Quantitative MCs_n~+ - SIMS for Direct Compositional Analysis of Interfaces of Low-Dimensional Structures

摘要

Excellent detection sensitivity, high dynamic range and good depth resolution make the Secondary ion Mass spectrometry (SIMS) technique extremely powerful for the chemical analysis of surfaces and interfaces of condensed matter systems. However, a serious problem in SIMS analysis is its "matrix effect" that hinders the quantification of a certain species in a sample and consequently, probing the composition of surfaces or interfaces by SIMS is greatly hindered. Appropriate corrective measures are therefore, needed to calibrate the secondary ion currents into respective concentrations for accurate compositional analysis. Working in the MCs~+-SIMS mode (M - element to be analyzed, Cs~+ - bombarding ions) can circumvent the matrix effect. The emission process for the species M~0 is decoupled from the MCs~+ ion formation process, in analogy with the ion formation in secondary neutral mass spectrometry (SNMS), resulting in a drastic decrease in matrix effect in the MCs~+- SIMS mode. Although this technique has found its applicability in direct quantification, it generally suffers from a low useful yield. In such cases, detection of MCs_n~+ (n = 2, 3,...) molecular ions offers a better sensitivity (even by several orders of magnitude), as the yields of such molecular ion complexes have often been found to be higher than that of MCs~+ ions. Several works have been reported on the emission of MCs_n~+ molecular ions, but a complete understanding on the formation mechanisms of these ion complexes is still lacking. However, irrespective of the formation mechanisms, MCs_n~+-SIMS technique in all its complexities has great relevance in the elemental analysis of materials. The talk will address on the possible formation mechanisms and potential applications of MCs_n~+ molecular ion complexes in the interfacial analysis of ultrathin films, metallic multilayers, semiconductor superlattices, quantum structures and also in the compositional analysis of MBE grown Si_(1-x)Ge_x alloys.