Search

Tungsten/carbon (W/C) multilayer thin films with a nominal d spacing varying from 2.5 to 14 nm were prepared by magnetron sputtering technique. The thicknesses of the W and C layers were varied from 0.5 to 12 nm. The multilayers were subjected to isochronal anneals in a quartz tube furnace in the range of 300 to 1000 C under high purity Ar flow conditions. X-ray diffraction, Raman scattering and Auger depth profiling were used to characterize the structure of the as-prepared and annealed films. It is found that an overcoat layer of silicon nitride (30-50 nm) prevents the multilayers from oxidation during the 1 hr heat treatment at temperatures as high as 1000 C in Ar flow. In all studied W/C multilayers, the carbon layers are amorphous (up to 12 nm). The tungsten layers are also amorphous when their thicknesses are less than 5 nm. Tungsten layers thicker than 5 nm show crystalline W peaks in addition to the amorphous W feature. Annealing of samples with a silicon nitride protective layer results in several structural changes which depend on annealing temperature, d spacing, the as-deposited W layer structure and the layer thickness ratio of W to C. For W layer thicker than C layer and W layer thickness > 4 nm and/or C layer thickness < 1 nm, the multilayers show the initial crystal formation of microcrystalline W2C occurring at C-W interfaces (that interface in which C was deposited on W) after 600 C anneal, followed by a second crystallization of a-W or a-W and WC at W-C interfaces (W was deposited on C) at the annealing temperature of 900 C. They reveal a relatively small (< 5 %) or essentially no layer expansion. For those multilayers having thin W layers (2 nm) and the same or thicker C layer thicknesses, the initial crystallization takes place at both W-C and C-W interfaces at 900 C or higher. The crystal formed is a-W or a-W and WC. The layer pair period of the multilayers in this group increases monotonically with increasing annealing temperature. Expansion is up to 16 % of the original d spacing and occurs in both W and C layers at approximately equal rates. The expansion in all multilayers is interpreted to be associated mainly with the structural ordering processes in the amorphous W and C layers.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We report on evidence that fluorine, properly incorporated into a-Si, replaces weakly bonded hydrogen and improves the material stability under light soaking. Our fluorinated amorphous silicon (a-Si:H:F) is made by if glow discharge at high deposition temperatures up to 430 C from a gas mixture of SiH4 or Si2H6 and F2. These a-Si:H:F films show much lower density of states in the light soaking saturated state than device quality a-Si:H prepared in the same deposition system. It is evident from our results that fluorine incorporated into the network at such high deposition temperature makes for a new configuration which minimizes dangling bonds and other defects.

The thermal activation energy, AE, of the conductivity in amorphous chalcogenide semiconductors is usually about half of the optical gap energy E. This is commonly interpreted as (intrinsic) conduction at the mobility edge by carriers whose concentration is governed by the Fermi energy EF which is pinned near the gap center. Small changes in glass composition and the presence of impurities usually have little effect on the properties except for small changes in EO and AE with composition. The insensitivity to impurities is ascribed to the fact that in a material lacking long-range order each foreign atom can satisfy its valency requirement,(1) and, thus, will not act as an electrically active center. The origin of the various states in the gap (DECs) (2,3) and the pinning of EF is attributed to those DECs which are inherent valence alternation defect centers.

The relationship between electronic conduction and stereochemical local bonding arrangements is illustrated by four examples: the unusual high conductance ON-state of the Ovonic Threshold Switch resulting from nonequilibrium excitation processes the high conductivity connected with the oxidation states of the Ovitron, the chemical modification of electronic transport in amorphous semiconductors and insulators, and the high temperature copper oxide ceramic superconductors. We discuss the effect of fluorination on raising the superconducting transition temperature to Tc=155K or higher, on eliminating oxygen diffusion and on orienting the crystallites in the YBaCuO superconductors.

The position of the Department of Energy (DoE) reiterated specifically at this meeting (1) is that photovoltaic solar energy conversion will not begin to make a large-scale contribution to our overall energy production until 1986 at the earliest, and more probably the year 2000. It is the theme of this paper that the basic materials problems which have led to such pessimistic projections have already been solved and that, given an intensive program of development, economically competitive photovoltaic cells can be mass produced in the near future.

The US Department of Energy is probably the best known body to have claimed that photovoltaic conversion devices cannot compete with fossil tuels or uranium until at the earliest 1986, or more likely 2000. Such statements are based on the known current technological problems of three types of silicon—single crystal, polycrystalline, and amorphous. With amorphous silicon the chief difficulty has been the high density of states (as explained by Taylor on p 672) which restricts the necessary current flow.

The crystallization properties of Te-Ge and Te-Ge-Sb alloys prepared by thermal evaporation were analyzed using various characterization techniques. Similar to previous results, our data for Te-Ge shows that alloys that deviate slightly from Te50Ge50 stoichiometry show drastically slower crystallization kinetics. Raman spectroscopy and x-ray diffraction show that alloys having non-stoichiometric atomic ratios phase separate during crystallization into a Te50Ge50 phase plus pure crystalline tellurium or germanium. It is this relatively slow process of phase segregation which limits the crystallization rate. Phase segregation during crystallization of non-stoichiometric Te-Ge can be eliminated by adding antimony to samples having a tellurium concentration of from 45 to 55 atomic percent over a wide range of Ge:Sb ratios. These alloys can have laser induced crystallization times of less than 50 nsec. The thermal crystallization temperature is reduced only slightly when antimony is substituted for germanium.

