Inorganic Electric-double-layer Gate Dielectrics And Its Application In Low-voltage Oxide Micro/Nanotransistors

Oxide semiconductor-based micro/nanotransistors have potential applications in flat-panel displays, E-paper and chemical-biological sensors because of their low processing temperature and high electron mobility. Traditional transistors are gated by themally grown SiO2which shows high operating voltage because of the week gate coupling of such dielectrics. It is not compitable with the portable electronics. In this dissertation, inorganic low-voltage oxide-based micro/nanotransistors gated by SiO2solid electrolyte with electric-double-layer (EDL) effect were stuied, including thin-film transistors (TFTs) and nanowire transistors. The obtained results are summarized below.(1) SiO2solid electrolytes with high specific EDL capacitance were deposited by specific plasma enhanced chemical vapor deposition (PECVD) technology. The polarization mechnisim of a SiO2solid electroclyte was developed. Three polarizations are indentified at different:EDL formation (low frequencies), ionic relaxation (intermediate frequencies) and dipole relaxation (high frequencies). The polarization response of the SiO2solid electroclyte was optimized by changing the deposition condition and the improved specific capacitance is1μF/cm2at1kHz and remains0.6μF/cm2at10kHz. We have also stuied the morphology of SiO2solid electrolyte. Ultralow-voltage (<1V) transparent In-Zn-O TFTs gated by such dielectrics were fabricated. The field-effect mobility, current on/off ratio and subthreshold swing were estimated to be46.2cm2/Vs,106and69mV/decade. Such TFTs hold promise to perate at1kHz because of the fast polarization response of the SiO2solid electroclyte.(2) The SiO2-based proton conductors and transparent In-Zn-O coplanar homojunction TFTs gated by such dielectrics were prepared. As the H3PO4concentration changed from0%to50%, the specific EDL capacitance of the SiO2-based proton conductors and the upper frequency limit of EDL formation were increased, and the field-effect mobility, operating voltage, subthreshold swing and current on/off ratio were also changed. The operating voltage of such TFTs was changed from1.5V to0.6V as the H3PO4concentration increased. The transparent high performance In-Zn-O TFTs with a field-effect mobility of12cm2/Vs and a average transmittance of75%were obtained.(3) The paper TFTs (Sb-doped SnO2and InGaZnO4channels) gated by SiO2solid electrolyte were fabricated and the operation mode of such devices were tuing by oxygen-modulation technology. Battery drivable low-voltage sb-doped SnO2-based paper TFTs with a near-zero threshold voltage (Vth=0.06V) gated by microporous SiO2dielectric with EDL effect were firstly fabricated by our group. The operating voltage is found to be as low as1.5V due to the huge gate specific capacitance (1.34μF/cm2at40Hz) related to EDL formation. The subthreshold gate voltage swing, current on/off ratio and field-effect mobility were found to be<80mV/decade,>105and>21cm2/Vs, respectively. InGaZnO4thin-film transistors (TFTs) on paper substrates gated by SiO2solid electrolyte were fabricated at room temperature and the impedance spectroscopy (ionic-conductivity-frequency and capacitance-voltage characteristics) and Fourier-transformed infrared spectroscopy of SiO2were characterized. Most importantly, both depletion-mode (Vth=-0.45V) and enhancement-mode (Vth=0.25V) operations were realized by rationally controlling the oxygen concentration in argon ambient during InGaZnO4channel deposition.(4) We introduce a simple nickel grid mask method to fabricate transparent single Sb-doped SnO2nanowire transistors without use of lithographic tools. Damage of the nanowire’s surface can be avoided without any thermal annealing and surface modification, which is very convenient for the fundamental electrical and photoelectric characterization of one-dimensional inorganic nanomaterials. The high specific capapcitance (2μF/cm2) SiO2solid electrolyte was used as the gate dielectric which results in the operating voltage was reduced to less than1.5V. The field-effect mobility, current on/off ratio and subthreshold swing were estimated to be175cm2/Vs,105, and116mV/decade, respectively. The static and dynamic bias stress measurements indicate that transparent SnO2nanowire FETs can operate at low-voltage with highly reproducibility.(5) Threshold voltage (Vth) instability and surface passivation effect of transparent In-Zn-O EDL TFTs were investigated. Unpassivated devices show a large negative threshold voltage shift of0.68V in the beginning of light-illuminated negative gate bias stress. Under longer time stress, anomalous positive Vth shifts were observed for both unpassivated and passivated TFTs, which was due to the mobile ions drifting in the SiO2-based solid-electrolyte gate dielectric. After surface passivation, the devices show neglectable negative Vth shifts of less than0.1V due to the protection of channel against the photodesorption of adsorbed oxygen ions.(6) Novel double-gate low-voltage paper TFTs were fabricated by self-assembled method. Such paper TFTs have a bottom-gate gate and an in-plane electrode gate and the electrical modulation effect of the in-plane electrode was investigated. The threshold voltage (Vth) of such paper TFTs was tuned from-0.98to0.94V as the voltage biases of the in-plane electrode was changed from2V to-2V. High electrical performance with a current on/off ratio of6×105～106, a subthreshold swing of0.14～0.19V/decade, and a mobility of8.64～9.45cm2/V s was obtained at different in-plane electrode voltage biases.