Production and characterization of shape memory alloy based thin film photodiodes

The subject of this doctoral thesis, which consists of a combination of shape memory alloys (SMAs), the most important class of smart materials, and photodiodes, one of the main research topics of nanotechnology, is about production&characterization of Schottky type photodiodes with nanosized SMA thin film components. While the mechanical functions of SMA films are utilized in devices called nano/microelectromechanical systems (N/MEMS) like nano/micro actuators/sensors, only the electrical&thermal properties of SMAs such as vacuum work functions tunable by alloy type&composition, electrical&thermal conductivities, and resistance varying by martensitic phase transformations can be benefited when SMAs are used as metal contact layers in hard semiconductor (e.g. silicon) based Schottky diodes. Destructive SMA thin film cracks in such hard (NiTi) SMA films coated on hard substrates occur by stresses generated by martensitic shape changes triggered by physical forces (heat, electrical field etc.). Although NiTi SMAs have superior shape memory properties, they have smaller electrical&thermal conductivities when compared to cheaper Cu-based SMAs with less shape memory abilities but softer. Therefore, using Cu-based SMA thin films in (photo)diodes seems to be a better alternative. In this first thesis on SMA-photodiodes, novel Schottky type Cu-based-SMA/n-Si/Al photodiodes were successfully produced by coating nano-films of Cu-based SMAs, some of them were new produced Cu-based ingot and strip SMAs, all with different compositions and martensitic transformation temperatures, on n-Si wafers via thermal evaporation and their basic Schottky diode and photodiode parameters were determined, and some of these parameters for SMA-photodiodes were determined for the first time, by performing photo-electrical current-voltage (I-V), current-time (I-t) and capacitance-voltage(C-V) measurements and high-quality photodiode features were obtained.