The Investigation Of Tin Monoxide Thin-film Transistors And CMOS-like Electronic Devices

Conventional silicon-based semiconductors can not meet the requirements of new fields like large-area transparent,flexible and ultra-definition display technology.Compared with silicon-based materials,oxide semiconductors show high transparency,large carrier mobility,excellent mechanical property,etc,are ideal materials for switching/driving thin-film transistors?TFTs?in new display technology.However,most oxides with excellent performance are n-type,p-type oxides with performance comparable to n-type oxides are rare.As a result,the applications of oxide semiconductors are confined to unipolar devices,inhibiting the development of oxides-based complementary devices or circuits etc.Therefore,it is of utmost importance to explore high performance p-type or bipolar oxide semiconductors.In this article,we focus on the investigation of p-type tin monoxide?SnO?.SnO films were prepared by magnetron sputtering,and the influence of sputtering power,O₂/?O₂+Ar?flux ratio?oxygen partial pressure,OPP?,work pressure,situ substrate bias,and situ substrate heating on the microstructure,optical,and electrical properties of SnO films were systematically investigated.Based on the films preparation,p-type SnO TFTs with bottom-gate structure were fabricated and the influence of different deposition conditions of channel layers on the performance of SnO TFTs was studied.The performance of p-type SnO TFTs was further improved by optimizing the annealing process.Based on the researches of p-type SnO TFTs,conversion of SnO TFTs from p-channel to ambipolar operation mode was achieved through the modification of SnO TFT back-channel surface.The mechanisms of operation mode conversion were intensively investigated.Based on the work of ambipolar SnO TFTs,complementary-like logic inverters were constructed by connecting two identical ambipolar SnO TFTs,high voltage gains in combination with wide nose margins wereobtained due to the balanced injection of holes and electrons into the SnO channel.Followings are the main research results:1.Properties of SnO films1)SnO films deposited at low OPP contain a large amount of metallic Sn,the content of Sn decreased but the content of SnO increased as OPP increased until the films became pure SnO.At high OPP,part of SnO in the matrix films was oxidized and changed into SnO₂,giving rise to the decrease of SnO content,and the mixture films of SnO and SnO₂ exhibit amorphous structure.Due to the phase composition transition of the films,the bandgap increased and the refractivity decreased as OPP increased.The pure SnO films display a bandgap of 2.6 e V and a refractivity of 2.7?wavelength 550 nm?.2)For Sn-SnO mixture films with dominated Sn phase?n-type conduction?,the electron concentration increased and the resistivity decreased as the content of SnO increased,the conduction of the films is similar to a degenerate semiconductor.This was probably originated from the generation of donor-like defect states in SnO under Sn-rich condition,leading to the increase of electron concentration.For Sn-SnO mixture films with dominated SnO phase?p-type conduction?,the hole concentration increased and the resistivity decreased as the content of Sn increased.This may because the Sn related defects would generate acceptor-like defect states in SnO films,leading to the increase of hole concentration.For SnO-SnO₂ mixture films with dominated SnO phase?p-type conduction?,the hole concentration decreased as the content of SnO₂ increased,due to the holes of SnO were compensated by the donors of SnO₂.The mixture films display high resistivity when the holes were almost completely compensated by the donors,and the mixture films exhibit n-type conduction when the concentration of donor was larger than that of the hole.The hole Hall mobility of Sn-SnO and SnO-SnO₂ mixture films was lower than that of pure SnO films,due to the holes were scattered by the Sn and Sn4+ related defect scattering centers.SnO films deposited by radio frequency?RF?40 W exhibit a relative large hole mobility of 5.6 cm2V-1s-1 with a hole density of 8.9×1017-6.0×1018 cm-3.3)SnO films deposited at high work pressure contain a large amount of SnO₂,leading to the increase of bandgap and the decrease of refractivity with increasing work pressure.The hole concentration and hole mobility decreased and the resistivity increased as work pressure increased,due to the introduction of SnO₂ phase into the SnO matrix.4)Excess oxygen will be introduced into SnO matrix under situ substrate bias,resulting in the re-oxidization of SnO into SnO₂ and the content of SnO₂ increased as bias power increased.As a result,the bandgap increased and the refractivity decreased as the bias power increase.In addition,the hole concentration and hole mobility decreased and the resistivity increased as the bias power increased.5)The texture property of SnO films can be changed and high quality SnO films can be obtained through situ substrate heating.However,the hole density of SnO films deposited by situ substrate heating was large?1020 cm-3?and is not in favor of TFT channel layer.2.Unipolar p-type SnO TFTs1)P-type SnO TFTs prepared by direct current?DC?sputtering display a low field-effect mobility of 0.37 cm2V-1s-1,due to the low Hall mobility of DC sputtered SnO films.In addition,the TFTs were difficult to be turned off and the on/off current ratio was low, 10.Therefore,the DC sputtering is not favorable to fabricate high hole mobility TFTs.2)SnO TFTs prepared by RF 20 W show a low field-effect mobility of 0.51 cm2V-1s-1 and an on/off current ratio of 10.SnO TFTs prepared by RF 60 W show a high field-effect mobility of 1.8 cm2V-1s-1.However,the on/off current ratio was low? 60?,due to the high intrinsic hole density of the channel layer.SnO TFTs prepared by RF 40 W exhibit a large field-effect mobility of 2.24 cm2V-1s-1 and an on/off current ratio of 2.4×103.In addition,p-type TFTs with channel layers composed of Sn-SnO or SnO-SnO₂ mixture films exhibit low field-effect mobility and on/off current ratio,due to the existence of large amount of structure defects in the mixture films.TFTs with pure SnO channel layer display relative large field-effect mobility and high on/off current