Photoconductivity in amorphous selenium blocking structures

Abstract

Photoconductive properties of the first practical a‐Se avalanche photosensor (hole drift mobility, photocurrent response time, quantum yield of charge photogeneration and avalanche multiplication gain) have been studied. It has been shown that hole transport and avalanche multiplication mechanisms in multilayered a‐Se blocking structure used in practical photosensors are similar to those observed in a‐Se monolayers confined between insulating layers and used for the first manifestation of avalanche effect in this material by Juska and Arlauskas, phys. stat. sol. (a) 59, 389 (1980). It is suggested that combination of high avalanche gain (up to 1000) with high quantum efficiency of charge photogeneration (almost 100%) and fast photocurrent time response (∼ 1 ns) makes a‐Se avalanche photosensor a promising candidate to replace vacuum photomultiplier tubes for application in medical radiation imaging. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Amorphous selenium (a-Se) avalanche photosensor with metal electrodes

“…3, hole mobilities in non-avalanche regime are identical for Type 1 and Type 2. Moreover the field dependence of hole drift mobilities at F b F AV is similar to that observed for a-Se insulating structures [2,7]: mobility increases rapidly with field and reaches the value~0.8 cm 2 /Vs at the threshold of avalanche multiplication F AV [1,7]. Because of breakdown it was impossible to measure hole mobility in avalanche regime (F > F AV ) for Type 1 sample.…”

“…For comparison Fig. 4 (X symbols) shows avalanche gain dependence on field for commercial HARP tube which utilizes electron beam readout [1,4,6]. The excellent agreement is obvious from Fig.…”

“…4 avalanche multiplication gain increases rapidly with the electric field and at F =104 V/μm g = 200 which is the maximum theoretical gain for 15 μm thick a-Se layer [1,2]. Further increase in electric field above 104 V/μm for this a-Se layer thickness will result in triggering of electrons impact ionization and self-sustaining Geiger breakdown process [2,8].…”

“…1(a)). In HARP the a-Se photoconductive layer is sandwiched between specially designed blocking layers to prevent injection of holes and electrons from the positive and negative contacts respectively (CeO 2 is hole blocking layer and Sb 2 S 3 is electron blocking layer) [1,4,5]. Type 2 structure: modified HARP phototarget on which a 1 μm thick RIL (using CA) was spin-coated prior to the deposition of a 0.8 mm 2 gold contact ( Fig.…”

“…The application of an electric field in excess of the avalanche multiplication threshold F AV of~70 V/μm initiates hole impact ionization and subsequent avalanche formation. The multiplication coefficient (avalanche gain) depends exponentially on the a-Se layer thickness and twice as exponentially on electric field F [1,2].…”