It is very difficult in 10 minutes to give you more than an overview of what is involved in the field of amorphous semiconductors. Also, I think you probably are more interested in applications at this conference rather than the physics involved.

Photovoltaic devices based on amorphous alloys of silicon with H and/or F have recently generated substantial interest in the photovoltaic community and the larger solid state physics and electronics communities as well. These developments have been reviewed recently by D. Carlson of RCA Labs(1). It was Carlson’s early collaboration with C. Wronski(2) which served to rekindle scientific and technological interest in a-Si:H produced by the glow discharge induced decomposition of SiH4 gas and the deposition of Si-rich fragments of this gas onto substrates held at 200–300°C(3–5).

On Sir Nevill’s 80th birthday, I wish to continue the discussion which 1 outlined in my contributions to his 65th and 75th birthday festschrifts [1,2], and I look forward to further exploring these concepts in the festschrift for his 85th.

The problems of our modern society are directly related to materials. Depletable fossil fuels, shrinking silver supplies, the high cost of gold, platinum and other precious metals all are interfering with industrial growth and creating societal problems. The fossil fuels and uranium supply us with energy without which no society or civilization can endure. However, there is a tendency to focus only on the fuel aspects of materials and ignore the larger materials picture, that is, that materials such as platinum, palladium and iridium, which are the core of the catalytic and electrochemical industries, affect not only the generation but the use of energy in a dramatic fashion.

During the past year a large number of models have been proposed to explain high Tc superconductivity in Cu-0 ceramic compounds. A nearly equal number of novel charge carrier pairing mechanisms have been invoked in these models as alternatives to the conventional phonon mediated process described in the BCS theory. Some of these models have recently been reviewed by Rice.(1)

We view amorphous materials as being ideal matrices for the encoding of information by optical or other means.1–10 The result can be the storage and reproduction of information serially or in parallel. The laser is ideal for the digital storage of information. One can achieve an exceedingly high density (one micron spots closely spaced) with a high signal to noise ratio, and utilize the same laser for reading, writing, and erasing. For higher densities, scanning electron beams are used.8

There are three key issues that challenge amorphous photovoltaics, namely, efficiency, stability, and production. In this paper we will review the current status of our amorphous photovoltaics at ECD in terms of these three issues and demonstrate how we have broken the efficiency-stability-production barrier.

The electronic nature of Ovonic threshold switching (1) has been definitively established.(2–8) Electronic action is also the initiating factor in our memories and the electronic concept has proven useful in establishing information encoding in an imaging manner as well.(9–22) It is now appropriate to elucidate the physical and chemical basis for such action.

It is fitting to discuss in this festschrift for Hellmut Fritzsche the phenomena which first interested him in the amorphous field.

It is difficult to do justice to the scientific and technological impact of the recent discoveries in high Tc superconducting ceramic oxides. It is now clear that the oxides that were the basis of the lanthanum copper oxide ceramics which moved the superconducting temperature from 23 to 30 to 40K (1) have been around for some time.(2,3)

It is well known in crystalline tetrahedral semiconductors that conventional incorporation of atoms by substitutional doping increases the electrical conductivity by many orders of magnitude. Such doping is the basis of the crystalline semiconducting industry and has recently been extended to amorphous silicon and germanium, with hydrogen appearing to play an important compensating role.(1–3)

The holographic response (Keneman, 1971; Ohmachi and Igo, 1972) of evaporated AS2S3 at 4880A has been ascribed to photo-polymerization of a hard-sphere As4S6 glass (deNeufville et al., 1973). The effect of illumination is to shift the location of the optical absorption edge (ΔE=-0.06eV), and to increase the refractive index (An=0.08). A comparable increase in refractive index occurs during annealing of evaporated As2S3 at 189 C, near its glass transition temperature, accompanied by a smaller edge shift (Δn=-0.03eV). While the optical absorption edge can be reversibly cycled by exposure at 25°C and annealing at 180°C (ΔE=-0.03eV), the accompanying index changes are relatively small (ΔE=**0.01). Furthermore, the X-ray diffraction pattern is unaffected by cycling, suggesting that the optical effects arise from the creation (exposure) and elimination (annealing) of a nonequilibrium concentration of trapped charge carriers rather than from an innate structural change.

An important difference between crystalline and amorphous structures is that while in crystals the local environment is the same everywhere, in amorphous materials one finds a large spectrum of 3-dimensionally varying spatial and bonding relationships. This gives rise to unique energy interactions, orbital overlaps and bonding configurations of the lone pair electrons associated with the group VI chalcogen atoms which do not occur in crystals. We present the ‘ball and spoke’ models we have built to test out the concepts of lone pair relationships discussed earlier (Ovshinsky 1972, Ovshinsky 1973, Ovshinsky and Fritzsche 1973).

There is a need for alternative energy sources. Photovoltaics are an attractive possibility, but their application has been limited by economic considerations in single-crystal materials, and for physical reasons such as grain boundaries in poly-crystalline materials. Amorphous semiconductors are especially attractive in this regard because they are basically much less expensive than their crystalline counterparts and because they possess a direct band gap with a high value for the optical absorption coefficient. We report here the development of a new alloy that eliminates the physical problems associated with the silicon-hydrogen alloys.