Alkali-metal-based molecular-ion secondary ion mass spectrometry for precise quantitative analysis of low-dimensional materials without standards

Abstract

Advancement in the understanding of fundamental aspects of Secondary Ion Mass Spectrometry (SIMS) has made this technique to be extremely powerful for quantitative materials analysis. The secondary ion emission process is a complex phenomenon involving various mechanisms to understand the mechanism. Among the existing mechanisms, the “electron-tunneling model” is the most accepted concept in the understanding of ionization probability for positive and negative ions. This model essentially concerns the survival probability of an ion emitted from the surface. As the intensity of secondary ions of a certain element depends strongly on the efficiency of ionization of a sputtered atom or molecule, the “local surface chemistry” plays a significant role in the secondary ion emission. This is purported as the “Matrix Effect” in SIMS, making the technique extremely challenging for quantification of materials in spite of the fact that it has the highest detection sensitivity (<ppb) and exceptional depth-resolution (<1[Formula: see text]nm). Therefore, it is required to correct the “matrix effect” for quantitative analysis. If the probing element (M) in its close proximity sees an alkali element (such as Li, Rb, K, Na and Cs, referred to as “A”) on the sample surface, the sputtered neutral atom (M0) of the probing element can attach an alkali-ion forming a quasi-molecular (MA)[Formula: see text] ion. Such a phenomenon can also occur if an alkali ion beam is chosen as the primary beam for sputtering. The MA[Formula: see text] molecular ion formation is directly related to the atomic polarizability of the element M. Since the sputtering of neutral atom (M0) is unconnected with the formation of MA[Formula: see text] ion, the “matrix effect” decreases dramatically in the analysis using MA[Formula: see text] ions. The phenomenon is very similar to the mechanism of “secondary neutral mass spectrometry” (SNMS). Although these (MA)[Formula: see text] molecular ions are potentially applicable in the direct quantification of materials without the need for “calibration standards”, it generally has a weak intensity. In such a case, the (MA)[Formula: see text] molecular ions are preferred in the analysis because of their higher detection sensitivity (by a few orders of magnitude in some cases), compared to that of (MA)[Formula: see text] molecular ions. Monitoring of the molecular ions is routinely preferred in conventional SIMS experiments to discriminate the sputtered ion species which are affected by mass interference or have poor dynamic ranges. For example, while making the SIMS analysis of GaAs, carbon as an impurity element is detected by monitoring (AsC)[Formula: see text] molecular ions instead of C− ions, because the C− ions have high background arising from the carbon-containing residual-gas species in the analysis chamber. Caesium is decidedly chosen for MCs[Formula: see text] or MCs[Formula: see text] molecular ions in SIMS because of the strongest reactivity and electropositive nature of caesium. This paper deals with a comprehensive discussion on the ion-emission phenomena in sputtering, the “matrix effect” and its compensation, and the potential application of the “MCs[Formula: see text]-SIMS” ([Formula: see text]) in the effective quantitative chemical analysis of materials. A special prominence has been given to the quantification of low-dimensional materials, superlattices and quantum-confined structures using this state-of-the-art “MCs[Formula: see text]-SIMS” approach in all complexities.