Showing papers by "Mahesh Kumar published in 2011"
TL;DR: In this article, the authors 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.
Abstract: 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.
49 citations
TL;DR: In this article, temperature dependent electrical transport properties were carried out for InN/GaN heterostructure based Schottky diodes were fabricated by plasma-assisted molecular beam epitaxy.
Abstract: InN/GaN heterostructure based Schottky diodes were fabricated by plasma-assisted molecular beam epitaxy. The temperature dependent electrical transport properties were carried out for InN/GaN heterostructure. The barrier height and the ideality factor of the Schottky diodes were found to be temperature dependent. The temperature dependence of the barrier height indicates that the Schottky barrier height is inhomogeneous in nature at the heterostructure interface. The higher value of the ideality factor and its temperature dependence suggest that the current transport is primarily dominated by thermionic field emission (TFE) other than thermionic emission (TE). The room temperature barrier height obtained by using TE and TFE models were 1.08 and 1.43 eV, respectively.
25 citations
TL;DR: The faster rise and decay time indicate that the InN NRs/n-Si heterojunction is highly sensitive to IR light.
Abstract: The present work explores the electrical transport and infrared (IR) photoresponse properties of InN nanorods (NRs)/n-Si heterojunction grown by plasma-assisted molecular beam epitaxy. Single-crystalline wurtzite structure of InN NRs is verified by the X-ray diffraction and transmission electron microscopy. Raman measurements show that these wurtzite InN NRs have sharp peaks E
2(high) at 490.2 cm-1 and A
1(LO) at 591 cm-1. The current transport mechanism of the NRs is limited by three types of mechanisms depending on applied bias voltages. The electrical transport properties of the device were studied in the range of 80 to 450 K. The faster rise and decay time indicate that the InN NRs/n-Si heterojunction is highly sensitive to IR light.
25 citations
TL;DR: In this paper, 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.
Abstract: 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 (β) and hexagonal (α) 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 °C when compared to the samples grown in the absence of silicon nitride buffer layer and with silicon nitride buffer layer grown at 600 °C. Core-level photoelectron spectroscopy of SixNy 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 modulat...
22 citations
TL;DR: In this article, a (0001)-oriented single crystalline wurtzite-InN layer was confirmed by high resolution X-ray diffraction (HRXRD) using different buffer layers.
20 citations
TL;DR: In this paper, a xenon-filled coaxial dielectric barrier discharge (DBD) has been studied to understand the high-pressure nonequilibrium nonthermal plasma discharge.
Abstract: In this paper, a xenon-filled coaxial dielectric barrier discharge (DBD) has been studied to understand the high-pressure nonequilibrium nonthermal plasma discharge. A quartz coaxial DBD tube (ID: 6 mm, OD: 12 mm) at 400-mbar xenon-filled pressure has been used in the experiment. A unipolar pulselike voltage up to a-6-kV peak working at 30 kHz has been applied to the discharge electrodes for the generation of microdischarges. A single discharge is observed per applied voltage pulse. Visual images of the discharge and electrical waveform confirm the diffused-type discharges. The knowledge obtained by dynamic processes of DBDs in the discharge gap explains quantitatively the mechanism that is obtained in the ignition, development, and extinction of DBDs. The behavior of different discharge parameters has also been analyzed. From the experimental results and equivalent electrical circuit, the dynamic nature of equivalent capacitance has been reported. The relative intensity analysis of the Xe peak in the optical emission spectra (172 nm) has also been carried out for different supplied powers, and it is found that the radiation power has increased with supplied power.
20 citations
TL;DR: In this article, the authors determined the band offsets in InN/p-Si heterojunctions by high-resolution x-ray photoemission spectroscopy, finding that the valence band of InN is 1.39 eV below that of Si.
Abstract: The band offsets in InN/p-Si heterojunctions are determined by high resolution x-ray photoemission spectroscopy. The valence band of InN is found to be 1.39 eV below that of Si. Given the bandgap of 0.7 eV for InN, a type-III heterojunction with a conduction band offset of 1.81 eV was found. Agreement between the simulated and experimental data obtained from the heterojunction spectra was found to be excellent, establishing that the method of determination was accurate. The charge neutrality level (CNL) model provided a reasonable description of the band alignment of the InN/p-Si interface and a change in the interface dipole by 0.06 eV was observed for InN/p-Si interface.
