanisms different...

Application of amorphous silicon field effect transistors in integrated circuits

Abstract: The work described in this paper is concerned with the possible applications in integrated circuits of thin-film field effect transistors (FETs) made from glow discharge amorphous (a-) silicon and silicon nitride. The construction and performance of inverter circuits, employing integrated a-Si load resistors, are described in some detail. The extension of this basic circuit to NAND and NOR gates, to a bistable multivibrator and to a shift register is reported. Based on the excellent photoconductive properties of a-Si an integrated addressable photosensing element has been constructed, which could have potential applications in imaging arrays.

Analysis of field-effect-conductance measurements on amorphous semiconductors

Abstract: A method for calculating the relationship between the density of localized states in an amorphous semiconductor and the experimentally measured field-effect conductance is presented. Commonly used simplifying assumptions of a constant space-charge density, zero-temperature statistics and a parabolic band-bending potential profile are removed and the errors that are introduced by using these approximations calculated. The method is used to show that the existence of a reported peak in the density of states at 0·4 eV below the conduction-band mobility edge of amorphous silicon (the Ex peak) cannot be proved through field-effect-conductance measurements.

P-Type semiconductor material having a wide band gap

Abstract: The production of improved photoresponsive amorphous alloys and devices, such as photovoltaic, photoreceptive devices and the like; having improved wavelength threshold characteristics is made possible by adding one or more band gap increasing elements to the alloys and devices. The increasing element or elements are added at least to a portion of the active photoresponsive regions of amorphous silicon devices. One increasing element is carbon which increases the band gap from that of the materials without the increasing element incorporated therein. Other increasing elements can be used such as nitrogen. The silicon and increasing elements are concurrently combined and deposited as amorphous alloys by vapor deposition, sputtering or glow discharge decomposition. A density of states reducing element allows the band gap increasing element(s) to be added to the alloy to adjust the band gap without reducing the electronic qualities of the alloy. The compensating or altering element(s) can be added during deposition of the alloy or following deposition. The addition of the increasing element(s) to the alloys increases the band gap to a widened utilization width for a particular device to increase the photoabsorption efficiency and to thus enhance the device photoresponse. The band gap increasing element(s) can be added in varying amounts, in discrete layers or in substantially constant amounts in the alloys and devices.

Dynamics of laser‐induced vaporization for ultrafast deposition of amorphous silicon films

Abstract: Atomic vapor produced by a frequency‐doubled, pulsed Nd:YAG laser with its beam focused on a silicon plate has been used to deposit amorphous silicon films on substrate at room temperature. Time‐resolved measurements on the emission lines from vaporized Si show that the laser‐irradiated surface is kept at temperatures high enough to maintain a significant amount of vaporization for approximately 40 ns after the 7‐ns laser pulse. Since intense pulsed atomic beams are used, the silicon films grow at high speed in excess of 106 A/s.

Formation of pin holes in hydrogenated amorphous silicon at high temperatures and the yield strength of a‐Si:H.

Abstract: The formation of holes in sputtered hydrogenated amorphous silicon films after isochronal annealing has been investigated. The holes are caused by the breaking of hydrogen‐containing bubbles formed at the film‐substrate interface as hydrogen diffuses from the film. The diameter of the holes is dependent on the film thickness, and the number of holes depends linearly on the initial hydrogen concentration. These simple relationships are used to derive the yield strength of a‐Si:H to be about 1.3×109 dyn/cm2.