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In recent times, memory devices based on resistive switching (RS) phenomena in dielectric materials have become a strong contender for the futuristic universal memory. Among other materials being explored for RS application, tantalum pentaoxide (Ta2O5) has emerged as potential candidate due to its large dielectric constant and compatibility with the existing complementary metal-oxide semiconductor process. In view of this, we have studied the resistive switching memory characteristics of Ta2O5 thin film in Cu/Ta2O5/Pt device configuration. About 200 nm thick films of Ta2O5were deposited on platinum-coated silicon (Pt/Si) substrate by Pulsed Laser Deposition (PLD) method. On the top of Ta2O5 thin film, Cu electrodes of radius ~100 µm and thickness ~ 100nm were grown by RF magnetron sputtering using shadow masking. The RS behaviour of Cu/Ta2O5/Pt devices was studied by current–voltage (I-V) measurements at room temperature. The as-fabricated Cu/Ta2O5/Pt devices showed repeatable and reliable, non-volatile bipolar resistance switching for 100 cycles, indicating good endurance. Due to virgin low resistance state of the device, the initial electroforming step was not required for bipolar RS. The mean resistance of high resistance state (HRS) and low resistance state (LRS) was ~300 MΩ and 500 Ω respectively with very high resistance ratio of ~106. The Cu/Ta2O5/Pt devices showed good data retention up to 103 s. The resistive switching mechanism in Cu/Ta2O5/Pt devices was understood in terms of redox reaction based formation and rupturing of conducting filaments constituting copper ions. The conduction mechanism in LRS was explained on the basis of Ohmic conduction, whereas Schottky emission and space charge limited conduction (SCLC) were found as possible conduction mechanisms in HRS. Our studies clearly show that Ta2O5-based resistive switching devices may have applications in futuristic universal non-volatile memory technology.

The interface at NiO and GaAs heterojunction is found to have claudetite phase of As2O3 with monoclinic structure, which increases with oxygen (O2) partial pressure. It is determined that rough interface, caused by recrystallization of GaAs surface along [111] direction, is not the main governing factor for the observed change in growth direction for NiO epitaxial layer from [111] to [001] direction with increase in O2 partial pressure. The out-of-plane and in-plane epitaxial relationships of [111] oriented NiO layer with respect to substrate are [111]NiO || [001]GaAs, [-1-12]NiO || [-1-10]GaAs and [-110]NiO || [-110]GaAs. The interfacial structure of NiO/GaAs heterojunction thus determined can have implications into the characteristics of optoelectronic devices based on this heterojunction.

For small samples, the modification of the X-ray reflectivity (XRR) profile by the geometric factors due to the profile and size of the beam and the size of the sample is significant. These geometric factors extend the spill-over angle, which is often greater than the critical angle for small samples. To separate the geometric factors, it is necessary to know the spill-over angle. Since the geometric factors are smoothly varying functions and extend beyond the critical angle, it is impossible to determine the spill-over angle from the XRR profile of small samples. It is shown that the spill-over angle can be determined by comparing the normal XRR profile of a small sample with the XRR profile taken with a surface-contact knife edge on the same sample. Thus, a procedure has been developed for data reduction for small samples and validated with suitable experiments. Unlike the methods used hitherto, which have drawbacks, this is a self-consistent method for data reduction.

We have observed weak localization induced by disorder in thin films of Gd doped ZnO (Gd:ZnO) deposited by pulsed laser deposition (PLD). Disorder parameter (Kfl) of all the Gd:ZnO thin films was found to be greater than 1, which indicates that thin films of Gd:ZnO in this study are in metallic side of metal-insulator transition. However, temperature dependent resistivity studies of all the Gd:ZnO thin films exhibits negative temperature coefficient of resistance (TCR) showing semiconductor like nature at low temperatures. This anomaly in temperature dependent resistivity behaviour has been attributed to the dominant presence of weak localization. Magnetoresistance measurements confirms the dominant influence of weak localization in the conducting mechanism of Gd:ZnO films. Phase coherent length was found to be varied with the concentration of Gd and reports the highest value at ~360 nm for Gd:ZnO thin film with 1 at.% of Gd concentration. .

