Undoped films of μc-Si deposited by RPECVD are n-type with a room temperature dark conductivity of ∼6×10-4 S/cm and an activation energy of ∼0.3 eV. This is due to native donor-like defects. We report on the conductivity and photoconductivity of boron-doped μc-Si, with emphasis on low doping levels that are designed to compensate exactly these native donor-like defects. We describe the dark conductivity and the photoconductivity as functions of dark conductivity activation energy and the average boron concentration, and present a model for the photoconductivity based on band off sets between the crystalline and amorphous regions of the μc-Si.

Current-voltage voltage characteristics of heterojunctions formed by remote plasma enhanced chemical vapor deposition (PECVD) of heavily doped μc-Si onto doped c-Si have been studied, as well as capacitance-voltage characteristics of MOS capacitor structures using heavily doped remote PECVD μc-Si and a-Si films as gate electrodes on thermally oxidized crystalline Si. Shifts in the flat-band voltages of MOS devices using the μc-Si and a-Si as gate electrodes relative to that of a reference Al/SiO2/c-Si structure are measured and explained in terms of a band structure model for the μc-Si and a-Si. Rectification and a photovoltaic effect observed in the pn heterojunctions are also explained in context of the same model.

A polycrystalline-silicon surface with hemispherical-grain (HSG) is deposited by low-pressure chemical vapor deposition at the transition temperature of the film structure from amorphous to polycrystalline. The surface area of the HSG-Si film is about twice as large as Si films deposited at other temperatures. It is found that the HSG-Si is not formed during deposition, but formed during annealing of amorphous Si surface after deposition. At the annealing temperature which is maintained to be the transition temperature, HSG-Si is formed by the nuclei generation on the amorphous-Si surface and the outward crystalline growth of grains dixough migration of surface Si atoms. By applying the HSG-Si film as the storage electrode for a 64Mbit dynamic random access memory (DRAM) stacked-capacitor with a SiO2Si3N4 dielectric film, twice the capacitance is obtained. The increase in the capacitance makes it possible to reduce the DRAM cell area, even by using a relatively thick dielectric film for higher reliability. Consequently, the HSG-Si technique is applicable to the fabrication process for 64Mbit and larger DRAMs.

Laser crystallization of a-Si thin films has been shown to produce materials with enhanced electrical properties and devices that are faster and capable of carrying higher currents. The quality of these polycrystalline films depends on a number of parameters such as laser energy density, shot density, substrate temperature, and the quality of the starting material. We find that the average grain size and transport properties of laser crystallized amorphous silicon films increase substantially with laser energy density, increase only slightly with laser shot density, and are unaffected by substrate temperatures of up to 400°C. The best films are those processed in vacuum but films of fair quality can also be obtained in air and nitrogen atmospheres.

Experimental results on a systematic investigation on the elemental composition, structural, optical and electrical properties of undoped and doped microcrystalline silicon carbide films deposited by Plasma Enhanced Chemical Vapor Deposition.

The doped samples show high values of dark conductivity accompanied by good optical properties so to satisfy the requirements for heterojunction window material.

A new analytical method for amorphous silicon solar cells, called DICE (dynamic inner collection efficiency), has been developed. The depth profile of the photovoltaic characteristics of solar cells under any operating condition are obtained by this method in a non-destructive way for the first time. It was experimentally confirmed by the DICE characterization that 1ight-induced deterioration does not occur homogeneously in the thickness direction of the I-layer, and that the main deterioration region in the I-layer changes according to the duration and load condition of light exposure.

The DICE method also contributed to an improvement in the conversion efficiency of two kinds of a-Si solar cells, an integrated-type submodule and a new type of ultralight flexible module. A total area efficiency of 10.6% was obtained with a size of 10 cm × 10 cm integrated-type a-Si solar cell submodule on a glass substrate, and the world's highest powet—to-weight ratio of 275 mW/g was achieved in the flexible module on a plastic film.

