Germanium Nanocrystals Embedded in Sapphire
- PDF / 1,288,764 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 91 Downloads / 221 Views
BB5.22.1
Germanium Nanocrystals Embedded in Sapphire Q. Xua,b, I.D. Sharpa,b, C.Y. Liaoa,b, D. O. Yi a,c , J.W. Ager IIIa, J.W. Beemana, K.M. Yua, 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 are formed in a sapphire matrix by ion implantation followed by thermal annealing. Transmission electron microscopy (TEM) of as-grown samples reveals that the nanocrystals are faceted and have a bi-modal size distribution. Notably, the matrix remains crystalline despite the large implantation dose and corresponding damage. Embedded nanocrystals experience large compressive stress relative to bulk, as measured by Raman spectroscopy of the zone center optical phonon. In contrast, ion-beam-synthesized nanocrystals embedded in silica are observed to be spherical and experience considerably lower stresses. Also, in situ TEM reveals that nanocrystals embedded in sapphire melt very close to the bulk melting point (Tm= 936 ˚C) whereas those embedded in silica exhibit a significant melting point hysteresis around Tm. INTRODUCTION Surface effects on nanostructure properties have been extensively studied for the simple reason that a large fraction of atoms reside on the surface in nanostructured materials. In the case of embedded Ge nanocrystals, the interface is particularly interesting because on the one hand, quantum mechanical models predict efficient size-dependent photoluminescence [1], but on the other hand, the reported photoluminescence from embedded Ge nanocrystals have mostly been attributed to oxide defects between the oxide and the nanocrystals. [2-3] In this context, it is important to determine the properties of interface defects and the roles they play in determining the optical properties. Freestanding nanocrystals have been reported to have a reduced melting point compared to bulk material due to their large surface area to volume ratio. [4-6] In contrast, small melting point elevations and hysteresis behaviors have been observed in some embedded metal nanocrystals systems. [7] The determining factors are the large
BB5.22.2
interface fraction and the difference in the interface energies between solid/liquid nanoparticles and the surrounding matrix. We have observed a 250˚C superheating and 100˚C undercooling thermal hysteresis of Ge nanocrystals embedded in amorphous silica.[8] This alteration in melting behavior well illustrates the influence the matrix and the interface have on the fundamental properties of nanocrystals. The study presented in this paper aims at comparing the effects of a crystalline matrix (sapphire) on Ge nanocrystals with our previous observations of those of amorphous silica matrix. EXPERIMENT
Intensity (a.u.)
A commercially available single crystalline sapphire substrate with an orienta
Data Loading...
