Eka Nurfani

Defect-induced excitonic recombination in Ti x Zn1− x O thin films grown by DC-unbalanced magnetron sputtering

We study the effects of Ti doping on the near-band-edge emission (NBE) and defect-related deep-level emission (DLE) of ZnO thin films grown by DC unbalanced magnetron sputtering. DLE in pure ZnO is contributed by zinc and oxygen vacancies (VZn+VO), as revealed by photoluminescence (PL) spectroscopy, current–voltage (I–V) characteristic measurement, and spectroscopic ellipsometry. The reduction in the number of VZn states is clearly observed upon Ti doping, resulting in the enhancement of green emission from VO. Interestingly, the thin film with a Ti concentration of 1 at. % shows a higher excitonic emission. Furthermore, the temperature dependence of PL spectra shows that the enhanced excitonic emission originates from the donor-bound exciton promoted by the Ti dopant and native VO. This study shows an important role of the defects in controlling the optical and electronic properties of ZnO films for future optoelectronic applications.

Polarity enhancement in high oriented ZnO films on Si (100) substrate

Zinc oxide films with hexagonal crystal structures have been grown on Si substrate (100) using the DC-unbalanced magnetron sputtering at temperature of 300°C with growth time variation. The films have been characterized using X-Ray Diffraction and Fourier Transform Infrared measurement to show the crystal parameter and atomic bonding. The results show that the films have a dominant orientation in the hkl plane (002), while peak positions are shift to lower diffraction angle with addition of growth time. In addition, the bonding between Zn and O atoms (Zn-O) tend to be weaker as indicated by transmittance peak shifting to lower energy. The weakening of Zn-O bonding is due to the contribution of addition of bond length. These conditions make the films tend to have a high polarity. Further analysis of these studies will bring us to have a good undestanding to explain the ferroelectric properties of the ZnO films.

Effect of Ta concentration on the refractive index of TiO2:Ta studied by spectroscopic ellipsometry

We have investigated optical properties of Ta-doped TiO2 thin film on LaAlO3 (LAO) substrate using Spectroscopic Ellipsometry (SE) at room temperature. Amplitude ratio Ψ and phase difference L1 between p- and s- polarized light waves are obtained by multiple incident angles measurement (60°, 70°, and 80°) at energy range of 0.5 – 6.5 eV. In order to obtain optical properties for every Ta concentrations (0.01, 0.4, and 5 at. %), multilayer modelling was performed simultaneously by using Drude-Lorentz model. Refractive index and optical dispersion parameters were determined by Wemple-DiDomenico relation. In general, refractive index at zero photon energy n(0) increases by increasing Ta concentration. Furthermore, optical band gap shows a significant increasing due to presence of Ta dopant. In addition, other optical constants are discussed as well.

Electrical properties of ZnO-based photodetector prepared by room temperature DC unbalanced magnetron sputtering

We study electrical properties of ZnO thin films on p-Si (100) substrates as ultraviolet (UV) photodetector. ZnO films with thickness of ~400nm were deposited by room temperature (RT) DC unbalanced magnetron sputtering (DC-UBMS), and we also used ZnO film deposited at 260°C for the comparison. Metal-semiconductor-metal (MSM) planar structure was fabricated by using silver (Ag) contact on ZnO thin films as photodetector. X-ray diffraction (XRD) spectroscopy shows the amorphous structure of RT growth of ZnO thin films in contrast to relatively good crystallinity of ZnO film grown at 260°C. I-V characteristics of reverse-forward bias of UV photodetector were taken in dark and light conditions. As the results, amorphous phase of RT ZnO shows the high value of dark current and photocurrent. High stability of photo detection is also observable over the specific time as investigated by switching condition. In addition, we found that the mechanism of photo detection is strongly related to the oxygen adsorption on the ZnO surface. This study provides an alternative way to have a simple and high sensitive UV photodetector.

