Investigation On Pulsed Laser Deposited Doped ZnO-Based Thin Films And Their Related Devices

Zinc oxide (ZnO), as a novel oxide semiconductor with a direct band-gap structure, has been considered as a very promising material for the next-generation short-wavelength optoelectronic devices, such as light-emitting diodes (LEDs) and laser diodes (LDs) due to a wide band-gap of 3.37 eV and a large excition binding energy of 60 meV. To realize these device applications, an imperative issue is to fabrication both high-qulity n-and p-type ZnO. Most of the previous studies related to ZnO-based transparent conducting thin films were carried out using metallic cation dopants such as Group IIIA (e.g., Al, Ga, and In). However, fluorine can be a promising n-type anion doping candidate. To the best of our knowledge, research on F-doped ZnO thin films is very limited. Therefore, we carried out some investigation on fluorine-doped ZnO transparent conducting thin films. On the other hand, it has proven to be difficulty in obtain high-quality, reproducible, and stable p-type doping in ZnO caused by the asymmetric doping limitation, which is the bottleneck of realizing such optoelectronic applications. Also, the optimal choice of acceptor species in ZnO remains to be determined. In this regard, the main work was focused on pursuing the appropriate doping species and doping techniques in p-type ZnO thin films in this dissertation, in an attempt to better understand the p-type doping mechanism by a combinatorial theory and experimental studies. The main work included:1. Highly transparent and conducting fluorine-doped ZnO thin films were prepared on glass substrates by pulsed laser deposition. The effects of oxygen pressure on the structural, electrical and optical properties of the films were investigated and discussed in detail. The minimum resistivity of 4.83x10"4Ω-cm. with a carrier concentration of 5.43×1020cm-3 and a Hall mobility of 23.8 cm2V"'s"', were obtained for fluorine-doped ZnO film prepared at the optimal oxygen pressure of 0.1 Pa. The average optical transmittance in the entire visible wavelength region was higher than 90%and the transmittance in 1500 nm was about 60%. In addition, a top-gated transparent ZnO-based thin-film transistor with a ZnO layer as the channel. Ta2O5 as the gate insulator and F-doped ZnO film as the gate/source/drain electrodes has been prepared.2. We realized the p-type ZnO thin films via Li-F codoping technique prepared by pulsed laser deposition. The effects of substrate temperature on the structural and electrical properties of the films were investigated in detail. An acceptable p-type conduction, with a resistivity of 23.45Ωcm. a Hall mobility of 0.35 cm3v-1s-1, and a hole concentration of 7.57x1017 cm-3 at room temperature, has been obtained for the Li-F codoped ZnO thin films grown at 550℃The possible formation mechanism of the p-type conduction in Li-F codoped ZnO has been proposed by first-principles calculations based on density functional theory.3. We reported on p-type Ag-doped ZnMgO thin films prepared on quartz substrates by pulsed laser deposition. The effects of substrate temperature on the structural, electrical and optical properties of the films were investigated in detail. XPS measurement confirmed that Ag principally occupies Zn site in the state of Ag+ ion and acts as an acceptor. An acceptable p-type conduction, with a resistivity of 24.96Ωcm. a Hall mobility of 0.32 cm2v-1s-1.and hole concentration of 7.89x1017 cm-3 at room temperature, was obtained for the film grown at the optimal substrate temperature of 400℃Meanwhile, a comparative study of p-type ZnO.Ag and ZnMgO:Ag films showed that the incorporation of Mg can result in the increase of the activation energy of the intrinsic donors and the suppression of charge-compensating oxygen-related defects, thus reduce the charge compensation effect of intrinsic defects, which is favorable to the realization of p-type ZnO thin films.