Experimental Evidence of Si Nanocluster delta-Layer Formation in Buried and Thin SiO₂ Films Induced by Ion Irradiation

Self-aligned Si nanocluster (NC) formation in thin MOSFET gate oxides by means of ion beam mixing and post-irradiation annealing of SiO₂-Si interfaces is proven for the first time by cross-section TEM. This study proves a recent prediction based on kinetic Monte Carlo (KLMC) simulations that a delta-layer of tiny Si NCs in SiO₂, located 2…3nm apart from the SiO₂-Si interface, can be fabricated by ion irradiation [1]. Such a self-aligned NC delta-layer configuration meets the requirements of non-volatile multi-dot floating-gate memory transistors [2], on which immense research effort has been spent in recent years.
By conventional TEM techniques the delta-layer of Si NCs in SiO₂ can not be observed due to the low mass contrast between Si and SiO₂ and the tininess of the Si NCs which have a mean diameter of 1.5nm. Here we present a method of mass contrast enhancement of such tiny Si NCs by alloying them with Ge. For this purpose, a thin Ge layer was embedded into the oxide far above the SiO₂-Si interface, i.e. outside the mixing range of the SiO₂-Si interface in order not to interfere with the original Si NC formation process. During post-irradiation annealing, diffusing Ge atoms are captured by the Si NCs due to the strong Si-Ge bond.