Optical Response of Polycrystalline Mercuric Iodide Photoconductive Detectors

Mercuric Iodide in its tetragonal form has received a lot of attention for many years as a prospective room temperature X-ray and y-ray detector. Its basic properties are well suited for this purpose. Its wide band gap of 2.1eV contributes to a high dark resistivity of 1012ohm-cm or higher. A high atomic number of its constituent atoms (Hg-80, I -53) and a density of 6.3g/cm3 result in its efficient interaction with incident X-ray or y-ray radiation. Single crystalline mercuric iodide has been thoroughly studied and successfully utilized in commercial radiation detectors. But with the urgent need for large area ,low cost efficient X-ray detectors, focus has now shifted towards the development and understanding of the properties of thin film Polycrystalline Mercuric iodide detectors. Such detectors also have the advantage of being most suited for direct X-ray detection i.e. a direct conversion of incident X rays into electric signals which are then used to obtain an equivalent image in digital X-ray imaging. They also can be used in applications where a scintillator intermediate is used to generate visible light from incident high energy photons.Therefore it is important to study their optical response in order to understand and evaluate their Optical Properties. The present work focuses on obtaining the Optical response of the thin film Mercuric iodide photoconductive detectors .These films were grown on TEC-15 LOF glass with a Tin Oxide (SnO2) coating on it, which acts as a growth surface for the films and also functions as the front contact of the detector.Palladium which is sputtered on top of this film acts as the back contact. There are a total of seven contacted devices on each film sample and each device has been tested for its optical response in terms of Spectral Response and I-V characteristics in both light and dark conditions.Results obtained have been tabulated, and some important samples have been analyzed. The aim in this research was to obtain films with high QE,low dark current and uniformity of the results for all devices on a sample.Typical values of high QE’s obtained for the samples ranged from 0.3 - 0.4 with a bias of -50V applied to the front contact. The photocurrents corresponding to these QE’s ranged from 2.83μA – 3.82μA as obtained from the spectral response data file. The dark currents that were measurable were typically below 50nA for the best sample