Research On Germanium-on-insulator(GOI) Material Fabricated By An Intermediate Ultra-thin Silicon Layer

Compared with silicon,Ge offer higher carrier mobility and more favourable absorption coefficient in the near infrared wavelength regime(1.3-1.5 ?m).Besides,it is compatible with Si CMOS process and can be used as a ?-? group material epitaxy template due to the small lattice mismatch between Ge and GaAs(0.07%).These make Ge a promising candidate for next-generation high-performance microelectronic and optoelectronic device.However,its smaller band gap of germanium causes a higher junction leakage current resulting in a large standby power,and some small size effects are almost inevitable with the further decrease of the device size.To overcome the drawbacks of Ge,germanium-on-insulator(GOI)was proposed due to it contains the merits of Ge and the structural advantages similar to SOI.Lots of papers presented the fabrication of low defect density GOI using Ge-SiO2 direct wafer bonding previously.However,the bonding strength is still too low and the voids at the interface cannot be effectively absorbed at low temperature(?400 ?)due to the fact that the surface hydrophilic treatment for Ge is difficult and the Ge native oxide(GeOx)is unstable.Besides,the interface state density at the Ge/SiO2 interface is high(>1012 eV-1 cm-2)due to the appearance of the unstable GeOx appear at the interface.This may degrade the electrical characteristics of the devices formed on the GOI substrate.In this thesis,we propose a low-temperature Ge-SiO2 bonding method to fabricate GOI with good bonding quality in which an ultra-thin Si layer is deposited on Ge surface(Si/Ge)to enhance the hydrophilic of the Ge surface and restrain the formation and decomposition of unstable GeOx layer.The main works and innovations are as follows:1.The effects of the ultra-thin Si film grown by UHV/CVD on the Ge/Si-SiO2 bonding have been detailedly investigated.The XPS results indicate that the formation of the GeOx is effectively restrained at the bonded interface due to the introduction of the ultra-thin Si film.Hence,the voids caused by the decomposition of GeOx have significantly vanished.After the the treatment of diluted NH4OH solution,the contact angle measurements show that surface hydrophilicity has been improved due to the introduction of the ultra-thin Si film.This leads to the amount of Si-O-Si bonds forming in Ge/Si-SiO2 bonding more than that of Ge-O-Si bonds forming in Ge-SiO2 bonding after post annealing(400 ?).In addition,the strength of Si-O-Si bond is higher than that of Ge-O-Si bond.Therefore,the greater amount of Si-O-Si bond and its higher strength ultimately can be responsible for the increase of the bonding strength(increasing to 4.39 MPa).2.The effects of the ultra-thin Si film with various thickness grown by magnetron sputtering on the Ge/Si-SiO2 bonding have been detailedly investigated.The surface hydrophilicity of the Si/Ge is better than that of bulk and improve with the increase of the thickness of the ultra-thin Si layer.The Si/Ge(5-nm-thick ultra-thin Si layer)has the most favourable surface hydrophilicity and good roughness.This leads to the high bonding strength of 3.9 MPa and the voids-free bonded interface are obtained after post annealing at 250 ?.3.The thinning processes of GOI and the electronic characteristics have been Systematically studied.The top Ge layer was firstly thinned from 170 ?m to 40?m by the mechanical grinding.It is is still complete except a small exfoliation at the edge.Then,the surface RMS roughness was reduced to 0.39 nm after a two-step chemical mechanical polishing.Finally,the top Ge layer was thinned to 0.5-1 ?m using the etching solution(H2O2:H3PO4:H2O=1:6:10).A flat GOI surface with few etching pits was obtained.The results of Hall-effect indicate that the hole mobility of the GOI is 460 cm2/V s,close to that of bulk Ge(523 cm2/V.s).The GOI Pseudo-MOSFET offers a transistor characteristics,whereas it is partial depleted because of the thick Ge layer.