S. Bhunia

Metalorganic vapor-phase epitaxial growth and characterization of vertical InP nanowires

Highly dense and free-standing InP nanowire structures of vertical orientation were grown by the metalorganic vapor-phase epitaxial technique using colloidal Au nanoparticles as the catalyst. Scanning electron microscopy and transmission electron microscopy showed that the nanowires were single crystalline with 〈111〉 growth direction and of uniform length of about 700 nm, and most of them had diameter in the range of 20–25 nm. Photoluminescence measurements, carried out at room temperature as well as at 77 K, showed a significant blueshift in the peak position compared to bulk InP due to the quantum confinement of the carriers in the nanowires. The successful growth of these nanowires opens up the possibility of realizing various nanoscale devices on the wafer scale in the bottom-up approach.

Polarized cathodoluminescence study of InP nanowires by transmission electron microscopy

Cathodoluminescence (CL) spectra and polarized monochromatic CL images of InP nanowires were studied using a transmission electron microscope combined with a CL detection system. Emission spectra from individual nanowires showed a broad single peak with a peak energy of 1.6 eV, which is higher than expected from the quantum confinement effect for an average nanowire with a diameter of 20 nm. An individual nanowire could be resolved in the monochromatic CL images. The emission is highly polarized along the wire axis with a degree of polarization of more than 50%.

Schottky barrier studies on single crystal ZnTe and determination of interface index

Detailed studies on metal-(111) p-ZnTe Schottky barriers have been carried out using In, Ag, Al, and Cu as barrier metals. Weak dependence of barrier height on metal work function was observed. The highest and lowest barrier heights of 0.99 and 0.80 eV were found for In and Cu respectively which had lowest and highest work functions. The ideality factor n was found to vary between 1.84 for In and 2.13 for Al contacts. The Fermi level was found to be pinned effectively by interface states, the density of which was calculated to be 4.7×1013 states/cm2/eV. From the current–voltage characteristics measured between 250 and 350 K, the effective Richardson constant A** was determined to be 72±6 A/cm2/K2. This agrees very well with the theoretically calculated value of A** for a hole effective mass mh*=0.6m0. The temperature variation of barrier height was also determined from the capacitance–voltage characteristics. The interface index a parameter used to describe the pinning strength of semiconductors was found...

Determination of well widths in a MOVPE-grown InGaAs/InP multi-quantum well structure using SIMS and photoluminescence

Abstract The well and barrier widths of MOVPE-grown In 1− x Ga x As/InP ( x =0.47) quantum well (QW) structures have been examined by secondary ion mass spectroscopy (SIMS). The well widths varying from 25 to 150 A were also estimated through photoluminescence (PL) measurements. It was found that the differences in estimation of well width by PL and SIMS are due to differences in sputtering rates between InGaAs and InP during SIMS measurements, the rate being found to be 1.38–1.97 times less for the former. The presence of interfacial layers of ∼2 monolayers width between well and barrier with low sputtering rate was also inferred.

Effect of Sol-Gel-Derived Nano-silica on the Properties of Natural Rubber-Poly Butadiene Rubber-Reclaim Rubber Ternary Blends/Silica Nanocomposites

Silica incorporation into natural rubber (NR)-polybutadiene rubber (PBR)-reclaim rubber (RR) ternary blend system was carried out by sol-gel technique at different temperatures. The effect of RR on silica reinforcement was studied for NR-PBR-RR blend systems. The physicochemical properties of sol-gel vulcanizates indicates that the reinforcing efficiency of the nanocomposites increases with increasing RR content. Sol-gel vulcanizates prepared at 50°C shows superior mechanical properties than others. The amount of silica incorporated by sol-gel technique was determined through thermogravimetry analysis, which indicates the increasing trend of thermal stability with silica content. SEM studies indicate the coherency and homogeneity in the NR-PBR-RR/SiO 2 nanocomposites.

