Unusual Surface Undulations Observed in Ge/Si(100) Multilayers
- PDF / 3,587,464 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 88 Downloads / 157 Views
Unusual Surface Undulations Observed in Ge/Si(100) Multilayers Woei Wu Pai, Y. H. Peng, W. F. Chung, S. Y. Wang, K. N. Chen, and H. H. Cheng Center for Condensed Matter Sciences, National Taiwan University Taipei, Taiwan 106 ABSTRACT Multilayer Ge/Si quantum wells grown on Si(100) at lower temperatures (300 C for Ge and 435 C for Si) exhibit unexpected large surface undulations. These groove-like features along directions are correlated to the underlying dislocation networks. Studies on the dependence of surface roughness versus Ge spacer thickness exclude kinetic limitation as the cause of the observed troughs. Further comparison of the surfaces for multilayer and singlelayer samples indicates a combination of the multilayer structure and the inhomogeneous surface strain field greatly enhances the undulations. INTRODUCTION Band structure engineering in silicon germanium strained heterostructures in recent years have attracted great attention [1]. To realize the full potential of SiGe based devices, growth of high quality samples becomes a critical issue. The difficulties lie in gaining good control over the strain relaxation mechanisms. Ge deposited on Si is compressively strained due to the 4.2% lattice mismatch. This mismatch is sufficiently large that the elastic strain energy quickly relaxes through either surface undulations, dislocation generation, or even intermixing (at higher temperature). Competition and balance between different strain relaxation mechanisms clearly affect the properties of samples. It is therefore still a topic of active experimental studies and theoretical simulations. One can take the interplay between elastic surface undulations and plastic misfit dislocation generation as an example. The classic Asaro-Tiller-Grinfeld (ATG) [2,3] instability has been invoked to explain the ripple structures in annealed strained layer films such as Si1-xGex on Si(100) [4,5], GeSi alloy films grown by liquid phase epitaxy [6] and by in situ CVD [7]. This ripple structure can pre-empt the driving force for dislocation generation, similar to the dislocation-free Stranski-Krastanov mode of island growth [8]. On the other hand, accompanied with the elastic strain relaxation, surface undulations also introduce non-uniformity in the strain field. This inhomogeneity can produce highly strained regions that serve as low barrier dislocation nucleation sites [9-11] or even to initiate fractures [12]. Introduction of dislocations can then either promote preferential nucleation [13-15], generate surface steps [16] or smooth a rippled surface. The interwoven relationships between dislocation generation, surface undulations, preferential nucleation, surface smoothing, and composition stress [17] etc. are clearly complicated phenomena. The main features of morphological evolution found in single strained layer film may remain true in multilayer structures. However, the propagation and dilation of strains across different spacers may very well alter the growth behaviors on successive interfaces, resulting in a very dif
Data Loading...
