| Zinc Oxide (ZnO) is a direct wide band gap semiconductor with the band gap located at 3.37 eV and the exciton binding energy of about 60 meV at room temperature. ZnO exhibits the excellent transparent conductive, photoelectric and piezoelectric properties, and the stable performance, low growth temperature. It is sensitive to doping, temperature, pressure, structure and the concentration of carrier. All of these make it be an ideal material of efficient UV/blue light-emitting diodes, sensors, photoelectric detector, and the laser devices.It is one of research hotspots in the field of semiconductor materials.The ZnO thin films on sapphire (Al2O3) and silicon (Si) substrates have been prepared by Metal Organic Chemical Vapor Deposition (MOCVD) in the same conditions, and the influence of the structure and optical properties of ZnO thin films based on different substrates were studied in this paper. Photoluminescence (PL), Time-resolved Photoluminescence (TRPL), and X-ray Absorption Fine Structure (XAFS) were used to analyze the differences between the ZnO thin films on different substrates. Their relevant optical properties were also characterized.The main results in this paper are as follows:(1)Temperature-dependent PL was excited by the 266 nm,325 nm laser, respectively. A slightly but abnormal blue shift of the PL peaks was observed around the temperature of 20K-50K excited by 266nm. The internal quantum efficiency (IQE) of ZnO thin films under certain temperature were calculated. The results show that the IQE of ZnO/sap is higher than that of ZnO/Si. Gaussian fittings of the temperature-dependent PL spectra excited by 325 nm laser indicate the temperature dependence of peak positions. There is small difference from the integral intensity between the ZnO/sap excited by 10% of the max output power and ZnO/Si excited by 50%, which means that the luminous quality of ZnO/sap is higher than that of ZnO/Si. Fitting of the temperature dependent integral intensity by the thermal quenching formula shows that the activation energy of ZnO/sap and ZnO/Si is 207 meV and 207 meV, respectively.(2)The power-dependent PL experiment was excited by 266 nm laser under 10 k and 300 k, respectively. The integral intensity of ZnO/sap is higher than that of ZnO/Si under the same conditions, and then the quantum efficiencies could be calculated.(3)The power dependent TRPL were excited by 266 nm laser at room temperature. The results of single exponential function fitting to the fluorescence decay curve show that the fluorescence lifetime of ZnO/sap are slightly longer than that of ZnO/Si. The linear fitting of the power-dependent PL integral intensity shows that the slope of ZnO/sap is 2.64 times that of ZnO/Si, which implies higher luminous efficiency.(4)XAFS of Zn K-edge were also performed. Based upon the analysis from EXAFS data, the bonding lengths of Zn-O and Zn-Zn were extracted and the lattice constants, a and c, were obtained. Then, the stress between the ZnO epitaxial layer and substrate was calculated from the lattice constants:ZnO/Si: εx= 7.49 × 10-5, εz= 2.52 × 10-5; ZnO/sap:εx=-6.4×10-6 εz= 5.76 × 10-8。 Comparing the XANES of theoretical model with the measured data of ZnO thin films, it is found that there are less O vacancies in the ZnO thin films in this paper. Moreover, there are less Zn vacancies in ZnO/sap, compared to ZnO/Si. |
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