Preparation of High T_c Nb-(Ge,Si) Thin Films at Low Substrate Temperature

Abstract

Deposition of Si substituted Al5 Nb-(Ge,Si) films has been attempted by using DC magnetron sputtering. The dependence of crystal structure and superconducting critical temperature Tc of these films on sputtering conditions, film composition and film thickness has been investigated. Using materials with amorphous structure as substrates, the authors have succeeded in depositing the single phase A15 Nb₃(Ge,Si) films which have a small lattice constant about 5.11 Å and high Tc of 23.1 K. The films about 6000 Å thick with Tc above 20 K are obtained in the following region of film preparation conditions: Si substitution ratio Si/(Ge+Si) about 0.12, (Ge+Si) content from 23 to 27 at.% and substrate temperature from 320 to 520°C. The range of substrate temperature for these high Tc films is much wider and 200 K lower than that for high Tc Nb₃Ge binary compound. Besides, T_c of these films does not depend significantly on film thickness and the films less than 100 Å thick deposited at. substrate temperature above 400°C have high Tc above 20 K.