Title: Superconducting magnesium diboride films on Silicon with Tc0 ~ 24K grown via vacuum annealing from stoichiometric precursors: by
Zhai et al., Submitted to Appl. Phys. Lett. Mar 28, 2001.
Superconducting magnesium diboride films on Silicon with Tc0 ~ 24K
grown via vacuum annealing from stoichiometric precursors
H.Y. Zhai,∗ H.M. Christen, L. Zhang, C. Cantoni, M. Paranthaman, B.C. Sales,
D.K. Christen, and D.H. Lowndes
Oak Ridge National Laboratory, Oak Ridge, TN 37931-6056
Superconducting magnesium diboride films with Tc0 ~ 24 K and sharp transition ~ 1 K were
successfully prepared on silicon substrates by pulsed laser deposition from stoichiometric MgB2
target. Contrary to previous reports, anneals at 630 °C and a background of 2×10-4 torr Ar/4%H2 were
performed without the requirement of Mg vapor or an Mg cap layer. This integration of
superconducting MgB2 films on silicon may thus prove enabling in superconductor-semiconductor
device applications. Images of surface morphology and cross-section profiles by scanning electron
microscopy (SEM) show that the films have a uniform surface morphology and thickness. Energy
dispersive spectroscopy (EDS) reveals these films were contaminated with oxygen, originating either
from the growth environment or from sample exposure to air. The oxygen contamination may account
for the low Tc for those in-situ annealed films, while the use of Si as the substrate does not result in a
decrease in Tc as compared to other substrates.
∗
Electronic Mail: [email protected]
1
Title: Superconducting magnesium diboride films on Silicon with Tc0 ~ 24K grown via vacuum annealing from stoichiometric precursors: by
Zhai et al., Submitted to Appl. Phys. Lett. Mar 28, 2001.
The discovery of superconductivity in the binary compound MgB2 is remarkable considering its simple composition,
the easily obtained starting materials, and the relatively high Tc among the classic superconductors.1-2 Recent attention has
been focused on physical properties and on potential large-scale applications and devices.3-5 Only weeks after the
discovery, the first superconducting MgB2 thin films were successfully fabricated.6-10 However, these efforts were
complicated by the low decomposition temperature (~ 650 °C) and the high equilibrium phase formation temperature (>800
°C) of MgB2. To date, all MgB2 films with bulk-like Tc > 38 K were prepared with an ex-situ method starting from boron
films, followed by a post-deposition annealing at high temperature (~ 900 °C) in the presence of Mg in a sealed
container.7,9-10
Silicon is an attractive substrate for the growth of
superconducting films due to the potential for device
integration. Attempts to integrate Si and high-Tc oxide
superconductors have failed, because Si-contamination into
those oxides depresses superconductivity. For MgB2 on Si,
post-deposition annealing at high temperature ~ 900 °C,
using the same procedure as with other substrates,7 has not
yielded superconducting material, and the only reported
MgB2 films grown on Si using ex-situ annealing show a low
Tc and a wide transition.11
This letter reports the systematic study of MgB2 films on
silicon substrates by pulsed laser deposition (PLD). The
measured Tc0 is 24 K with a narrow transition width of
about 1 K, which is comparable to MgB2 films prepared on
other substrates (LaAlO3, SrTiO3, and Al2O3) using an insitu annealing procedure on Mg-rich precursor films, and
either an Mg cap layer6,8 or an Mg plasma.7 The present
technique for fabricating superconducting MgB2 on Si
differs remarkably from those previous reports: (1) The
starting precursor films of MgB2 were prepared from a
MgB2 target; (2) no excess of Mg or MgB2 was present
during the annealing; (3) the films were annealed in 2×10-4
torr of Ar/4%H2 (with a base pressure of 5×10-6).
Resistance versus temperature curves were measured using
a standard four-probe method in a small refrigerator system
capable of reaching 11 K. X-ray diffraction θ-2θ scans were
used to study the structure of the MgB2 films. Surface
morphologies and cross-section profiles of MgB2 films
reacted at different annealing temperatures were examined
by scanning electron microscopy (SEM). Film
compositions at different sites (surface, middle of the film,
MgB2/Si interface) were studied by energy-dispersive
spectrometry (EDS). Details of growth mechanisms and
composition for MgB2 films fabricated with different
processes and substrates will be reported elsewhere.12
Stoichiometric targets were prepared using MgB2
powders obtained either from Alfa Aesar or by reacting
elemental Mg turnings with amorphous B powder. Both
conventionally sintered and hot-pressed pellets were used.
