Investigation Of The Characteristics Of The Y₂O₃/SiO₂/4H-SiC Stack Dielectric MOS Capacitors

Silicon carbide?SiC?metal-oxide-semiconductor field effect transistor?MOSFET?,one of the mainstream power semiconductor device currently,is still facing the leakage current and reliability problems of the gate dielectric layer under the high electric field.In this thesis,the thickness of the gate dielectric layer is increased by using a high-k material Y₂O₃ instead of the conventional SiO₂ gate dielectric,thereby reducing the electric field applied to the gate dielectric layer.However,the small band offset between Y₂O₃ and SiC will cause the serious leakage current problem,which will violate the purpose of using high-k material.To address this issue,in this paper,the interlayer of SiO₂ was prepared to increasing the band offset between gate dielectric and SiC.In this paper,the effect of different thicknesses of SiO₂ on the Y₂O₃/SiO₂/SiC stack gate dielectric MOS capacitor was studied and a suitable SiO₂ transition layer was selected.The samples were fabricated by the thermally oxidized Si O₂ on P-type 4H-SiC epitaxial layer,the thickness of three SiO₂ films about 2 nm,3.5 nm and 5 nm obtained by spectroscopic ellipsometer,then deposited 9 nm Y₂O₃ thin film on the surface of Si O₂ by radio frequency?RF?magnetron sputtering,finally the electrodes was deposited to complete the preparation of MOS capacitor.The RF-magnetron sputtering Y₂O₃ films were tested by atomic force microscope?AFM?,the result shows that the thin films have an even surface and effectively avoid the leakage current problem caused by the high electric field in some area.The characteristic of Y₂O₃film was investigated by the high temperature annealing,it is concluded that the sample annealing at 400?has the best material and leakage current characteristics.The post-deposition annealing of Y₂O₃ films were analyzed by X-ray photoelectron spectroscopy?XPS?,the result shows that the RF-magnetron sputtering Y₂O₃ films post-deposition annealing at 400?with the high purity and quality.For the Y₂O₃/SiO₂/SiC structure,although Y₂O₃ and SiO₂ can mutual diffusion and reaction in high temperature environment,a portion of the SiO₂ layer is not fully reacted for the 5 nm SiO₂ interlayer due to the limited diffusion depth.The method of XPS data fitting was used to get the band gap and valence band offset of gate dielectric,the result shows that the band gap and valence band offset of transition layer increases with the increase of the thickness of interlayer.The band information of different samples is obtained by data fitting and equation calculating,and draw the band diagram of the corresponding samples.The electrical characteristics for the preparation MOS capacitor structure were tested,the doping concentration of the epitaxial layer is1.55?10^{1 5}cm^-3,which obtained by mercury probe C-V test.In order to avoid the influence of parasitic resistance in the process of capacitance-voltage?C-V?test,the test data was amended and the revised C-V results show that the different samples have lower oxide traps,and the fixed oxide charge and interface trap density are reduced after the introduction of the SiO₂ transition layer.Current-voltage?I-V?results found that introduction of SiO₂ transition layer significantly reduces the leakage current,increasing the breakdown electric field.Compared the influence of the different thickness of SiO₂ Y₂O₃/SiO₂ dielectric on the interface characteristics and leakage current characteristics of MOS capacitance,it's found that the gate dielectric breakdown is SiO₂breakdown,and the magnitude of the breakdown electric field is positively correlated with the thickness of the SiO₂.This result has carried on the leakage mechanism analysis,it's found that the leakage mechanism leading order is space-charge-limited?SCL?conduction mechanism,schottky emission?SE?,Poole-Frenkel?P-F?emission and Fowler-Nordheim?F-N?tunneling,the leakage current under the mechanism of SCL will remain in a small leakage current order of magnitude,once the emission mechanism involved,leakage current will increase rapidly.The thin transition layer will breakdown under a low electric field due to the emission mechanism,the thicker SiO₂ happen SE mechanism require the higher electric field,due to the thicker SiO₂ has the greater valence band offset,which will block hole emission effectively,for the 5 nm SiO₂ interlayer,the breakdown electric field up to 7.2 MV/cm,the leakage current of the sample keep in 10^-7 order of magnitude before breakdown,on account of the 5 nm SiO₂ transition layer with large valence band offset,reduce the probability of hole emission under the high electric field,the gate dielectric breakdown at F-N mechanism dominant.