We report the preparation of α-(Si.Ge):H alloy films by decomposition of SiF4, GeF4, and H2 in a d.c. glow discharge. Germanium is incorporated very efficiently from GeF4. The germanium content and optical gap canbe controlled by varying the GeF4 flow while keeping the SiF4 and H2 flows constant. The films, all prepared at 300 ºC substrate temperature, exhibit high photo- to dark-conductivity ratios for compositions of up to -25% germanium.

Hydrogenated amorphous silicon germanium (a—SiGe:H) films were deposited by photo—chemical vapor deposition (Photo—CVD) of SiH4 and GeH4 with mercury sensitizer. Their band gap was controlled from 0.9 eV to 1.9 eV by changing the gas ratio of SiH4 and GeH4. High quality opto—electrical properties have been obtained for thea—SiGe:H films by Photo—CVD. Hydrogen termination and microstructure of a-SiGe:H were investigated by infrared absorption and transmission electron microscopy. Ana—Si:H solar cell and an a—Si:H/a—SiGe:H stacked solar cell were made, each of which has conversion efficiency 5.3% and 5.1%, respectively.

Hydrogenated amorphous silicon germanium alloys a-Si1-xGex:H are being actively investigated for their application as a low band gap material in cascade solar cells [1,2]. To date, such alloys produce material of reasonable electronic quality only if the Ge-content is kept low (<40 at.%) such that the band gap is not decreased much below 1.5 eV. Conversion efficiencies of -5% have been obtainedwith alloys having such a band gap, and tandem cells have shown conversion efficiencieswhich are lower than those of good quality single layer a-Si:H devices. Thus, the performance of alloys is well below that necessary to achieve conversion efficiencies of >16%, which are ultimately hoped to be obtained using the cascade approach [2]. Other low band gap alloys such as a-Si1-xSnx:H have been shown to be even less suitable with regard to their electronic properties [3]. The cause of the degradation in electronic properties with increased alloying is not yet understood. Factors such as preferential attachment of H to Si rather than Ge [4] or microstructure observed in alloys have been suggested as a cause for the electronic degradation, [5,6] but no unique correlations have been established between such findings and the electronic properties.

A new type of hydro—fluorinated amorphous silicon—carbide (a-SiC:F:H) is produced by the glow discharge decomposition of gas mixture of CF4, H2 and intermediate species SiF2. The electrical, optical and structural properties of this a—SiC:F:H are studied and the results are compared with the similar results for a—SiC:F:H produced from gas mixture of CH4, H2 and SiF2 and also for hydrogenated amorphous silicon carbide (a—SiC:H) produced from CH4 and SiH4 gas mixture. It is found that the optical band gap can be increased without degradation of photo—conductive properties only when amorphous silicon carbide is produced from CF4, H2 and SiF2 gas mixture.

We have deposited a-SiC:H thin films from a silane-methane glow discharge plasma in a planar reactor using high (13Mhz and low (12kHz) frequency plasma excitation. Films produced at these two frequencies have significantly different properties as measured by vacuum UV reflectivity measurements, XPS, and infrared spectroscopy. Samples produced at the low excitation frequencies, where higher ion fluxes and energies are expected, have higher VUV reflectivities, higher bulk plasmon energies, and fewer IR bands due to organic fragments. In order to relate the VUV reflectivity data to the material properties, it was fitted to a simple Lorentz model using parameters taken from XPS data. We will discussour interpretation of these data and also the dependence of film properties on the reactant gas composition.

Hydrogen evolution and infrared absorption studies of a-Si:C:H films prepared from mixtures of hydrocarbon (CH4' C2H6' C2H4) and silane gases (SiH4 'Si2H6)by the glow-discharge process show that both the absolute hydrogen content and the hydrogen bonding is uniquely determined by the carbon content. Up to a carbon concentration of ∼45 % the widening of the optical gap is mainly due to an increase of the hydrogen content. For a hydrogen density NH > 1- 2x1022cm-3 the incorporated hydrogen atoms lead to the formation of voids, resulting in thermal instability of the bonded hydrogen and in a deterioration of the electronic quality of the amorphous material.

