Si/SiGe n-MODFETs on thin SiGe virtual substrates prepared by means of He implantation

Si/SiGe n-type modulation-doped field-effect transistors grown on a very thin strain-relieved Si0.69Ge0.31 buffer on top of a Si(100) substrate were fabricated and characterized. This novel type of virtual substrate has been created by means of a high dose-He ion implantation localized beneath a 95-nm-thick pseudomorphic SiGe layer on Si followed by a strain relaxing annealing step at 850 degreesC. The layers were grown by molecular beam epitaxy. Electron mobilities of 1415 cm(2)/Vs and 5270 cm(2)/Vs were measured at room temperature and 77 K, respectively, at a sheet carrier density of about 3 X 10(12)/cm(2). The fabricated transistors with Pt-Schottky gates showed good dc characteristics with a drain current of 330 mA/mm and a transconductance of 200 mS/mm. Cutoff frequencies of f(t) = 49 GHz and f(max) = 95 GHz at 100 nm gate length were obtained which are quite close to the figures of merit of a control sample grown on a conventional, thick Si0.7Ge0.3 buffer.