Review
MgB2 thin films by hybrid physical–chemical vapor deposition

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Abstract

Hybrid physical–chemical vapor deposition (HPCVD) has been the most effective technique for depositing MgB2 thin films. It generates high magnesium vapor pressures and provides a clean environment for the growth of high purity MgB2 films. The epitaxial pure MgB2 films grown by HPCVD show higher-than-bulk Tc due to tensile strain in the films. The HPCVD films are the cleanest MgB2 materials reported, allowing basic research, such as on magnetoresistance, that reveals the two-band nature of MgB2. The carbon-alloyed HPCVD films demonstrate record-high Hc2 values promising for high magnetic field applications. The HPCVD films and multilayers have enabled the fabrication of high quality MgB2 Josephson junctions.

Introduction

Magnesium diboride, MgB2, is an exciting superconductor [1]. It is a conventional BCS superconductor, in which the Cooper pairs are formed through electron–phonon coupling [2], with a high transition temperature Tc of 39 K [1]. It has multiple bands with weak interband scattering: the two-dimensional σ bands and the three-dimensional π bands [3]. They couple to the B–B stretch modes of E2g symmetry with different strengths, resulting in different superconducting energy gaps [4], [5]. The existence of the “two bands” and “two gaps” not only effects various properties of MgB2 [6], [7], it also leads to new physics that does not exist in single-band superconductors [8], [9], [10], [11], [12], [13]. For electronic applications, the high Tc of MgB2 allows operation of MgB2 devices and circuits above 20 K, substantially reducing the cryogenic requirements compared to the Nb-based superconducting electronics, which have to operate at 4.2 K [14], [15]. MgB2 Josephson junctions [16] and superconducting quantum interference devices (SQUIDs) [17] with excellent properties well over 20 K have been demonstrated. For applications in high magnetic field, carbon-alloyed MgB2 films have shown higher upper critical field Hc2 values than those of the Nb-based superconductors at all temperatures [18]. MgB2 is of particular interest for magnets in cryogen-free magnetic resonance imaging (MRI) systems [19]. It is further recognized that the high Tc and low resistivity make MgB2 an attractive material for RF cavity applications [20], [21].

High quality MgB2 thin films are important for both fundamental research and electronic, high-field, and RF cavity applications. Much effort has been devoted to the deposition of MgB2 thin films and tremendous progress has been achieved by various deposition techniques [22]. The deposition techniques used for MgB2 films include high-temperature ex situ annealing of B or Mg–B precursor films in Mg vapor [23], [24], [25], [26], [27], [28], [29], intermediate-temperature in situ annealing of Mg–B precursor films [30], [31], [32], [33], [34], [35], [36], low-temperature in situ deposition [36], [37], [38], [39], [40], and high- and intermediate-temperature in situ deposition [41], [42], [43]. An analysis of the pros and cons of each techniques has been given in Ref. [22]. Of all these techniques, hybrid physical–chemical vapor deposition (HPCVD) [43], [44] has been the most effective one for MgB2 films. The HPCVD MgB2 films on single crystal substrates such as c-cut SiC and sapphire are epitaxial [43], [45]. The pure films show Tc values at almost 42 K, higher than the bulk samples [46], and residual resistivity much lower than 1 μΩ cm [9]. When alloyed with carbon, the upper critical field Hc2 of the HPCVD films increases dramatically from that of the pure films to reach over 60 T at low temperatures [18], [47]. Similar results have been obtained in polycrystalline MgB2 films on polycrystalline substrates [48], [49], including metallic substrates. A wide range of works by various groups using HPCVD films have played important roles in the research of MgB2.

In this paper, we describe the principles and key elements of the HPCVD technique and discuss the properties of MgB2 films deposited using HPCVD, including clean epitaxial films, carbon-alloyed films, polycrystalline films, and multilayers of MgB2 with other materials. A short summary of MgB2 Josephson junctions using HPCVD films will be presented, while additional information can be found in the review on epitaxial MgB2 tunnel junctions and SQUIDs by Brinkman and Rowell [50].

