Raudah Abu Bakar

A FREE-SPACE METHOD FOR COMPLEX PERMITTIVITY MEASUREMENT OF BULK AND THIN FILM DIELECTRICS AT MICROWAVE FREQUENCIES

A free-space, non-destructive method for measuring the complex permittivity of a double-layer bulk dielectrics and thin fllm oxide layers at microwave frequencies have been developed. The method utilizes a spot-focusing antenna system in conjunction with a vector network analyzer in the range of 18{26GHz. The bulk dielectric was measured using the Transmission Method and Metal-Backed Method, while the Metal-Backed Method was used to investigate the thin fllms. Both types of samples were sandwiched between two quarter-wavelength Te∞on plates to improve the mismatch at the frequencies of measurement. The thin fllm sample arrangement was backed by an additional metal plate. The double-layer bulk dielectric samples were Te∞on-PVC and Plexiglas-PVC, while the thin fllm samples consisted of SiO2 layers of difierent thicknesses grown on doped and undoped Si wafer substrates. The relative permittivity obtained for PVC ranged between 2.62 to 2.93, while those for Plexiglas exhibited values between 2.45 to 2.63. The relative permittivity of SiO2 deposited on these wafers was between 3.5 to 4.5. All these values are in good agreement with published data. The advantage of the method is its ability to measure the dielectric properties of the fllms at the mid- frequency band irrespective of the substrate type used. Simulations of the measurement setup were carried out using CST Microwave Studio and the simulation results agreed closely with the measurements.

The Efficiency Enhancement on the Direct Flow Evacuated Tube Solar Collector Using Water-Based Titanium Oxide Nanofluids

In the present study, the efficiency of an evacuated-tube solar collector was investigated experimentally by using water and water-based titanium oxide nanofluids as the working fluids. The titanium oxide nanofluids with the average nanoparticles size between 30-50nm were prepared and tested at volume concentration of 1.0%, 2.0% and 3.0%. The volume flow rate of the working fluids in the solar collector varied from 2.0 to 3.0 liter/min. The experiments were performed at outdoor conditions according to the ASHRAE standard 93-2000. The result shows the efficiency of evacuated solar collector have increased up to 42.5% by using 2.0% concentration nanofluids compared to its base fluid at 2.0litre/min flow rate. In conclusion, the efficiency of collector shows greater enhancement at the low volume flow rate and concentration of nanofluids compared to its base fluid which was water.

A free-space method for measurement of complex permittivity of double-layer dielectric materials at microwave frequencies

Two methods for the calculation of complex permittivity of double-layer dielectric materials measured by a spot-focusing free-space measurement system have been developed. Standard materials have been used to test these methods and further works will be carried out on SiO2 wafer. The dielectric constants obtained were close to the published values. The two methods developed are Transmission Method and Metal-Backed Method. The S11, S21, and S22 are measured for Transmission Method, while only S11 is measured for Metal-Backed Method. In both methods, the samples are sandwiched between two Teflon plates which are quarter wavelength at mid-band frequency. Results are reported in the frequency range of 18–26GHz.

Effect of film thickness on the memristive behavior of spin coated titanium dioxide thin films

The paper presents the memristive behavior of spin-coated titania thin films. The spin speed during the deposition was varied and their effect on the film thickness and thus, to the memristive behavior was studied. Sample deposited at 5000 rpm produced thinner film thickness which result in better switching behavior. The sample showed larger OFF to ON resistance ratio of 5. All samples were measured 3 times for each positive and negative bias. It was found that the memristive behavior was repeatable for 2nd and 3rd measurement. The crystallinity of the films was characterized using Raman spectroscopy. In our work, it was observed that the film thickness mainly affects the switching behavior of memristive device instead of the crystallinity of the films.

Effect of Post-Deposition Annealing Process on the Resistive Switching Behaviour of TiO2 Thin Films by Sol-Gel Method

The paper presents the memristive behaviour of spin-coated titania thin films on ITO substrate. The sample was annealed in air ambient at different annealing temperature and duration of 250 and 450 °C for 20 and 60 min. The effect of post-annealing process to the physical thickness and crystallinity of the films towards switching behaviour was studied. It was found that the thickness and crystallinity of the films increases as the post annealing process increases. Sample annealed at 250 °C for 20 min with thinner film showed better switching behaviour even though the sample is still in amorphous form. Thus, in our work, we believed that the crystallinity of the films does not affect the switching behaviour of the sample. The reliability of device performance was studied by repeating the measurement for three times.

