Robert E. Johanson

The influence of deposition conditions and alloying on the electronic properties of amorphous selenium

Electronic properties of a-Se as a function of the source (boat) temperature and as a function of As (up to 0.7%) and Cl (up to 40 wt ppm) concentrations have been experimentally studied by carrying out conventional and interrupted field time-of-flight (IFTOF) transient photoconductivity measurements that provide accurate determinations of the drift mobility and the deep trapping time (lifetime). No variation in electron and hole lifetimes and mobilities for pure a-Se was observed with the source temperature, that is, no dependence was observed either on the deposition rate or on the vapor composition. The addition of As reduces the hole lifetime but does not change the hole mobility. At the same time, As addition increases the electron lifetime while reducing the electron mobility. The electron range μτ, however, increases with the As content, which means that the overall concentration of deep electron traps must be substantially reduced by the addition of As. Cl addition in the ppm range increases the hole lifetime but reduces the electron lifetime. The drift mobility of both carriers remains the same. We interpret the results in terms of a shallow-trap-controlled charge transport in which deep traps are due to potential under-and overcoordinated charged defects that can exist in the structure.

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.

Recombination of drifting holes with trapped electrons in stabilized a-Se photoconductors: Langevin recombination

Stabilized amorphous selenium (a-Se) is one of the x-ray photoconductors that is currently used in recently developed direct conversion flat panel x-ray image detectors. We have studied the recombination of free holes with trapped electrons in stabilized a-Se. Electrons were deeply trapped in a-Se by carrying out repetitive electron time-of-flight (TOF) transient photoconductivity experiments. By using conventional and interrupted field hole time-of-flight (IFTOF) transient photoconductivity techniques in a TOF, IFTOF, TOF sequence, we were able to develop a technique that allows the measurement of the capture coefficient between free holes and trapped electrons. We find that the capture process of holes by trapped electrons closely follows the Langevin recombination mechanism.

Properties of a-SbxSe1−x photoconductors

We have examined the thermal stability, and carrier drift mobilities and lifetimes (and hence electron and hole ranges) of a-SbxSe1−x photoconductors. We have found that the most thermally stable a-SbxSe1−x alloys can be obtained for x between 1 and 1.5 at.% which therefore limits the amount of Sb that can be added to improve the X-ray absorption and hence the X-ray photoconductivity of these alloys. On the other hand, we have found that the photoconducting films fabricated from a-SbxSe1−x alloys exhibit better electron and hole ranges when compared with the starting pure a-Se material. The X-ray sensitivity of a-Se:1 at.% Sb is greater than that of the pure a-Se.

X-ray induced effects in stabilized a-Se X-ray photoconductors

Abstract We report on X-ray induced changes in the electrical properties of stabilized amorphous selenium typical of the material used in direct conversion X-ray imaging devices. Carrier mobility and deep-trapping lifetime were measured using time-of-flight and interrupted-field time-of-flight (IFTOF) measurements. The hole and electron drift mobility is not affected by up to 1 R of exposure to 50 kV p X-rays. The hole lifetime decreases from 50 to 27 μs after exposure to 0.48 R. The hole lifetime slowly recovers to its initial value after circa 3000 min. The electron lifetime does not change after exposure to 1 R. The results are explained by an accumulation of excess hole traps due to capture of electrons in deep localized states. We also observe small changes in the dark current after exposure to X-rays.

1/f conductance noise in n-type amorphous silicon

Abstract We have measured conductance fluctuations in n-type hydrogenated amorphous silicon using current densities J in the range 0·1 < J < 0·55 A cm−2 at various temperatures from 13°C to 170°C. The noise power density varies as 1/f from 2 Hz to 5 kHz and increases as the square of the bias current at all temperatures studied as expected for flicker noise. The magnitude of the noise power increases with temperature and, if assumed to follow an Arrhenius curve, has an activation energy of 0·1 eV. For current densities exceeding about 1 A cm −2, we find large abrupt changes in the conductance superimposed on the flicker noise reminiscent of random telegraph noise.

