| AVS 54th International Symposium | |
| Nanometer-scale Science and Technology | Wednesday Sessions |
| Session NS+EM-WeM |
| Session: | Nanoscale Devices and Nanowires I |
| Presenter: | F.S. Flack, University of Wisconsin-Madison |
| Authors: | F.S. Flack, University of Wisconsin-Madison C.S. Ritz, University of Wisconsin-Madison M.G. Lagally, University of Wisconsin-Madison H.J. Kim, University of Wisconsin-Madison K. Turner, University of Wisconsin-Madison M. Huang, University of Utah F. Liu, University of Utah |
| Correspondent: | Click to Email |
Self-assembly in nanostructures is a linchpin of the bottom-up design technique essential to the advance of nanoscale fabrication. In particular, the self-assembly demonstrated by quantum dots (QDs) in strain-mismatched systems has potential for applications in lasers, high-density memory, and thermoelectric devices. Self-assembled arrays of QDs have been fabricated by several techniques with varying degrees of success and usability. We demonstrate an entirely new route for investigating the nucleation and ordering of QDs: growth on ultrathin semiconductor nanomembranes, substrates that are so thin, usually on the order of tens of nm, that they allow the strain created by a QD to penetrate the entire structure,. Such freestanding thin films are truly compliant and, when thin enough, regions of high strain will bend. Thus, we must address the effects of both strain and bending on the preferential nucleation of SiGe QDs. Tethered Si nanomembranes (cantilevers, ledges, wires, and ribbons) with thicknesses ranging from 20 nm to 60 nm are fabricated from silicon-on-insulator (SOI) substrates. SiGe QDs are then grown epitaxially by ultrahigh vacuum chemical vapor deposition. To ensure appropriate strain, the QDs are grown to have dimensions of the same order as the membrane thickness - roughly 8 nm in height. CVD allows for QD nucleation simultaneously on both sides of the membrane, so that nucleation of a QD on one side influences nucleation on the other. In addition, the nanomembranes are thin enough to be semi-transparent in an SEM, granting us a direct view of the alignment of QDs on the top surface to those on the bottom. We model this growth system with finite element analysis and see that a SiGe island nucleated on the top creates regions of high strain on the bottom along preferential crystallographic directions. These strained regions act as sites for preferential nucleation for QDs on the bottom, which will then seed the next QDs on the top layer and so on to create highly ordered, anticorrelated, arrays on both the top and bottom of the membrane. We discuss the simulations and observations in the limits of QD nucleation due to substrate bending and strain modulation. Research supported by DOE and AFOSR.