The fabrication and characterization of strained-Si material grown on a relaxed Si0.79 Ge0.21/graded Si1-x- Gex/Si virtual substrate, using reduced pressure chemical vapor deposition, are presented. The Ge concentration of the constant composition SiGe layer and the grading rate of the graded SiGe layer are estimated with double-crystal X-ray diffraction and further confirmed by SIMS measurements. The surface root mean square roughness of the strained Si cap layer is 2.36nm,and the strain is about 0.83% as determined by atomic force microscopy and Raman spectra, respectively. The threading dislocation density is on the order of 4 × 10^4cm^-2. Furthermore, it is found that the stress in the strained Si cap layer is maintained even after the high thermal budget process, nMOSFET devices are fabricated and measured in strained-Si and unstrained bulk-Si channels. Compared to the co-processed bulk-Si MOSFETs at room temperature,a significant low vertical field mobility enhancement of about 85% is observed in the strained-Si devices.
The paper describes the growth of a germanium （Ge） film on a thin relaxed Ge-rich SiGe buffer. The thin Ge-rich SiGe buffer layer was achieved through a combination of ultrahigh vacuum chemical vapor deposition （UHVCVD） SiGe epitaxial growth and SiGe oxidation. A lower Ge content strained SiGe layer was first grown on the Si （001） substrate and then the Ge mole fraction was increased by oxidation. After removal of the surface oxide, a higher Ge content SiGe layer was grown and oxidized again. The Ge mole fraction was increased to 0.8 in the 50 nm thick SiGe layer. Finally a 150 nm thick pure Ge film was grown on the SiGe buffer layer using the UHVCVD system. This technique produces a much thinner buffer than the conventional compositionally graded relaxed SiGe method with the same order of magnitude threading dislocation density.
Biaxial strain technology is a promising way to improve the mobility of both electrons and holes, while （100） channel direction appears as to be an effective booster of hole mobility in particular. In this work, the impact of biaxial strain together with （100） channel orientation on hole mobility is explored. The biaxial strain was incorporated by the growth of a relaxed SiGe buffer layer,serving as the template for depositing a Si layer in a state of biaxial tensile strain. The channel orientation was implemented with a 45^o rotated design in the device layout,which changed the channel direction from （110） to （100） on Si （001） surface. The maximum hole mobility is enhanced by 30% due to the change of channel direction from （110） to （100） on the same strained Si （s-Si） p-MOSFETs,in addition to the mobility enhancement of 130% when comparing s-Si pMOS to bulk Si pMOS both along （110） channels. Discussion and analysis are presented about the origin of the mobility enhancement by channel orientation along with biaxial strain in this work.
Strain-relaxed SiGe virtual substrates are of great importance for fabricating strained Si materials. Instead of using graded buffer method to obtain fully relaxed SiGe film, in this study a new method to obtain relaxed SiGe film and strained Si film with much thinner SiGe film was proposed. Almost fully relaxed thin SiGe buffer layer was obtained by Si/SiGe/Si multi-structure oxidation and the SiO2 layer removing before SiGe regrowth. Raman spectroscopy analysis indicates that the regrown SiGe film has a strain relaxation ratio of about 93% while the Si cap layer has a strain of 0.63%. AFM shows good surface roughness. This new method is proved to be a useful approach to fabricate thin relaxed epilayers and strain Si films.