Section snippets

Key requirements in deposition of MgB2 films

The most important requirement for the deposition of MgB2 films is to provide a sufficiently high Mg vapor pressure for the thermodynamic phase stability of MgB2 at elevated temperatures. Fig. 1 is a Mg pressure–temperature phase diagram calculated by Liu et al. [51]. The MgB2 film deposition parameters should fall within the growth window marked by “Gas + MgB2” where the thermodynamically stable phases are MgB2 and Mg gas. The Mg pressures for this growth window are very high for temperatures

Hybrid physical–chemical vapor deposition

Fig. 2a is a schematic of the HPCVD setup [44]. It consists of a water cooled quartz tube reactor and a susceptor, which is inductively heated. During the deposition, the carrier gas is purified H2 with a flow rate of the order of 400 sccm to 1 slm at a pressure of about 100 Torr. The high total pressure makes it possible to generate a high Mg vapor pressure necessary for the phase stability of MgB2. The reducing environment is important for preventing oxidation during the deposition. Bulk pure Mg

Structural properties

The high deposition temperatures in HPCVD made possible by the high Mg vapor pressure result in excellent epitaxy and crystallinity in the MgB2 films deposited by this technique [43], [45]. Fig. 3 shows a high-resolution transmission electron microscope (HRTEM) image for the interface between a MgB2 film and (0 0 0 1) 6H–SiC substrate. The insets are selected area electron diffraction (SAED) patterns from the film (top) and substrate (bottom). The result shows that the MgB2 film grows epitaxially

Carbon-alloyed HPCVD films

Although clean MgB2 films are desirable for many fundamental studies and applications, good superconducting properties in high magnetic field require the superconductors to be dirty. Clean MgB2 has low Hc2 [7], but in high resistivity MgB2 films Hc2 is substantially higher [94]. Because of the multiple impurity scattering channels and the two-gap nature of superconductivity in MgB2, Hc2 can be enhanced well above the estimate Hc2(0)=0.69TcHc2(Tc) of one-gap theory. The HPCVD technique can not

Polycrystalline MgB2 films

The extraordinarily high Hc2 of over 60 T shown in Fig. 15 was obtained in textured MgB2 films [47]. It is important that such result can be translated into practical high-field wires or tapes. Most likely such wires or tapes will be made from polycrystalline MgB2 materials. Using HPCVD, we have fabricated MgB2 coated-conductor fibers, and the carbon-alloyed fibers show high Hc2 (55 T) and high Hirr (near 40 T) at low temperatures [48]. Fig. 17 shows SEM images of a pure MgB2 coated-conductor

Epitaxial boride heterostructures

The family of boride materials consists of a variety of structures [113] and possesses a wide range of properties from refractory highly conductive metal to semimetal, narrow-band-gap semiconductor, and wide-band-gap semiconductors [114]. Besides the two-band superconductivity in MgB2 [6], there are extremely diversified magnetic properties in transition metal and rare-earth borides [115], [116], [117], self-healing of radiation-induced defects in some icosahedral borides [118], [119], and

Josephson junctions and SQUIDs

Several types of MgB2 Josephson junctions have been made using HPCVD films: nanobridge constrictions in epitaxial MgB2 films [17], ion-damaged weak link planar junctions [127], planar superconductor–normal metal–superconductor (SNS) junctions using epitaxial MgB2/TiB2 bilayers [16], and MgB2-barrier–Pb superconductor–insulator–superconductor (SIS) trilayer junctions [60]. SQUIDs fabricated from the nanoconstrictions show stable SQUID voltage modulation up to 38.8 K [17]. A 20-junction series

Concluding remarks

HPCVD has shown to be a very effective technique for MgB2 thin films. The scope of its impact has been remarkably broad: from the cleanest MgB2 material to the highest Hc2 values in carbon-alloyed films; from the studies of new physics due to the two-band nature of MgB2 to the investigations of practical applications such as in MgB2 coated conductors; and from superconducting digital circuits using Josephson junctions to cryogen-free superconducting magnets for MRI systems. There is no doubt

Acknowledgements

We would like to thank our many colleagues and collaborators, whose published works are quoted in this paper. This work is supported in part by ONR under Grant Nos. N00014-00-1-0294 (XXX), N00014-07-1-0079 (XXX), and N0014-01-1-0006 (JMR), by NSF under Grant Nos. DMR-0306746 (XXX and JMR), DMR-0405502 (QL), DMR-0514592 (ZKL), DMR-0507146 (DGS and XXX), and DMR-9871177 (XQP), and by AFOSR under Grant No. FDFA9550-05-1-0436 (RCD).

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