TC suppression in superconducting films for use in transition edge sensors

Transition edge sensor (TES) microcalorimeters have proven their value as photon detectors in several wavelength regimes. The central element of a TES is a superconducting film with a transition temperature designed to be at a specific temperature, usually 100 - 500 mK. These films are typically fabricated by depositing bilayers or multilayers of superconducting and normal metals (e.g., molybdenum and copper) whose relative thicknesses have been engineered to produce the desired superconducting transition temperature (T_C) through the proximity effect. Although these fabrication methods have been widely applied, bilayer and multilayer deposition is often difficult, and care must be taken in controlling thicknesses and surface interface quality. An alternative method is to dope the superconducting film with dilute magnetic impurities in order to suppress its T_C to the desired temperature. This technique has been used successfully in tungsten, but tungsten films are not well-suited to all TES applications. We have thus undertaken a project to investigate T_C suppression in molybdenum and aluminum films. We present our results from impurity doping using iron and manganese in these films.