The Basic Research Of The ZnO Thin Film Transisor

Information image display is a way for people getting information, and is also a medium between human-computer interactions. Compared with the bulky cathode ray tube display device, the application of the flat panel display device is a great progress. Flat panel display device used as an information terminal has many advantages, such as full planar, light weight, thin thickness and less electricity. These advantages are the future direction of the image display devices development. In recent years, active-driven flat-panel display technology has become increasingly widespread in electronics products. The most representative applications are the thin film transistor (TFT) liquid crystal display driver (TFT-LCD) and organic light emitting display (TFT-OLED). With the 3G technology in large-scale commercial use, thin-film transistor monitor will have a new development opportunity.There are many advantages in ZnO based active matrix driving display devices. The ZnO thin film transistor usually has high carrier mobility, which can improve the response speed to display high-definition screen. Also, the ZnO has good light stability and is visible light transparent. This makes the ZnO based transparent thin film transistor (TTFT) have high transmittance in the visible range. The transparent thin-film transistor is helpful to increase the pixel aperture ratio, which will make the screen look more bright and clear. On the other hand, ZnO thin films can be prepared at relatively low temperature, thus the ZnO-TFT can combine with the OLED craft. So it is suitable for manufacturing a flexible display options. In this thesis, the MgO film was used as the insulator layer, and the zinc oxide film was used as the active layer. They are both grown by photon assisted metal organic chemical vapor deposition system (PA-MOCVD). The ITO glass was used as the substrate and the gate.Then, MgO insulation was deposited on it by PA-MOCVD method. The biggest changes are the controlling the oxygen flow rate and the growth temperature, which can greatly impact the MgO surface morphology and optical transmittance. It is found that the intensity of the photon assisted source has great affect on the surface morphology of the MgO films. With photon assistance, the surface of the MgO film distributes many different sizes islands, and the surface is very rough. Without photon assistance, the MgO thin film surface becomes more flat. Based on the light advantage theory, the photon can directly send its energy to the reactant. This is helpful to make the reaction proceed totally. So the MgO film trend to form crystal structure. Thus the island growth pattern emerges. But when the light is removed, the energy is greatly reduced, which lower down the activation of the MgO molecular. It would cause the MgO film accumulating layer by layer. Its crystal character becomes worse, but the surface seems much falter. After considering all of the factors, we have found the suitable condition on growing MgO insulator film.When the MgO insulator layer is prepared, n-ZnO film was deposited on it in MOCVD system, too. Then Al source and drain electrodes were made on the top of the ZnO film by thermal evaporation method. So the bottom gate TFT device could be described as ITO/MgO/ZnO/Al. Then, the optical character is investigated by UV-1700 spectrophotometer, and the transfer and output characteristics are measured using Keithley 4200 Semiconductor Characterization System. The transmission of the ZnO-TFT device is more than 90%, which has already met the requirements of liquid crystal displays. The on/off current ratio is at 103 magnitude, the threshold voltage is 2.5V, and the Field-effect mobility can reach 11.57cm2/Vs.In addition, we find the oxygen flow rate in MOCVD system has great affect on the performance of ZnO-TFT devices. If the oxygen flow is excessive, many defects would introduce into the MgO film and the chemical dosage rate of the MgO would change. It makes the insulation effect become worse. When the gate voltage increases, the leakage current is serious. But, if the oxygen flow rate is small, the reaction of the Mg organic source would be incomplete. Numerous carbon impurities would remain in the MgO film. And they could make the ZnO-TFT's transmission decrease, thus impact the performance of the whole devices.