Single electron transistors with high-quality superconducting niobium islands

Deep submicron Al/AlOx/Nb tunnel junctions and single electron transistors with niobium islands were fabricated by electron beam gun shadow evaporation. Using stencil masks consisting of the thermostable polymer polyethersulfone and germanium, high quality niobium structures with good superconducting properties and a gap energy of up to 2ΔNb=2.5 meV were achieved. The I(U) characteristics of the transistors show special features due to tunneling of single Cooper pairs and significant gate modulation in both the superconducting and the normal state.