GOALI/FRG: Nanoscale Morphological Control of Strained Semiconductor Surfaces

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This FRG/GOALI project is a collaborative effort between researchers at the University of Virginia and IBM, Yorktown Heights, NY. Additional collaborations are established with U. IL, Urbana-Champaign; Longwood College, Farmville, VA; UCB; and Sandia National Laboratories. The approach is to investigate pathways for strain-relaxation of lattice mismatched heteroepitaxial films with the goal of gaining sufficient fundamental understanding of the roles that surface roughening and interfacial dislocations play as the epilayer relaxes to allow morphological 'programming' of heteroepitaxial films. The investigators seek to acquire the knowledge and ability to define the final morphology of an epilayer by introducing perturbations--surface steps, interfacial dislocations and/or lithographically defined patterns--onto the initial growth surface. Applications of such programmed surfaces for templating of subsequent nanoscale structures such as patterned quantum dot arrays and functional biological molecules will also be explored. This programmability places total reliance on neither spontaneous self-assembly nor lithography. Instead, it seeks out synergistic effects that may provide breakthroughs in practical nanostructure fabrication. Initially, experiments will concentrate upon the GeSi/Si system. A range of growth techniques (molecular beam epitaxy, gas source molecular beam epitaxy, and ultra-high vacuum chemical vapor deposition) will be studied, to enable general mechanisms to be inferred, rather than those particular to a particular growth chemistry or configuration. Studies on the evolution of uniform (i.e. non-perturbed) surfaces will focus upon both microscopic mechanisms of strain relief and macroscopic measurements of the overall strain state. The competition and inter-dependence of morphological evolution and misfit dislocation injection will be examined in detail. Experimental techniques will include ex-situ and in-situ transmission electron microscope (TEM), in-situ low energy electron microscope (LEEM), two dimensional reciprocal lattice mapping with X-Ray diffraction, in-situ wafer curvature and spectroscopic light scattering measurements, finite element analysis (FEA) calculations, and local strain mapping using TEM imaging and diffraction. Quantitative measurements of strain relaxation by epilayer roughening will be incorporated into an existing simulator, developed under a previous NSF grant, that models misfit dislocation injection into lattice-mismatched heterostructures. The combined simulator will provide a complete description of strain relaxation, morphological evolution and dislocation generation in a range of heteroepitaxial systems. Subsequent studies of perturbation mechanisms and programmable surfaces will apply many of the same tools, with emphasis on locally sensitive probes such as TEM-, LEEM- and FEA-based techniques.%%%The project addresses basic research issues in a topical area of materials science with high technological relevance. The basic knowledge and understanding gained from the research is expected to contribute to improving semiconductor materials performance in current and future device and circuit applications. An important feature of the program is the integration of research and education through the training of students in a fundamentally and technologically significant area. The multidisciplinary (materials science, electrical engineering, physics) and industrially-connected nature of this GOALI program offers unique educational opportunities for students to experience a teamwork-oriented research environment from both academic and industrial perspectives.***