Gap Electron Mobility Research Articles

Amorphous Silicon with Extremely Low Absorption: Beating Thermal Noise in Gravitational Astronomy.

Amorphous silicon has ideal properties for many applications in fundamental research and industry. However, the optical absorption is often unacceptably high, particularly for gravitational-wave detection. We report a novel ion-beam deposition method for fabricating amorphous silicon with unprecedentedly low unpaired electron-spin density and optical absorption, the spin limit on absorption being surpassed for the first time. At low unpaired electron density, the absorption is no longer correlated with electron spins, but with the electronic mobility gap. Compared to standard ion-beam deposition, the absorption at 1550nm is lower by a factor of ≈100. This breakthrough shows that amorphous silicon could be exploited as an extreme performance optical coating in near-infrared applications, and it represents an important proof of concept for future gravitational-wave detectors.

Thermal‐Sensing Fiber Devices by Multimaterial Codrawing

Thermal sensing and thermography yield important information about the dynamics of many physical, chemical, and biological phenomena. Spatially resolved thermal sensing enables failure detection in technological systems when the failure mechanism is correlated with localized changes in temperature. Indeed, IR-imaging systems have become ubiquitous for applications where line-of-sight contact can be made between the measured object and the camera lens. Nevertheless, many critical applications do not lend themselves to radiative IR imaging because of the subterraneous nature of the monitored surface, spatial constraints, or cost considerations. The recent challenge of monitoring the skin temperature beneath the thermal tiles on the space shuttle represents a good example in which high-spatial-resolution information is required on very large surface areas but which cannot be obtained using traditional thermal-imaging systems. Thus, the problem of continuously monitoring and detecting a thermal excitation on very large areas (100 m) with high resolution (1 cm) is one that has remained largely unsolved. We present a new methodology for measuring spatially resolved temperature information on large areas with high spatial resolution and low cost. Underlying our approach is a new fiber material that senses heat along its entire length and generates an electrical signal. This is in contrast to all previous work on thermal sensing using fibers, which require the use of optical probing signals. Although the fibers are produced by thermal drawing, they contain a set of materials that have not been traditionally associated with this process. The use of thermal drawing guarantees the production of extremely long fibers, while the innovation in preparation of the preform and choice of materials allows the incorporation of novel functionalities. Specifically, both thermal and electrical functionalities are obtained in the fibers studied in this communication, while optical and optoelectronic functionalities in alternative designs have been obtained previously and are reported elsewhere. The fibers are produced by a novel fabrication technique that enables the incorporation of materials with widely disparate electrical and thermal properties in a single, macroscopic, cylindrical preform rod, which subsequently undergoes thermal drawing to give solid-state microstructured fibers with high uniformity. The main requirements in the materials used in this preform-to-fiber approach are as follows: 1) the component which supports the draw stress should be glassy, so as to be drawn at reasonable speeds in a furnace, with self-maintaining structural regularity; 2) the materials must be above their respective softening or melting points at the draw temerature to enable fiber codrawing; and 3) the materials should exhibit good adhesion/wetting in the viscous and solid states without delamination, even when subjected to thermal quenching. According to these requirements, we identified suitable semiconducting, insulating, and metallic materials. The insulating material is a 75 lm thick polymer film: polysulfone (Ajedium, USA), having a glass-transition temperature, Tg = 190 °C. The chosen semiconducting glass, Ge17As23Se14Te46 (GAST), was arrived at by optimizing the composition formula GexAs40–xSeyTe60–y (10 < x < 20 and 10 < y < 15) under constraints of compatibility of Tg and viscosity with the codrawn polymer. Metallic electrodes are made of the alloy 96 %Sn–4 %Ag, which has a low meltingtemperature range (TM = 221–229 °C) below the fiber-drawing temperature of 270 °C. The chemical composition of the glass is chosen such that the electronic mobility gap of the amorphous semiconductor is small, yielding high electrical responsivity to small changes in temperature. The fabrication process (see Fig. 1A) begins with preparing cylindrical rods of the glass (see “Amorphous Semiconductor Synthesis” in the Experimental section). A cylindrical shell of polymer having an inner diameter equal to that of the glass rod is prepared with four slits removed from the walls. Four thin rods of the metal alloy are then placed in these slits. The glass rod is inserted into the polymer shell (Fig. 1A(a)), and a polymer sheet is then rolled around the resulting cylinder to provide a protective cladding (Fig. 1A(b)). Finally, the cylinder is thermally consolidated (Fig. 1A(c)) and subsequently is drawn in a fiber-draw tower producing hundreds of meters of C O M M U N IC A IO N S

Electronic conduction in shock-compressed water

The optical reflectance of a strong shock front in water increases continuously with pressure above 100 GPa and saturates at ∼45% reflectance above 250 GPa. This is the first evidence of electronic conduction in high pressure water. In addition, the water Hugoniot equation of state up to 790 GPa (7.9 Mbar) is determined from shock velocity measurements made by detecting the Doppler shift of reflected light. From a fit to the reflectance data we find that an electronic mobility gap ∼2.5 eV controls thermal activation of electronic carriers at pressures in the range of 100–150 GPa. This suggests that electronic conduction contributes significantly to the total conductivity along the Neptune isentrope above 150 GPa.

Soft atomic modes in glasses

Abstract The concept of atomic soft modes in glass dynamics and general assumptions and some basic results of the soft-mode dynamics theory of glasses are discussed in the light shed by experimental observations. Negative- U centres as actual electronic mobility-gap centres, vast pressure-induced effects in dynamics, quasilinear frequency dependence of acoustic (dielectric) absorption and high frequency acoustic-like excitations associated with the boson peak are shown to be essentially related to the soft modes in glasses. Some unsolved problems of the soft-mode dynamics theory of glasses are noted in conclusion.