Lattice parameter of NiO layers deposited using pulsed laser deposition is determined accurately using three parallel Bragg reflections. A small decrease in out of plane lattice parameter is found as O 2 partial pressure is increased, which is attributed to the lattice contraction due to Ni vacancies. The NiO layers are found to be oriented along (111) direction on (0001) oriented Al 2 O 3 substrate. The presence of Pendellosung oscillations indicates high crystalline and interfacial qualities of NiO layers, which gradually degrade with increase in O 2 partial pressure. The NiO layer thickness determined from both x-ray diffraction and x-ray reflectivity show similar trend with O 2 partial pressure. The optical band gap does not vary much due to the presence of very small compressive strain.

Weak localization arising due to disorder and leading to phase coherent electron transport was clearly observed in sparsely doped (Zn:Ga)O thin films grown by pulsed laser deposition. All the Ga doped ZnO ((Zn:Ga)O) thin films have shown metal like behavior with a change in sign of temperature coefficient of resistivity (TCR) in temperature dependent resistivity measurements. This has been explained by considering quantum corrections to conductivity where weak localization contribute significantly which was confirmed by temperature dependent magneto-transport measurements at varying magnetic fields. Phase coherent length as extracted from magneto-transport measurement was found to vary with Ga concentration in the ZnO film. Maximum phase coherent length (at 5 K) was found to be ∼337 nm at Ga concentration ∼ 0.75 at %. By reducing film thickness much below the phase coherent length in (Zn:Ga)O (Ga ∼0.75 at %) film, a clear signature of dimensional crossover of weak localization resulting in phase coherent electron transport along film thickness was observed. Temperature dependence of phase coherent length has suggested electron-electron scattering as the major phase breaking mechanism in the (Zn:Ga)O thin films. Observation of varying phase coherent length with Ga concentration in ZnO film and dimensional crossover of weak localization in ultra thin films may be useful in futuristic phase coherent transport applications based on ZnO.

We report quantised conductance in Au/Ta2O5/Pt structures grown by pulsed laser deposition. Quantum conduction phenomenon was observed during reset switching event of resistive switching in tantalum oxide (Ta2O5) thin films. Nonvolatile and repeatable electric field controlled unipolar resistive switching was clearly seen in Au/Ta2O5/Pt structures with good endurance. The quantum conduction phenomenon has been attributed to formation of nanometer scale conducting filaments which constraints the motion of electron in transverse direction resulting in quantisation of device conductance in half integer multiple of fundamental unit of conductance G0 (2e2/h). These quantised conductance results in well separated resistance states, suitable for realising futuristic multi-bit resistance switching memory.

X-ray multilayer mirrors of period ranging from 9.6 to 1.7 nm, deposited using ion beam sputtering, have been examined using grazing incidence X-ray reflectivity (GIXRR) and grazing incidence X-ray diffraction. Detailed analysis of GIXRR data revealed that significant amount of re-sputtering of Si layer takes place while W deposition is underway. Re-sputtering is mainly due to bombardment of high-energy neutrals getting reflected from the W target. Due to re-sputtering interface of the multilayer becomes asymmetric. This puts a major hindrance in avoiding the intermixing and achieving sharp interfaces at shorter periods. Maximum thickness of Si which gets lost due to re-sputtering during deposition is ∼0.8 nm. The shortest period multilayer estimated, that could be deposited without intermixing, was 2.7 nm. These results are of significance for developing low period W/Si multilayers.

