| Thin-film transistor is composed by the metal gate (gate, G), insulated layer or semiconductor depleted layer, channel,source(S)and drain(D) with ohmic contracts. The main application of thin-film transistor is employed as switching device for pixel. Each pixel department must need a thin film transistor which controls light to pass or block .These"on" and "off" states are controled by electric field. At present, amorphous silicon thin film transistors are the most widely employed pixel switch devices , but the mobility of amorphous silicon thin film transistor is very small, generally smller than 1cm2V-1s-1. Also they need additional mask layers,due to serious photosensitive degenerative effects. What`s more, a -Si material is opaque. Therefore, a -Si will occupy a certain area of pixels, and rereduce the effective display area, which cause the the open ratio of pixel usually can not reach 100%. Since a-Si: TFT have many drawbacks, one of the most possible candidated solutions is to adopt transparent semxonductor material ZnO as active layers. ZnO film is a direct wide band gap (room temperature band gap of 3.37eV) transparent oxide semiconductor.This material has many advantages , such as a low growth temperature, strong radiation resistance,weak photosensitive degenerative effects without mask layers, chemical stability,high mobolity and so on . ZnO thin film with transmission rate more than 80% in the visible range is preparated on transparent substrate, The film has a mobility up to 36cm2 / V·s, and on / off current ratio greater than 106. In addition, ZnO have excellent piezoelectric, optical, pressure sensitive and other properties. But there are still some disadvantages of ZnO-TFT ,such as larger off-state current and the poor repeatability.At present, although the research of zinc oxide thin film transistors has made some achievements, but there are still a lot of work to do, for example:(1) The crystal quality of ZnO, including grain size and surface morphology, is a vary important factor of a good TFT. The sizes of the grains should be uniform and appropriate, and if the grains in ZnO channel layer have a smaller sizes, many grain boundaries will be in present, which will lead to lower electronic mobility.(2) ZnO surface morphology can not be too rough; if it is too rough the field-effect mobility will be decreased. Thickness of ZnO film should be appropriate, because a thicker film will have a larger average grain size and larger roughness of surface, and generate higher fault current. Thus defects in ZnO thin film will be increased and field-effect mobility will be decreased.This thesis studies main issues of ZnO thin film transistor. Silicon dioxide dielectric layer is prepared by thermal oxidation and ZnO active layer is fabricated by molecular beam epitaxy (MBE), the detailed contents and conclusions are as follows:(1) Silicon dioxide is prepared by thermal oxidation, which makes surface more compact and more uniform. Thus the dielectric constant is greatly increased and passivation effect is better. And the breakdown voltage of silicon dioxide grown by thermal oxidation is about 50 volts.(2) The source and drain of ZnO thin film transistor is prepared by lithography. Through our exploration, electrode of zinc oxide thin-film transistor is successfully prepared.(3) Using MBE method crystal quality of ZnO thin film has been greatly improved, grain size of ZnO film is more uniform, the surface of ZnO film is more flat, and defect is decreased. And the ZnO thin film transistor shows a better I-V characteristic. |
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