Hellmut Fritzsche

Localization and metal-insulator transitions

One: Metal-Insulator Transitions: Experimental.- The Disordered Insulator: Electron Glasses and Crystals.- Tuning the Metal-Insulator Transition in N-Type Silicon with a Magnetic Field.- Metal-Semiconductor Transitions in Doped IV-VI Semiconductors.- Metal-Insulator Transitions in Pure and Doped V02.- Composition-Controlled Metal-Insulator Transitions in Metal Oxides.- Pressure-Induced Insulator-Metal Transition.- The Metal-Insulator Transition and Superconductivity in Amorphous Molybdenum-Germanium Alloys.- On the Nature of the Metal-Insulator Transition in Metal-Rare-Gas Mixture Films.- Electrical Conductivity of Discontinuous Metal Films.- Metal-Nonmetal Transition and the Critical Point Phase Transition in Fluid Cesium.- The Semiconductor-to-Metal Transition in Liquid Se-Te Alloys.- Localization and the Metal-Nonmetal Transition in Liquids.- Diffusion and Conduction Near the Percolation Transition in a Fluctuating Medium.- Counter-Cation Roles in Ru(IV) Oxides with Perovskite or Pyrochlore Structures.- The Mott Mobility Edge and the Magnetic Polaron.- Two: Metal-Insulator Transitions: Theoretical.- Metal-Nonmetal Transitions and Thermodynamic Properties.- Metal-Insulator Transition and Landau Fermi Liquid Theory.- Long-Range Coulomb Interaction Versus Chemical Bonding Effects in the Theory of Metal-Insulator Transitions.- The Metal-Insulator Transition in Liquid Doped Crystalline and Amorphous Semiconductors: The Effect of Electron-Electron Interaction.- Exciton Condensation and the Mott Transition.- Metal-Insulator Transition in Doped Semiconductors.- Flux Quantization in Rings, Cylinders and Arrays.- Localization and Heavy Fermions.- Anderson Localization.- Effect of Phase Correlations on the Anderson Transition.- Electron-Lattice-Interaction Induced Localization in Solids.- Density Correlations Near the Mobility Edge.- An Alternative Theory for Thermoelectric Power in Anderson-Mott Insulators.- Transport Properties Near the Percolation Threshold of Continuum Systems.- Three: Quasi-One-Dimensional and Quasi -Two-Dimensional Systems.- First-Order Phase Transition to the Metallic State in Doped Polyacetylene: Solitons at High Density.- The Germanium Grain Boundary: A Disordered Two-Dimensional Electronic System.- Structural Properties of Two-Dimensional Metal-Ammonia Liquids in Graphite.- Physical Properties of the Quasi-Two-Dimensional Compound La2NiO4.- One Electron Band Structure of a Collection of Resonant States.- Inelastic Scattering and Localization in Two Dimensions.- Existence of a Sharp Anderson Transition in Disordered Two-Dimensional Systems.- Fluctuation Kinetics and the Mott Hopping.- Aspects of 2D and 3D Conduction in Doped Semiconductors.- Localization Phenomena and AC Conductivity in Weakly Disordered Quasi-One-Dimensional and Layered Materials and in Anisotropic Low Dimensional Systems.- of Companion Volumes:.- Tetrahedrally-Bonded Amorphous Semiconductors, Edited by David Adler and Hellmut Fritzsche.- Physics of Disordered Materials Edited by David Adler, Hellmut Fritzsche, and Stanford R. Ovshinsky.- Author Index.

Radiation Hardness of Ovonic Devices

Ovonic threshold switches have been exposed to pulses of flash X-rays to levels of 1.8×1011 rads/sec and to fast neutron fluences of as high as nvt = 1.2×1017 n/cm2. The switching devices have continued to function during the transient of the X-ray flash and experienced no permanent damage or change of their electrical parameters in excess of the resolution of the experimental testing procedure which was about ±100% in the case of the neutron exposure.

Electron beam creation of metastable defects in hydrogenated amorphous silicon: hydrogen collision model

Abstract Metastable dangling bond (DB) creation during keV electron-beam irradiation of hydrogenated amorphous silicon (a-Si:H) is explained with the H collision model of light-induced metastability and several physically reasonable assumptions. Each incident electron creates an electron–hole pair every nm as it traverses the sample, resulting in a 1-nm-radius ‘hot tube’ extending from the front to the back surface of a film

Photoinduced stress in hydrogenated amorphous silicon films

Photo-induced compressive stress ΔS in hydrogenated amorphous silicon (a-Si:H) has been studied using films deposited by plasma-enhanced or hot-wire chemical vapor deposition on crystalline silicon microcantilevers. The kinetics of ΔS(t) first rises with exposure time as t0.5 and follows a stretched exponential. The saturation values ΔSsat correspond to volume changes of about 10−3, which excludes the possibility that ΔS is a consequence of the light-induced creation of coordination defects. The highest-quality films have large initial stress, small values of the Young’s modulus, and a rapid approach of ΔS(t) towards saturation.

