Mar 10, 2009
Manipulation of quantum-dot ordering in amorphous multilayers
A recent study of germanium quantum-dot (QD) formation in an amorphous silica matrix showed that under specific deposition conditions a self-ordering growth regime occurs and three-dimensionally (3D) ordered QDs can be produced. Technological aspects of this research were published recently in Nanotechnology and the fundamental background has been covered in Phys. Rev. B.
Ordered germanium quantum dots are produced by magnetron sputtering deposition of (Ge+SiO2)/SiO2 multilayers using different substrate temperatures. The figure above combines transmission electron microscopy and grazing incidence small angle X-ray scattering (GISAXS) images of the multilayers formed after the deposition and subsequent annealing at 800 °C. The appearance of Bragg-like spots in the GISAXS maps of samples deposited at 500 °C confirms a 3D correlation in the dot positions. A precise analysis shows the formation of slightly disordered QD lattices with rhombohedral structure. The deposition temperature proved to be a critically important parameter for controlling mechanisms that produce the 3D arrangement of QDs.
The appearance of the correlation in the positions of the dots is explained by the influence of the positions of buried dots on the morphology of the growing surface, which affects the surface diffusion of the adatoms and consequently influences the nucleation of new dots. This mechanism is completely different from the structuring process that occurs in crystalline multilayers, where the anisotropy of the stress caused by the mismatch of the lattices of the dot material and the elastically anisotropic crystalline matrix causes a 3D ordering of quantum dots. However, such an anisotropy-mediated mechanism is not effective in an inherently isotropic amorphous matrix.
The physical properties of the formed dots depend highly on the degree of their spatial correlation. The study published in Nanotechnology shows a narrowing of the size distribution with an increase in the degree of correlation of the dots. An even more interesting physical property is the appearance of collective-behavior vibrations of dots, which are visible in their low-frequency Raman spectra. Due to the spatial correlation, dots vibrate in-phase, which results in an enhancement of the acoustical vibrational modes.
The presented method of the production of regular dense arrays of quantum dots offers a new perspective in the semiconductor technology of new micro and optoelectronic devices. The production method is quite general and can be applied to different kinds of semiconductor QDs in amorphous matrices.
About the author
Dr Maja Buljan is a researcher in the Division of Material Science at Ruder Boškovic Institute in Zagreb, Croatia, led by Dr Nikola Radic. Recently, she finished her PhD study supervised by Dr Uroš Desnica, and started a post-doctoral study at the Charles University of Prague, in the group of Prof. Václav Holy. She deals with the study of self-ordering of nanoparticles (quantum dots) in amorphous systems, the crystalline structure of these dots and she develops experimental methods based on X-ray scattering for the study of such materials.