Zulkafli Othaman

Optical behavior of self-assembled high-density Ge nanoislands embedded in SiO2

The radio frequency magnetron sputtering method is used to prepare well-dispersed pyramidal-shaped Ge nanoislands embedded in amorphous SiO2 sublayers of various thicknesses. The estimated size and number density of Ge nanoislands in SiO2 sublayer thicknesses beyond 30 nm are approximately 15 nm and 1011 cm-2, respectively. Atomic force microscopy (AFM) reveals root mean square (RMS) roughness sensitivity as the SiO2 sublayer thickness varies from 30 to 40 nm. The formation of nanoislands with high aspect ratios is attributed to the higher rate of surface reactions between Ge adatoms and nucleated Ge islands than reactions associated with SiO2 and Ge. The Ge nanoisland polyorientation on SiO2 (50-nm thickness) is revealed by X-ray diffraction (XRD) patterns. Photoluminescence (PL) peaks of 2.9 and 1.65 eV observed at room temperature (RT) are attributed to the radiative recombination of electrons and holes from the Ge nanoislands/SiO2 and SiO2/Si interfaces, respectively. The mean island sizes are determined by fitting the experimental Raman profile to two models, namely, the phonon confinement model and the size distribution combined with phonon confinement model. The latter model yields the best fit to the experimental data. We confirm that SiO2 matrix thickness variations play a significant role in the formation of Ge nanoislands mediated via the minimization of interfacial and strain energies.

The Effect of Sintering Temperature on the Structural and Magnetic Properties of Ni-Mg Substituted CoFe2O4 Nanoparticles

This study evaluates the structural and magnetic properties of Ni-Mg substituted Cobalt ferrite samples prepared through the co-precipitation method. The nominal compositions Co0.5Ni0.5−xMgx Fe2O4 in the range x = 0.1 have been synthesized and then was sintered at temperature at 700 and 1000°C in the furnace for 10 hour with a heating rate of 5°C/min. The prepared nanoferrites were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and vibration sample magnetometer (VSM). XRD confirmed formation of single phase spinel ferrite with average crystalline size in the range of 27–33 nm. The lattice constant (a), cell volume (V) and X-ray density (ρx) are also calculated from XRD data. Lattice constant (a) decreases with an increase of sintering temperature. Further information about the structure and morphology of the nanoferrites was obtained from FESEM and results are in good agreement with XRD. Saturation magnetization showed increasing trend with sintering temperature from 700 to 1000°C.

The Effect of V/III Ratio on the Crystal Structure of Gallium Arsenide Nanowires

Gallium arsenide (GaAs) nanowires were grown vertically on GaAs (111)B substrate by gold particle-assisted using metal-organic chemical vapour deposition. Transmission electron microscopy and X-Ray diffraction analysis were carried out to investigate the effects of V/III ratio and nanowire diameter on structural properties and crystallinity changes. Results show that GaAs nanowires grow preferably in the wurtzite crystal structure than zinc blende structure with increasing V/III ratio. Additionally, XRD studies have revealed that wurtzite nanowires show prominent peaks especially at (222) orientation. The optimum V/III ratio was found to be 166 with less defect structure, uniform diameter and peak prominence. The nanowires with high quality are needed in solar cells technology for energy trapping with maximum capacity.Keywords : Nanowire; crystal structure; Gallium arsenide; Vapor Liquid Solid

Structure and electrical characterization of gallium arsenide nanowires with different V/III ratio growth parameters

Gallium arsenide (GaAs) nanowires were grown vertically on GaAs(111)B substrate by gold-assisted using metal-organic chemical vapour deposition. Field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and conductivity atomic force microscopy (CAFM) analysis were carried out to investigate the effects of V/III ratio on structural properties and current-voltage changes in the wires. Results show that GaAs NWs grow preferably in the wurtzite crystal structure than zinc blende crystal structure with increasing V/III ratio. Additionally, CAFM studies have revealed that zincblende nanowires indicate ohmic characteristic compared to oscillation current occurred for wurtzite structures. The GaAs NWs with high quality structures are needed in solar cells technology for trapping energy that directly converts of sunlight into electricity with maximum capacity.

The Effect of Indium Mole Fraction on the Growth Behavior of Your InxGa1-xAs Nanowires (NWs) Grown Using MOCVD

InxGa1-xAs NWs have been grown with various indium mole fractions (x) using MOCVD. The morphology of InxGa1-xAs NWs was observed using Field Emission-Scanning Electron Microscopy (FE-SEM) in order to study the growth behavior of the NWs. FE-SEM results show that the NWs growth mechanism has changed due to changing of indium mole fraction. At low indium mole fraction, the NWs grew via direct impinging mechanism which has produced NWs with relatively uniform diameter. By increasing the value of x the growth mechanism has transformed to the combination of direct impinging and diffusion of source atoms from the surface of substrate causing tapering of NWs. The degree of tapering increases with increasing value of indium mole fraction. InxGa1-xAs NW grown at x = 0.65 has the highest tapering factor, TF = 12.82, whereas NW grown at x = 0.41 has the lowest tapering factor, TF = 2.76.

