Inge M. Sutjahja

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.

Polarization behavior of zinc oxide thin films studied by temperature dependent spectroscopic ellipsometry

We report on the influence of temperature on the polarization behavior of highly oriented ZnO thin film. First, the investigation of crystal structure change is studied, providing supporting information on the macroscopic-scale polarization of the ZnO thin film. Here, the lattice distortion is investigated by using X-ray diffraction. Furthermore, the role of temperature on the polarization behavior of the ZnO thin film is comprehensively studied by using temperature dependent spectroscopic ellipsometry. Here, the temperature dependent dielectric function analysis and electronic excitation models are used to understand the mechanism of polarization. We found an interesting temperature dependence of electronic transition, where the red-shift absorption and exciton-phonon interaction are observed on the system. This interaction is responsible for the increase of polarization response, which is confirmed by dielectric susceptibility spectra. These results provide important understanding for the control of the polarization dependence on the working temperature of ZnO thin film, which is the essential key in the fabrication of switchable optical devices.

Thermophysical Parameters of Organic PCM Coconut Oil from T-History Method and Its Potential as Thermal Energy Storage in Indonesia

The thermophysical parameters of organic phase change material (PCM) of coconut oil (co_oil) have been studied by analyzing the temperature vs time data during liquid-solid phase transition (solidification process) based on T-history method, adopting the original version and its modified form to extract the values of mean specific heats of the solid and liquid co_oil and the heat of fusion related to phase transition of co_oil. We found that the liquid-solid phase transition occurs rather gradually, which might be due to the fact that co_oil consists of many kinds of fatty acids with the largest amount of lauric acid (about 50%), with relatively small supercooling degree. For this reason, the end of phase transition region become smeared out, although the inflection point in the temperature derivative is clearly observed signifying the drastic temperature variation between the phase transition and solid phase periods. The data have led to the values of mean specific heat of the solid and liquid co_oil that are comparable to the pure lauric acid, while the value for heat of fusion is resemble to those of the DSC result, both from references data. The advantage of co_oil as the potential sensible and latent TES for room-temperature conditioning application in Indonesia is discussed in terms of its rather broad working temperature range due to its mixture composition characteristic.

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.

The role of chemical additives to the phase change process of CaCl2.6H2O to optimize its performance as latent heat energy storage system

CaCl2.6H2O is one of salt hydrate based phase change material (PCM) which is suitable for room air-temperature stabilizer because it has the melting temperature just above the human comfort zone temperature (Tm ~ 29 oC) and a relatively large heat entalphy (AH ~ 190 kJ/kg). This paper reports the role of the type of chemical additives to PCM CaCl2.6H2O to the phase change process throughout the solidification process or heat release in order to optimize its performance as latent heat energy storage system. In this research we used several kinds of chemical additive, namely SrCl2.6H2O (1.0 wt%), BaCO3 (0.5 wt%), and K2CO3 (0.5 wt%). In terms of its latent time for phase change process the order the effectiveness of those chemical additives are reduced from SrCl2.6H2O, BaCO3and K2CO3. We found that this is also related to their role in suppression supercooling and phase separation effects which occurs during crystallization process of CaCl2.6H2O.

Room temperature analysis of dielectric function of ZnO-based thin film on fused quartz substrate

A set of sample consist of pure ZnO and Cu-doped ZnO film were grown on fused-quartz substrates using pulsed laser deposition (PLD) technique. Here, we report room temperature spectroscopic ellipsometry analysis (covering energy range of 0.5 to 6.3 eV) of pure ZnO film and Cu doped ZnO film at 8 in at. %. The thickness of pure ZnO and Cu-doped ZnO film using in this study is about 350 nm. To extract the dielectric function of ZnO thin film, multilayer modeling is performed which takes into account reflections at each interface through Fresnel coefficients. This method based on Drude-Lorentz models that connect with Kramers-Kronig relations. The best fitting of Ψ (amplitude ratio) and Δ (phase difference) taken by SE measurement are obtained reasonably well by mean the universal fitting of three different photon incident angles. The imaginary part of dielectric function (e2) show the broad peak at around 3.3 eV assigned as combination of optical band energy edge with excitonic states. The exitonic states c...

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.

Thermophysical parameters of coconut oil and its potential application as the thermal energy storage system in Indonesia

The high consumption of electric energy for room air conditioning (AC) system in Indonesia has driven the research of potential thermal energy storage system as a passive temperature controller. The application of coconut oil (CO) as the potential candidate for this purpose has been motivated since its working temperature just around the human thermal comfort zone in the tropical area as Indonesia. In this research we report the time-dependent temperature data of CO, which is adopting the T-history method. The analysis of the data revealed a set of thermophysical parameters, consist of the mean specific heats of the solid and liquid, as well as the latent heat of fusion for the phase change transition. The performance of CO to decrease the air temperature was measured in the thermal chamber. From the results it is shown that the latent phase of CO related to the solid-liquid phase transition show the highest capability in heat absorption, directly showing the potential application of CO as thermal energy storage system in Indonesia.

The effectiveness of organic PCM based on lauric acid from coconut oil and inorganic PCM based on salt hydrate CaCl2.6H2o as latent heat energy storage system in Indonesia

A latent heat energy storage system utilizing phase change materials (PCM) is an alternative strategy to reduce the use of Air Conditioning (AC) system in big cities in Indonesia in order for energy conservation in the future. In this research we used two kinds of materials, namely organic PCM based on lauric acid from coconut oil (CO) and inorganic PCM based on salt hydrate CaCl2.6H2O, because they have thermophysical parameters suitable for humans thermal comfort application in the building. The CO which contained more than 50% lauric acid has the melting temperature (Tm ) of about 26 °C and heat entalphy (ΔH) around 103 kJ/kg, while CaCl2.6H2O has the melting point of 29 °C and heat entalphy of 190 kJ/kg. In this paper we report the effectiveness of those two kinds of PCM in reducing the air temperature as one of some criteria for humans thermal comfort. The experiments were performed in a close and adiabatic room and the time-temperature measurements were done automatically using Arduino microcontroller and LM35 temperature sensor connected to the PC.