Hole‐blocking mechanism in high‐gain avalanche rushing amorphous photoconductor (HARP) film

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)