Solid State Phenomena Vol. 301

Abstract: The fabrication of porous GaN (PGaN) by UV-assisted electrochemical etching with a variations of current densities (40, 60, and 80 mA/cm²) for 60 min in electrolytes consisting of 4% KOH are reported. Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-Ray (EDX), Atomic Force Microscopy (AFM) and X-ray Diffraction (XRD) were used to characterize the morphological and structural characteristics of the PGaN. All PGaN sample prepared by electrochemical etching technique produced a hexagonal-like pore shape. FESEM images demonstrated that the pore uniformity and porosity are affected significantly by the current density. The PGaN sample fabricated with 80 mA/cm² produces a uniform and high porosity structure compared to other PGaN sample. This shows that the morphology and structural characteristic of PGaN are increase with the increase of current density. The EDX result revealed significant Ga and N atom presence in all samples. However, the O atom only presence in sample etched with 80 mA/cm² implying that the etching process is occur vigorously in this sample. The AFM verified that the surface roughness and the pore depth are increased as current density increased. There were relatively large variations of the peak intensities for 2Theta-scan patterns as exposed by XRD. The peak shift for PGaN sample relative to as-grown was inconsistent and the changed was relatively small. Raman intensity found to be enhanced with the increase in current density and among the PGaN sample, the E₂(high) peak for sample prepared with 60mA/cm² and 80mA/cm² was observed to be slightly shifted to lower frequency. The PL spectra displayed that the porosity has high impact on the PL peak intensity. . Overall, this proved that with the usage of low power UV light, the pore structure still can be produced as good as pore structure fabricated with high power UV light.

Abstract: A set of n-type porous silicon (PS) layers were fabricated by photoelectrochemical etching using direct current (DC) and pulse current (PC) techniques. The study aims to compare the effect of different resistivity (5 Ω and 10 Ω) on the formation of the PS structure. The samples were etched in a solution of HF:C₂H₆O with a composition ratio of 1:4. The etching process were done for 30 minutes with the current density of J = 10 mA/cm². In the time of PC etching process, the current was supplied through a pulse generator with 14 ms cycle time (T) which the on time (T_on) set to 10 ms and pause time (T_off) set to 4 ms respectively. The samples were then being characterized in terms of surface morphology by using FESEM, AFM and XRD. Through the FESEM results, it can be seen that sample with 10 Ω resistivity which using PC form a more homogeneous structure of pores as compared to other samples.

Abstract: Lanthanum Oxide-Polyvinyl alcohol (La₂O₃-PVA) phosphor nanofibers were prepared by electrospinning and investigation of the effect of needle diameter on the optical properties and morphological structure at the nanofibers was carried out. The average diameter of phosphor nanofibers have been evaluated for five different needle diameters. The resulting phosphor nanofibers were observed and anaylzed by X-ray diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM), Ultraviolet-visible Spectroscopy (UV-Vis) and HPC-2 Lightsource Colorimeter. The La₂O₃-PVA phosphor nanofibers were used in light down-conversion of UV light (365 nm) for the white light-emitting diode (WLED) application. The chromaticity coordinates (CIE) and correlated colour temperature (CCT) were measured for different phosphor nanofibers with different sizes of needle diameter (0.45,0.55,0.65,0.75,1.1) mm. The results revealed that the phosphor nanofibers formed by needle diameter of 0.65 mm places macromolecule in a better order and the average diameter increase from 40.08 nm to 87.27 nm after annealing followed by increasing in atomic percentage of lanthanum. The energy gap of resulting phosphor nanofibers is 3.5365 eV. An optimum colour rendering index (CRI) value of 70.20 is obtained. The white phosphor exhibited CIE values of 0.3536, 0.407 and CCT of 4890 K.

Abstract: Gallium oxide (β-Ga₂O₃) nanostructures (NSs) have been successfully obtained through a simple scalable synthesis via thermal evaporation of gallium (III) oxide powder in hydrogen-ambient chemical vapour deposition (HACVD) without the presence of carrier gas. β-Ga₂O₃ was deposited on Si substrate by evaporating the source material at 1000 C in a regulated hydrogen reducing atmosphere, for 120 min growth time. Hydrogen ambient was regulated by varying the flow to observed changes in the morphological, structural and optical properties of films. The samples were characterized using high resolution X-ray diffraction (HR-XRD), field-emission scanning electron microscope (FE-SEM) and UV-vis-NIR spectrophotometer. The density and quality of NSs was observed to increase with hydrogen gas supply. The rarely reported 1) dominant XRD peak of β-Ga₂O₃ was obtained in the event of eliminating the carrier gas which is part of the usual recipe for CVD technique. The average crystallite size and energy gap of the synthesized material was found to decrease with increased hydrogen flow rate from 176.5 to 39.8 nm and 5.47 to 4.83 eV, respectively.

