Defesa de Dissertação de Mestrado #614 – Rafael Ricardo Rojas Lopez – 11/09/2018

Optical properties of transition metal dichalcogenides on GaAs

Autor: Rafael Ricardo Rojas Lopez

Banca Examinadora

Prof. Paulo Sergio Soares Guimarães (Orientador)

DF-UFMG

Prof. Leonardo Cristiano Campos

DF-UFMG

Dra. Juliana Caldeira Brant

DF-UFMG

Orientação

Prof. Paulo Sérgio Soares Guimarães (Orientador)

DF-UFMG

Resumo do Trabalho

Transition metal dichalcogenides (TMDs) are crystalline structures of the form MX2, where M is a transition metal, like Mo or W, and X is a chalcogenide, as S or Se.
They are multilayer structures, with X-M-X covalent bonds within a layer and Van der Waals bonds at the interlayer interfaces. As the thickness is reduced from
several layers to a single monolayer, a transition is observed from a semiconductor with indirect band gap to a direct band gap in the visible or near infrared
frequencies. Therefore, the TMDs are good candidates for the implementation of ultra-thin optoelectronical devices. For this reason, and also for their interest in the
study of new physical phenomena, they have been extensively investigated over the last decade. It is already well established that the optical properties of
monolayer TMDs are strongly affected by the substrate. In this work, we will present how the optical properties of three different TMDs, MoS2, WS2 and WSe2, are
modified by placing them on GaAs substrates. We investigate samples on three types of substrates: p-doped GaAs, n-doped GaAs and semi-insulating GaAs. The
monolayers were obtained by mechanical exfoliation and transfered to the substrates. Since the refraction indexes of GaAs and the TMDs are similar, we had to
establish a procedure to locate and deal with suitable monolayers. We present a spectral analysis of the luminescence emission for each case, as well as their Raman
spectra. We find that the GaAs substrates produce significant changes in the luminescence of the TMDs, which can be understood with basis on the band alignment
of the TMD with GaAs. We conclude that GaAs/TMDs heterojunctions have a great potential for applications as photodetectors and solar cells.