The electrical and optical properties of heavily boron-doped hydrogenated amorphous silicon (a-Si:H) films deposited at 100°C with a gas phase doping concentration between B2H6/SiH4=0.001 and 0.1 have been investigated. The absorption edge exhibits the characteristic Urbach form and the width of exponential absorption tail, Eo, increases with increasing doping concentration. The optical gap has a linear relationship with the Eo and it is expected to be 2.20 eV when the Eo is zero. Thus, the disorder determines the optical gap directly whether the sample is doped or undoped, the hydrogen and boron in the film change the disorder. The conversion efficiency of n-i-p cell is improved by 15% if the p-layer is deposited at 100°C after preparing the n- and i-layers ∼250°C.

Measurements of the drift mobility μ of photo-excited electrons in n-type hydrogenated amorphous silicon (a-Si:H) as a function of light intensity are reported. The value of μ increases as the quasi Fermi level is moved closer to the transport states in accordance with the multiple trapping theory. The drift mobility decreases with increasing doping as well as with an increase in the concentration of metastable dangling bonds defects by strong light exposures. This decrease in μ between 300 and 360K can be explained by a corresponding decrease in the microscopic mobility, by an increase in the density of tail states within 0.35eV below the electron mobility edge, or by a combination of both these effects.

Absorption measurements of as deposited and photodegraded intrinsic amorphous hydrogenated silicon films were made using photothermal deflection spectroscopy (PDS). The films were light-soaked in situ using HeNe laser light to simulate AM1 illumination. An increase in subbandgap absorption occurred predominantly near energies of 1.2eV. A simple model was developed in which a density of states function is hypothesized and the resulting optical absorption at subgap energies is calculated. The measured absorption could be well matched in all cases by assuming a single peak of defect states at or slightly below the Fermi level. Further, the observed changes in optical absorption due to degradation could be modeled by increasing the density of the single peak of defect states and moving the Fermi level towards the valence band.

The discussion about the influence of oxygen on the degradation be-havior of a-Si:H pin solar cells is now as before controversial. Therefore, we prepared a-Si:H pin solar cells having different amounts of O contami-nation in the i-layer by adding O2 gas to the silane gas during the glow-discharge deposition of the i-layer. The admixture of O2 gas was in the 0 ppm to 1000 ppm range resulting in an O concentration in the i-layer of 1.10190/cm3 to 3.1020 0/cm3 determined by SIMS measurements. Then we inves-tigated the long-term degradation behavior of these cells, exposing them for about 270 h to white light of 100 mW/cm2 intensity. During the first 20 h the cell efficiencies dropped relatively fast to about 80 % of the initial value and then more slowly to about 60 % after 270 h. The degradation of the cells may be slightly different during the first 20 h, but this has nothing to do with the O concentration. The normalized efficiency values meet all at the 60 % level after the long-term light exposure for 270 h. We there-fore conclude that the degradation behavior of pin a-Si:H solar cells after long-term exposure is not dependent on the O concentration in the range investigated. At O concentrations higher than about 5.1020 cm−3, the den-sity of states increases rapidly and the electrical properties of the solar cells deteriorate, making it difficult to clarify the true influence of oxygen on the degradation behavior.

Lihgt-induced change in photovoltaic performance of p-i-n solar cell was studied. The behavior of degradation of the solar cell strongly depends on the intensity and the spectrum of illumination light. The results of these light soaking tests are presented and discussed. On the basis of these results, the superiority of double-stacked a-Si/a-Si tandem solar cells is discussed.

Light-induced changes in vibrational absorption of SiHn have been studied for a-Si:H films. The stretching mode absorption near 2000 cm−1 decreases after light exposure. The magnitude of the change increases as the substrate temperature is lowered, and little change is observed for a-Si:H films deposited above 200°C. From the changes both for the peak and the spectrum, it is suggested that the hydrogen in the bulk diffuses to the microvoids, where it can combine to form H2 in low substrate temperature sample. The B-H modes increase after light soaking, Which is related with the increase in doping efficiency of boron, which is confirmed from the conductivity change with time during illumination.

The kinetics of the Staebler-Wronski effect in a-Si:H p-i-n solar cells are well known [1]. We report measurements of the kinetics of light-induced degradation and annealing of a-Si films in which we have monitored the changes in subgap absorption by means of photothermal deflection spectroscopy (PDS). The kinetics of films and cells are found to be very similar, suggesting that cell degradation is largely a material, as opposed to a device effect.

A series of systematic trials to improve the light emission efficiency has been made on thin film visible a-SiC LEDs. A wide variety of the approaches, such as a superlattice structure, p-i-n/p-i-n tandem structure, an efficient injection electrode with a wide-gap layer etc., have been done. From the results, two practically available new technologies for bright LEDs have been developed, that is; a) a hot carrier injection structure by inserting a highly- resistive layer between p-and i- or i- and n-layers and b) a wide-gap highly-conductive p-type injector layer prepared by ECR (Electron Cyclotron Resonance) CVD deposition. With these trials, the brightness of yellow LEDs have been increased by more than one order of magnitude to be about 5cd/m2

The current-voltage characteristics of amorphous silicon alloy n-π-n and p-i-p diodes have been investigated. These diodes show highly nonlinear conduction characteristics which are attributed to the punch-through effect. The dependence of the non-linearity on the thickness and doping of the intermediate active layer support this interpretation. An experimental 108 × 108 pixel liquid crystal display was built with punch-through diodes as pixel switches.

