Research Topic task started on Wed Sep 27, 2006 at 3 - INFN-LNL

ATTEMPTS TO DEPOSIT MgB2 BY LOW PRESSURE CVD
Laura Crociani #, Gilberto Rossetto, Pierino Zanella, Giovanni Carta, Istituto di Chimica Inorganica e
delle Superfici, CNR, C.so Stati Uniti 4, 35127 Padova, Italy e Consorzio INSTM
Naida El Habra, Marco Guidolin, Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo
1, 35131 Padova, Italy
Saulius Kaciulis, Alessio Mezzi, Istituto per lo Studio dei Materiali Nanostrutturati (ISMN) CNR via
Salaria Km 29,300 00016 Monterotondo Stazione (RM) Italy
Vincenzo Palmieri, INFN-Laboratori Nazionali di Legnaro, viale dell’Università 2, 35020 Legnaro (PD)
Italy
Giovanni Giunchi, Edison SPA R&D Foro Buonaparte 31 -20121 Milano Italy
Abstract
Chemical Vapour Deposition, CVD, is a technique
offering the advantages of high grown rates and excellent
conformal coverages with the possibility of employing
simple apparatus by using a single source precursor [3].
Thanks to our chemical background and skills and to our
experience with CVD we have been thinking of preparing a
suitable compound to use as single source precursor for
CVD deposition of MgB2.
We have chosen Mg(BH4)2 which contains Mg and B in
the right ratio and sublimates at 230 °C at 10-3 Torr [4].
Here we report a new method for the synthesis of
Mg(BH4)2*xEt2O, the application of Mg(BH4)2 as CVD
precursor and the preliminary results of the XPS
investigation.
The Chemical Vapour Deposition (CVD) process is a
versatile method for the deposition of several different
materials.
In particular our attention has been focused to MgB2 which
offers a new class of low-cost, high performance
superconductor.
Efforts to fabricate MgB2 belong to several different
methods being mainly typical difficulties in its preparation
the presence of MgO and the formation of MgBx,
compounds, X>2.
Thanks to our experience in the CVD process we decided
to start investigating the opportunity of depositing MgB2 via
CVD by choosing a possibly suitable compound which
singled Mg(BH4)2 out.
Experimental
Here we report a new synthesis of the adduct
Mg(BH4)2*xEt2O whose desolvation produced Mg(BH4)2, Mg(BH4)2 synthesis
the application of Mg(BH4)2 as CVD precursor and the
All the manipulations were carried out under an oxygenpreliminary results of the XPS investigation of the deposits
and
moisture-free atmosphere in a MBraun MB 200G-II
obtained on Si(100).
dry-box with an organic solvents scavenger. Et2O was dried
before its use, following standard procedures [5]. Tl(BH4)
Introduction
was synthesized as reported in the literature [6]. MgI2
MgB2 is a well known compound first synthesized in the (Fluka) was used as received.
Microanalyses were performed at Istituto di Chimica
50’s, which is now the object of an intense scientific
research because of its superconductivity properties and its Inorganica e delle Superfici, CNR, Padova with a EA 1108
Instrument
potential applications [1].
11
B NMR were recorded with a Bruker AMX-300
Many efforts have been made to prepare pure MgB2 thin
spectrometer,
operating at 96.251 MHz, using BF3*Et2O as
films mainly by using physical methods like Molecular
an
external
standard.
Beam Epitaxy, sputtering, Plasma Laser Deposition or
mixing physical and chemical method like in the Mg(BH4)2: To a cold solution of MgI2 (750 mg, 2.7 mmol)
in Et2O (10 mL) was added Tl(BH4) (1.302 g, 3 mmol) and
electrochemical synthesis or the Hybrid Physical-Chemical
the mixture was stirred overnight at room temperature.
Vapour Deposition [1b, 2].
Problems associated with such techniques are the After filtration, the ethereal solution was concentrated by
presence of MgO as impurity, different vapour pressure of distillation and the solvent completely removed as reported
in the literature [4]. Yield 134 mg, 92%.
B and Mg, formation of MgBx x#2, the use of dangerous
Bulk decomposition of Mg(BH4)2 was achieved by
substances like diborane.
