Surface Deformation and Defect Reduction in Annealed Epitaxial Germanium

Abstract: Epitaxial germanium (epi-Ge) on Si is an ideal material for transistor and optoelectronic devices such as photodetectors due to its higher carrier mobility compared to Si, and its compatibility with existing Si-based processes. However, epi-Ge suffers from surface roughness and crystal defects such as dislocations due to its 4% lattice mismatch with Si. The wide adoption of epi Ge-based devices will depend in a significant part on successfully growing smooth Ge films - or functional structures - directly onto Si substrates with a low defect density. Previous studies have shown that defects in hetero-epitaxial Ge on Si can be removed by annealing in an H2 ambient, or by selective epitaxy and defect-necking through a high- aspect-ratio aperture etched through an SiO₂ mask. Given the efficacy of either method, it would be logical to investigate processes utilizing elements of both methods. In this study, we demonstrate two examples of three-dimensionally shaped epi-Ge structures upon which post-growth annealing was applied, resulting in clear alteration of the surface morphology and reduction in defect density. In the first example, high-aspect-ratio holes were etched into a thick layer of epi-Ge on Si (Fig.1), then annealed at 850^oC in a hydrogen atmosphere, resulting in topological changes that closed the hole openings and yielded a continuous Ge film on the surface and smooth nanocavities below the surface (Fig.2). In the second, mesa-like structures of epi-Ge were selectively grown through patterned openings in a thermal silicon dioxide layer, then annealed at temperatures above 700^oC in a hydrogen atmosphere. Whereas the as-grown mesas were rough with crystalline growth facets or morphological non-uniformities related to the selective epitaxy conditions, and contained multiple growth defects such as stacking faults and twins (see Fig.3), the post-annealed structures were smooth and rounded, and showed a drastic decrease in defect density (Fig.4). The post- annealed microstructural and morphological changes observed in both cases are attributed to bulk and surface diffusion and rearrangement of Ge atoms according to the Gibbs-Thomson effect, by which local surface energy is minimized.

2 Pages

• External Posting Date: External Posting Date: April 6, 2015 [Abstract Only]. Approved for External Publication - External Copyright Consideration
• Internal Posting Date: Internal Posting Date: April 6, 2015 [Fulltext]

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