Pseudogap Effects in Disordered Niobium Nitride Superconducting Thin Films
The pseudogap region in the disordered Niobium Nitride (NbN) superconductors has been a subject of contemporary interest and its existence for higher disorder films have been established. Niobium Nitride (NbN) thin films have been deposited by reactive direct current magnetron sputtering at 0.266 Pa total pressure in a mixture of argon and nitrogen. The Glancing Incidence X-ray Diffraction (GIXRD) analysis indicates the formation of NbN cubic fcc B1 structure. All the films were found to be superconducting with a maximum superconducting transition temperature (Tc) of 12 K. At higher temperatures, all the films exhibited a negative temperature coefficient of resistance indicating the presence of disorder in the films. The downturn of resistance due to the appearance of superconducting state is observable in the resistance versus temperature plot at a temperature above Tc defined as T*. It has been observed the downturn in resistance versus temperature plot of NbN thin film happens at a temperature around 20 K (T*) for a sample which is superconducting at 11.5 K (Tc).
Received: July 19, 2012;
Accepted: January 21, 2013;
Published: February 21, 2013
Superconductivity has been a fascinating subject for over hundred years since
the discovery of the first superconductor by Kammerlingh Onnes in 1911. Even
after the discovery of High Temperature Superconductors (HTSC), interest in
Low Temperature Superconductors (LTSC) has not waned. The prime reason for this
interest is the wide spectrum of applications of these LTSC superconductors
and the rich physics, which governs the observation of new phenomena. Niobium
nitride (NbN) is such a material that has a significant application potential
and is being investigated both from the point of view of basic physics as well
as applications (Chand et al., 2012; Abdo
et al., 2006; Villegier et al., 2009).
Though deposition of thin films of NbN by reactive DC/RF sputtering has been
well-understood (Wang et al., 1996), the properties
of these NbN thin films have not been fully unraveled. Superconducting state
is characterized by the existence of a gap in the electronic density of states
and this energy gap is expected to vanish at Tc. With the discovery
of HTSC materials, it has been noted that this gap structure extends above Tc,
not as a full gap but as a depression in electronic density of states; this
phenomenon is termed as pseudogap (Ding et al., 1996).
This pseudogap region in the disordered superconductors has been a subject of
contemporary interest and has been investigated by many experimental techniques
including measurement of electrical resistivity (Passos et
al., 2006). Pseudogap state in conventional superconductors like NbN
has been shown to exist at temperatures T* higher than Tc. when the
disorder in thin films is strong while T* merges with Tc in thin
films which are only weakly disordered according to the phase diagram (Chand
et al., 2012). In these highly disordered NbN thin films, the maximum
T* does not exceed the maximum Tc exhibited by low disorder NbN films.
The pseudo gap temperature T* has been identified in Hg-Re system using resistivity
measurements (Passos et al., 2006), where the derivatives
of resistance with temperature serve to establish a criterion for identifying
the T*. In this study, the existence of T* which is larger than the maximum
Tc observed in NbN system has been identified using measurements
of electrical resistivity.
DEPOSITION OF THIN FILMS
The NbN thin films were deposited by reactive sputtering in a load locked deposition
chamber with a base pressure reaching 8x10-8 mbar. The sputtering
is carried out using a 100 mm diameter niobium target of 99.999% purity. The
target to substrate distance is around 0.2 m. The sputtering is carried out
in a mixture of argon and nitrogen with a steady argon flow of 33 SCCM and a
Nitrogen flow of 7 SCCM at a total pressure of 0.266 Pa. The NbN thin films
were deposited on 19x25 mm rectangular glass substrates and oxidized Silicon
wafers of size 20x10 mm. The substrates were sputter cleaned in the load lock
chamber using Argon plasma at a RF power of 80 W for 10 min before transferring
the substrates to the Deposition chamber (DPC) using a magnetic manipulator.
The substrates were positioned in 80 mm square steel holder capable of holding
multiple substrates and rotated about a central axis to ensure uniformity in
the thickness of the deposited film. The substrate holder is cooled by flowing
water to avoid undesirable rise in temperature. The rate of deposition as well
as the total thickness of NbN thin films was measured by the in-situ
Quartz crystal monitor. NbN thin films with a nominal thickness of 140 nm were
deposited on these substrates at a deposition rate approximately 0.2 nm sec-1.
