Capacitance effect due to electric-field-induced surface modification in TlGaSe_2 layered crystal.

What is it about?

We have investigated the effect of an external electric polling on the surface of TlGaSe_2 single crystal. The performance of TlGaSe_2 is limited by electrically active native deep-level defects, which may destabilize the TlGaSe_2 crystal lattice if their ionization states are controlled by external electric field. Surface modification in the TlGaSe_2 layered semiconductor is originated from the Se anions electromigration from the subsurface bulk to the topmost surface layer. Se ions drift under electrical stress during polling process can lead to accumulation of a high - density of Se - chalcogen vacancies (considering as positively charged defect) in the subsurface bulk. The increase of the Se vacancy concentration together with Se anions drift - related migrations induced by the external field polling must provide the capacitance properties of nearly surface region of the present material. Se atoms deficient and/or a high density of Se chalcogen vacancies model considered in this work for polled TlGaSe_2 sample must give rise to small deformations in the crystal lattices surrounding of each removing Se atom.

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Photo by Axel Richter on Unsplash

Photo by Axel Richter on Unsplash

Why is it important?

Generally, no native vacancies or intrinsic defects in the thermodynamically stable phase of TlGaSe_2 crystal can spontaneously migrate (or drift) at ambient conditions due to high diffusion barriers preventing their spontaneous migration (drifting). However, some native defects and other intrinsic imperfections can migrate (diffuse) inside the lattice structure of materials from initially stable sites inside the unit cell to other sites and be positioned there under external perturbations such as extremely high temperature or an applied external electric. Within this context, an applied external electric can be considered an external perturbation that destabilizes the stable states of native defects (Se vacancies) and triggers their reversible diffusion. The unusual properties of TlGaSe_2 such as a diode-type nonlinear current-voltage associated with a metal-insulator- TlGaSe2 semiconductor (MIS) or MISIM structures in which the rectification direction only depends on the polarity of previously applied bias to the metal electrode and that is independent of electrode materials have been spontaneously formed in TlGaSe_2 sample. Really, the electromigration of charged entities (native Se vacancies or selenium ions) leading to the formation of a thin insulator layer near the correspondently biased TlGaSe2 sample surface. Additionally, the field-effect transistor behavior like to MOSFET at the metal electrode-surface insulator interfaces controlled by an external electric field applied to TlGaSe2 can be easily induced in the exanimated samples.

Perspectives