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Room-temperature synchrotron X-ray diffraction and subsequent detailed line profile analysis of Fe powder were performed for microstructural characterization. The peak shapes of the diffraction pattern of Fe were found to be super-Lorentzian in nature and the peak widths were anisotropically broadened. These peak profile features of the diffraction pattern are related to the microstructural parameters of the material. In order to elucidate these features of the diffraction pattern, detailed line (peak) profile analyses were performed using the Rietveld method, modified Williamson–Hall plots and whole powder pattern modelling (WPPM), and related microstructural parameters were determined. Profile fitting using the Rietveld and WPPM methods with a single microstructural (unimodal) model shows systematic deviation from the experimentally observed diffraction pattern. On the basis of Rietveld analysis and microstructural modelling it is revealed that the microstructure of Fe consists of two components (bimodal profile). The microstructural parameters of crystallite/domain size distribution, dislocation density, nature of dislocations and phase fraction were evaluated for both components. The results obtained using different methods are compared, and it is shown that diffraction peak profile analysis is capable of modelling such inhomogeneous bimodal microstructures.

International audience; For phase change materials (PCMs) to become a viable universal memory candidate and obsolete von-Neumann architecture, materials with very low crystallization speed are needed. Moreover, introducing pre-structural ordering inside the material is also being touted as one of the promising techniques to reach the speed of SRAM or further down. In this aspect, GaSb alloys are showing much promise for not only having very low crystallization times by themselves and also showing an enormous drop in the programming time while crystallizing the re-amorphized material. Here we demonstrate how the threshold switching behaviour changes for the fully amorphous film in contrast with the disordered film with nucleation sites using conductive-Atomic Force Microscopy. It is found that the required power and programming current for memory switching with nominally stoichiometric GaSb (44:56) are 23 nW and 6.2 nA, respectively. As expected for a nucleation oriented PCM, the deposited thin film has two voltage thresholds during the local programming process, one for conduction and another for memory switching. However, when the nucleation sites are introduced inside the disordered film, the conduction and memory switching become simultaneous. This is also found to be reducing threshold power and SET current by 93% (1.5 nW) and 91% (550 pA), respectively. Furthermore, we also reveal the origin behind this behavioural change observed between these two thin films by using high-resolution transmission electron microscopy and synchrotron radiation photoelectron spectroscopy.

In this paper, experimental study for shape control of silicon mirror is done by nonlinear piezoceramic actuator. A piezoelectric unimorph-based silicon mirror (PUSiM) is fabricated using APC 850 piezoceramic for shape control analysis. The piezo response function of all the actuators and profile of PUSiM are measured by confocal sensor at various applied electric fields to verify the nonlinearity of piezoceramic actuator. For comparison, electromechanical coupled finite element model of PUSiM is developed. Recently proposed iterative piezo response function-based optimization is used to calculate the electric field of piezoceramic actuators to achieve the target elliptical profile of PUSiM. At optimum voltage the achieved profile of PUSiM is quite close to the target elliptical profile with 193 nm root-mean-square error. Knowing the nonlinearity inherent in the piezoceramic actuators at high voltages the proposed measurement technique is a way forward for the shape control of structures and deformable mirrors.

Crystallographic and electronic structures of phase pure ternary solid solutions of Ni1−xCoxO (x = 0 to 1) have been studied using XRD, EXAFS and XAS measurements. The lattice parameter of the cubic rock-salt (RS) Ni1−xCoxO solid solutions increases linearly with increasing Co content and follows Vegard's law, in the complete composition range. A linear increase in the bond lengths (Ni/Co–O, Ni–Ni and Ni–Co) with “x”, closely following the bond lengths determined from virtual crystal approximation (VCA), is observed, which implies that there is only a minimal local distortion of the lattice in the mixed crystal. The optical gap of the ternary solid solution determined from diffuse reflectivity measurements shows neither a linear variation with Co composition nor bowing, as observed in many ternary semiconductors. This trend in the variation of optical gaps is explained by probing the conduction band using XAS at the O K-edge. We have observed that the variation in the onset energy of the conduction band edge with “x” is very similar to the variation in the optical gap with “x”, thus clearly indicating the dominant role played by the conduction band position in determining the optical gap. The variation in the intensities of the pre-edge peak in the XANES spectra measured at Ni and Co K-edges, and the L1/2 peak in XAS spectra measured at Ni and Co L-edges, is found to depend on the unoccupied O 2p-metal-(Ni/Co) 3d hybridized states and the bond lengths.

