F. Wooten

Electrical Conductivity Induced in Insulators by Pulsed Radiation

The minimum prompt photoconductivity induced by pulses of x rays, gamma rays, and energetic electrons in various amorphous and disordered insulating organic and inorganic materials is predicted on the basis of data for the scattering of hot electrons in solids and the band gap for insulators. For total doses of 3 × 104 to 30 × 104 rad or greater, the minimum prompt photoconductivity is predicted to be linear with dose rate, ¿, and is given by ¿(¿-1cm-1)=5×10-19¿0¿/Eg2, where ¿0 is the density (g/cm3) and Eg is the optical band gap (eV). This formula agrees well with data for a variety of plastics, mica, and borosilicate glass under widely different irradiation conditions. The formula considerably underestimates absolute values of prompt conductivities observed for Al2O3, MgO, and certain plastics, because the model does not hold for ordered materials.

Properties of Thin Antimony Films Deposited in High Vacuum

The properties of thin artimony films deposited at pressures down to 10/ sup -9/ mm Hg and below are described. Depositions at 10/sup -7/ mm Hg and higher confirm the report that the evaperation rate of 100 A/sec controls the quality of the film, but with rates of 1 or 2 A/sec at lO/sup -8/ mm Hg only 3 out of 7 depositions prodaced heterogeneous films. Of 10 depositions at 10/sup - 9/ mm Hg and less, no heterogeneous films were produced, even with rates of 0.1 to 1 A/sec. The purity did not affect film quality. The high evaporation rate prevents deleterious reactions of the antimony with atmosphere impurities, probably water. (T.R.H.)

Enhanced Photoemission and Photovoltaic Effects in Semitransparent Cs3Sb Photocathodes

Measurements were made of photoemission and photovoltaic effects in thin (300–1250 A) films of Cs3Sb, on a glass substrate, by scanning with a 20 μ‐diam light spot. Enhanced photoemission and photovoltaic effects are obtained in regions within 1 to 3 mm from an Al electrode contact. Enhancement is about 25% with white light, and as much as a factor of 2.5 at 6200 A. A band model is proposed in which the top of the valence band is fixed with respect to the Fermi level at the vacuum surface, but bends up at the glass substrate and, even more, at the electrodes.

Photoemission and electron scattering in Cs3Bi and Cs3Sb

Photoemission studies of Cs3Bi and Cs3Sb have been made for the spectral region from 5 to 11.5 eV. Structure in the energy distribution as a result of structure in the valence‐band density of states is the same as that measured previously at lower energies. As the photon energy is increased, this structure moves to higher energy by an amount equal to the increase of h ν. This further supports the argument that conservation of k is an unimportant selection rule for these materials. Monte Carlo calculations of photoemission from these materials have made possible a determination of the scattering parameters and density of states. Photoemission from Cs3Sb can be understood quantitatively only by including scattering of electrons at acceptor sites. For both Cs3Sb and Cs3Bi, the higher lying valence‐band peak has an area twice that of the lower lying peak. However, the electron‐electron scattering cross section happens to increase with increasing electron energy so as to reduce the magnitude of the higher lyin...

Role of MnO Substrates in Enhanced Photoemission from Cs3Sb

Manganese oxide films are frequently used as substrates for Cs3Sb photocathodes prepared on glass windows. Such photocathodes have significantly enhanced sensitivity at wavelengths in the range 5500–6500 A. Photovoltaic and photoemission measurements indicate that the enhanced photosensitivity results from band bending in the Cs3Sb at the MnO substrate interface. Band bending provides an electrostatic field, which reduces the barrier height for photoemission from near the substrate.

Visualization techniques for molecular dynamics

Electron and X-ray diffractometry visualize atoms in severely stressed single-crystal silicon and help analyze the resulting phase transformations. Massively parallel computers simulate both diffraction techniques. We study these phase transitions with a number of simulation techniques. We calculate the position of the atoms in the material during the process and display atomic images of the crystal structure as a function of time. Simulated diffraction patterns enable us to follow structural transformations more easily. In addition, we have developed several diagnostic imaging techniques that aid the analysis of phase transformations: the pair-correlation function, bar-code plotting, ring statistics and subvolume visualization. >

Secondary electron emission from Cs3Sb

Monte Carlo calculations of secondary electron emission from Cs3Sb have been made for primary electron energies in the range 0.6–10 eV. All parameters in the model were determined by analysis of independent photoelectric emission experiments. Good agreement is obtained between calculated and measured secondary emission yields. Structure in the measured yield related to the threshold for electron pair production also shows up in the calculations. The results indicate that the semiclassical three‐step model for photoelectric emission (excitation, scattering, and escape) also provides an accurate description of secondary electron emission.

Optical Properties Of Silicon Clusters Deposited On The Basal Plane Of Graphite

Laser ablation was used to deposit of silicon on highly oriented pyrolytic graphite surfaces in an ultra high-vacuum environment equipped with Auger electron spectroscopy (AES), scanning tunneling microscopy (STM) and luminescence spectroscopy. For deposition of up to several monolayers, post annealing produced silicon clusters, whose size distribution was determined vs annealing time and temperature using STM. Pure silicon clusters ranging from 1 to 10 nm showed no detectable photoluminescence in visible range. Exposure to oxygen at 10{sup {minus}6} Torr and for up to 8 hours showed adsorption on the surface of the clusters without silicon oxide formation and no detectable luminescence. Hydrogen termination of these clusters was accomplished by exposing them to atomic hydrogen beam but did not result in any photoluminescence. Prolonged exposure of these clusters to ambient air, however, resulted in strong photoluminescence spectra with color ranging from red to greenish-blue depending on average cluster size. Auger electron spectra revealed the existence of partially oxidized silicon clusters. This luminescence could be due to either an oxide phase or to changes in electronic structure of the clusters as a result of quantum confinement effect.

Computational Diagnostics for Detecting Phase Transitions During Nanoindentation

We study nanoindenmtion of silicon using nonequilibrium molecular dynamics simulations. with up to a million particles. Both crystalline and amorphous silicon samples are considered. We use compumtional diffraction pattems as a diagnostic tool for detecting phase transitions resulting from structural changes. Simulations of crystalline samples show a transition to the amorphous phase in a region a few atomic layers thick surrounding the lateral faces of the indenter, as has been suggested by experimental results. Our simulation results provide estimates for the yield strength (nanohardness) of silicon for a range of temperatures.