Quaternary Tl2CdGeSe4 selenide: Electronic structure and optical properties of a novel semiconductor for potential application in optoelectronics

Insights from Experiment and Theory on Peculiarities of the Electronic Structure and Optical Properties of the Tl2HgGeSe4 Crystal

“…The presence of this essential hybridization degree of the above-mentioned PDOS causes the existence of a significant covalent component (furthermore to ionic component) in the total combination of chemical bonding in the Tl 2 HgGeSe 4 crystal, being in fair correspondence with the conclusion retrieved based on XPS evaluations of the binding energies listed in Table . Such a feature of the chemical bonding of the Tl 2 HgGeSe 4 crystal is similar to that established earlier for the related Tl 2 HgSnSe 4 and Tl 2 CdD IV Se 4 (D IV = Ge and Sn) compounds. ,, …”

“…Furthermore, the main maximum of the Ge Kβ 2 XES band is located in the vicinities of the theoretical feature C. Therefore, the main contribution of Ge 4p states is experimentally detected in the central area, with some contributions of these electronic states in the upper part of the valence band of the Tl 2 HgGeSe 4 crystal, again confirming the theoretical suggestions for the location of Ge 4p electronic states (Figure c). Similar features of filling the valence band ranges by electronic 4p states associated with selenium and germanium atoms were detected by XES measurements and/or first-principles computations of other related Ge-bearing selenides, in fact Tl 2 CdGeSe 4 , and Cu 2 HgGeSe 4 . It should be noted that, in accordance with the theoretical computing data plotted in Figure , contribution of electronic s-like states associated with mercury and thallium is expected to be also essential in the lower part of the valence band of Tl 2 HgGeSe 4 .…”

“…As a result, in the present experiments, the charging surface effect has been compensated by using a specially constructed flood gun available in the SPECS UHV analysis system as we used to do such a technique in the case of Ge-bearing Cu(Tl) 2 B II D IV X 4 -type chalcogenides. ,, Following the findings on the electronic structure of quaternary Ge-bearing selenides Cu(Tl) 2 B II GeSe 4 indicating that among electronic states associated with these atoms one could expect essential input of Se(Ge) 4p-like states in the valence band area, ,, we have recorded the fluorescent XES Se(Ge) Kβ 2 spectra originating due to the transition from the N II,III shell to the K level and supplying information regarding the peculiarities of energy distributions of valence Se(Ge) p-like electronic states. , The latter bands were derived with a DRS-2 M spectrometer in the third order of reflection and employing for spectral excitation a BHV-7 X-ray tube with a Au anode. The operating conditions of the DRS-2 M spectrometer are completely the same as they were chosen for the similar XES experiments of related Cd-containing selenide, Tl 2 CdGeSe 4 . The edge of the optical absorption of the Tl 2 HgSnSe 4 crystal was measured using an MDR-206 monochromator with the spectral resolution not worth than 0.2 nm using the technique reported in ref .…”

“…The computing iterations were carried out until reaching the condition q = ∫ |ρ n − ρ n−1 | dr ≤ 10 −4 , where ρ n (r) and ρ n−1 (r) are ascribed to the charge density of the present and previous iterations, respectively, as it is generally proposed to follow in the case of (Cu,Tl) 2 B II D IV X 4 -type sulfides and selenides. 6,34,50,52,61 When reaching the best coincidence of the given theoretical data with the experimental measurements regarding features of the energy allocation of electronic states in the vicinities of the valence band area of Tl 2 HgGeSe 4 and E g value, then, we calculate the optical constants of this compound using a 5000 k-point grid. In particular, the imaginary portion of the complex dielectric function is expressed by the following equation (1 ) ( )…”

“…We used a grid amounting to 1000 k -points for Brillouin zone sampling via its irreducible wedge. The computing iterations were carried out until reaching the condition q = ∫|ρ n – ρ n –1 | d r ≤ 10 –4 , where ρ n ( r ) and ρ n –1 ( r ) are ascribed to the charge density of the present and previous iterations, respectively, as it is generally proposed to follow in the case of (Cu,Tl) 2 B II D IV X 4 -type sulfides and selenides. ,,,, …”