Influence of thermal annealing on the ion beam sputtered FeCo(Nb)B quaternary alloy thin films having thickness ∼20 nm and 80 nm was investigated. Regardless of amorphous microstructure, the as-deposited films possess uniaxial magnetic anisotropy (UMA) in the film plane. Annealing at temperature ∼503 K results in amorphous to nanocrystalline phase transformation with precipitation of bcc-FeCo phase. With increasing annealing temperature, magnetic anisotropy gradually decreases and thermal annealing at 773 K results in complete disappearance of UMA. In-plane strain distribution in such nanocrystalline FeCo(Nb)B thin films was obtained by using laboratory and synchrotron X-ray diffraction (XRD) measurements. In-plane and out-of-plane X-ray diffraction measurements suggest, growth induced long range non-uniform tensile stress, as the possible origin of in-plane uniaxial magnetic anisotropy in such films. It is demonstrated that the stress distribution becomes uniform after annealing at 773 K and results in complete disappearance of magnetic anisotropy. The study further suggests that the presence of Nb in FeCoB film improves the thermal stability, inhibits grain growth and stabilizes bcc-FeCo phase up to elevated temperatures. This information is useful for magnetic tunnel junction application, where annealing of FeCoB magnetic electrode is essential.

W/B4C multilayers (MLs) consisting of ten layer pairs with varying boron carbide layer thicknesses have been investigated. The ML structures were characterized using grazing-incidence hard X-ray reflectivity (GIXR), resonant soft X-ray reflectivity (RSXR), hard X-ray photoelectron spectroscopy (HAXPES) and X-ray absorption near-edge spectroscopy (XANES). Depth-resolved spectroscopic information on the boron carbide layer in W/B4C MLs was extracted with sub-nanometre resolution using reflectivity performed in the vicinity of the B K-edge. Interestingly, these results show that the composition of boron carbide films is strongly dependent on layer thicknesses. HAXPES measurements suggest that most of the boron is in the chemical state of B4C in the multilayer structures. XANES measurements suggest an increase in boron content and C—B—C bonding with increase in boron carbide layer thickness.

In this work, polycrystalline NiCrGa half Heusler alloy, which is predicted to be half-metallic ferromagnet from first principles calculations, has been synthesized by arc meting technique and its structural, magnetic as well as the electronic properties have been studied. The measured x-ray diffraction (XRD) pattern shows the signature of a disordered structure. From the magnetization measurements, there is no evidence of ferromagnetic ordering observed in this system down to the lowest temperature studied. Instead, the system shows the signature of an antiferromagnetic ordering at very low temperature. The experimentally observed structural and magnetic properties are found to be significantly different from the theoretically predicted properties of the ordered cubic C1b structure. To probe the possible disorder present in the system and its effect on the magnetic properties, we have carried out first principles calculations using the spin-polarized-relativistic Korringa-Kohn-Rostoker method (SPR-KKR). Using a combination of XRD, photoelectron spectroscopy, magnetization measurements and first principles calculations, we conclude that NiCrGa has significant amount of atomic disorder. Although, the ordered structure is energetically more stable than the disordered structures, we find that after synthesis, the system tends to stabilize in a disordered structure. With this atomic disorder present in the sample, the ferromagnetic ordering is disturbed and the calculated spin polarization is consequently reduced., Comment: 18 pages, 5 figures, 3 tables

Shape control of complex structures by optimizing the electrode potential is not achievable directly by analytical solutions and piezo response function base-optimization techniques due to the nonlinear response of piezoactuators. In the present work, a metaheuristic iterative piezo response function (iPRF)-based optimization technique is developed to achieve the arbitrary shape of piezoelectric unimorph (PU) using nonlinear piezoactuators. In this regard, a PU is fabricated using APC 850 piezoactuator to verify the nonlinear response in bending mode and nonlinear analytical model of PU. After verification, length of the inactive layer and number of piezoactuators in PU are modified to study the shape control. iPRF-based technique is used for the optimization of electric potential to achieve the target shape of modified piezoelectric unimorph (MPU) with various piezoactuators. The results of iPRF-based technique are compared with the results of simulated annealing (SA)-based direct optimization technique. Unlike SA-based direct optimization technique, prior knowledge of nonlinear coefficients of piezoactuator is not required in iPRF-based technique. Optimum values obtained from both the direct nonlinear solution- and iPRF-based optimization methods are same for all MPUs. Furthermore, the number of iterations of iPRF-based optimization approach is not affected by the number of piezoactuators used to achieve the desired shape.

