Electric-field-induced surface modification in TlGaSe2 layered semiconductor: Capacitive effect caused by electromigration of native defects

Abstract

This paper reports the changes in morphology and topographic roughness on the surface of a pristine TlGaSe2 layered crystal caused by an external electric field applied perpendicular to the layer plane at room temperature. These electric-field-induced surface reconstructions and modifications in the TlGaSe2 sample were monitored through x-ray reflectivity, x-ray diffraction, and atomic force microscopy techniques. Two distinct electric-field-induced surface responses have been observed: the shifting of the XRD peaks to the higher Bragg angles and the variations in the XRR curves depending on the strength and polarity of the applied external electric fields. AFM results show that the applied electric field leads to a reproducible transformation of the surface roughness of the TlGaSe2 single crystal from smooth to disheveled, with well-defined depth protrusions. The relaxation time of these surface topological configurations induced under an applied dc electric field was found to be much longer than a few days. The electrostatic capacitive behavior of this two-dimensional semiconducting material is believed to be caused by lattice distortions and the formation of inner stresses (strains) during electric field poling, as well as a drop in the unit-cell characteristics of TlGaSe2. The current-voltage (I-V) measurements show a pronounced nonlinear relationship for a previously poled sample. This nonlinearity is attributed to the field-effect-induced capacitance in TlGaSe2. Electromigration of intrinsic defects such as Se-anion vacancies, which are already present in the crystal lattice structure of virgin TlGaSe2, may diffuse into the sample surface from a bulk (or vice versa) during electric field applications. Finally, employing DFT simulations, we present that the Se-anion vacancy model may be beneficial because changes in the charge state of metal ions positioned around selenium vacancies could be expected. The slightly asymmetric capacitance with respect to the polarity of the bias potential applied to the top surface of TlGaSe2 is justified by our theoretical calculations.