S. Das

Effect of reactive‐ion bombardment on the properties of silicon nitride and oxynitride films deposited by ion‐beam sputtering

Silicon nitride and oxynitride films of very low hydrogen content have been deposited on silicon at low temperatures (150–200u2009°C) using ion‐beam sputtering. A dual‐ion‐beam sputtering technique, making simultaneous use of an energetic argon‐ion beam to sputter silicon nitride from a target and a low‐energy oxygen or nitrogen ion beam to react with the sputtered films on the substrate, has been employed to control the composition of the films. A precise control of film composition independent of deposition rate has been achieved through the control of oxygen/nitrogen ion‐beam parameters and gas flow ratios. The films have been characterized by the measurement and study of refractive index, chemical etch rate, infrared absorption, and x‐ray photoelectron spectra. A direct correlation between film properties with oxygen content has been obtained for silicon oxynitride films. The electrical properties have been studied by the measurement of the characteristics of metal‐insulator‐semiconductor capacitors fabri...

Memory effect in a junction-like CdS nanocomposite/conducting polymer poly[2-methoxy-5-(2-ethylhexyloxy)1,4-phenylene-vinylene] heterostructure

The operation of a nonvolatile memory device is demonstrated using junction-like CdS nanocomposites embedded in a polymer matrix. The capacitance-voltage characteristics of Al/conducting polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene]/CdS nanocomposites in a polyvinyl alcohol matrix/indium tin oxide device exhibit hysteresis, which is attributed to the trapping, storage, and emission of holes in the quantized valence band energy levels of isolated CdS nanoneedles. The characteristics at different operating frequencies show that the hysteresis is due to trapping of charge carriers in CdS nanocomposites rather than in the interfacial states. The memory behavior in the inorganic/organic heterostructure is explained on the basis of a simple energy band diagram.

Evolution of strain and composition of Ge islands on Si (001) grown by molecular beam epitaxy during postgrowth annealing

Self-assembled Ge islands have been grown using a Stranski–Krastanov growth mechanism on Si (001) substrates by solid source molecular beam epitaxy. We performed time varying annealing experiments at a representative temperature of 650 ° C to study the shape and size evolution of islands for a relatively high Ge coverage. Islands are found to coarsen due to heat treatment via structural and compositional changes through continuous strain relaxation. Different island morphologies, namely, “pyramids,” “transitional domes,” and “domes” are found during the annealing sequence. The dominant coarsening mechanisms for the temporal evolution of islands of as-grown and annealed samples are explained by the comprehensive analysis of Rutherford back scattering, Raman spectroscopy, high-resolution x-ray diffraction, and atomic force microscopy. A correlation of the morphological evolution with the composition and strain relaxation of grown islands is presented.

Microstructural characteristics and phonon structures in luminescence from surface oxidized Ge nanocrystals embedded in HfO2 matrix

Ge nanocrystals embedded in HfO2 matrices were prepared by rf magnetron sputtering technique. Transmission electron micrographs reveal the formation of spherical shape Ge nanocrystals of 4–6 nm diameters for 800u2009°C and 6–9 nm for 900u2009°C annealed samples. X-ray photoelectron spectroscopy confirms the formation of surface oxidized Ge nanocrystals. Embedded Ge nanocrystals show strong photoluminescence peaks in visible and ultraviolet region even at room temperature. Spectral analysis suggests that emission in 1.58 and 3.18 eV bands originate from TΣ(TΠ)→S0, and TΠ′→S0 optical transitions in GeO color centers, respectively, and those in the range 2.0–3.0 eV are related to Ge/O defects at the interface of the oxidized nanocrystals. Temperature dependent photoluminescence study has revealed additional fine structures with lowering of temperature, the origin of which is attributed to the strong coupling of electronic excitations with local vibration of germanium oxides at the surface.

Solution-processed, hybrid 2D/3D MoS2/Si heterostructures with superior junction characteristics

We report a theoretical and experimental investigation of the hybrid heterostructure interfaces between atomically thin MoS2 nanocrystals (NCs) on Si platform for their potential applications towards next-generation electrical and optical devices. Mie theory-based numerical analysis and COMSOL simulations based on the finite element method have been utilized to study the optical absorption characteristics and light-matter interactions in variable-sized MoS2 NCs. The size-dependent absorption characteristics and the enhancement of electric field of the heterojunction in the UV-visible spectral range agree well with the experimental results. A lithography-free, wafer-scale, 2D material on a 3D substrate hybrid vertical heterostructure has been fabricated using colloidal n-MoS2 NCs on p-Si. The fabricated p-n heterojunction exhibited excellent junction characteristics with a high rectification ratio suitable for voltage clipper and rectifier applications. The current-voltage characteristics of the devices under illumination have been performed in the temperature range of 10-300 K. The device exhibits a high photo-to-dark current ratio of ∼3xa0×xa0103 and a responsivity comparable to a commercial Si photodetector. The excellent heterojunction characteristics demonstrate the great potential of MoS2 NC-based hybrid electronic and optoelectronic devices in the near future.

