Majority Carrier Type Change In Carbon Nanotube Thin Film Transistors And Flexible Devices

Single-walled carbon nanotubes?SWNTs?have advantages of superior electrical and mechanical properties for using as channel semiconducting material in future electronic applications.The field effect transistors based on SWNTs behave as P-type devices when exposed to air due to influence of adsorbed oxygen and water molecules.Therefore,we focused on fabricating N-type devices in a way that do not require high temperature process,applicable to flexible substrate and compatible with photoetching process.Here,we developed a bilayer dielectric structure of MgO/Al₂O₃?or HfO₂?layer,which could achieve air-stable N-type carbon nanotube thin-film transistors?CNT-TFT?at low temperature of 120? or 90?.The Al₂O₃?or HfO₂?layer was deposited by atomic layer deposistion?ALD?As-made N-type devices show average mobility of 14.9±5.2 cm2/Vs,on/off ratio of 104.7.The N-type property maintain unchanged after 104 days in air.The advantage of this method is the low temperature and compatability to photoetching process,thus facilitating circuit integaration.By integrating both P-type and N-type devices together,CMOS inverter,NOR gate and NAND gate have been achieved.The CMOS inverter with local top gate shows gain of 23.As the bilayer structure can be realized at low temperature,we fabricated N-type carbon nanotube thin-film transistors on flexible PET substrate,which show on/off ratio of 103,subthreshold swing of 248 mV/dec,and average mobility of 7.07 ± 1.73cm2/Vs.By investigate the influence of MgO and Al₂O₃?or HfO₂?layer on the electrical behavior of devices,we studied the mechanism behind majority carrier type change.Firs,Assimilation of water molecules by MgO layer can reduce the trap state density and deplete the initial hole carriers.Second,the MgO layer can prevent the influence of water molecule in ALD process on the carbon nanotube.Third,the positive fixed charge in Al₂O₃?or HfO₂?layer can electrostatically dope carbon nanotubes.Also,Al₂O₃?or HfO₂?layer can prevent the penetration of oxygen and water molecules from the air.