Study On The Co-doping Of ZnO Films And Fabrication Of ZnO Based Thin-film Transistors

Zinc oxide?ZnO?is a very important wide-bandgap semiconductor material.Its advantages of excellent optoelectronic properties,low growth temperature,high radiation resistance,non-toxic,environmentally friendly,and abundant make it a promising candidate in the area of optoelectronics,flexible electronics,and high-voltage electronics.However,the lack of stable and reliable p-type ZnO materials limits its practical application in the field of optoelectronics.ZnO-based flexible thin film transistors?TFTs?and high voltage thin film transistors?HV-TFTs?require better and performance and stabilities.To this end,we have carried out research work from both basic science and application technology.From the perspective of basic science,we used the co-doping method to study the point defect behavior in ZnO,trying to provide a possible solution for p-type doping.We successfully prepared Na-F co-doped ZnO single crystal thin films by ratio frequency plasma-assisted molecular beam epitaxy.Combining the results of X-ray photoelectron spectroscopy,secondary ion mass spectrometry and variable temperature Hall effect,the improvement of the solid solubility of the Na impurity and the suppression of the interstitial Na compensation donor were verified.The doping concentration of F and Na up to 10²⁰ cm^-3 did not affect the lattice constant.The introduction of F improved the crystal quality,surface roughness,and the Hall mobility of ZnO thin films.F doping increased the Fermi level of ZnO thin films,which increased the formation energy of Na interstitials,while decreased that of substitutional Na.Therefore,most of the Na impurities occupied the lattice position of Zn,and the proportion of Na interstitial was much smaller than 2.3%.The high doping concentration and low electron concentration of the Na-F co-doped ZnO film indicated that most of the F donors are compensated by acceptors.The high mobility and the uniform depth distribution of Na and F indicated that Na and F form(F_O⁺-Na^-_Zn)⁰neutral complex defect.Doping also affected the luminescence of ZnO thin films.The D⁰X peaks related to F donors around 3.352 eV appeared in the low-temperature photoluminescence spectrum of ZnO:F and ZnO:Na-F.The intrinsic ZnO exhibit deep-level luminescence?DLE?associated with Zn vacancies(V_Zn),while the formation of F_O⁺-(1_Zn^2—complex defects in ZnO:F eliminated isolated V_Zn defects.The formation of(F_O⁺-Na^-_Zn)⁰ in ZnO:Na-F resulted in a large amount of V_Zn,enhancing the DLE.From the perspective of application technology,we have fabricated ZnO-based flexible thin film transistors and explored two high-temperature-resistant flexible substrates,where the performance and stability of the device were improved by annealing.Flexible thin film transistors are mainly fabricated on polymer substrates.However,they cannot withstand high processing temperatures.Susceptibility to gas permeation and large thermal expansion coefficient are the other drawbacks of polymers.We first investigated the flexible ZnO TFT on the metal copper substrate and obtained the 25?m-thick flexible copper foil by the electroplating,which solved the problem of rough surface of the metal substrate.The performance and stability were improved after annealing.The TFT performance remained stable after 10,000 bending cycles.Then we studied the flexible transparent InGaZnO?IGZO?TFT on the Muscovite mica substrate,which has an average transmittance of 86.75%in the visible range.As the annealing temperature increased,the electrical properties and stability of the TFT increased.The 15?m-thick mica has excellent mechanical properties and the performance of the TFT remained stable after 10,000 bending cycles.These two high-temperature-resistant substrate are beneficial to the practical application of high performance and high stability flexible electronic devices.From the perspective of application technology,we have also fabricated high voltage thin film transistors.We have used a double-layer channel structure to passivate the dielectric/channel interface defect states and realized high performance HV-TFTs.We first studied the effects of different thicknesses of the two layers on the performance and stability of the TFT.The low-resistance IGZO layer had an electron concentration of 10¹⁹ cm^-3,which passivated the interface defect states.The extremely smooth surface morphology provides a good homojunction interface,The mobility,subthreshold swing,threshold voltage,and hysteresis voltage were greatly improved.The thick high-resistance IGZO layer increased the contact resistance between the source/drain electrodes and the channel.It also has a great impact on the hysteresis,which changes from clockwise to counterclockwise.Then we prepared the HV-TFT with the optimal dual-layer structure,and achieved a ON/OFF ratio of up to 10¹⁰ and a breakdown voltage higher than 200 V.The offset structure only affected the current in the threshold region while the threshold voltage,hysteresis voltage,and off-state current were not affected.The comparison between the TCAD simulation and the experimental results showed that the high on-state current originated from the conductive path provided by the low-resistance IGZO layer.Finally,we demonstrated the phenomenon of negative drain current at off-state,which existed widely in thin film transistors.By establishing a dynamic equivalent circuit of the TFT in the off state,we concluded that the negative drain current is derived from the discharge effect of the gate dielectric capacitance.The smaller drain voltage,smaller initial gate voltage,larger scanning rate,larger gate dielectric resistance,larger gate dielectric capacitance,and smaller drain contact resistance would accelerate existence of the negative off-state drain current.Above all,we revealed the point defects behavior in ZnO,which is helpful for understanding the electrical and optical properties of ZnO.It is also useful for the study of related materials.We have improved the performance and stability of flexible thin film transistors.The high-temperature-resistant substrates are not limited to TFT applications.We have introduced a dual-active-layer structure in high-voltage thin-film transistors,which improved the breakdown voltage and ON/OFF ratio.we have demonstrated the negative off-state drain current phenomenon and explained the mechanisms.