Structural and optical properties of deposited zinc oxide on integrated pulsed electrochemical porous silicon / Nurul Hanida Abd Wahab

This thesis focuses on the study of the structural and optical properties of deposited zinc oxide on porous silicon substrate which has been prepared by using two different etching method. This work has been divided into two parts where the first part was the fabrication of porous silicon with two different methods used known as direct current (DC) electrochemical etching and pulse current (PC) electrochemical etching. During the pulse current etching process, the introduction of delay time has been introduced to each samples with variation of 0 minutes, 2 minutes and 4 minutes delay. This time delay was applied during the etching process before applying the current source to the cell. During this time delay, the silicon wafer was left to react with the chemical solution mixture which acts as an electrolyte for the etching process. The different range of time delay has resulted in the different pattern in terms of their surface morphology, surface roughness, elemental composition, as well as optical properties of the porous silicon structure. The structural analysis showed that all the porous silicon samples which has been fabricated using pulse current etching exhibited uniform circular shape and the surface roughness has also increased as compared to the as grown silicon sample. The sample with introduction of time delay for 2 minutes has been identified to have the most homogeneous pore structure as compared to other samples. Furthermore, stronger Raman peak obtained from the sample with 2 minutes delay time as compared to other fabricated samples with slightly left shifted from the infrared (IR) region. The quantum confinement effect for this sample has been observed by the increase in band gap value of 1.60eV. The second part of these research work was the deposition of zinc oxide (ZnO) inside the porous silicon structures in order to synthesize the ZnO nanostructures in terms of structural and optical properties. The deposited process has been done to all porous silicon samples by using thermal evaporation method. During the thermal evaporation method, the zinc (Zn) plate with 99.9% purity was placed in the tungsten boat which later being attached in the bell jar. The Zn plate was melted hence evaporated as ZnO on the surface of porous silicon substrate. This deposited structure has formed different morphology on the surface where flower petals like structure were obtained in all samples. The Atomic Force Microscopy (AFM) results showed that the sample with 2 minutes delay time has the roughest surface as compared to other samples which similar to previous results before deposited process is performed. The X-Ray Diffraction (XRD) pattern of the deposited porous silicon samples reveals that all of the samples are polycrystalline. The peaks that appeared at 2θ of 31.8o, 34.5o, 36.3o, 47.6o, 56.7o, 63.0o, 68.1o and 69.2o represent the (100), (002), (101), (102), (110), (103) and (112) phases respectively. These phases indicating the hexagonal close-packed crystal structure of the ZnO according to JCPDS no. 36-1451. Sample with 2 minutes delay time has constantly showed better optical properties as compared to other samples for having highest intensity in the PL spectra and blueshift has been identified. By comparing the PL results between the PSi samples and the deposited ZnO/PSi samples of 2 minutes delay time, it can be concluded that the PL peak has been experienced blue shift with broader peak at 630nm which falls in the red region for deposited sample ZnO/PSi as compared to the PSi samples where the peak was located at 766nm.