Reliability Of Indium Zinc Oxide Thin Film Transistors Based On Low-frequency Noise

Indium zinc oxide thin film transistor(a-IZO TFT)has been widely used in flexible integrated circuits and flat panel displays due to its high mobility,large switching ratio,good light transmittance and good electrical stability.In the development of IZO TFT,reliability of devices is the main bottleneck restricting its further expansion of application fields.This thesis mainly researches on the reliability of IZO TFT under different stresses(illumination,electrical stress,temperature and hydrogen environment)based on low frequency noise.The major works and conclusions of this research can be summarized into these points:(1)The electrical characteristics of IZO TFT under negative bias illumination stress are discussed.And low frequency noise of the device before and after stress was also measured.The results of the individual illumination experiments show that the variations of current–voltage characteristics and electrical parameters are dominated by the stress-induced ionized oxygen vacancies and the acceptor-like states.The dependence of degradation on light wavelength is also investigated.More negative shift of threshold voltage and greater sub-threshold swing are observed with the decrease of light wavelength.At the same time,the negative bias illumination stress experiment shows that the degradation of the device is aggravated due to the decrease of recombination effects between ionized oxygen vacancies and free carriers,which reduced by the negative gate voltage.Moreover,the contributions of ionized oxygen vacancies and acceptor-like states are separated by using the mid-gap method.The results show that negative shift of threshold voltage is dominated by the ionization oxygen vacancies.In addition,ionized oxygen vacancies are partially recombined at room temperature and fully recombined at high temperature.Finally,low-frequency noise is measured before and after negative bias illumination stress.Experimental results show that the oxide trapped charge is almost unchanged during stress.It is proved that the generation of ionized oxygen vacancies during stress is the reason of the increase of low-frequency noise,which is consistent with the proposed degradation mechanism of the electrical characteristics of the device.(2)The transfer characteristics of IZO TFT were measured in the temperature range of 10-400 K.The variation of electrical parameters with decreasing temperature were then extracted and analyzed.Moreover,the dominated carrier transport mechanisms at different temperature regions were investigated.The experimental data showed that the carrier transport mechanism may change from trap-limited conduction to variable range hopping conduction at lower temperature.In addition,the temperature dependence of low frequency noise(LFN)characteristics in indium zinc oxide thin film transistors is investigated in the range from 10 K to 300 K.In the range of 10-300 K,the measured noise increases with the decrement of temperature,which follows the M-N rule in the range of 80-200 K and follows Mott's law below 80 K.Moreover,the measured noise shows that the device may be varied from an interface dominated device to a bulk dominated device at lower temperature,which may induce the variation of the power coefficient of normalized noise against the effective gate voltage.Finally,the variations of flat-band voltage spectral densities and average Hooge's parameters with temperature are studied.(3)The stability of the electrical parameters of IZO TFT in hydrogen environment was researched.The results show that hydrogen can penetrate the passivation layer into the device,which affects the electrical parameters of the device.Hydrogen combine with oxygen ions in the IZO film to form hydroxyl groups,releasing an electron and generating an ionized oxygen vacancy in the process.The variation of threshold voltage is caused by hydroxyl groups and ionized oxygen cacancy.The hydrogen anion is combined with the metal ions in the channel to form the M-H structure,which is the main factor for increasing the subthreshold swing of the device.In addition,the quantitative and qualitative discussion of the low-frequency noise characteristics of the device is carried out.The measured noise shows that the device may be varied from a bulk dominated device to an interface dominated device.The trapped charges of gate oxide have almost no change under the hydrogen stress,which means that the interface states at channel is the dominant factor for the increase of low frequency noise.That is,the increase of low-frequency noise caused by the generation of oxygen vacancies due to the infiltration of hydrogen,resulting in more random carrier random capture/release processes.