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