Second-harmonic generation spectroscopy for interface studies of dielectric thin films on silicon

Abstract

Most modern semiconductor devices, for example field-effect transistors, memory devices, and wafer-based solar cells, rely on silicon as the principle semiconductor material. Typical semiconductor devices are built-up from various functional materials with the leading dimensions on the nano- and micrometer scale. In recent years, there is a trend of introducing new materials in various fields of silicon-based semiconductor technology which holds in particular for the dielectric thin films employed. This inherently changes the interface and material properties which may impact the performance of the semiconductor devices. Three common aspects related to dielectric thin films that affect device performance are the trapping of charges in dielectric films, the creation of defects at the silicon-film interface, and the formation of (SiO2) interlayers between the film and the silicon. A fundamental insight into the interface and material properties allows for optimizing device performance and may ultimately lead to better control of the material properties such that the device functionalities can be tailored to order. This dissertation concerns the development and the use of the nonlinear optical technique of second-harmonic generation (SHG) to obtain a fundamental insight into the interface and material properties of dielectric-thin-film systems on silicon. SHG is an excellent candidate for this because of its interface specificity for centrosymmetric materials, such as crystalline silicon and amorphous films, and its sensitivity to internal electric fields. The research focuses on technologically relevant topics with the rationale to improve the functionality and performance of semiconductor devices through in-depth understanding. Specifically, the merits and opportunities of SHG spectroscopy in the vicinity of the E1 and E2 critical points of bulk silicon are addressed. Because the measured SH intensity spectra are composed of contributions related to both the silicon interface as well as the electric field of the silicon space-charge region (SCR) an optical model is required to isolate them. An existing optical model was further developed to be able to analyze the SH intensity spectra for more complex sample geometries. SHG spectroscopy studies were performed for SiO2, a-SiNx:H, and Al2O3 thin films which contain different number densities of either positive or negative charges depending on the material system and the processing conditions. Knowledge about the strength and polarity of the electric field in the silicon SCR induced by the built-in charges was obtained under steady-state conditions from the electric-field-induced contribution to the SH response. The sensitivity to the space-charge field was used to distinguish between the influence of field-effect passivation and chemical passivation for ultrathin Al2O3 films on silicon, which is of significance for solar cells. It was shown that the field-effect passivation is virtually unaffected by the Al2O3 thickness down to 2 nm, indicating that the reduced passivation performance for