Transmission Electron Microscopy (TEM) Studies of Ge Nanocrystals
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Transmission Electron Microscopy (TEM) Studies of Ge Nanocrystals Q. Xua,b, I.D. Sharpa,b, C.Y. Liaoa,b, D. O. Yic, J.W. Ager IIIa, J.W. Beemana, Z. Liliental-Webera, K.M. Yua, D. Zakharova, D. C. Chrzana,b, E.E. Hallera,b a Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, U.S.A. b Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, U.S.A. c Applied Science and Technology Group, University of California, Berkeley, CA 94720, U.S.A. ABSTRACT 74
Ge nanocrystals were formed by ion beam synthesis in SiO2. Transmission Electron Microscopy was used to characterize the structure and properties of these Ge nanocrystals before and after liberation from the matrix. The liberation from the SiO2 matrix was achieved through selective etching in a HF bath. High-resolution micrographs and selective area diffraction confirm that the crystallinity is retained in this process. Transfer of released nanocrystals is achieved through ultrasonic dispersion in methanol and deposition onto lacey carbon films via evaporation of methanol. In an effort to determine the melting point of Ge nanocrystals and observe the growth and evolution of nanocrystals embedded in the amorphous SiO2 during heat treatment, as-grown nanocrystals were heated in-situ up to 1192˚C±60˚C in a JEOL 200CX analytical electron microscope. Electron diffraction patterns are recorded using a Charge-Coupled Device. A large melting hysteresis was observed around the melting temperature of bulk Ge. INTRODUCTION Embedded Ge nanocrystals have attracted strong interest worldwide due to promising non-volatile memory applications [1-3] and quantum mechanical predictions of efficient size-dependent photoluminescence [4]. Among various methods of embedded nanocrystal fabrication, ion beam synthesis has the advantages of compatibility with existing microfabrication processes and precise control of isotopes and spatial distribution. Ion-beam synthesized nanocrystals can exhibit a large compressive stress after post implantation annealing. [5-6] The stress state of these nanocrystals can be controlled by post-growth thermal treatments so as to finely tune the energy band structure. [7] However the origin of this stress is still unclear. It is known that Ge has a 5% volume increase upon solidification; therefore it is vital to determine the state of these nanocrystals at growth temperature. Freestanding nanocrystals have been reported to have a reduced melting point compared to bulk material due to the large surface area to volume ratio. [8-10] In contrast,
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observations of embedded nanocrystals have shown hysteresis behavior around the bulk melting temperature. [11] The fundamental thermal properties are influenced by the surface phonon modes of the nanocrystals, thus it is very important to obtain a better understanding of the growth process at annealing temperature. EXPERIMENT 500 nm silicon dioxide thin films were grown by wet oxidation of (100) oriented Si substrates. Isotopically
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