#
decomposing
in a Schlenk tube about 200 mg of Mg(BH4)2
e-mail: [email protected]
at 430 ÷ 440 °C and at 10-3 Torr for 1h: the powder was
characterized by means of X-ray diffraction.
collected
I = f(KE).
and
CVD deposition
We set a suitable CVD apparatus by taking a Schlenk
tube which was connected to a diffusive pump. The tube
was closed by a quartz tube with a quickfit cone on which
grease was applied to seal the whole system. Inside the
quartz tube a resistance was put to heat the substrate (T =
500 °C). The precursor (about 100 mg) was put in the
bottom and heated at ca 280 °C in a furnace at 10-3 Torr for
15, 30, 60, 90, 120, and 240 minutes.
Sample characterization
The samples were characterized by means of X-Ray
Electron
Diffraction
and
X-Ray
Photoelectron
Spectroscopy.
XRD spectra were performed by using a Philips PW1830
powder diffractometer in Bragg-Brentano geometry using
Cu Kα radiation (40 kV, 30 mA, λ = 1.54056 Å). The
detector was a Xe gas proportional counter equipped with a
secondary curved graphite monochromator. The patterns
were collected in the 25º ÷ 70º 2θ range at grazing
incidence (Ω = 2º). Peak positions were determined with a
statistical error of d(2θ) = 0.02º. Phase identification was
performed by using standard spectra reported in the JCPDS
database.
XPS measurements (including X-ray induced Auger
spectra) were carried out by using an ESCALAB MkII (VG
Scientific Ltd., U.K.) spectrometer, equipped with a
standard Al Kα excitation source and a 5-channeltron
detection system. Core level photoelectron and Auger
spectra were collected at constant pass energy of the
analyser set to 20 eV and a base pressure in analysis
chamber of 1x10-10 mbar, which was increased up to
10-7 mbar during the depth profiling. The energy of the ion
gun (Ar+) was set to 2.0 keV and the sample current density
- to about 2 µA/cm2. Selected area of the sample was
sputtered by using a gold mask with a circular hole of 3 mm
in diameter. The binding energy (BE) scale was calibrated
by measuring the C 1s peak (BE = 285.0 eV) from the
surface contamination. Before investigating the samples, the
energy scale was calibrated using sputter-cleaned Au foil
with Au 4f7/2 peak set to BE = 84.0 eV. The accuracy of the
measured BE was ± 0.1 eV. More experimental details
have been reported elsewhere [7].
The spectra were processed by the CasaXPS v. 2.2.84
software, using a peak-fitting routine with symmetrical
Gaussian-Lorentzian functions. Background intensity was
subtracted from the photoelectron spectra by using Shirley
method, while for Auger spectra was used the method of
linear subtraction. Auger spectra of Mg KL23L23 line were
processed
in
integral
mode
Results and discussion
Mg(BH4)2 is an inorganic compound which sublimates
above 230 °C at 10-3 Torr [4]. Many synthetic
methodologies have been reported in the literature, but
usually they require long reaction times or severe
conditions, what prompted us to develop a new strategy of
synthesis [8].
Therefore we have set up a new, easy way to prepare the
adduct by reacting Tl(BH4) with MgI2 in Et2O. The yield is
high and the adduct was characterized by means of 11BNMR spectroscopy. Finally the ether was removed in vacuo
to produce Mg(BH4)2.
Once set the synthesis up, we first studied decomposition
of Mg(BH4)2 in vacuo. We decomposed about 200 mg of
Mg(BH4)2 at 430 ÷ 440 °C at 10-3 Torr and we obtained a
dark grey-brown powder which was analyzed by mean of
X-ray diffraction. This spectra shows that the powder
contains crystalline MgB2. Fig.1, what prompted us to
proceed with the film deposition.
Counts
400 100
'
101
300
110
200
100
0
30
40
50
60
Position 2Theta/°
2Theta
70
Fig. 1.
XRD spectrum of the product of bulk
decomposition of Mg(BH4)2
We prepared six films 1 on Si(100) by using different
deposition times ranging from 15 up to 240 minutes
The samples were first analyzed by mean of XRD but
unfortunately XRD spectra show the presence of only
crystalline MgO. (Fig.2).
Counts
Counts 1000
**
MgO
MgO
(220)
(220)
800
600
400
MgO
MgO
(111)
(111)
MgO
MgO
(200)
(200)
**
200
0
-200
33 34 35 36 37 38 39 40 41 42 43 44 60 61 62 63 64 65
33
Position2Theta
2Theta [ [° ]°]
Position
2Theta/°
Fig. 2.
Typical XRD spectrum of the films.
