Audrey GROCKOWIAK. Grenoble, France

Transcription

1 Audrey GROCKOWIAK Grenoble, France Ins8tut Néel, CNRS : Thierry KLEIN SPSMS/LATEQS, CEA : Christophe MARCENAT Slovak Academy of Sciences, Kosice : Jozef KACMARCIK

2 OUTLINE Samples prepara8on Superconduc8vity of Si:B Normal state proper8es of Si:B 2

3 1964 : theoretical prediction of superconductivity in group-iv SC (M.L Cohen, Phys. Rev. 134 A511) 1965 : superconductivity in Si under high P and in a different X phase (Tc : 13K) (Buckel et al., Phys. Lett. 17, ) 2001 : discovery of superconductivity in MgB 2 (40K) = > interest for covalent metals (Nagamatsu et al., Nature (London), 410, 63) 2004 : discovery of superconductivity in B-doped diamond (Tc = 11.4K) (Ekimov et al., Nature 428 ) 3

4 2006 : discovery of superconductivity in B-doped silicon (Tc =0.6K, cb ~8at%) (Bustarret et al, Nature 444,465) 2009 : Possible (but still controversial) discovery of SiB superconductive at 145K (Bagraev et al., Low-Dimensional Systems, 05/09) Silicon : one of the most widely studied materials with different dopants Why was its superconductivity so lately discovered? 4

5 a. Solubility limit problem 5

6 a. Solubility limit problem Boron solubility limit in silicon : at/cm 3 Minimum B concentration nb necessary to trigger superconductivity : at/cm3 b. Solution : Gas Immersion Laser Doping Use out-of-equilibrium technique c Developped by the Institut d Electronique Fondamentale, Paris 6

7 b. The GILD process Si (001) in vaccum chamber, 500µm BCl3 precursor gas : chemisorbed at the surface of the wafer UV laser pulse (308nm) : melts the Si wafer forces diffusion of B Number of pulses concentration Duration of pulses thickness of layer Cooling : re-crystallisation of Si with some B in substitution Formation of a thin Si:B layer on top of Si wafer (Thickness nm, surface : 2mmx2mm) 7

8 c. Boron concentration measurement SIMS : Secondary Ion Mass Spectroscopy. Performed by INSA, Lyon Principle : shoot oxygen beam at sample and count outcoming ions to obtain a concentration profile 8

9 a. Resistivity measurements Tc α nb Transition incomplete for nb < 2at% Linear dependency for nb < 6at% Plateau for nb > 6at% Gradual appearance of partial then total superconductive state 9

10 b. Cristallinity control : XRD diffraction measurements B in substitution = distorsion of Si lattice modification of XR diffraction angle Si wafer peak Doped layers peaks For nb < 6at% : Peak position α nb Confirmation of presence of a cristalline doped layer For nb > 6at% : Amorphous layer BUT : no indication of : -exact position of B (homogeneous repartition vs clusters) -number of B atoms in substitution vs interstitial 10

11 c. STM measurements Performed by F.Dahlem (Institut Néel) Normalized di/dv Tc = 510mK 2 positions spaced by 1µm : homogeneous SC Fit BCS prediction : Δ(0) k B T c =

12 d. Evolution of Tc wrt the electron-phonon coupling constant λ Combination of : -Ab initio calculations : λ wrt nb (X. Blase) -Experimental results : Tc wrt nb Doesn t fit with BCS theory : Tc α exp - (1/λ) Nor with McMillan : Tc α exp - (1+λ/λ- μ*) Feigel man power law : Tc α λ 1/γ with 0<γ<1 Understand discrepancy between STM and ab initio calculations 12

13 e. Influence of other dopants Study e doping (P, As) vs hole (Al, Ga) doping Study size of dopant Resistivity measurements : Si:As and Si:P not superconductive down to 300mK XRD measurements : Si:As, Si:P and Si:Al show same peak as Si:B. No peak for Si:Ga 13

14 a. Resistivity measurements Anomaly in resistivy measurements Only GILD Si:B samples Characteristics : Linear metallic decrease Minimum around K Sharp increase at 50K Plateau Superconductive transition (not shown) 14

15 a. Resistivity measurements Behaviour when a magnetic field is applied : Characteristics : Magnetoresistance of 100% Anomaly is erased with increasing field Plateau and value at HT independent on B Same features if H is parallel of orthogonal to sample 15

16 b. Hall effect measurements Hall effect measurements: R H = E y j x B = r ne r = Hall factor Sharp increase of RH between 50 and 100Kfor 0<H<2T For H > 2T, RH tends to value obtained for 4.2K Variation of r with H and T? Variation of sign and/or value of n? 16

17 b. Hall effect measurements Usually for doped silicon : -4 < r H < 2 Coll. J.Pernot Variation by a factor 50! 17

18 c. Hypothesis? Anomaly in resistivity and Hall Effect for 50K<T<350K and 0T<H<9T Structural phase transition? Explains resistivity curves at H = 0T only Possibility to do XRD at low temperatures Anderson localization? Usually occurs close to metal-insulator transitions Would be destroyed by an applied field, not enhanced Granular media properties? Analysis on Si:B implanted samples with tomographic atom probe, nb = Insterstitial boron tend to form clusters with nb = 7at% on average and core level reaching 10at% (O.Cojocaru et al., Scr Mater 60 (2009) ) 18

19 *Superconductive state : -probe at lower temperatures the influence of other dopants -probe influence of the thickness of the layer -point contact spectroscopy to get gap value -specific heat measurements on Si:B membranes -XRD at lower temperatures *Normal state : -qualitative study of anomaly with controlled contacts 19