Investigations On Microstructures Of ZnO Films And One-dimensional ZnO On Silicon Substrates

Zinc oxide (ZnO) is a semiconductor with a wide direct wide-bandgap of 3.37eV at room temperature, and exciton binding energy of 60 meV which is much larger than other wide-gap semiconductors such as GaN and 2.3 times that of the room-temperature thermal energy ( k_B = 26meV ). It is a potential candidate for applications in short-wavelength optoelectronic devices, including light emitting diodes (LEDs) and laser diodes (LDs), because its large exciton binding energy could lead to lasing action based on excition recombination even above room temperature. Furthermore, many significant exciton effects may be expected in low-dimensional ZnO nanostructures due to quantum confinement effects, and so the improvement in device performance can be predicted. ZnO nanostructures have promising potentials in extensive applications in nano-optoelectronics and nano-electronics devices, nanosized gas sensors, and field emitters etc.However, its applications to optoelectronic devices has not yet materialized due chiefly to the lack of reliable and high quality p-type epitaxial layers of ZnO. An imperative issue is that crystal quality of epitaxial ZnO layers should be improved and p-type conductivity films with good properties should be achieved, which could potentially make inroads into the world of optoelectronics..In this thesis, to make use of modern transmission electron microscopy, ZnO films by magnetron sputtering and nano-ZnO by Metal Organic Chemical Vapor Deposition (MOCVD) have been investigated for structure, properties and developing mechanism on micrometer, nanometer and atomic scales. The grain boundary, as-growth defects and processing defects by annealing also have been investigated. The influences of sputtering conditions and post-annealing on microstructures for ZnO films have been investigated in order to find way to improve crystallinity, and the developing mechanism for nano-ZnO materials have been investigated to control structure and self-assembled crystal.The research topics and main results are as follows:1. In optimized condition, ZnO films possess columnar grains with strong[0001] preferred orientation. The reason of formation for texture is mainly that only ZnO nucleus with selecting [0001] orientation grow on amorphous oxidation on Si due to the lower surface free energy of (0001) plane. It is indicated that single stacking fault is a kind of as-growth defect which could be surrounded by partial dislocations with Burgers vector b =1/6[02|-23]. The [0001]-tilt grain boundaries in ZnO films have been investigated symmetrically. It is indicated that boundaries can be classified into three types: low-angle boundaries described as an irregular array of edge dislocations with Burgers vector b =1/3[21|-1|-O], boundaries of near 30 degreeangle with {101|-0} facet structures, and large-angle boundaries with symmetric structure which have been investigated by low Σ Coincidece Site Lattice as a low free energy ones to explain why it is straight or zigzag arrangements. Zn_1-xCd_xO films have been investigated, too. It is indicated that the wurtzite-type structure of Zn_1-xCd_xO can be stabilized up to Cd content x = 0.05 without a cubic CdO phase separation.2. It is indicated that films grow still with preferred orientation but with dense defects at low temperature, and deviation angles of ZnO [0001] orientation from normal increase at temperature more than 400℃. And ZnO films degrade with too larger power due to having random oriented polycrystalline ZnO layer at the film-substrates interfaces. A growth process with two steps is suggested that c-oriented layer is deposited at low temperature and then epitaxial ZnO film is developed at temperature more than 400 °C with low defects and preferred c-oriented.3. The crystallinity of ZnO films could be improved by annealing, however, in too high temperature, high concentration of oxygen vacancies formed because oxygen atoms have to diffuse from the surrounding lattice to stacking fault when Zn atoms precipitate which lead to that n-type carrier concentration increases. The three layer stacking fault is a kind of processing defects by annealing because interstitial Zn atoms precipitated. Besides, double stacking fault is formed when interstitial Zn atoms prefer to precipitate on as-growth single stacking fault. It is proposed that the precipitation is Zinc-blende ZnO of about 1nm thick.4. Microstructures of well-aligned ZnO nanowires, quasi-alignedone-dimensional nano-ZnO composed of nanotubes and nanobelts grown by MOCVD method on Si substrates have been investigated. It has hexagonal cross sections, surrounded by {101|-0}planes. The diameter of ZnO nanowires has been controlled by simply varying the reactant sources flow rate, and nanowires with a 10 nm diametical tip possess of excellent field emission properties. The nanotubes have hexagonal cross sections with sheets of about 10 nm thick, but surrounded by {21|-1|-0} planes. A new mechanism growth is proposed, which can be used to control other nanotubular and self-assembled structures, that the ZnO nanobelts have been limited to grow at top of ZnO islands with a diameter of 40～60 nm on Si substrates to lead to form nanotubes.