In order to achieve more information about the
composition of the filsm they were analyzed by means of
X-ray photoelectron spectroscopy. First XPS spectra of
MgB2 commercial pellets were recorded showing 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 B
boride (BE = 188.4 eV) (fig. 3a) can be easily separated
by the peak-fitting of B 1s line, while MgO and boride
peaks (fig. 3b) are overlapping in Mg 2p line (~ 51.0
eV);
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), fig. 3c, although it
is difficult to quantify from Auger peaks .
However, the concentration of Mg attributed to boride
(Mgbor) can be calculated from the intensity of the principal
components separated by peak-fitting routine using the
following formula
9k
B 1s
Intensity, cps
7k
oxide
6k
5k
4k
198 195 192 189 186 183 180
Binding Energy, eV
Intensity, cps
6k
b)
Mg 2p
5k
4k
3k
2k
1k
62 60 58 56 54 52 50 48 46
Binding energy, eV
90k
oxide Mg KL L
c)
23 23
80k
Intensity, cps
Mgbor(atomic %) = Mgtot(atomic %)·[Iboride /(Iboride + Ioxide)
where Mgtot is the total atomic concentration of Mg
calculated by XPS quantitative analysis and Iboride and Ioxide
are the intensity in cps (count per second) of the main
component of Mg KLL of boride and oxide respectively.
We could therefore determine the composition of the
surface and of the bulk of the films (table 1).
On the surface we can observe a lower B/Mg than in the
bulk, which indicates that the surface is richer in Mg and it
consists mainly of MgO. There is an oxide layer which is
more or less constant apart the first and last samples: this is
due to the presence of oxygen in the system because of a
not very good sealing of the apparatus.
By sputtering with Ar ion we could also determine the
composition of the bulk of the films.
boride
a)
8k
70k
60k
50k
40k
30k
20k
10k
boride
11671170117311761179118211851188
Kinetic Energy, eV
Fig 3. a) XPS signals of boron oxide and B boride. b) XPS
Mg 2p line; .c) Auger peak of Mg KLL.
Table 1 Surface and bulk composition of the films expressed as B/Mg ratio and oxide thickness.
Surface
Bulk
Surface
Bulk
Sample label
oxide thickness
composition composition* composition composition*
and
in sputtering
deposition time
minutes
Btot / Mgtot Btot / Mgtot Bboride/Mgboride Bboride/Mgboride (1 min ~ 0.2 nm)
(min)
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
*Such compositions were obtained after sputtering the samples until a constant composition value was observed.
We could observe that the inner part was richer in B and
B/Mg ratio was quite higher than 2 which is expected in
MgB2, being the film probably a mixture of different
magnesium boride.
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 during film deposition
under vacuum. Mg migrates to the surface partly reacting
with the oxygen present in the reactor as impurity and partly
evaporating.
Conclusions
We were able to set up a new easy and quick way to
prepare Mg(BH4)2, whose bulk decomposition 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 reached the hot
substrate: decomposition occurred but because of the sizes
of the particles and/or the little amount of substance
deposited and/or formed, other phenomena such as Mg
segregation prevailed on the formation of MgB2 yielding a
deposit not well identifiable.
References
[1] a) J. Nagamatsu, N. Nakagawaa, T. Muranaka, Y.
Zenitani, J. Akimitsu, Nature (2001), 410, 63. b) M. Naito,
K. Ueda, Supercond. Sci. Technol. (2004), 17, R1.
[2] S.H. Pawar, P.M. Shirage, D.D. Shivagan, A.B. Jadhav,
Modern Physics Letters B (2004), 18, 505.
[3] Chemical Vapor Deposition: Principles and
Applications (Eds M.-L. Hitchman, K.-F Jensen), Academic
Press, London (1993).
[4] J. Plešek, S. Hermanek, Collect. Czech. Chem. Commun.
(1966), 31, 3845.
[5] Purification f Laboratory Chemicals 2nd Edition D.D.
Perrin, W.L.F. Armarego, Pergamon Press, (1980).
[6] E. Wiberg, O. Dittmann, H. Nöth, M. Schmidt, Z.
Naturforsch. (1957), 12B, 56.
[7] S. Kaciulis, A. Mezzi, G. Montesperelli, F. Lamastra,
M. Rapone, F. Casadei, T. Valente, G. Gusmano, Surf.
Coat. Technol. (2006), 201, 313.
[8] B.D. James, M.G.H. Wallbridge, Prog. Inorg. Chem.
(1970), 11 , 99. M. Bremer, H. Nöth, M. Warchold, Eur. J.
Inorg. Chem. 2003, 111.

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