Doped n-type a-Se alloys for use in x-ray image detectors: preparation, thermal and electrical properties

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The proposed NSERC-CRD grant is in collaboration with the Analogic Canada (Anrad) Corporation in Montreal, and has the overall goal of synthesizing and characterizing stabilized n-type a-Se alloys in which the stabilization is achieved by increasing the As content from 2% to 10%A. The n-type a-Se is used in one of the key-layers in the a-Se x-ray image detector next to the positively biased metal electrode to reduce the dark current. However, the doping of a-Se with n-type dopants tends to crystallize the structure over time, and it is more difficult to fabricate than the i-layer. The goal of this CRD is to synthesize a range of Se(1-x)As(x) alloys with As content varied from 0.3 to 10%, and the alloys doped with various IA and IIA elements, and characterize their structural, thermal and electrical properties. The CRD project involves the following research: (a) synthesis of the base Se(1-x)As(x) alloys; (b) n-type doping of these Se(1-x)Se(x) alloys; (c) structural and thermal characterization of the new n-type alloys through differential scanning calorimetry (DSC) and temperature-modulated DSC to obtain the glass transition, crystallization and melting temperature, the heat capacity, relaxation and crystallization enthalpies, crystallization kinetics; (d) electrical properties through charge transport measurements (such as transient photoconductivity type time-of-flight measurements), dark current measurements, and excess noise experiments. The experimental work will be supplemented by the formulation of new useful models for the dark current so that it can be predicted as a function of time and applied field from the intrinsic material properties for x-ray detectors that will have the new n-type a-Se layer. The proposed CRD grant builds on the expertise gained in the previous CRD and therefore starts from an already established know-how and builds on strength. The results are expected to lead to enhancements in detector performance in terms of having the n-layer in the detector thinner and higher production yield. The research directly impacts a-Se x-ray detectors currently used in mammography; and also therefore impacts the healthcare of women (breast cancer screening).