250 research outputs found
Valence Band Onset and Valence Plasmons of SnO2 and In2-xSnx O3 Thin Films
Valence band onset (Ev), valence band tail (VBT) and valence plasmons (VPs)
have been studied as a function of sputtering of SnO2 and In2-xSnxO3 (ITO) thin
films, using ultraviolet photoemission spectroscopy (UPS). Decrease in Ev with
respect to the Fermi level and increase in the density of energy levels of VBT
have been observed after 5 minutes of sputtering using Ar+ ions (500V). Bulk
and surface components of VPs of Sn, SnO and SnO2 in sputtered SnO2 thin films
have been observed in UPS spectra. Similarly, bulk and surface components of
VPs of In, Sn and oxygen deficient ITO in sputtered ITO thin films have been
observed in UPS spectra. Possible roles of Ev and increase in the density of
energy levels of VBT are discussed in the mechanisms of current transport
through heterojunctions of SnO2 with semiconductors.Comment: To be published (2014
In situ soft x-ray absorption spectroscopic study of Fe/MgO interfaces
Interfacial reaction between iron and MgO has been studied in situ during
deposition of iron on MgO surface, using soft x-ray absorption spectroscopy.
High sensitivity of the technique which is capable of detecting even a sub
monolayer of a given phase, combined with in situ measurements as a function of
iron layer thickness, allowed one to make a quantitative estimate of
interfacial phases. Two different substrates namely, MgO (001) single crystal,
and a polycrystalline MgO film on Si substrate have been used in order to
elucidate the role of the state of MgO surface in controlling the interface
structure. It is found that at the interface of iron and MgO film, about two
monolayers of Fe3O4 is formed. Fe3O4 being the oxide of iron with the highest
heat of formation, the reaction appears to be controlled thermodynamically. On
the other hand, on the interface with MgO (001) surface, FeO is formed,
suggesting that the reaction is limited by the availability of oxygen atoms.
Magnetic behaviour of the FeO layer gets modified significantly due to
proximity effect of the bulk ferromagnetic iron layer.Comment: 6 figure
Origin of resistivity minima at low temperature in ferromagnetic metallic manganites
The resistivity and magnetoresistance measurements were carried out on thin
film of La0.7Ca0.3MnO3 to investigate the possible origin of low temperature
resistivity minimum observed in these samples. We observed large hysteresis in
the magnetoresistance at low temperature; 5K and the sample current I has large
effect on resistivity minima temperature. The observation of hysteresis at low
temperatures suggests the presence of inhomogeneity at low temperatures. These
in-homogeneities consist of regions of different resistive phases. It appears
that the high resistive phase prevents the tunneling of charge carriers between
two low resistive regions and thus giving rise to the resistivity minimum in
these samples.Comment: Figure-3 and figure-4 are important new result
Tuning the Magnetic Ground State by Charge Transfer Energy in SrCoO2.5 via Strain Engineering
SrCoO2.5 (SCO) is a charge transfer insulator with 3d6 ground state
configuration leading to antiferromagnetic nature. It is observed that
substrate induced strain engineering modifies the ground state of SCO thin film
with 3d7L (L:O-2p hole) configuration causing negative charge transfer
energy.The consequent strong hybridization between O-2p and Co-3d bands causes
a hole in O-2p band leading to hole mediated unconventional ferromagnetic
ordering in SrCoO2.5 thin film. This opens up a new avenue to tune the
electronic structure vis a vis magnetic property via strain engineering.Comment: 15 pages, 3 figure
Electrical and magnetic transport properties of Fe3O4 thin films on GaAs (100) substrate
Thin films of magnetite (Fe3O4) are grown on single crystal GaAs (100)
substrate by pulsed laser deposition. X ray diffraction (XRD) result shows the
(111) preferred orientation of the Fe3O4 film and x-ray photoelectron
spectroscopy confirm the presence of single phase Fe3O4 in the film. The
electrical transport property of the film shows the characteristic Verwey
transition at 122 K and below 110 K, the transport follows variable range
hopping type conduction mechanism. The film shows room temperature
magnetization hysteresis loop suggesting the ferrimagnetic behavior of the film
with saturation magnetization value close to 470 emu/cc.Comment: 16 pages, 5 figur
Density and Microstructure of Amorphous Carbon Thin Films
In this work, we studied amorphous carbon (-C) thin films deposited using
direct current (dc) and high power impulse magnetron sputtering (HiPIMS)
techniques. The microstructure and electronic properties reveal subtle
differences in -C thin films deposited by two techniques. While, films
deposited with dcMS have a smooth texture typically found in -C thin films,
those deposited with HiPIMS consist of dense hillocks surrounded by a porous
microstructure. The density of -C thin films is a decisive parameter to
judge their quality. Often, x-ray reflectivity (XRR) has been used to measure
the density of carbon thin films. From the present work, we find that
determination of density of carbon thin films, specially those with a thickness
of few tens of nm, may not be accurate with XRR due to a poor scattering
contrast between the film and substrate. By utilizing neutron reflectivity (NR)
in the time of flight mode, a technique not commonly used for carbon thin
films, we could accurately measure differences in the densities of -C thin
films deposited using dcMS and HiPIMS.Comment: 6 pages, 5 figures, 2 table
