Study Of Optics Effect In Wide Gap Zinc Oxide Semiconductor Induced By Ion Implantation

Optical waveguide structure is a fundamental unit of the integrated optics devices. The research content the integrated optics mainly focus on the optics phenomenon and optical system based on waveguide structure, and optical waveguide plays an important role in the field of moden optical communication because of its excellent characteristics, possibility in integration and rather low cost in manufacturing. Researchers are trying to fabricating waveguide structure on various materials by a variety methods due to its practical application. The development of new waveguide materials depends not only on materials engineering at a practical level, but also on a clear understanding of the properties of materials, and the fundamental science behind these properties. It is the properties of a material that eventually determine its usefulness in an application. The series therefore also includes such titles as electrical conduction in solids, optical properties, thermal properties, and so on, all with applications and examples of materials in electronics and optoelectronics.Wurtzitic ZnO is a wide band gap semiconductor material (Eg=3.437eV at RT) that has many applications, including light emitting diode, solar cell window, phosphors, and transparent conduction films. Most of these applications only require polycrystalline materials; however, recent successes in producing large-area single crystals make possible the production of blue and UV light emitters and high power transistors. The main advantage of ZnO as a light emitter is its large exciton binding energy (Eb=60meV). This binding energy is three times larger than that of the20meV exciton of GaN. ZnO also affords superior radiation hardness compared with other common semiconductor materials, such as Si, GaAs, CdS, and even GaN, enhancing the usefulness of ZnO for space applications. Optically pumped UV laser action in ZnO has already been demonstrated at both low and high temperatures although efficient electrically induced lasing awaits further improvements in the experimental ability to grow high quality p-type ZnO material. Over the past years, a number of reseach groups have proposed that ZnO might be a leading candidate device material for the next generation of optoelectronics, owing to the many similarities between the optical, electrical and structural properties of ZnO and GaN, including their band gaps (3.44eV for ZnO and3.50for GaN at RT) and their lattice constants. In addition, still others have noted that ZnO has a free excition binding energy of60meV, approximately twice that of GaN, which could lead to highly efficient, ZnO-and MgZnO-based, UV injection lasers (UV laser diodes and detectors) at room temperature.Scientists and engineers are trying to find various ways to fabricate high-quality optical waveguide due to its importance in practical application. Several alternative methods can be employed to fabricate optical waveguide. The common technique are epitaxial layer growth, diffusion, ion exchange and ion implantation, and so on. The basic principle of epitaxial layer growth method is to grow a higher index thin film epitaxially on a lower index substrate, but this method is not always reliable, the common problem of epitaxial growth, especially for crystal, is relatively high scattering loss at the interface between the optical film and substrate due to their in-plane lattice mismatch. The principle of diffusion or ion exchange method is to increase the index above that of the substrate by chemical means, which implies that the guiding region must to some extent be contaminated, the presence of impurities (in the diffusion case) or the change in composition (ion exchange) would be expected to affect the crystalline properties of the substrate. However, the ion implantation method can be used to overcome these drawbacks.Ion implantation is an outstanding method to modify the surface property of materials since it offers accurate control of both the depth and lateral concentrations of dopant structural modification at any selected temperature. The dominant effect of ion implantation on refractive index is usually due to the partial lattice disorder produced by nuclear damage processes, which leads to a decrease in physical density and hence to a low index optical barrier. The region between this barrier and the surface is therefore surrounded by regions of low index and is able to act as awaveguide. Ion implantation can be used to fabricate optical waveguides in a wide variety of substrates, including non-linear, electro-optic and laser host materials.Increasing interest has been centered on the Tm-doped materials due to their potential applications integrated optics and optical communications. Trivalent thulium has an unfilled4f electronic shell that is shielded by the filled5s and5p electronic shell, so it has a little influence of the suerounding materials, Tm3+has attracted much attention due to its stable excited levels which is suitable for blue and ultraviolet emitting devices.In this dissertation, we report the fabrication and the properties of the planar waveguide, and the optical confinement in ZnO crystal is observed by ion implantation. Furthermore, the rare-earth doped ZnO is investigated and the phenomenon of photoluminiscence