Processing and Optical Uses of Van der Waals Layered Materials

Abstract

This thesis is mainly divided into two parts: processing and optical uses of (1) transition metal dichalcogenides and (2) group IV-VI monochalcogenides. Wefirst study processing of large-area polycrystalline MoS₂ and TiS₂ film for photonics and microelectronics, which mainly focus on lowering processing temperature by controlling oxygen concentration. We identify opposite roles of oxygen during the sulfurization process as a reaction catalyst or an inhibitor, for MoS₂ and TiS₂ formation respectively. In MoS₂, O₂ promotes the crystallization of MoS₂ at lower temperature (as low as 400 °C). On the other hand, the kinetic barrier of replacing Ti-O bonds by Ti-S bonds increases with adoption of oxygen in the system. Thus we design the system to lower oxygen background, enabling to lower sulfurization temperature as low as 500° and then form smooth film by suppressing roughening. Beyond investigating roles of oxygen during thin film growth, it is crucial to thoroughly understand the processing and properties of native oxide for designing semiconductor devices. Combining modeling with experimentation, we calculate the oxidation rate and uncover the mechanisms of spontaneous oxidation of bulk single crystals of ZrSxSe₂₋ₓ alloys and MoS₂. ZrSxSe₂₋ₓ alloys oxidize rapidly, and the oxidation rate increases with Se content. Oxidation of basal surfaces is initiated by favorable O₂ adsorption and proceeds by a mechanism of Zr-O bond switching, that collapses the van der Waals gaps, and is facilitated by progressive redox transitions of the chalcogen. The rate-limiting process is the formation and out-diffusion of SO₂. In contrast, MoS₂ basal surfaces are stable due to unfavorable oxygen adsorption. Furthermore, we investigate the role of various processing parameters involved in thermal oxidation and non-thermal oxidation, revealing the growth mechanism. In the second part, new switching mechanism for light-controlling-light is examined in IV-VI monochalcogenides and black phosphorous for photonic integrated circuits. This work is inspired by recent theoretical work suggesting that the in-plane crystal orientation in such materials can be switched through an ultra-fast, displacive (i.e. non-diffusive), non-thermal, and lower-power mechanism by strong electric fields, due to in-plane dielectric anisotropy. We use numerical device modeling to study device concepts based on switching the crystal orientation of SnSe and bP in photonic integrated circuits. Furthermore, we experimentally observe a preliminary switching behavior in bulk single crystal SnSe using single THz pump-probe spectroscopy.