Finding pathways for stoichiometric Co4N thin films
In this work, we studied the pathways for formation of stoichiometric
\tcn~thin films. Polycrystalline and epitaxial \tcn~films were prepared using
reactive direct current magnetron (dcMS) sputtering technique. A systematic
variation in the substrate temperature (\Ts) during the dcMS process reveals
that the lattice parameter (LP) decreases as \Ts~increases. We found that
nearly stoichiometric \tcn~films can be obtained when \Ts~= 300\,K. However,
they emerge from the transient state of Co target (3\,inch). By reducing
the target size to 1\,inch, now the \tcn~phase formation takes place from
the metallic state of Co target. In this case, LP of \tcn~film comes out to be
99\p~of the value expected for \tcn. This is the largest value of LP
found so far for \tcn. The pathways achieved for formation of polycrystalline
\tcn~were adopted to grow an epitaxial \tcn~film, which shows four fold
magnetic anisotropy in magneto-optic Kerr effect measurements. Detailed
characterization using secondary ion mass spectroscopy indicates that N
diffuses out when \Ts~is raised even to 400\,K. Measurement of electronic
structure using x-ray photoelectron spectroscopy and x-ray absorption
spectroscopy further confirms it. Magnetization measurements using bulk
magnetization and polarized neutron reflectivity show that the saturation
magnetization of stoichiometric \tcn~film is even larger than pure Co. Since
all our measurements indicated that N could be diffusing out, when \tcn~films
are grown at high \Ts, we did actual N self-diffusion measurements in a CoN
sample and found that N self-diffusion was indeed substantially higher. The
outcome of this work clearly shows that the \tcn~films grown prior to this work
were always N deficient and the pathways for formation of a stoichiometric
\tcn~have been achieved.Comment: 12 pages, 18 figure
Effect of strain on the electronic structure of La0.7Ca0.3MnO3
The effect of substrate strain on the electronic valence band structure of
La0.7Ca0.3MnO3 thin films has been investigated. For this purpose
La0.7Ca0.3MnO3 thin films have been simultaneously grown on SrTiO3 and LaAlO3
substrates using pulsed laser deposition technique. These samples were
characterized by x-ray diffraction, x-ray reflectivity, core level x-ray
photoelectron spectroscopy, and scanning electron microscopy measurements. Our
experiments confirmed that all these samples have same chemical composition but
different strain configuration. The electronic structure of these samples is
probed through valence band spectroscopy measurements on Indus-1 synchrotron
x-ray source. We observe that strain has large effect on the valence band
electronic structure of La0.7Ca0.3MnO3. The results are explained on the basis
of strain induced orbital occupancy and change in the crystal field splitting
due to Mn-O bond length
Studies on temperature dependent semiconductor to metal transitions in ZnO thin films sparsely doped with Al
For a detailed study on the semiconductor to metal transition (SMT) in ZnO
thin films doped with Al in the concentration range from 0.02 to 2%, we grew
these films on (0001) sapphire substrates using sequential pulsed laser
deposition. It was found that the Al concentration in the films increased
monotonically with the ratio of ablation durations of the Alumina and ZnO
targets used during the deposition. Using X-ray photo electron spectroscopy it
was found that while most of the Al atoms occupy the Zn sites in the ZnO
lattice, a small fraction of the Al also gets into the grain boundaries present
in the films. The observed SMT temperature decreased from ~ 270 to ~ 50 K with
increase in the Al concentration from 0.02 to 0.25 %. In the Al concentration
range of ~ 0.5 to 2 % these doped ZnO films showed metallic behavior at all the
temperatures without undergoing any SMT. A theoretical model based on thermal
activation of electrons and electron scatterings due to the grain boundaries,
ionic impurities and phonons has been developed to explain the observed
concentration and temperature dependent SMT.Comment: 5 figures, 24 page
Magnetism and structure of in-situ grown FeN films studied using N K-edge XAS and nuclear resonance scattering
We studied the structural and magnetic properties of \textit{in-situ} grown
iron mononitride (FeN) thin films. Initial stages of film growth were trapped
utilizing synchrotron based soft x-ray absorption near edge spectroscopy
(XANES) at the N -edge and nuclear resonant scattering (NRS). Films were
grown using dc-magnetron sputtering, separately at the experimental stations of
SXAS beamline (BL01, Indus 2) and NRS beamline (P01, Petra III). It was found
that the initial stages of film growth differs from the bulk of it. Ultrathin
FeN films, exhibited larger energy separation between the t and e
features and an intense e feature in the N -edge pattern. This indicates
that a structural transition is taking place from the rock-slat (RS)-type FeN
to zinc-blende(ZB)-type FeN when the thickness of films increases beyond 5\,nm.
The behavior of such N -edge features correlates very well with the
emergence of a magnetic component appearing in the NRS pattern at 100\,K in
ultrathin FeN films. Combining the \textit{in-situ} XANES and NRS measurements,
it appears that initial FeN layers grow in RS-type structure having a magnetic
ground state. Subsequently, the structure changes to ZB-type which is known to
be non-magnetic. Observed results help in resolving the long standing debate
about the structure and the magnetic ground state of FeN.Comment: 9 pages, 5 figure
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