Characterization and Manipulation of Exposed Ge Nanocrystals
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Characterization and Manipulation of Exposed Ge Nanocrystals I.D. Sharpa,b, Q. Xua,b, C.Y. Liaoa,b, D.O. Yia,c, J.W. Ager IIIa, J.W. Beemana, K.M. Yua, D.N. Zakharova, Z. Liliental-Webera, D. C. Chrzana,b, E.E. Hallera,b a Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 b Department of Materials Science and Engineering, University of California, Berkeley, CA 94720 c Applied Science and Technology Graduate Group, University of California, Berkeley, Berkeley, CA 94720 ABSTRACT Isotopically pure 70Ge and 74Ge nanocrystals embedded in SiO2 thin films on Si substrates have been fabricated through ion implantation and thermal annealing. Nanocrystals were subsequently exposed using a hydrofluoric acid etching procedure to selectively remove the oxide matrix while retaining up to 69% of the implanted Ge. Comparison of transmission electron micrographs (TEM) of as-grown crystals to atomic force microscope (AFM) data of exposed crystals reveals that the nanocrystal size distribution is very nearly preserved during etching. Therefore, this process provides a new means to use AFM for rapid and straightforward determination of size distributions of nanocrystals formed in a silica matrix. Once exposed, nanocrystals may be transferred to a variety of substrates, such as conducting metal films and optically transparent insulators for further characterization. INTRODUCTION With the exception of those synthesized by chemical means, semiconductor nanocrystals are typically embedded in a host matrix, usually SiO2. While this may be desirable for the fabrication of conventional solid-state devices [1], it is not conducive to comprehensive surface and electronic characterization or manipulation. Therefore, it is desirable to develop a method to selectively remove the matrix and obtain free-standing nanocrystals. Such a process will provide a means to directly and individually contact nanocrystals, thereby significantly increasing the number of available characterization techniques and providing a means for nanomanipulation [2]. It is often desirable to transfer nanocrystals to other substrates for further characterization; Lacy carbon grids allow for rapid characterization using transmission electron microscopy (TEM), extremely flat and conducting substrates are required for scanning tunneling microscopy (STM), and optically transparent substrates are required for optical absorption measurements. Once liberated, nanocrystals may be transferred to these, or other, substrates. Two methods are currently available for determining the size distributions of nanocrystals: TEM and Raman spectroscopy. TEM requires painstaking sample preparation and has a very limited sampling of nanocrystal sizes. Fitting Raman spectra using phonon confinement models is relatively inaccurate owing to the dependence of the weighting function on the specific form of the confining function [3] and is typically only
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used to obtain a rough estimate of the average nanocrystal size. Using the liberation process,
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