Annealing temperature dependence of resistive switching behavior for sol-gel spin coated zinc oxide thin films

This work focuses on the resistive switching behavior of sol-gel spin coated zinc oxide (ZnO) thin films on ITO substrate. The deposited ZnO thin films were annealed at various temperatures from 300°C to 500°C in a furnace for 60 minutes in order to study the effect of annealing temperature on the resistive switching behavior of ZnO thin film. The electrical property of the thin film was characterized using 2-point probe current-voltage (I-V) measurement. The surface morphology and film thickness were examined and measured using atomic force microscopy (AFM) and surface profiler respectively. The I-V characteristic showed that the heat treatment on the ZnO thin films at 300 and 400°C resulted in the resistive switching characteristic behavior. Further increasing the temperature up to 500°C on the other hand leads to the formation of asymmetrical hysteresis loop.

Memristive Behavior of TiO2 Nanostructures Grown at Different Substrate Positioning by Immersion Method

In this paper, the physical characteristics and memristive behavior of TiO2 nanostructures grown at different substrate positioning by wet chemical solution were investigated. TiO2 thin film as a seed layer for TiO2 nanostructures growth was first deposited on ITO-coated substrate by RF magnetron sputtering method. TiO2 nanostructures were then grown by immersing the TiO2 thin film/ITO/glass sample in 10M NaOH solution at 80 °C while studying the effect of the substrate position to the nanostructure growth and thus its memristive behavior. Characterization on the growth morphology of TiO2 nanostructures was observed using scanning electron microscopy (FESEM). The current-voltage (I-V) measurement of the device was investigated for its memristive behavior. Different growth morphology of TiO2 nanostructures was observed at different substrate positioning. It was found that sample immersed with TiO2 layer facing down the vessel result in the formation of TiO2 nanowires and exhibit better memristive behavior.

Switching Behavior of Titania-Zinc Oxide Composites Thin Films

Single layer of titanium dioxide (TiO2) is common metal oxide in fabricating memristor device. In this study, two types of memristor with composite metal oxide thin films will be demonstrated. The two types of memristor are titanium dioxide (TiO2) thin film coated on zinc oxide (ZnO) thin film and ZnO coated on TiO2 thin film. Sol-gel spin coating method was to coat metal oxide thin film and sputtering method for depositing the metal contact. Platinum (Pt) was selected as the top electrode and indium tin oxide (ITO) as the bottom electrode. The electrical characteristics were defined by performing I-V measurement using two point probe equipment. I-V characteristics showed shape of pinched hysteresis loop for both samples. Sample with TiO2 coated on ZnO has slightly higher Roff/Ron ratio than sample ZnO coated on TiO2 which means it more memristive than another one. The cross-section of sample with TiO2 coated on ZnO had been performed as well by using Field-Emission Scanning Electron Microscopy (FESEM).

Effect of TiO2 Seed Layer Thickness to the Growth of TiO2 Nanostructures by Immersion Method for Memristive Device Application

TiO2 nanostructures were successfully grown on TiO2 thin film by solution-based method at low temperature. TiO2 thin film as a seed layer for the nanostructures growth was deposited on ITO substrate by RF magnetron sputtering method at 40 and 60 nm thicknesses. Then the TiO2 nanostructures were synthesized on the samples by keeping them floating with TiO2 layer facing down the vessel in 10M NaOH solution at 80°C for 45 min. Effect of seed layer thickness to the growth of TiO2 nanostructure and its memristive behaviour were investigated. Surface morphology and current-voltage measurement for its memristive behaviour were measured by FESEM image and Keithley 4200 semiconductor characterization system. It was found that 60 nm-TiO2 thin film result in the formation of dandelion-like morphology of TiO2 nanowires and gives better memristive behavior with larger switching loops when positive voltage was applied to the sample.

Effect of Electrode Types on the Resistive Switching Behavior of Titania Thin Films

The resistive switching or memristive behavior of sputtered titania thin films sandwiched in between of three types of metal electrodes (Au, Pt and Ti) was investigated. The active region of the device consisted of two titania thin films, in which, the first layer was exposed to a plasma treatment to create the oxygen vacancies, before the deposition of the second layer. The whole active layer sputtering deposition process was conducted in a one-flow process without exposing the sample to the room ambient. From the I-V measurements, titania thin films in between Ti and Au did not show any resistive switching, but those sandwiched between Pt electrodes exhibit a noticeable memristive behavior. This may due to the metal work function of the platinum itself.