THz Emission Spectroscopy of 2D Bismuth and Tellurium Layers

Thin bismuth films of various thicknesses between 5 and 32 nm grown by molecular beam epitaxy on Si (111) substrates were investigated. Due to the 2D confinement, the electron energy band structure depends on the thickness. Using the terahertz excitation spectroscopy method, direct bandgap were determined to be in the range from 0.25 to 0.5 eV-much greater than the indirect bandgaps of the layers. A simple model was used to describe the nature of the THz emission from these films, which is the cause of uncompensated lateral photocurrents occurring due to diffusive electron scattering at the Bi/Si interface. In addition 150 nm and thinner layers of tellurium were grown using two different technologies: deposition on various substrates from a chemical solution and thermal evaporation in a vacuum. The mobility and lifetime of the layers obtained by chemical deposition were significantly higher than the values of these parameters in the thermally evaporated Te layers. The lifetimes in the layers of the first of the mentioned types were greater than 500 ps, while in the thermally evaporated layers of the second type, they did not exceed 40 ps. The carrier mobilities also differed several times in both cases.