The Modification Of The Properties Of SnO Thin Films And Their Ambipolar Transistors

In an ambipolar thin film transistor, the injection and transportation of both hole and electron carriers can be tuned by changing the polarity and magnitude of the gate voltage and source-drain voltage. Thus, ambipolar thin film transistors with thereby simplified circuit design and fabrication processes can reduce the complexity of device and circuit processing to the minimum. In recent decades, oxide thin film transistors are highly desirable due to their high field-effect mobility, small subthreshold swing, large on-off current ratio, good transparency, low-temperature process, and good compatibility with the Si-based technology. However, the majority of oxide thin film transistors is unipolar(either n-type or p-type), while ambipolar thin film transistors based on oxide semiconductor are rarely reported. The main challenges to realize ambipolar oxide thin film transistors lie in the performance optimization of the channel, and delicate design of the device structure. In this work, tin monoxide(SnO) is chosen as the channel material, and the investigations on the above-mentioned two key points are carried out. The main conclusions are listed as follows.(1) The tunable crystallographic grain orientation and the identification of Raman characteristic peak of polycrystalline SnO thin films are successfully implemented in this study.(001) or(101)-orientated polycrystalline SnO films were fabricated by the e-beam evaporation technique, respectively. The preferred orientation conversion was observed by modifying the stoichiometry of the SnO films. It was revealed that the O-rich and Sn-rich SnO films favor(001) and(101) grain orientations, respectively. Moreover, it is found that there is a close relationship between the Raman characteristic peak and crystallographic orientation of SnO. The intensity of 110 cm-1 Raman peak of SnO increases with the relative texture coefficient of the(101) grain orientation but decreases with that of the(001) one, which agrees well with the theoretical variation of the Eg mode on the grain orientation derived based on the Raman selection rule. Based on both theoretical and experimental results, it is verified that the 110 cm-1 Raman peak should be assigned to the Eg mode of SnO.(2) In order to improve the performance of the SnO ambipolar TFTs, one must find a way to balance and optimize both the injection and the transport of the hole and electron carriers. In this study, we mainly focused on the effect of the stoichiometric ratio of SnO and the barrier height of carrier injection, through modulating the Ar gas flow during rapid thermal annealing, the capping layer of Al2O3 on the back-channel surface, and the inserting layer of Al2O3 in between the sourcedrain electrode and channel layer to achieve optimized ambipolar properties of SnO TFTs. The results showed that, stoichiometric SnO channels are good for the improvement to the field-effect mobility, on-off current ratio and symmetric output of the ambipolar transistors, while Sn-rich or O-rich SnO channels may degenerate the ambipolar performance due to the existence of metal Sn defects and cause a conversion of the transistor properties to the high resistance or n-type characteristics because of the partial oxidization of SnO into SnO2, respectively. In addition, the introduction of Al2O3 inserting layer can reduce the surface state of SnO and then increase the injection barrier height of holes, which in turn improves the output symmetry and on-off current ratio of the SnO ambipolar transistors. The SnO ambipolar transistor with the optimized performance has field effect mobility of 2.85 cm2V-1S-1 and on-off current ratio of 1.3×103 in the nchannel operation and 0.90 cm2V-1S-1 and 1.7×103 in the p-channel region, respectively.