Field-effect and electron density modulation of the superconducting transition in composite In/InOx thin films

Composite In/InOx films 5 nm thick were deposited by reactive ion beam sputtering of In in a controlled partial pressure of O2, incorporation of which reduces the free-electron density to 1020 cm−3. Previous microscopy on similar-prepared thick films showed a composite of amorphous In and crystalline indium oxide, with 295 K resistivity ∼10−3 ohms cm and superconducting Tc⪅3.4 K. The present field-effect device uses a mylar-insulated gate to modulate electrical conductance with charge, interpreted as a field-effect mobility μ = 40 cm2V−1s−1.The charge-induced variation in Tc, measured to be at most 8%, correlates with the dependence of Tc on the 295 K sheet resistance, a dependence determined by the controlled reversible desorption/adsorption of O2 in the films near 300 K. For an areal electron density in the as-deposited films near 3 × 1013 cm−2, a 20% change should induce a factor of 2 change in Tc.