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Solid state X-ray detectors based on large-area amorphous semiconductors such as amorphous silicon or selenium have been developed. The requirements for various applications in medical diagnosis determine the boundary conditions for different detectors. Key parameters are image receptor size, spatial resolution, image frequency, signal-to-noise ratio and long-term stability. Conventional thorax radiographs are 43×35 cm 2 in size. Therefore, to replace film, large detectors are required. Mammograms need to display microcalcifications of some 100 μm in detail. Fluoroscopic images are taken at a rate of 30 images/s or even faster. Moreover, all detectors need to deliver optimum image quality at the lowest tolerable dose. The lifetime should be at least 10 years. In this paper we discuss different types of detectors that could fulfill those needs. Two fundamental concepts are compared: A scintillator coupled to a photodiode and a direct converting semiconductor, both being arranged on an amorphous silicon switching matrix.

Amorphous silicon (a-Si) has proven to be the most suitable semiconductor for large-area devices. Our detector prototype with a pixel pitch of 200 μ m and an active area of 20×20 cm 2 uses one PIN photodiode and one PIN switching diode per pixel for readout. Cesium iodide is used as scintillator. Evaluation of the detector was performed in the laboratory as well as in a clinical site where it was integrated in a C-arm for cardiological investigations. In this paper, modulation transfer function, dynamic behavior, noise figures, and quantum yield will be discussed. The performance of these detectors represents a first step towards the goal of replacing existing fluoroscopic or radiographic X-ray systems for medical diagnosis.

The constant photocurrent method (CPM) 1 is now widely used for the basic characterization of amorphous hydrogenated silicon (a-Si:H). For detailed deconvolution procedures 2 reliable data are of great importance. We therefore studied the influence of different contacts on the measured spectra of identical samples. Undoped a-Si:H samples were covered with coplanar contacts made from NiCr, Ag, or Ti. Different contact spacings were used to investigate the influence of scattering in the samples 3 . An additional n + layer between a-Si:H and metal was used on some samples to ensure good ohmic contacts as a reference. The influence of SF 6 plasma etching (necessary for removal of the n + layer) was also tested. We found that for all the above arrangements, the subbandgap shoulder increases with increasing distance of coplanar contacts 3 . However, significant differences were found in the magnitude and slope of the shoulder for different contact metals. These differences are not related to contact ohmicity (addition of n + layer has negligible influence). The preliminary results indicate that this effect can be at least partially explained by the reflection of light at the contact edges, whose height and shape can be critical in the subbandgap region.

The new possibilities and advantages of SCLC limited TOF transients are demonstrated. First, the effects related with the realistic absorption profile, screening current and effective thickness are used to explain the shift of the “cusp” position with the increasing laser pulse intensity. The model of a full charge extraction is used to explain the observed “extraction” time (t*) and exemplified by c-Si diode. The first results on a-Si:H p-i-n junctions together with the corresponding theory are presented.

In connection with the development of solar cells made from amorphous silicon (a-Si:H), we investigated the temperature dependence of the photocurrent under reverse bias of different diodes. From the saturation behavior of the current-voltage characteristics we conclude that the reverse-bias photocurrent is of primary nature. Therefore, it is expected to be independent of temperature. A slight decrease with rising temperature might be due to increasing absorption within the p+ layer. Surprisingly, we found an increase of the photocurrent with temperature in all samples investigated. The dependence of this effect on bias voltage, illumination wavelength and intensity suggests the following picture. Using weakly absorbed light, temperature dependent absorption can explain the positive temperature coefficient (>5·10−3/K). Strongly absorbed light leads to a small temperature coefficient of the order of +10−4/K. The photo-generated carriers influence the internal field distribution and may thus cause an additional increase of the current.

We have studied quenching and enhancement effects of photoconductivity (PC) and photoluminescence (PL) by IR-excitation from the non-equilibrium trap populations in glow discharge deposited a-Si:H films. In a dual beam arrangement (hν1 = 1.92 eV, hν2

Large-area optical scanners require sensors with high photoconductivity and fast response. By a-Si:H sandwiched between a metal and an ITO electrode we obtained a low dark current for a high signal-to-noise ratio. The influence of problems like crosstalk and Staebler-Wronski effect is described.

a-Si:H/Pd and a-Si:H/ITO interfaces were examined by TEM. In the case of Pd, the influence of an oxide layer on silicide formation and the silicide structure were investigated. It is shown that the stability and electrical performance of the ITO contact on a-Si:H are mainly dependent on surface roughness and an interfacial oxide layer.

We have studied the temperature dependence of the photoluminescence steady state and transient photoconductivity of a-Si:H down to 4 K. Below 50 K we observe photoconductivity with ημτ ⋍ 10 −11 cm 2 / V , which is independent of temperature and varies only little with the defect density in the films. We propose that this conduction arises predominantly from the drift of the photoexcited electrons and holes during thermalization in the extended states prior to the localization in states below the mobility edge. An important implication of the present data is that, even at helium temperatures, a considerable part of the carriers recombines in a non-geminate process.

The temperature dependence of the photoluminescence, steady state and transient photoconductivity of a-Si:H has been studied between 4 and 500 K. Below 50 K a temperature independent ημτ-product of 10−11cm2/V is observed, which varies only little with the defect density. We propose that this mode of photoconduction arises from the drift of photoexcited electrons and holes during their thermalization near the mobility edges. It is shown that photoexcitation of deeply trapped holes by IR-radiation leads to an enhancement of the photoconductivity at low temperature. Time dependent studies yield information on the mechanism of trap emptying.

The stability of image sensors made from a-Si:H was studied. If senssor are light-soaked in moist ambient, the reverse dark current rises by orders of magnitude. Contrary to the Staebler-Wronski effect, this increase disappears at room temperature between 1 minute and several days. We suggest: Ambient-induced defect states emerge at the interface a-Si:H/ITO in presence of H 2 O. They act as hole traps 1.0 … 1.4 eV above E v . Thermal emission governs the relaxation of the dark current.