[Show abstract][Hide abstract] ABSTRACT: Modified Williamson-Hall (WH) analysis is used to determine the reliable values of the microstructures for Zincblende epilayers grown on non-polar substrates. Systematic high resolution X-ray diffraction (HRXRD) ex-periments are performed for several skew symmetric reflections which enable an accurate measurement of the values of vertical coherence length (VCL) and microstrain of GaAs epilayers grown on Si. Furthermore, a simple method based on the orientation of Burgers vector is proposed for estimating the ratio of tilt and twist. In this method, the twist can be found easily once tilt is known. It is rather quick and the measured values of twist are very similar to those which are otherwise estimated by acquiring numerous HRXRD scans along with tedious fitting procedures. Presence of 60 mixed dislocations is confirmed from the cross sectional high resolution transmission electron microscope images of GaAs / Si samples. Furthermore, the estimated value of VCL is equivalent to the layer thickness measured by the surface profiler.
[Show abstract][Hide abstract] ABSTRACT: Epitaxy of ZnO layers on cubic GaP (111) substrates has been demonstrated
using pulsed laser deposition. Out of plane and in-plane epitaxial relationship
of ZnO layer with respect to GaP substrate determined using phi scans in high
resolution X-ray diffraction measurements are (0001) ZnO || (111) GaP and (-1 2
-1 0) ZnO || (-1 1 0) GaP respectively. Our results of epitaxy of ZnO and its
intense excitonic photoluminescence with very weak defect luminescence suggest
that (111) oriented GaP can be a potential buffer layer choice for the
integration of ZnO based optoelectronic devices on Si(111) substrates.
[Show abstract][Hide abstract] ABSTRACT: Hydrogen interstitials and oxygen vacancies defects create energy levels in the band gap of alumina. This limits the application of alumina as a high-k dielectric. A low thermal budget method for removal of hydrogen from alumina is discussed. It is shown that bombardment of alumina films with low energy oxygen ion beam during electron beam evaporation deposition decreases the hydrogen concentration in the film significantly.
[Show abstract][Hide abstract] ABSTRACT: We have carried out photoemission experiments to obtain valence band spectra of various crystallographic symmetries of BaTiO3 system which arise as a function of temperature. We also present results of a detailed first principle study of these symmetries of BaTiO3 using generalized gradient approximation for the exchange-correlation potential. Here we present theoretical results of density of states obtained from DFT based simulations to compare with the experimental valence band spectra. Further, we also perform calculations using post density functional approaches like GGA + U method as well as non-local hybrid exchange-correlation potentials like PBE0, B3LYP, HSE in order to understand the extent of effect of correlation on band gaps of different available crystallographic symmetries (5 in number) of BaTiO3.
[Show abstract][Hide abstract] ABSTRACT: The electronic structures of Co2.01Ni1.05Ga0.94 and Co1.76Ni1.46Ga0.78 Heusler alloys have been investigated by resonant photoemission spectroscopy across the 3p-3d transition of Co and Ni. For the Ni excess composition Co1.76Ni1.46Ga0.78, the valence band peak shows a shift of 0.25 eV as compared to the near stoichiometric composition Co2.01N11.05Ga0.94. Also an enhancement is observed in the Ni related satellite features in the valence band for the Ni excess composition. Due to hybridization of Co and Ni 3d states in these systems, the Co and Ni 3p-3d resonance energies are found to be higher as compared to Co and Ni metals. Theoretical first principle calculation is performed to understand the features in the valence band and the shape of the resonance profile.
