B. Polischuk

An interrupted field time‐of‐flight (IFTOF) technique in transient photoconductivity measurements

An interrupted field time‐of‐flight (IFTOF) technique that enables displacement‐current‐free measurement of the photocurrent in the conventional transient photoconductivity experiment is described and implemented. During the drift of the photoinjected charge carriers across the sample, the applied field is removed at time T1 and reapplied at time T2=T1+ti, where ti is the interruption time. During the interruption period ti, the charge carriers interact with the deep traps so that the recovered photocurrent when the field is reapplied at T2 = T1 + ti indicates the concentration of carriers remaining in the transport band. Although the IFTOF technique has a number of distinct advantages for studying charge trapping and release processes in high‐resistivity solids, it has generally not been adopted as a convenient experiment since the sample capacitance results in large displacement currents at switching.The present paper describes a Schering‐type bridge network that is excited by a switchable floating high...

Metallic electrical contacts to stabilized amorphous selenium for use in X-ray image detectors

We investigate the junction between a-Se and a variety of metals with the aim of determining whether the choice of metal can improve the performance of X-ray image detectors, in particular minimizing the dark current. The samples consist of 500-μm thick stabilized a-Se with top electrodes formed from (in order of increasing work function) Ca, Mg, In, Bi, Al, Zn, Sn, Cr, Ag, Cu, Au, Pd, Ni, and Pt. For each metal, current transients following application of voltages from 30 V to 3000 V are measured for up to an hour. We find that dependencies of current on voltage typically follow power laws at high fields but vary from metal to metal. The dark currents span a range of five orders of magnitude depending on the metal used. The current is controlled by hole injection at the metal-Se junction but there is no consistent trend with the metals work function possibly because of the formation of an intermediate metal selenide layer.

Drift mobility relaxation in a‐Se

Time‐of‐flight (TOF) hole drift mobility (μd) measurements have been carried out on vacuum deposited and identically aged a‐Se photoreceptor type films over a temperature range encompassing the glass transformation region to study the nature of mobility controlling shallow traps in this elemental chalcogenide semiconductor. Differential scanning calorimetry (DSC) experiments using both heating and cooling scans have also been carried out on the same films to correlate the enthalpy relaxation phenomenon with the TOF drift mobility dependence on temperature. By considering the heating rate dependence of the minimum peak in the μd vs T behavior and the heating and cooling rate dependence of the DSC glass transition temperature, it is shown that the mean retardation times, τμ and τH, associated with the relaxation of the shallow traps and the enthalpy, respectively, have similar temperature dependencies, essentially Vogel–Tammann–Fulcher type, with negligible structural contribution. Correlation is also estab...

X-ray sensitivity of a-Se for x-ray imaging with electrostatic readout

Currently there is much interest in stabilized amorphous selenium (a-Se alloyed with 0.2–0.3% As and doped with 5–20 ppm Cl) as an x-ray photoconductor in various x-ray imaging systems. Although optical photogeneration in a-Se is well characterized, experiments on its x-ray sensitivity have been limited and, further, have resulted in widely differing conclusions especially for the electron hole pair (EHP) creation energy EEHP. In this work, we first introduce absolute and incremental sensitivity (S and s, respectively) concepts within the electrostatic readout mode of operation and then establish models linking these two sensitivities to the energy EEHP absorbed to create a free EHP. X-ray irradiation induced xerographic discharge experiments have been carried out on vacuum deposited stabilized a-Se x-ray photoconductor films to examine how the sensitivity depends on the charge carrier schubwegs (μτF where μ is the drift mobility, τ is the lifetime, and F is the electric field), the mean photon energy, an...

Determination of the deep-hole capture cross-section in a-Se via xerographic and interrupted-field time-of-flight techniques

Abstract Combined techniques based on first-cycle xerographic residual potential, cycled-up saturated xerographic residual potential, and interrupted field time-offlight (IFTOF) measurements are used to determine the hole-drift-mobility-lifetime product μτ deep trapping time,τ, integrated deep-trap concentration, N p trap capture coefficient and trap capture radius in a range of a-Se films. Very good agreement is observed between the μτ products determined via xerographic and IFTOF methods. Ballistic and diffusional deep-trapping models applied to this prototype amorphous semiconductor indicate that the capture radius is 2-3 [Abreve], indicative of neutral trapping centres with an integrated density of about 1013cm−3

A high-voltage interrupted-field time-of-flight transient photoconductivity apparatus

The authors describe a floating bridge technique and a high-voltage apparatus for carrying out interrupted-field time-of-flight (IFTOF) transient photoconductivity measurements with applied voltages up to 1 kV. Switching transient voltages appearing as a result of the removal and reapplication of the applied bias voltage, which otherwise would obliterate the normal TOF photocurrent signal, have been reduced to an undetectable level by using a floating Schering bridge network with two nulling capacitors. The bias voltage is applied from a single triggerable floating high-voltage switch which utilizes n-channel TMOS transistors and can deliver up to 1 kV in a switching time of approximately 150 ns. By switching the gain of an amplifier during the interruption time, even very small recovered photocurrent signals can be measured with respect to the pre-interrupted signal. The technique has been successfully applied to measure both hole and electron trapping lifetimes in a-Se:0.5%As electroradiographic photoreceptors, and to determine the range of the charge carriers. The distinct advantage of the present apparatus, as well as its shortcomings, over the complementary bias and single nulling capacitor method, which was described previously, are also addressed.

X-ray sensitivity of halogenated a-Se:As photoreceptors for electroradiography

The present work reports measurements on the x-ray sensitivity of Cl doped a-Se:0.1–0.3% As x-ray receptor films as a function of electric field, film thickness, and electron and hole schubwegs.

Progress in the science and technology of direct conversion a-Se X-ray sensors

Abstract The present paper provides a critical review and discussion of some of the recent advances in the science and technology of stabilized a-Se (alloyed with 0.2–0.5% As and doped with small amounts of Cl) based flat panel X-ray sensors. Although a-Se is now a well-established X-ray photoconductor used in flat panel direct conversion X-ray sensors that have produced excellent images, there are many scientific aspects of the work that have not been resolved. We identify and discuss several further experimental and theoretical studies that must be undertaken to advance the science of X-ray photoconductors to a point that the new knowledge can be incorporated into enhanced device design.

Thermal, mechanical and electrical relaxation phenomena in a-Se films

Abstract Time-of-Flight (TOF) hole drift mobility ( μ d ), Differential Scanning Calorimeter (DSC) and Thermomicrohardness (TμH) measurements have been carried out on vacuum deposited and identically aged a-Se photoreceptor type films over a temperature range encompassing the glass transformation region to study the nature of structural relaxations in this elemental chalcogenide semiconductor.

Study of photogenerated charge carrier dispersion in chlorinated a‐Se:0.3%As by the interrupted field time‐of‐flight technique

Dispersion of photogenerated charge carriers due to their mutual Coulombic repulsion hitherto has been neglected in considering the spread of the photocurrent in a typical drift mobility experiment such as the time‐of‐flight transient photoconductivity measurement. Interrupted field time‐of‐flight experiments carried out on a high‐resistivity semiconductor material (a‐Se) with a well‐defined mobility clearly show that the overwhelming mechanism for the dispersion of the charge packet in the absence of an applied field is due to Coulombic repulsion. Dispersion is directly proportional to time and the amount of photogenerated charge and puts a lower limit to the resolution of photodetector devices operating in the constant mobility regime.