Optical and electrical properties of SnS semiconductor crystals grown by physical vapor deposition technique

Tin sulfide (SnS) is a material of interest for use as an absorber in low cost solar cells. Single crystals of SnS were grown by the physical vapor deposition technique. The grown crystals were characterized to evaluate the composition, structure, morphology, electrical and optical properties using appropriate techniques. The composition analysis indicated that the crystals were nearly stoichiometric with Sn-to-S atomic percent ratio of 1.02. Study of their morphology revealed the layered type growth mechanism with low surface roughness. The grown crystals had orthorhombic structure with (0 4 0) orientation. They exhibited an indirect optical band gap of 1.06 eV and direct band gap of 1.21 eV with high absorption coefficient (up to 10(3) cm(-1)) above the fundamental absorption edge. The grown crystals were of p-type with an electrical resistivity of 120 Omega cm and carrier concentration 1.52 x 10(15) cm(-3). Analysis of optical absorption and diffuse reflectance spectra showed the presence of a wide absorption band in the wavelength range 300-1200 nm, which closely matches with a significant part of solar radiation spectrum. The obtained results were discussed to assess the suitability of the SnS crystal for the fabrication of optoelectronic devices. (C) 2011 Elsevier B.V. All rights reserved.

Conductance of Tomonaga-Luttinger liquid wires and junctions with resistances

We study the effect that resistive regions have on the conductance of a quantum wire with interacting electrons which is connected to Fermi liquid leads. Using the bosonization formalism and a Rayleigh dissipation function to model the power dissipation, we use both scattering theory and Green's function techniques to derive the DC conductance. The resistive regions are generally found to lead to incoherent transport. For a single wire, we find that the resistance adds in series to the contact resistance of h/e(2) for spinless electrons, and the total resistance is independent of the Luttinger parameter K-W of the wire. We numerically solve the bosonic equations to illustrate what happens when a charge density pulse is incident on the wire; the results depend on the parameters of the resistive and interacting regions in interesting ways. For a junction of Tomonaga-Luttinger liquid wires, we use a dissipationless current splitting matrix to model the junction. For a junction of three wires connected to Fermi liquid leads, there are two families of such matrices; we find that the conductance matrix generally depends on K-W for one family but is independent of K-W for the other family, regardless of the resistances present in the system. Copyright (c) EPLA, 2011

Time-Resolved Photoluminescence and Microwave Conductivity at Semiconductor Electrodes: Depletion Layer Effects

Analytical expressions which include depletion layer effects on low-injection carrier relaxation are being presented for the first time here. Starting from the continuity equation for the minority carriers, we derive expressions for the output signal pertinent to time-resolved microwave and luminescence experiments. These are valid for the time domain that usually overlaps with the time scales of surface processes, such as charge transfer and trapping. Apart from the usual pulse form of illumination, theoretical expressions pertaining to other forms of illumination such as switch-on and switch-off transient modes, a periodic mode, and a steady state and their various inter-relationships are derived here. The expressions obtained are seen to be generalizations of existing flat-band low-injection results in the Limit of early or initial band bendings. The importance of the depletion layer as an experimental parameter is clearly seen in the limit of larger band bendings wherein it is shown, unlike the flat-band case, to exhibit pure exponential forms of carrier relaxation. Our results are consistent with the main conclusions of the numerical and experimental work published recently. Furthermore, this work provides the actual functional relationships between the applied potential and observed carrier decay. This should enable one to extract the surface kinetic parameters, after deciding on the dominant mode of carrier relaxation at the interface, whether charge transfer or trapping, by studying the potential dependence of the fate of relaxation.

