Solution processed metal oxide-based thin film transistor for CMOS

Thin-film transistors (TFTs) using oxide semiconductor channels have intensively been investigated as oxide semiconductors such as In–Ga–Zn–O (IGZO) show field effect electron mobilities in excess of 10 cm2 V−1 s−1, higher than that of hydrogenated amorphous silicon. Additionally, such oxide semiconductors can be deposited by a wide range of large-area compatible vacuum-based techniques. Despite however the tremendous potential, further advancements have been hampered by a lack of hole-transporting oxides with similar or comparable transport characteristics to their n-type counterparts, limiting the applications of oxide-semiconductor-based TFTs to relatively simple devices and circuits such as backplanes for active-matrix information displays where n-channel TFTs only are needed. Although there are a reports on p-type doping of traditional n-type oxides, the subject still remains controversial as doping of metal oxides is typically one-sided due to self-compensation so alternative metal oxides that show intrinsic p-type characteristics are required. To date, only a few compounds such as SnOx and Cu2O have been realised and incorporated into p-type TFTs. Both compounds however show relatively low hole field-effect mobilities in the range between 0.001 and 4 cm2 V-1 s-1 and the TFTs show significantly high off currents. Collectively, these results reaffirm that field-effect mobility of >1 cm2 V-1 s-1 is generally achievable with Cu2O transistors. In the case of Cu2O this is further supported by the fact that Hall Effect measurements showed mobilities exceeding 100 cm2 V-1 s-1 reaching values as high as 250 cm2 V-1 s-1. This presentation reports on the deposition of both p-type Cu2O-based as well as n-type ZnO-based thin films by spray coating in air at substrates temperatures in the range between 100 oC and 450 oC from acidic and alkaline solutions of Cu2O and ZnO nanoparticles. The films’ properties were investigated by means of UV–Vis spectroscopy, x-ray diffraction, AFM and field-effect measurements and the effects of the solution’s pH and the substrate’s temperature are discussed in respect to the thin films’ structure and electronic transport properties. Furthermore, TFT’s employing spray coated cubic Y2O3 gate dielectrics and Cu2O and ZnO-based semiconducting channels show excellent operating characteristics, in particular low operation voltage, high charge carrier mobility on the order of 4 cm2 V-1 s-1, low off currents (1 nA) and high current modulation ratio >10^4. The deposition methodology as well as the results that are presented here, demonstrate an alternative route for the manufacturing of large-area oxide electronics and represents a significant step towards the development of low-cost, large-area complementary metal-oxide-semiconductor circuits at moderate temperatures