Peter J. Walsh

Experimental results in amorphous semiconductor switching behavior

Abstract Experimental investigations carried out at Picatinny Arsenal on the electronic and switching behavior of chalcogenide amorphous films are summarized. The I−V off state behavior of the material indicate two distinct processes, A and B, each contributing an exponential type current dependence on voltage. At different temperatures, the off state conduction process A, found at fields below about 3 × 10 5 V/cm, can be normalized to a single I − V curve displaying an ohmic and exponential region, each a single aspect of one single activated process. The exponential slope is intimately connected with the dielectric properties of the medium. It is found that at the switching threshold the effective shift in activation energy is substantially independent of temperature, of hydrostatic pressure, of thickness and of limited material variations. The conduction by process B at high fields can also be normalized. Both process A and process B have essentially the same activation energy. Analysis of the conduction by process B indicates that an empirical criterion for the onset of switching is that the activation energy for process B be essentially eliminated by the field.

Photolysis of Fe(CO)5 adsorbed on GaAs at 77 K

It is shown experimentally that pentacarbonyl iron can be adsorbed and desorbed on a GaAs substrate at low temperatures and photolyzed with ultraviolet illumination to yield Fe film deposit. A simple double beam laser technique is used to monitor the sequence of events during adsorption, desorption, and UV illumination. This work is the first evidence for the direct formation of a thin metal film on a cold semiconductor surface by ultraviolet photolysis of a condensed organometal.

NEGATIVE CAPACITANCE IN AMORPHOUS SEMICONDUCTOR CHALCOGENIDE THIN FILMS

Certain amorphous materials exhibit rapid electrical switching. The variation in their capacitance was evaluated as a function of temperature and electric field in the high‐resistance state. The zero‐field capacitance follows a Curie‐Weiss law above room temperature. At low temperatures, and at fields near the threshold of switching for higher temperatures, the open‐circuit incremental ac capacitance becomes negative.

Equations governing threshold switching in amorphous semiconductors

Extensive data are collected and presented to test the coupled‐carrier equations and attending density‐switching criterion proposed to describe nonthermal threshold switching in amorphous semiconductors. The equations and switching criterion predict in magnitude and functional form a wide range of phenomena found in the off‐state, during the switching‐on, in the on‐state, and during the switching‐off of amorphous chalcogenide films. The critical temperature noted in a variety of experiments is identified as the elevated temperature at which the switching criterion is fulfilled thermally in the presence of a small field.

Analytical model for low-pressure gas discharges: application to the Hg + Ar discharge.

A general technique for analyzing complicated gas discharges has been developed and applied to the Hg + Ar (fluorescent lamp) discharge. The theoretical model includes electron excitation and deexcitation, two-state ionization through a saturated metastable level, and proper treatment of the self-absorption of the resonance radiation. The analysis yields simple analytic expressions for the electron temperature, the resonance radiation, and the electric field. When applied to Hg + Ar discharges, these analytic expressions yield good quantitative agreement with the available absolute data on the dependence of the electron temperature, the Hg 2537-A radiation, and the electric field on mercury pressure and current.

Voltage‐current relationship for pulsed arc discharges

A theoretical treatment of the electrical behavior of pulsed discharges in gas‐filled flashlamps is presented. The theory is based on a simple model of the plasma and a balance of electrical power input and radiation output. It is assumed that the plasma radiates as a greybody with an emissivity that depends on the electron density. The principal result is an analytic expression for the arc voltage as a function of current, with initial voltage, arc length and diameter, and atom density as parameters. The expression describes the entire voltage pulse, including the initial fall, the following minimum and the peak when the current peaks. The predicted voltages substantially agree with measurements, including dependences on lamp length and diameter and initial voltage. The theory is an improvement on the empirical V‐I relation commonly used in flashlamp circuit analyses.

Optical and electrical effects during polymerization and depolymerization in liquid sulfur: indications for the nonuniformity model for covalent liquids

The polymerization and depolymerization of liquid sulfur refers, respectively, to the processes of chain growth and chain scission promoted by changing thermodynamic conditions. These structural transitions cause major optical and electrical consequences, including: (1) color change from yellow to orange to red, (2) peaked high angle and low angle scattering and minimum in transmission of He-Ne laser light at the structural changes, (3) a decrease in the current induced by a pulsed ruby laser above the polymerization temperature T(p) compared with below T(p), (4) two changes in the sign of the temperature coefficient of resistance in the vicinity of T(p) and the depolymerization temperature T(d), and (5) superlinear current-voltage characteristics above T(d). The data are interpreted as being suggestive of the structure of a liquid being better described by a nonuniformity rather than random network model.

Radiation-Conserving Incandescent Lamps

In this paper, the technology of an energy-conserving incandescent lamp is described. The lamp has a spherical envelope coated with a multilayer heat mirror of TiO2-Ag-TiO2. The coated envelope focuses the infrared radiation back to a centered compact filament of high emissivity while transmitting the visible wavelengths. The energy savings depend on the emissivity of the filament and the effective reflectance and visible transmittance of the coated envelope. Performance has been evaluated both in a model lamp system and in actual lamps.

Theoretical model of visible radiation from rare gas flashlamps.

A simple theoretical model of visible light emission from xenon flashlamps is presented. The continuum light emission is calculated from the rate of electron-ion recombination in the xenon plasma, which is treated as a greybody in thermal equilibrium. The effect of radiation reabsorption is calculated in terms of the temperature-dependent greybody emissivity. The model predictions of radiated power and energy are compared to measured data. Reasonable agreement is obtained over a wide range of parameters of practical interest. Thus the model provides a useful analytical tool for first-order engineering design of xenon flash-lamp illumination systems.

Amorphous semiconductor threshold on−state properties as functions of decay time, ambient temperature, and polarity

The on−state conductance and radiative emission properties of an amorphous semiconductor threshold switch have been investigated. The transient on−state conductance GT and differential conductance GT are found to be constant for time intervals τ of up to 120 nsec during which the voltage is below the low−frequency holding voltage but above the knee voltage. The knee voltage is the voltage at which the transient on−state current−voltage curve coalesces into the transient off−state curve in the down−voltage direction. For τ≳120 nsec, GT and GT vary asymptotically with time. As ambient temperature is increased from 300 to 400 °K, GT is observed to remain constant or increase slightly as would be expected in dGT/dT for a metal. If the on−state is assumed to have a band gap, conductance vs temperature measurements indicate that the gap would have a value less than 1.2×10−4 eV, and thus for practical purposes the gap could be considered to be essentially zero. The intensity of the nonthermal radiative emissi...