Stress and internal friction associated with light-induced structural changes of a-Si:H deposited on crystalline silicon microcantilevers

The study of light-induced changes of the mechanical properties of a-Si:H should be valuable in the search for the microscopic mechanisms behind the Staebler-Wronski (SW) effect. We have developed a sensitive technique for studying such changes by depositing a-Si:H films onto commercial scanning probe microscope Si microcantilevers. The detection system of the microscope provides for measurements of beam bending, oscillation resonant frequency, and in-resonance damping factor. The internal friction of an a-Si:H film is much larger than that of crystalline Si and is the largest damping factor of the bilayer beam. We observed an increase in relative volume, ΔV/V, with photocarrier generation rate, G, and exposure time, t, following ΔV/V G 0.7 t 0.45 in intrinsic as well as in 1 ppm PH 3 /SiH 4 doped a-Si:H. The volume changes could be reversed by annealing and were the same for CW and pulsed light exposures using 400 μs long square pulses at a rate of 200 s -1 . Based on the magnitude of ΔV/V and the fact that it does not saturate we suggest that the structural changes causing ΔV/V permeate the whole film and are not limited to defect sites.

Light induced stress in a-Si1-xGex:H alloys and its correlation with the Staebler-Wronski effect

Abstract Photoinduced compressional stress ΔS has been studied in hydrogenated amorphous silicon and silicon–germanium alloys, a-Si1−xGex:H with x=0, 0.4 and 0.67. The films were deposited by plasma enhanced chemical vapor deposition onto 4 μm thick crystalline silicon microcantilevers commonly used in scanning probe microscopy. The initial stress S0 of the films was obtained from the initial bending and the Youngs modulus from the cantilever resonance frequency. The kinetics of ΔS(t) follow a stretched exponential. ΔS(t) cannot be a consequence of photoinduced defect creation because ΔS continues to rise when defect creation has saturated and the largest ΔS corresponds to a relative volume change ΔV/V=1×10−3, too large for 4×1017 cm−3 defects. We observe a significant decrease in ΔS between x=0.4 and x=0.67 alloy composition just where defect creation is greatly diminished. We suggest that defect creation is associated with the time dependent large local strains in and around the volume elements of electron–hole recombination. ΔS is the time and spatial average of the local configuration changes.

Early Research On Amorphous Silicon: Errors and Missed Opportunities

It is instructive to reflect on the past research of an entirely new material such as hydrogenated amorphous silicon in order to analyze which impediments lay in the way of reaching our present understanding. What in retrospect appears to be clear and important evidence was often not recognized as such and, therefore, not incorporated into a coherent picture for a surprisingly long time. One reason for the inability to see and understand is ones mode of thinking, which is conditioned by training and prior experience. Other impediments to progress are some persuasive, but erroneous, views of respected members of the scientific community. A third factor that tends to slow progress is a false sense of competition which, for example, prevents research laboratories from a fruitful exchange of materials and samples. It appears that human faults run neck to neck with natures intricacies in slowing the pace of our understanding.

Space charges resulting from photocurrents exceeding the thermionic emission currents in a-Si:H

Abstract When the photocurrent exceeds the thermionic emission current of electrons and holes, a photoconductor cannot remain neutral. A positive space charge appears near the cathode when the contribution of holes to the photocurrent is less than that of electrons. That happens in a-Si:H at all light intensities at low temperature and, at room temperature and above, at high light intensity. As long as the space charge is limited to a narrow region near the cathode, the photocurrent remains ohmic. We found conditions, however, under which the space charge spreads through a significant fraction of the sample causing the photocurrent to become subohmic. We have measured the total space charge as a function of photocurrent in the ohmic and subohmic regime and explored the conditions for a spatial spread of the charge resulting in subohmicity, as a function of temperature, light intensity and the concentration of dangling bond defects. Available models predict that the photocurrent, I, is proportional to the square root of the voltage, U, in the subohmic regime. Experimentally, we find I∝Um with m=0.25±0.04. We suggest directions for improving the models.