Misfit dislocation dissociation and Lomer formation in low mismatch SiGe/Si heterostructures
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Using transmission electron microscopy we observe the dissociation of 60° misfit dislocations at the interface of SiGe/Si multilayers, extending into the substrate for distances of 5.0–7.5 nm. Analysis using elasticity theory shows that this dissociation is the equilibrium configuration for individual 60° misfit dislocations, as it is for 60° mixed dislocations in bulk Si, and that the compressively strained multilayer film serves mainly to position the partial dislocations and stacking fault with respect to the free surface. We observe both undissociated 60° and Lomer edge dislocations after annealing, and conclude that these result from dislocation climb in the interface. Since the dislocations move off their slip plane during climb, they cannot remain dissociated. Significant climb and Lomer dislocation formation for these low misfit layers is observed at temperatures above 850 °C and for samples with a high initial dislocation density, such as found in thicker as-grown samples. The dislocation configuration formed during annealing is distinct from that reported to form during growth of higher mismatch films: the Lomer dislocations tend to be segmented, with the segments connected by perfect 60° dislocations.
I. INTRODUCTION
The occurrence of misfit dislocations in strained-layer semiconductor heterostructures is of fundamental interest in understanding thin film growth behavior and of considerable practical interest due to potential effects on device performance. Although the geometrical constraints of an epitaxial system tend to simplify misfit dislocation morphologies relative to bulk material, these dislocations nonetheless exhibit a complex array of characteristics and interactions depending on misfit strain and synthesis conditions (see, for example, the review of the SiGe system by Hull et al. or LeGoues1,2). The ability of the typical 60° glide dislocations to dissociate into partials separated by stacking faults is one such complicating behavior: classical treatment of strain relaxation in [001] oriented semiconductor films concludes that, for films under tension, stacking faults may extend through the film thickness, whereas for compressive films, such as SiGe/Si, the partials are pushed together at the interface and the dislocations are considered to be perfect 60° dislocations.3,4 However, for the compressive system InGaAs on GaAs, several researchers have observed
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0065 J. Mater. Res., Vol. 20, No. 2, Feb 2005
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and studied in detail the narrow dissociation of 60° dislocations at the interface and extending into the substrate.5–7 These results were observed for low-strain conditions, i.e., those with low In concentration, where the films exhibit layer-by-layer growth and smooth surfaces. Dissociation has also been occasionally demonstrated for isolated dislocations in SiGe on Si.8–10 Reported dissociation widths are generally less than 10 nm and there
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