Charge transport in stabilized A-Se films used in X-ray image detector applications
Haugen, Christopher Jon
The suitability of stabilized amorphous selenium (a-Se:0.2-0.5%As, 10-20 ppm Cl) as an x-ray imaging receptor is largely determined by its charge generation, transport, and trapping properties. The product of the charge carrier drift mobility, [mu], deep trapping lifetime, [tau], and applied electric field, F, also known as the schubweg, represents the average distance that a charge carrier will travel in the conduction band before being trapped in deep localized states within the mobility gap. Initial measurements on stabilized a-Se layers using TOF and IFTOF techniques showed that the hole and electron schubwegs were longer than the layer thicknesses. These measurements confirmed the electronic quality of the stabilized a-Se layers studied and schubweg limitations did not effect any of the subsequent experiments. Irradiation of the selenium layers causes a negative bulk space charge to build up within the bulk of the layer. The exponential distribution of this charge was probed with TOF current pulses. The distribution of charge and rate of detrapping was examined as a function of temperature, photon energy, and with infrared illumination. It had been speculated that the detrapping of the bulk space charge was the origin of persistent x-ray photocurrents in imaging systems. The mean detrapping current was found to be approximately 11 pA, orders of magnitude smaller than the expected dark current. However, the trapped charge modifies the electric field at the top metal electrode causing it to become more injecting as injection is very field sensitive. This is a possible origin for the observed persistent photocurrents. TOF measurements were used to determine the nature and field dependence of hole dispersion as the hole charge packet traversed a selenium layer. The measured dispersion increased linearly with an increase in the injected charge and a significant amount of the measured dispersion was due to coulombic repulsion even when small signal conditions were maintained. When the effect of coulombic repulsion was removed, the observed dispersion was attributed to a combination of classical diffusion and multiple trapping and release. The amount of dispersion observed is not expected to impact on the speed of fluoroscopic imaging systems. The energy required to create a free electron-hole pair, WEHP, was measured by integrating the x-ray induced photocurrent to find the number of free charge carriers and dividing that by the energy absorbed in the selenium layer. WEHP evidenced a strong field dependence which was extrapolated at the highest fields to show the intrinsic electron-hole pair creation energy was 5.9 eV. WEHP was shown to be temperature independent over the range of 263K to 300K This result supports the columnar recombination theory as the origin of the field dependence of WEHP.