19 citations
TL;DR: In this paper, a small residual magnetic moment is observed for these nanoparticles, which may be attributed to size confinement at nanoscale and to spin canting, supported by M-H loop measurements.
Abstract: Room temperature Mossbauer spectra of (BiFeO3)1−x (PbTiO3)x (x = 0, 0.1, 0.2, 0.3) nano multiferroic systems prepared by solution combustion technique reveal the long range ferromagnetic ordering, supported by M-H loop measurements. Magnetic hyperfine interaction field and isomer shift for these nanocrystallite indicate the presence of Fe3 + ion only, likewise confirmed by from X-ray photoelectron spectroscopy study, which is essential for enhancement of multiferroic properties. A small residual magnetic moment is observed for these nanoparticles, which may be attributed to size confinement at nanoscale and to spin canting. These nanomaterials are of great importance in basic as well as applied research.
18 citations
TL;DR: Negative differential capacitance (NDC) has been observed in n-GaN/p-Si heterojunctions grown by plasma assisted molecular beam epitaxy (PAMBE) as discussed by the authors.
16 citations
TL;DR: In this paper, the growth of non-polar a-plane GaN epilayers was confirmed by high resolution X-ray diffraction (HRXRD) study and the effect of growth temperature on structural, morphological and optical properties has been studied.
16 citations
TL;DR: In this paper, the structural and optical properties of a-plane GaN film grown on a r-plane sapphire substrate by plasma-assisted molecular beam epitaxy were reported.
TL;DR: In this paper, 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 currentvoltage (I-V) characteristics based on thermionic emission (TE), and found to be temperature dependent.
TL;DR: In this paper, single-crystalline structure of InN QDs was verified by transmission electron microscopy, and the chemical bonding configurations were examined by x-ray photoelectron spectroscopy.
Abstract: 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]
TL;DR: In this article, a systematic manipulation of the morphology, optical emission, and structural properties of InN/Si (111) QDs is demonstrated by changing the growth kinetics parameters such as flux rate and growth time.
Abstract: One of the scientific challenges of growing InN quantum dots (QDs), using Molecular beam epitaxy (MBE), is to understand the fundamental processes that control the morphology and distribution of QDs. A systematic manipulation of the morphology, optical emission, and structural properties of InN/Si (111) QDs is demonstrated by changing the growth kinetics parameters such as flux rate and growth time. Due to the large lattice mismatch, between InN and Si (~8%), the dots formed from the Strannski–Krastanow (S–K) growth mode are dislocated. Despite the variations in strain (residual) and the shape, both the dot size and pair separation distribution show the scaling behavior. We observed that the distribution of dot sizes, for samples grown under varying conditions, follow the scaling function.
TL;DR: In this article, the temperature-dependent photoluminescence spectra showed an anomalous behavior with an S-like shape of free exciton (FX) emission peaks, which indicated the silicon nitride layer, which not only acts as a growth buffer layer, but also prevents Si diffusion from the substrate to GaN epilayers.
TL;DR: In this article, the effect of annealing of silicon nitride surface was investigated with core-level photoelectron spectroscopy, revealing a characteristic series of components for the Si species, not only in stoichiometric Si 3 N 4 (Si 4+ ) but also in the intermediate nitridation states with one (Si 1+ ) or three (Si 3+ ) nearest neighbors.
TL;DR: In this article, the effect of N/Ga flux ratio on structural, morphological, and optical properties of GaN films on c-plane sapphire by plasma-assisted molecular beam epitaxy (PAMBE) was studied.
