Mott barrier behavior of metal–TlGaSe₂ layered semiconductor junction

Abstract

We report on the characteristics of metal-p-type high resistance TlGaSe2 semiconductor junction barrier fabricated by deposition of indium and gold metals. The electrical properties of /TlGaSe2/ semiconductor contacts were monitored as a function of temperature in the range of ∼80–300 K using current–voltage ( I − V ) and capacitance–voltage ( C − V ) measurements. Device characteristics of I n /TlGaSe2/ A u showed rectification properties. From forward bias I − V characteristics it was revealed that the increase of current is slower than the predictions by Schottky barrier theory. The rectification properties of I n /TlGaSe2/ A u semiconductor device were simulated by the Mott barrier model where the presence of an undoped layer on the semiconductor surface is assumed, and measurements and computational simulation agreed on the validity of this model. C − V measurements showed that at higher temperatures I n /TlGaSe2/ A u barrier showed a capacitance from accumulation toward depletion mode, whereas at low temperatures the barrier capacitance degradation was found. The observed C − V of I n /TlGaSe2/ A u does not significantly change over the entire bias range ± 30 V, confirming that the I n /TlGaSe2/ A u is fully depleted, thus the structures are Mott barrier. First-principles electronic band diagram calculations showed that introduction of S e -atom vacancies in the various sites of TlGaSe2 unit cell greatly affects the electronic band structure of this semiconductor in an increasing manner in band gaps with respect to stoichiometric compound. Consequently, the S e - vacancies localized inside the thin surface layer of TlGaSe2 are responsible for high average surface electrical resistivity of the material and could be proposed as a physical basis for the existence of native insulator layer on TlGaSe2 surface. Thus we propose a model for TlGaSe2 crystal which is comprised from an electrically conducting bulk semiconductor sandwiched between two high resistivity thin surface sheets. We suggest that the selenium vacancy defects on the surface of TlGaSe2 layered semiconductor are also responsible for the memristive behavior of this compound.

A Study of Thermoelectric Performance of TlGaSe2 Layered Dichalcogenides from First‐Principles Calculations: Vacancy Defects Modeling and Engineering

“…[ 1–4 ] In bulk form, is a p‐type semiconductor with optical bandgap in the visible spectral range (≈2.1 eV) at room temperature. [ 5–9 ] grows in the form of multilayers where layers are weakly interacting through van der Waals forces along the direction. Within the layers, every atom is strongly bonded with neighbor ones via ion‐covalent chemical bonding mechanism.…”

“…At room temperature the electronic transport mechanism in the direction parallel to the layers is directly related to the presence of localized states in the bandgap created by native structural defects. [ 7–19 ] The electrical conduction perpendicular to the layers is expected to be highly sensitive to inevitable structural defects such as: planar defects presented between the layers due to a slipping of successive layers, point defects, charged impurities, deep level traps, dislocations as well as to many other crystal lattice imperfections that are mainly attributed to the interlayer stacking faults. [ 20–22 ] Only at extremely high temperatures, due to the thermally activated free carriers from the valence band the intrinsic electrical conduction is determined by a band‐to‐band mechanism.…”

“…A conversion from p‐type conduction to n‐type by doping techniques has also appeared in the literature, but added impurities do not alter its p‐type conduction. [ 7,23 ] Even addition of impurities in large concentrations can't switch the conduction type from p to n. The origin of p‐type conduction is not yet fully understood and is still controversial.…”

“…In our recent study, [ 5,7 ] the selenium inhomogeneities at surfaces of pure single crystals have been identified as favorable characteristics of this dichalcogenides. It was experimentally demonstrated that the real van der Waals surfaces of suffer from the presence of both selenium inclusions (segregations) and the regions with a very low concentration of Se constituent atoms on the outermost layers, which likely exist due to poor solubility of selenium during thermo‐physical processes of crystal grown.…”

“…It can be expected that a small amount of selenium in host can be considered as presence ‐vacancy defects, intrinsic in nature, randomly distributed in crystal unit cells. [ 5,7 ] These chalcogen vacancy defects severely affect the charge transport properties of and could be responsible for two nontrivial electronic features of this compound. [ 7,8,33,34 ] Thus, a sets of selenium vacancies should be localized in various atomic sites of unit cell due to compositional inhomogeneities forming during crystal synthesis or post‐growth treatment.…”