| Thin-film oxide semiconductors are a family of electronic materials.In recent years,many researches have focused on oxide-semiconductor-based electronic devices.In this thesis work,electronic devices based on two oxide semiconductors,InGaZnO?IGZO?and gallium oxide?Ga2O3?,are studied.IGZO has a wide bandgap?3.4 eV?,high carrier mobility,good visible light transparency,high uniformity over large areas,and room-or low-temperature deposition process?making it compatible with flexible substrates?,and hence is particularly advantageous for flexible,transparent,and wearable electronics.IGZO-based transistors have already been commercialized for the backplane driver of displays.However,despite of intensive research on oxide transistors,studies on IGZO-based Schottky diodes?SBDs?are yet very limited.Ga2O3 has ultra-wide bandgap??4.8 eV?and much higher breakdown field?8 MV/cm?than those of well developed wide bandgap semiconductors like GaN and SiC,and is an ideal candidate for next gerneration power electronics.?-Ga2O3 single crystals have a layered structure with two cleavage planes of?100?and?001?,and thus high quality Ga2O3 single crystalline thin films can be prepared by mechanical exfoliation.With the breakthrough of crystal growth technology of Ga2O3 single crystals in the last several years,electronic devices based on exfoiled Ga2O3 single crystalline films or homoepitaxial films started to be developed.At present,the research on Ga2O3 electronic devices is still in the initial stage,and high-quality Schottky interface for Ga2O3 SBDs is still challenging.A detailed investigation of the electrical properties and physical mechanisms of Ga2O3 Schottky interfaces is timely.In this thesis,the electrical and physical properties of the Schottky interface are analyzed by means of capacitance-voltage?C-V?,current density-voltage?J-V?,frequency and voltage-dependent capacitance and conductance?C-V-f?G-V-f?,Schottky capacitance spectrum,low frequency noise?LFN?,scanning electron microscopy?SEM?,atomic force microscopy?AFM?and X-ray photoelectron spectroscopy?XPS?measurements.High performance SBDs based on IGZO and Ga2O3 have been achieved,and IGZO-based Schottky-barrier thin-film transistors?SB-TFTs?and Schottky-barrier electric-double-layer transistors?SB-EDLTs?with low power consumption,low driving voltage are further fabricated.The key experiments in this thesis are as follows:1.Fabrication of high performance rigid and flexible IGZO SBDs?1?High performance rigid IGZO SBDsEffects of oxygen partial pressure during sputtering IGZO.Oxygen vacancy-related defects exist commonly in oxide semiconductors,and have serious effects on the metal/oxide semiconductor Schottky interfaces,for example,causing Fermi level pinnings and thus resulting in low barrier heights.In this thesis work,the effects of oxygen partial pressure?Po?during the IGZO deposition on the SBD performances are investigated.For PO = 0%,the SBDs show poor rectification characteristics with low IOn/off of only?1 0 atą1 V As Po increased from 2.5 to 20%,the Schottky barrier height(?BJV)increased from 0.79 to 0.92 eV,Ion/off decreased from 5.4 × 106 to 6.0 × 103,and the ideal factor?n?degraded from 1.11 to 1.68.Overall,the SBD with PO of 2.5%has the highest performances.This indicates that the introduction of an appropriate amount of oxygen during the sputtering process helps to form high ?BJV,large Ion/off,and ideal n,but over-high PO leads to degraded n and Ion/off.Effects of oxidation treatments on anode metal.Because most metals have a certain chemical affinity for oxygen,the diffusion of oxgen from semiconductor subsurface to metal is common in metal-semiconductor contacts.An increase of oxygen defect density at the metal-oxide semiconductor interface can lead to a degradation of device performance.To avoid this problem,the surface of the Pd anode metal was pretreated by oxidation.In the thesis work,the effects of different oxidation treatments?oxygen plasma treatment or UV-ozone treatment?on the quality of Pd/IGZO Schottky interface are systematically studied.XPS results indicate that both treatments lead to Pd surface oxidation.The oxygen plasma treatment results in more complete oxidation,and thus ensures better oxygen