Effects of Stress on Step Energies and Surface Roughness
- PDF / 1,478,075 Bytes
- 4 Pages / 576 x 777.6 pts Page_size
- 27 Downloads / 301 Views
MRS BULLETIN/APRIL 1996
dulating film has a larger surface area so that this reduction is balanced by the increased surface free energy of the film. Based on such a competition, linear theory predicts a critical thickness for coherent islanding, both in the presence and absence of elastic distortions of the substrate. If the growth rate of the film is appreciable, the instability may be postponed to a larger thickness because of a kinetic stabilization of the growing film.8 Strain-induced shape changes will in-
fluence the surface morphology, even during the latter stages of the growth. Going beyond linear theory, Yang and Srolowitz9 have shown that roughening can lead to the formation of grooves or pits, in which the stress concentrates and the nucleation of dislocations is facilitated.10 Another intriguing feature is that during growth, strain relaxation in heteroepitaxial systems can lead to changes in the shapes of islands. In semiconductor systems, the coherent islands are often faceted and characterized by large aspect ratios. For example, Mo and co-workers11 have obtained beautiful scanning tunneling microscopy (STM) images of "hutlike" Ge islands grown on Si(100) surfaces. These clusters display (105) facets and are characterized by aspect ratios as large as 8:1. Another example is provided by Ag clusters on Si(100), which during their later stages of growth, display aspect ratios as large as 50:1.12 Through a calculation of the balance between the surface energy of the facets and the elastic energy inherent in the islands, Tersoff and Tromp13 were able to show that strained islands are likely to undergo a shape transformation during growth. Below a critical island size, the energy balance favors compact,
0.1 um Figure 1. Cross-sectional transmission electron micrograph of 50-A GeasSio.s films on various substrates. On Ge substrate under 2% tensile strain (top panel), on Geo.25Sio.75 relaxed buffer under 1% compressive strain (middle panel), and on a Si substrate under 2% compressive strain (lower panel). Photograph courtesy of Y.H. Xie.
27
Effects of Stress on Step Energies and Surface Roughness
symmetric islands while elongated islands with high aspect ratios are the preferred shape for large islands. Thus the shape transformations taking place as a result of the system relaxing its strain may provide a convenient way of producing quasi-one-dimensional quantumwire structures. Similar shape changes have also been observed in the ordered domain structures that form in latticemismatched binary alloy solutions, following a temperature quench.14 Roughening in Ge/Si(100) Systems Recent experiments by Xie and coworkers5 demonstrate that the strain-induced roughening in semiconductor systems may depend strongly on the sign of the strain. In Ge/Si(100) systems, films under compression were found to be rough while those under tension remained relatively flat. The films were produced as follows. On top of clean Si(100) substrates, an epitaxial compositionally graded GexSii_v buffer layer was grown at 900°C eq
Data Loading...