Minimum metallic conductivity of fluid hydrogen at 140 GPa (1.4 Mbar)

Electrical conductivity measurements indicate that fluid hydrogen achieves the minimum conductivity of a metal at 140 GPa, ninefold initial liquid-${\mathrm{H}}_{2}$ density, and 2600 K. Metallization density is defined to be that at which the electronic mobility gap ${E}_{g}$ is reduced by pressure to ${E}_{g}\ensuremath{\sim}{k}_{B}T,$ at which point ${E}_{g}$ is filled in by fluid disorder to produce a metallic density of states with a Fermi surface and the minimum conductivity of a metal. High pressures and temperatures were obtained with a two-stage gun, which accelerates an impactor up to 7 km/sec. A strong shock wave is generated on impact with a holder containing liquid hydrogen at 20 K. The impact shock is split into a shock wave reverberating in hydrogen between two stiff ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ anvils. This compression heats hydrogen quasi-isentropically to about twice its melting temperature and lasts \ensuremath{\sim}100 ns, sufficiently long to achieve equilibrium and sufficiently short to preclude loss of hydrogen by diffusion and chemical reactions. The measured conductivity increases four orders of magnitude in the range 93 to 140 GPa and is constant at 2000 (\ensuremath{\Omega} ${\mathrm{c}\mathrm{m})}^{\mathrm{\ensuremath{-}}1}$ from 140 to 180 GPa. This conductivity is that of fluid Cs and Rb undergoing the same transition at 2000 K. This measured value is within a factor of 5 or less of hydrogen conductivities calculated with (i) minimum conductivity of a metal, (ii) Ziman model of a liquid metal, and (iii) tight-binding molecular dynamics. At metallization this fluid is \ensuremath{\sim}90 at. % ${\mathrm{H}}_{2}$ and 10 at. % H with a Fermi energy of \ensuremath{\sim}12 eV. Fluid hydrogen at finite temperature undergoes a Mott transition at ${D}_{m}^{1/3}{a}^{*}=0.30,$ where ${D}_{m}$ is the metallization density and ${a}^{*}$ is the Bohr radius of the molecule. Metallization occurs at a lower pressure in the fluid than predicted for the solid probably because crystalline and orientational phase transitions in the ordered solid do not occur in the fluid and because of many-body and structural effects. Tight-binding molecular dynamics calculations by Lenosky et al. suggest that fluid metallic hydrogen is a novel state of condensed matter. Protons are paired transiently and exchange on a timescale of a few molecular vibrational periods, $\ensuremath{\sim}{10}^{\ensuremath{-}14}\mathrm{sec}.$ Also, the kinetic, vibrational, and rotational energies of the dynamically paired protons are comparable.

Electronic mobility gap structure and deep defects in amorphous silicon-germanium alloys

Amorphous hydrogenated silicon-germanium alloys have been studied using a variety of junction-capacitance techniques to establish the dependence of the mobility gap electronic structure and the density of deep defects on the germanium content. The Urbach tail slope is observed to be nearly constant over the whole alloy range. The energy position of the dominant deep defect band near midgap is deduced and evidence for a shallower unoccupied defect band undergoing a large lattice relaxation is also observed. The total density of deep defects is found to increase exponentially with increasing germanium content and the details of this increase are shown to be consistent with a weak bond to dangling bond conversion model.

Electronic mobility gap structure and the nature of deep defects in amorphous silicon-germanium alloys grown by photo-CVD

Amorphous hydrogenated silicon-germanium alloys grown by photo-CVD have been studied using a variety of steady-state and transient junction-capacitance techniques. The dependence of the electronic mobility gap structure, the density of deep defects, and the carrier trapping properties on the germanium content has been investigated systematically. The Urbach tail slope is found to be nearly constant over the whole alloy range. The dominant defect band is found to track the midgap energy position, and the density of deep defects increases exponentially with increasing germanium content. These results are fully consistent with weak bond breaking theories and suggest that the quality of amorphous silicon-germanium alloys and a-Ge:H is inherently inferior to pure a-Si:H materials.

On the electrical conduction and current-controlled negative differential resistance with memory in bulk samples of glassy semiconductor Cu0.05As0.50Te0.45: NTC thermistor-type behaviour

The essential characteristics of NTC thermistor-type behaviour, besides the transition from a high resistance state to a permanent low resistance or memory state, have been analysed in the bulk modified glassy chalcogenide semiconductor Cu0.05As0.50Te0.45. Experimental results have shown a strong decrease in electrical resistivity (which ranges from 104 to 105 Omega cm in AsTe glassy alloys and its value for this alloy is 7.9*103 Omega cm at room temperature) and in the semiconductors electron mobility gap (which ranges from 0.4 to 0.5 eV for AsTe glassy semiconductors and is 0.37 eV for this alloy), due to the presence of the metallic element copper. Also, it has been shown that there are no marked non-ohmic effects in the range of voltages under study (up to 75 V for an interelectrode distance of 2.3 mm). Furthermore, the current-controlled negative differential resistance phenomenon has been justified in terms of a thermal model and confirmed by turnover voltage measurements (28.4 V for 24 degrees C and 16.9 V for 52 degrees C). Finally, several features of this glassy alloy, such as an easy synthesis, processing and low electrical resistivity values, make it appropriate for its use in the manufacture of NTC thermistors.

GaP electron mobility empirically related to donor concentration and temperature

GaP electron mobility empirically related to donor concentration and temperature