stoichiometry at the Schottky interface and a higher anode work function,leading to higher device performance.Effects of substrate and anode metal annealing.The effects of SiO2/Si substrate and Pd anode metal annealing on IGZO SBD properties are further studied on the basis of the optimizations mentioned above?oxygen plasma treatment on Pd and PO = 2.5%during IGZO sputtering?.The results show that the root mean square?RMS?roughness of the as-deposited Pd?47 nm?on the annealed substrate?300 ?,30 min?is the smallest?0.69 nm?.The corresponding device showed the best performance with ?BJV of 0.88 eV,Ion/off of 7.21 × 107,and n of 1.09,this is,to the best of our knowledge,the highest performance among the reported SBDs with as-deposited IGZO.However,the RMS roughness of Pd increased from 0.69 to 0.73 nm after annealing of the Pd anode,resulting in a slight degradation in device performance with ?BJV droping to 0.86 eV,Ion/off droping to 3.48 × 107,and n increasing to 1.13.The C-V measurements indicate that the surface roughness is correlated with the trap state density at the Schottky interface.?2?High performance flexible IGZO SBDsOn the basis of the optimization of rigid IGZO SBDs,including oxygen plasma treatment on Pd and PO = 2.5%during IGZO sputtering,high performance flexible IGZO diodes on polyethylene terephthalate?PET?and polyimide?PI?have been fabricated with Ion/off of 7.3 × 106 and 2.6 × 104,?BJV of 0.79 and 0.76 eV,and n of 1.22 and 1.19,respectively.Interestingly,these flexible diodes show even improved performance after storing in ambient air for two years,and fittings on the reverse currents indicate an improved barrier uniformity.The improvement is most likely due to a slow filling up of oxygen vacancies in the region close to the IGZO Schottky interface.The flexible diode on PET achieves a high Ion/off of 2 × 107,a large ?BJV of 0.80 eV,a close to unity n of 1.09,a high breakdown voltage of 7.5 V.To the best of our knowledge,this is the highest performance in reported flexible diodes to date.In addition,these flexible diodes show high stability upon tensile and compressive stresses.2.Fabrication of high performance Ga2O3 SBDsFor Ga2O3 SBD power devices,the key factor affecting their performance is the quality of the Schottky interface,including n,?BJV,and interface state density.In this thesis work,a new type of high quality SnOx/Ga2O3 Schottky contacts are realized.It provids an effective way to fabricate high quality Schottky contacts on Ga2O3,and lays a foundation for the research of Ga2O3 high-power SBDs.?1?Mechanical exfoliating and properties of Ga2O3?-Ga2O3 crystal has a large lattice constant of 12.23 A in the[100]direction,that is,has a cleavage plane.Therefore,quasi-two-dimensional Ga2O3 single crystal film/flake can be obtained by mechanical exfoliation.Based on the high quality bulk Si-and Cr-doped ?-Ga2O3 single crystal grown in the State Key Laboratory of Crystal Materials of Shandong University,high quality Ga2O3 flakes are prepared by mechanical exfoliation.Hall effect measurement results show that the Si-and Cr-doped Ga2O3 wafers have carrier concentrations of 1.0× 1018 cm-3,and carrier mobilities of 68 and 90 cm2/Vs,respectively.AFM analysis indicates that the surface of the exfoliated Ga2O3 flakes are atomically flat with RMS roughnesses of 0.3-0.37 nm,which is suitable for fabricating electronic devices.In addition,a synergic study on UV-vis optical transmission spectroscopy,Raman spectroscopy,and X-ray diffraction of the Ga2O3 flakes illustrates that the flakes have wide band gaps of 4.6-4.8 eV and high single crystal qualities.?2?Pd/Ga203 SBDsFirstly,Pd/Ga2O3 SBDs with bottom Schottky contacts?Pd schottky electrode is prepared first,then Ti/Au ohmic electrode is prepared?are prepared based on the exfolied Ga2O3 flake.The devices show a low ?BJV of 0.86 eV,a low Ion/off of?106,a large n of 1.22,and a high series resistance?Rs?of 0.7 ?cm2.The Schottky capacitance spectroscopy and LFN measurments indicate that the Pd/Ga2O3 Schottky interface has a high trap state density of>1013 eV-1cm-2.Secondly,in order to reduce the influence of subsequent processes on Schottky contact,Pd/Ga2O3 SBDs with top schottky contacts?Ti/Au ohmic electrode is prepared first,then