Antibonding-Induced Anomalous Temperature Dependence of the Band Gap in Crystalline Ge₂Sb₂Te₅

Abstract

The band gap of most semiconductors decreases with temperature, but a few semiconductors exhibit an increase of the gap with temperature. This abnormal behavior is usually attributed to thermal expansion, band inversion, or d-states at the valence band maximum (VBM). However, the temperature-induced increase of the band gap in hexagonal (hex) crystalline Ge2Sb2Te5 cannot be understood by the current available mechanisms. Here, we propose a new mechanism, i.e., the interplay between antibonding states and phonon processes. In hex-Ge2Sb2Te5, the abnormal presence of antibonding-like states below the Fermi level induces weaker two-phonon processes than one-phonon processes at the VBM, resulting in a decrease in energy of the VBM with temperature. Moreover, p–d orbital hybridization leads to a comparable strength of two-phonon processes with one-phonon processes at the conduction band minimum (CBM), and thus this makes the CBM position almost temperature-independent. Furthermore, by considering the effects of the temperature-dependent electronic structure (TDES), the calculated electrical conductivity is in good agreement with the experiment, while the conventional rigid band approximation overestimates the values of the electrical conductivity over a wide range of temperature. Our work reveals the origin of the increase of the band gap with temperature in hex-Ge2Sb2Te5 and demonstrates the importance of TDES in electrical conductivity calculations.

Uniaxial Pressure‐Induced 2D–1D Dimensionality Change in GaSe and Related Materials

“…The antibonding nature of chalcogen atoms has been reported earlier. [ 25 ] For this reason, the forced formation of bonds across the gap requires rupture of preexisting bonds, resulting in the formation of a structure with a lower (q1D) dimensionality. This structure is maintained up to a pressure of approx.…”

First-principles study of the temperature-induced band renormalization in thermoelectric filled skutterudites

“…22,23 Combing the lattice expansion and vibration effects is significant, and there are many examples showing its rationality for estimating the temperature-dependent band structure in functional materials. 18,23,24,26,27 First-principles calculations are all performed using the Vienna ab initio Simulation Package (VASP). [28][29][30] The projector augmented-wave method 28 is applied with a plane-wave cutoff energy of 350 eV.…”

“…22,23 Combing the lattice expansion and vibration effects is significant, and there are many examples showing its rationality for estimating the temperature-dependent band structure in functional materials. 18,23,24,26,27…”

Chemical bonding within AIIIBVI materials under uniaxial compression

“…This observation is consistent with the welldocumented antibonding nature of chalcogen atoms. 67 To quantitatively analyze the changes in bond strength, the integrated -COHP (ICOHP) was calculated as a function of pressure for different types of chemical bonds in GaSe, as shown in Fig. S6 (ESI †).…”