Attempts to prepare MgB 2 by low pressure CVD

1. Attempts to prepare MgB2by low pressure CVD Dr. Laura Crociani Istituto di Chimica Inorganica e delle Superfici C.So Stati Uniti 4, 35127 Padova (Italia) E-mail: crociani@icis.cnr.it The International Workshop on: THIN FILMS AND NEW IDEAS FOR PUSHING THE LIMITS OF RF SUPERCONDUCTIVITY Legnaro INFN 9-12 October 2006

2. Preparation of MgB2 thin films Molecular Beam Epitaxy (Mg+B metal) Sputtering (Mg, B two targets) Plasma Laser Depositon (Mg+B pressed pellet) Electrochemical Synthesis (Mg acetate+H3BO3) Hybrid Physical-Chemical Vapour Deposition (Mg metal + B2H6) Problems: MgO impurity, different vapour pressure of B and Mg, formation of MgBx x>2, use of dangerous substances Chemical Vapour Deposition (CVD): High grown rates and excellent conformal coverages; Simple apparatus thanks to the use of a single source precursor.

3. Mg(BH4)2 Tsub = 230 °C at 10-3 Torr Et2O ClMgEt + NaBH4 (BH4)MgEt + NaCl Et2O (BH4)MgEt + B2H6 Mg(BH4)2 . xEt2O + “EtBH2” ……or with drastic conditions Et2O MgH2 + B2H6 Mg(BH4)2 . xEt2O SYNTHESIS OF THE CVD PRECURSOR ….about the preparation of Mg(BH4)2 In the literature Mg(BH4)2 is obtained by desolvation of the ether adduct Mg(BH4)2 . xEt2O whose preparation may be achieved in several but tedious ways one week reaction! high pressure of diborane

4. SYNTHESIS OF Mg(BH4)2 Et2O Tl(OEt) + LiBH4 Mg(BH4)2 . xEt2O+ 2TlI Et2O MgI2 + 2 Tl(BH4)2 yield 97% Tl(BH4) + LiOEt The etherate complex has been characterized by mean of 11B-NMR spectroscopy in Et2O: the signal is a quintet with JB-H= 82.3 Hz. - 41.71 (ppm) - 39 - 40 - 41 - 42 - 43 - 44 - 45 The coordinated ether was removed in vacuo.

5. Counts ' 400 100 101 300 110 200 100 0 30 40 50 60 70 Position [°2Theta] BULK DECOMPOSITION OF Mg(BH4)2 Condition: Mg(BH4)2 powder ca 200 mg p= 10-3 Torr T= 430 °C

6. A B C A: quartz tube where an electric resistance is inserted B: substrate C: substance DEPOSITION CONDITIONS Precursor Mg(BH4)2 (100 mg) Substrate Si(100)Heating T = 280 °C Pressure = 10-3 Torr Deposition temperature = 500 °C Deposition time = 15 ,30, 60, 90, 120, 240 minutes PREPARATION OF THE FILMS

7. XRD characterization of the films shows only the presence of crystalline MgO. The peaks with the stars are those of the substrate.

8. Mg 2p X-Ray Photoelectron Spectroscopy (XPS) Analysis XPS characterization of commercial MgB2 pellets XPS spectra (B 1s, Mg 2p and Mg KLL) showed that i)The sample remained partially oxidized even after 2 h sputtering with 4 KeV energy Ar ion in UHV. ii) The XPS signals of boron oxide (BE = 193.3 eV) and metallic B (BE = 188.4 eV) can be easily separated by the peak-fitting of B 1s line, while MgO and boride peaks are overlapping in Mg 2p line (~ 51.0 eV). B 1s

9. Mg KL23L23 iii) Fortunately, the chemical states of MgO and MgB2 can be easily distinguished from Auger peak of Mg KLL (1181.5 and 1184.5 eV, respectively), although it is difficult to quantify from Auger peaks. However, the ratio boride:oxide can be calculated from the intensity ratio of the principal components separated by peak-fitting routine. Mgboride(atomic %) = Mgtot(atomic %)·[Iboride /(Iboride + Ioxide)] Mgtot is the total atomic concentration of Mg calculated by XPS quantitative analysis Iboride and Ioxide are the intensity in cps (count per second) of the main component of Mg KLL of boride and oxide respectively.

10. 0.7 16.2 0.05 6.8 0.5 7.2 0.7 3.5 0.1 2.3 0.6 0.5 Surface composition of the samples expressed as B/Mg ratios and oxide thickness. The surface is richer in Mg (mostly as MgO) and it consists mainly of an oxide layer.

11. Bulk composition* Bulk composition* Bboride/Mgtot Bboride/Mgboride 5.1 7.6 5.6 8.5 12.9 9.9 8.2 6.2 Bulk composition* 6.0 7.6 Btot /Mgtot 7.3 10.0 7.3 8.6 8.7 5.7 6.2 The inner part is richer in B. 8.8 Bulk composition of the samples expressed as B/Mg ratios. *Such compositions were obtained after sputtering the samples until a constant composition value was observed: sputtering times range from 55 minutes (MgB1) up to 255 minutes ( MgB6). Bboride/Mgboride ratio is quite higher than in MgB2, being the film probably a mixture of different magnesium boride.

12. We are not able to rationalize the B/Mg ratio and oxide thickness (magnesium and boride oxides) with the length of time deposition: the high B/Mg ratio in the bulk may be ascribed to Mg segregation occurring in the film under vacuum. Mg migrates to the surface partly reacting with the oxygen present in the reactor as impurity and partly evaporating. Surface and bulk composition of the samples expressed as B/Mg ratios and oxide thickness. *Such compositions were obtained after sputtering the samples until a constant composition value was observed: sputtering times range from 55 minutes (MgB1) up to 255 minutes ( MgB6).

13. We have set up a new easy and quick way to prepare Mg(BH4)2. Decomposition of bulk solid Mg(BH4)2 under vacuum produced MgB2. Decomposition of Mg(BH4)2 under CVD conditions produced complex films consisting of magnesium borides covered by an oxide layer in which it is not possible to exclude also the existence of MgB2. We think that because of the low volatility of the precursor a small amount of Mg(BH4)2 reaches the hot substrate: decomposition occurs but because of the size of the particles and/or the little amount of substance deposited and/or formed, other phenomena such as Mg segregation prevail on the formation of MgB2 yielding a deposit not well identifiable. IN CONCLUSION

14. The structure of MgB2 is hexagonal and contains sheets of B and Mg, which are alternating along the c axis. This architecture provokes, according with band structure calculations, a stabilisation of the p orbital perpendicular to the plane (pz) shifting their value below the p-s bands, corresponding to a hole-doping effect of these levels. This condition allows to work with a very large vortex state that extends from values of the Hc1 (lower critical field) about 25 mT up to 32 T for Hc2 (upper critical field). SUPERCONDUCTVITY OF MgB2