Estudo das propriedades de transporte elétrico em nanoestruturas compostas de Bi2Te3 e de PbTe

Abstract

This work presents a study on the magneto-transport properties of nanostructures based on Bi2Te3 and PbTe, both narrow gap semiconductors. Electrical transport in a n-type PbTe quantum well was investigated using light excitation in different temperatures and applied magnetic fields. Under illumination, the sample exhibited metal-insulating transition around 100 K and Shubnikov-de Haas oscillations were observed in the magnetoresistance curves. Through the fast Fourier transform (FFT) analysis, four frequencies were obtained that compose the oscillations observed and one of which corresponds to a second harmonic. The cyclotronic masses of the other three frequencies were calculated using the reduced Lifshitz-Kosevich (LK) equation, which together with the ratio between these frequencies allowed us to assign one to the longitudinal valley and another to the oblique valley. Finally, using the full expression of LK the last frequency was assigned to a non-trivial Berry phase. Bi2Te3 samples with thickness varying between 15 and 150 nm with a protective BaF2 caplayer and a Bi2Te3 film without a layer with a thickness of approximately 156 nm were investigated. The sample without cap layer exhibited linear magnetoresistance followed by Shubnikov-de Haas oscillations at low temperatures. Using the FFT, it was found that the observed oscillations are composed of two close frequencies. With the LK equation it was possible to extract the cyclotronic masses of these frequencies through the thermal damping of the FFT amplitude and also to calculate the Berry phases of the carriers. Through the variation of the angle of incidence of the field it was verified that the oscillations have their origin from the bulk rather than the surface states and the appearance of the two close frequencies in the FFT and the non-zero Berry phase found were attributed to the Rashba effect caused by surface degradation. Analyzing the influence of the film thickness of the Bi2Te3 samples with BaF2 protective layer (CP) it was found that the mobility of the carriers decreased with the reduction of the film thickness and that the magnetoresistance curves exhibited a weak localization effect and only the thinnest sample exhibited the weak antilocalization (WAL). The WAL effect observed in regions of low magnetic field in the sample with CP and thickness of 15 nm was analyzed using the Hikami Larkin Nagaoka model, which allowed us to verify that the transport occurs in a two-dimensional regime and what electron-electron scattering is the dominant transport mechanism. None of the samples with CP showed quantum oscillations up to 9 T.