RUI: Halogen-Etched and Chemically-Sputtered Germanium Surfaces

University Of Wisconsin-Oshkosh, Oshkosh WI

Investigators

Abstract

This RUI project is a multifaceted study of single crystal germanium surfaces modified by reactive halo-gen etchants and energetic ion bombardment. Using scanning probe microscopy (SPM), reflection high-energy electron diffraction (RHEED), and photoemission spectroscopy (PES) a sequence of experiments on etched germanium substrates are planned. Initial stages will investigate the morphology of Ge(100)-(2x1) and Ge(111)-c(2x8) surfaces etched by diatomic halogen molecules (Cl2 , Br2 , I2 ). Through statis-tical analyses (correlation functions, size distributions, etc.) of SPM images from the etched surfaces, comparisons with theoretical models of surface roughening will be made to assess competing smoothing and roughening mechanisms. These data will also be compared to data collected from etched silicon sur-faces with the same orientations. The chemical similarity of germanium and silicon is predicted to yield the same halogenated surface species upon exposure to the halogen etchant-this will be verified by PES measurements to be carried out with collaborators at the University of Wisconsin Synchrotron Radiation Center. Structural differences, particularly on the (111)-oriented surfaces are expected to yield different surface structures, thereby yielding insight into structural mechanisms of surface evolution. Chemical sputtering (the simultaneous exposure of a surface to energetic ions and neutral halogen molecules) will be carried out on Ge(100)-(2x1) surfaces. SPM results from these experiments in conjunction with those from simple etch systems and those from simple sputter systems will identify similarities and differences in surface structure. Features unique to the chemical sputtering method are expected due to the synergis-tic enhancements in etching that can occur in the presence of both energetic ions and reactive chemical etchants. Dynamic RHEED measurements taken in situ during etching will enable monitoring of surface crystallinity. Etching parameters can be tuned so that RHEED oscillations are obtained, indicative of layer-by- layer removal. PES measurements will be used to assess concentrations of highly halogenated species after chemical sputtering. Unique surface morphologies result from material removal. Etch pits, surface defects, regrowth dimer chains, and regrowth islands are examples of the kinds of nanostructures that can be controlled via etchant flux, etchant fluence, and substrate temperature. Large domains of these surface features can be created and used as templates for thin-film growth. The goal of the research is to explore the influence of these nanostructures on the nucleation and growth of thin films. Statistical analysis of SPM data will allow monitoring evolution of surface morphology. Questions being addressed include whether or not these surface structures enhance nucleation density, how island coalescence is af-fected, and how interface abruptness is affected. Before the thin-film growth experiments take place, re-sidual halogen atoms must be removed from the surfaces, preferably without disturbing the structure of the etched surface. Extraction of halogen atoms by atomic hydrogen has been shown to be effective using spectroscopic techniques; however, the effect of this procedure has not been explored from a morpho-logical perspective. SPM and PES data will be collected in order to verify the effectiveness of this tech-nique with respect to surface modification. %%% The project addresses fundamental research issues in a topical area of materials science having techno-logical relevance. Undergraduate students will play a primary role in the project, with opportunities for research integrated directly into their academic program, and continued full time in the summer. The re-search program will enable forefront research and access to state-of-the-art surface science equipment to undergraduate students. An important feature of the project is the strong emphasis on education, and the integration of research and education. ***