Low-Frequency Noise in a-Se Based X-Ray Photoconductors

We report on the excess, low-frequency noise in pin-like amorphous selenium alloy structures that are used in direct-conversion x-ray imaging detectors. These are the first measurements of the noise power density spectrum in these structures under reverse bias. Of the two samples measured, one has a noise spectrum that fits well to a 1/f(alpha) power law with alpha near one. The 1/f noise is not atypical except for a nonlinear dependence on d.c. current at fields above 5 V mu m. The variance in correlated double sampling measurements of the noise signal is calculated and related to the 1/f noise spectrum. The other sample has a white noise spectrum down to 10(-2) Hz. The white noise of the second sample is larger than the 1/f noise and is likely masking the 1/f noise over the measured frequency range. The origin of the white noise is not known.