Study On The Thin-film Transistors Gated By Proton Conducting Films

Recently, oxide thin-film transistors (TFTs) have attracted a lot of attentions because of their high electron mobility, good compatibility with flexible substrates, optical transparency at visible light and low temperature fabrication process in large-size area. However, oxide TFTs gated by conventional dense dielectrics often need a large operation voltage, which great affects their applications in low-power electronics. Therefore, it is highly desirable to find an appropriate dielectric with effective electrostatic modulation and to develop new devices structure with new processings for obtaining low voltage driven TFTs with high electrical performances.In order to improve the electrostatic modulation, proton conductors are proposed as the gate dielectrics in electric-double-layer (EDL) TFTs in this thesis. The proton conductors have a strong EDL modulation effect, which can greatly decrease the operation voltage. In this thesis, different proton conductors are proposed as the gate dielectrics, different oxide channels are used, new devices structures are proposed. The main results are listed as follows.Firstly, nanogranular Al2O3films were deposited by plasma-enhanced chemical vapor deposition. A big proton conductivity of～1.4×10-4S/cm is obtained. The nanogranular Al2O3films have a typical capacitance-frequency dependence, similar to that of ionic liquid. A large electric-double-layer (EDL) capacitance of1μF/cm2is obtained. At the same time, chitosan solutions are obtained with acetic acid as solvent. Then the chitosan films are coated on glass substrate. Both a big proton conductivity of～2.3×10-3S/cm and a big electric-double-layer (EDL) capacitance of～4μF/cm2are obtained.Secondly, patterned ITO films are sputtered on nanogranular Al2O3coated conductive glass with a metal shadow mask. The patterned ITO films are used as the source/drain electrodes. Due to the diffraction effects, a thin ITO channel could be self-assembled between the source and drain electrodes. The bottom conductinve layer could be used as the bottom gate. Thus, a bottom-gate ITO TFT is obtained. The device could be driven at2.5V. Similarly, a double layered nanogranular SiO2/nanogranular Al2O3films are also used as the gate dielectrics. The bottom-gate ITO TFTs gated by the double layer could be driven at1.5V. The field-effect mobility, subthresold swing and current ON/OFF ratio are estimated to be～20.8cm2/Vs,～85mV/decade and～5×106, respectivelyThirdly, patterned ITO films are sputtered on nanogranular Al2O3coated conductive substrate with a metal shadow mask. The two terminals of the ITO films could be deemed as the source and drain electrodes. The bottom conductive layer could be deemed as the gate dielectrics. Thus, a bottom-gate junctionless ITO TFTs are obtained. The device have good electrical performances, such as low driven voltage at2V, a high current on/off ratio of6.9×106, a low small subthreshold swing of80mV/decade and a high field-effect mobility of26.4cm2/Vs. At the same time, nanogranular Al2O3gated coplanar-gate junctionless ITO TFTs are also obtained with good electrical performances, such as a low-voltage operation of2V, a field-effect mobility of11.1cm2/Vs, a small subthreshold swing of128mV/decade and a high current on/off ratio of1.1×10.Fourthly, patterned IZO films are sputtered on chitosan coated conductive glass with a metal shadow mask. A thin IZO films could be self-assembled between the source and drain electrode due to the diffraction effects. Thus, a bottom gate IZO TFT is obtained. The device have good electrical performances, such as low driven voltage at1.5V, a high current on/off ratio of3.7×107, a small subthreshold swing of116mV/decade and a large field-effect mobility of26.3cm2/Vs. At the same time, single coplanar-gate TFTs gated by chitosan are also fabricated with high performances with a low voltage operation of1V, a large field-effect mobility of34cm/Vs, a small subthreshold swing of87mV/decade and a high current on/off ratio of1.5×108.Fifthly, lateral IZO/chitosan/IZO structure shows a large electric-double-layer (EDL) capacitance of～0.55μF/cm2, indicating a strong lateral electrostatic coupling effect for such structure. Chitosan films are deposited on a paper substrate as gate dielectrics. Then, IZO source/drain electrodes, lateral gate electrodes and self-assembled channel were deposited by radio frequency magnetron sputtering with a one-step process through a shadow mask. Then a laterally coupled IZO TFT is obtained. A good electrical performance is obtained, such as a low voltage operation of1.5V, a high current on/off ratio of3.2×106, a low small subthreshold swing of136mV/decade, a high field-effect mobility of0.36cm2/Vs. Furthermore, no degradation is observed for the devices under bending, indicating its potential flexible electronics applications. When the devices are worked in the dual lateral gate mode, a robust "AND" gate operation is obtained. Furthermore, free-standing chitosan filmes with thickness of～16.4μm are obtained. Then, free-standing IZO TFTs with lateral gate are obtained. Good electrical performances are obtained, such as a low voltage operation of1V, a large field-effect mobility of1.2cm2/Vs, a small subthreshold swing of119mV/decade, a high current on/off ratio of105and a interface state density of7.7×1012cm-2. Similarly,"AND" logic operation are realized on the laterally gated IZO TFT work in dual lateral gate mode.Sixthly, GaN TFTs gated by nanogranular SiO2were prepared. I-GaN films are deposited on PALE-AlN buffer. The effects of the thickness of PALE-AlN buffer layer are investigated. I-GaN epilayers with high quality are grown on200period PALE-AlN buffer layer. Then, a lightly doped n-type GaN layer is epitaxially deposited on the I-GaN layer. Such n-GaN shows a low roughness of～0.58nm, an electron concentration of～8.2×1017cm-3and a large field-effect mobility of178cm2/Vs. At last, top-gate GaN TFTs gated by nanogranular SiO2were obtained.In this thesis, nanogranular Al2O3, chitosan and nanogranular SiO2films are obtained with high proton conductivities. A series of low-voltage driven oxide TFTs with different structures are obtained on low cost substrate by single step magnetron sputtering using metal mask. Good electrical performances are obtained. These devices have potential applications in low-cost, low-power consumption, biological electronical systems and portable electronics.