Strain relieved SiGe buffers for Si-based heterostructure field-effect transistors

The transport properties of strained and relaxed Si1-xGex alloys have been investigated for x = 0 ... 1 by using n-type and p-type modulation doped field-effect transistor (MODFET) structures grown by MBE on lop ni. strain relieved graded buffers. The mobility in slightly n-doped strain relaxed layers is mainly affected by alloy scattering. The electron mobility at a doping concentration of N-d = 1 x 10(17) cm(-3) varies from 200 cm(2)/V s to 1700 cm(2)/V s at room temperature (RT) for x = 40-100%, respectively. Double sided doped Si channel n-MODFETs grown on various buffers show RT mobilities between 2700 and 1050 cm(2)/V s for sheet carrier densities of 0.5-7 x 10(12) cm(-2). The electron confinement and the mobility are both proportional to the conduction band offset. In F-type MODFETs with Si1-yGey channels, alloy scattering is again the limiting factor for the transport properties. A mobility of 1880 cm(2)/V s at RT for a sheet carrier density of 2.1 x 10(12) cm(-2) has been achieved with a pure Ge channel layer on a strain relieved Si0.4Ge0.6 buffer. With this structure a record cut-off frequency of f(max) = 85 GHz in p-MODFETs has been achieved. (C) 1999 Elsevier Science B.V. All rights reserved.