Structural properties of Ge nanocrystals embedded in sapphire

Abstract

Isotopically pure 74 Ge nanocrystals were formed in a sapphire matrix by the ion beam synthesis method. In contrast to those embedded in amorphous silica, sapphire-embedded nanocrystals are clearly faceted and are preferentially oriented with respect to the crystalline matrix. In situ transmission electron microscopy of heated samples reveals that the nanocrystals melt at 955 °C ± 15 °C, very near to the bulk Ge melting point. Raman spectra indicate that the sapphireembedded Ge nanocrystals are under compressive stress in the range of 3 -4 GPa. The 2 magnitude of the stress is consistent with that expected for hydrostatic pressure arising from solidification. Stress relaxation was not observed for sapphire-embedded Ge nanocrystals; this is attributed to the slow self-diffusion rate of the alumina matrix atoms at temperatures below the nanocrystal melting point.

Estimation of Ge nanocrystals size by Raman, X-rays, and HRTEM techniques

“…The Raman spectrum ( figure 4 (a)), for the same sample, shows a blue shift relative to Ge bulk , giving confirmation of the fact that the NCs are actually under the effect of compressive stress. This effect in Ge NCs due the SiO 2 matrix was already studied by other groups [8], but in Al 2 O 3 matrix this effect has not yet been studied. So, the red shifts caused by phonon confinement would be compensated by the blue shifts which are caused by the compressive stress on the Ge NCs embedded in Al 2 O 3 .…”

“…So, the red shifts caused by phonon confinement would be compensated by the blue shifts which are caused by the compressive stress on the Ge NCs embedded in Al 2 O 3 . The stress may also affect the linewidth of the Raman peak for this kind of systems [8]. Further detailed studies are in course in order to clarify both effects of the stress, and the amorphous phase or the microcrystalline one of the matrix on the peak position and the linewidth in Raman and XRD spectra.…”

Microstructure and dielectric functions of Ge nanocrystals embedded between amorphous Al2O3films: study of confinement and disorder

“…The Raman peaks are also blue shifted with compare to bulk Ge (300.7 cm À1 ) (Figure 3). Similar blue shift of Raman spectrum, which is not in agreement with the phonon confinement theory, has been reported in the literature for sapphire embedded [8,19,20] and silica-embedded [10,21] Ge-NCs grown by RF magnetron sputtering technique followed by high temperature (800 C-900 C) processing. The behaviour has been attributed [10] to the matrix-induced compressive stress on embedded NCs.…”

Partial melting mechanisms of embedded nanocrystals

“…This could specifically be the case in transmissionelectron microscopy ͑real-space images͒ 4,52 and Rutherford backscattering/channeling experiment. 8,11,14,54 On the other hand, methods which measure the average atomic shortrange order such as Raman and certain x-ray approaches 14,56 would be expected to interpret nanocrystals to still remain crystalline during the defect motion state.…”

“…2,3 While nanoclusters in vacuum are well known to usually melt at temperatures much below the normal bulk melting point 1,4 ͑with the exception of some very small systems with only a few tens of atoms 5,6 ͒, embedded nanoclusters have been variously reported to melt below or above the bulk melting temperature. 4,[7][8][9][10][11][12][13][14][15][16][17][18][19][20] Perhaps the most intriguing is the experimental 4 and computational evidence 21 of embedded or coated nanoclusters which shows that even in the same system, the melting point can be either lowered or increased depending on the nature of the interface. Several different theoretical models have been proposed to explain the melting mechanisms in embedded nanoclusters but it is not clear which, if any, of them has general validity ͑for a recent review see Ref.…”