In situ growth of MgB/sub 2/ thin films by hybrid physical-chemical vapor deposition

Abstract

We have carried out thermodynamics studies of the Mg-B system with the calculation of phase diagrams (CALPHAD) modeling technique and found that the superconductor MgB 2 phase is thermodynamically stable only under fairly high Mg pressures at elevated temperatures. This has lead us to the investigation of chemical vapor deposition in which the pressure during the film deposition can be high. Although the initial effort on metal-organic chemical vapor deposition (MOCVD) was not successful due to carbon contamination, a unique hybrid physical-chemical vapor deposition (HPCVD) technique has successfully produced high quality in situ MgB 2 films. The epitaxially-grown MgB 2 films show high transition temperature and low resistivity comparable to the best bulk samples, and their surfaces are smooth. In this paper, the details of the technique and the results of the HPCVD films are presented.

Superconductivity and normal state magnetoresistance in superconducting FeSe:Sb

“…However, for a less clean sample, the scattering is strong and MR could be suppressed considerably. For MgB 2 , only very clean epitaxial films, which have very low normal state resistance, show large MR [31]. In our case, we suggest that the samples with Sb are actually cleaner than the sample without Sb as far as the transport is concerned.…”

Momentum-dependent multiple gaps in magnesium diboride probed by electron tunnelling spectroscopy

“…In the clean MgB 2 film grown by HPCVD, the mean free path can be as large as 280 nm (ref. 24), which is most likely the reason that the fine structure in the gap distributions becomes observable in our junctions. The sensitivity of the fine structure in the gap distribution to the scattering rate shown in Figure 4a and the smearing out of the structure by the increasing number of scattering centres shown in Figure 4b are all in agreement with the multiband theory of Mazin et al 14 Another factor that can affect the fine structure in the gap distribution is the scattering at the superconductor/insulator interfaces, which may cause all the states in the σ or π bands to mix.…”

“…To fabricate the MgB 2 /native oxide/Pb junctions, a 100-nm thick MgB 2 bottom electrode layer was first epitaxially grown on a MgO (211), SiC (0001), or c-axis-8°-tilted SiC substrate by HPCVD. Because of the high purity of the Mg and B sources and the reducing hydrogen atmosphere during the deposition, MgB 2 films, prepared by HPCVD, are exceptionally clean 24,28 . The mean free path of the MgB 2 films, as estimated by the residue resistivity, can be greater than 280 nm and, in our case, is limited by the thickness of the film (~100 nm) 29 .…”

Growth of high-quality large-area MgB2thin films by reactive evaporation

“…One is that the growth of MgB 2 appears to be kinetically limited, [11] wherein the vapor pressure of Mg over MgB 2 is far lower than over Mg metal, so that Mg does not escape the MgB 2 film once it has formed at a reasonable growth T. The growth of MgB 2 also appears to be adsorption limited, so that as long as sufficient Mg vapor is present the MgB 2 phase will form automatically and additional Mg will not be incorporated into the film. [12] The growth technique of HPCVD makes use of these properties by providing Mg vapor near the substrate and introducing B by a carrier gas. This technique has produced very clean, epitaxial films with the lowest reported residual resistivity values and highest T c values.…”