Study On Hf-based High-k Gate Dielectric And Its Interface Improvement For MoS₂ Field-effect Transistors

According to Moore's law,Si-based CMOS devices have been continuous scaling for decades,with their performance continuous improvement and cost continuous reduction,which has greatly promoted the development of electronic information technology.However,with the device size entering the 10nm node,the scaling is facing severe challenges.Si-based devices are approaching their physical limits,so it is urgent to find the candidate channel materials for the next generation of transistors.MoS₂ is an emerging two-dimensional semiconductor material,because of its atomic level thickness,which can effectively improve the gate control ability and suppress the short channel effect,and the advantages of high carrier mobility and suitable band gap width,making it have great potential to replace Si as the channel candidate material for the next generation of transistors.However,it is difficult to deposit a dielectric with thickness of 10nm on the surface of MoS₂ film because there is no dangling bond.In addition,the interface quality of MoS₂/dielectric has not been improved effectively due to the lack of treatment of dielectric itself and its surface,which results in the electrical performance of MoS₂ transistor much lower than the theoretical prediction.In order to solve these problems,this paper focuses on the improvement of Hf O₂ dielectric and the interface quality of Hf O₂/MoS₂ to improve the electrical performance of the transistor,and explores a new process to improve the dielectric quality deposited on the MoS₂ surface.In the aspect of doping modification and preparation of gate dielectric materials,the following research work has been carried out:(1)the Hf_1-xTi_xO dielectric is prepared by alternating deposition of Ti O₂ and Hf O₂ by ALD to increase the k value,and the Ti content is optimized.The device performance is improved due to the Coulomb impurity scattering screening effect.However,excessive Ti will introduce charge impurities and deteriorate the interface quality.Therefore,considering the balance between dielectric screening effect and interface quality,the best electrical performance is obtained at 0.1 Ti content:?=31 cm²/Vs.(2)In order to further improve the interface quality,Hf_1-xAl_xO dielectric was prepared by alternating deposition of Al₂O₃ and Hf O₂ by ALD,and the Al content was optimized.It is found that the MoS₂ transistor with 50%Al content achieves the best interface improvement,which reduces the D_it to 2.9×10¹² e V^-1cm^-2 and increases the?to 49.3cm²/Vs.In the aspect of interface passivation between dielectric and MoS₂:(1)Hf O₂ is nitrided by NH₃ plasma to form Hf ON for improving its k value,and N,NH,H generated by NH₃decomposition can passivate defects and traps on the surface and inside of Hf O₂,thus improving the interface quality.Therefore,the transistor performance can be improved from two aspects of k value and interface quality.Compared with the samples treated with N₂ and O₂ plasma,the effect of NH₃ plasma on improving the interface quality is the best.The D_itdecreases to 2.79×10¹² e V^-1cm^-2,the?increases to 61.3cm²/Vs.Furthermore,the temperature,time and power of NH₃ plasma were optimized to obtain the best device performance.By considering interface quality and mobility,the conditions of NH₃ plasma are determined as follows:temperature of 300?,time of 5min and power of 80W.(2)On the basis of(1),the NH₃ plasma treated Hf O₂ is used as the top gate dielectric,the electrical performance is greatly improved:the I_on/I_off ratio reached 1.6×10⁷,the?increased to 87cm²/Vs,and SS decreased to 72 m V/dec.In the aspect of MoS₂ surface functionalization:(1)1-nm metal Al was deposited on the surface of MoS₂ and oxidized to form Al₂O₃ as buffer layer to deposit the TG Hf O₂ dielectric,with different doping concentration of Si-substrate as gate electrode.The results show that adding Al₂O₃ buffer layer can improve the interface quality;The Si gate electrode with high doping concentration can effectively screen the phonon scattering in Hf O₂ dielectric,making the carrier mobility of back-gate and top-gate improve.(2)The high/low temperature deposition method is adopted:the 1-nm gate dielectric is deposited at low temperature(95°C)as the nucleation layer,and then the 14 nm gate dielectric is deposited at high temperature(250°C).Thus,the top gate MoS₂ transistors with Hf O₂,Hf_0.5Al_0.5O and Al₂O₃ as the gate dielectric are prepared.The results show that the introduction of nucleation layer greatly improves the interface quality between dielectric and MoS₂,and the small molecular characteristics of Al₂O₃ making it easy to infiltrate into the interval of Hf O₂ along the surface,resulting in the formation of flat and dense Hf_0.5Al_0.5O film,and Al doping can reduce the trap charge(oxygen vacancy)in Hf O₂,suppress the Coulomb scattering,and make Hf_0.5Al_0.5O MoS₂ transistor has the best electrical performance:high?of 83.3cm²/Vs,low D_it of 1.08×10¹² e V^-1cm^-2.