STEADY-STATE AND TIME-RESOLVED PHOTOCONDUCTIVITY MEASUREMENTS OF MINORITY CARRIER LIFETIME IN ZNTE

Minority carrier lifetime in ZnTe has been determined from steady-state and time-resolved photoconductivity (PC) measurements. Three types of single crystal p-ZnTe (E G = 2.26 eV ) grown by the Bridgman technique were studied : i) as-grown - before and after hydrogen passivation, ii) Zn annealed and iii) In - doped semi-insulating. Steady-state photoconductivity was studied between 80 - 300 K and showed that for as-grown samples, the lifetime went through a sharp maximum of 4.5 x 10 -7 s at 220K, decreasing to 2.5 x 10 -8 s at 300K. For hydrogen passivated samples, the lifetime remained high at 4.5 x 10 -7 s at 300K, due to passivation of deep acceptors O Te . Time-resolved photoconductivity measurements gave values in reasonably good agreement viz. 4.6 x 10 -8 s and 3.2 x 10 -7 s respectively for as-grown and Zn annealed samples. The radiative recombination constant B was thus found to be 1.4 x 10 -9 cm 3 s -1 and 4 x 10 -10 cm 3 s -1 respectively at 300 K for the two samples. PC spectral response studies showed a maximum at 2.41 eV at 300K with slight shift to 2.43 eV occurred on H passivation.

Real-time observation of surface morphology of indium phosphide MOVPE growth with using X-ray reflectivity technique

Abstract The results of real-time X-ray reflectivity measurements of MOVPE grown indium phosphide surface are presented. At the low growth temperature of 450°C, large decreases of reflectivity were observed, suggesting the formation of indium islands. At higher growth temperature of 550°C, only small changes were observed at high growth rate, indicating the step-flow growth mode. Oscillations longer than mono-layer growth were also observed at 500°C and 550°C, and roughness changes obtained from these oscillations were less than 0.01-nm, suggesting small islands formation on the terrace or step-edge fluctuation during the growth.

High-resolution x-ray diffraction analysis of epitaxially grown indium phosphide nanowires

Indium phosphide (InP) nanowires epitaxially grown on InP (111)B were investigated by using grazing incidence x-ray diffraction. A broad scattering at the tail of InP (22¯0) diffraction and an additional peak at the low angle side were observed, showing the formation of nanowires and alloys of the gold catalysts and indium. Scattering intensity along the [11¯0] direction was compared with calculations based on a cylinder model. The best fit was obtained for a 5.5-nm radius with a 2.5-nm deviation, which was smaller than the values determined from the secondary electron microscopy and photoluminescence spectroscopy measurements. This result is explained by an oxide layer on the nanowire sidewalls and the low quantum efficiency of photoluminescence yields for small nanowires since x-ray diffraction directly detects crystalline structure of nanowires.

Microstructure and dielectric functions of Ge nanocrystals embedded between amorphous Al2O3 films: study of confinement and disorder

Ge nanocrystals (6–9 nm) embedded between amorphous Al2O3 films were produced in a cluster beam deposition system. The microstructural evaluation and compressive stress experienced by the Ge nanocrystals due to the presence of an oxide layer, nanoparticle size distribution and their changes due to thermal annealing were studied by X-ray diffraction, HRTEM and Raman spectroscopy. Spectroscopic ellipsometry was used to measure the dielectric functions of the deposited films. A multilayer model based on the effective medium approximation was used to analyze the variation of percentage of defects and the extent of disorder with particle size of the nanocrystals. The correlation between the microstructural characteristics and optical properties was established by evaluating standard sum rules. Germanium nanocrystals show visible photo luminescence at room temperature around 3.0 and 2.8 eV. However, a peak shift towards lower energies with increasing particle size due to thermal annealing was not detected. The experimentally observed luminescence is presumably originated due to the presence of oxide-related defect centers at the interface between the germanium nanocrystals and the embedded oxide layers.

High resistivity In-doped ZnTe: electrical and optical properties

Semi-insulating <111> ZnTe prepared by In doping during Bridgman growth was found to have a resistivity of 5.74 × 107 ohm-cm, the highest reported so far in ZnTe, with hole concentration of 2.4 × 109/cm3 and hole mobility of 46 cm2 /V.s at 300 K. The optical band gap was 2.06 eV at 293 K compared with 2.26 eV for undoped semiconducting ZnTe. Thermally stimulated current (TSC) studies revealed 2 trap levels at depths of 202–222 meV and 412–419 meV, respectively. Photoluminescence (PL) studies at 10 K showed strong peaks at 1.37 eV and 1.03 eV with a weak shoulder at 1.43 eV. Short anneal for 3 min at 250°C led to conversion to a p-type material with resistivity, 14.5 ohm-cm, indicating metastable behaviour. Raman studies carried out on undoped and In-doped samples showed small but significant differences. Possible models for semi-insulating behaviour and meta-stability are proposed.