| About this Abstract |
| Meeting |
2011 Electronic Materials Conference
|
| Symposium
|
2011 Electronic Materials Conference
|
| Presentation Title |
S10, LATE NEWS: Two Coherent Limits in Core-Shell Semiconductor Nanowires |
| Author(s) |
Shadi A Dayeh, Jian Wang, Wei Tang, Greg Swadener, Karen L Kavanagh, S. Tom Picraux |
| On-Site Speaker (Planned) |
Shadi A Dayeh |
| Abstract Scope |
The epitaxial growth of different materials and interfaces or electrical conduction type on one-dimensional semiconductor nanowires allows engineering the local strain, quantum size, and electronic band offsets that in turn can be tailored to control the spatial distribution of charge carriers and achieve novel opto-electronic devices. It is therefore particularly important, experimentally and theoretically, to assess the coherent limits for the growth of such epitaxial shells and to identify the mechanisms by which the strains are relaxed. Here, we observe two types of relaxed shell structures using HRTEM and explain their formation mechanisms by proposing two critical thicknesses to relax axial and radial strain components, and validate these observations by molecular dynamics simulations. Our system constitutes of a [111] oriented Ge nanowire core grown by chemical vapor deposition through the vapor-liquid-solid growth mechanism and subsequent in-situ Si shell deposition. The Ge NW core diameter is 30 nm and the Si shell thickness was varied from 1-10 nm and was post-annealed at 600 ⁰C for evaluation of the equilibrium critical thickness. As the shell thickness increases to 2.7 nm, we generally find perfect crystalline core/shell structures. At a thickness of 3.2 nm up to 5 nm, we observe extra Si (111) planes along the axis of the NW that indicate a perfect dislocation loop with a Burgers vector of a/2<110>. The axial separation of these additional planes was reproducibly found to be 26 (111) bi-layers or ~ 8.5 nm consistent with and larger than a predicted spacing of 7.5 nm along the [111] direction for such dislocation loops. The dislocation loop nucleates at the shell surface and introduces an extra plane at the edge of the shell, and then glides on one of the three {111} planes (besides the growth direction) at a 19.5 degrees from the growth axis to form an extra plane on the other edge of the NW. Our HRTEM analysis verifies the presence of several of such loops that form elliptically around the Ge core with a total axial length of ~ 85 nm consistent with the expected 30nm/tan(19.5). However, the additional (111) planes in the axial direction relieve only axial strain and radial strain remains to be relieved. Since the insertion of additional (111) Si planes radially is not supported, defected structures (such as twin, grains) evolve instead and surface roughening of the shells happens as verified experimentally for Si shell thicknesses exceeding 7-8 nm. With such comprehensive understanding, we can better predict the distribution of strains in both core and shell and energy band-edge profiles for different core/shell nanowire dimensions. |
| Proceedings Inclusion? |
Undecided |