Influence of structural disorder on soft x-ray optical behavior of NbC thin films

Abstract

Structural and chemical properties of compound materials are modified, when thin films are formed from bulk materials. To understand these changes, a study was pursued on niobium carbide (NbC) thin films of different thicknesses deposited on Si (100) substrate using ion beam sputtering technique. Optical response of the film was measured in 4-36 nm wavelength region using Indus-1 reflectivity beamline. A discrepancy in soft x-ray performance of NbC film was observed which could not be explained with Henke's tabulated data (see http://henke.lbl.gov/optical-constants/). In order to understand this, detailed structural and chemical investigations were carried out using x-ray reflectivity, grazing incidence x-ray diffraction, x-ray absorption near edge structure, extended x-ray absorption fine structure, and x-ray photoelectron spectroscopy techniques. It was found that the presence of unreacted carbon and Nb deficiency due to reduced Nb-Nb coordination are responsible for lower soft x-ray reflectivity performance. NbC is an important material for soft x-ray optical devices, hence the structural disorder need to be controlled to achieve the best performances.