Using Spectroscopic Ellipsometry as an In Situ Technique to Control the Growth of Chalcogenide Films

While there are several ex-situ experimental tools to characterize chalcogenide thin films, it is imperative to use in-situ techniques, ideally operated in-operando during growth, to obtain immediate feed-back to control the quality, thickness and alloy concentration of films. Spectroscopic ellipsometry (SE) is a non-destructive optical method that can be employed to determine the dielectric function and thickness of films. The dielectric function obtained from SE can be further analyzed to extract fundamental band gap as well as the higher order electronic transitions, providing insights into the band structure of the material under study. More importantly, the dielectric function obtained from SE can be correlated with other results, such as X-ray diffraction and Raman experiments, to deduce its functionality on thickness, alloy concentration and quality of the chalcogenide film in question. Consequently, having access to such calibrated dielectric functions will enable the incorporation of SE as an in-situ technique to provide immediate-information on thickness, alloy concentration and quality of the chalcogenide film being grown.<br/>In this presentation, we will discuss in-situ SE results on several molecular beam epitaxy-grown chalcogenide films including, Bi₂Se₃, In₂Se₃, (Bi_1-xIn_x)₂Se₃and PtSe₂. SE spectra of both single-layer films and multi-layer heterostructures were analyzed at each step of the growth cycle, including the spectra of the substrate prior to the growth of the film. The dielectric function found for each film via SE is modelled as a collection of oscillators, each of which is associated with an electronic transition in the material. This procedure allows us to infer the quality of films via the broadening parameter of oscillators. Specifically, we find that the Bi₂Se₃ films grown on a buffer layer of (Bi_0.7In_0.3)₂Se₃ have a higher quality compared to Bi₂Se₃ grown directly on sapphire. Besides gathering information on the quality of films, in-situ SE predicts the thickness variance of a sample as growth proceeds. Extensive work performed on Bi₂Se₃ films show that in-situ SE can detect unintentional desorption of layers as thin as a quintuple layer of Bi₂Se₃ during a temperature ramp-up, and subsequent cool-down back to the growth temperature. With regard to the multilayer structures, the optical models developed for these structures can decipher minute perturbations in layers as the growth progresses. For instance, our models show that a ~7 nm Bi₂Se₃ layer grown next to a sapphire substrate seems to disappear as the structure is annealed at 600 °C. In addition to the quality and thickness, in-situ SE can determine the alloy concentration of chalcogenide ternary compounds. We are able to predict the alloy concentration of (Bi_1-xIn_x)₂Se₃at the initial stages of its growth by analyzing in-situ SE spectra. In addition, by examining SE spectra obtained every 15 seconds during the growth, we can monitor if the alloy concentration fluctuates during the growth of the structure. Finally, we will discuss our results on the growth of ultra-thin MoS₂, and show how in-situ SE can be used to control the deposition of monolayers and bilayers of this material.