Gate-modulated optical transitions of tellurium-based field-effect transistors

Tellurium ($\mathrm{Te}$) is a two-dimensional semiconducting material with significant potential for tunable device applications. Here, we perform optical transmission measurements on a $\mathrm{Te}$-based field-effect transistor (FET) and reveal three major optical changes induced by electrical gating. (i) Intraband transition in $\mathrm{Te}$ evolves from localized Drude-Smith states to delocalized, standard Drude states as the hole carrier density increases. (ii) Interband transition and defect-level transition both shift to higher energies at distinct rates. (iii) The optical phonon peak of the ${\mathrm{Si}\mathrm{O}}_{2}$ dielectric layer undergoes a subtle yet distinct shift. We calculate the variation of dc dielectric constant $\ensuremath{\epsilon}(0)$ of ${\mathrm{Si}\mathrm{O}}_{2}$ induced by the optical phonon shift and support that the FET operates in practice within the linear charge response regime.