1. MgB 2 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 FOR PUSHING THE LIMITS OF RF SUPERCONDUCTIVITY Legnaro National Lab, Padova, October 9-12, 2006

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

3. MgB 2 properties-I MgB 2 properties-I Crystalline structure Fermi surface E 2g phonon mode T c  40 K Simple layered structure Covalent bonding between B atoms Conventional superconductivity (isotopic effect) Coupling with vibrational modes of B atoms (  bands) 3D  bands 2D  bands 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 J.Nagamatsu et al. Nature (2001) 410

4. MgB 2 properties-II MgB 2 properties-II Two distinct energy gaps closing at the same T c    2 meV ; 2    KT C  1.6    7 meV ; 2    / KT C  4 M.Iavarone et al., PRL 89, 187004 (2002) F.Bouquet et al., PRL 87 (2001) 047001 Two gaps from the specific heat Two gaps from STM  

5. MgB 2 Nb T c (K)399.2  0 (  cm) 0.1-100.05 RRR3-30300   (meV) 2, 7 1.2    K B T c (meV) 1.6, 43.9   (nm) 50,1240 (nm) 8580  0 H c2 (T) 6-500.2 R BCS s @ 4K, 500MHz (n  ) 2.5/2.3x 10 -5 69 A comparison with conventional SC for RF applications from F.Collings et al. SUST 17 (2004)

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

7. Deposition techniques Two-step method In-situ techniques Deposition of an amorphous precursor (boron or mg+B) at room temperature Post-annealing in Mg atmosphere (usually ex-situ) + Advantages: Possibility to use high temperatures for the phase crystallization High T c, good structural properties Disadvantages: Difficult to extend to large area Growth of MgB 2 at low temperature Two main problems in depositing MgB 2 thin films: 1.sensitivity of Mg to oxydation 2.High Mg vapour pressure required for phase stability 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 MgB 2 growth: Single crystalline: c-cut Al 2 O 3, 4H and 6H SiC, (111) MgO

8. Two step methods Group and reference Growth technique SubstratePrecursor Reaction temperature(°C) and time T c (K) Kang et al. Science 292, 1521 (2001) PLD SrTiO 3 (100), Al 2 O 3 r cut boron900, 10-30 min37-39 Eom et al. Nature 411,558 (2001) PLDSrTiO 3 (111)Mg+B850, 15 min34-36 Ferdeghini et al. Physica C 378, 56 (2002) Ferrando et al. SUST 16, 241(2003) PLD Al 2 O 3 c cut, MgO(111) Mg+B stoich. 850-900, 30 min35-38 Paranthaman et al APL 78,3669 (2001) E-beamAl 2 O 3 r cutboron890, 10-20 min38-39 Moon et al.APL 79, 2429 (2001) E-beam Al 2 O 3 c cut, MgO(111) boron700-950, 30 min39 Zhai et al. J.Mater.Res. 16, 2759 (2001) E-beam, PLDAl 2 O 3 r cutB, Mg+B900, 1h 39,25- 28 Vaglio et al. SUST 15,1236 (2002) Magnetron sputtering Al 2 O 3 r cut, MgO Mg+B stoich. 830,10 min in situ35

9. P.C.Canfield et al. PRL 86, 2324 (2001) A feasible two step method Two stage CVD Reaction of a boron coating in Mg vapor 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 H 2 and BCl 3 ). A similar approach could be applied to the formation of a MgB 2 film on the surface of an RF cavity previously coated with B using established CVD technology B filament Reacted filament:MgB 2 High critical temperature; T C onset = 39.4 K and ΔT c =0.9K Low resistivity (  0 =0.38µΩcm) High RRR (25)

10. In-situ techniques Group and reference MethodCommentsSourceSubstrate Growth temperature (°C) T c (K) Ueda et al. APL 79, 2046 (2001), JAP 93,2113 (2003) MBECo-deposition Mg and B metal Al 2 O 3, STO, Si28033-36 Saito et al. J.JAP 41,L127 (2002) Sputtering Carousel sputtering MgB 2 targetAl 2 O 3 25028 Grassano et al. SUST 14,762 (2001) PLDBlue plume Mg+B pressed target Al 2 O 3, MgO40025 Jo et al. APL 80, 3563 (2002) MBECo-deposition Mg and B metal Al 2 O 3 30034 Erven et al. APL 81, 4982 (2002) MBECo-deposition Mg and B metal Si + MgO seed layer 30035 Moeckly et al. SUST 19, L21 (2006) Reactive evaporation Large area films Mg and B metal Single and poly crystals, metallic 400-60038-39 Zeng et al.Nat.Mat. 1,35 (2002) HPCVD Clean epitaxial films Mg and B 2 H 6 SiC, Al 2 O 3 72039-41

11. Boron Plume Deposition zone Magnesium vapor Heater pocket Rotating shaft B. Moeckly et al. SUST 19, L21(2006) A promising in situ method 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. 1.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. Advantages: 1.high Mg pressure provided locally near the substrates 2.Mg temperature independent of the substrate temperature 3.double-sided deposition 4.Growth of large area films 5.Growth on metallic substrates 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. Superconductor Tecnologies Inc.

12. Two steps deposition process @ LAMIA followed by PLD deposition of an MgB 2 precursor layer from stoichiometric target (prepared with pure 11 B) at room temperature in UHV annealing in Mg vapour In Ar atmosphere in a sealed Ta tube at 850-900 °C The reaction temperature is crucial for the quality of the samples Best samples at 900 °C Mg Quartz tube Vacuum pump Furnace Ta crucible Films Ta case The PLD apparatus

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

14. MgB 2 has two bands, weak interband scattering  two channels conducting in parallel: H c2 (0) is determined by the band with the lowest diffusivity H c2 does not depend on resistivity Similar H c2 in samples with very different  0 V.Ferrando et al. Phys. Rev.B 68, 094517 (2003) Tuning MgB 2 properties by disorder Four samples whose resistivity differs of more than one order of magnitude Introducing selectively disorder in  or  band one can have samples with low  0 and high upper critical field Interesting for RF applications

15. Conclusions Due to its high critical temperature and its non granular behaviour, MgB 2 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 H c2.