Temperature dependent transport studies in InN quantum dots grown by droplet epitaxy on silicon nitride/Si substrate

InN quantum dots (QDs) were fabricated on silicon nitride/Si (111) substrate by droplet epitaxy. Single-crystalline structure of InN QDs was verified by transmission electron microscopy, and the chemical bonding configurations of InN QDs were examined by x-ray photoelectron spectroscopy. Photoluminescence measurement shows a slight blue shift compared to the bulk InN, arising from size dependent quantum confinement effect. The interdigitated electrode pattern was created and current-voltage (I-V) characteristics of InN QDs were studied in a metal-semiconductor-metal configuration in the temperature range of 80-300K. The I-V characteristics of lateral grown InN QDs were explained by using the trap model. (C) 2011 American Institute of Physics. [doi:10.1063/1.3651762]

Experimental evidence of Ga-vacancy induced room temperature ferromagnetic behavior in GaN films

We have grown Ga deficient GaN epitaxial films on (0001) sapphire substrate by plasma-assisted molecular beam epitaxy and report the experimental evidence of room temperature ferromagnetic behavior. The observed yellow emission peak in room temperature photoluminescence spectra and the peak positioning at 300 cm(-1) in Raman spectra confirms the existence of Ga vacancies. The x-ray photoelectron spectroscopic measurements further confirmed the formation of Ga vacancies; since the N/Ga is found to be >1. The ferromagnetism is believed to originate from the polarization of the unpaired 2p electrons of N surrounding the Ga vacancy. (C) 2011 American Institute of Physics. [doi:10.1063/1.3654151]

Substrate nitridation induced modulations in transport properties of wurtzite GaN/p-Si (100) heterojunctions grown by molecular beam epitaxy

Phase pure wurtzite GaN films were grown on Si (100) substrates by introducing a silicon nitride layer followed by low temperature GaN growth as buffer layers. GaN films grown directly on Si (100) were found to be phase mixtured, containing both cubic (beta) and hexagonal (alpha) modifications. The x-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy studies reveal that the significant enhancement in the structural as well as in the optical properties of GaN films grown with silicon nitride buffer layer grown at 800 degrees C when compared to the samples grown in the absence of silicon nitride buffer layer and with silicon nitride buffer layer grown at 600 degrees C. Core-level photoelectron spectroscopy of Si(x)N(y) layers reveals the sources for superior qualities of GaN epilayers grown with the high temperature substrate nitridation process. The discussion has been carried out on the typical inverted rectification behavior exhibited by n-GaN/p-Si heterojunctions. Considerable modulation in the transport mechanism was observed with the nitridation conditions. The heterojunction fabricated with the sample of substrate nitridation at high temperature exhibited superior rectifying nature with reduced trap concentrations. Lowest ideality factors (similar to 1.5) were observed in the heterojunctions grown with high temperature substrate nitridation which is attributed to the recombination tunneling at the space charge region transport mechanism at lower voltages and at higher voltages space charge limited current conduction is the dominating transport mechanism. Whereas, thermally generated carrier tunneling and recombination tunneling are the dominating transport mechanisms in the heterojunctions grown without substrate nitridation and low temperature substrate nitridation, respectively. (C) 2011 American Institute of Physics. [doi:10.1063/1.3658867]

Synthesis, characterization and spectroscopy of composites of graphene with CdSe and CdS nanoparticles

Nanocomposites of few-layer graphene with nanoparticles of CdSe and CdS have been synthesized by two different methods, one involving ultrasonication of a mixture of graphene and the chalcogenide nanoparticles, and another involving assembly at the organic-aqueous interface. The nanocomposites have been examined by electron microscopy, electronic absorption and photoluminescence spectroscopies as well as Raman spectroscopy. Electron microscopy reveals that the nanoparticles are dispersed on the graphene surface. Raman spectra show the presence of definitive electronic interaction between the nanoparticles and graphene depending on the capping agent. Photoluminescence spectra are markedly influenced by the interaction of the nanoparticles with the graphene surface, depending on the capping agent.

The fluorescence of Nd3+ in lead borate and bismuth borate glasses with large stimulated emission cross section

The stimulated emission cross section σp for the 1060 nm transition of Nd3+ in lead borate and bismuth borate glasses has been determined from fluorescence measurements. The compositional dependence of σp, which has been evaluated using radiative transition probability, refractive index of the host glass, effective fluorescence linewidth, and position of the band, with PbO/Bi2O3 content is investigated. The σp values of the 1060 nm band of Nd3+ for lead borate and bismuth borate glasses are found to be in the range 2.6–5.7×10−20 cm2 at 298 K and 3.0–6.3×10−20 cm2 at 4.2 K. The σp values are comparatively large suggesting the possible utilization of these materials in laser applications.

