Conductivity Mechanism Of Ambipolar And P-type SnO Thin Film Transistors

Oxide semiconductor thin film transistors(thin film transistors,TFTs)have advantages in high mobility(?1-100 cm2/Vs),high transparency in visible light region,low process temperature and thus can be prepared on flexible substrates,large area uniformity(e.g.by sputtering,solution method,etc.),and low cost.Therefore,oxide semiconductor TFTs have great potential in the application fields including flat panel displays,wearable electronics,and internet of things.Compared with n-type oxide TFTs which have already been commercialized,the progress of p-type oxide TFTs are much behind so far.Moreover,ambipolar oxide TFTs which can realize both p-type and n-type conductivity in one channel are very rare.Due to the lack of p-type and ambipolar oxides,the development of oxide based CMOS circuits are limited severely.Therefore,high-performance p-type and ambipolar oxide semiconductors have always been a major challenge for oxide semiconductors.There are two main reasons for the high challenge of achieving p-type oxides.One is that the valence band maximum(VBM)of most oxide semiconductors is usually composed by anisotropic and localized O 2p orbital,resulting in the low hole mobility.The other is that for most oxide semiconductors,there exist a number of oxygen vacancy donor defects but very limited number of metal vacancy acceptor defects,leading to high electron concentration but low hole concentration.This is because that the formation energy of oxygen vacancy is usually much lower than that other defects such as metal vacancies and interstitial atoms.SnO has a special band structure with that its VBM is formed by the hybridization of Sn 5s orbital and O 2p orbital.The isotropic and high delocalized 5s orbital enables SnO to achieve high hole mobility.In addition,the formation energy of Sn vacancy acceptor is low in SnO so that a sufficiently high hole concentration can be formed.At present,SnO is regarded as one of the most potential p-type oxide semiconductors.In recent years,SnO TFTs have been found to be able to achieve both n-type and p-type,defined as ambipolar,conductivity under gate voltage regulation in one channel,which is favorable to some low-cost CMOS circuits with not very high performance requirements.As far as we know,SnO is also the only oxide semiconductor which achieves ambipolar conductivity.Understanding ambipolar conduction mechanism is the basis for fabrication of high performance ambipolar SnO transistors.However,studies on the ambipolar mechanism of SnO is very limited,and the reported mechanisms are even in debateRegarding to these problems and challenges mentioned above,this thesis work fabricated high performance ambipolar SnO TFTs as well as p-type SnO TFTs by optimizes of the processing parameters.Based on these high performance ambipolar SnO TFTs,we have constructed CMOS-like inverters and analyzed their potential in the circuit applications.Moreover,by comparatively analyzing the following properties between ambipolar and p-type SnO channels,including field effect conductivities,surface morphology by atomic force microscopy(AFM),UV-Vis-NIR optical transmittance,X-ray diffraction,and X-ray photoemission electron spectroscopy,the ambipolar conduction mechanism of SnO was discussed.The main contents of this thesis work are as follows(1)Considering the low and imbalance of hole and electron mobilities of ambipolar SnO TFTs,the annealing(in air atmosphere)time and temperature were optimized.The results show the optimized annealing temperature and time were 225?and 2h,respectively.The optimized ambipolar SnO TFTs show high and balanced mobitly,u,low subthreshold swing SS,and high on/off current ratio of 0.7 cm2v-1s-1,3.6 V dec-1 and 1.5×103 for the p-type region,and 0.75 cm2v-1s-1,3.1 V dec-1 and 1.5×13 for the n-type region,respectively.At the same time,the hole mobility ?h,SS and the on/off ratio of the fabricated p-type SnO TFTs reach 1.16 cm2v-1s-1,6.14 V dec-1 and 2.5×104,respectively.(2)Based on the optimized process conditions of ambipolar SnO TFTs,the CMOS-like inverters based on two ambipolar SnO TFTs was fabricated,and their electrical properties were studied.The results show that the voltage gain of CMOS-like inverters can reach-30.(3)A systematic study was made on the influences of Al2O3 passivation layer and various annealing processes on the electrical conductivity of the SnO TFTs.The various annealing processes including:Process A,anneal SnO TFTs firstly and then deposit 0-30 nm Al2O3 passivation layer;Process B,deposit 0-30 nm Al2O3 layer firstly and then anneal the TFTs;Process C,anneal the TFT firstly,then deposit 30 nm Al2O3 layer,and then annealed again(twice annealing).The study shows that only when Al2O3 passivation layer exceeds a certain thickness(4 nm),and the SnO TFTs cover with passivation layer firstly,and then anneal,the SnO TFTs show ambipolar conductivity;other processes include:Process A,Process B with thin passivation layer(? 