Light-induced changes in current-voltage characteristics of amorphous single-junction solar cells made of silicon alloys, a-SiC, a-Si, and a-SiGe have been studied systematically. The effect of the light intensity and the bias voltage on the light-induced degradation in the conversion efficiency and other photovoltaic parameters has been clarified quantitatively, and it has been shown that the light-induced degradation characteristics of the photovoltaic parameters can be described by a single function of some normalized parameters of exposure condition. Theoretical background for the experimental results is also examined and discussed by applying a defect creation model modified to an active layer of the a-Si alloy solar cells. Utilizing these analytical formula, we propose an accelerated test method by solar simulator indoor measurement instead of natural sunlight outdoor testing.

Three junction a-Si based photovoltaic modules have been made using a design which provides high initial efficiency and good resistance to photodegradation. The cells have a Si/Si/SiGe configuration in which the i-layer in the middle junction is 4000Å thick. The most efficient module measured to date has an aperture area efficiency of 9.82%. This design will limit light-induced efficiency losses to 15% or less, based on small area results, however defect related problems have increased this value to 17 – 23%. Preliminary data on the effect of shunts on stability is presented. We discuss various concerns related to large area deposition and scale-up.

The sub-bandgap primary photocurrent and the solar cell performance of a-Si:H p-i-n devices have been studied before and after light induced degradation. The results indicate significant discrepancy between the two methods when used to estimate the degree of degradation and the defect density in the i-layers. A preliminary explanation is proposed.

The simulation of a-Si:H solar cells has the potential to make a major contribution to the development of improved devices. However, to achieve this potential, simulations must run quickly on personal computers. This will require simplified approaches which are justified by the correspondence of their results to more elaborate but slower running simulations.

a-Si:H solar cells were irradiated with 1.00 MeV proton fluences in the range of 1.00E14 to 1.2 5E15 cm-2. Annealing of the short-circuit current density was studied at 0, 22, 50, 100 and 150 °C. Annealing times ranged from an hour to several days. The measurements confirmed that annealing occurs at O °C and the initial characteristics of the cells are restored by annealing at 200 °C. It is proposed that the degradation in the short-circuit current density with irradiation is due to carrier recombination through the fraction of D° states bounded by the guasi-Fermi energies. The time dependence of the rate of annealing in the short-circuit current density appears to be consistent with the interpretation that there is dispersive transport mechanism which leads to the annealing of the irradiation induced defects.

We have investigated the influence of substrate temperature on the optoelectronic and structural properties of heavily doped μc-Si:H, prepared with the Very High Frequency Glow Discharge process. At substrate temperatures as low as 160°C we obtain, for films with 0.5μm thickness, maximum conductivities of 100 S/cm and 20 S/cm for <n> and <p> material, respectively. Starting from these values the deposition parameters were optimised for ultrathin layers having thicknesses in the range of 100 to 500Å. We observe that boron doping plays a critical role in the crystallisation of ultrathin films. The thinnest layers investigated so far show conductivities of 0.2 S/cm at d=100Å for <n>, and 0.2 S/cm at d=250Å for <p> material. These properties make μc-Si:H films attractive candidates to form tunnel junctions in tandem solar cells.

The conversion efficiency of a-Si:H P-I-N photovoltaic devices was improved using aluminum doped zinc oxide thin films between the n-layer and the metal electrode. The improvement is due to the increase in the quantum efficiency, mainly in the wavelengths between 550 – 750 nm, and is attributed to the increased reflectivity of the zinc oxide/metal layer over the conventional metal electrode. The dependence of the enhancement, on the thickness of the zinc oxide layer and the back metal electrode was studied and the optimal configuration was identified as 800 angstroms of zinc oxide backed by a silver electrode. Photostability studies of devices containing these back-reflectors showed that their stability to light-soaking was comparable to conventional devices establishing the use of zinc oxide back-reflectors as a viable way of increasing the efficiency of a-Si:H devices without compromising their stability.

We have studied the spectrum of various types of a-SiGe:H alloys for pin and Schottky barrier photodlodes, in which the band gaps of the a-SiGe:H vary between 1.75 eV and 1.35 eV. It has been found that the spectral re-ponse of the Schottky barrier diode shifts significantly to longer wavelength and the quantum efficiency decreases with an increase in i layer thickness. However, for the pin diode, an increase in the i layer thickness can hardly shift the spectrum to longer wavelength. For both pin and Schottky barrier diodes, the quantum efficiency can be increased by increasing the reverse bias. Therefore, an enhanced spectrum with a maximum at 800nm and a tail to lun can be achieved for a reverse-biased a-SiGe:H Schottky barrier diode. The results indicate that a-SiGe:H has a great potential for a low cost infrared photodetector.

