The Study On Thin Film Transistors And Memristive Devices Based On The InGaZnO/InGaZnO:N Double-layer Structure

The appearance of the information age was inseparable with the development of the information display and storage technology. In the field of information display technology, to cater to the requirements for high definition and large screen display of devices, higher mobility and stability of thin film transistor as the core element was being pursued, because the traditional silicon material based device could not meet this requirement. In the field of information memory, memristor has the extensive application prospect in the next-generation nonvolatile storage technology and the synapses simulation. However, the memristive application still faced some problems, such as: the fluctuation of the resistive switching(RS) parameters and the material selection of the neural synapse simulation device. While the amorphous InGaZnO materials offered certain advantages, including easily controlled electrical properties, high uniformity over a large area and excellent flexibility, which were considered as candidate materials for the new generation transistors and the memory devices. Based on the different electrical properties of the IGZO and IGZO:N films, we had constructed the IGZO/IGZO:N thin film transistors and memristive devices to realize the performance optimization and neural synaptic emulation. The specific content was shown as follows:A thin film transistor based on IGZO channel was designed and fabricated. The effect of preparation conditions such as chemical compositions, oxygen partial pressure and substrate temperature on the performance of the transistor were discussed in detail, for example:filed-effect mobility on/off drain current ratio, threshold voltage, gate voltage swing, on-current, off-current. The results indicated that: Firstly, the excess indium incorporation into IGZO could increase in conducting path ways to improve the saturation current and mobility of the device; Secondly, as the oxygen partial pressure reduced the carrier concentration of the channel films had also reduced, thus threshold voltage was shifted to the negative; Thirdly, the atomic order improved significantly for the IGZO films deposited on a heated substrate, and leading to a increase in electron mobility. We had already ensured the optimized conditions and the permance parameter as followed: Ion~3.5×10-4 A, Ioff~2×10-10 A,Ion/off~1.7×106,μFE=2.92 cm2/V·s,Vth=-15 V,SS=3.47 V/decade.To balance the mobility and the carrier concentration of the IGZO channel film, a thin film transistor based on IGZO/IGZO:N double-layer structure channel was designed and fabricated. by the insertion of an ultrathin nitrogenated a-IGZO(a-IGZO:N) layer at the channel/dielectric interface. The double-layer channel device showed higher saturation mobility 5.74 compared to its single-layer. The improvement could be attributed to three aspects:(1) the IGZO layer with high carrier concentration could ensure the amount of the carrier,and the IGZO:N layer with high hall mobility had the smoother carrier transfer patch(2) the passivation effect of nitrogen doping on the surface between the channel and the dielectric(3) the higher surface flatness of the double-layer channel. Also the results showed the double device had the smaller shift of the threshold voltage(2.6 V to 7.2 V). On the one hand, the IGZO layer with the high carrier concentration could effectively prevent the channel from exposing to the atmosphere and retarding interactions with ambient oxygen species. On the other hand, the a-IGZO:N layer taken the advantage of the passivation effect of nitrogen doping could effectively reduce interfacial trap density between the active channel and the gate insulator.According to the resistance change mode, the memristor could be divided into digital RS and simulative RS, the former realized the switching between the high resistance state and low resistance state, using for the high-density memory; the latter realized the continuous adjustability of the resistance state, using for the neural synaptic emulation. Based on the double-layer IGZO/IGZO:N memristor, we had designed and fabricated the digital RS device and simulative RS device in this thesis. For the digital RS device, the double-layer IGZO/IGZO:N RRAM devices showed the small forming and switching voltages and the uniform RS. It can attribute to the different migration velocity of the oxygen ions, leading to the defference of the size of the conduction filaments(CFs) in the IGZO and IGZO:N films(it could form the thinner CFs in the IGZO:N layer, while the thicker CFs in the IGZO layer). Thus, the region of the RS was located in the IGZO:N layer, reducing to the randomness of the forming and rupture of the CFs, improving the device performance. For the simulative RS device, based on the migration of the conductive filaments medol using the migration and diffusion of the oxygen ions, it realzed the continuous change of the thickness of the IGZO:N insulation layer and the continuous adjustability of the resistance state. Based on this IGZO/IGZO:N neural synaptic emulation device, several essential synaptic functions were achieved, including the nonlinear transmission characteristic, spike-timing-dependent plasticity(STDP), long-term/short-term plasticity and “learning-experience” behavior. It showd the similar transmission characteristic with cognitive rules of organism.