Structural characterization of Si1−xGex alloy layers grown by molecular beam epitaxy on Si(001) substrates

Abstract

Residual strain, surface roughness, and dislocations of Si1−xGex alloy layers grown by molecular beam epitaxy on Si(001) substrates at 550 °C have been characterized by x-ray diffraction, atomic force microscopy, and cross-sectional transmission electron microscopy (XTEM). Two kinds of samples were grown. One is a series of Si1−xGex alloy layers with x⩽0.3 and a thickness of 5000 Å directly grown on Si(001) substrates, and the other is a series of Si0.7Ge0.3 alloy layers with a thickness of 2000 Å grown on Si(001) via compositionally graded Si1−yGey buffer layers (0⩽y⩽x). The Ge grading rate gr in the buffer layer, defined by gr% Ge/μm, ranged from 22 to 76. In the case of direct growth, the surface morphology changes from a wavy ripple pattern to a cross-hatch pattern with increase in x, and islandlike patterns appear at x=0.3. The residual strain decreases with increase in x, whereas the surface roughness increases with x. In the case of Si0.7Ge0.3 alloy layers grown with buffer layers, the surfaces of all samples display cross-hatch pattern. The surface roughness is highest for a grading rate of about 35, and it decreases for both lower and higher grading rates. The residual strain also shows a similar dependence on the grading rate. XTEM images are correlated to the residual strain and surface roughness.