The reduction of the ITO (Indium Tin Oxide) by the deposition of an a-Si:C:H p layer by means of a Plasma Enhanced Chemical Vapour Deposition (PECVD), is considerably diminished if a thin silicon monoxide layer (estimated to be between 5–50Å thick) is applied as a diffusion barrier. The amount of reduced indium diminishes (its concentration is two times lower), while the amount of silicon oxide is less although silicon monoxide was added on purpose. The influence of the silicon monoxide layer on the electrical properties of the p/i junction is shown by its J-V characteristics (in the dark and under illumination). We can see that with a suitable silicon monoxide thickness we can improve the short circuit current density (Jsc) and the rectifying ratio.

The fast changes observed in the stabilized state of a-Si:H solar cells and modules at temperatures below 70°C are inconsistent with the commonly accepted picture of “defect annealing.” The fast changes observed in the stabilized state, for example when the temperature is altered, are explained in terms of converting the charge state of the dangling bond defects that are already present in the material. It is suggested that the slow degradation of solar cells arises from the creation of new defects and can be described by fitting stretched exponential curves to the solar cell performance data.

Amorphous Hydrogenated Silicon (a-Si:H) photovoltaic thin films have been deposited onto ultra low density (ULD) silica aerogel substrates using plasma enhanced chemical vapor deposition (PECVD). The techniques and results of ULD aerogel fabrication, surface treatment, thin film deposition and photovoltaic (PV) characterization are discussed. DC photbcurrent and transient time of flight measurements were performed for the PV characterization, while ellipsometry, transmission optical spectrophotometry, scanning electron microscopy (SEM) and low magnification optical microscopy (×64) were used to obtain qualitative and quantitative surface morphology, microstructure and film thickness data. This research indicates that ULD aerogels may be used as substrates for dielectric and metallic thin films which have electrical, optical and mechanical properties comparable to films deposited onto conventional substrates such as glass, metals or plastics. These data taken together with the lightweight of the ULD aerogels may be of interest to the space based sensor community as a possible route to large, lightweight photonic sensors or PV arrays. These results thus indicate that ULD aerogels could be used as substrate materials for other thin films of interest to the PV community, for example, we have produced ultra lightweight reflective optics using this fusion of solgel and thin film technologies [1].

Extremely high-sensitive image pickup tubes with sensitivities 1000 times higher than those of conventional tubes are fabricated using the avalanche phenomenon in a-Se as photoconductive targets. The excess avalanche noise of a video signal is found to be much less than that expected, based on the carrier ionization rates. The frequency spectra of the noise currents of both the pickup tubes and sandwich-type photocells are examined. The results are compared with those of a simulation, and it is found that the excess noise can be reduced by the charge-storage operation of imaging devices.

A thin film of amorphous silicon (a-Si:H) has been developed for application as the charge retention layer in electrostatic loudspeakers. The vibrating diaphragm in this type of loudspeakers is usually coated with a thin film in order to assure homogeneous charge distribution across the surface. This thin film should be capable of maintaining a high surface charge density and possess a high lateral resistivity to prevent charge displacement. Furthermore, it should be mechanically stable, capable of accomodating mechanical deformation, and resistant against humidity. Conventionally, a graphite layer is used in these applications. However, the conventional layers are frequently unstable and suffer from charge displacement effects eventually leading to electric breakdown. Further requirements are that the film can be deposited continuously and homogeneously over large areas in excess of 1 m2 and that the deposition technique is compatible with the properties of the thin membrane. We investigated small-area prototypes of electrostatic loudspeakers with a-Si:H thin films deposited on polyimide substrates in the Utrecht deposition system, ASTER. Plasma-deposited amorphous silicon films fabricated under certain conditions are shown to meet all of the above requirements.

Both Cr/p+/V and Cr/p+/Cr devices exhibit fast electrically programmable memory switching with a programmable voltage range, ΔVS, which depends only on the choice of the top metal. Despite a difference in ΔVS, both types of device have very similar electrical properties, but show one major difference in room temperature I-V characteristics, where under certain conditions, discontinuities are observed.