Electric field controlled forming free and unipolar resistive memory switching was observed in Au/TiO2/Pt devices containing ultrathin TiO2 films of thickness ~ 4 nm grown by atomic layer deposition. These devices showed a large resistance ratio of ~ 103 between high and low resistance states along with appreciable time retention for ~ 104 seconds and endurance. The spread of reset and set voltages was from ~ 0.4-0.6 V and 1.1-1.5 V respectively with a clear window between them. The resistive switching mechanism was explained based on conductive filamentary model.

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.

The valence band offset value of 2.3 ± 0.2 eV at epitaxial NiO/Al2O3 heterojunction is determined from photoelectron spectroscopy experiments. Pulsed laser deposited thin film of NiO on Al2O3 substrate is epitaxially grown along [111] direction with two domain structures, which are in-plane rotated by 60° with respect to each other. Observation of Pendellosung oscillations around Bragg peak confirms high interfacial and crystalline quality of NiO layer deposited on Al2O3 substrate. Surface related feature in Ni 2p3/2 core level spectra along with oxygen K-edge soft X-ray absorption spectroscopy results indicates that the initial growth of NiO on Al2O3 substrate is in the form of islands, which merge to form NiO layer for the larger coverage. The value of conduction band offset is also evaluated from the measured values of band gaps of NiO and Al2O3 layers. A type-I band alignment at NiO and Al2O3 heterojunction is also obtained. The determined values of band offsets can be useful in heterojunction based l...

The influence of buffer layer on interface roughness has been investigated. With Si buffer layer, NbC/Si multilayer mirrors show 28% increase of reflectivity at 1st Bragg peak for 1.54A0. Reduction in physical and chemical roughness in the multilayer with buffer layer was calculated by fitting both specular and diffused scattering data.

Effect of oxygen ion beam bombardment on the hydrogen impurity distribution (as a function of depth) in alumina thin films is discussed. It is shown that the bombardment of oxygen ion beam during electron beam evaporation significantly decreases the hydrogen content in the films. The observed peak of hydrogen distribution at the silica alumina interface is explained on the basis of morphological changes happening to the silica alumina interface due to the oxygen ion beam bombardment. It is also shown that stoichiometry of these films, irrespective of the oxygen ion beam energy, remains same as that of sapphire crystal.

The optical constants of sapphire crystal (α-Al(2)O(3)) and amorphous Al(2)O(3) in the soft x-ray region (67-85 eV) around the aluminum LsubII,III/subabsorption edge (73.1 eV) are determined by angle-dependent x-ray reflectivity. The differences between the optical constant values of both the samples are discussed. The fine structures obtained in the absorption of crystalline sapphire are explained. An absorption feature at 70.2 eV is observed for the first time for crystalline alumina. Both datasets are compared to the tabulated values of Henke et al. [At. Data Nucl. Data Tables 54, 181 (1993)], Weaver et al. [Physik Daten, Physics Data: Optical Properties of Metals (Fach-information zentrum, 1981), Vols. 18-1 and 18-2], and [Handbook of Optical Constants of Solids II (Academic, 1991)].

Uranium is reported to be present in phosphate fertilizers. The recovery of uranium from the fertilizers is important because it can be used as fuel in nuclear reactors and also because of environmental concerns. For both these activities suitable method of uranium determinations at trace levels in these fertilizers are required. Studies have been initiated for such TXRF determination of uranium and the results are reported in the present paper. For TXRF determinations the fertilizer samples were processed with nitric acid and the uranium present in it was removed by solvent extraction using tri-n-butyl phosphate as the extractant. The organic phase containing uranium was equilibrated with 1.5% suprapure nitric acid to bring out uranium in aqueous phase. This aqueous phase was mixed with internal standard Y and the TXRF spectra were measured by depositing samples on float glass supports. The amounts of uranium in four fertilizer samples of Hungarian origin were determined by processing these TXRF spectra. Uranium concentrations in two fertilizer samples were found to be in the range of 4–6 μg/g, whereas two fertilizer samples did not show the presence of uranium. The precision of the TXRF determination of uranium was found to be better than 8% (1σ).