Squeeze film damping is a dominant mechanism to examine the losses in the micro-opto-electro-mechanical systems. It plays an important role in understanding the dynamic performance of vertical-comb drive (VCD) microactuator. In the present paper, finite element method is used to estimate the Q factor and damping ratio at varying ambient pressures ranging from atmospheric (105 Pa) to vacuum (0.1 Pa) in COMSOL Multiphysics. The effect of ambient pressure on dynamic performance of the VCD is discussed in detail. Based on the applications, two different VCDs (VCD-L and VCD-SR) with extended micromirrors have been proposed in the current study for the manipulation of light. VCD-L having lower eigen frequency of 670 Hz can be used in laser and biomedical applications. VCD-SR with higher eigen frequency nearly 90 kHz has the potential use in a synchrotron radiation source. In addition, a parametric study is done to understand the performance of VCD for varying process parameters such as variable offset length among the comb fingers, spring dimensions, and orientation of single crystal silicon. For spring design, the von-Mises stress theory is preferred over maximum shear stress theory.

Crystallographic and electronic structures of phase pure ternary solid solutions of Ni

Growth of GaN epitaxial layers is optimized in an indigenously developed nitride metal organic vapour phase epitaxy (MOVPE) system. The motivation of present work is to develop a simple MOVPE reactor which is though affordable but can deliver GaN epilayers of desired quality. The design and fabrication methodology of nitride MOVPE system is briefly discussed. MOVPE growth of GaN epilayers is carried out through a two-step growth process where influence of the growth parameters of low temperature buffer layer on the crystalline properties of high temperature GaN epilayer is discussed in detail. Optimum values of the growth temperature, annealing duration and thickness of GaN buffer layer are achieved. High resolution X-ray diffraction measurements (HRXRD) confirmed that the crystalline quality of GaN epitaxial layers is reasonable. A good surface morphology of the optimized sample is strongly corroborated by the results obtained from photoluminescence (PL) and HRXRD techniques. Usefulness of a systematic Williamson-Hall analysis in the optimization of growth of GaN epitaxial layers is demonstrated where a high value of lateral coherence length of GaN buffer layer is found to be a key parameter. Such an optimization process leads to the good crystalline quality of GaN epitaxial layers with low dislocation density. It is found that the high temperature GaN epilayers grown on the optimized GaN buffer layer are compressively stressed which is also revealed by the PL measurements.

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.

The epitaxial NiO layers deposited with higher fluence values are found to be strained, and the strain increases with the fluence values. The X-ray diffraction (XRD) profile taken from the synchrotron beam shows the presence of relaxed grains of NiO in addition to the strained grains, where the fraction of relaxed grains gradually increases with the fluence values. The presence of Pendellosung fringes in the XRD profile for the layers deposited at lower fluence values confirms good interfacial and crystalline qualities. As the fluence value is increased, the Pendellosung fringes start merging indicating relatively poor interfacial and crystalline qualities. The NiO layers are of epitaxial nature and grown along [111] direction with two domain structures that are in-plane rotated by 60° with respect to each other. The analysis of local structures from extended X-ray absorption fine structure measurements also indicates that the NiO lattice is strained at higher fluence values. The Ni–O bond distance does not change with the fluence values; however, Ni–Ni bond distance increases with the fluence values in corroboration with XRD results. The surface topography shows island growth of NiO at lower fluence values giving larger roughness, and these islands start merging with an increase in the fluence values leading to relatively smoother layers.

We report the structural and optical properties of ternary α-(GaxCr(1-x))2O3 with 0 ≤ x ≤ 0.45 which is synthesized by solid state reaction method. Single phase with space group R-3c is obtained for the gallium composition range of 0 ≤ x ≤ 0.45 and further increase in Ga concentration results in appearance of secondary C2/m phase corresponds to Ga2O3. Variations in the in-plane ’a’ and out of plane ’c’ lattice parameters shows a clear daviation from the Vegard's law with the bowing parameters of −0.0510 A and 0.0305 A respectively. Extended X-ray absorption fine structure (EXAFS) spectroscopy shows larger values of Ga-O bond lengths, which elucidate the deviation in lattice parameters from the Vegard's law and immisibility of Ga in Cr2O3 lattice. Diffuse reflectance spectroscopy (DRS) confirms a blue shift of 0.5 eV in the band gap of α-(GaxCr(1-x))2O3. It has also observed that Cr 3d level shifted towards the O 2p level in the valence band which has been reconfirmed by photo electron spectroscopy. The observed shift indicates enhancement of mixing between these levels which can leads to further delocalization of hole states in the valence band responsible for p-type conduction in the α-Cr2O3. These results suggest that ternary α-(GaxCr(1-x))2O3 (0 ≤ x ≤ 0.45) can be useful in the field of UV transperant electronics and photodetectors.

