Resumo:
In this work, we study the photoconductivity effect in p-type IV-VI semiconductors
SnTe/P b0,9Eu0,1T e. The samples were grown using molecular beam epitaxy, resulting
in high quality 30 nm-wide structures. Photoconductivity measurements are made for
temperatures below 200 K, with radiation varying from infrared (IR, 940 nm) to ultraviolet
(UV, 398 nm) wavelengths. A positive photoresponse is observed for temperatures greater
than 10 K, with persistent photoconductivity when the light is turned off. Analysis of
the photoconductivity decay curves showed that defect states are present in the energy
band structure of SnTe, and that different defects are activated, as functions of the
radiation energy. For temperatures below 10 K, a transition from positive to negative
photoconductivity is observed when the sample is illuminated by UV light. To explain the
negative effect, Hall measurements were made in order to obtain the carrier concentration
and mobility under light and dark conditions, comparing the variation of these parameters
using UV radiation. With this information, the negative photoconductivity effect is
explained with a classical transport model that predicts the transition for temperatures
below 4 K. Magnetoresistance measurements showed that there is no contribution of the
topological surface states for the electrical transport under illumination. In a second sample,
photoconductivity measurements were conducted for lights ranging from IR, through visible
to UV, in order to verify the dependence of negative photoconductivity on radiation energy.
It was observed that the negative effect indeed manifests only for radiations with energies
greater than 2 eV, starting with yellow. This leads to the conclusion that the negative
photoconductivity is originated from the energy bands above 2 eV.
