Paving the way to dislocation reduction in Ge/Si(001) heteroepitaxy using C-based strained layer superlattices

Epitaxial Ge films were grown on Si(001) substrates by molecular beam epitaxy. During epitaxial growth, two carbon interlayers were deposited at varying substrate temperatures ( 140 - 620 mml:mspace width=".1emmml:mspace degrees C) and with varying C quantity ( 0 - 1.5 mml:mspace width=".1em"mml:mspace monolayers). The influence of the second carbon interlayer on in-plane strain was investigated using high-resolution x-ray diffraction and transmission electron microscopy (TEM). All samples exhibited compressive strain, which was attributed to substitutional incorporation of carbon atoms. In-plane strain decreases with increasing substrate temperature during carbon deposition, indicating that enhanced surface mobility of carbon adatoms leads to formation of carbon clusters. This was confirmed by cross-sectional TEM investigations. Variation of C quantity at 180 mml:mspace width=".1em"mml:mspace degrees C reveals maximum strain at an intermediate quantity of 0.8 monolayers. Omission of the second C interlayer results in much lower strain, indicating a mismatch between the two Ge layers separated by a C interlayer. This could be used to enforce dislocation filtering following the principle of strained layer superlattices. An upper estimate of 1 x 10 - 3 was found for the mismatch strain, resulting in a critical thickness for dislocation filtering of h c = 153 mml:mspace width=".1em"mml:mspace nm. A sample just exceeding h c exhibited a clear dislocation reduction effect as shown by TEM.