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.

Oriented thin films of β-(Ga1−x Fe x )2O3 were deposited by radio frequency magnetron sputtering on c-Al2O3 and GaN substrates. The itinerant character of the Fe 3d states forming the top of the valence band (VB) of the Fe-substituted β-Ga2O3 thin films has been determined from resonant photoelectron spectroscopy. Further, the admixture of the itinerant and localized characters of these Fe 3d states has been obtained for larger binding energies; i.e. deeper in the VB. The bottom of the conduction band (CB) for β-(Ga1−x Fe x )2O3 has been also found to have strongly hybridized states involving Fe 3d and O 2p states compared to that of Ga 4s in pristine β-Ga2O3. This suggests that β-Ga2O3 transforms from a band-like system to a charge-transfer system with Fe substitution. Furthermore, the bandgap red shifts with Fe composition, which has been found to be primarily related to the shift of the CB edge.

We have investigated the non-commutativity of the band offset in RF magnetron sputter deposited all oxide epitaxial α-Cr2O3/β-Ga2O3 heterojunction (HJ). The core-level x-ray photoelectron spectroscopy technique has been employed to probe the electronic structure of the interface formed between α-Cr2O3 and β-Ga2O3. Valence and conduction band offsets of 2.6 ± 0.2 and 0.9 ± 0.2 eV, respectively, for α-Cr2O3/β-Ga2O3 HJ have been determined from Kraut's method. These values are different from those reported for β-Ga2O3/α-Cr2O3 HJ, thus indicating that the α-Cr2O3/β-Ga2O3 HJ does not follow the band commutativity with respect to the growth sequence of the constituting layers forming the HJ. Furthermore, the band alignment at α-Cr2O3/β-Ga2O3 HJ is still type-II like β-Ga2O3/α-Cr2O3 HJ but with lower band offset values. Therefore, this HJ would also be able to confine the electrons and holes in β-Ga2O3 and α-Cr2O3 layers, respectively, with lower turn on voltage.

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.

Bond length behavior for Zn substituted NiO is determined through extended x-ray absorption fine structure (EXAFS) measurements performed at ambient conditions. We report bond length value of 2.11±0.01 A for Zn-O of rock salt (RS) symmetry, when Zn is doped in RS NiO. Bond length for Zn substituted NiO RS ternary solid solutions shows relaxed behavior for Zn–O bond, while it shows un-relaxed behavior for Ni–O bond. These observations are further supported by first-principles calculations. It is also inferred that Zn sublattice remains nearly unchanged with increase in lattice parameter. On the other hand, Ni sublattice dilates for Zn compositions up to 20% to accommodate increase in the lattice parameter. However, for Zn compositions more than 20%, it does not further dilate. It has been attributed to the large disorder that is incorporated in the system at and beyond 20% of Zn incorporation in the cubic RS lattice of ternary solid solutions. For these large percentages of Zn incorporation, the Ni and the Zn atoms re-arrange themselves microscopically about the same nominal bond length rather than systematically increase in magnitude to minimize the energy of the system. This results in an increase in the Debye-Waller factor with increase in the Zn concentration rather than a systematic increase in the bond lengths.

We report the compositional analysis of boron carbide thin films grown on different substrates Si, fused silica and sapphire using ion beam sputtering. The compositions of grown films are estimated quantitatively in non-destructive manner using resonant soft x-ray reflectivity measurements. Interestingly, our results show that the composition of deposited thin films is closely dependent on substrate used for deposition. We could not observe any compositional difference between the films grown on Si and fused silica. Boron carbide thin films grown on sapphire are boron deficient as compared to films grown on Si and fused silica.

Intentional doping and unintentional impurities in intrinsic semiconductors generate carriers that enable device operations. Fundamental to the electronic activity of dopants and impurities is the introduction of defect states inside the forbidden energy gap of semiconductors having shallow and/or deep characteristics, which fundamentally define the ability to engineer its physical properties and associated device performance. Here we demonstrate that unintentional electron doping by oxygen (${\mathrm{O}}_{\mathrm{N}}$) impurities and intentional magnesium hole doping ($\mathrm{M}{\mathrm{g}}_{\mathrm{Sc}}$) in scandium nitride (ScN) do not introduce any defect states inside its fundamental bandgap and that the rigid-band electronic structure remains unchanged. Employing a combination of spectroscopic techniques as well as first-principles density functional theory analysis, we show that the ${\mathrm{O}}_{\mathrm{N}}$ and $\mathrm{M}{\mathrm{g}}_{\mathrm{Sc}}$ defect states in ScN are located inside the bands, which leaves behind the virgin ScN bandgap as well as the valence and conduction band edges that are important for electronic transport. The rigid-band electronic structure of ScN with respect to the electron and hole doping results in high electron and hole concentrations due to the free movement of Fermi level and results in tunable electronic and thermoelectric properties necessary for device applications.

