Mechanism Investigation And Process Development Of Nitrogen-doped Amorphous Oxide Semiconductor Thin Film Transistors

Amorphous oxide semiconductors（AOS）,including amorpous InGaZnO（a-IGZO）,amorphous InZnO（a-IZO）etc.,have been regarded as a glorious candidate to replace the traditional Si-based semiconductors as the channel materials for the next-generation thin film transistors（TFTs）.So far,AOS TFTs exhibited outstanding features such as high field-effect mobility(>10 cm² V^-1·s^-1),room temperature deposition,and good uniformity.However,there are still some concerns（bias instability,thermal instability,and illumination instability,etc.）in AOS TFTs.Recently it was reported that nitrogen-doping（N-doping）technology could effectively improve the device stability of AOS TFTs.However,the related physical mechanism of N-doping still remains unclear.In addition,it is quite meaningful to develop the N-doping process which is fit for mass production of AOS TFTs.In this study,we aimed to investigate the N-doping mechanism in AOS TFTs,based on which we seeked more processing methods and novel materials to further improve the device stability.These results built theoretical fundations for the commercial application on N-doped AOS TFTs in flat panel displays（FPDs）.Firstly,we investigated the N-doping effect on the electrical properties of AOS TFTs.Here,we carried out the comparative study of N-doping and oxygen-doping（O-doping）for the typical AOS TFTs,i.e.,a-IGZO TFTs.It was found that the O-doping dramastically decreased the concentration of oxygen vacancy（V_O）in the a-IGZO films,along with more bulk and interface trap states.However,the N-doping moderately adjusted the concentration of V_O and brought about almost no trap states in the a-IGZO films.Hence,the O-doping resulted in the seriously degraded electrical performance of a-IGZO TFTs,while the N-doping showed no adverse influence on the device performance.The N-doped a-IGZO TFTs exhibited better electrical stability than the O-doped a-IGZO TFTs,especially during thermal stress.The XPS characterization results indicated that N-doping caused lesser variation of V_O(smaller O₂/O_total in O 1s spectra)with temperature increasing compared with O-doping.Hence,the N-doped a-IGZO TFTs had much better stability than those with undoped and O-doped channel layers.Secondly,we investigated the chemical bonds in N-doped AOS（AOS:N）thin films,including N-doped a-IGZO（a-IGZO:N）and N-doped a-IZO（a-IZO:N）.We explored the N-doping mechanism in AOS TFTs by device tests and film characterization.As for a-IGZO:N,nitrogen atoms could preferentially combine with Ga cations and form stable Ga-N bonds along with few N-related defects for low N-doping,but turned to additionally forming less stable In-N and Zn-N bonds as well as excess N-related defects for high N-doping.Accordingly,the stable Ga-N bonds along with few defects made V_O variation more difficult and hence achieved better device stability for low N-doping.The less stable In-N and Zn-N bonds as well as excess defects led to easier change of V_O and thus more unstable a-IGZO:N TFTs for high N-doping.However,there existed unstable In-N and Zn-N bonds rather than stable Ga-N bonds in the a-IZO:N,which was quite different from the case in the a-IGZO:N.Therefore,with the N-doping concentration increasing in the a-IZO:N films,there produced many more unstable Metal-N（In-N,Zn-N）bonds and N-related defects,leading to larger V_O variation in the films and thus achieving more deteriorated properties of a-IZO:N TFTs during light-illumination and thermal stress.Thirdly,we combined N-doping with double-stacked channel layers（DSCL）to prepare more stable devices（a-IZO/a-IGZO:N TFTs and a-IZO:N/a-IGZO:N TFTs）.The DSCL AOS:N TFTs exhibited the superior electrical performance and stability.The large field-effect mobility might be due to the DSCL structure composed of a high-defect-density channel layer（a-IZO,a-IZO:N,etc.）and a low-defect-density channel layer（a-IGZO:N）.In addition,the a-IGZO:N films well suppressed the variation of defects and V_O in the channels of TFT devices,weakening the influence of light-illumination and ambient gas on the device channels,leading to the better device stability than the other samples.Finally,we tried to develope novel passivation layers（MoZnO（MZO）,AlMnZnO（AMZO）,etc.）to improve the light-illumination stability of N-doped AOS TFTs.We combined N-doping with MZO（or AMZO）passivation layers,making the AOS TFTs more stable,especially for the light-illumination stability.This might be due to the fact that the MZO（or AMZO）acted as not only an isolator for ambient gas,but also a shielding layer for ultra violet（UV）light,suppressing the defect formation in the bulk channel layers.