Pd Schottky electrode is prepared?are prepared.Device performances are improved significantly with n,?BJV,Rs,and Ion/off of 1.18,1.34 eV,30.6 m?cm2,and>1010,respectively.?3?SnOx/Ga2O3 SBDsCompared to the high work function metals,such as Au,Pd,and Pt,etc.,Sn has a significantly low cost and is easily to be oxidized.As a schottky electrode,the oxided metal can ensure the oxygen-rich Schottky interface and reduce oxygen deficiency-related defects,so as to improve the quality of the schottky contact.For the first time,high performance Ga2O3 SBDs are fabricated using SnOx as Schottky electrodes.By adjusting PO during sputtering,the chemical composition and electrical properties of the SnOx films are adjusted.A synergic study on Raman,XPS,and UV-vis optical transmission spectroscopy of the SnOx films illustrates that:the films with PO = 0-3.1%consist mainly SnO and Sn with high conductivity;the films with PO = 4.6-5.4%are composed of both p-type SnO and n-type SnO2 with high resistance;the films with PO= 10.0-13.1%are mainly dominated by n-type SnO2 with low resistivity.In addition,as PO increased from 0 to 3.1%,the SBDs performances are significantly improved due to that the SnO-dominated films reduce effectively the oxygen-deficiency at the Ga2O3 interfaces and the related interface state density.With PO = 5.4%,the high resistive SnOx results in degraded diode performance because that:a)the high-resistance layer reduces the current of the device;b)the SnO2 component with oxygen vacancy defects may aggravate the oxygen-deficiency at the Schottky interface.By carefully adjusting the SnOx electrode chemical composition to be conductive SnO-dominated,with the optimized PO of 3.1%,the Si-doped Ga2O3 SBD shows the highest performance with a large ?BJV of 1.12 eV,a near unity n of 1.22,and a high Ion/off of>1010.With the same optimization,the Cr-doped Ga2O3 SBD exhibits even higher performance with n of 1.02,?BJV of 1.17 eV,and Ion/off of>1010.C-V-f and G-V-f analysis show that Cr-doped Ga2O3-based SBD has a rather low interface state density of 2.46 × 1012 eV-1cm2.3.IGZO SB-TFTsSB-TFTs can achieve low voltage and low power consumption,and have great application value in the field of mobile and portable electronics.In this thesis work,SB-TFTs based on IGZO have been studied.The barrier height formed between the Pd source/drain metal and the IGZO channel is adjusted by annealing,consequently,the performances of the IGZO SB-TFTs are adjusted.After annealing at 100 ?,the barrier height between Pd and IGZO is 0.54 eV,and SB-TFTs exhibit the best performance,low voltage and low power consumption.The off current of?10-13 A,Ion/off of?107,subthreshold swing?SS?of 0.93 V/dec,threshold voltage?VTH?of 0.25 V,and saturated drain voltage of<1.5 V,are achieved.At the gate bias stress of+10/-10 V for 3000 s,YTH is shifted by 1.19 V under positive bias stress?PBS?,and almost no deviation under negative bias stress?NBS?.Negative-bias-illumination tests show that the devices exhibit good stability in the red?650 nm?and green?532 nm?illuminations,except for the large VTH shift?-4.87 V?under the blue?450 nm?illumination.4.IGZO SB-EDLTsEDLTs are another effective way to achieve low voltage.Firstly,IGZO EDLTs are fabricated on rigid substrates,and the gate bias stress stability of the devices is investigated.At the stress of+1/-1 V for 24 min,VTH is shifted by 0.57 V under NBS,the high stability under PBS is observed.Secondly,a simple and low cost"carving,cutting,and flip-chip bonding" technique is developed for the fabrication of flexible EDLTs.The device exhibits high performance at very low gate bias of-1-+1 V with a large Ion/off of 1.4 × 107 and SS as low as 76 mV/dec?close to the theoretical minimum of 59 mV/dec?.On this basis,SB-EDLTs are preliminarily prepared by using Pd as source/drain electrode.Compared to devices with as-deposited IGZO,the off current is reduced from 1 × 10-7 to 5 × 10-9 A and the SS is reduced from 170 to 80 mV/dec for the SB-EDLTs with annealed IGZO layer.The results show that this Schottky source/drain device has better device performance than conventional ohmic contact devices?such as SB-EDLTs with as-deposited IGZO?. |
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