X-ray reflectivity study of semiconductor interfaces

The results of an X-ray reflectivity study of thick AlAs-AlGaAs and thin GeSi-Ge multilayers grown using metal-organic vapour-phase epitaxy and ion-beam sputtering deposition techniques, respectively, are presented. Asymmetry in interfaces is observed in both of these semiconductor multilayers. It is also observed that although the Si-on-Ge interface is sharp, an Si0.4Ge0.6 alloy is formed at the Ge-on-Si interface. In the case of the III-V semiconductor, the AlAs-on-AlGaAs interface shows much greater roughness than that observed in the AlGaAs-on-AlAs interface. For thin multilayers it is demonstrated that the compositional profile as a function of depth can be obtained directly from the X-ray reflectivity data.

Phase transformation and semiconductor-metal transition in thin films of VO2 deposited by low-pressure metalorganic chemical vapor deposition

Thin films of the semiconducting, monoclinic vanadium dioxide, VO2(M) have been prepared on ordinary glass by two methods: directly by low-pressure metalorganic chemical vapor deposition (MOCVD), and by argon-annealing films of the VO2(B) phase deposited by MOCVD. The composition and microstructure of the films have been examined by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Films made predominantly of either the B or the M phase, as deposited, can only be obtained over a narrow range of deposition temperatures. At the lower end of this temperature range, the as-deposited films are strongly oriented, although the substrate is glass. This can be understood from the drive to minimize surface energy. Films of the B phase have a platelet morphology, which leads to an unusual microstructure at the lower-deposition temperatures. Those grown at similar to370 degreesC convert to the metallic, rutile (R) phase when annealed at 550 degreesC, whereas those deposited at 420 degreesC transform to the R phase only at 580 degreesC. (When cooled to room temperature, the annealed films convert reversibly from the R phase to the M phase.) Electron microscopy shows that annealing leads to disintegration of the single crystalline VO2(B) platelets into small crystallites of VO2(R), although the platelet morphology is retained. When the annealing temperature is relatively low, these crystallites are nanometer sized. At a higher-annealing temperature, the transformation leads to well-connected and similarly oriented large grains of VO2(R), enveloped in the original platelet. The semiconductor-metal transition near 68 degreesC leads to a large jump in resistivity in all the VO2(M) films, nearly as large as in epitaxial films on single-crystal substrates. When the annealed films contain well-connected large grains, the transition is very sharp. Even when preferred orientation is present, the transition is not as sharp in as-deposited VO2(M), because the crystallites are not densely packed as in annealed VO2(B). However, the high degree of orientation in these films leads to a narrow temperature hysteresis. (C) 2002 American Institute of Physics.

Optical and Electronic Properties of Conjugated Polymer -Nanocluster Semiconductor Hybrid Systems

Conjugated polymers are intensively pursued as candidate materials for emission and detection devices with the optical range of interest determined by the chemical structure. On the other hand the optical range for emission and detection can also be tuned by size selection in semiconductor nanoclusters. The mechanisms for charge generation and separation upon optical excitation, and light emission are different for these systems. Hybrid systems based on these different class of materials reveal interesting electronic and optical properties and add further insight into the individual characteristics of the different components. Multilayer structures and blends of these materials on different substrates were prepared for absorption, photocurrent (Iph), photoluminescence (PL) and electroluminscence (EL) studies. Polymers chosen were derivatives of polythiophene (PT) and polyparaphenylenevinylene (PPV) along with nanoclusters of cadmium sulphide of average size 4.4 nm (CdS-44). The photocurrent spectral response in these systems followed the absorption response around the band edges for each of the components and revealed additional features, which depended on bias voltage, thickness of the layers and interfacial effects. The current-voltage curves showed multi-component features with emission varying for different regimes of voltage. The emission spectral response revealed additive features and is discussed in terms of excitonic mechanisms.

Barrier height inhomogeneities in InN/GaN heterostructure based Schottky junctions

The electrical transport properties of InN/GaN heterostructure based Schottky junctions were studied over a wide temperature range of 200-500 K. The barrier height and the ideality factor were calculated from current-voltage (I-V) characteristics based on thermionic emission (TE), and found to be temperature dependent. The barrier height was found to increase and the ideality factor to decrease with increasing temperature. The observed temperature dependence of the barrier height indicates that the Schottky barrier height is inhomogeneous in nature at the heterostructure interface. Such inhomogeneous behavior was modeled by assuming the existence of a Gaussian distribution of barrier heights at the heterostructure interface. (C) 2011 Elsevier Ltd. All rights reserved.