Relaxation of the electrical properties of vacuum-deposited a-Se1−xAsx photoconductive films: Charge-carrier lifetimes and drift mobilities

The authors have examined the relaxation of the electrical properties of vacuum-deposited x-ray photoconductor-type a-Se1−xAsx (x=0%–0.5%) films in terms of the time evaluation of the deep trapping time τ, i.e., carrier lifetime, and the drift mobility μ from the time the samples were brought to 23 °C after they had been annealed at 55 °C (above the glass-transition temperature Tg) for 30 min. The changes in the lifetime and drift mobility arise from structural-relaxation processes and have been modeled using a stretched exponential-relaxation process, i.e., τ=τ∞+(τo−τ∞)exp[−(t/τsr)β], where τ∞ is the lifetime when the sample is fully relaxed (the final “equilibrium” value), τo is the initial lifetime, τsr is the characteristic structural-relaxation time that controls the relaxation of the observed property, and β is the stretching factor. The authors have examined the relaxation of τ and μ as a function of composition. Within experimental errors, the structural-relaxation time associated with electron an...

Active pixel image sensor for large-area medical imaging

The most widely used pixel architecture is a passive pixel sensor (PPS) where the pixel consists of a detector and an a-Si:H thin-film transistor readout switch. While the PPS has the advantage of being compact and amenable towards high-resolution imaging, the data line capacitance, resistance, and the column charge amplifiers add a large noise component to the PPS that reduces the minimum readable sensor input signal. Building upon previous research into active pixel sensor (APS) based amplified pixel readout circuits, this work investiates a current-mediated APS (C-APS) x-ray detection array for diagnostic medical imaging applications. Preliminary tests indicate linear performance, and a programmable circuits gain via choice of supply voltage and sampling time. In addition, the performance of C-APS amplified pixels is measured from both, a-Si TFT metastability and noise performance perspectives. Theory and measurements indicate that the C-APS pixel architecture is promising for diagnostic medical imaging modalities including low noise, real-time fluoroscopy.

1/f noise in bismuth ruthenate based thick-film resistors

We report measurements of 1/f noise in bismuth ruthenate (Bi/sub 2/Ru/sub 2/O/sub 7/) thick-film resistors in the frequency range of 1 Hz to 40 kHz including the dependence of the noise on the bias current, resistance in the range 1 k/spl Omega/ to 400 k/spl Omega/ and laser trimming effects. The noise spectra show that the magnitude of the noise parameterized by the relative noise index parameter C has the approximate dependence given by C=3/spl times/10/sup -15/ (R/sub s/n/sub s/)/sup 0.6/, where n/sub s/ is the aspect ratio and R/sub s/ is the sheet resistance. Technologically this is an important result since it allows the prediction of the approximate level of noise for any resistor made from the BIROX series compounds from the ink resistivity and the dimensions of the resistor. The link between the noise magnitude and noise models is examined.

X-ray-induced recombination effects in a-Se-based x-ray photoconductors used in direct conversion x-ray sensors

Stabilized amorphous selenium (a-Se) is currently used as an x-ray photoconductor in direct conversion flat-panel digital x-ray image detectors. Therefore, there is much interest in x-ray-induced effects in a-Se, especially changes in charge carrier lifetimes that result from x-ray exposure. We have observed that the exposure of an a-Se x-ray detector sample to x rays induces negative capture centers in the bulk and thereby reduces the hole lifetime. By using conventional and interrupted field time-of-flight (IFTOF) transient photoconductivity techniques in a TOF-IFTOF-TOF sequence, we were able to develop a technique that allows the measurement of the capture coefficient Cr between free holes and x-ray-induced negative centers, which we believe to be trapped electrons. We find that the capture process follows the Langevin recombination mechanism, the same recombination mechanism that has been observed in the case of recombination between free holes and free electrons in a-Se. We have shown that the conce...