Viswanath Aiyah

Amorphous chalcogenide Se1−x−y TexPy semiconducting alloys: thermal and mechanical properties

Thermal and mechanical properties of ternary Se rich Se1−x−yTexPy semiconducting glasses (Te < 20 at % and P < 10 at %) in vitreous bulk and film form have been studied by differential scanning calorimetry (DSC) and microhardness measurements. Bulk vitreous samples were prepared by conventional melt quenching techniques and the amorphous photoreceptor films were prepared by vacuum deposition onto oxidized aluminum substrates whose electrophotographic properties were reported previously. We measured the glass transition temperature Tg starting from a well defined thermal history and using both heating and cooling scans as a function of composition. Tg increases monotonically with both Te and P content. Both bulk and film samples evince similar compositional Tg dependence. The increase in Tg with the P content in the glasses follows the Tanaka rule, that is, P addition has a networking effect due to the trivalent nature of the P atom and increases the mean coordination number. Both Te and P additions initially inhibit crystallization but at high Te contents (∼20 at %) the crystallization behavior is comparable to the pure a-Se case. Glasses with ∼10 at % Te seem to have the greatest resistance to crystallization. The crystallization behavior does not correlate with the Tg behavior over the whole composition range. The Vickers microhardness HV increases with both Te and P content. HV vs. Te and P behavior is similar to that of Tg vs. Te and P content . The compositional dependence of both HV and Tg can be explained by the same factors that reduce Se chain mobility.

X-RAY INDUCED HOLE TRAPPING IN ELECTRORADIOGRAPHIC PLATES

Changes in the electrical properties of pure a‐Se and Cl‐doped a‐Se:0.35% As electroradiographic layers upon exposure to x‐ray radiation has been examined using xerographic techniques based on the first residual potential VR1 and the cycled‐up residual potential VR∞ Hole lifetime τ as determined from the Warter expression VR1=L2/2μτ has been found to decrease sharply with the x‐ray dose, tending to saturate at higher dose levels in both pure a‐Se and Cl‐doped a‐Se:0.35%As. The hole liftime following x‐ray exposure recovers toward its equilibrium value at the anneal temperature. The recovery time is dependent on the dose and is accelerated appreciably by temperature, indicative of a thermally activated process. Cycled‐up xerographic residual‐potential measurements have indicated that the deep‐hole‐trap population increases with exposure to x rays.Comparison of cycled‐up saturated residual potential and first‐cycle residual‐potential measurements have shown that as a result of x‐ray exposure, the apparent c...

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

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.

Thermal and mechanical properties of amorphous selenium films in the glass transformation region

Thermal and mechanical properties of photoreceptor-type amorphous selenium (a-Se) films have been studied via differential scanning calorimetry (DSC), thermomechanical analysis (TMA) as well as thermomicrohardness analysis (T mu HA). The a-Se films were vacuum deposited by thermal evaporation onto heated Al substrates and identically aged. The experiments were carried out over a temperature range encompassing the glass transformation to examine the nature of physical processes controlling the thermal and mechanical properties. In DSC experiments, the glass transformation kinetics was investigated via both heating and cooling schedules. By applying the present thermoanalytical methods-Ritland and Kissinger analyses-it proved possible to identify a typical Vogel-Tammann-Fulcher ( approximately exp(A/(T-T0))) type of response in the structural retardation times controlling the relaxation of the enthalpy. It was found that over a common temperature range, the temperature dependence of the mean relaxation time tau (T), whether determined from heating or cooling schedules, followed the viscosity-temperature data for a-Se suggesting that tau (T) approximately eta (T). The temperature dependence of the microhardness, Hv(T), on a semilogarithmic plot evinced a sharp decrease at a temperature Tg in the glass transformation region. The origin of this sharp decrease in the microhardness has been attributed to a structural relaxation process which has a mean retardation time proportional to the viscosity as in the enthalpy relaxation process. In the case of TMA however, it was found that the shift in the softening temperature with the heating rate indicated an apparent activation energy less than that for the viscosity. Two possible models are discussed for the interpretation of the TMA softening temperature which are based on the relaxation of the elastic modulus and an elastic deformation to viscous flow transition. By comparing the results from the present thermal and mechanical studies with the reported viscosity-temperature data, it is concluded that the rate of relaxation of enthalpy and microhardness of a-Se in the glass transformation region is inversely proportional to the melt viscosity which can be adequately described over a temperature range above approximately 30 degrees C by a Vogel-Tammann-Fulcher expression.

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.

Doped amorphous selenium based photoreceptors for electroradiography: Determination of x-ray sensitivity

Abstract There is a renewed interest in the use of amorphous Se based alloys in x-ray imaging applications. The present work reports the dependence of the x-ray sensitivity of Cl doped a-Se: 0.2% As detectors on the x-ray exposure, applied field, energy of the incident x-ray photons, and thickness of the detector, which are technologically important parameters. Further, the dependence of x-ray energy required for free electron-hole-pair creation on the applied field and x-ray photon energy is examined. The present set of measurements complement the dependence of x-ray sensitivity on charge transport parameters (charge-carrier ranges) reported in the 14th ICAS.

Charge carrier trapping in chalcogenide semiconductors

Interrupted field time-of-flight (IFTOF) transient photoconductivity measurements have been performed in conjunction with first residual and cycled-up xerographic measurements on pure a-Se and chlorinated a-Se:7.4%Te and a-Se:0.2–0.3%As electrophotographic films. The hole lifetime has been measured as a function of Cl doping and location in the sample. Cycled-up xerographic measurements were used in conjunction with IFTOF experiments to estimate the capture coefficient C t and diffusional capture radius r c .