Abstract: GaN films were grown on c-plane sapphire by plasma-assisted molecular beam epitaxy (PAMBE). The effect of N/Ga flux ratio on structural, morphological, and optical properties was studied. The dislocation density found to increase with increasing the N/Ga ratio. The surface morphology of the films as seen by scanning electron microscopy shows pits on the surface and found that the pit density on the surface increases with N/Ga ratio. The room temperature photoluminescence study reveals the shift in band-edge emission toward the lower energy with increase in N/Ga ratio. This is believed to arise from the reduction in compressive stress in the films as is evidenced by room temperature Raman study. The transport studied on the Pt/GaN Schottky diodes showed a significant increase in leakage current with an increase in N/Ga ratio and was found to be caused by the increase in pit density as well as increase in dislocation density in the GaN films.
TL;DR: In this paper, a surface bandgap of InN QDs was estimated from scanning tunneling spectroscopy (STS I-V curves and found that it is strongly dependent on the size of QDs.
Abstract: InN quantum dots (QDs) were grown on Si (111) by epitaxial Stranski-Krastanow growth mode using plasma-assisted molecular beam epitaxy. Single-crystalline wurtzite structure of InN QDs was verified by the x-ray diffraction and transmission electron microscopy. Scanning tunneling microscopy has been used to probe the structural aspects of QDs. A surface bandgap of InN QDs was estimated from scanning tunneling spectroscopy (STS) I-V curves and found that it is strongly dependent on the size of QDs. The observed size-dependent STS bandgap energy shifts with diameter and height were theoretical explained based on an effective mass approximation with finite-depth square-well potential model.
TL;DR: In this paper, the electrical transport behavior of n-n indium nitride nanodot-silicon (InN ND-Si) heterostructure Schottky diodes is reported.
Abstract: The electrical transport behavior of n-n indium nitride nanodot-silicon (InN ND-Si) heterostructure Schottky diodes is reported here, which have been fabricated by plasma-assisted molecular beam epitaxy. InN ND structures were grown on a 20 nm In N buffer layer on Si substrates. These dots were found to be single crystalline and grown along [0 0 0 1] direction. Temperature-dependent current density-voltage plots (J-V-T) reveal that the ideality factor (η) and Schottky barrier height (SBH) (ΦB) are temperature dependent. The incorrect values of the Richardson constant (A**) produced suggest an inhomogeneous barrier. Descriptions of the experimental results were explained by using two models. First one is barrier height inhomogeneities (BHIs) model, in which considering an effective area of the inhomogeneous contact provided a procedure for a correct determination of A**. The Richardson constant is extracted -110 A cm-2 K-2 using the BHI model and that is in very good agreement with the theoretical value of 112 A cm-2 K-2. The second model uses Gaussian statistics and by this, mean barrier height Φ0 and A** were found to be 0.69 eV and 113 A cm-2 K-2, respectively.
TL;DR: In this paper, the removal of native oxide from Si (1 1 1) surfaces was investigated by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectra (SIMS) depth profiles.
TL;DR: In this paper, the incorporation of oxygen in the outer shell of indium nitride (InN) quantum dots was confirmed by the results of transmission electron microscopy (TEM), X-ray photoemission spectroscopy (XPS).
Abstract: 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
TL;DR: In this paper, the valence band offset (VBO) of wurtzite InN/Ge heterojunction is determined by X-ray photoemission spectroscopy (XPS).
22 Dec 2011
TL;DR: In this work the image is decomposed using DT-CWT to produce eight complex sub-bands and the features are extracted from the magnitude of low frequency band (LL) using Principal Component analysis (PCA).
Abstract: In this paper the suitability of Dual Tree Complex Wavelet Transform for Face Recognition is studied. In contrast to the discrete wavelet transform (DWT), the design of Dual Tree Complex Wavelet Transform poses good directional properties for diagonal features and is rugged to shift Invariance. These features of DT-CWT motivated to study their suitability for Face Feature Extraction and Recognition, as the features of face are oriented in different directions. In this work the image is decomposed using DT-CWT to produce eight complex sub-bands and the features are extracted from the magnitude of low frequency band (LL) using Principal Component analysis (PCA). ORL database is used, as the database consists of variation in pose and expression. Recognition rate achieved on ORL database using DT-CWT is satisfactory.