Self Organized Compound Semiconductor Nanocrystallite Distributions in SiO 2 on Silicon Synthesized by Ion Implantation
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Self Organized Compound Semiconductor Nanocrystallite Distributions in SiO2 on Silicon Synthesized by Ion Implantation H. Karl, I. Großhans and B. Stritzker Institut für Physik, Universität Augsburg, D-86135 Augsburg, Germany ABSTRACT
The temporal and spatial evolution of sequentially and single ion implanted Cd and Se concentration distributions into thermally grown SiO2 on (001)-Si were investigated. Ex-situ rapid thermal annealing was performed to initiate the reaction-diffusion driven material transport, nanocrystal nucleation and growth. Finally this leads to the formation of buried distinct layers of CdSe nanocrystals. The spatiotemporal evolution of the concentration distributions were quantitatively analyzed by dynamic Secondary Ion Mass Spectrometry (SIMS). It will be shown, that there is a correlated diffusion of Cd and Se when both elements were implanted in overlapping concentration profiles, whereas the single implanted Cd and Se exhibit a completely different diffusion behaviour. In the region of the supersaturated solid solution reaction to thermodynamically stable CdSe clusters takes place. The steep concentration gradient provokes indiffusion of the stoichiometric compound. In the case of a surplus of Cd over Se and long annealing times self-organized, nearly periodic and correlated concentration variations of Cd and Se can be observed. Comparison of the profiles indicate that this pattern formation is controlled by the diffusion and precipitation of the over stoichiometric Cd.
INTRODUCTION Phase separation processes are of great technological and scientific interest and intensively investigated for many years. The study of the involved nucleation and growth processes of precipitates are usually performed in systems with macroscopically homogeneous concentration distribution, i.e. the length scale of the initial spatial variation of the concentration is large compared to the microscopic reaction-diffusion and nucleation regions. After mixing of the components and thermal quenching a homogeneous distribution of precipitates can be obtained, which is far away from its thermal equilibrium. Further heat treatment will lead to Ostwald ripening and a temporal evolution of the size distribution and so to a coarsening of the elemental distribution on a nano- up to microscopic length scale. This process is mainly controlled by the total surface energy of the precipitates and the interplay of diffusive transport and chemical reaction rate. In contrast to the above mentioned case of a homogeneous concentration distribution, ion implanted concentration profiles are highly inhomogeneous. Under those initial conditions the dynamics of the nucleation and growth of precipitates varies locally. In addition diffusion of material according to the macroscopic concentration gradients on the flanks of the profile takes place. It is long known, that similar reaction-diffusion systems hold potential for selforganization [1]. The conditions for self-organizing phenomena to be observed are nonlinear couplings betw
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