Study On NMOSFET Device And Process Technology On Ge Platform

As feature sizes of devices in silicon?Si?integrated circuits become smaller and smaller,methods for increasing carrier mobility by introducing strain are increasingly limited.Therefore,high mobility channel materials,such as germanium?Ge?,are gradually becoming the candidate material to replace silicon as a channel material.Although Ge pMOSFET devices have made great progress in surface passivation and stress engineering,there are still some problems in the research of Ge nMOSFET devices.These problems includes:electron mobility degradation of the channel in Ge nMOSFET devices,and the large series resistance of source/drain due to physical problems such as Fermi level pinning.Therefore,this paper focuses on the channel engineering of Ge nMOSFET,the metallic source/drain process,and the fabrication and analysis of self-aligned nMOSFET devices on GeOI?Ge-on-Insulator,GeOI?based on metallic source/drain.The suppression of Fermi level pinning by inserting an interlayer between metal and n-Ge,as well as the reliability and improvement of the metal/insertion layer/n-Ge ohmic contact were studied.A two-step low-temperature oxidation method is proposed to improve the problem of high channel roughness caused by the prior process.After this process,the channel portion roughness can be reduced to 0.33 nm.The GeO₂ prepared by Ge-based direct thermal oxidation method has poor quality and cannot achieve effective passivation effect.High-quality low-interface state-density MOS capacitor is realized by firstly depositing 1nm Al₂O₃,and then 500^oC rapid thermal oxidation method for 45s,the interface state density of the MOS capacitor is 7.3×10¹¹eV^-1cm^-2 under the optimal process conditions.The formation process of ohmic contact of NiGe and TiGe metallic junction were studied.The self-aligned nMOSFET device on GeOI was fabricated based on ohmic contact of NiGe alloy.This is the first result of the Ge-based nMOSFET device reported by the domestic research institutes.The minimum specific contact resistivity?_c=1.14×10^-4?·cm² can be obtained by alloying 350°C NiGe on heavily doped n-Ge for 30 s.A source/drain self-aligned nMOSFET device on GeOI was fabricated using NiGe as the source/drain metal.After annealing the device with channel length of 5?m and channel width of 2?m at 450^oC,I_d of3.5?A/?m,and I_on/I_off ratio of 500 is obtained,at V_g=2V and V_d=0.5V.We proposed a method to increase the current density of Al/n-Ge contact,by depositing ZnO between Al electrode and n-Ge by ALD,and further increase in the contact current was realized by treating ZnO with Ar plasma.It has been found that the insertion of 2 nm ZnO can obtain the maximum current.Ar plasma treatment of ZnO can increase the oxygen vacancies in ZnO and improve the conductivity of ZnO.Furthermore,the surface adsorbed oxygen is removed,and the ZnO/n-Ge conduction band offset can be reduced to 0.17eV,so that the current density of Al/ZnO/n-Ge contact can be further improved.The Ar plasma-treated Al/ZnO/n-Ge CTLM structure was prepared on heavily doped n-Ge,and the specific contact resistivity was extracted to be?_c=2.86×10^-5?·cm².The process optimization and reliability of Al/TiO₂/n-Ge contact by ALD insertion of TiO₂between Al and n-Ge were systematically studied.The effects of ALD deposition temperature on the properties and electrical properties of Al/TiO₂/n-Ge materials were investigated.It is concluded that the deposition of TiO₂ at lower temperature is beneficial to inhibit the growth of GeO_x,and it is also beneficial to increase the oxygen vacancies in TiO₂and improve the conductivity.It is therefore beneficial to increase the current density of the Al/TiO₂/n-Ge contact.The effects of different oxygen source in ALD TiO₂ process on the material properties and electrical properties of Al/TiO₂/n-Ge contact were investigated.It was found that during the ALD TiO₂ process with oxygen plasma,the oxygen plasma had strong penetrability,thus forming thicker GeO₂ between TiO₂ and n-Ge,and this GeO₂ layer increases the whole series resistance.The effect of passivation of ammonium sulfide solution on material properties and electrical properties before deposition by TiO₂ was investigated.It was revealed that n-Ge surface dangling bond is passivated by the pre-deposition ammonium sulfide solution treatment,and dipole is introduced to increase the electron concentration on the n-Ge surface,resulting in the Fermi level depinning of n-Ge.The lowest specific contact resistivity on n⁺-Ge achieved by implanting P⁺ions with dose of 1×10¹⁵cm^-2 was obtained by pretreat with ammonium sulfide solution before inserting TiO₂,which was1.82×10^-5?·cm².The failure mechanism of Al/TiO₂/n-Ge at high temperature was studied,and the single-layer graphene was inserted as a diffusion barrier between TiO₂ and n-Ge before TiO₂ deposition by ALD to improve Al/TiO₂/n-Ge contact.Thermal stability increases to above 350^oC without electrical characteristic degradation.The reason for the performance degradation of Al/TiO₂/n-Ge contact under the total dose of Co⁶⁰?-ray irradiation was studied.It is concluded that the inter-diffusion of Al and TiO₂ is the cause of MIS contact failure.