X-Ray Sensing Properties of a Lead Oxide Photoconductor Combined With an Amorphous Silicon Tft Array
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low. For example, at an electric field of 4 V/pm it takes 100 eV of absorbed x-ray energy to produce an electron-hole pair. For comparable device geometries, the signal obtained at the same dose using a CsI scintillator with a photodiode array would be three times higher. Moreover, careful optimization of selenium doping structures is necessary to obtain an efficient and fast charge collection on the pixel capacitances /6/. On the other hand, scintillators show a trade-off between sensitivity and spatial resolution; the thicker the scintillating layer, the higher the signal and the higher the spatial spread of the light output. It would be highly desirable to have a technology making the best of both worlds, i.e. a highly sensitive x-ray detector, with fast response and the capability of maintaining these features down to very small pixel sizes, i.e. having a high spatial resolution. The most straightforward method is to use a highly sensitive x-ray photoconductor. To alleviate the problem of charge collection on the pixel electrode, it appears attractive to have a charge collecting electrode as large as possible. This was realized in our arrays by vertically separating the collecting electrode from the rest of the array using a thick insulating layer. The same technology has proven suitable for increasing the active area of the photodiodes in a photodiode TFT array /7/. PbO has been chosen as x-ray photoconductor due to its high x-ray absorptivity. A favorable estimate on dark current, temporal response and potential charge conversion efficiency was obtained from the long time experience with PbO as photoconductor in plumbicon camera tubes /8/. The technological challenge was to produce layers with the desired thickness in a versatile, upscalable process and maintain the physical properties of the material. 321
Mat. Res. Soc. Symp. Proc. Vol. 507 ©1998 Materials Research Society
LEAD OXIDE TECHNOLOGY The lead oxide layers are deposited in a vacuum thermal evaporation process. Starting material is lead oxide powder. A major difficulty with PbO is to obtain a blocking electrical contact on top of the layer. Moreover, the lead oxide material is known to be sensitive to air and humidity. To solve these problems the layer is passivated with a Cl-doped selenium layer which effectively blocks electron injection from the (negatively biased) top Au contact. The Se-layer also prevents degradation of the lead oxide, due to contact with air. Optimization of the growth of lead oxide layers was done on plain glass substrates covered with a continuous Al electrode. These layers were characterized for dark current, sensitivity and temporal response to an x-ray pulse. The measurements were performed in a simple arrangement using a high voltage supply for establishing the electrical field across the lead oxide layer together with a Keithley current meter.
Figure 1 shows the dark resistivity and the (inverse) efficiency of one PbO sample deposited on an unstructured Al electrode on glass. The inverse efficiency is given in absor
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