Liberation of Ion Implanted Ge Nanocrystals from a Silicon Dioxide Matrix via Hydrofluoric Acid Vapor Etching
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Liberation of Ion Implanted Ge Nanocrystals from a Silicon Dioxide Matrix via Hydrofluoric Acid Vapor Etching I.D. Sharpa,b, Q. Xua,b, C. Y. Liaoa,b, J.W. Ager IIIa, J.W. Beemana, K.M. Yua, D. Zakharova, Z. Liliental-Webera, E.E. Hallera,b a Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 b Department of Materials Science and Engineering, University of California, Berkeley, CA 94720 ABSTRACT A method to liberate germanium (Ge) nanocrystals from silicon dioxide (SiO2) thin films by hydrofluoric acid (HF) vapor etching is presented. Multi-energy implantation of mass separated Ge ions into 500-nm-thick wet oxide layers on silicon (Si) substrates followed by thermal annealing produces nanocrystals that are 2 to 8 nm in diameter. Raman spectra exhibit the expected asymmetric line shapes due to the phonon confinement effect, but with a higher peak frequency than predicted. To free the nanocrystals, samples are etched in HF vapor to selectively remove the SiO2 matrix and expose the nanocrystal surfaces. Raman spectra of etched samples display peak frequencies consistent with relief of compressive stress. The liberated nanocrystals show long-term stability under ambient atmospheric conditions. Ge nanocrystals can be removed from etched surfaces using an ultrasonic methanol cleaning procedure. The nanocrystal-containing solution is applied to a TEM grid and the solvent is evaporated. Subsequently obtained electron diffraction patterns confirm that the nanocrystals survive this transfer step. Thus, liberated Ge nanocrystals are expected to be accessible for a wide range of manipulation processes and direct characterization techniques. INTRODUCTION Ge nanocrystals have been widely investigated due to their potential optoelectronic and non-volatile memory applications [1,2], as well as their value in exploring the basic physical properties of nanocrystalline materials [3]. A variety of techniques, including ion implantation [4,5], rf co-sputtering [6,7], and inverse micelle liquid chemistry [8] have been used to prepare Ge nanocrystals. In particular, fabrication via implantation of energetic Ge ions into SiO2 has attracted considerable interest due to its compatibility with existing microfabrication processes. Ion implantation also permits fabrication of isotopically controlled Ge nanocrystals that may be used for spintronics investigations of 73Ge, selective doping of nanocrystals via neutron transmutation doping (NTD), and precise analysis of nanocrystal stress states. Since Ge nanocrystals formed by means of ion implantation are embedded in a solid (often SiO2) matrix, only certain optical and x-ray techniques are available for nondestructive characterization. Direct access to the nanocrystal surfaces, which would allow for contact formation and manipulation to construct 2-D arrays, relies on processes to liberate nanocrystals. The aim of this study is to develop a process to liberate Ge nanocrystals from SiO2 films and transfer them between surfaces in order to expand the rang
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