Controllable Growth And Characteristic Of ZnO And ZnMgO Quantum Dots

ZnO is a direct semiconductor with band gap of 3.37 eV at room temperature and a large exciton binding energy of 60 meV. It has gained more and more attention in the fields of transparent conducting films, solar batteries, untra-vioIet(UV) light-emitting diodes, UV light detectors, piezoelectricity transition, sound-wave surface devices and so on. As a new nanostructure material, ZnO quantum dot(QD) has attracted a lot of attention because of its unique optical and electrical properties. Its fascinating applications depend on several factors, namely the particle size, size dispersivity and the density. So the controlled size, size dispersivity and density of ZnO quantum dots are the key issues of the future development. In addition, tunable band gap can increase the possibility of improvement of devices, such as QD light-emitting diodes and QD light diodes. Thus, research on tunable band gap of ZnO QD by doping Mg is very important.In this thesis, based on a comprehensive review of the research history and current status of ZnO quantum dots, we conducted a detailed study on ZnO and ZnMgO QDs grown on Si (111) substrates by MOCVD and its properties. The controllable growth of ZnO QD is described. Also the growth and tunable band gap of ZnO QDs by doping Mg are investigated which is a quite new subject since very few reports have been published till now.The main content of this thesis is listed as follow:1. High quality self-assemble ZnO QDs have been grown on the Si (111) substrates by metalorganic chemical vapor deposition (MOCVD). The diameter of ZnO QDs is about 10 ran in average. Cross-TEM shows that the height of ZnO QDs is about 5 nm. Compared with ZnO thin film, the photoluminesce (PL) of ZnO QDs reveals a 3-nm blue-shift because of quantum size effect.2. The growth of ZnO QDs is controlled by growth duration. As growth duration increases from 180 s to 600 s, at first, ZnO QDs are invisible, then the diameter of ZnO QDs increases from 9 nm to 20 nm and the density also increases from 2.9×10⁹ cm^-2 to 2.4×10¹¹ cm^-2 . As a result, by adjusting the growth duration, we control the growth of the ZnO QDs.3. The growth of ZnO QDs is controlled by the Zn flow rate. As the Zn flow rate increases, the diameter and density of ZnO QDs increase and the diameter dispersivity is improved. As the Zn flow rate is increased from 7.5 sccm to 20 sccm, the diameter ofZnO QDs increases from 10 nm to 20 nm and the density increases from 3.6×10¹⁰cm^-2 to 9.2×10¹⁰cm^-2 gradually.4. It is studied that how the density and diameter of the ZnO QDs change while using different oxygen sources and different flow rate, respectively. When using N₂O or NO instead of O₂ as oxygen source, the probability for ZnO QDs to nucleate and grow is increased because of the creation of active nascent oxygen. So the diameter and density of ZnO QDs are increased. In addition, it is found out that as the O₂ flow rate increases, the diameter of ZnO QDs increases.5. The diameter and density of ZnO QDs are influenced by different substrates. When the native oxide on Si (111) is removed, there are more bared steps so it is easier for ZnO QDs to nucleate and grow. As a result, the diameter increases from 9-12 nm to 18-20 nm. Apparently, the diameter dispersivity is also improved which is very important in the future development of QD devices.6. The growth of ZnMgO QDs and the tunable band gap by doping Mg are investigated. High quality self-assemble Zn_0.98Mg_0.02O QDs have been grown on the Si (111) substrates by MOCVD. Mg is introduced into ZnO QDs, as confirmed by X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) measurements. There have been few reports on this issue so far.It should be noted that these results have been consistently repeated. It demonstrates that our method is highly reproducible, which makes the further development of ZnO and ZnMgO QDs-based devices possible.