An indirect flat-panel imager (FPI) with avalanche gain is being investigated for low-dose x-ray imaging. It is made by optically coupling a structured x-ray scintillator CsI(Tl) to an amorphous selenium (a-Se) avalanche photoconductor called HARP (high-gain avalanche rushing photoconductor). The final electronic image is read out using an active matrix array of thin film transistors (TFT). We call the proposed detector SHARP-AMFPI (scintillator HARP active matrix flat panel imager). The advantage of the SHARP-AMFPI is its programmable gain, which can be turned on during low dose fluoroscopy to overcome electronic noise, and turned off during high dose radiography to avoid pixel saturation. The purpose of this paper is to investigate the important design considerations for SHARP-AMFPI such as avalanche gain, which depends on both the thickness d(Se) and the applied electric field E(Se) of the HARP layer. To determine the optimal design parameter and operational conditions for HARP, we measured the E(Se) dependence of both avalanche gain and optical quantum efficiency of an 8 microm HARP layer. The results were used in a physical model of HARP as well as a linear cascaded model of the FPI to determine the following x-ray imaging properties in both the avalanche and nonavalanche modes as a function of E(Se): (1) total gain (which is the product of avalanche gain and optical quantum efficiency); (2) linearity; (3) dynamic range; (4) gain nonuniformity resulting from thickness nonuniformity; and (5) effects of direct x-ray interaction in HARP. Our results showed that a HARP layer thickness of 8 microm can provide adequate avalanche gain and sufficient dynamic range for x-ray imaging applications to permit quantum limited operation over the range of exposures needed for radiography and fluoroscopy.

Although the effect of the impact ionization and the consequent avalanche multiplication in amorphous selenium ͑a-Se͒ was established long ago and has led to the development and commercialization of ultrasensitive video tubes, the underlying physics of these phenomena in amorphous semiconductors has not yet been fully understood. In particular, it is puzzling why this effect has been evidenced at practical electric fields only in a-Se among all amorphous materials. For instance, impact ionization seems much more feasible in hydrogenated amorphous silicon ͑a-Si: H͒ since the charge carrier mobility in a-Si: H is much higher than that in a-Se and also the amount of energy needed for ionization of secondary carriers in a-Si: H is lower than that in a-Se. Using the description of the avalanche effect based on the lucky-drift model recently developed for amorphous semiconductors we show how this intriguing question can be answered. It is the higher phonon energy in a-Si: H than that in a-Se, which is responsible for the shift of the avalanche threshold in a-Si: H to essentially higher fields as compared to a-Se.

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HARP photoconductive film made of amorphous selenium (a‐Se), which makes use of the avalanche multiplication phenomenon, has been developed for the ultrahigh‐sensitivity television cameras that are used to report breaking news at night or to produce nature and science programs. We have tried to reveal the hole‐blocking mechanism in HARP films in the present work to improve their characteristics. It is important to reduce the dark current in HARP film to improve its sensitivity. HARP film has a hole‐blocking layer to suppress dark current, which interrupts the injection of holes to the a‐Se layer. Hole injection is considered one of the main factors related to dark current. The hole‐blocking layer consists of cerium dioxide (CeO2), which is an n‐type wide‐gap material. We have recently succeeded in producing improved CeO2 whose hole‐blocking capabilities are superior to the abilities of conventional normal CeO2. This paper describes the hole‐blocking mechanism in HARP film. To investigate this, the relationship between dark current and the film thickness of the CeO2 layer were measured with each HARP film using the two different types of CeO2. Furthermore, we analyzed the Ce 3d core‐level photoemission spectra for both types of CeO2 layers in HARP film by using hard X‐ray photoelectron spectroscopy (HAX‐PES) at Spring‐8. As a result, we found that the hole‐blocking capabilities of the film could be improved by reducing the number of defect levels generated from oxygen vacancies in the CeO2 hole‐blocking layer (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The kinetics of the photodarkening effect has been studied experimentally for amorphous selenium ͑a-Se͒ layers at room temperature and at an elevated temperature ͑35°C͒ close to the glass transition. By switching an intense pumping light on and off with a period of 100 s, we have studied the kinetics of both the buildup of photodarkening and its relaxation ͑recovery͒. It was found that at 35°C, only a reversible component of photodarkening has been observed. This result has been interpreted within the framework of a phenomenological model assuming that photodarkening is caused by light-induced transitions of structural units from their ground states into metastable states. Our estimate for the energy barrier E B between these states obtained for the photodarkening process ͑E B ϳ 0.8 eV͒ coincides with that obtained from the analysis of the relaxation process. At room temperature, an irreversible component of photodarkening has been observed along with the reversible one. The energy barrier responsible for the relaxation of the reversible component at room temperature appears the same as at 35°C. This suggests that the energy barrier identified represents a fundamental feature of the photoinduced structural metastability in amorphous selenium.