Stress-driven surface evolution in heteroepitaxial thin films: Anisotropy of the two-dimensional roughening mode
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Stress-driven surface evolution in heteroepitaxial thin films: Anisotropy of the two-dimensional roughening mode Cengiz S. Ozkan Process Development Group, Applied Micro Circuits Corporation, San Diego, California 92121
William D. Nix Materials Science and Engineering Department, Stanford University, Stanford, California 94305
Huajian Gao Mechanical Engineering Department, Stanford University, Stanford, California 94305 (Received 12 November 1998; accepted 17 May 1999)
We have analyzed the anisotropic behavior of surface roughening in Si1−xGex /Si(001) heterostructures by use of methods of elastic analysis of undulated surfaces and perturbation analysis on the basis of global energy variations associated with surface evolution. Both methods have shown that the two-dimensional stage of surface roughening preferentially takes place in the form of ridges aligned along the two orthogonal 〈100〉 type directions. This prediction has been confirmed by ex situ experimental observations of surface evolution by use of atomic force microscopy and transmission electron microscopy in both subcritically and supercritically thick Si1−xGex films grown on Si(001) substrates. Further experiments in supercritically thick films have revealed a remarkable interplay between defect formation and surface evolution: the formation of a network of 〈110〉 misfit dislocations in the latter stages alters the evolution process by rotating the ridge formations toward the 〈110〉 type directions.
I. INTRODUCTION
In the last two decades, much attention has been devoted to the development of novel semiconductor devices based on lattice mismatched heteroepitaxial thin films. The lattice mismatch typically results in a few percent elastic strain within the film, which can be used to tailor the electrical properties of semiconductor devices.1–4 For example, the electronic band gap of the devices can be tailored to have a specific value as a function of the elastic strain in the film, which is a function of the film composition. Applications of strained heteroepitaxial thin film structures (or heterostructures) include the fabrication of various electronic and optoelectronic devices, such as heterojunction bipolar transistors (HBTs),5,6 resonant tunneling diodes (RTDs),7 and vertical cavity surface-emitting lasers (VCSELs).8 The structural stability of heteroepitaxial films is influenced by the elastic mismatch strain in the film, which is the main driving force for the formation of defects, such as dislocations.9–14 Device fabrication requires that the density of the defects be kept to a minimum because otherwise the electrical properties and the performance of the devices will be adversely affected. Extensive efforts have been made to understand the mechanisms of defect formation and propagation in various heteroepitaxial thin film systems such as Si1−xGex/Si,15–18 J. Mater. Res., Vol. 14, No. 8, Aug 1999
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In1−x−yGaxAsy/GaAs,19–21 and CdTe/GaAs.22–24
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