Polycrystalline diamond films as prospective UV photocathodes

Polycrystalline diamond films grown by chemical vapor deposition have been considered recently for a number of UV detection applications. Negative electron affinity, chemical and mechanical stability and relative ease of fabrication make such films attractive candidates for effective and stable UV photoconverters. In this paper we present our study of the absolute quantum efficiency of a thin film diamond reflective photocathode in the spectral range of 25-200 nm. Modification of the surface by microwave hydrogen plasma etching resulted in a substantial increase of the photocathode sensitivity. The quantum efficiency of the photocathode at ~40 nm was as high as 37 percent and the sensitivity cut off was found to be about 200 nm. We also verified that the photocathode is relatively stable under air exposure. The relative QE degradation in the spectral range studied did not exceed 15 percent after the sample was left in ambient air for 18 hours. In addition, the diamond photocathode appeared to be chemically stable and mechanically robust: alcohol and water ultrasonic cleaning, followed by the same surface activation in hydrogen plasma, did not result in any degradation of the sample UV sensitivity. The photoyield from the diamond film at 256 Å exhibited an increase with the angle of radiation incidence, which is in agreement with the results of our calculations.