Crystallization Process And Stability Of P-type SnO In Thermal Annealing

In recent years,with the progress of electronic industry,oxide semiconductors are highly attractive for a wide range of device applications,such as backplane driver of displays and flexible/transparent electronics,due to their advantages including high mobility(around 1-100 cm2/Vs),low temperature and even room-temperature processability,visible light transparency,large area processability,and low cost manufacturing.Almost all reported oxide semiconductors exhibit n-type conductivity,and development of p-type oxides are far behind.In order to develop complernentary metal oxide semiconductor(CMOS)technology and p-n junction components,p-type oxide semiconductor with similar performance to n-type counterpart are highly needed.Recently,SnO has been proven to be a promising p-type oxide semiconductor.SnO exhibits high hole mobility and excellent p-type conductivity due to combined contributions of delocalized Sn 5s and O 2p orbitals to the valance band maximum.This thesis work mainly studied the optimization of process parameters in SnO thin film transistors(TFTs),as well as the crystallization process and stability of SnO during thermal annealing,the main work and achievements are as follows:(1)The optimization of process parameters in SnO TFTsIn this paper,we fabricated a series of SnO thin films using reactive magnetron sputtering based on metal Sn target and Ar/O2 mixed gas.The sputtering power and partial pressure of oxygen(P0)can control the deposition rate and the degree of oxidation reaction,and thus play key roles in controlling the stoichiometry and conductivity of the thin film.Firstly,we optimized the sputtering power(when the oxygen partial pressure Po was fixed at 11.5%),the SnO TFTs were fabricated and tested at different sputtering power from 50 to 110 W.The optimized sputtering power was found to be 100 W according to parameters of the thin-film transistors(mobility,on current,off current,subthreshold swing and threshold voltage).However,the metal Sn target was prone to deteriorate during high power reactive sputtering because of its low melting point of 232? and easy oxidation under oxygen-included atmosphere.Thereby,in this work,we decided to fabricate SnO TFTs with low sputter power of 50 W,based on the comprehensive consideration of device performance,target stability,and device repeatability.Then,we optimized the oxygen partial pressure with the fixed sputtering power of 50 W.Our results revealed that:(1)When 1.5%?Po?4.2%,the SnO TFTs presented p-type conduction.(2)When 5.4%?Po ? 6.9%,the SnO TFTs exhibited a rather high resistivity.(3)When 10.1%? Po? 13.1%,the SnO TFTs presented n-type conduction.Comparing synthetically every parameter in SnO TFTs which exhibited p-type conduction,the optimized Po was found to be 3.1%.(2)The crystallization process and stability of SnO in thermal annealingThis paper systemically studied the detail of crystallization process and stability during thermal annealing at 200 ? in air atmosphere in a time range of 0?1500 minutes,by investigating the transfer characteristics of SnO thin film transistors,the optical microscope,scanning electron microscope images,X-ray diffraction patterns,and micro Raman spectra of the SnO channel.The channel started to be modulated by gate bias after being annealed for 5 min.The 10 min annealing leaded to the uniform crystalline SnO film with the highest hole mobility of 0.9 cm2/Vs.After the crystallization process of SnO was completed,the reduced Sni concentration and increased SnO2 acting as hole traps explained well the reduced hole mobility,on current and off current with annealing time increased.With annealing time increased from 10 to 1000 min,the subthreshold swing and on/off ration of SnO TFTs show inconspicuous variation,which indicated threshold voltage and output current can be adjusted by annealing time.With the annealing time beyond 1000 min,the increased SnO2 became predominant,leading to obvious performance degradation with annealing time increased.These results chould give significant guidance on how to achieve high performance p-type SnO via thermal annealing.