Optoelectronic and Structure Properties of Diluted Magnetic Semiconductor Based on ZnO Nanorods

Abstract

In this dissertation in order to improve structure quality the ZnO-based nanorods were fabricated by two processes including annealing treatment and doping method Transition-metal-doped (TM-doped) ZnO ultraviolet (UV) photodetectors (PDs) were also studied This dissertation is divided into four parts; the first of which investigates the surface quality of ZnO nanorods The second part discusses the growth of TM-doped ZnO nanorods and the third part analyzes the effect of doping concentration on the bulk quality of TM-doped ZnO nanorods The final part discusses TM-doped ZnO UV PDs To begin with the surface quality of ZnO nanorods which were grown in aqueous solution After post-annealing ZnO nanorods were analyzed by second harmonic generation (SHG) with the assistance of photoluminescence (PL) spectroscopy and the X-ray photoelectron (XPS) spectroscopy SHG is sensitive to the quality of surface structure and involves the elimination of surface defects and restructuring Oxygen (O)-deficient and surface defects were generated during the growth of ZnO nanorods PL and XPS analyses results indicated that the surface defects on the nanorods were reduced at annealing temperatures above 600 °C The bulk crystal structure was repaired for high activation temperatures (> 700 °C) based on X-ray diffraction (XRD) results The SHG results revealed the relationship between surface restructuring of ZnO nanorods and annealing temperature Then room-temperature ferromagnetism (RTFM) was observed in Co-doped ZnO (Co:ZnO) and Fe-doped ZnO (Fe:ZnO) vertically aligned nanorod arrays that were grown via hydrothermal synthesis The evolution of RTFM properties and nanorod qualities were studied at different growth temperatures and doping concentrations At the growth temperature of 80 °C the vertically aligned ZnO nanorods were well-formed; Co or Fe was readily substituted for Zn in the nanorod arrays The weak RTFM of Co:ZnO and Fe:ZnO nanorod arrays was determined via magnetization measurements The morphology and quality of the nanorods were examined using structure and composition analysis tools Co or Fe atoms were readily incorporated into the ZnO lattice without any precipitation or segregation of the secondary phase in vertically aligned ZnO nanorod arrays The properties of the nanorods were enhanced at low doping concentrations (1%) at the growth temperature of 80 °C Furthermore high-quality ZnO nanorods that were free from post-annealing treatment were fabricated via low-temperature hydrothermal synthesis by using dilute Co dopants Detailed analyses on the bulk quality of ZnO nanorods were performed with SHG via XRD and PL SHG provided a more sensitive differentiation between nanostructures and thin films compared with XRD The structure of ZnO nanorods shows a high surface-to-volume ratio which resulted in large surface dipole moments in the radial direction of the nanorods Moreover high variations in the gradient of the electrical field were observed around the nanorod structure enhancing the SHG signal ZnO nanorods with dilute Co-doping concentrations (1%) exhibited a higher bulk crystal quality than pure ZnO nanorods and those with high doping concentrations Finally Co:ZnO and Fe:ZnO nanorod metal-semiconductor-metal (MSM) UV PDs were fabricated with respective ratios of UV-to-visible rejection of 11700 and 25000 upon biasing at 1 V with a sharp cutoff at 380 nm Moreover the dark and photo noise equivalent power (NEP) of the fabricated Co:ZnO nanorod MSM PDs were 1 3 × 10?13 and 1 8 × 10?11 W at corresponding dark detectivities (D*) and photo D* of 1 1×1014 and 7 3×1011 cm‧Hz0 5‧W-1 respectively Co:ZnO nanorod UV PDs exhibited lower dark currents and better flicker noise characteristics compared with ZnO nanorod PDs

Date of Award	2014 Jul 11
Original language	English
Supervisor	Shoou-Jinn Chang (Supervisor)