Deposition was carried out at room temperature in a
background of 10-4 Torr of Ar/4%H2. Laser energies ranged
from 200 – 400 mJ per pulse at a wavelength 248 nm,
corresponding to an energy density of 1.7 – 3.3 J/cm2 on the
target, and the laser repetition rate was 15 Hz. Typical
thickness of these films was 600 nm. N-type, (100)-oriented
Si with a resistivity of 0.2-0.3 Ω⋅cm was used as the
substrate material for all samples described here. Details
regarding the experimental system and the target
preparation can be found elsewhere.6,8 In order to start from
exactly the same condition, all 6 samples annealed at
different temperatures were cut from a single MgB2-coated
Si chip measuring about 8 mm by 11 mm. Five of these
samples (1-2 mm by 8 mm) were then annealed in the same
laser deposition chamber at 500 °C, 550 °C, 600 °C, 630
°C, and 670 °C respectively, with the remaining one serving
as a reference. Each anneal was performed with a
temperature ramp of 100 °C/min in a 2×10-4 Ar/4%H2
background, and the sample was held at the annealing
temperature for 20 minutes. The chamber was then vented
with 1 atm Ar/4%H2 and cooled to below 200 °C at 50
°C/min. Considering the relatively slow decomposition of
MgB2 even at elevated temperature,14 these rates are
sufficiently high to exclude effects related to the ramp time
in our study.
With the current growth conditions, a stoichiometric
MgB2 precursor cannot be obtained above 300 °C, and the
superconducting MgB2 phase is not formed at room
temperature.8 Plotted in Fig. 1 are the resistance versus
temperature curves of MgB2 films grown on Si from the
stoichiometric target at room temperature and annealed at
500 °C (Fig. 1a), 550 °C (Fig. 1b), 600 °C (Fig. 1c), 630 °C
(Fig. 1d), and 670 °C (Fig. 1e). The film grown at room
temperature (not shown) has a flat R-T curve and shows no
sign of a transition above 11 K. The resistance of samples
annealed above 500 °C increases when the temperature is
decreased, presumably due to carrier freeze-out in the Si
substrate. Zero resistance is not observed on films annealed
at 500 °C, 550 °C, and 670 °C. The film annealed at 600 °C
has a Tc0 > 18 K with Tconset ~ 25 K, and the film annealed at
630 °C has a Tc0 at 24 K and Tconset at 25.5 K. It is
particularly interesting to note that Mg-rich precursors on
Al2O3 with Mg cap layers annealed in 1 atm. of Ar/4%H2
do not yield superconducting films when annealed above
630 °C,8 whereas 630 °C is the optimum temperature for
the current films. (Note that the system used for this work is
the same as that used in references 6 and 8.)
2
Title: Superconducting magnesium diboride films on Silicon with Tc0 ~ 24K grown via vacuum annealing from stoichiometric precursors: by
Zhai et al., Submitted to Appl. Phys. Lett. Mar 28, 2001.
10
Figure 2 shows SEM images of MgB2 film surface
morphologies for different treatments. The MgB2 film
deposited at room temperature without high temperature
treatment shows a uniform layer with typical micron-size
droplets from PLD, as shown in Fig. 2a. The MgB2 films
annealed at 500 °C, 550 °C, 600 °C, 630 °C, and 670 °C all
show a uniform and identical surface morphology except
for the typical micron-size PLD particles, as shown in Fig.
2b for the sample annealed at 630 °C. We also observed
cracks in these films and the films grown on other
substrates (Al2O3 etc.) using the in situ procedure.8 The
cracks usually originate at a droplet or encircle a droplet,
possibly implying chemical composition variations in that
area in addition to the different thermal expansion
coefficients between the substrate (Si) and MgB2 films.
o
500 C
0
20
o
Resistance [Ω]
550 C
0
20
o
600 C
0
20
20
o
630 C
10
0
20
22
24
26
28
30
0
200
o
670 C
0
0
50
100
150
200
250
300
Temperature [K]
Fig. 1. Resistance – Temperature curves for MgB2 films prepared on n-Si
(100) using standard four-probe method. Measurements down to 11 K
were performed. Measured resistivity of original Si substrate at 25 K is ~
300 Ω⋅cm corresponding to 2 kΩ as the same size substrate we used in our
samples.
