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The physics of a-Se

Alla Reznik

Overview

The use of amorphous selenium (a-Se) in advanced X-ray detectors [1] continues to motivate the study of the fundamental physics of this photoconductor. We are interested in the quantum efficiency of charge generation, the nature of photo-induced defects and the mechanisms of charge transport in a-Se. These research topics are linked by the common goal of understanding the behavior of a-Se at high electric fields (F > 70 V/µm), where impact ionization occurs.

Quantum efficiency

Indirect conversion X-ray detectors use a-Se to record light from a scintillation layer (e.g. CsI, LYSO). The absorption of light photons in a-Se produces mobile electrons and holes. We study the efficiency of this process as a function of photon energy and electric field by analyzing the magnitude of the photo-induced signal from a-Se samples. The figure below shows a typical experimental setup in which the intensity of monochromatic illumination is kept constant via neutral density filters under feedback control.

Our work indicates that the field-enhanced release of holes from states in the valence band tail may be significant for sub-bandgap photoexcitation [2].

Photo-induced defects

Perfect material stability under continuous illumination is a desired characteristic of a-Se detectors. This property is particularly important for a-Se targets of HARP (high-gain amorphous rushing photoconductor) video cameras, which are subject to prolonged exposure to light.
We are investigating the effect of temperature on the initiation of photo-induced defects, and have identified reversible and irreversible components of this phenomenon. Experimental work is ongoing and is accompanied by the development of a theoretical model of defect creation. Transition states associated with light-induced metastability phenomena form the basis of the model [3].

Charge transport

The time-of-flight (TOF) technique is a proven and powerful tool for evaluation of the transport of charge carriers in amorphous semiconductors. We study TOF waveforms to investigate the following:

  1. the non-linear effect of electric field on the transport of both electrons and holes (on their mobilities, in particular);
  2. factors that govern the electric field in a-Se under avalanche conditions and their implications for detector gain; and
  3. the details of charge transport and the nature of charge trapping; for example, the effect of temperature on charge trapping and on the development of defects in a-Se.

We use a pulsed N2 laser to create electrons and holes at the surface of a-Se samples. This charge is drawn apart by an applied electric field and produces a current. When holes (positive charge) are created at the anode face of the a-Se, they traverse the full cross-section of the detector. The duration of this trip is referred to as the time of flight and can be determined from a record of current magnitude versus time.

A variety of interesting scenarios arise under avalanche conditions, when holes in transit create additional electrons and holes at intermediate depths throughout the a-Se. These new charge carriers contribute their own signature to the observed current. TOF measurements captured under different operating conditions enable conclusions to be drawn regarding the physics of charge transport in a-Se.

References

  1. W. Zhao, D. Li, A. Reznik, B.J.M. Lui, D.C. Hunt, Y. Okawa, K. Tanioka, J.A. Rowlands, “Indirect flat-panel detector with avalanche gain: Fundamental feasibility investigation for SHARP-AMFPI (scintillator HARP active matrix flat-panel imager),” Med. Phys., 32(9), 2954-2966 (2005).
  2. A. Reznik, B.J.M. Lui, J.A. Rowlands, Y. Ohkawa, K. Tanioka, “The quantum efficiency of photo-charge generation in a-Se avalanche photodetectors,” New Developments in Photodetection (NIM-A), Beaune, France, June 19—24, 2005.
  3. A. Reznik, B. J. M. Lui, V. Lyubin, M. Klebanov, K. Tanioka, J. A. Rowlands, “The effect of temperature on photo-induced metastability in avalanche a-Se layers,” 21st International Conference on Amorphous and Nanocrystalline Semiconductors (ICANS21), Lisbon, Portugal, September 4—9, 2005.