Attempts to prepare MgB 2 by low pressure CVD Dr. Laura Crociani Istituto di Chimica Inorganica e...

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Attempts to prepare MgB 2 by low pressure CVD Dr. Laura Crociani Istituto di Chimica Inorganica e delle Superfici C.So Stati Uniti 4, 35127 Padova (Italia) E-mail: [email protected] The International Workshop on: THIN FILMS AND NEW IDEAS FOR PUSHING THE LIMITS OF RF SUPERCONDUCTIVITY Legnaro INFN 9-12 October 2006

Transcript of Attempts to prepare MgB 2 by low pressure CVD Dr. Laura Crociani Istituto di Chimica Inorganica e...

Page 1: Attempts to prepare MgB 2 by low pressure CVD Dr. Laura Crociani Istituto di Chimica Inorganica e delle Superfici C.So Stati Uniti 4, 35127 Padova (Italia)

Attempts to prepare MgB2 by low pressure CVD

Dr. Laura CrocianiIstituto di Chimica Inorganica e delle Superfici

C.So Stati Uniti 4, 35127 Padova (Italia)E-mail: [email protected]

The International Workshop on: THIN FILMS AND NEW IDEAS FOR PUSHING THE LIMITS OF RF SUPERCONDUCTIVITY Legnaro INFN 9-12 October 2006

Page 2: Attempts to prepare MgB 2 by low pressure CVD Dr. Laura Crociani Istituto di Chimica Inorganica e delle Superfici C.So Stati Uniti 4, 35127 Padova (Italia)

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.

Page 3: Attempts to prepare MgB 2 by low pressure CVD Dr. Laura Crociani Istituto di Chimica Inorganica e delle Superfici C.So Stati Uniti 4, 35127 Padova (Italia)

SYNTHESIS OF THE CVD PRECURSOR

….about the preparation of Mg(BH4)2

Mg(BH4)2

Tsub = 230 °C at 10-3 Torr

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

ClMgEt + NaBH4 (BH4)MgEt + NaClEt2O

Et2O (BH4)MgEt + B2H6 Mg(BH4)2 . xEt2O + “EtBH2”

MgH2 + B2H6

Et2O Mg(BH4)2 . xEt2O

……or with drastic conditions

Page 4: Attempts to prepare MgB 2 by low pressure CVD Dr. Laura Crociani Istituto di Chimica Inorganica e delle Superfici C.So Stati Uniti 4, 35127 Padova (Italia)

The coordinated ether was removed in vacuo.

Et2O

SYNTHESIS OF Mg(BH4)2

Mg(BH4)2 . xEt2O + 2TlI

Tl(OEt) + LiBH4

Et2

O Tl(BH4) + LiOEt

MgI2 + 2 Tl(BH4)2

yield 97%

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) -45 -44 -43 -42 -41 -40 -39

Page 5: Attempts to prepare MgB 2 by low pressure CVD Dr. Laura Crociani Istituto di Chimica Inorganica e delle Superfici C.So Stati Uniti 4, 35127 Padova (Italia)

Position [°2Theta]

30 40 50 60 70

Counts

0

100

200

300

400 '100 101

110

BULK DECOMPOSITION OF Mg(BH4)2

Condition: Mg(BHCondition: Mg(BH44))22 powder ca powder ca 200 mg 200 mg p= 10p= 10-3-3 Torr T= 430 °C Torr T= 430 °C

Page 6: Attempts to prepare MgB 2 by low pressure CVD Dr. Laura Crociani Istituto di Chimica Inorganica e delle Superfici C.So Stati Uniti 4, 35127 Padova (Italia)

PREPARATION OF THE FILMS

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 TorrDeposition temperature = 500 °CDeposition time = 15 ,30, 60, 90, 120, 240 minutes

Page 7: Attempts to prepare MgB 2 by low pressure CVD Dr. Laura Crociani Istituto di Chimica Inorganica e delle Superfici C.So Stati Uniti 4, 35127 Padova (Italia)

XRD characterization of the films shows only the presence of crystalline MgO.

