Infrared ultrafast spectroscopy of solution-grown thin film tellurium

Abstract

Several materials studied intensively in their bulk forms several decades ago have re-emerged in recent years in their thin film and monolayer manifestations. Tellurium, like black phosphorus, is one such elemental two-dimensional material with promising semiconducting properties for electronic and optoelectronic applications. To study fundamental carrier properties such as hot carrier relaxation and recombination, we performed ultrafast femtosecond pump-probe spectroscopy on thin flakes of solution-grown tellurium. To access the low band gap of tellurium, we used infrared, near-band-gap and below-band-gap probes to monitor the relaxation processes. Sweeping the probing wavelengths across the band gap helps to shed light on the anisotropic band structure of the material. We find that relaxation in flakes of 60--160 nm thickness is on the order of 100 s of picoseconds. Thinner flakes (10--20 nm), on the other hand, exhibit fast relaxation times of sub-20 ps. Radiative recombination is identified as the relaxation mechanism in thick flakes, whereas midgap trap states arising from surface defects and impurities are responsible for the fast relaxation in thin flakes. A diffusion-recombination model accounting for the surface defect and radiative recombinations explains the experimental data well.