The structural and electrical characterisation of SiGe heterostructures deposited on strain relaxed virtual substrates
The influence of lateral dimensions on the relaxation mechanism and the resulting effect on the surface topography of limited-area, linearly graded Si1-xGeX virtual substrates has been investigated for the first time. A dramatic change in the relaxation mechanism of such buffer layers has been observed for depositions on Si mesa pillars of lateral dimensions of 10μm and below. For such depositions, misfit dislocations are able to extend, unhindered, and terminate at the edges of the growth zone. In this manner, orthogonal misfit dislocation interactions are avoided, yielding a surface free of the problematic surface cross-hatch roughening. However, as the lateral dimension of the growth zone is increased to 20μm, orthogonal misfit interactions occur and relaxation is dominated by the Modified Frank-Read (MFR) multiplication mechanism. The resulting surface morphology shows a pronounced surface cross-hatch roughening. It is proposed that such cross-hatch roughening is a direct consequence of the cooperative stress fields associated with the MFR mechanism. It is postulated that the method of limited-area, linearly graded buffer layers provides a unique opportunity, by which 'ideal' virtual substrates, free of surface cross-hatch and threading dislocations, may be produced to any Ge content. In addition, a unique method by which the electrical performance of low temperature, strained layer depositions may be optimised is discussed. The method relies on the elimination, 'of as-grown lattice imperfections via a post growth thermal anneal treatment. A 25-fold increase in low temperature hole mobility of a Si0.5Ge0.5/Si0.7Ge0.3 heterostructure has been demonstrated using a 30 minute, 750°C in-situ, post growth anneal.