Improvement Of Dielectric Interface And Doping Modification For Al-based High-k Gate MoS₂ FETs

With the characteristic length of Si based CMOS technology scaling to 5nm node,more serious short channel effect,quantum tunneling effect and large number of dangling bonds becoming the defect centers limit its continuing scaling down.It is urgent to explore new materials as the channel layer in transistors.As a kind of two-dimensional layered material,MoS₂ has many advantages like suitable band gap,monolayer thickness is 0.65nm,free of dangling bonds and short channel effect.So,MoS₂ has been widely studied as a promising candidate of Silicon in recent years.However,there are two major problems in the current MoS₂transistors:the experimental electrical performance can't reach the theoretical limit,resulting the miss of requirements of high mobility and low power consumption for the highly integrated circuits;the widely used mechanical exfoliation can't satisfy the mass production of the devices and integrated circuits.For solving the above two problems,this paper makes great efforts on:improving the electrical performance of MoS₂ transistors by improving the interface quality of MoS₂/high-k gate dielectrics with plasma treatment or doping modification;optimizing the MoS₂ growth condition for obtaining high-quality and large-size MoS₂ thin films on high-k gate dielectrics.Firstly,for improving the mobility of the device,the interface quality of MoS₂/high-k gate dielectric has been improved by plasma treatment,including two methods:(1)NH₃plasma treatment was used for effectively passivating the oxygen vacancies in Al₂O₃ and decreasing the dangling bonds at the Al₂O₃ surface,thus reducing the traps at/near the Al₂O₃/MoS₂ interface;increasing the k value of the dielectric by N incorporation to enhance the screening effect on the Coulomb impurity scattering;as compared with the N₂ and O₂plasma treatment sample,the NH₃-treated device having the best results:enhanced mobility from 23.8 to 39.3 cm²/Vs,decreased D_it from 4.1×10¹² to 1.3×10¹²ev^-1cm^-2.(2)Based on the good interface characteristics between Al₂O₃ and MoS₂,the Al₂O₃/Zr O₂ laminated gate dielectric was designed as the gate dielectric of MoS₂FETs,CF₄/O₂ plasma was also used to treat the stacked gate dielectric Al₂O₃/Zr O₂/Si for further improving the electrical performance of MoS₂ FETs,especially for the F-treated Zr O₂device:high mobility of 53.7 cm²/Vs,small subthreshold swing of 117 m V/dec and high on/off ratio of 2.7×10⁷.The involved main mechanisms lie in the facts that:(i)the MoS₂/dielectric interface is improved through the F^-passivation on the oxygen vacancies in the dielectric by forming Zr-F and Al-F bonds;(ii)a moderate k value of the stacked gate dielectric for the F-treated Zr O₂sample can get a better balance between the gate-screening effect on the CI scattering and the reduction of the SO-phonon scattering.Secondly,in order to simplify the gate dielectric preparation process,the effects of Zr O₂incorporation into Al₂O₃ forming a Zr AlO binary mixture as the gate dielectrics was studied:(1)Zr AlO serves as the positive capacitance gate dielectric of MoS₂ field-effect transistors(FETs)when it is as deposition,and a large improvement in device performance is achieved by the Al-doped Zr O₂ gate dielectric with Zr:Al=1:1:high carrier mobility of40.6 cm²/Vs(41%higher than that of the control sample),small subthreshold swing of 143m V/dec and high on/off current ratio of 6×10⁶.These are attributed to the fact that Al incorporation into Zr O₂ can decrease its oxygen vacancies,thus reducing the traps at/near the Zr_0.5Al_0.5O_y/MoS₂ interface and the gate leakage current.(2)After annealing,Zr AlO was used as the gate dielectric to fabricate back-gated MoS₂ negative-capacitance FETs(NCFETs),in which Al₂O₃ exhibits a positive capacitance and Zr O₂ crystals formed through rapid thermal anneal of 550?exhibit a negative capacitance,resulting in the excellent device performances:small subthreshold swing(SS)of 28 m V/dec and high on/off ratio of 6.4×10⁷.Moreover,the SS is lowered to 18 m V/dec and nearly free hysteresis when a remote NH₃plasma is used to treat the Zr AlO thin film.At last,from the practical consideration of MoS₂ FETs,CVD-MoS₂growth has been investigated:(1)The crystallinity and size are improved when the temperature increases to220?,the fabricated transistor with the triangle MoS₂ achieves the electrical properties of I_on/I_off?10⁶,V_th?7.4V and a carrier mobility of 2.62 cm²/(V·s);(2)On the basis of(1),high k material Al₂O₃ is used as substrate,an effective way of chemical modification on the dielectric surface(Al₂O₃)is investigated to CVD-grow high-quality monolayer MoS₂ and the relevant back-gated FETs are fabricated without MoS₂ transfer.As a result,the size of the triangle MoS₂is increased(from 10?m to 20?m)and its quality is improved as the surface of Al₂O₃is treated by H₂SO₄.And the fabricated transistor with the H₂SO₄-treated Al₂O₃ as gate dielectric achieves better electrical properties:high carrier mobility of 12.9 cm²/Vs(?10 times higher than the untreated sample,?5.2 times higher than the Si O₂ gate-dielectric sample),small subthreshold swing of 110 m V/dec,high on/off ratio of 3×10⁶.This simple chemical-modification treatment will open up an effective approach of combining the high-crystallinity CVD-MoS₂ with the high-k dielectric without MoS₂ transfer required.