Abstract: The potential shape memory alloy Ni50Mn34In16 is studied with partial substitution of Mn with Fe and Cr to investigate the effect of such substitution on the martensitic transition in the Ni–Mn–In alloy system. The results of ac susceptibility, magnetization and electrical resistivity measurements show that while the substitution with Cr increases the martensitic transition temperature, the substitution with Fe decreases it. Possible reasons for this shift in martensitic transition are discussed. Evidence of kinetic arrest of the austenite to martensite phase transition in the Fe substituted alloys is also presented. Unlike the kinetic arrest of the austenite to martensite phase transition in the parent Ni50Mn34In16 alloy which takes place in the presence of high external magnetic field, the kinetic arrest of the austenite to martensite phase transition in the Fe doped alloy occurs even in zero magnetic field. The Cr substituted alloys, on the other hand, show no signature of kinetic arrest of this phase transition.
Abstract: The microstructure of epitaxial InN layers has been analyzed by high-resolution X-ray diffraction. Various mosaic block parameters like the tilt and twist between the blocks and an estimate of their lateral coherence lengths have been obtained for a large number of InN epitaxial layers deposited under different V/III ratios, temperatures and reactor pressures. Based on the detailed analysis of the microstrain, we have arrived at a set of optimized deposition parameters for InN in a close-coupled showerhead reactor. We also conclude that excessively high V/III ratio, as mentioned in a few earlier reports, is not a prerequisite for the deposition of high-quality InN layers. In fact, all deposition parameters that lead to an increase in the dissociation of ammonia beyond a critical value lead to increase in the screw dislocation density as indicated by an increase in the tilt value. Interestingly, we find that the density of edge dislocation, indicated by the twist value of the epilayers remains nearly the same irrespective of the deposition parameters.
Abstract: Polycrystalline ZnO films were deposited on quartz substrates by reactive sputtering of zinc target. X-ray powder diffraction, pole figure analysis and high resolution measurements along with transmission electron microscopy, Raman and photoluminescence studies were carried out to study the microstructure, crystallinity and optically active defects in the films. All the films deposited in the substrate temperature range from room temperature to 600 °C exhibited strong c-axis preferred orientation. The changes in preferred orientation of crystallites with substrate temperature were attributed to its being determined by preferential nucleation at lower temperatures and surface diffusion at higher temperatures. A detailed microstructural analysis showed that with increase in substrate temperature from 300 °C to 600 °C, a significant reduction in micro-strain to not, vert, similar 10− 3 takes place, along with a marginal increase in crystallite size. Raman and photoluminescence studies have shown that the films deposited below 300 °C possessed poor crystalline quality. The film deposited at 600 °C yielded the most intense and narrow (not, vert, similar 102 meV) band edge luminescence at room temperature, though it did not exhibit the strongest c-axis orientation of crystallites. This is attributed to its superior crystalline quality and absence of oxygen-deficiency related defects.
Abstract: Gallium phosphide (GaP) homoepitaxial layers are optimized by varying the growth temperature, growth rate and V/III ratio in a low-pressure metal organic vapour phase epitaxy (MOVPE) process. It is observed that we need high growth temperature, low growth rate and an optimum value of the V/III ratio in order to obtain a mirror finish surface morphology. The surface morphology is mirror finish for an epitaxial layer grown ≥820 °C when viewed under a stereo-zoom microscope for a V/III ratio of about 100 or more. A sharp and intense excitonic photoluminescence (PL) feature and a fine structure associated with donor–acceptor pair recombination confirm a high optical quality of the grown layer, which is supported by high-resolution x-ray diffraction measurements. For silicon doping of GaP using silane in low-pressure MOVPE, our results confirm theoretical predictions available in the literature and the values of a distribution coefficient lie in a similar range known for other conventional semiconductors. This optimization provides high-quality GaP (1 1 1)B epilayers needed for the development of either nanostructure-based optoelectronic devices on transparent substrates or polarization-sensitive infrared photodetectors based on two-photon absorption.
Abstract: A two-step growth process of gallium phosphide (GaP) epilayer on silicon substrate is carried out using metal organic vapor-phase epitaxy (MOVPE). This process includes the growth of a low-temperature GaP nucleating layer and a high-temperature GaP epilayer. In the first step, a GaP nucleating layer of thickness not, vert, similar80 nm with high V/III ratio not, vert, similar1725 is grown at 425 °C. This is followed by the growth of a GaP layer of thickness not, vert, similar760 nm with V/III ratio not, vert, similar100 at 770 °C. The total thickness of the epilayer is not, vert, similar845 nm as measured on a cross-section by a scanning electron microscope (SEM). A GaP layer grown by the two-step growth process shows significant improvement in morphology compared with that grown by the single-step process, as confirmed by Raman and SEM studies. High-resolution X-ray diffraction studies show that the epilayer is of single crystalline nature and structurally coherent with the silicon substrate. Epilayers grown using the two-step process show reduced dislocation density and micro-strains, with significant improvements in their structural properties. These layers also show n-type behavior with an electron density of not, vert, similar8.5×1017 cm−3, while the single-step-grown layers show p-type behavior. This change in type is explained by a reduced/increased incorporation of silicon into phosphorus/gallium sites, respectively, i.e. a predominant donor nature of silicon in GaP grown by the two-step growth process. Photoluminescence (PL), surface photovoltage spectroscopy (SPS) and transport measurements confirm a significant reduction in sub-band gap states and carrier density.
Abstract: 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.
Abstract: 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.
Abstract: Growth of gallium phosphide layer on silicon substrate has been carried out using metal-organic vapor phase epitaxy. Epitaxial layers were grown at 845 °C with a V/III ratio of 100 and a growth rate of 1.7 Å s−1at a reactor pressure of 30 mbar. The nominal thickness of the gallium phosphide epitaxial layer is not, vert, similar600 nm as measured by cross-sectional scanning electron microscopy. Growth of gallium phosphide epilayer is confirmed by Raman spectra studies. High-resolution X-ray diffraction studies show that the epilayer is of single crystalline nature and structurally coherent with silicon substrate. It is also inferred from these measurements that the in- and out of plane strain arising from small mismatch confirms a relaxed epilayer. As-grown epilayer shows p-type behavior with a hole carrier density of not, vert, similar1.2×1018cm−3 and hole mobility 114 cm−2V s−1 at room temperature. Annealing at 550 °C for 10 min shows significant improvements in crystalline quality of the epilayer. The annealed layer shows a reduced hole density (not, vert, similar6.7×1017cm−3) and increased hole mobility (155 cm−2 V s−1).