The thickness of the deposited NbN films is measured subsequently using a DEKTAK
3030 A surface profiler. The thin films have been patterned into standard four-probe
geometry for electrical resistivity measurements using lift-off photolithography.
CHARACTERIZATION OF THIN FILMS
The deposited thin films were first characterized using Glancing Incident X-ray diffraction measurements (GIXRD). The electrical resistivity measurements were carried out using the standard four probe geometry patterned on the thin films. A constant current of 20 μA is passed through the sample using a constant current source (Time Electronics Model: 5018 Multifunction calibrator) and the voltage developed across the sample is measured using a nanovoltmeter (Agilant, Model: 34420 A). The sample is fixed firmly on the cooper sample holder mounted on a dipstick. A silicon diode thermometer is mounted on the reverse side of the sample holder for measuring the temperature of the sample. The sample holder is surrounded by a radiation shield to reduce fluctuations of temperature. The temperature of the sample is varied by dipping the sample holder inside a liquid helium dewar which is partially filled. By varying the height of the sample holder above the surface of liquid helium inside the dewar, different temperatures are attained. The resistance and the temperature data is acquired using a labview program and is stored in a computer for subsequent analysis.
RESULTS AND DISCUSSION
Figure 1 shows the GIXRD measurements for a typical NbN thin
film deposited on oxidized silicon substrate and indicates the formation of
cubic fcc B1 structure. All the NbN thin films deposited were found to be superconducting
with a maximum Tc of about 12 K. The plot of resistance versus temperature
for the NbN film deposited in accordance with the procedure described above
is shown in Fig. 2.
|| GIXRD pattern of NbN thin film
||Measured electrical resistance of the NbN sample with temperature.
The inset shows the dependence of the electrical resistance with temperature
in the range of 15 to 30 K
The electrical resistivity of the sample is 1254 μΩ-cm at 300 K.
The superconducting transition temperature of this film is 11.5 K with a RRR
of 0.65 indicating the presence of substantial disorder. The inset in Fig.
2 shows the dependence of resistance on temperature in the range 15 to 30
K. Conspicuous changes in the temperature dependence of resistance have seen
below a temperature of 25 K and the down turn in resistance appears at about
20 K. The plots of the first derivative of resistance with temperature and second
derivative of resistance with temperature are shown in Fig. 3.
The upturn in the first derivative plot with a nearly constant, second derivative
is a clear indication of T* and is observed at around 20 K. This shows that
in these relatively disordered NbN thin films, the pseudogap appears at temperatures
of about 20 K while superconductivity (Tc) sets in at a lower temperature
of about 11.5 K.
||Dependence of first derivative of resistance with temperature
(+) and second derivative of resistance with temperature (o)
between 15 and 30 K
The behavior of most of the disordered NbN thin films investigated during
the present study conforms to this overall scenario with only marginal variations
in Tc and T*.
NbN thin films have been deposited on glass and oxidized silicon substrates
by reactive DC magnetron sputtering. The NbN thin films have been characterized
by GIXRD, which shows the formation of cubic fcc B1 structure. Variation of
resistance with temperature from 300 to 4.2 K has been measured using a simple
dipstick cryostat. Most of the NbN thin films exhibit a negative temperature
coefficient of resistance in the 300 to 20 K temperature range. The NbN thin
films were found to be superconducting with a maximum Tc of about
12 K indicating the presence of substantial disorder in these thin films. The
electrical resistivity measurements indicate that pseudogap state appears at
a temperature of about 20 K, which is higher than the experimentally observed
superconducting transition temperature of 11.5 K. Further measurements and detailed
analysis of the experimental data are being carried out to look for the existence
of such states at even higher temperatures, since the resistivity measurements
show anomalous behavior in this temperature range.
We sincerely thank Dr. S. Rajagopalan and Ms D. Sunita for providing the GIXRD data on these thin films. We also thank Ms S. Kalavathi for preliminary GIXRD measurements and for identification of the structure in these NbN thin films. The authors thank Dr. C.S. Sundar for his continued support.
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