Structural and optical properties analysis of MoS2 nanoflakes on quartz substrate as prepared by mechanical exfoliation

We study the structural and optical properties of MoS2 nanoflakes on quartz substrate as prepared by mechanical exfoliation method. The structural and morphological properties of MoS2 nanoflakes were characterized by SEM, EDS, and XRD, while the high-resolution spectroscopic ellipsometry (SE) with the photon energy of 1.27 to 6.53 eV is used to study its optical characteristics. As the results, SEM data shows that MoS2 appears to be nanoflakes covering around 25-35% on the surface of quartz substrate, and XRD spectra shows the dominant orientation along c-axis (002). Based on spectroscopic ellipsometry analysis, the average thickness of MoS2 nanoflakes is around 12 nm or about 6-8 layers. By using Tauc plot method, we confirm that MoS2 nanoflakes have a semiconductor characteristic with the optical bandgap as high as 1.68 eV. Our study shows the important of structural and optical properties of MoS2 nanoflakes that can be utilized for future optoelectronic devices and energy-harvesting purposes.

The effect of surface morphology in copper oxide nanostructure to photo detector characteristics

We study the effect of structural properties of copper oxide nanostructure and its sensitivity for photo detector application. Copper oxide (CuO) nanostructures were prepared by combining the thermal evaporation with dry oxidation. We select CuO system at different thickness (around 90, 170, and 270 nm) and X-ray diffraction spectra confirms their polycrystalline structure. Scanning electron microscope images show the distinct CuO morphology surface for each sample on which the grain shape is changed and the size is grown with the increasing of the CuO thickness. Fourier-transform infra-red spectra reveals the high penetration depth of oxygen at ~90 nm thick sample indicated by large Si-O-Si bond intensity. Photo current characterization for each sample is carried out to compare its light sensing properties. We found that the thickness of CuO nanostructure system is related to the sensitivity of device during the light exposure in which thinner samples have better performance. Furthermore different band gap for each CuO sample is predicted from its sensitivity at continuous light exposure. We suggest that the grain boundary and the distinct morphology might promote a unique confined nanostructure effect, resulting in band gap widening. These studies demonstrate a new approach for tunable photonic device efficiency that could be beneficial in reducing energy loss during energy conversion.

The enhancement of ferromagnetism in Ta-doped anatase TiO2 system by iron co-doping

The ferromagnetic properties of Ta-doped and (Ta,Fe)-doped anatase TiO2 as diluted ferromagnetic materials has been studied within spin-polarized generalized gradient approximation (GGA) and GGA + U method. We observed a ferromagnetic properties in Ta- doped anatase TiO2, with a total magnetic moment of 1.00 μ B /supercell, which mainly arised due to Ti 3d and O 2p states upon Ef. Furthermore, the enhancement of ferromagnetism, mainly arising due to Fe 3d states, in (Ta,Fe)-doped anatase TiO2 was observed up to 5.00 μB /supercell, 5 times larger than that in Ta-doped TiO2. Our results confirmed the important role of Fe doping for the ferromagnetism enhancement in Ta-doped anatase TiO2.

Optical properties analysis of Ta-doped TiO2 thin films on LaAlO3 substrates

We study optical properties of Ta-doped TiO2 thin film on LaAlO3 substrate using spectroscopy ellipsometry (SE) analysis at energy range of 0.5 – 6.5 eV. Room temperature SE data for Ψ (amplitude ratio) and Δ (phase difference) between p- and s- polarized light waves are taken with multiple incident angles at several spots on the samples. Here, absorption coefficient has been extracted from SE measurements at photon incident angle of 70° for different Ta concentration (0.01, 0.4, and 5 at. %). Multilayer modelling is performed which takes into account reflections at each interface through Fresnel coefficients to obtain reasonably well the fitting of Ψ and Δ data simultaneously. As the results, we estimate that film thickness increases by increasing Ta concentration accompanied by the formation of a new electronic structure. By increasing Ta impurities, the blueshift of absorption coefficient (α) peaks is observable. This result indicates that TiO2 thin film becomes optically resistive by introducing Ta do...