Comparison Of Mercuric Bromide And Lead Bromide Layers as Photoconductors For Direct X-ray Imaging Applications

Films of PbBr₂ and HgBr₂ were grown by the physical vapor deposition method onto ITO-coated or palladium-coated glass substrates 1"times1" in size. The best growth conditions were a source temperature of 500 degC, a growth temperature of 220 degC and a growth time of 192 hours, with an initial pressure of 4 times 10^-3 Pa, for PbBr₂, and a source temperature of 150 degC, a growth temperature of 50 degC and a growth time of 2 hours with an initial pressure of 4 times 10^-3 Pa, for HgBr2 layers. Layers thicknesses and grain sizes gave values ranging between 40 and 60 mum (10%), and between 2 and 10 mum, respectively, for PbBr₂, and about 300 mum and between 20 and 50 mum, respectively, for HgBr₂. Layers were characterized by optical and scanning electron microscopies. The dark current density is lower than 10 pA/mm² for electric fields below 0.2 V/mum for HgBr₂, and below 5 V/mum for PbBr₂, and the resistivity is 5.4 times 10¹³ Omega-cm for PbBr₂ and 4.9 times 10¹² Omega-cm for HgBr₂. Signal to noise is 14 at 7.5 V/mum for PbBr₂ films and 0.31 at 0.2 V/mum for HgBr₂ films. Dark current density, resistivity and response to X-rays of the films are acceptable taking into account that this is the first report of layers of these materials as X-ray detectors, and indicate these heavy metal bromides as promising candidates for the application.