Tail and defect states in the gap of a-SixCl-x:H alloys have been studied by measurements of spectral photoconductivity. The variation of defect—state density versus x is found to be negligible. By comparison with PDS results the ητ product has been determined and found to be almost independent of photonenergy and to strongly decrease with inclusion of carbon. This effect is attributed to changes in the transport mechanism combined with an increased recombination rate associated with the widening of the gap.

The electrical and optical properties of a—SiNx:H films prepared by the glow discharge of Si2H6-NH3 gas mixture have been investigated in a wide range of the molar ratio of NH3 to Si2H6 The maxima of the dark—and the photo—conductivity are observed at the molar ratio of about 10-1. At this molar ratio the deposition rate is about33 Å/sec. The optical gap and the compositional ratio of nitrogen to silicon change smoothly from those of a—Si:H to those of a—Si3N4. It is found that the valency control is possible in a—SiNx:H films prepared at a high deposition rate.

The photoconductivity of boron doped a—SiNx:H(x=O.07)has been studied by steady state secondary photocurrent, xerographic photodischarge, the time of flight measurement, and ESR. The dominant carrier of the photocurrent in a—SiNx:H is found to be electron. The μτ product of the electron decreases with increasirg doping amount of boron, while that of the hole increases and becomes largerthan 10-8cm2/V. From these results it is found that the life time of the hole becomes longer with increasing the amount of doping boron. The intensity of the ESR signal due to the neutral dangling bonds decreases monotonically with doping boron, and this decrease of the neutral dangling bond density is considered to be due to not only change of the occupation statistics, but mainly to the chemical or electrical interaction between boron and nitrogen.

The energy levels of defect centers in amorphous silicon nitride have been calculated. The results are related to recent photoemission and light-induced electron spin resonancedata. The Si dangling bond is argued to be the memory trap in MNOS devices and to be responsible for the electron accumulation at interfaces with amorphous silicon and for the n-type chargetransfer doping of amorphous silicon-silicon nitride superlattices.

Silicon and Silicon-Tellurium films were prepared in an ultrahigh vacuum system. The d.c. electrical conductivity of these films were measured under vacuum before exposing them to the air. The thermal activation energies of Silicon-Tellurium films were always found to be greater than that of Silicon films. Furthermore the optical measurements on Silicon-Tellurium films indicated that the optical gap of these films decreases as the concentration of tellurium in the films increases. Electron microscope examinations revealed that the films were amorphous in structure and homogeneous in composition.

In this paper we report for the first time the discovery of metallic conductivity in some tetrahedrally bonded diamond–like ternary oompounds 1 v vl with the general formula A3B C4 · Such compounds are characterized by a Valence Electron Concentration (VEC) of four valence electrons per atom. The preparation and characterization of one of the investigated compounds Cu3AsSe4 will be presented.

The transition metal atoms in such a compound are connected by non-transition metal atoms (Se) via coordinate locallized bonds in a tetrahedral environment. Such an environment should lead to a splitting in the 3d filled orbitals of the transition metal because of lattice field. A delocalization of the 3d electrons is expected to take place via back donation, through an overlap with the empty antibonding orbitals on the non-metal atoms to minimize the negative formal charge on the transition metal atoms. If such an overlap leads to continuous electron path through the whole material, metallic conductivity could be visualized. The detailed bonding model will be discussed.

A review is given on recent progress in the amorphous silicon solar cells and their technologies. Firstly, some unique advantages of amorphous silicon as a low cost solar cell material are pointed out, and its significant position in the photovoltaic project are discussed. Secondly, newly developed key technologies for improving the photovoltaic performance are demonstrated from the film quality improvement to new junction structure solar cells with a wide and narrow energy gap amorphous silicon alloys. Then, current state of the art in the cell performance are summarized. In the final part, recent feature of the industrializations in both consumer and power application fields are overviewed.

