Epitaxial growth of superconducting niobium thin films by ultrahigh vacuum evaporation

Abstract

A series of thin films of niobium have been grown on various orientations of single crystal sapphire by e-beam evaporation in an ultrahigh vacuum molecular beam epitaxy system. The films were grown at temperatures between 600 and 950 °C and at growth rates from 0.1 to 10 Å/s. The films were single-crystal as determined by both in situ reflection high energy electron diffraction patterns and by Read photographs. The best films had rocking curve widths of the order of 1/10 of a degree. The superconducting transition temperature was measured as a function of film thickness and crystal orientation and ranged up to 9.35 K for a thick sample grown on [11̄02] sapphire.

Very high residual resistivity ratios of heteroepitaxial superconducting niobium films on MgO substrates

“…The interval between pulses is 4 s. A typical coating of 1 μm requires ∼4000 shots. Though the average deposition rate at this low duty cycle is of the order of 0.1 nm s −1 , which is comparable with that in e-beam evaporation [8,21] sources, the instantaneous deposition rate (within the arc pulse) is 300 nm s −1 . This is to be contrasted with rates of the order of 1 nm s −1 in DC arc [5,6], ECR [7] or magnetron [22] sources.…”

“…Over two decades, several techniques [5][6][7][8][9] have been used to deposit superconducting thin films of Nb on superconducting RF cavities [10] or on different crystal substrates, such as a-plane sapphire and MgO. A key figure of merit for such films is the residual resistivity ratio (RRR).…”

“…RRR is thus a useful figure of merit for superconductors. Over two decades of development, RRR in Nb films has been shown to increase from ∼10 to ∼100 [5][6][7][8][9]. One isolated measurement of RRR = 199 was reported [13] but on a-sapphire, with MgO stated to be poorer than sapphire, achieving RRR < 30.…”

High-RRR thin-films of NB produced using energetic condensation from a coaxial, rotating vacuum ARC plasma (CEDTM)

“…2) Fresh, highly reactive Nb on the walls of the chamber acts like a getter reducing the background pressure. 3) Despite the low average deposition rate of ~ 0.1 nm/s , the instant deposition rate is 300 nm/s, much higher than other standard techniques [4][5][6][7][8] so impurities introduced during the discharge pulse are much lower, according to the expression [18]:…”