Atomic-Level Control during Film Growth under Highly Kinetically Constrained Conditions: H Mediation and Ultrahigh Dopin
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Atomic-Level Control
during Film Growth under Highly Kinetically Constrained Conditions: H Mediation and Ultrahigh Doping during Si1xGex GasSource Epitaxy J.E. Greene
The following article is an edited transcript of the presentation given by Joseph E. Greene (University of Illinois), recipient of the MRS David Turnbull Lecturer Award at the 1999 MRS Fall Meeting in Boston on December 2, 1999. Greene was cited for “contributions to the use of nonthermal methods in the growth of thin films and the engineering of their phase, composition, and microstructure; and for excellence in teaching and writing.”
Introduction We are living in the golden era of materials science. To cite but one example, consider the field of thin-film physics. Crystal growers have been moving inexorably closer to being able to deposit layers and hence to control film properties on an atom-by-atom basis. We are nearing an era in which it will be possible to deposit
MRS BULLETIN/OCTOBER 2001
“designer” materials with a specified set of properties. Much of the remarkably rapid progress in thin-film crystal growth over the past decade has been due to the confluence of three achievements. One is the development of an ever-increasing array of in situ surface-science probes that provide the ability to interrogate both structure and chemistry at the nanoscale and thereby to intelligently alter growth conditions during film growth. Second is the maturation of computational materials science. Third, crystal growers have begun to devise hybrid growth techniques in which the trajectory of the reaction path is highly kinetically constrained. The goal of the modern crystal grower is, then, to design the kinetic constraints in order to achieve the desired film properties. For gas-phase crystal
growth, this may involve, as one example, selecting and integrating specific advantages inherent in molecular-beam epitaxy (MBE) (e.g., ultrahigh vacuum), chemical vapor deposition (CVD) (site-selective reaction chemistry), and sputter deposition (energy transfer to the growth surface).
Case Study In this article, I will focus on a single case study: ultrahigh p-type doping of Si(001) and Si1x Gex (001) with H atoms mediating the surface reaction kinetics during CVD. The goal here is not only to deposit layers under conditions arbitrarily far from equilibrium while controlling filmgrowth kinetics and nanoscale chemistry, but also to be able to quantitatively predict, with no fitting parameters, the growth conditions required to do so. If the physics and chemistry—that is, the materials science—underlying the growth process is well understood, the models should be transportable to any growth platform. Ultrahigh doping of semiconductors, the incorporation of n- or p-type dopants at concentrations well over an order of magnitude higher than their equilibrium solubility limits, is of intense technological interest for future generations of devices. Example applications include use as emitter layers in Si bipolar transistors; base layers in Si/Si1x Gex heterostructu
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