Tapas Ganguli

Magnetocaloric effect in Heusler alloys Ni50Mn34In16 and Ni50Mn34Sn16

We present results of detailed ac susceptibility, magnetization and specific heat measurements in Heusler alloys Ni50Mn34In16 and Ni50Mn34Sn16. These alloys undergo a paramagnetic to ferromagnetic transition around 305 K, which is followed by a martensitic transition in the temperature regime around 220 K. Inside the martensite phase both the alloys show signatures of field-induced transition from martensite to austenite phase. Both field- and temperature-induced martensite–austenite transitions are relatively sharp in Ni50Mn34In16. We estimate the isothermal magnetic entropy change and adiabatic temperature change across the various phase transitions in these alloys and investigate the possible influence of these transitions on the estimated magnetocaloric effect. The sharp martensitic transition in Ni50Mn34In16 gives rise to a comparatively large inverse magnetocaloric effect across this transition. On the other hand the magnitudes of the conventional magnetocaloric effect associated with the paramagnetic to ferromagnetic transition are quite comparable in these alloys.

Structural evolution and the kinetics of Cu clustering in the amorphous phase of Fe-Cu-Nb-Si-B alloy

An attempt has been made to investigate the evolution of the structure of the amorphous phase of Fe73.9 Cu0.9 Nb3.1 Si13.2 B8.9 (finemet) alloy by a combination of wide-angle x-ray scattering, small angle x-ray scattering (SAXS), Mossbauer spectroscopy and X-ray absorption near edge spectroscopy on the supposition that they would provide complementary information. Before the onset of nanocrystallization, the amorphous phase undergoes a structural relaxation resulting in small increase in the hyperfine field and a decrease in the width of the first diffraction maxima. There is an increase in the topological ordering in the system, though chemical inhomogeneity sets-in due to the clustering of Cu atoms in the pure amorphous state of this alloy. Annealing at 400 °C (well below the crystallization temperature) for different time durations results in occurrence of Cu clusters having fcc structure. Kinetics of Cu clustering is studied using SAXS. The incubation time for the clustering at 400 °C is ∼120 min. Wit...

Distorted wurtzite unit cells: Determination of lattice parameters of nonpolar a-plane AlGaN and estimation of solid phase Al content

Unlike c-plane nitrides, “nonpolar” nitrides, e.g., those grown in the a-plane or m-plane orientation encounter anisotropic in-plane strain due to the anisotropy in the lattice and thermal mismatch with the substrate or buffer layer. Such anisotropic strain results in a distortion of the wurtzite unit cell and creates difficulty in accurate determination of lattice parameters and solid phase group-III content (xsolid) in ternary alloys. In this paper we show that the lattice distortion is orthorhombic, and outline a relatively simple procedure for measurement of lattice parameters of nonpolar group III-nitrides epilayers from high resolution x-ray diffraction measurements. We derive an approximate expression for xsolid taking into account the anisotropic strain. We illustrate this using data for a-plane AlGaN, where we measure the lattice parameters and estimate the solid phase Al content, and also show that this method is applicable for m-plane structures as well.8 Unlike c-plane nitrides, “non-polar” nitrides grown in e.g. the a-plane or m-plane orientation encounter anisotropic in-plane strain due to the anisotropy in the lattice and thermal mismatch with the substrate or buffer layer. Such anisotropic strain results in a distortion of the wurtzite unit cell and creates difficulty in accurate determination of lattice parameters and solid phase group-III content (xsolid) in ternary alloys. In this paper we show that the lattice distortion is orthorhombic, and outline a relatively simple procedure for measurement of lattice parameters of non-polar group III-nitrides epilayers from high resolution x-ray diffraction measurements. We derive an approximate expression for xsolid taking into account the anisotropic strain. We illustrate this using data for a-plane AlGaN, where we measure the lattice parameters and estimate the solid phase Al content, and also show that this method is applicable for m-plane structures as well.

The role of hydrostatic stress in determining the bandgap of InN epilayers

This letter establishes a correlation between the internal stress in InN epilayers and their optical properties such as the measured absorption band edge and photoluminescence emission wavelength. By a careful evaluation of the lattice constants of InN epilayers grown on c-plane sapphire substrates under various conditions by metalorganic vapor phase epitaxy, the authors find that the films are under primarily hydrostatic strain. The corresponding stress results in a shift in the band edge to higher energy. The effect is significant and may be responsible for some of the variations in InN bandgap reported in the literature.

Band alignment and interfacial structure of ZnO/Ge heterojunction investigated by photoelectron spectroscopy

Studies on band-offset and band-alignment of heterojunction of highly c-axis oriented ZnO thin films grown on n-Ge (1 1 1) by pulsed laser deposition show a type-II band alignment with the valence band offset (ΔEV) of 3.1 ± 0.2 eV. The valence band spectra of this heterojunction show band onsets corresponding to Ge, interfacial GeOx, and ZnO layers. This observation also enabled us to determine ΔEV of ZnO/GeOx heterojunction to be 1.4 ± 0.2 eV. These studies provide further insight into the band alignment of ZnO/GeOx/Ge system wherein the observed large value of ΔEV of ZnO/Ge can be used for heterojunction based optoelectronic devices.

Photoluminescence study of β-Ga2O3 nanostructures annealed in different environments

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.

Observation of electron confinement in InP/GaAs type-II ultrathin quantum wells

The issue of type-II band alignment for InP/GaAs heterostructure is addressed by means of simple layer architecture of ultrathin quantum wells (QWs). From specific signatures of the radiative recombination in type-II QWs especially the cube root dependence of blueshift in the lowest excitonic transition energy on excitation power in photoluminescence measurements indicates that the observed luminescence is originating from spatially separated electrons and holes. Such a blueshift is seen to increase with the QW thickness again confirming a type-II band alignment. A direct evidence of electron confinement in the conduction band of InP is provided by the capacitance voltage measurements.

Non-intrinsic superconductivity in InN epilayers : Role of Indium Oxide

In recent years there have been reports of anomalous electrical resistivity and the presence of superconductivity in semiconducting InN layers. By a careful correlation of the temperature dependence of resistivity and magnetic susceptibility with structural information from high-resolution x-ray diffraction measurements, we show that superconductivity is not intrinsic to InN and is seen only in samples that show traces of oxygen impurity. We hence believe that InN is not intrinsically a superconducting semiconductor.

High pressure x-ray diffraction and extended x-ray absorption fine structure studies on ternary alloy Zn1−xBexSe

The ternary semiconductor alloy Zn1−xBexSe has been studied under high pressure by x-ray diffraction and extended x-ray absorption fine structure (EXAFS) at the Zn and Se K-edge in order to determine the bulk and bond-specific elastic properties. Our measurements on samples with x=0.06–0.55 show pressure induced phase transformation from zinc blende to NaCl. The phase transformation pressure increases linearly with x. Murnaghan equation of state fitting to the data yields the unit cell volume at ambient pressure and bulk modulus, both of which follow the Vegard’s law. Nearest neighbor bond distances derived from EXAFS do not show sharp phase transition except for x=0.06. Bond modulus derived for the Zn–Se bonds shows them to be apparently stiffer than the bulk alloy, which is nontrivial. This tendency increases with increasing x and a strong positive bowing from the Vegard’s law is observed. We attribute the observed anomalies to the contrastingly different properties of the two components ZnSe and BeSe.

Large dielectric permittivity and possible correlation between magnetic and dielectric properties in bulk BaFeO3−δ

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.