The prospect of developing thin film diodes (TFDs) and thin film transistors (TFTs) by using indigenously developed aerosol assisted chemical vapor deposition system is reported in this paper. Optical, structural, elemental and current (I) - voltage (V) characterization of fabricated structures manifest distinctly the properties of both TFT and TFT. UV-VIS spectroscopy shows that the average transmittance of single and multilayer thin films range between 80 and 95% implying that highly transparent deposition can be achieved. X-ray photoelectron spectroscopy manifest the deposition of ZnO, Al:ZnO and Al2O3 thin films. X-ray diffraction of the films confirm the polycrystalline type of deposition. I-V characteristics of In/n-ZnO/n+-Al:ZnO/In bi-layer structure suggests the fabrication of a TFD. Linear dependency of barrier height (BH) and exponential decrease of ideality factor of fabricated TFD with increasing temperature are observed. Modified Richardson plot is used to measure the inhomogeneity in BH and Richardson constant of In/n-ZnO/n+-Al:ZnO/In TFD. Transfer and output characteristics of the present TFT structure is found to be in agreement with those for a p-channel TFT system. The threshold voltage, saturation field effect mobility, subthreshold swing and current on/off ratio of the TFT were measured as 2.2 V, 24.3 cm2 V-1 s−1, 0.23 V/decade and 36, respectively.

In the present work, optical, electrical, structural and morphological characterization of polycrystalline thin films of ZnO and Al:ZnO have been reported. Aerosol assisted chemical vapour deposition technique has been adopted for the growth of the films on the glass substrate at 480 °C. Al-fraction in Al:ZnO thin films was determined by using energy dispersive x-ray analysis. In the visible to near-infrared regions, all the films were found to exhibit high average transmittance ≥80%. Blue shift of energy bandgap, from 3.20 to 3.50 eV, has been observed with increasing Al-fraction in the Al:ZnO thin films. For highly doped Al:ZnO thin films, both theoretical and experimentally measured values of carrier concentration (≈ 1020 cm−3) were greater than Mott's critical value (≈ 1019 cm−3) and supports the blue shift of the bandgap. The figures of merit of the films are found to be around 10−2 Ω−1 suggesting the applicability of Al:ZnO thin films as a transparent electrode. The observed variation of carrier effective mass with increasing Al-doping agrees quantitatively well with the theoretical calculations. At high Al-fraction (≥ 12 at.%), the compressive strain generated in ZnO lattice is due to the presence of amorphous Al2O3 or AlOx at the grain boundaries. X-ray diffraction suggests that the films were single phase and polycrystalline in nature. Atomic force microscopy of Al:ZnO films reveals a systematic change in surface morphology with increasing Al-fraction in the films. The results demonstrate the potentiality of producing thin films of transparent conducting oxides with good electrical, optical, structural and morphological properties via a low-cost deposition technique.

Resonant photoemission spectroscopy has been used to investigate the character of Fe 3d states in FeAl alloy. Fe 3d states have two different character, first is of itinerant nature located very close to the Fermi level, and second, is of less itinerant (relatively localized character), located beyond ∼2 eV below the Fermi level. These distinct states are clearly distinguishable in the resonant photoemission data. Comparison between the results obtained from experiments and first principle based electronic structure calculations show that the origin of the itinerant character of the Fe 3d states is due to the ordered B2 structure, whereas the relatively less itinerant (localized) Fe 3d states are from the disorders present in the sample. The exchange splitting of the Fe 3s core level peak confirms the presence of local moment in this system. It is found that the itinerant electrons arise due to the hybridization between Fe 3d and Al 3s and 3p states. Presence of hybridization is observed as a shift in the Al 2p core-level spectra as well as in the X-ray near edge absorption spectra towards lower binding energy. Our photoemission results are thus explained by the co-existence of ordered and disordered phases in the system.

Valence band offset of 2.3 ± 0.4 eV at strained NiO/MgO heterojunction is determined from photoelectron spectroscopy (PES) measurements. The determined value of valence band offset is larger than that is predicted from first principle calculations, but is in corroboration with that obtained from band transitivity rule. Our PES result indicates a larger value of the valence band offset at strained NiO/MgO heterojunction and can be used to predict accurately carrier transport and electroluminescence mechanisms for n-ZnO/MgO/p-NiO and p-NiO/MgO/n-GaN heterojunction light emitting diodes.

