Accessing High Performance P-type And Ambipolar SnO Thin-film Transistors

Oxide semiconductors used as active layer of thin-film transistors(TFTs),have recently attracted significant attention especially in the perspective of fast growing flexible and/or transparent electronics,because of their low processing temperature compatible with plastic substrates,high carrier mobility(-1-100 cm2V-1s-1),high optical transparency in visible light region,and low cost in comparison to amorphous silicon.However,one major challenge for oxide semiconductors is that most of them are n-type,and only a very limited number of oxides show p-type conductivity.The reported TFTs based on p-type oxides usually show lower mobilities and much lower on/off ratios than their n-type counterparts.Such unbalanced development of n-and p-type oxides strongly hampers the implement of p-n oxide junction based electronics and complementary metal oxide semiconductor integrated circuits with low-power consumption.Among the reported p-type oxide semiconductors,tin monoxide(SnO)is considered the most promising due to its native p-type conductivity,high mobility,good stability and reproducibility.SnO TFTs with bottom-gate structure were fabricated in this thesis work.Thermally-oxidized SiO2(-100 nm)on heavily p+-doped Si wafers are used as the gate dielectric(SiO2),gate electrode(p+-Si),and substrate.SnO thin-films were deposited by reactive radio-frequency(RF)magnetron sputtering.The as-deposited SnO films were annealed to achieve field effect modulations.Metal source and drain electrodes were deposited by an electron-beam evaporator.In this thesis work,various efforts on processing parameters have been made to optimize the performance of SnO TFTs,including optimizations of annealing temperature,sputtering power,oxygen partial pressure(Opp)and materials of metal electrods.In addition,we have systematically explored the influence of active layer thickness and passivation layers on the performance of SnO TFTs.Based on these investigations,both high performance p-type and bipolar SnO TFTs have been fabricated.The main work and achievements are as follows:(1)Effects of thermal annealing temperature.The as-deposited SnO thin-films via reactive sputtering were amorphous,and too conductive to be modulated by gate bias.This is because that during sputtering,due to inadequate oxidation of Sn,excess of metallic Sn existed in the films and contributed to the conductivity.These deposited SnO films were thermal annealed in ambient air to achieve p-type gate modulation.Our results indicated that as temperature increased from 150 to 250?,the films firstly showed high resistivity and then converted to p-type characteristics.Our results suggested that metal Sn was precipitated and oxidized to SnO and the films are poly crystallized during annealing process.(2)Effects of RF sputtering power.As the RF sputtering power control the deposition speed and thus influence the oxidization extent during the deposition.As sputtering power increased,the deposition speed increased,and a part of metallic Sn was not fully oxidized to SnO,thereby,the content of Sn in the films was increased.(3)Effects of Opp.Opp directly influences the oxidation extent of metallic Sn during the sputtering process.Our results revealed that with a RF power of 50 W,p-type SnO transport occurred in the range of 1.5%?Opp?4.2%,and the optimized Opp is 3.1%.(4)Effects of materials of source and drain electrods.The ohmic contact between channel layer and electrods is important for TFTs.The devices with Ti showed optimal performance by experiment.This may be due to the formation of the Schottky barrier,resulting in a significant reduction in the off-state current of the device.(5)Effects of active layer thickness on the performance of p-type SnO TFTs.The device with 20 nm SnO showed the best performance.Both the mobility and on/off ratio of the 15 nm SnO TFT dropped significantly by one order of magnitude.When the film thickness increased from 20 to 30 nm,the on/off ratio of decreased and the mobility increased.On the one hand,the decreasing thickness of the SnO channel layer led to the reduction of hole carriers.On the other hand,surrounding environment could affect the top surface of the channel layer.Both the adsorption of water molecules and the presence of SnO2 can act as electron donors,resulting in deterioration of device performance.(6)Effects of back channel passivation layers(A12O3 and PMMA)on the performance of p-type SnO TFTs.After passivating,the devices showed improved performance.The passivation process on the back-channel surface of the bottom-gate TFTs was indispensable for suppressing the surface states and blocking the interactions between the semiconductor channel and the surrounding atmosphere,and thus led to high p-type performance for SnO TFTs.The experiment suggested that the PMMA passivation was more effective than A12O3 to reduce shallow trap states on the SnO top surface,while A12O3 was more effective than PMMA in reducing the deep traps on the top surface of the SnO film.(7)Effects of thickness of back channel passivation layer A12O3 and annealing on the p-type and ambipolar SnO TFTs.When applying thermal annealing in ambient air to the SnO passivated by Al2O3 with a thickness over 3 nm,the channel showed bipolar conductivity.This suggested that such passivation can highly suppress the surface states and trap states caused by the back-channel surface and surrounding atmosphere,as well as reduce the formation of SnO2 during annealing.The extremely low subgap states of the annealed SnO channel with thick Al2O3 passivated layer offered the opportunity to facilitate the Fermi level shifting between the valence band maximum and conduction band minimum by adjusting gate voltages.