Fabrication Of ZnO Thin Film Transistors And Study On The Optical Properties Of CdS_xSe_1-x Quantum Dots

ZnO, a kind of semiconductor material with a direct wide band-gap, possesses larger exciton binding energy than GaN and is easy to realize ultraviolet excitonic emission at room temperature. ZnO has been extensively studied in recent years for various optoelectronic applications, such as ultraviolet light-emitting diodes (LEDs), laser diodes, thin film transistors (TFTs) and ultraviolet detectors, etc. Recently, a research on the TFTs with ZnO thin film as an active channel layer has been made both in and abroad. It is noteable that this kind of TFTs can be made at a low temperature so it can be deposited at low temperature and do not require substrate rigorously. Furthermore, this transistor can be made entirely transparent. If ZnO-based TFTs can be applied in active-matrix liquid crystal display (AMLCD), it will simplify the processing of TFT-LCD, reduce the cost, and enhance the reliability of transistors.In this work, high quality ZnO films were deposited on SiO₂/Si and Al₂O₃ substrates by pulsed laser deposition (PLD) with optimized technique. The preparation of so high quality ZnO films is the base of the realization of the ZnO optoelectronic devices. Moreover, on the base of the present study of ZnO-TFT as well as the conditions of our lab, we designed a reasonable structure for ZnO-TFT. That is first growing one insulated layer (SiO₂) on p-type silicon (111) substrate, then deposit ZnO layer on this insulated layer by PLD, and prepare the drain and source polar to form channel by vaporation equipment, lithography and peeling technology in the end. By electric test it was found that the transistor worked well on n-channel enhancement mode and exhibits excellent saturation and pinch-off characteristics. I_on/I_off ratio is about 10³, the threshold voltage is 13.2 V, and the mobility rate is 1.02 cm²/Vs. As was stated, ZnO, a promising material, has a good future and can be used in TFTs.At the beginning of bur research work, we tried to test the electroluminescence (EL) properies of CdS_xSe_1-x quantum dots (QDs), in order to fabricate CdS_xSe_1-x QDs based optoelectronic devices. But the fact that large pores exist among the particles leads to poor electrical properties, thus the corresponding devices could not be made out successfully. Then we turned to discusse the temperature-dependent photoluminescence (PL) properties of CdS_xSe_1-x QDs. The result shows that when the temperature is increased from 10 K to 300 K, the peak wavelength redshifts, the FWHM becomes broader. We have also made a qualitative analysis for the anomalous increasing behavior of the PL intensity with increasing temperature in the range of 180-200 K. Moreover, the parameters of the Varshni relation for CdS_xSe_1-x materials are obtained by PL peak energy as a function of the room temperature and the best-fit curve:α= (3.5±0.1)10^-4 eV/K, andβ= 210±10 K. which is well close to the Debye temperatureθ_D of the material. Finally, we valued the ratio of S to Se for CdS_0.9Se_0.1 QDs by the results from XRD pattern and PL peak energy at room temperature according to Vegard Law. These results obtained from our experiments are important for the proposed optoelectronic applications of CdS_xSe_1-x QDs.