In this paper phosphorous doped amorphous and microcrystalline silicon are used as emitter material for npn bipolar transistors. A heterojunction is formed between emitter and base, resulting in a higher current gain β for the same base parameters in comparison with conventional transistors. Because the amorphous silicon results in a too high emitterresistance, a compromise solution is microcrystalline silicon (μc-Si). HREM-micrographs give credit to the true heterojunction concept and show epitaxial reorganization, especially after annealing of the amorphous silicon.

Nonvolatile, associative electronic memory based on neural network models promises high (−109 bits/cm2 ) information storage density since the information is stored in a matrix of only two-terminal, passive interconnections (synapses). An electronic memory switch at each interconnect would be ideal as a programmable synapse. The massive parallelism in the architecture, however, requires that the ‘ON’ state of a synaptic connection must be unusually ‘weak’ (i.e., highly resistive). For example, a binary synapse should be 106Ω in its ‘ON’ state and >109Ω in the ‘OFF’ state for a 1024 × 1024 matrix (−256 K bits of programmable read only memory, PROM). The small deliverable switching energy dictated by the resistive ‘ON’ state requirement is a new constraint for switching in thin films. Memory switching in hydrogenated amorphous silicon (a-Si:H) along with ballast (current limiting) resistors patterned from resistivity-tailored, amorphous Ge-metal alloys are investigated for a binary PROM matrix. A lμm2 area of a-Si:H could be switched from a −1010Ω (OFF state) to −105Ω (ON state) by a voltage pulse of lpsec duration, with the switching energy of −1 nanojoule, delivered through a 106Ω ballast resistor. Programmable, read-only, 1600 synapse (40x40) test arrays of uniform connection strengths (variation ±2%) and a-Si:H switching elements have been fabricated. Suitability of the memory switching in a-Si:H for high-density neural networks is discussed.

We report the development of a very high speed high resolution contact line imager using amorphous silicon alloy PIN diodes both as photosensing elements and as isolation diodes in the multiplexing scheme. High speed is achieved by reading the integrated photocurrent in 8μsec and using current integration times less than Imsec. For 200 dots/inch, the scan speed at an illumination of 5×10−4W/cm2 is over 1000 lines/sec. This allows the reading of an A4 (8½″×11″) page in less than 2.0 sec. At this light level, the signal to noise ratio is greater than 40dB. The photosensor array can be used in the true direct contact mode and the multiplexed addressing scheme gives a substantial reduction in the number of peripheral IC chips necessary for operation.

Amorphous silicon p-i-n diodes were illuminated with LED-generated trains of rectangular light pulses under various duty-cycle conditions. Our results indicate that such diodes are able to handle modulation rates in excess of 10MHz over a wide range of operating temperatures. Measurements of the dark I-V characteristics show that the shot-noise in properly designed junctions can be reduced to a level comparable to c-Si photodiodes.

We have observed several steps in the I-V characteristics of nonmetallic Sb doped Si near the metal-nonmetal transition. The steps appear at equal intervals in the bias voltage of the low temperature I-V curve and the intervals in the voltage are independent on magnetic field. Such strongly nonlinear conduction can be explained by neither the Joshphson junction effect nor the Stark ladder effect. We have compared it with tunneling models in order to study a new application to a current or a voltage standard using quantum effect transport.

Positive charging amorphous silicon alloy photoreceptors require intentionally fabricated electron blocking layers at the photoreceptor/substrate interface. Thin insulating films and lightly boron doped (300ppm) a-Si alloy layers have been previously employed either singly or in multilayers as back blocking layers. We report here, for the first time, the use of thin (≈300Å) heavily doped microcrystalline (≈10 Ω−1 cm−1) Si back blocking layers [1], with enhanced photoreceptor performance. Investigation of the temperature and electric field dependence of the dark decay of photoreceptors with amorphous or microcrystalline silicon back blocking layers as a function of blocking layer thickness and boron doping is interpreted within a comprehensive model of blocking layer operation.

The attraction for amorphous Silicon based alloys (a-Si) stems in large part from its ease in its depositing uniformly over large areas in a cost effective manner. We here at Glasstech Solar, Inc. (GSI) wish to report on the development of a-Si films for electrophotographic applications. For this specific purpose we have developed thick films (˜30–40 μm) of intrinsic a-Si deposited on TCO at deposition rates of up to 14 Å/S We tested the material for its photoconductivity and breakdown voltage, and its suitability for the application at hand with one measured parameter, the 400 volt switching time, TΔ400. Essentially the goal consisted of developing a high voltage photoconductive switch, that would swing 400 volts at the terminals in a few μsec.