Reverse graded SiGe/Ge/Si buffers for high-composition virtual substrates

The effect of compositional grading rate on reverse linear graded silicon germanium virtual substrates, grown by reduced pressure chemical vapor deposition, is investigated. For a Si(001)/Ge/RLG/Si(0.22)Ge(0.78) buffer of 2.4 mu m total thickness the threading dislocation density (TDD) within the top, fully relaxed, Si(0.22)Ge(0.78) layer is 4x10(6) cm(-2), with a surface roughness of 3 nm. For a thicker buffer, where the grading rate is reduced, a lower TDD of 3x10(6) cm(-2) and a surface roughness of 2 nm can be achieved. The characteristics of reverse graded Si(0.22)Ge(0.78) virtual substrates are shown to be comparable to, or exceed, conventional buffer techniques, leading to thinner high-quality high Ge composition SiGe virtual substrates.