Advances in Light-Emitting Doped Semiconductor Nanocrystals

Insertion of just a few impurity atoms in a host semiconductor nanocrystal can drastically alter its phase, shape, and physical properties. Such doped nanomaterials now constitute an important class of optical materials that can provide efficient, stable, and tunable dopant emission in visible and NIR spectral windows. Selecting proper dopants and inserting them in appropriate hosts can generate many new series of such doped nanocrystals with several unique and attractive properties in order to meet current challenges in the versatile field of luminescent materials. However, the synthesis of such doped nanomaterials with a specific dopant in a predetermined host at a desired site leading to targeted optical properties requires fundamental understanding of both the doping process as well as the resulting photophysical properties. Summarizing up to date literature reports, in this Perspective we discuss important advances in synthesis methods and in-depth understanding of the optical properties, with an emphasis on the most widely investigated Mn-doped semiconductor nanocrystals.

Effect of surface on the conductance characteristics of Au‐Bi2Sr2CaCu2O8+δ (single crystal) point contact junctions

We report the effect of surface treatments on the dynamic conductance curves (G=dI/dV‐V) of Au‐Bi2Sr2CaCu2O8+δ (single crystal) point contact junctions of variable junction conductances (100 mS≳G≳100 μS). We find that if the crystal surface is cleaved freshly just prior to making contacts, all irreproducible sharp multiple features often observed in tunneling data of Bi(2212) oxide superconductors disappear. If the cleaved crystal surfaces are left under ambient conditions for a few days and the tunneling experiments are repeated, these multiple features reappear. We also find that if the current in the junction is made to pass predominantly through the bulk (and not along the surface), gap features are sharper. The observed conductance curves are fitted to a modified model [G. E. Blonder et al., Phys. Rev. B 25, 4515 (1982)] and estimated gap values are Δ≂28 to 30 meV corresponding to the ratio 2Δ/kBTc ≂ 7.5 with lifetime broadening Γ/Δ≂0.2. We conclude that the sharp multiple features observed in Bi(2212) tunneling curves has no intrinsic origin in the bulk and they arise from the surface only.

Investigations on multimagnetron sputtered Zn1-xMgxO thin films through metal-ferroelectric-semiconductor configuration

The effect of Mg doping in ZnO is investigated through structural, electrical, and optical properties. Zn1−xMgxO (0<×<0.3) thin films were deposited on Si (100) and corning glass substrates using multimagnetron sputtering. Investigations on the structural properties of the films revealed that the increase in Mg concentration resulted in phase evolution from hexagonal to cubic phase. The temperature dependent study of dielectric constant at different frequencies exhibited a dielectric anomaly at 110 °C. The Zn0.7Mg0.3O thin films exhibited a well-defined polarization hysteresis loop with a remnant polarization of 0.2 μC/cm2 and coercive field of 8 kV/cm at room temperature. An increase in the band gap with an increase in Mg content was observed in the range of 3.3–3.8 eV for x = 0–0.3. The average transmittance of the films was higher than 90% in the wavelength region λ = 400–900 nm.

Effect of gate-drain/source overlap on the noise in 90 nm N-channel metal oxide semiconductor field effect transistors

In this paper, we have studied the effect of gate-drain/source overlap (LOV) on the drain channel noise and induced gate current noise (SIg) in 90 nm N-channel metal oxide semiconductor field effect transistors using process and device simulations. As the change in overlap affects the gate tunneling leakage current, its effect on shot noise component of SIg has been taken into consideration. It has been shown that “control over LOV” allows us to get better noise performance from the device, i.e., it allows us to reduce noise figure, for a given leakage current constraint. LOV in the range of 0–10 nm is recommended for the 90 nm gate length transistors, in order to get the best performance in radio frequency applications.