The peaks with the stars are those of the substrate.

Page 8: Attempts to prepare MgB 2 by low pressure CVD Dr. Laura Crociani Istituto di Chimica Inorganica e delle Superfici C.So Stati Uniti 4, 35127 Padova (Italia)

X-Ray Photoelectron Spectroscopy (XPS) Analysis

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).

XPS characterization of commercial MgB2 pellets

B 1s Mg 2p

Page 9: Attempts to prepare MgB 2 by low pressure CVD Dr. Laura Crociani Istituto di Chimica Inorganica e delle Superfici C.So Stati Uniti 4, 35127 Padova (Italia)

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.

Mg KL23L23

Page 10: Attempts to prepare MgB 2 by low pressure CVD Dr. Laura Crociani Istituto di Chimica Inorganica e delle Superfici C.So Stati Uniti 4, 35127 Padova (Italia)

Sample label and

deposition time (min)

Surface composition

Surface composition

Surface composition

Btot / Mgtot Bboride /Mgtot Bboride/Mgboride

MgB1 – 15 1.3

MgB2 – 30 0.9

MgB3 – 60 1.2

MgB4 – 90 1.0

MgB5 – 120

0.8

MgB6 – 240

0.2

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.

1.3

0.9

1.2

1.0

0.8

0.2 0.6

0.1

0.7

0.5

0.05

0.7

0.5

2.3

3.5

7.2

6.8

16.2

oxide thickness in sputtering

minutes(1 min ~ 0.2

nm)

38

12

12

16

15

145

Page 11: Attempts to prepare MgB 2 by low pressure CVD Dr. Laura Crociani Istituto di Chimica Inorganica e delle Superfici C.So Stati Uniti 4, 35127 Padova (Italia)

Sample label and

deposition time (min)

Surface composition

Btot / Mgtot

MgB1 – 15 1.3

MgB2 – 30 0.9

MgB3 – 60 1.2

MgB4 - 90 1.0

MgB5 – 120 0.8

MgB6 – 240 0.2

8.8

6.2

5.7

8.7

8.6

7.3

Btot /Mgtot

Bulk composition*

10.07.3

7.66.0

6.28.2

9.912.9

8.55.6

7.65.1

Bboride/MgborideBboride/Mgtot

Bulk composition*

Bulk composition*

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.

The inner part is richer in B.

Page 12: Attempts to prepare MgB 2 by low pressure CVD Dr. Laura Crociani Istituto di Chimica Inorganica e delle Superfici C.So Stati Uniti 4, 35127 Padova (Italia)

Sample label and

deposition time (min)

Surface composition

Bulk composition*

Surface composition

Bulk composition* oxide thickness

in sputtering minutes

(1 min ~ 0.2 nm)Btot / Mgtot Btot / Mgtot Bboride/Mgboride Bboride/Mgboride

MgB1 – 15 1.3 7.3 8.4 7.6 38

MgB2 – 30 0.9 8.6 7.7 8.5 12

MgB3 – 60 1.2 8.7 8.2 9.9 12

MgB4 - 90 1.0 5.7 2.5 6.2 16

MgB5 – 120 0.8 6.2 7.6 7.6 15

MgB6 – 240 0.2 8.8 4.3 10.0 145

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).

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.

Page 13: Attempts to prepare MgB 2 by low pressure CVD Dr. Laura Crociani Istituto di Chimica Inorganica e delle Superfici C.So Stati Uniti 4, 35127 Padova (Italia)

IN CONCLUSIONIN CONCLUSION

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.

Page 14: Attempts to prepare MgB 2 by low pressure CVD Dr. Laura Crociani Istituto di Chimica Inorganica e delle Superfici C.So Stati Uniti 4, 35127 Padova (Italia)

SUPERCONDUCTVITY OF SUPERCONDUCTVITY OF MgB2

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).