[Show abstract][Hide abstract] ABSTRACT: We have studied the changes in various properties of Poly (methyl methacrylate) (PMMA) after irradiation with electron beam using several techniques and tried to understand the correlation of modification with electron beam energy and current. This work was carried out with an aim to fabricate 3D structures in freestanding PMMA thin films by lithography. Chemical, structural and electrical alterations in PMMA thin film after irradiation with different doses were monitored using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and frequency dependent dielectric measurements, respectively. All the measurements invariably showed that the irradiation induced degradation of PMMA resulted in breaking of parental chain and associated changes. FTIR indicated increase in un-saturation (C
[Show abstract][Hide abstract] ABSTRACT: We report structural, magnetic, and dielectric properties of oxygen deficient hexagonal BaFeO3−δ. A large dielectric permittivity comparable to that of other semiconducting oxides is observed in BaFeO3−δ. Magnetization measurements indicate magnetic inhomogeneity and the system shows a paramagnetic to antiferromagnetic transition at ∼160 K. Remarkably, the temperature, at which paramagnetic to antiferromagnetic transition occurs, around this temperature, a huge drop in the dissipation factor takes place and resistivity shoots up; this indicates the possible correlation among magnetic and dielectric properties. First principle simulations reveal that some of these behaviors may be explained in terms of many body electron correlation effect in the presence of oxygen vacancy present in BaFeO3−δ indicating its importance in both fundamental science as well as in applications.
[Show abstract][Hide abstract] ABSTRACT: Synchrotron based photoemission spectroscopy study shows a type-II band alignment at pulsed laser deposited epitaxial ZnO layer and GaP substrate. Additionally, the ZnO/GaP hetero-junction is comprised of an interfacial GaOx layer. Valence band onsets corresponding to ZnO, GaOx, and GaP materials enabled us to determine the valence band offset (ΔEV) values of 2.81 eV, 0.35 eV, and 2.46 eV at ZnO/GaP, ZnO/GaOx, and GaP/GaOx hetero-junctions, respectively. ΔEV values determined directly from the difference of valence band onsets are in corroboration with that obtained by Kraut's method. The band alignment and interface study of ZnO/GaP hetero-junction can be used to design optoelectronic devices.
[Show abstract][Hide abstract] ABSTRACT: The influence of Co addition on the atomic ordering of bcc FeCo-phase in partially nanocrystalline Fe 81-x Co x Nb 7 B 12 alloys (x ¼ 20.25, 27, 40.5, 54, 60.75) was investigated using synchrotron radiation based x-ray diffraction (SR-XRD) and 57 Fe M€ ossbauer spectroscopy (MS). The results indicate that as the Co content in the alloy increases, there is a decrease in the thermal stability against crystallization of the amorphous phase. In the pure amorphous state, topological ordering as well as compositional homogeneity of the amorphous phase increases with Co addition (x ! 40.5). In the partially nanocrystalline state (isothermally annealed at 823 K for 1 h), the anomalous diffraction measurements close to Fe K-edge provide unambiguous evidence of atomically ordered a 0 -FeCo phase (CsCl type) as the nanocrystalline ferromagnetic phase by the presence of (100) super-lattice reflection for the alloys with x ! 40.5. However, alloys with x 27 present a disordered structure as confirmed by the absence of superlattice reflection. The long range order parameter (S) possess maximum value for the alloys close to equi-atomic composition (x ¼ 40.5 and 54). By predominant low hyperfine field contribution (close to 33.7 T), M€ ossbauer spectroscopy measurement also validates the atomically ordered FeCo nanocrystalline phase in Fe 81-x Co x Nb 7 B 12 alloy with x ! 40.5.
[Show abstract][Hide abstract] ABSTRACT: The influence of Co addition on the atomic ordering of bcc FeCo-phase in partially nanocrystalline Fe81-xCoxNb7B12 alloys (x = 20.25, 27, 40.5, 54, 60.75) was investigated using synchrotron radiation based x-ray diffraction (SR-XRD) and 57Fe Mössbauer spectroscopy (MS). The results indicate that as the Co content in the alloy increases, there is a decrease in the thermal stability against crystallization of the amorphous phase. In the pure amorphous state, topological ordering as well as compositional homogeneity of the amorphous phase increases with Co addition (x ≥ 40.5). In the partially nanocrystalline state (isothermally annealed at 823 K for 1 h), the anomalous diffraction measurements close to Fe K-edge provide unambiguous evidence of atomically ordered α′-FeCo phase (CsCl type) as the nanocrystalline ferromagnetic phase by the presence of (100) super-lattice reflection for the alloys with x ≥ 40.5. However, alloys with x ≤ 27 present a disordered structure as confirmed by the absence of superlattice reflection. The long range order parameter (S) possess maximum value for the alloys close to equi-atomic composition (x = 40.5 and 54). By predominant low hyperfine field contribution (close to 33.7 T), Mössbauer spectroscopy measurement also validates the atomically ordered FeCo nanocrystalline phase in Fe81-xCoxNb7B12 alloy with x ≥ 40.5.