Abstract: In this work, ZnO thin films were prepared by the low-cost sol-gel deposition method onto six different substrates (glass, ITO coated glass, sapphire (Al₂O₃), p-Si, p-GaN and polyethylene terephthalate (PET)) to study the effects of these substrates on the morphological and structural properties of the produced films. Precursor solution is Zinc acetate dihydrate based dissolved in ethanol with monoethanolamine (C₂H₇NO) added to act as a stabilizing agent to the sol. The corresponding ZnO thin films were characterized using field emission scanning electron microscopy (FESEM), high resolution X-ray diffraction (XRD) and atomic force microscopy (AFM). Results revealed distinct morphological and structural properties of ZnO thin films deposited on each substrate. The most uniform morphology was identified on glass, owing to the acquisition of the averagely stable grain sizes (58 nm – 61 nm) and thin film thicknesses (280 nm – 325 nm). High resolution XRD analysis showed that the films deposited on glass, ITO, p-Si, and p-GaN were attributed to hexagonal crystallite structures while the films deposited on sapphire and PET substrates exhibited amorphous phases. Amongst the samples, the ZnO thin film spin coated on p-Si demonstrated preferred orientation in (002) direction.

Abstract: This study presents an investigation on copper oxide (CuO) composited with polyethylene glycol (PEG) thin film. The main objective for this compositing thin film is to explore and determine the characteristic of the thin film when the value of PEG is varied. In this study, PEG plays a role as phase change material (PCM), where this type of material has the capability to absorb and release heat based on its surrounding condition. Due to this characteristic, the amount of PEG is vary from 0.5, 1.0, 1.5 and 2.0 ml to confirm its influence towards CuO thin film. As for comparison, CuO thin film without PEG is also fabricated. Both of the materials used were prepared using sol-gel route and deposited onto indium tin oxide (ITO) substrate using spin coating method. The influences of the PEG values on CuO thin film were investigated using field emission scanning electron microscope (FESEM), x-ray diffraction (XRD), ultra-violet visible microscope (UV-Vis), and current-voltage (IV) measurement. Based on the investigation conducted, the characteristics of CuO thin film produced in this study are affected with the different value of PEG.

Abstract: White light-emitting diode (LED) is used in a new generation of solid-state lighting due to its advantages in energy saving and environmental friendliness. Based on this assumption, Emphasis was put on trying to establish a laboratory technique to convert UV-blue light into white light by using polymeric materials. In this work, an laboratory technique to convert UV-blue light into white light by using polymeric materials, consisting of red (R), green (G) and blue (B) for a white light generation. The project employed the use Colouring polymorph plastic as an active materials mixed with Silicone Sealant in different ratio and pumping by UV-Blue light. Colour rendering index (CRI) and correlated colour temperature (CCT) as main measurement parameters to evaluate the performance of the white light. The best white light appearance an indicated by photo and colour meter were achieved by mixing red (R), 0.05gm and green (G), 0.1gm the optimum results were CCT =3606k, CRI =70.3, x=0.3661, y=0.2925, and by mixing red (R), 0.005gm and green (G), 0.005gm the results were CCT=4891 k, CRI =63.8 and x=0.3359, y=0.2405.

Abstract: The behaviors of yttrium aluminum garnet (YAG) phosphor powder doped by cerium (YAG:Ce³⁺) was studied. Here, the YAG:Ce³⁺ nanopowders have been synthesized using microwave combustion (MW) according to the formula, Y_(3-0.04) Al₅O₁₂: 0.04Ce³⁺ to produce white light emitting diode (WLEDs) by conversion from blue indium gallium nitride light emitting diode (InGaN LED, 445 nm) chips; the whole process took only 20 min. Pure YAG phase was obtained after annealing at 1050 °C for 5 h with nonaggregated and spherical particles. Thermogravimetry and derivative thermogravimetry (TG/DTG), X-ray powder diffraction (XRPD), transmission electron microscope (TEM), photoluminescence (PL), electroluminescence (EL) emissions and standard CIE 1931 chromaticity diagrams have been used to characterize the samples. The highest WLEDs emission was achieved for the annealed YAG:Ce³⁺, together with proper color rendering index (CRI), and tunable correlated color temperature (CCT). Finally, we conclude that the decreasing EL intensity at increased injection current is caused by the thermal ionization from the 5d₁ level to the conduction band.

Abstract: White LEDs (WLEDs) have been produced from the combination of blue LED chips and phosphor converter. In the present work, considerable amount of yttrium aluminium garnet (YAG) phosphor powders doped with 0.3 mol% of cerium (Ce) have been synthesized via Microwave Induced Combustion Synthesis (MICS) method with different fuel sources such as urea and mixed fuel of urea and glycine. The effects of different fuel sources on the crystallinity, structure, luminescent properties and Commision International de L’Eclairage (CIE) chromaticity was characterized and studied using high resolution X-ray diffraction (HR XRD), field emission-scanning electron microscopy (FE SEM), energy dispersive X-ray spectroscopy (EDX), electroluminescence (EL) and standard CIE 1931 chromaticity diagram, respectively. The highest EL intensity can be observed from the sample prepared by mixed fuel technique. In contrast, the experimental enhancement in the aforementioned properties was demonstrated by the WLED synthesized using mixed fuel technique. Keywords: White LEDs, phosphor converter, microwave induced combustion, mixed fuel technique.