and thermal annealing are also discussed. The main contents of this dissertation are given as follows:(1) ZnO is considered as one of the most outstanding material in optoelectronics applications owing to its remakable electrical and optical properties. O ions is the common ions used to fabricated waveguide by implantation. To our knowledge, it is the first time to report formation of planar optical waveguides in the O+ion-implanted ZnO single crystal. Optical confinement in ZnO is observed by implantation with implantation energy of2MeV,4MeV and6MeV. The optical properties of the waveguide are studied by the end-face coupling and the prism-coupling technology at the wavelength of633nm, The results shows that the first sharp mode TEo means a good confinement of the light, which corresponds to the real waveguide mode. The ZnO lattice damage in near surface region induced by the O+ions implantation is investigated by the Rutherford backscattering/Channeling technique. A theoretical model is developed to explain the refractive-index changes in the ZnO and the refractive index profile in the planar waveguide is reconstructed accordingly.(2) He ions is the common ions used to fabricated waveguide by implantation. Waveguide effect is observed in the ZnO crystal by He+ion implantation with different energies and doses. Computer code is employed to simulate the process of He+ion implantation into ZnO. The Rutherford backscattering/Channeling is used to analyze ZnO lattice damage in the guiding region caused by the ion implantation.The waveguide properties are measured by the prism coupling and end-face coupling technique. The optical properties as well as the electric field intensity distribution of the propagation mode in the implanted waveguide are also investigated by using finite-difference beam propagation method, The results show that only one mode is observed for500KeV implantation and most distribution of the TE field extends into the substrate, but for2MeV implantation, most of field of fundamental TEo is restricted within the guide region. The refractive index profile of the waveguide is reconstructed and the results shows that the ordinary index decreases at the near surface region after implantation, the nuclear energy loss plays the major role in the crystal refractive index change rather than that from electronic energy loss. (3) ZnO waveguide are formed at room temperature by6MeV Si+implantation with dose of5×1013ions/cm2,5×1014ions/cm2and5×1015ions/cm2, The width of waveguide is2.4μm. Dark modes are studied by prism couping method at633nm, The relationship between modes and doses is also presented. The computer code is employed to simulate implantation prosess of Si+into ZnO. The ion concentration and energy loss of Si+is analyszed. The Rutherford backscattering/Channeling is used to measure ZnO lattice damage in the surface region caused by Si+implantation.(4) ZnO crystals were implanted by Tm+ions at500KeV with different doses at room temperature. The damage profiles in ZnO induced by Tm+implantation are studied using Rutherford backscattering/Channeling technique and we found that damage profile shows good consistence with the distribution of Tm+ions. The lattice damage tends to saturate at the implantation dose3×1015ions/cm2, indicating that ZnO is very resistive to high dose and high energy irradiation. After implantation post-implant annealing at temperature from800to1050℃is performed to activate Tm ion optically. Results show that annealing at800℃exerts no obvious effect on Tm+ions distribution, as well as the lattice damage. But annealing higher than950℃resulted in out-diffusion of Tm ions. Complete damage recovery is found in the ZnO with low fluence after annealing at1050℃. Photoluminescence was measured at room temperature with UV and green excitation, significant luminescence of transition3H4→3H6at794nm from Tm3+and concentration quenching behavior is observed in samples suffering800℃annealing for30minutes. Typical Broad band emission from ZnO are detected in both virgin and the implanted samples, and the red shift of peak indicates that deep defects introduced by implantation contribute to the enhanced red emission.(5) Planar optical waveguides were formed in ZnO crystal doped with Tm+ions by500KeV Tm+implantation combining with a subsequent implantation of4.0MeV O+ions. The distributions of Tm+in as-implanted and annealed ZnO samples are measured by Rutherford backscattering/Channeling technique. A shift of Tm+peak towards sample surface and out-diffusion are observed after thermal treatment. Waveguide properties was determined by using prism coupling method after O+implantation in Tm+-doped ZnO crystal and two guided modes were detected. The refractive index profile in the waveguide region was reconstructed according to computer code simulation. The result shows that the refractive index contrast of waveguide comes from both the index increase in guiding region and the index decrease in reduced index barrier.The experimental result and theoretical analysis in this dissertation are significant for the further study of ZnO material, and a chievement of optical waveguide structure in ZnO crystal makes it possible to expand its application in ZnO-based integrated optics and optoelectronic devices in attempt to control the propagation of light and to enhance the optical efficiency.