Journal of Applied Physics 08/2013; 114(8). · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: b-Ga2O3 nanostructures were synthesized via vapor transport method on gold coated Silicon
substrate in N2 ambient. The as synthesized products were investigated by grazing incident X-ray
diffraction, scanning electron microscopy and photoluminescence (PL) spectroscopy. It is shown that
the intensity of photoluminescence from the ensemble of b-Ga2O3 nanostructures in oxygen gas
ambience is correlated with the oxygen pressure through the Langmuir equation. This correlation is
found to be reversible and reproducible. This phenomenon, which was not observed in the bulk
b-Ga2O3 single crystal, is attributed to the oxygen related shallow trap surface states of the
nanostructures with energies at about 4.2 eV above the valance band. Based on the changes in the PL
intensity with the oxygen pressures, a possible mechanism for the observed photoluminescence is
suggested. The present results provide a route for room-temperature response of oxygen in the
gallium oxide nanostructures.
Journal of Applied Physics 08/2013; 114:74309. · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Epitaxial GaN films were grown on sputtered ZnO buffer layers of thickness 25–200 nm over c-plane sapphire by reactive sputtering of GaAs in nitrogen at 700 °C. The epitaxial quality and microstructure have been studied by high resolution X-ray diffraction in phi (ϕ) and omega (ω) scan geometries. The surface morphology of epilayers was studied by atomic force microscopy and scanning electron microscopy and their crystalline quality was assessed by Raman spectroscopy. These studies have shown that ZnO buffer layers of 50–100 nm facilitate growth of GaN epilayers of high crystalline quality, compared to those grown on thinner and thicker ZnO buffer layers.
[Show abstract][Hide abstract] ABSTRACT: BiFeO3 sample prepared by rapid liquid phase sintering (RLPS) technique is found to posses better dielectric properties than the subsequently annealed sample. These samples also show ferromagnetism at room temperature. Detailed X-ray diffraction and Rietveld refinement of the data show that oxygen–iron–oxygen (O–Fe–O) bond angle is considerably more distorted in samples prepared by rapid liquid-phase sintering technique than in the annealed sample. In addition, synchrotron X-ray diffraction as well as high resolution transmission electron microscopy studies show higher concentration of impurity phases in the annealed samples, which is the likely reason for inferior dielectric properties. Further transmission electron microscopy results indicate that high crystalline order is not necessary for better dielectric properties. However weak ferromagnetism is observed in all these samples and appears to be the intrinsic property of samples prepared by rapid liquid phase sintering technique.
[Show abstract][Hide abstract] ABSTRACT: A high pressure XRD measurement setup in the angle dispersive geometry has been setup in the Angle Dispersive X-ray diffraction beamline (BL-12) in Indus-2 synchrotron facility. The X-Ray beam is collimated inside the diamond anvil cell (DAC) using a pair of cross-slit collimators and 100 micron orifice in a 400 micron thick Ta sheet. With the use of an adaptive optics to ensure a converging beam at the sample position, the need of an x-ray beam collimator is eliminated making the alignment of the DAC quite easy. The alignment of the DAC with respect to the incident x-ray beam is made by placing it on a computer controlled sample mounting and alignment stage developed specifically for this setup. Interactive software has been developed to make the alignment of the X-ray through the DAC very easy and accurate. NIST standard LaB6 powder was used for test runs, and a few fine pieces of gold served as pressure calibrator. The data was recorded on a MAR345 Image plate detector.
Journal of Physics Conference Series 03/2013; 425(11):112001.
