Enlargement of Kinetic Inductance of NbN Superconducting Thin Films for Device Applications

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Abstract
Experimental studies to enlarge kinetic inductance of NbN thin films using the effects of quality and thickness of the film are made by measuring Fiske current steps in the current-voltage (I-V) characteristics of NbN/Pb-In Josephson junctions. Experiments demonstrate that the specific kinetic inductance L K can be enlarged over a wide range by changing stoichiometry of the film, as well as by making the film thickness thin, and that this enlargement becomes larger by both effects when the film thickness is in an ultra thin regime. This inductance is shown to be well fitted to L K=µ0λ2/d, w h e r e µ_0 i s t h e v a c u u m p e r m e a b i l i t y, d i s t h e t h i c k n e s s, a n d λ i s t h e m a g n e t i c p e n e t r a t i o n d e p t h i n t h e d i r t y l i m i t. I n t h e p r e s e n t e x p e r i m e n t w e a t t a i n e d t h e i n d u c t a n c e a s h i g h a s L_K=130 p H b y t h e c o m b i n e d e f f e c t s o f i m p u r i t y a n d t h i c k n e s s f o r a f i l m w i t h d=30 n m.