Investigation On P-type Doping Of ZnO Thin Films And Controllable Growth Of ZnO Nanodots

With a large exciton binding energy of 60 meV and a wide bandgap of 3.37 eV at room temperature, ZnO is considered as a promising material for short-wavelength optoelectronic devices, such as light-emitting diodes (LEDs) and laser diodes (LDs). In addition, ZnO has many other advantages, including the availability of large-area single crystal, the amenability to wet chemical etching, the high radiation resistance, the relatively low costs, and the availability of alloys system (ZnMgO and ZnCdO) for bandgap engineering, etc. On the other hand, by virtue of numerous unique properties expected in the low-dimensional system, ZnO nanometer-scale materials, such as nanowires, nanotubes, nanobelts, and nanodots promise to be important in the next-generation electronic, optoelectronic, and medical applications.Due to the asymmetric doping limitations, n-type ZnO, with a high electron concentration, has been well prepared. The realization of p-type ZnO, however, has proven difficult and thought to be the bottleneck in the development of ZnO-based devices. Also, the optimal choice of acceptor species in ZnO remains to be determined. In this regard, the work was focused on the growth and characterization of p-type ZnO thin films in this dissertation, in an attempt to better understand the p-type doping mechanism. In addition, the controllable growth of ZnO nanowires and nanodots were reported. The work included:1. Al-N codoped p-type ZnO thin films were prepared by dc reactive magnetron sputtering. The effects of growth temperature and Al content on properties of Al-N codoped ZnO were discussed.2. The author demonstrated the reproducible growth of Li-doped p-type ZnO thin films by dc reactive magnetron sputtering. Under the oxygen rich condition and a proper Li content, the formation of Li_Zn acceptor could be enhanced, resulting in low-resistivity, p-type ZnO thin films. In addition, the doping mechanism for the p-type ZnO.Li was proposed tentatively.3. Intrinsic p-type ZnO thin films were grown by plasma-assisted metalorganic chemical vapor deposition (MOCVD). The increment of the oxygen concentration in the intrinsic p-type ZnO, compared with the intrinsic n-type layer, was well confirmed by secondary ion mass spectroscopy. The origin of intrinsic p-type behavior was ascribed to the formation of zinc vacancy and some complex acceptor center.4. N-doped, p-type ZnO thin films were grown by plasma-assisted MOCVD by using NO plasma. The intrinsic zinc vacancy and extrinsic nitrogen acceptor, identified by low-temperature photoluminescence (PL), contributed to the p-type conductivity in the ZnO:N simultaneously. Also, the author demonstrated comparative study on ultraviolet photoconductivity of p-type ZnO:N thin films and n-type ZnO epilayer. The surface adsorption and photodesorption process, combined with a competition between holes and electrons, in the p-type ZnO:N was proposed, providing qualitative agreement with the observed behaviors. Furthermore, the author developed a plasma-free MOCVD method to grow reproducible N-doped, p-type ZnO thin films. The typical rectifying I-V characteristics and room-temperature electroluminescence from the ZnO homojunction LEDs were observed.5. Well-aligned ZnO nano wires were grown on silicon substrates by MOCVD without catalysts. The mechanism of the catalyst-free growth of ZnO nanowires on silicon substrates was discussed. By modulating the flux of the reactant, the diameter of ZnO nanowires could be well tailored.6. ZnO and ZnMgO nanodots (NDs), with the diameter of 5～10 nm, were grown on silicon and sapphire substrates by MOCVD. The density of the NDs was shown to be temperature-dependent. By adjusting the Mg content, a tailored optical bandgap was achieved for the ZnMgO NDs. Also, the Ga donor and N acceptor were introduced into ZnO NDs, respectively. The author demonstrated, with a combination of valence band x-ray photoelectron spectroscopy and scanning tunneling microscopy, that the electrical properties as well as Fermi level of the ZnO NDs could be well tuned by the donor/acceptor doping. Finally, ZnO/MgO quasi core-shell NDs were grown by a MOCVD method. The free exciton (FX) emission from the NDs showed remarkable enhancement after the growth of MgO shell layer. Also, the FX emission exhibited an evident blueshift with the decreased ZnO size due to the quantum confinement effect. An exciton binding energy of 118 meV was obtained from temperature-dependent PL, which was almost two times as that of the bulk materials.