Structure And Optical Properties Of Mg-doped Nanocrystals

ZnO is an n-type Ⅱ-Ⅵ semiconducting compound with a wide band gap (3.37eV) and a large exciton binding energy (60meV) at room temperature. It has extensive applications in optoelectronics and microelectronics field owning to their superior thermal stability, conductivity and performance of ultraviolet absorption. Incorporating Mg into ZnO has already been proved to be a feasible technique to realize band gap tuning the band gap of ZnO. In this thesis, the preparation, structural changes and optical properties of Mg-doped ZnO nanocrystals have been studied by X-ray diffraction, Raman spectra, Uv-vis absorption spectra, photoluminescence spectra, et al. The main results are as follows:1. MgxZn1-xO (MZO, x=0.00,0.02,0.04,0.06,0.08,0.10) nanocrystals were prepared by sol-gel method. The XRD results showed that the lattice constant a increased but c decreased with the Mg content x increasing. When the Mg content x is more than0.10, a mixed phase MgO appears. Only8%of Zn2+can be substituted by Mg2+in wurtzite structure of MgxZn1-xO nanocrystals prepared by sol-gel method.2. The Raman spectra of the pure ZnO sample at room temperature excited by325,488、514.5、532、632.8and785nm laser line, respectively. When excitation condition is632.8nm laser line, the intensities of204and332cm-1peak appear stronger. Besides, the intensities of536cm-1peak appear stronger by514.5nm laser line. The results showed that photon energy of632.8and514.5nm are resonant with the electronic interband defect level of wurtzite ZnO.3. For the nonresonant Raman, as Mg content x increases to0.10, the99.5,204,379,529and579cm-1modes blue-shift to102,213,383,544and590cm-1, attributed to increasing force constant of atom vibration. As the Mg content x is more than0.10,277cm-1mode related to MgO appears in the MgxZn1-xO samples.4. From resonant Raman scattering spectra (RRS), LO phonon overtones up to the fifth order (1LO、2LO、3LO、4LO and5LO) are observed in MgxZn1-xO at room temperature. As Mg content x increasing, the LO phonon overtones all show significant blue and the intensity ratio of second-to first-order LO phonons(I2LO/I1LO) decrease.5. Uv-vis absorption spectra of MgxZn1-xO shows that band gap of increase with Mg content increasing.6. PL spectra of MgxZn1-xO at room temperature show that the spectra consist of two parts:the near band edge emissions (NBE) in the UV region; the green and orange peak of deep level emissions (DLE) in visible light region. A weaken green emission locates at539nm (2.30eV) and is attributed to the oxygen vacancies. The other strong orange emission locates606nm (2.05eV) and is attributed to the oxygen interstitials. As the Mg content increases to0.10, the NBE emission blue-shifts to365nm (3.40eV), which can be explained by an increase of band gap energy.7. PL spectra of MgxZn1-xO at low temperature (16K) show that with increasing x to0.8the peak of free exciton blue-shifts to353nm (3.52eV), the intensity reaches the maximum and the full width at half maximum decreases. This implies that Mg dopant is beneficial to the free exciton emission.8. In Mg0.02Zn1.002O sample, the362nm (3.43eV) and367nm (3.38eV) peaks are attributed to free exciton (FX) recombination in areas with and without Mg+, respectively. Above140K, the free excitons in area with Mg2+transfer to area without Mg2+, so the362nm (3.43eV) peak quenches.