Gray track formation in KTiOPO4 by swift ion irradiation

Potassium titanyl phosphate single crystals were irradiated with 48 MeV lithium ions at fluences varying from 5×1012 to 1016 ions/cm2. The defects created in the crystal have been characterized using x-ray rocking curve measurements, optical transmittance, and photoluminescence spectroscopy. From x-ray rocking curve studies, the full width at half maximum for the irradiated samples was observed to increase, indicating lattice strain caused by the energetic ions. Optical transparency of these samples was found to decrease upon irradiation. The irradiated samples exhibited a broadband luminescence in the 700–900 nm region, for fluences above 5×1013 ions/cm2. The results indicate that ion-beam-induced optical effects in KTiOPO4 single crystals are very similar to the ones obtained for crystals with “gray tracks,” which are attributed to the electronic transitions in the Ti3+ levels.

Novel thiophene derivative hybrid composite solar cells

In this work composites of poly(3-hexylethiophene) (P3HT) and a thiophene derivative (7, 9-di (thiophen-2-yl)-8H-cyclopenta[a]acenaphthylen-8-one) (DTCPA) having donor acceptor architecture (DAD) were prepared. Photovoltaic properties of these hybrid composites were evaluated. DTCPA, which is a highly crystalline organic molecule with wide absorption range, was observed to improve the open circuit voltage of the solar cell. Furthermore, DTCPA crystals acts as a nucleating center and increases the molecular ordering of P3HT in the composite. Improved charge separation efficiency was observed by photoluminescence spectroscopy. Because of high built in potential in these devices, large open circuit voltage was observed. (C) 2011 Elsevier B.V. All rights reserved.

Indium flux, growth temperature and RF power induced effects in InN layers grown on GaN/Si substrate by plasma-assisted MBE

In the present work, we report the growth of wurtzite InN epilayers on GaN/Si (1 1 1) substrate by plasma-assisted molecular beam epitaxy (PAMBE). The growth parameters such as indium flux, substrate temperature and RF power affect the crystallographic and morphological properties of InN layers, which were evaluated using high resolution X-ray diffraction (HRXRD) analysis and atomic force microscopy (AFM). It is found that excess indium (In) concentrations and surface roughness were increased with increase in In flux and growth temperature. The intensity of HRXRD (0 0 0 2) peak, corresponding to c-axis orientation has been increased and full width at half maxima (FWHM) has decreased with increase in RF power. It was found that highly c-axis oriented InN epilayers can be grown at 450 degrees C growth temperature, 450 W RF power and 1.30 x 10(-7) mbar In beam equivalent pressure (BEP). The energy gap of InN layers grown by optimizing growth conditions was determined by photoluminescence and optical absorption measurement. (C) 2011 Elsevier B.V. All rights reserved.

Evidences for ambient oxidation of indium nitride quantum dots

Investigations were carried out on the ambient condition oxidation of self-assembled, fairly uniform indium nitride (InN) quantum dots (QDs) fabricated on p-Si substrates. Incorporation of oxygen in to the outer shell of the QDs was confirmed by the results of transmission electron microscopy (TEM), X-ray photoemission spectroscopy (XPS). As a consequence, a weak emission at high energy (similar to 1.03?eV) along with a free excitonic emission (0.8?eV) was observed in the photoluminescence spectrum. The present results confirm the incorporation of oxygen into the lattice of the outer shell of InN QDs, affecting their emission properties. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Structure and Photocatalytic Activity of Ti 1- x M x O 2±δ (M = W, V, Ce, Zr, Fe, and Cu) Synthesized by Solution Combustion Method

The W, V, Ce, Zr, Fe, and Cu metal ion substituted nanocrystalline anatase TiO2 was prepared by solution combustion method and characterized by XRD, Raman, BET, EPR, XPS, IR TGA, UV absorption, and photoluminescence measurements. The structural studies indicate that the solid solution formation was limited to a narrow range of concentrations of the dopant ions. The photocatalytic degradation of 4-nitrophenol under UV and solar exposure was investigated with Ti1-xMxO2±δ. The degradation rates of 4-nitrophenol with these catalysts were lesser than the degradation rates of 4-nitrophenol with undoped TiO2 both with UV exposure and solar radiation. However, the photocatalytic activities of most metal ion doped TiO2 are higher than the activity of the commercial TiO2, Degussa P25. The decrease in photocatalytic activity is correlated with decrease in photoluminescence due to electron states of metal ions within the band gap of TiO2.

