K. Jeganathan

Raman scattering of phonon-plasmon coupled modes in self-assembled GaN nanowires

We report the determination of free-electron concentration and mobility of free-standing GaN nanowires (NWs) by line shape analysis of the coupled longitudinal optical phonon-plasmon Raman modes (L+). The E2high phonon mode at 566.9 cm−1 with a sharp linewidth of 2.8 cm−1 indicates strain free NWs with high crystalline perfection. The lattice temperature of the NWs was varied between 313 and 472 K by varying the excitation laser beam power. For unintentionally doped samples at room temperature, an average electron concentration and mobility of strain free NWs were found to be ∼2×1017 cm−3 and 460 cm2/V s, respectively. We have shown that the electron concentration does not change significantly over a temperature range between 313 and 472 K. The electron mobility decreases at high temperatures, in agreement with literature data for compact layers. For Si-doped NWs, the L+ phonon peak is strongly upshifted indicating a higher free-carrier concentration of about 1×1018 cm−3. Asymmetric broadening observed at...

Electrical transport properties of single undoped and n-type doped InN nanowires

Electrical transport properties of undoped and n-type doped InN nanowires grown by molecular beam epitaxy were studied by current-voltage and back-gate field-effect transistor measurements. The current-voltage characteristics show ohmic behavior in the temperature range between 4 and 300 K. Down to about 120 K a linear decrease in resistance with temperature is observed. The investigation of a large number of nanowires revealed for undoped as well as doped wires an approximately linear relation between the normalized conductance and diameter for wires with a diameter below 100 nm. This shows that the main conduction takes place in the tubular surface accumulation layer of the wires. In contrast, for doped wires with a diameter larger than 100 nm a quadratic dependence of conduction on the diameter was found, which is attributed to bulk conductance as the main contribution. The successful doping of the wires is confirmed by an enhanced conduction and by the results of the back-gate field-effect transistor measurements.

Probing the electron density in undoped, Si-doped, and Mg-doped InN nanowires by means of Raman scattering

We report a Raman scattering determination of the electron density in InN nanowires from the analysis of longitudinal optical-phonon-plasmon coupled modes. A Raman peak assigned to the L− coupled mode is observed in both undoped and doped InN nanowires. This peak exhibits a shift to higher (lower) frequencies in the Si-doped (Mg-doped) nanowires and allows us to estimate the electron density in the nanowires. A significant residual electron density is found in the undoped nanowires, which increases in Si-doped nanowires and is partially compensated in Mg-doped nanowires.

Raman scattering on intrinsic surface electron accumulation of InN nanowires

An intrinsic property of vertically aligned InN nanowire (NW) ensembles have been investigated by analysis of coupled longitudinal optical (LO) phonon mode using μ-Raman scattering. Spectra were recorded in backscattering geometries in parallel and perpendicular to the axis of the NWs. The width of surface accumulation layer is estimated from the LO phonon peak intensity ratios. The carrier concentration is extracted to be 6.7×1016 cm−3. The pronounced peak at 627.2 cm−1 is related to the interaction of phonons with surface electrons. The surface charge density, Nsc is calculated to be ∼2.55×1013 cm−2 which provides surface accumulation field strength of 5.5 Mv/cm.

Investigation of Molybdenum Doped ZnO Thin Films Prepared by Spray Pyrolysis Technique

Molybdenum doped zinc oxide thin films were deposited on the Corning glass substrates pre-heated to 400°C by spray pyrolysis technique. The effect of molybdenum doping concentration (0–2 at.%) on the structural and optical properties of zinc oxide thin films was studied. X-ray diffraction analysis confirmed that the molybdenum doped ZnO films belong to the hexagonal crystal structure of ZnO. Atomic force microscopic studies revealed the effect of molybdenum doping on the surface morphology of these films. The 1 at.% Mo doped ZnO exhibited highest average transmittance of ∼87% in the visible region (400–800 nm). Optical bandgap energy of the ZnO and Mo doped ZnO films lies in the range of 3.28–3.33 eV. Annealing of the high resistive ZnO and Mo doped ZnO films at 400°C in vacuum for 60 min improved their resistivity. The minimum resistivity of 4.7 × 10−2 Ω cm and the maximum carrier concentration of 1.27 × 1019 cm−3 were achieved for 1 at.% Mo doped ZnO films.

Selective area growth of Bernal bilayer epitaxial graphene on 4H-SiC (0001) substrate by electron-beam irradiation

We report selective area growth of large area homogeneous Bernal stacked bilayer epitaxial graphene (BLEG) on 4H-SiC (0001) substrate by electron-beam irradiation. Sublimation of Si occurs by energetic electron irradiations on SiC surface via breaking of Si–C bonds in the localized region, which allows the selective growth of graphene. Raman measurements ensure the formation of homogeneous BLEG with weak compressive strain of −0.08%. The carrier mobility of large area BLEG is ∼5100 cm2 V−1 s−1 with a sheet carrier density of 2.2 × 1013 cm−2. Current-voltage measurements reveal that BLEG on 4H-SiC forms a Schottky junction with an operation at mA level. Our study reveals that the barrier height at the Schottky junction is low (∼0.58 eV) due to the Fermi-level pinning above the Dirac point.

Properties of uniform diameter InN nanowires obtained under Si doping

High quality, well-separated, homogeneous sizes and high aspect ratio Si-doped InN nanowires (NWs) were grown by catalyst-free molecular beam epitaxy (MBE) after optimization of the growth conditions. To this end, statistical analysis of NW density and size distribution was performed. The high crystal quality and smooth NW surfaces were observed by high resolution transmission electron microscopy. Spectral photoluminescence has shown the increase of the band filling effect with Si flux, indicating successful n-type doping. A Raman LO scattering mode appears with a pronounced low energy tail, also reported for highly doped InN films.

Ferromagnetism in undoped One-dimensional GaN Nanowires

We report an intrinsic ferromagnetism in vertical aligned GaN nanowires (NW) fabricated by molecular beam epitaxy without any external catalyst. The magnetization saturates at ∼0.75 × emu/gm with the applied field of 3000 Oe for the NWs grown under the low-Gallium flux of 2.4 × 10−8 mbar. Despite a drop in saturation magnetization, narrow hysteresis loop remains intact regardless of Gallium flux. Magnetization in vertical standing GaN NWs is consistent with the spectral analysis of low-temperature photoluminescence pertaining to Ga-vacancies associated structural defects at the nanoscale.

Self-assembled growth of GaN nanowires

GaN nanowires (NWs) have been grown on Si(111) substrates byt plasma-assisted moleucular bearn epitaxy. The Morphology and optical properties of the NWs are influenced by te growth parameters as investigated by the scanning electron microscope. The nucleation process of GaN-NWs is explained in terms of nucleation density and wire evolution with time. The wire length in the nucleation stage shows a linear time dependence. The wire density increases rapidly with time and then it saturates. We explain GaN-NWs growth by making use of the diffusion-induced (D-I) mechanism that explains the dependence of the length on wire diameter for a deposition time longer than the nucleation stage.