Detailed structural and optical properties of α-(AlxCr1−x)2O3 (0 ≤ x ≤ 1) synthesized by the solid state reaction method have been investigated. Single phase α-(AlxCr1−x)2O3 with space group R 3 ¯ c is obtained for the full composition range of 0 ≤ x ≤ 1. Variations in the lattice parameters a and c have been determined. Lattice parameter c follows Vegard’s law, while the lattice parameter a shows a clear deviation with a bowing parameter of −0.035 A. This behavior of the lattice parameters of α-(AlxCr1−x)2O3 with x is explained in detail by studying the local structure. Extended x-ray absorption fine structure spectroscopy shows a reduction in the values of Cr–O bond lengths with composition x. Optical absorption measurements of α-(Al1−xCrx)2O3 for 0 ≤ x ≤ 1 show a large bandgap tunability of 1.9 eV (from 3.4 eV to 5.3 eV). The photoemission spectroscopy data and the analysis of partial density of states obtained from first principles electronic structure calculations suggest that the valence band maxima is mainly composed of Cr 3d levels, which hybridize with the O 2p levels. Increased contribution of O 2p partial density of states is observed with Al substitution, which is expected to enhance p-type carrier conduction in the α-(AlxCr1−x)2O3 system as compared to the parent α-Cr2O3 system. Thus, the large bandgap, its tunability in the UV region, and the predicted enhancement of p-type conductivity in the α-(AlxCr1−x)2O3 system make it a potential candidate for application in UV based photo-detectors and transparent electronics.

Lattice distortion and epitaxial nature of β-Ga2O3 layers deposited on GaN templates using radio-frequency magnetron sputtering have been evaluated. Determined values of out-of-plane (tensile type) and in-plane (compressive type) strain are found to decrease with increase in deposition temperature indicating better relaxation of unit cell lattice at higher temperatures. The obtained value of angle β is invariably higher than its bulk value suggesting a distortion in the unit cell of β-Ga2O3 layer. The columnar type of growth for β-Ga2O3 layer on GaN template has been observed. Analysis of epitaxial nature reveals that six domains of grown layer are in-plane rotated by 60o ± δo (δ=2-3o) with each other, which is explained due to the presence of three non-equivalent oxygen atoms on (-201) plane of β-Ga2O3. The calculated values of δ nearly match with experimentally observed values; however, smaller difference between the two is related to residual strain in the layer. The out-of-plane and in-plane epitaxial relationship for β-Ga2O3 layer with respect to GaN are (-201)Ga2O3 || (0001)GaN, (010)Ga2O3 || (11-20)GaN respectively.

Semiconducting metal oxides are attractive for various applications since most oxides are non-toxic, stable, and easy to deposit. Wide band-gap materials have been studied more extensively, compared to low bandgap materials, which introduces limitations to the applications of oxides. Study of low band-gap semiconducting oxides can propel the usage of oxides in a wider range of applications. Here, the electronic, structural, and optical properties of Zn2Mo3O8 (ZMO) are investigated. Stoichiometric polycrystalline films of ZMO are deposited using pulsed laser deposition system at room temperature. The unintentionally n-doped films show a hall electron mobility of 0.7 cm2 V−1 s−1 and have a bandgap of 2.1 eV. The photoelectron spectra contain complex peak profiles which are explained to be a manifestation of final state effects. The orbital contribution to the valence band of ZMO is probed using resonant photoelectron spectroscopy, which confirms that the valence band is composed of Mo 4d levels. The conduction and valence band edges are predicted to be at 4.2 eV and 6.3 eV, so most of the conventional wide band-gap oxides can be used as hole-blocking layers with ZMO. Under A.M. 1.5 illumination, single-sided Schottky diode with Fluorine-doped tin oxide/TiO2/ZMO/Au structure shows no photovoltaic action, possibly due to high exciton binding energy and low carrier drift lengths. However, the Schottky device shows a higher current under illumination, which suggests that with improvement in carrier drift lengths, ZMO can find applications in low-cost optoelectronic devices on flexible substrates like plastic or Polyethylene terephthalate.

We report on optical and electrical properties for ternary α-(AlxCr1-x)2O3 (0 ≤ x ≤ 1) solid solutions synthesized using solid sate reaction method. Single R-3c phase was obtained for the Aluminum composition of 0 ≤ x ≤ 1. Optical absorption spectra for the solid solutions have showed a blue shift of∼1 eV in the optical gap. Two observations have been made from the optical characterization. First, Blue shift of transition level with the Al substitution, which is related to decrease in the Cr and O bond lengths, has been confirmed by EXAFS measurements. Second, enhancement of mixing in the d and p levels, which is responsible for p-type conduction in wide band gap semiconductors, has been verified by enhancement in the electrical conductivity. These results suggests that ternary α-(AlxCr1-x)2O3 (0 ≤ x ≤ 1) can be useful in the field of UV transparent electronics and UV photodetectors.