Microstructures and magnetic properties of rapidly solidified Ni54Fe27−2xGa19+2x ferromagnetic Heusler alloys

Rapidly solidified Ni54Fe27−2xGa19+2x (x=0, 1, 2, 3, and 4) ferromagnetic shape memory alloys were made by melt-spinning with variation of Fe and Ga contents to report on the martensitic phase transformation, microstructures, and magnetic properties. Rapid solidification produced pure L21 phase by preventing the formation of γ-phase. To study the effect of heat treatment on the phase transitions, microstructures, and the magnetic properties, the melt-spun ribbons were partly heat treated at different temperatures of 800, 900, 1000, 1100, and 1200 K with holding times of 5, 10, and 15 min followed by either water quenching or air cooling. The microstructures of the as-spun ribbons as revealed by electron microscopy studies exhibited a gradual transition from cellular to dendritic structure with increasing Ga concentration and with the presence of some internal martensitic twin bands at higher Ga content. The ribbons exhibited very low coercivity with high saturation magnetization, as high as ∼87u2002emu/g (dec...

Influence of magnetic frustration and structural disorder on magnetocaloric effect and magneto-transport properties in La1.5Ca0.5CoMnO6 double perovskite

The rare existence of a magnetocaloric effect, a Griffith phase, and frustrated magnetism in the antisite disorder compound La1.5Ca0.5CoMnO6 have been investigated in detail in this work. The nature of the observed Griffith phase (at TGu2009∼u2009226u2009K) can be best understood in terms of ferromagnetic (FM) entities within the globally paramagnetic network above the Curie temperature. From the isothermal magnetization measurement around Curie temperature (TCu2009∼u2009157u2009K) and cluster glass transition temperature (Tgu2009∼u200951u2009K), we have determined the maximum entropy change (−ΔSM) as ∼2.2u2009J/kg K and ∼1.2u2009J/kg K, respectively, for a magnetic field variation of 7u2009T. Interestingly, a sudden drop of resistivity curve at Tg, associated with magnetic frustration or magnetic disorder, can be related to the ferromagnetic (FM) phases with antiferromagnetic antiphase boundaries, giving rise to a large negative magnetoresistance (∼67%) at 45u2009K.

Role of Gd spin ordering on magnetocaloric effect and ferromagnetism in Sr substituted Gd2CoMnO6 double perovskite

The phenomenon of magnetocaloric effect occurring around the ordering temperatures for Gd2−xSrxCoMnO6 (xu2009=u20090 and 0.5) has been investigated in order to disclose the effects of Sr substitution on the individual collinear Gd spin order. It is shown that 25% Sr substitution of Gd ions significantly suppresses the independent spin order, leading to enhancement of ferromagnetism. As a result, magnetic entropy change (−ΔSM) increases from +0.9 J kg−1u2009K−1 to +3.7u2009Ju2009kg−1u2009K−1 at Curie temperature for a change in magnetic field (ΔH) = 6.9u2009T. On the other hand, the −ΔSM changes sign at Neel temperature due to the effect of Sr substitution. We have also observed noticeably large −ΔSM (∼+25.84u2009Ju2009kg−1u2009K−1) and (∼+21.01u2009Ju2009kg−1u2009K−1) for ΔH = 6.9u2009T at 4f-4f negative coupling temperature (T < 10u2009K) of xu2009=u20090 and xu2009=u20090.5 samples, respectively.The phenomenon of magnetocaloric effect occurring around the ordering temperatures for Gd2−xSrxCoMnO6 (xu2009=u20090 and 0.5) has been investigated in order to disclose the effects of Sr substitution on the individual collinear Gd spin order. It is shown that 25% Sr substitution of Gd ions significantly suppresses the independent spin order, leading to enhancement of ferromagnetism. As a result, magnetic entropy change (−ΔSM) increases from +0.9 J kg−1u2009K−1 to +3.7u2009Ju2009kg−1u2009K−1 at Curie temperature for a change in magnetic field (ΔH) = 6.9u2009T. On the other hand, the −ΔSM changes sign at Neel temperature due to the effect of Sr substitution. We have also observed noticeably large −ΔSM (∼+25.84u2009Ju2009kg−1u2009K−1) and (∼+21.01u2009Ju2009kg−1u2009K−1) for ΔH = 6.9u2009T at 4f-4f negative coupling temperature (T < 10u2009K) of xu2009=u20090 and xu2009=u20090.5 samples, respectively.

Conducting force microscopy and hole charging in Ge islands grown by RF sputtering

We have grown self assembled nearly monomodal Ge islands on Si(100) using RF magnetron sputtering at a much higher pressure compared to molecular beam epitaxy. Raman spectra of the grown sample exhibits the formation of strained Ge islands intermixed with Si. Both the Both the conducting atomic force microscopy image and I-V characteristics of individual Ge islands exhibit hole charging in isolated form.