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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.

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.

Anisotropic distribution of microstructure in GaP/GaAs epitaxial layers is investigated by performing high resolution x-ray diffraction (HRXRD) experiments for symmetric reflections at different azimuths. The observed anisotropy leads to the different values of lattice relaxation along the [011¯] and [01¯1¯] directions as revealed by the reciprocal space maps recorded at 0° and 90° azimuths for (42¯2¯) and (422¯) reflections. The anisotropic relaxation process causes large differences in the full width at half maximum of (400) diffraction peaks of omega scans along the [011¯] and [01¯1¯] directions. It therefore provides large differences in the values of microstructural parameters of the GaP epitaxial layer along the two in-plane orthogonal directions. A systematic Williamson-Hall analysis of HRXRD data confirms the presence of large anisotropy of lattice relaxation process along the [011¯] and [01¯1¯] directions. Due to the large lattice mismatch, the GaP/GaAs epilayer presented here possesses grain bou...

A detail cross-sectional transmission electron microscopy of as-grown α-Fe2O3 nanowire sample, synthesized on iron substrate by thermal annealing method, was carried out to understand the mechanism of growth in this system. Iron undergoes sequential oxidation to form a layered structure of Fe/FeO/Fe3O4/α-Fe2O3. α-Fe2O3 nanowires grow on to the top of α-Fe2O3 layer. It was found that subsequent oxide layers grow topotaxially on the grains of iron, which results in a direct orientation relationship between the α-Fe2O3 nanowire and the parent grain of iron. The results also showed that the grains of α-Fe2O3 layer, which were uniquely oriented in [110] direction, undergo highly anisotropic growth to form the nanowire. This anisotropic growth occurs at a twin interface, given by (−11−1), in the α-Fe2O3 layer. It was concluded that the growth at twin interface could be the main driving factor for such anisotropic growth. These observations are not only helpful in understanding the growth mechanism of α-Fe2O3 nanowires, but it also demonstrates a way of patterning the nanowires by controlling the texture of iron substrate.

Microstructure and composition analysis of periodic multilayer structure consisting of a low electron density contrast (EDC) material combination by grazing incidence hard X-ray reflectivity (GIXR), resonant soft X-ray reflectivity (RSXR), and transmission electron microscopy (TEM) are presented. Measurements of reflectivity at different energies allow combining the sensitivity of GIXR data to microstructural parameters like layer thicknesses and interfacing roughness, with the layer composition sensitivity of RSXR. These aspects are shown with an example of 10-period C/B4C multilayer. TEM observation reveals that interfaces C on B4C and B4C on C are symmetric. Although GIXR provides limited structural information when EDC between layers is low, measurements using a scattering technique like GIXR with a microscopic technique like TEM improve the microstructural information of low EDC combination. The optical constants of buried layers have been derived by RSXR. The derived optical constants from the measured RSXR data suggested the presence of excess carbon into the boron carbide layer.

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 in-situ growth of CuO nanowires was studied by Energy Dispersive X-ray Diffraction (EDXRD) to observe the mechanism of growth. The study was carried out for comparison at two temperatures—at 500 °C, the optimum temperature of the nanowires growth, and at 300 °C just below the temperature range of the growth. The in situ observation revealed the successive oxidation of Cu foil to Cu2O layer and finally to CuO layer. Further analysis showed the presence of a compressive stress in CuO layer due to interface at CuO and Cu2O layers. The compressive stress was found to increase with the growth of the nanowires at 500 °C while it relaxed with the growth of CuO layer at 300 °C. The present results do not support the existing model of stress relaxation induced growth of nanowires. Based on the detailed Transmission Electron Microscope, Scanning Electron Microscope, and EDXRD results, a microstructure based growth model has been suggested.

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 Mossbauer 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 superlatti...

β-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 β-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 β-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.

β-Ga2O3 nanostructures (nanowires, nanoribbons, and nanosheets) were synthesized via vapor transport method on gold coated silicon substrate in N2 ambient and these β-Ga2O3 nanostructures grown on silicon substrates were taken as the starting material to study the effect of annealing in the different environments (oxygen, water vapour, and ammonia solution) on the structural front and photoluminescence (PL) properties. The PL spectra of β-Ga2O3 nanostructures exhibit a UV-blue emission band whose intensity is strongly affected by the annealing in different environments. Annealing modifies the surface of the nanostructures by creating surface states which quench the PL by creating competitive nonradiative paths. This study also indicates the dominance of the formation of water induced surface states over ammonia induced surface states. The irreversible nature of these defects significantly affects the applicability of this system in moist high temperature environments.

InAsxP1−x/InP quantum wells (QWs) with excellent crystalline and interfacial quality are grown by metal organic vapor phase epitaxy as confirmed from the cross-sectional transmission electron microscopy, high resolution x-ray diffraction and photoluminescence measurements. The electron confinement in InAsxP1−x/InP QW states is determined by capacitance voltage measurements, where we find that the electron accumulation increases with increasing QWs thickness and arsenic composition. This is explained by the variation of the band offset and hence the effective change in the position of the electronic energy level from Fermi level with QWs composition and thickness. The conduction band offset (ΔEc) for InAsxP1−x/InP QWs has been obtained by solving the self consistent set of Schrodinger and Poisson equations and fitting the theoretical carrier density profile with the apparent carrier density measured from experiments. The ΔEc values in strained InAsxP1−x/InP QWs have been obtained which fits to the expressi...

Quantum states in InP/GaAs type-II ultrathin quantum wells (QWs) are investigated through temperature dependent capacitance–voltage (C–V) measurements. We observe a well-defined peak in the apparent carrier density (ACD) profile for the ultrathin QWs at low temperatures in the vicinity of QWs. ACD peak value is found to decrease with the reduction in QW thickness, indicating quantum confinement effect. Decrease in the ACD peak value and increase in its width with increasing temperature confirms that the observed peak in the ACD profiles is related to the two dimensional electrons occupying the quantum states formed in the ultrathin QWs. We do not observe appreciable peak shift in ACD profiles with temperature, which is attributed to the less temperature dependence of the Debye length because of the high doping density used in the barrier region of InP/GaAs ultrathin QWs. We determine a strained value of 180 ± 30 meV for the conduction band discontinuity by simulating the C–V profile through the self-consi...