ratio.3)P-type TFTs with thinner SnO channel layer display larger field-effect mobility and higher on/off current ratio.However,SnO films will deviate from stoichiometry severely and were high resistive when the thickness of the channel layer was too thin.Therefore,the optimized channel layer thickness for p-type SnO TFTs is 15-20 nm.4)The hole concentration of SnO films can be reduced and the on/off current ratio of SnO TFTs will be improved by extending the annealing time-duration.However,the gate-insulator-semiconductor interface qualities will become deteriorated due to long-time annealing,resulting in intense interface scattering and low field-effect mobility.Short-time annealing lead to a poor crystallization quality of SnO channel layers with a large density of atom vacancies,interstitials structure defects,etc.As a result,the TFTs exhibit a relative low field-effect mobility and on/off current ratio.The SnO TFTs show a relative large field-effect mobility and on/off ratio of 2.44 cm2V-1s-1 and 6.0×103,respectively,after an optimal 2-hours annealing.5)The turn-on voltage of p-type SnO TFTs was difficult to be shifted by changing the work function of source-drain metal electrodes or insertion of ultra-thin insulator layer between the metal electrodes and the SnO layer,indicating that the Fermi level in SnO surface or bulk was pinned and the metal-SnO barriers were independent of the metal work function.3.Ambipolar SnO TFTs1)The ambipolar operation of SnO TFTs can be achieved through dielectric passivation?e.g.Al₂O₃?on the back-channel surface.The operation mode conversion is mainly determined by the fabrication process:?i?SnO TFTs annealing without passivation?AWP?show unipolar p-channel operation mode,?ii?SnO TFTs annealing before passivation?ABP?display distinct but weak n-channel inversion feature,?iii?SnO TFTs annealing after passivation?AAP?exhibit excellent ambipolar operation performance.The TFTs with Sn-SnO or SnO-SnO₂ mixture channel layers exhibit relative poor ambipolar operation symmetry.The TFTs with pure SnO channel layers show the optimum ambipolar performance,with an n-and p-channel field-effect mobility of 1.64 cm2V-1s-1 and 0.65 cm2V-1s-1,an on/off current ratio of 1700 and 560,respectively,and a turn-on voltage of 8.5 V.2)Based on the band-diagram analyses and results of device simulation,the excellent ambipolar operation performance of AAP TFT was originated from suppression of formation of SnO surface defect states through back-channel surface passivation and modulation process,rather than the shielding effect of the passivation layer to the environmental oxygen or vapor atmosphere etc.The positive charges at the Al₂O₃-SnO interface?confirmed by C-V characterization?will help to deplete the holes in SnO channel and is further in favor of n-channel inversion and ambipolar operation.To sum up,the Al₂O₃ passivation layer has three functions:?i?the passivation layer can improve the stability of SnO TFTs by inhibiting the charge transfer and chemical reaction between the SnO channel layer and the environmental atmosphere,?ii?the passivation layer can suppress the formation of surface defect states and reduce the subgap DOS in SnO bandgap,enabling the ease shift of Fermi level from valence band maximum to conduction band minimum and achieving n-channel inversion and ambipolar operation,?iii?the positive charges at Al₂O₃-SnO interface is favorable to deplete the holes in SnO channel and lower the turn-on voltage,further enhance the n-channel inversion and ambipolar operation.In additon,the hole concentration of AAP films was much lower than that of AWP and ABP films,which is favorable to hole depletion and the n-channel inversion and ambipolar operation.3)For ambipolar SnO TFTs,low work function drain-source contact?Ni-SnO contact?is favorable to electron injection,high work function drain-source contact?Ti-SnO contact?is favorable to hole injection.Metallic Mg has a work function lower than that of Ni,but the barrier layer at Mg-SnO interface was higher than that of Ni-and Ti-contact,giving rise to a poor injection of carriers,due to the easy oxidization of Mg.Among the Al₂O₃,Ta₂O₅,SiO₂ and Hf O₂ passivation materials,the Al₂O₃ layers passivated TFTs exhibit the optimum ambipolar performance.The Ta₂O₅,SiO₂ and Hf O₂ layers passivated TFTs display large turn-on voltages with low field-effect mobility and on/off current ratio for n-and p-channel modes.4)Ambipolar SnO TFTs exhibit good stability under negative gate-bias stress,however,the transfer curves display parallel and positive shift without shape change under positive gate-bias stress?PGBS?.The instability under PGBS was originated from the multiple trap states in the SnO channel or/and at the SnO-SiO₂?gate insulator?interface.The stress-time evolution of the turn-on voltage shift induced by PGBS under different stress voltages and temperatures could be described by the stretched exponential model,in which the extracted relaxation time and the activation energy is 1.6×104 s and 0.43 e V,respectively.4.Complementary-like logic inverters1)Based on the researches of ambipolar SnO TFT,complementary-like logic inverters were constructed by using two identical ambipolar TFTs.The inverters exhibit large voltage gain?> 100?in combination with wide noise margin?15.3 V?,due to balanced injection of holes and electrons into the SnO channel.The output signal of inverters responded well to a low frequency input signal?3.8 Hz?,however,the device can not respond to a high frequency input signal?16.6 Hz?promptly,due to the low field-effect mobility of ambipolar SnO TFTs.At low OPP,TFTs with Sn-SnO mixture channel layers exhibit relative poor ambipolar operation symmetry,leading to the degeneration of voltage gain,switching threshold voltage,noise margin and transition width as OPP decreased.2)The ambipolar SnO inverters display excellent stability in air.After being exposed in air over 10 months,the inverters nearly did not show any degeneration in the voltage gain,switching threshold voltage,noise margin and transition width.These results imply that the passivation layer can further improve the stability of SnO TFTs by effectively inhibit the charge transfer and chemical reaction between the SnO channel layer and the environmental atmosphere.