External-cavity semiconductor laser sensor

The spectral characteristics of a diode laser are significantly affected due to interference caused between the laser diode output and the optical feedback in the external-cavity. This optical feedback effect is of practical use for linewidth reduction, tuning or for sensing applications. A sensor based on this effect is attractive due to its simplicity, low cost and compactness. This optical sensor has been used so far, in different configuration such as for sensing displacement induced by different parameters. In this paper we report a compact optical sensor consisting of a semiconductor laser coupled to an external cavity. Theoretical analysis of the self- mixing interference for optical sensing applications is given for moderate optical feedback case. A comparison is made with our experimental observations. Experimental results are in good agreement with the simulated power modulation based on self-mixing interference theory. Displacements as small as 10-4 nm have been measured using this sensor. The developed sensor showed a fringe sensitivity of one fringe per 400nm displacement for reflector distance of around 10cms. The sensor has also been tested for magnetic field and temperature induced displacement measurements.

Near-resonant two-photon absorption in luminescent CdTe quantum dots

We report the nonlinear optical absorption studies in two differently sized water-soluble cadmium telluride quantum dot (QD) samples, exhibiting first excitonic absorption peaks at 493 nm and 551 nm, respectively. An optical limiting behavior is observed for near-resonant excitation at 532 nm using nanosecond laser pulses, originating from the effective two-photon absorption (TPA) mechanism. The effective TPA coefficient (beta(eff)) is measured to be in the range of 10(-12) m/W. This is one order of magnitude higher than the TPA coefficient (beta) reported for off-resonant excitation. At this excitation wavelength, the smaller QD shows a relatively weaker photoluminescence and stronger nonlinear absorption. (C) 2012 American Institute of Physics. [doi:10.1063/1.3687695]

Band-Structure Lineup at In0.2Ga0.8N/Si Heterostructures by X-ray Photoelectron Spectroscopy

In0.2Ga0.8N layers were directly grown on Si(111) substrate by plasma-assisted molecular beam epitaxy (PAMBE). Structural characteristics of the as-grown InGaN epilayers were evaluated high resolution X-ray diffraction and composition of InGaN was estimated from photoluminescence spectra using the standard Vegard's law. High-resolution X-ray photoemission spectroscopy measurements were used to determine the band offset of wurtzite-In0.2Ga0.8N/Si(111) heterojunctions. The valence band of InGaN is found to be 2.08 +/- 0.04 eV below that of Si. The conduction band offset (CBO) of InGaN/Si heterojunction is found similar to 0.74 eV and a type-II heterojunction. (C) 2012 The Japan Society of Applied Physics

Gallium and indium co-doped ZnO thin films for white light emitting diodes

Ga and In co-doped ZnO (GIZO) thin films together with ZnO, In-doped ZnO (IZO), Ga-doped ZnO (GZO), and IZO/GZO multilayer for comparison, were grown on corning glass and boron doped Si substrates by PLD. The photoluminescence spectra of GIZO showed a strong white light emission and the current-voltage characteristics showed relatively lower turn-on voltage and larger forward current. The CIE coordinates for GIZO were observed to be (0.31, 0.33) with a correlated colour temperature of 6650 K, indicating a cool white light, and establishing a possibility of white light emitting diodes. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Cobalt-doped ZnO nanowires on quartz: Synthesis by simple chemical method and characterization

The synthesis of cobalt-doped ZnO nanowires is achieved using a simple, metal salt decomposition growth technique. A sequence of drop casting on a quartz substrate held at 100 degrees C and annealing results in the growth of nanowires of average (modal) length similar to 200 nm and diameter of 15 +/- 4 nm and consequently an aspect ratio of similar to 13. A variation in the synthesis process, where the solution of mixed salts is deposited on the substrate at 25 degrees C, yields a grainy film structure which constitutes a useful comparator case. X-ray diffraction shows a preferred 0001] growth direction for the nanowires while a small unit cell volume contraction for Co-doped samples and data from Raman spectroscopy indicate incorporation of the Co dopant into the lattice; neither technique shows explicit evidence of cobalt oxides. Also the nanowire samples display excellent optical transmission across the entire visible range, as well as strong photoluminescence (exciton emission) in the near UV, centered at 3.25 eV. (C) 2012 Elsevier B.V. All rights reserved.