For the sample annealed at 630 °C (Tc0 ~ 24K), an SEM
cross-section image is shown in Fig. 3. The film is dense
with a uniform thickness of 600 nm. EDS performed at
different sites show oxygen contamination throughout the
film. Because EDS is insensitive to light elements, the B
percentage can only be determined semi-quantitatively.
Nevertheless, the Mg:B ratio in the film clearly deviates
from the bulk value of 0.5, measuring about 0.3 throughout
most of the volume and decreasing slightly near the surface.
No anomalous Mg-depletion near the film/substrate
interface is observed. However,
Figure 3. Cross-section SEM images and EDS composition analysis results
(inset) on the film annealed at 630 °C.
Figure 2. SEM images of surface morphologies of MgB2 films grown
at room temperature (a) and annealed at 630 °C (b).
Mg diffuses substantially into the Si substrate, consistent
with the reported formation of an Mg-Si compound at 650
°C.13 Note that these results may be numerically accurate
because EDS typically samples a volume comparable to the
present film thickness.
3
Title: Superconducting magnesium diboride films on Silicon with Tc0 ~ 24K grown via vacuum annealing from stoichiometric precursors: by
MgB2(002)
S
MgB2(101)
MgB2(100)
MgB2(001)
Zhai et al., Submitted to Appl. Phys. Lett. Mar 28, 2001.
S
o
Intensity [arb. Log10]
MgB2/Si 670 C
o
630 C
o
600 C
o
550 C
o
500 C
Mg
as-deposition
20
30
40
50
60
2θ
Figure 4. X-ray diffraction θ-2θ scans showing the broad and weak peaks
of MgB2 (101) and (002) in films annealed above 500 °C, and the
presence of elemental Mg in the as-deposited film.
The films’ structure was also studied using X-ray
diffraction (Figure 4). As expected from powder
diffractograms of polycrystalline MgB27, the MgB2 (101)
reflection is strongest for annealed films, with the (002)
peak also being detected. Considering the film thickness of
about 600 nm, the broad and weak peaks imply that only a
small volume fraction of the layers is crystalline MgB2, as
was observed previously for Mg-rich MgB2 films annealed
in-situ with an Mg cap layer.6,8 Moreover, this result also
explains why the resistance of these film is much larger
than that of stoichiometric MgB2 materials (Fig. 1).
Magnesium deficiency and oxygen contamination are the
most likely reasons for the incomplete crystallization, and
the oxygen contamination could be caused by oxygen in the
growth chamber or in the target materials, or could result
from the exposure of the precursor film to air before the
anneal. However, films on other substrates, for which the
precursor was not exposed to air, showed comparable Tc,
indicating that exposure to air may not be the dominating
factor. Oxygen contamination has also been identified in a
transmission electron microscopy study by Eom et a., and
the laser induced MgB4/MgB7 formation has been proposed
as possible contamination in PLD-grown MgB2 films.8,15
In summary, several issues have been identified as
limiting factors in the growth of MgB2 films on Si
substrates. In particular, the formation of a binary Mg-Si
compound at the film/substrate interface restricts the
current approach to annealing temperatures below 650 °C,
and our data indicate that the Mg+B precursor films do not
fully react to form the correct MgB2 phase in anneals at 630
°C. Oxygen contamination and Mg-deficiency are identified
as possible reasons for incomplete crystallization, leading to
a further decrease in the transition temperatures.
Nevertheless, these MgB2 films prepared on n-type, (100)oriented Si substrates show Tc ≈ 24 K and sharp transitions
with widths of about 1 K. Comparison to films grown on
other substrates shows that Si does not suppress the
superconducting when annealed at 630 °C. This approach
does not rely on an additional Mg vapor source or Mg cap
layers for the anneals, contrary to earlier studies, and will
thus facilitate the integration of MgB2 layers into
heterostructures, possibly enabling the formation of
junctions and devices.
This research is sponsored by the U.S. Department of
Energy under contract DE-AC05-00OR22735 with the Oak
Ridge National Laboratory, managed by UT-Battelle, LLC,
and by the DOE Office of Energy Efficiency and
Renewable Energy, Office of Power Technologies –
Superconductivity Program.
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Web:
http://www.aps.org/meet/MAR01/mgb2/talks3.html#talk54
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7
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4
Title: Superconducting magnesium diboride films on Silicon with Tc0 ~ 24K grown via vacuum annealing from stoichiometric precursors: by
Zhai et al., Submitted to Appl. Phys. Lett. Mar 28, 2001.
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12
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13
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5