| Wide-gap semiconductors such as silicon carbide(SiC),gallium nitride(GaN),gallium oxide(Ga2O3),and diamond are ideal materials for the new generation of power electronics and ultraviolet detectors because of their advantages of wide band gap and high breakdown electric field strength in comparison to those of traditional silicon.Among them,Ga2O3 has an ultra-wide band gap of 4.5?4.9 eV,an absorption edge located at deep ultraviolet solar-blind band(?250 nm),a Baliga's figure of merit(BFOM=3444)which is much higher than that of SiC and GaN,an ultra-high breakdown field strength of 8 MV·cm-1,and a high electron saturation drift velocity of?1.5×107cm·s-1.Thus,it has great application potentials in high-voltage and high-power electronics,microwave power electronics,and photodetectors.However,due to the lack of effective p-type doping of Ga2O3,Ga2O3-based field effect transistors(FETs)are usually in n-type depletion-mode,and thus are unfavorable for safe operation and low static power consumption comparing with enhancement-mode FETs.In addition,most of the reported Ga2O3 FETs have electron mobility(?)much lower(?1 order of magnitude)than the theoretical value of 300 cm2V-1s-1,which is very unfavorable for obtaining high BFOM(proportional to ?)to realize high-power electronics and microwave power electronics.Therefore,developing enhancement-mode Ga2O3 FETs with high electron mobility is currently an important requirement for high-power and microwave power Ga2O3 electronics.In this thesis,based on the mechanically exfoliated Ga2O3 films,we successfully prepared enhancement-mode Ga2O3 FETs by introducing a heterojunction to control the conductivity of the Ga2O3 channel,and achieved a high ? of 191.57 cm2V-1s-1.This value broke the highest record of the mobility of Ga2O3 FETs at room temperature in the published literature.The main research contents of the thesis are as follows:1.Preparation of bottom-gate Ga2O3 FETs and optimization of the ohmic contactIn this thesis,Cr-doped ?-Ga2O3 single crystals with an electron concentration of?1 × 1018cm-3 is used.The Ga2O3 single crystal films of different thicknesses were prepared by mechanically exfoliation method and transferred to the p++-Si/SiO2 substrate,as well as the gate/gate insulator,to fabricate bottom-gate type FETs.The source and drain electrodes are vacuum deposited Ti/Au and patterned by photolithography.To enhance the ohmic contacts,inductively coupled plasma(ICP)etching are used before the source and drain deposition.Before evaporating the source and drain electrodes,the regions of Ga2O3 under the source and drain are treated with ICP to increase the electron doping concentration and thus enhance the ohmic contacts.In this thesis work,the ICP treatment time are optimized to be 3 min to form good ohmic contacts.2.Ga2O3 FETs with back-channel surface passivation by aluminum oxide(Al2O3)On the back-channel surface of Ga2O3,30 n,Al2O3 was prepared by atomic layer deposition(ALD)as the passivation layer,and Ga2O3 FETs with Al2O3/Ga2O3 heterostructure were prepared.After surface passivation,the threshold voltage(VTH)of the FET shifts negatively by 8.42 V.This is because of that the Al2O3 passivation can compensate a large number of dangling bonds on the Ga2O3 surface and thus restrain the surface depletion.As a result,the number of carriers in the channel are increased and thus the VTH shifts negatively.Such negative shift of VTH proves the existence of surface depletion for Ga2O3.In addition,with the Al2O3 passivation,the subthreshold swing(SS)of the device decreases from 0.36 V/dec to 0.30 V/dec,the hysteresis decreases from-5.20 V to-4.59 V,and the ? remains unchanged at 9.11 cm2V-1s-1.The overall performances of the FET is improved.3.Ga2O3 FETs with indium gallium zinc oxide(IGZO)n-n heterojunctionGa2O3 FETs with IGZO/Ga2O3 heterojunction in the back-channel surface were prepared.Before introducing the IGZO/Ga2O3 heterojunction,IGZO thin film transistors(TFTs)were fabricated firstly to verify its n-type conductivity.Based on the interface energy band theory,the introduction of IGZO/Ga2O3 heterojunction will induce the electron transfer from the Ga2O3 to the IGZO at the interface,and lead to the Ga2O3 energy band bend upward.Thus,it enhances the Ga2O3 channel depletion,reduces the carrier numbers in the channel,and debilitates the electron scattering during carrier transporting in the channel.The experimental results are in good agreement with the theoretical analysis.With the IGZO(12 nm)/Ga2O3 heterojunction,the VTH of the FET shifts positively from?-35 V to-19.83 V,and the ? increases from 1.94 cm2V-1s-1 to 3.17 cm2V-1s-1.Although enhancement-mode Ga2O3 FET has not yet been realized,it is illustrated as an effective solution for adjusting the VTH and improving the performances of the Ga2O3 FETs.4.Ga2O3 FETs with SnO/Ga2O3 p-n junctionIn order to enhance the depletion of the Ga2O3 channel and realize enhancement-mode operation,in this thesis work,Ga2O3 FETs with SnO(4? 12 nm)/Ga2O3 p-n junctions were prepared.SnO TFTs were firstly fabricated to verify its p-type conductivity.The VTH of the Ga2O3 FETs with SnO/Ga2O3 p-n junction shifts positively significantly in comparison with that of the Ga2O3 FETs without SnO/Ga2O3 p-n junction,and the ? of the Ga2O3 FETs improves significantly with SnO/Ga2O3 p-n junction.This is because of that p-type SnO can deplete significantly the electron carriers in the Ga2O3 channel and weaken the scattering effect during electron transporting in the channel.With the 4 nm SnO p-n junction for 185 nm Ga2O3,the VTH of the Ga2O3 FET shifts from-11.44 V to 0.79 V,and the ? increases from 1.40 cm2V-1s-1 to 2.71 cm2V-1V-1.With the 8/12 nm SnO p-n junction for 280 nm Ga2O3,the VTH of the Ga2O3 FET shifts positively significantly from?-35 V to-0.57 V and 5.30 V,respectively,and the ? increases significantly from 8.29 cm2V-1s-1 to 36.3 cm2V-1s-1 and 191.57 cm2V-1s-1,respectively.To the best of our knowledge,this mobility 191.57 cm2V-1s-1 is a record high value for the reported Ga2O3 FETs at room temperature.Therefore,with SnO/Ga2O3 p-n junction,enhancement-mode Ga2O3 FETs with ultra-high electron mobility have been realized.This provides a new effective solution for the preparation of high-power and microwave power Ga2O3 FETs in the future. |
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