Investigation On The Preparation And Properties Of MgZnO Thin Film Field-Effect Transistors

The rapid development of the information society promots the development of flat panel display technology. Thin film field-effect transistors（TFTs） are core components driving advanced display technologies, such as active matrix liquid crystal display（AMLCD）. After the development of amorphous silicon TFT, poly-silicon TFT and organic TFT, transparent oxide TFT is considered as a very potential candidate to be the next-generation TFT technology. Because of good stability, high mobility, simple process and suitable for low temperature manufacturing, ZnO based oxide TFTs have attracted much attention since early beginning. Because MgZnO-TFT has better stability than ZnO-TFT, MgZnO film with a certain content of Mg is considered to be an ideal material as the active layer of TFT devices. In this paper, MgZnO thin films prepared in different growth conditions were characterized, the structure and fabrication technology of MgZnO-TFT were studied, and the performance of the device is further optimized. The main contents are as follows:（1） MgZnO films were prepared on Si substrate by pulse laser deposition（PLD）. The effect of oxygen pressure, oxygen flow,and substrate temperature on optical and electrical properties was investigated by using test methods such as X ray diffraction（XRD）,scanning electron microscopy（SEM）, UV-visible spectrophotometer（UV-vis） and photoluminescence spectroscopy（PL）.To reduce the effects of the oxide layer and the mismatch,an Mg O buffer layer was induced between the substrate and MgZnO film. The results show that the crystal of MgZnO thin films has been greatly improved by employing the Mg O buffer layer.（2） The MgZnO-TFT device of bottom-gate structure was designed and fabricated by using heavily doped p-type silicon wafer as substrate and gate electrode, thermally oxide Si O2 as insulating layer, MgZnO thin film as active layer grown by pulsed laser deposition and metallic Al as the source and drain electrodes deposited by resistance heatingevaporation.The effects of active layer growth condition（oxygen pressure, substract temperature etc.） and N2 annealing processes on device properties were studied.A higher quality of MgZnO-TFT with threshold voltage of 26.5V, mobility of 2.10 cm²V^-1 s^-1, and current on/off ration of 4.2x10⁵ is obtaind.（3） Light stability and forward bias stability of TFT devices was studied. Under illumilation with different visible light wavelengths, the stability of MgZnO-TFT and ZnO-TFT were compared.The results show that the shorter the wavelength of the light is, the worse the stability of the device is, mainly reflecting the decrease of threshold voltage and the increase of leakage current.In contrast to ZnO-TFT, MgZnO-TFT shows better stability under various wavelengths of light irradiation.The stability of MgZnO-TFT at different gate biases was studied. With increasing gate bias from 40 V to 60 V, the transfer characteristics curve drifts to forword bias direction and the drift margins become bigger and bigger.The reason is attributed to some of the carriers captured in the defect states on the interface between the active layer and insulating layer.