2 nm),and Process C,all show p-type conductivity.(4)The effects of different passivation and annealing processes on the surface roughness of SnO films(20 nm,the same as the TFT channel thickness)was investigated by AFM.The results show that annealing without or with a relatively thin(<2 nm)passivation layer lead to a large surface roughness(RMS:3.26?8.60 nm),while samples annealed after covering a sufficiently thick(4?30 nm)passivation layer have a relatively low surface roughness(RMS:0.74?1.20 nm).This is due to that SnO films will be further oxidized when annealing without air isolation,which will change the composition ratio and morphology of the film and results in high surface roughness.The surface roughness of the sample covered with the 2 nm passivation layer was the largest,indicating that the 2-nm-thick Al2O3 probably is not continuous,and the exposed and covered SnO showing large roughness after annealing.(5)Optical transmittance and band gap of SnO films prepared by different processes were studied by UV-Vis-NIR transmittance spectra.The results show that the transmittance of annealed p-type SnO films prepared without passivation layer or with thin passivation layer(2 nm)was very high,close to 100%,and the corresponding optical band gap was relatively large(2.75?2.81 eV),while the ambipolar SnO films annealed with sufficiently thick passivation layer(4?30 nm)have a low transmittance between 60 and 75%,and the corresponding optical band gap is relatively small(2.62?2.65eV).The reason is that the p-type SnO films annealed without or with thin(2 nm)passivation layer unable to isolate air absolutely,and the films are partly oxidized of during annealing,with the composition of the film is dominated by SnO,but with part of SnO2.The SnO films with thick passivation layer can be isolated from the water and oxygen in air atmosphere,and thus the oxidation reaction is inhibited during annealing,and thus the film composition is dominated by SnO,but with part of Sn,and the metallic Sn components reduce the transmittance.Meanwhile,the dispersed Sn(combined with XRD?XPS analysis)in the film acts as a dopant,which increases the band-tailing state density and leads to the narrowing of the band gap(6)The components of 20 nm thick ambipolar and p-type SnO films and their valence band electron states were studied by XPS.The results showed that:A,the ambipolar SnO film is dominated by SnO,and contains small part of the metallic Sn but without SnO2;while the p-type SnO film is dominated by SnO,and includes a small part of SnO2 but without metallic Sn.The existence of SnO2 in p-type SnO lead to a high band gap state density,which is consistent with the high SS of p-type SnO TFT,which is also a reason of hard to achieve ambipolar conductivity.B,The high resolution XPS valence band spectrum indicates that the valence band state density near the Fermi level of the ambipolar SnO is significantly higher than that of the p-type SnO,resulting in that the ambipolar SnO has a narrower band gap than the p-type SnO,and this agrees well with the results in(5).Combined with the narrow indirect bandgap width(0.7 eV)of SnO itself,the narrower band gap width of SnO containing Sn component is one of the main reason for its ambipolar conductivity.(7)The components and crystallinity of 1-?m-thick SnO films fabricated by different processes were studied by XRD.The results show that the thick ambipolar SnO are polycrystalline with strong SnO crystalline peaks but very weak metallic Sn crystalline peak,indicating that the Sn tends to exist in the form of dispersed clusters(doping),while the thick p-type SnO are polycrystalline with strong SnO crystalline peaks and strong metallic Sn crystalline peak(do not match with the XPS results of 20-nm-thick p-type SnO without Sn,and this is because that the air cannot penetrate into the entire 1-?m-thick SnO film without the passivation layer when annealing,thus the relatively deep metal Sn cannot be fully oxidized).Based on the above analysis,high performance ambipolar SnO TFTs can be prepared by the annealing process with a thick Al2O3 passivation layer(?4 nm),and the SnO films prepared by the annealing process without or with a thin Al2O3 passivation layer(0-2 nm)are all p-type.The p-type SnO films are dominated by SnO with a small amount of SnO2,and thus with high band gap state density and high SS in the corresponding TFTs,low valence band top state density and wide band gap.However,the ambipolar SnO films are dominated by SnO with a small amount of dispersed Sn.The doping of Sn lead to a significant increase in valence band tailing state density,and thus lead to a narrower band gap.In addition,due to the absence of SnO2 in ambipolar SnO films,they have low band gap state density and low SS in the corresponding TFTs.Combined with the small and narrower indirect band gap width of SnO itself(--0.7 eV)and the low band gap state density,the corresponding SnO TFTs show ambipolar conductivity under gate voltage modulation.