Research On Electrical Properties And Integrated Circuits Of Transparent Zinc Oxide Thin Film Transistors

Metal oxide semiconductors have attracted wide attention in scientific and industrial communities due to low fabrication temperature,low cost and high mobility.Metal oxide thin film transistors（TFTs）have gradually replaced amorphous silicon TFTs in large scale displays and been applied in low temperature poly oxide（LTPO）with polysilicon TFTs,which is often used in small size and high-resolution active matrix organic light-emitting diodes（AMOLED）displays.Moreover,metal oxide semiconductors have great potential in flexible,transparent and wearable electronics due to their wide band gap and diverse substrate.Metal oxide,including indium gallium zinc oxide（IGZO）,zinc tin oxide（ZTO）,indium tin zinc oxide（ITZO）,indium oxide（In₂O₃）,and zinc oxide（ZnO）,mainly exhibits N-type characteristic.ZnO is one of the most promising metal oxide semiconductors due to its low price,high reserves,simple atomic layer deposition（ALD）process,and excellent electrical properties.However,ZnO TFTs also need to have better stability,higher mobility,and the feasibility of large-scale circuits.Therefore,this work systematically studied the factors that affect the stability of ZnO TFTs under stress,ZnO TFTs with high stability and mobility,a variety of high-performance inverters based on ZnO TFTs,and a simple one-bit microprocessor that implements bubble sorting algorithms.The main research content and results of this article are as follows:（a）Aluminum oxide and silicon oxide were deposited by ALD and magnetron sputtering as first insulator layer with ALD ZnO as active layer to study the effect of insulator on device performance and stability.It was found that devices with silicon oxide have better stability under negative bias illumination stress（NBIS）due to large valance band offset（VBO）.By employing laser irradiation of 650 nm,532 nm,488 nm,450 nm,and 405 nm,the degradation mechanism of threshold voltage of ZnO TFTs under NBIS is clearly clarified as the ionization of shallow/deep level defects and the implantation of valence band holes.Therefore,this work adopted O₃ ALD ZnO instead of hydrothermal ALD ZnO to reduce the defects of oxygen vacancies,and further increased the VBO by proposing an innovative material of ALD SiAlO_Xas holes-blocking layer to improve stability of devices.The optimized ZnO TFTs exhibit high mobility(43.8 cm²V^-1s^-1),negligible hysteresis voltage（<0.01 V）,and good stability(NBIS:ΔV_TH=-1.09 V)。（b）An innovative method of in-situ ozone annealing was proposed to improve the performance of ZnO TFTs.The effects of annealing time and temperature on devices’perfoemance were systematically studied to determine the optimal annealing condition.The results of X-ray photoelectron spectroscopy（XPS）,atomic force microscope（AFM）,and scanning electron microscope（SEM）showed that ozone in-situ annealing can effectively reduce oxygen vacancies defects,decrease surface roughness of ZnO film and enhance the crystallization of ZnO nanocrystals.Finally,ZnO TFTs annealed in ozone at 300℃for 500seconds achieved the maximum fileld effect saturation mobility of 71.27±6.36 cm²/Vs,an increase of 257.24%compared with traditional devices,and good stability under various stresses for 40000 seconds.（c）High-performance depletion-load inverters,Pseudo CMOS inverters and novel feedback inverters have been designed and fabricated.The threshold voltage of the devices is regulated by shallow donor doping and electrostatic field with deuterium and oxygen plasma,respectively.The feedback inverter utilizes the principle of positive feedback to enhance the direct current（DC）voltage gain and to increase noise margin of inverter without additional bias voltage.With bias voltage of 10 V,pseudo CMOS inverter achieves uncertain region of 0.03 V accounting for 0.6%of the amplitude,which means large noise tolerance.The depletion-load inverter and feedback inverter achieved DC voltage gain of up to 304 V/V and 380 V/V,respectively.（d）To verify the feasibility of manufacturing integrated circuits by ZnO TFTs,a simple one-bit microprocessor implementing bubble sort algorithm was designed and fabricated by depletion-load inverter using oxygen plasma.The process followed the design rules of digital circuits based on CMOS.Firstly,a transisitor-level circuit was designed and simulated using Spice model extracted from ZnO TFTs.And then the layout was prepared and the circuits were fabricated and tested.The one-bit microprocessor consumed area of 6.86 mm² including 10instructions and 771 transistors.With operating voltage of 5 V,the microprocessor can perform bubble sort at clock frequency of 500 Hz with power consumption of 37.15μW.When the operating voltage decreases to 3.2 V,the microprocessor achieved ultra-low power consumption of 682.56 n W at 1 Hz.The average static power consumption of single device is less than 1 n W.