Preparation And Physical Properties Of Zno-based Thin Films & Multiple Quantum Well Structures By Pulsed Laser Deposition

ZnO is an II-VI group semiconductor with wide bandgap （3.37 eV） and high exciton binding energy （⁶0 meV） at room temperature （RT）. Its excellent optical property makes it possible to obtain the ultraviolet （UV） emission at RT and even higher, so it can be used to fabricate short-wavelength optoelectronic devices. In this dissertation, our works focus on the structural and optical properties of ZnO, MgZnO films, MgZnO/MgO and ZnO/MgO strained multiple quantum wells （SMQW） grown by pulsed laser deposition （PLD）.The effects of substrate temperature, oxygen pressure and laser power density on the structural and optical properties of ZnO films were systematically studied, and the optimized growth conditions were obtained. Based on the optimized conditions, plasma assisted PLD technology was employed to nitrogenize the Si（111） surface via nitrogen plasma prior to growth of high quality ZnO film. The results show that the introduction of nitrogenized layer play a key role in overcoming the problem of interface charge imbalance when growing a polar semiconductor on an nonpolar one, reducing the density of interface defects at the initial stage of growth and then obtaining high quality ZnO film. The X-ray photoelectron spectroscopy （XPS） depth analysis confirmed the presence of the N layer. Combining the results of XPS and theoretical simulation of natural bond orbital （NBO）, a reasonable stable bonding configuration in the sequence of Si-N-Zn-O at the interface of ZnO/Si（111） was proposed and confirmed adequately.To broaden the optical band gap of ZnO in the range of UV and deep UV （DUV）, a series of Mg_xZn_1-xO films with different Mg content were grown by PLD. The structural and optical properties of the Mg_xZn_1-xO films were studied systematically. The Mg_xZn_1-xO alloy crystallizes in wurtzite structure when x<40%, while rock-salt structure for x>50%. However, the structure of Mg_xZn_1-xO is mixed phase in the range of 40%<x<50%. The optical band gap of Mg_xZn_1-xO is tunable continuously in the range of UV and DUV when the Mg content changed in full components. The UV emission of Mg_xZn_1-xO blue shift from 3.3 to 3.8 eV with the Mg content increasing. When Mg content reaches at 40%, the UV emission peak no longer bule shift and is held at ³.8 eV. This may result from the Zn-rich localized states in disorder alloy. To investigate the application of Mg_xZn_1-xO in optical detection, Au/Mg_0.21Zn_0.79O and Au/NiO/Mg_0.21Zn_0.79O structured UV photodetectors were constructed and their responsivities were investigated. The results show that the introduction of the NiO layer enhanced the responsivity and higher quantum efficiency of the photodetector. The enhancement may result from the avalanche process of the hot carriers accelerating in the high electric field of NiO layer.The MgZnO/MgO and ZnO/MgO SMQWs nanorods were prepared on c-Al₂O₃ and m-Al₂O₃ substrates by PLD. The nanorods were growth perpendicular to the surface of substrates. Z-contrast scanning transmission electron microscopy observations and line-scan compositional analysis reveal that the nanorods are compositionally modulated along their length and have a multi-quantum-well structure. The high resolution transmission electron microscope indicates that a coherent epitaxial relationship with a sharp interface is established between MgZnO, ZnO and MgO layers with large lattice mismatch. It is worth noting that the MgZnO and ZnO layers undergo a structural transition from hexagonal to cubic phase with their thicknesses decreasing. The results of X-ray diffraction and theoretical calculations reveal that a large in-plane compressive stress dominates such an interesting phase transition process, stabilizes the low Mg-content MgZnO alloy and even ZnO in the anomalous cubic phase, and also leads to a broadening of their band gaps. As a result, the wavelength-tunable deep-UV emission in the range of 261-331 nm is obtained from the MgZnO/MgO SMQW. To the best of our knowledge, the 261 nm is known to be the shortest emission wavelength ever reported for MgZnO material. This research proves a feasible method that will be used to prepare cubic ZnO and single-phase Mg_xZn_1-xO alloy in full components.