MS-1-P-1619 High resolution HAADF investigation of Ga droplets on Si(001) surfaces

The growth of III/V material on Si substrates opens a wide field of new applications and devices [1]. The initial stages of the nucleation and especially the type of element (III or V) it is started with may have a crucial impact on the interface structure and therefore a device´s performance [2]. In this study Ga was deposited on Si substrates without the presence of group V elements to investigate the processes occurring during these early stages of growth.
The samples were grown via metal organic vapor phase epitaxy in an AIX 200 GFR reactor. To investigate the impact on the morphology two different precursors, triethylgallium and trimethylgallium, were used and the growth temperature was varied between 400 and 500°C. Electron transparent samples were prepared along an <110> axis of the silicon by conventional mechanical grinding and final ion milling in a Gatan PIPS. The samples were characterized in a double C S-corrected JEOL JEM 2200 FS scanning transmission electron microscope (STEM) operating under high angle annular dark field (HAADF) conditions resulting in Z-contrast.
On the surface of the Si Ga droplets form which can be identified in HAADF images by their bright contrast, due to the higher atomic number of Ga with respect to Si (Fig. 1). The number of droplets clearly scales with amount of supplied precursor during the growth. Moreover, the droplets are not only confined to the surface but penetrate into the Si forming a pyramidal structure with boundaries on {111} lattice planes. Complementary energy dispersive X-ray measurements confirm that these pyramids contain Ga. The observed morphology can be explained by the fact that the liquid Ga etches the Si at the growth temperature. By addition of a precursor for group V after the Ga deposition, crystallization of the droplets can be enforced. Therefore, the droplets can serve as nucleation sites for the growth of low dimensional materials like nanowires.
This contribution will show how HAADF STEM can be used to investigate etching processes on an atomic scale.

References

[1] Liebich et al., Appl. Phys. Lett. 99, 071109 (2011).
[2] Volz et al., J. Cryst. Growth 315, 37 (2011).