W/B

Williamson–Hall (WH) analysis is a well established method for studying the microstructural properties of epilayers grown on foreign substrates. However, the method becomes inapplicable in specific cases where the structure factor considerations and the presence of anti-phase domains forbid the data acquisition for certain reflections in conventional high-resolution X-ray diffraction (HRXRD) measurements. Here, this limitation is overcome by exploiting the large intensity (25 µW mm−2) and high photon energy (15.5 keV) of the X-ray beam obtained from a synchrotron radiation source. The lateral coherence length, vertical coherence length, tilt and micro-strain of GaAs epilayers grown on Si substrate have been successfully measured using the conventional WH analysis. The microstructure information obtained from the conventional WH analysis based on the data acquired at the synchrotron radiation source is in reasonable agreement with the results obtained from atomic force microscope and surface profiler measurements. Such information cannot be obtained on a laboratory-based HRXRD system where modification of the WH method by involving a set of parallel asymmetric crystallographic planes is found to be essential. However, the information obtained from the modified WH method is along a different crystallographic orientation.

The electronic structure of a ferromagnetic Heusler alloy Co 2 MnSn has been investigated using resonant photoemission spectroscopy to probe the Mn 3 d and Co 3 d states in the valence band region. The motivation of the present work is to understand the low value of spin polarization reported for this system, which mainly depends on the density of states present at the Fermi level. Valence band of Co 2 MnSn shows a large density of delocalized Co 3 d states present near the Fermi level. The Mn 3 d states are more localized and exist below the Fermi level. First principles calculations using density functional theory with GGA and GGA + U formalisms have been performed to understand and analyze the experimental results. The experimental density of states and magnetic moment show a good matching with the calculation carried out with on-site Coulomb correlation U of ∼3 eV on the Mn 3 d states. The presence of U on the Mn 3 d states also reduces the spin polarization at the Fermi level, which is one of the reasons for the observed low value of spin polarization reported for this system.

We have studied the electronic structure of Mo rich Mo$_{1-x}$Re$_x$ alloys (0$\leq$ x $\leq$0.4) using valence band photoemission spectroscopy in the photon energy range 23-70 eV and density of states calculations. Comparison of the photoemission spectra with the density of states calculations suggests that with respect to the Fermi level $E_F$, the $d$ states lie mostly in the range 0 to -6 eV binding energy whereas $s$ states lie in the range -4 to -10 eV binding energy. We have observed two resonances in the photoemission spectra of each sample, one at about 35 eV photon energy and other at about 45 eV photon energy. Our analysis suggest that the resonance at 35 eV photon energy is related to the Mo $4p$-$5s$ transition and the resonance at 45 eV photon energy is related to the contribution from both the Mo $4p$-$4d$ transition (threshold: 42 eV) and Re $5p$-$5d$ transition (threshold: 46 eV). In the CIS plot, the resonance at 35 eV incident photon energy for binding energy features in the range of $E_F$ (B.E. = 0) to -5 eV becomes progressively less prominent with the increasing Re concentration $x$ and vanishes for $x>$ 0.2. The difference plots obtained by subtracting the valence band photoemission spectrum of Mo from that of Mo$_{1-x}$Re$_x$ alloys, measured at 47 eV photon energy, reveal that the Re $d$ like states appear near $E_F$ when Re is alloyed with Mo. These results indicate that interband $s$-$d$ interaction, which is weak in Mo, increases with the increasing $x$ and influences the nature of superconductivity in the alloys with higher $x$., 15 pages, 7 figures

We report on structural and optical properties for ternary α-(AlxCr1-x)2O3 (0 ≤ x ≤ 1) solid solutions synthesized by using solid sate reaction method. Single R-3c phase was obtained for the Aluminum composition of 0 ≤ x ≤ 1. Due to difference in the ionic radia of Al3+ and Cr3+, in plane lattice parameter showed deviation from the vegard’s law. Optical absorption spectra for the solid solutions showed a blue shift of ∼ 0.5 eV in the optical gap. It has also been observed that Cr 3d level shifted towards the O 2p level in the valance band which indicates the enhancement of hybridization in the d and p levels, which is related to the delocalization of hole states, responsible for p-type conduction in wide band gap semiconductors. The results suggests that ternary α-(AlxCr1-x)2O3 (0 ≤ x ≤ 1) can be useful in the field of UV transparent electronics and UV photodetectors.

