MgB 2 thin films: growth techniques and peculiar properties

1. MgB2 thin films: growth techniques and peculiar properties C. Ferdeghini CNR-INFM Lamia, Genova, Italy Coworkers: V.Ferrando, C.Tarantini, I.Pallecchi, M.Putti The International Workshop on: THIN FILMS APPLIED TO SUPERCONDUCTING RF AND NEW IDEAS FORPUSHING THE LIMITS OF RF SUPERCONDUCTIVITY Legnaro National Lab, Padova, October 9-12, 2006

2. Outline • Magnesium diboride and its intriguing properties • Thermodynamic of MgB2 • Challenges in MgB2 thin film deposition • Two steps methods: examples • In situ methods: examples • Results obtained @ Lamia on MgB2 thin films

3. MgB2 properties-I Crystalline structure Fermi surface Tc  40 K J.Nagamatsu et al. Nature (2001) 410 3D p bands 2D s bands • Simple layered structure • Covalent bonding between B atoms • Conventional superconductivity (isotopic effect) • Coupling with vibrational modes of B atoms (s bands) E2g phonon mode Weak interband scattering due to different symmetry of the two bands The two bands are two conducting channels in parallel: crucial role of disorder in coupling them

4. MgB2 properties-II Two distinct energy gaps Dp2 meV ; 2Dp /KTC 1.6 Ds7 meV ; 2Ds/KTC 4 closing at the same Tc Dp Ds Two gaps from STM Two gaps from the specific heat M.Iavarone et al., PRL 89, 187004 (2002) F.Bouquet et al., PRL87 (2001) 047001

5. A comparison with conventional SC for RF applications from F.Collings et al. SUST 17 (2004)

6. Challenges in MgB2 thin films growth gas +MgB2 : Mg excess does not condense on the film surface and MgB2 is stable Z.-K. Liu et al., APL 78(2001) 3678. Mg MgB2 Kinetically limited Mg M. Naito and K. Ueda, SUST 17 (2004) R1 optimal T for epitaxial growth ~ Tmelt/2 For MgB2 , 540°C → it requires PMg ~11 Torr Too high for UHV deposition techniques (PLD, MBE...) At PMg = 10-4-10-6 Torr, compatible with MBE, Tsub ~ 400°C MgB2 is stable, but no MgB2 formation: Mg atoms re-evaporate before reacting with B Kinetic of Mg is also important evaporation Mg pressure from MgB2 < decomposition curve of MgB2 < Mg vapor pressure At P=10-6 Torr and T> 250°C no accumulation of Mg will take place on the substrate and the growth of the superconducting phase is very slow due to a large kinetic energy barrier. At low Mg pressure only extremely low deposition temperatures can be used

7. Deposition techniques Two main problems in depositing MgB2 thin films: • sensitivity of Mg to oxydation • High Mg vapour pressure required • for phase stability Two-step method Deposition of an amorphous precursor (boron or mg+B) at room temperature Post-annealing in Mg atmosphere (usually ex-situ) + • Possibility to use high temperatures for the phase crystallization • High Tc, good structural properties Advantages: Disadvantages: • Difficult to extend to large area In-situ techniques Growth of MgB2 at low temperature Advantages: for some of them, possibility to deposit large area films Disadvantages: • low growth temperature (except for HPCVD, see next talk) • sometimes low Tc, poor structural properties Substrates for MgB2 growth: • Single crystalline: c-cut Al2O3, 4H and 6H SiC, (111) MgO

8. Two step methods

9. A feasible two step method Two stage CVD Reaction of a boron coating in Mg vapor • High critical temperature; TC onset= 39.4 K and ΔTc=0.9K • Low resistivity (0=0.38µΩcm) • High RRR (25) B filament The B fibers are made by a CVD technique: drawing of a W filament (the ‘substrate’), heated to 1200 °C, through a Boron gaseous compound (mixture of H2 and BCl3). Reacted filament:MgB2 A similar approach could be applied to the formation of a MgB2 film on the surface of an RF cavity previously coated with B using established CVD technology P.C.Canfield et al. PRL 86, 2324 (2001)

10. In-situ techniques

11. A promising in situ method Rotating shaft Heater pocket Deposition zone Magnesium vapor Boron Plume Superconductor Tecnologies Inc. Use of a rotating pocket heater ( similar to that developed for deposition of large area HTS thin films) containing a rotating platter that holds the substrates and spins them through a quasiblack-body radiative oven. • The substrate is exposed to the vacuum chamber via a window and hence to the evaporated flux of boron. 2. Then it passes through the heater and is exposed to a pocket with Mg vapour only into the interior of the heater. B. Moeckly et al. SUST 19, L21(2006) The Mg vapour is relatively well sealed inside the heater pocket by means of a small gap between the platter and the heater body, and the single B e-beam source is therefore free to operate in a vacuum environment. • Advantages: • high Mg pressure provided locally near the substrates • Mg temperature independent of the substrate temperature • double-sided deposition • Growth of large area films • Growth on metallic substrates

12. Two steps deposition process @ LAMIA The PLD apparatus PLD deposition of an MgB2 precursor layer from stoichiometric target (prepared with pure 11B) at room temperature in UHV followed by annealing in Mg vapour In Ar atmosphere in a sealed Ta tube at 850-900 °C Furnace Ta crucible Ta case Vacuum pump Mg Quartz tube Films The reaction temperature is crucial for the quality of the samples Best samples at 900 °C

13. Properties of the films grown @ LAMIA q-2q scan f scan Rocking curve Good structural properties: c axis orientation, single in plane orientation Presence of an epitaxial interlayer of MgO Tc close to the bulk value

14. Tuning MgB2 properties by disorder MgB2 has two bands, weak interband scattering  two channels conducting in parallel: Four samples whose resistivity differs of more than one order of magnitude Similar Hc2 in samples with very different r0 Hc2 does not depend on resistivity Hc2(0) is determined by the band with thelowest diffusivity Introducing selectively disorder in s or p band one can have samples with low r0 and high upper critical field V.Ferrando et al. Phys. Rev.B 68, 094517 (2003) Interesting for RF applications

15. Conclusions • Due to its high critical temperature and its non granular behaviour, MgB2 is an emerging superconducting material for applications. • In principle thin film deposition is not an easy task. • Nevertheless, several different techniques were developed in the last years. Some of them can be suitable for deposition on large areas. • In form of thin film, this material can present very low resistivity along with considerable Hc2.