Optical And Electrical Properties, Doping Of ZnO Thin Films

The semiconductor ZnO has gained substantial interest in the research community in part because of its wide-bind gap (3.37 eV at room temperature) and large exciton binding energy (60 meV) which could lead to prospective applications in shortwave optoelectronics. However, the application of ZnO is hindered mainly due to the difficulty in achieving reliable and reproducible p-type ZnO and ZnO p-n junction. Moreover, some basic issues such as defect physics, the relationship between defects and luminescence, ZnO conduction mechanism is also very important and need further research. This thesis is based on the background surrounding the ZnO p-type doping, defect physics and luminescence issues. The contents of this dissertation are listed as follows:1. Ag doped ZnO films have been fabricated on single-crystal Si (100) substrates by magnetron sputtering. XRD measurements show that the ZnO:Ag films have high crystallization quality without new phase related to Ag appearing. Doping with Ag can clearly decreases the neutral donor bound exciton emission as the low temperature (10K) photoluminescence spectra show; a neutral acceptor bound exciton emission has been observed at 3.315eV which is attributed to the formation of acceptor defect (Ag substitutes Zn). The level of Agzn acceptor is estimated to be about 242 meV above valence-band maximum. Hall measurements show that Ag doped ZnO films are p-type conductivity with the resistivity about 0.1Ωcm, hole concentration about 1.7×1018cm-3, and mobility about 36cm2/Vs.The two-layer structured ZnO p-n homojunctions have been prepared on Si(100) substrates by depositing Ag doped p-type ZnO film on intrinsic n-type ZnO film using magnetron sputtering. The current-voltage (I-V) characteristics derived from the intrinsic ZnO/Ag doped ZnO two-layer structure clearly show the rectifying characteristics of typical p-n junctions.2. Ag-S codoped ZnO thin films on Si substrate have been fabricated by radio frequency (RF) magnetron sputtering using thermal oxidizing method. XRD and SEM measurements showed that the sample is hexagonal wurtzite structure with a preferential (002) orientation and the surface is composed of compact and uniform grains. Agzn-nSo defect complexes were observed in the Ag-S codoped ZnO films by XPS analysis. Low temperature PL spectra showed neutral acceptor bound exciton emission related to Agzn-nSo The corresponding acceptor ionization energy of 158 meV is much lower than that of Ag monodoped (242 meV), which is favorable for p-type doping of ZnO.3. Zn diffusion method was employed to investigate optical and electrical properties resulted form the diffusion of Zn into ZnO crystals. We found that UV emission intensity in ZnO is dramatically enhanced by Zni which is a shallow donor with ionization energy of-50meV. In the mean time Zno can be found during Zn diffusing into ZnO and contribute to the visible emission peaked at 2.43eV. Secondly, We tried to find the best way to enhance the UV emission intensity by diffusing Zn into ZnO crystal.4. The experiments find that the ZnO thin films deposited by DC-magnetron sputtering have different conduction types after annealing at high temperature in different ambient. Hall measurements show that ZnO films annealed at 1100℃in N2 and in O2 ambient became n-type and p-type, respectively. It is due to the generation of different intrinsic defects by annealing in different ambient. XPS and PL measurements indicate that zinc interstitial becomes a main defects after annealing at 1100℃in N2 ambient, and these defects play an important role for n-type conductivity of ZnO. While the ZnO films annealed at 1100℃in O2 ambient, the oxygen antisite contributes ZnO films to p-type.5. Transparent conducting Al-doped zinc oxide (AZO) thin films have been fabricated on quartz substrates by single source CVD system. Effects of annealing temperature and Al-doping concentration on the optical and electrical properties of AZO films were investigated. AZO film with Al/Zn ratio of 1% annealed at 700℃has high optical transmittance of 88.2% and low electrical resistivity of 1.85×10-2 Qcm. Hall measurements and absorption edge analysis indicate that resistivity of AZO films is determined by carrier concentration rather than carrier mobility.