The lattice parameter variation for phase pure cubic rocksalt (RS) Ni1-xZnxO ternary solid solutions is observed to be perfectly governed by Vegard's law. X-ray absorption near edge spectroscopy confirms the RS symmetry of Zn atoms in the cubic lattice of Ni1-xZnxO. The optical gap bowing parameter for RS Ni1-xZnxO ternary solid solutions is determined, using diffuse reflectivity, to be -0.93 ± 0.05 eV. The negative value of the bowing indicates a repulsive interaction between the ligand O-2p and the metal Ni-3d orbitals. The determined value of the bowing parameter can be useful in designing UV photodetectors based on Ni1-xZnxO solid solutions.

The synchrotron powder X-ray diffraction (XRD) and subsequent detailed Rietveld analysis of lead magnesium niobate (PMN) samples were performed to study the microstructural properties of polar nanoregions (PNRs) of the R3m phase. PMN samples were synthesized under different sample processing conditions. The line profile broadening analysis of room-temperature synchrotron powder XRD patterns was performed using the multi-phase Rietveld refinement method for isotropic microstructural evaluation of different PMN samples. The two phases of perovskite PMN considered in the Rietveld refinement approach for satisfactorily fitting the XRD patterns are the paraelectric cubic phase (Pm\overline 3m) and the local rhombohedral phase (R3m) which corresponds to the PNRs. It is observed that the contributions of the Gaussian component of size broadening of the polar rhombohedral phase (R3m) and the Lorentzian component of strain broadening of the paraelectric cubic phase (Pm\overline 3m) are apposite for satisfactory Rietveld refinement of the synchrotron XRD data for all PMN samples. The volume-average crystallite size of PNRs (R3m phase) is almost invariant (approximately 12 nm) with increasing processing temperature while their weight percentage increases. The values of the apparent microstrain in the paraelectric cubic phase (Pm\overline 3m) are larger for hot-pressed samples.

Epitaxy of ZnO layers on cubic GaP(1 1 1) 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 (ϕ) scans in high resolution X-ray diffraction measurements are (0 0 0 1)ZnO ‖ (1 1 1)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 (1 1 1) oriented GaP can be a potential buffer layer choice for the integration of ZnO based optoelectronic devices on Si(1 1 1) substrates.

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 C bonds) and formation of carbon dioxide in irradiated samples with increase in electron beam energy and dose. The radial distribution function (RDF) analysis showed a change in the short range order with increase in dose, ascribed to the increase in the breaking of polymer chain. Frequency dependent dielectric measurements revealed that the conductivity was more for un-irradiated PMMA. All the measurements invariably showed that the amount of damage increased with increase in electron energy and dose.

Epitaxial growth of (111) oriented NiO layers on (−201) oriented β-Ga2O3 and vice versa have been carried out to obtain an all oxide p-n heterojunction (HJ) consisting of NiO/β-Ga2O3 and β-Ga2O3/NiO interfaces, respectively. Careful investigations by minimizing the effect of differential charging phenomena during x-ray photoelectron spectroscopy measurements yield a valence band offset (VBO) value of 1.6 ± 0.2 eV for both NiO/β-Ga2O3 and β-Ga2O3/NiO HJs. Thus, the VBO value is practically independent of the growth sequence for p-type NiO/n-type β-Ga2O3 HJs and follows band commutativity. The band diagram shows a staggered (type-II) band alignment and the value of the conduction band offset is found to be small (0.3 ± 0.2 eV). Our results are useful to design optoelectronic devices based on all oxide p-type NiO/n-type β-Ga2O3 HJs.

Epitaxial growth of α-Cr2O3(p-type) on c-Al2O3 and β-Ga2O3 (n-type) on α-Cr2O3(p-type) has been carried out to make an all oxide epitaxial n-type β-Ga2O3/p-type α-Cr2O3 heterojunction using RF sputtering. A valence band offset of 3.38 ± 0.2 eV at the heterojunction is determined using Kraut's method. From the bandgap measurements of α-Cr2O3 and β-Ga2O3, the conduction band offset of 1.68 ± 0.2 eV at the heterojunction is obtained. Thus, the band alignment at this heterojunction is found to be staggered (Type-II), which leads to the confinement of electrons and holes in the β-Ga2O3 layer and α-Cr2O3 layer, respectively. Our results provide a pathway to design all oxide optoelectronic devices based on a p-n heterojunction consisting of n-type β-Ga2O3 and p-type α-Cr2O3.

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.