The desired properties of solar cells are discussed, and a relative figure of merit for the comparison of cells fabricated using different technologies is described. The advantages of utilizing amorphous silicon alloys as the active material in solar cells are enumerated. Selected materials properties of these alloys are described and the physics of their electronic structure is discussed in detail. The necessary steps for achieving commercially viable cells based on amorphous silicon alloys are listed, and it is demonstrated how each of them has been achieved using a technology that incorporates fluorine throughout the entire process. The chemical and physical basis for the superiority of fluorinated material is presented in detail. Continuous web large-area high-efficiency multijunction solar cells are in production. Dual band gap multijunction cells have been tested under continuous air mass 1 exposure for over 2000 hours and show essentially no degradation. Some recent results are presented.

A survey of the materials issues involved in the development of thin film Si:H power modules is presented. Though there are significant opportunities for advancement of Si:H itself, especially in terms of new alloys, the range of opportunities is extended dramatically when all aspects of the product -- from gaseous fuels through encapsulants -- are included. Examples from each of these areas are presented to Indicate the current state of development as well as the needs for the future. Viable products exist today, but the extent of greater success depends critically on materials innovation.

High performance a-Si solar cells were developed. A conversion efficiency of 11.5% was achieved for a textured TCO/p-SiC/in/Ag structure with a size of 1 cm2 using the high quality i-layer fabricated by a new consecutive, separated reaction chamber apparatus. A conversion efficiency of 9.0% was obtained with a size of 10cm × 10cm. A high quality a-SiGe:H:F, which is a new narrow bandgap material for a-Si solar cells, was fabricated by a glow discharge decomposition of SiF4 + GeF4 + H2.

A photo-CVD method was investigated in order to improve the interface properties of a–Si solar cells. A conversion efficiency of 11.0% was obtained with a solar cell in which the p-layer is fabricated by the photo-CVD method. a-SiGe:H films were fabricated by the photo-CVD method for the first time as a narrow bandgap material for multi-bandgap a-Si solar cells.

The Amorphous Silicon Research Project (ASRP), established at the Solar Energy Research Institute (SERI) in 1983, is responsible for all U.S. DOE government supported research activities in the field of amorphous silicon photovoltaics. The objectives and research directions of the project have been established by a Five Year Research Plan developed at SERI in cooperation with the Department of Energy in 1984. In order to accomplish project goals, research is performed by a combination of i) multi-year programs consisting of multi-disciplinary research teams based on strong government/industry partnerships and ii) basic research performed in university, government and industrial laboratories. A summary of recent research progress in the ASRP program is presented.

It is well known that the performance of hydrogenated amorphous silicon (a-Si) p-i-n type devices is determined by the sequence of deposition. For instance, the stainless steel/p-i-n configuration generally shows a larger value of the open circuit voltage (up to 200mV) compared to the n-i-p sequence of deposition \1,2]. Explanations of this phenomena such as the Dember potential \1], self field effect \2], residual doping \3], hydrogen effusion effects associated with the p+ layer deposition process \4, 5] are unable to satisfactorily explain the difference in performance.

Continuous progress is being made in the conversion efficiencies of a- Si:H solar cells and efficiencies in excess of 11% have been achieved. Because of these advances and the development of a-Si:H cell technologies there is an increased interest in the long term performance of a-Si:H cells and the mechanisms responsible for their degradation. The reversible light-induced changes in a-Si:H solar cells are generally associated with the Staebler-Wronski effect (SWE) (1). This effect has been studied on a wide range of a-Si:H materials using a variety of different experimental techniques and this talk reviews the results that have been obtained on a- Si:H films and solar cells (2). It discusses in greater detail recent studies on a-Si:H solar cell structures in which simultanous measurements have been made on the changes in both the photovoltaic properties as well as their electronic properties and densities of gap states. In particular it focuses on several results obtained with semitransparent metal-undoped a-Si:H Schottky barrier solar cell structures (3).

The dependence of the creation and the annealing of metastable dangling bonds in hydrogenated amorphous silicon on various material parameters will be discussed in the context of a recently proposed model. After a brief review of the kinetic behaviour governing defect creation and annealing in undoped a- Si:H, a number of special cases will be analyzed: the influence of alloying with O, N, C, and Ge, changes introduced by doping and compensation, and the role of mechanical stress. Finally, possibilities to increase the stability of a-Si:H based devices will be examined.