The Atomic Force Microscope Study of Self-Assembled Silicon Nanodots Growth Using Magnetron Sputtering System

In this paper self-assembled silicon nanodots have been grown on silicon substrate using radio-frequency magnetron sputtering system. This system were settled at varying experimental conditions such as substrate temperature, time of deposition, RF power and fixed argon flow rate. Then the surface roughness was measured by AFM which resulted average dots size of 113 nm. However, the presence of a small amount of grain atoms formed on the surface was confirmed using SEM measurement. The crystalline Si-NDs with (100) plane contributed sharp diffraction peak located at 69.5° was confirmed using XRD measurement. These results of Si-NDs structural properties support the possible growth technique of radio-frequency magnetron sputtering.

The Growth Mechanism of Silicon Nanodots Synthesized by Sputtering Method

Silicon quantum dots have been grown on sapphire substrate using a self‐assembly method of physical vapour deposition. The samples were fabricated at low sputtering rate and varying experimental conditions. Apparently, the onset of nucleation took place during the first 5 minutes of deposition, followed by a further growth of stable islands so‐called nanodots, with the measured radii comparable to the predicted values. Other measurement results confirmed the existence of these dots, including the bandgap energy ∼1.80 eV from PL and a 5% at. silicon from EDX. The nucleation parameters were predicted as follows: Free energy change per unit volume ΔGv∼−2.4×105u2009Jmol−1; Surface energies per unit area, γLNu2009=u20091.48u2009Jm−2, γNSu2009=u200921.6−88.3u2009Jcm−2 and γLSu2009=u20090.82×10−2u2009Jm−2; Critical energies ΔG*u2009=u20096.83×10−16−3.68×10−14u2009J; Critical radii r*u2009=u200920−72u2009nm. This experimental evidence strongly support the early stage growth model of silicon quantum dot deposited on corning glass substrate.

THE EFFECT OF In0.1Ga0.9As UNDERLYING LAYER ON THE STRUCTURAL PROPERTIES OF SELF-ASSEMBLED In0.5Ga0.5As QUANTUM DOTS

The effect of a thin In0.1Ga0.9As underlying layer on the structural properties of single layer In0.5Ga0.5As quantum dots (QDs) was investigated using atomic force microscopy (AFM), transmission electron microscopy (TEM) and high-resolution X-ray diffraction (HR-XRD) characterization. The size of dots formed on the surface is uniform but the density increases with the addition of In0.1Ga0.9As underlying between In0.5Ga0.5As QDs and GaAs buffer layer. This is consistent with the TEM characterization. The existence of thin underlying layer has caused the dots to have different crystal orientation as shown in TEM characterization. From the HR-XRD characterization, broad peak of In0.1Ga0.9As underlying layer and QDs has been observed. The wider width of the layer peak than the expected one has been attributed to the strain-relaxation-induced defects. The growth of a thin In0.1Ga0.9As underlying layer in the In0.5Ga0.5As/GaAs structures strongly affects the structural properties, which was also believed to influence the optical properties of QDs.

Morphology and structure of self-assembled inxGa1-xAs quantum dots grown on gaAs (100) using MOCVD

Stacked InxGa1−xAs quantum dots (QDs) were grown by metal‐organic chemical vapor deposition (MOCVD) via Stranski ‐Krastanov growth mode. AFM images show that the QDs structures were formed and the stacked structure of InxGa1−xAs QDs were confirmed by the HR‐XRD analysis. Composition of the dots on the top most layers was related to the number of stack layers. The observed PL peak position was blue‐shifted due to different number of stacked QDs. The PL intensity also dramatically increase, which shows the possibility of the QDs application in optical devices at room temperature

Influence of substrate orientation on the structural properties of GaAs nanowires in MOCVD

In this study, the effect of substrate orientation on the structural properties of GaAs nanowires grown by a metal organic chemical vapor deposition has been investigated. Gold colloids were used as catalyst to initiate the growth of nanowiresby the vapour-liquid-solid (VLS) mechanism. From the field-emission scanning electron microscopy (FE-SEM), the growth of the nanowires were at an elevation angle of 90°, 60°, 65° and 35° with respect to the GaAs substrate for (111)B, (311)B, (110) and (100) orientations respectively. The preferential NW growth direction is always B. High-resolution transmission electron microscope (HRTEM) micrograph showed the NWs that grew on the GaAs(111)B has more structural defects when compared to others. Energy dispersive X-ray analysis (EDX) indicated the presence of Au, Ga and As. The bigger diameter NWs dominates the (111)B substrate surface.