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.

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Although avalanche amorphous selenium (a-Se) is a very promising photoconductor for a variety of imaging applications, it is currently restricted to applications with electron beam readout in vacuum pick-up tube called a High-gain Avalanche Rushing Photoconductor (HARP). The electron beam readout is compatible with high definition television (HDTV) applications, but for use in solid-state medical imaging devices it should be replaced by an electronic readout with a two-dimensional array of metal pixel electrodes. However, due to the high electric field required for avalanche multiplication, it is a technological challenge to avoid possible dielectric breakdown at the edges, where electric field experiences local enhancement. It has been shown recently that this problem can be overcome by the use of a Resistive Interface Layer (RIL) deposited between a-Se and the metal electrode, however, at that time, at a sacrifice in transport properties. Here we show that optimization of RIL deposition technique allows for electroded avalanche a-Se with transport properties and time performance previously not achievable with any other a-Se structures. We have demonstrated this by detailed analysis of transport properties performed by Time-of-Flight (TOF) technique. Our results showed that a stable gain of 200 is reached at 104 V/μm for a 15-μm thick a-Se layer, which is the maximum theoretical gain for this thickness. We conclude that RIL is an enabling technology for practical implementation of solid-state avalanche a-Se image sensors.