[Show abstract][Hide abstract] ABSTRACT: It was found that by changing the substrate temperature from room temperature to ∼850 °C, ZnO thin films with widely varying resistivity values could be grown on sapphire substrates using DC-discharge-assisted pulsed laser deposition (PLD) in oxygen ambient. The resistivity of the film grown at room temperature was too high to measure using our existing setup. However, as the growth temperature was increased from 550 °C to 750 °C, the resistivity first decreased slowly from ∼14.0 to 4.4 Ω m and then dropped suddenly to get saturated at ∼2.0×10−3 Ω m as the growth temperature was further increased. In contrast to these, when there was no DC-discharge, the variation of resistivity for ZnO thin films grown by PLD was marginal up to the substrate temperature of ∼850 °C. The reason for these observations was found to be the combined effects of reduction in donor defect densities like oxygen vacancies and zinc interstitials, introduction of acceptor type defects like interstitial oxygen and zinc vacancies, and the resultant poor carrier mobility at lower growth temperatures. At higher growth temperatures (800 °C and above), the appearance of oxygen vacancies and increase in mobility due to better crystalline quality were found to be responsible for reducing the resistivity. The PL of these films had significant emission in the green and red regions of the spectrum due to the aforesaid defect related transitions. Such highly resistive and luminescent films might be suited for applications such as resistive RAM, UV-photo detector, TFT, piezoelectric, transparent phosphor, and broadband LED applications.
[Show abstract][Hide abstract] ABSTRACT: Growth of CuO nanowires by annealing method has been studied in-situ by
grazing incidence Energy Dispersive X-ray Diffraction (EDXRD) technique
on Indus-2. It was observed that Cu slowly oxidized to Cu2O
and finally to CuO. The data was taken as a function of time at two
annealing temperatures 500°C where nanowires form and 300°C
where nanowires don't form. We found that the strain in the CuO layer
may be a principal factor for the spontaneous growth of nanowires in
Journal of Applied Physics. 02/2013; 114(14):306-307.
[Show abstract][Hide abstract] ABSTRACT: β-Ga2O3 nanostructures including nanowires,
nanoribbons and nanosheets were synthesized via vapor transport method
on gold coated Silicon substrate in N2 ambient. The as
synthesized products were investigated by Grazing Incident X-Ray
Diffraction, Scanning Electron Microscopy, Transmission Electron
Microscopy and photoluminescence. The PL spectra of
β-Ga2O3 nanostructures exhibit a UV-blue
emission band which is strongly affected by the oxygen pressure in terms
of PL intensity. We believe that because of desorption of oxygen,
surface of the nanostructures is reconstructed and this may create
surface states which quench PL by creating competitive nonradiative
paths. It provides an alternate route of oxygen sensing at room
temperature and low pressure conditions. The luminescence mechanism, for
this, is also discussed.
[Show abstract][Hide abstract] ABSTRACT: The electronic structure of Co2MnSn Heusler alloy has been
investigated by using resonant photoemission spectroscopy across the
3p-3d transition of Mn and Co. Angle dispersive X-ray diffraction
pattern corroborates with the valence band photoemission spectra and
confirms a conventional L21 structure for this system. In addition to Mn
and Co 3p-3d transitions, the constant initial-state spectra shows two
more features related to the multiplet structure of Mn and the small
impurity phase of Co, which is confirmed by the diffraction data.
Theoretical first principle calculation is performed to understand the
features in the valence band and the shape of the resonance profile.
[Show abstract][Hide abstract] ABSTRACT: Lead Magnesium Niobate (PMN) is prepared by two step solid state
reaction i.e. Columbite route. X-ray diffraction pattern of powder of
calcined PMN at 800 °C and powder of PMN pellet sintered at 1200
°C have shown some systematic differences in their relative
intensity ratios measured by a 2D detector on Indus-2 synchrotron
source. Rietveld method is used to study these differences taking
split-atom approach. We have observed that statistical disorder observed
in both cases is different. Such multisite disorder is consistent with
other Pb based systems like PZN-PT.