Optical properties of pseudobinary GeTe,Ge2Sb2Te5,

Abstract

Fe doped GST has shown experimentally the ability to alter its magnetic properties by phase change. In this work, we use screened exchange hybrid functional to study the single neutral substitutional 3d transition metal (TM) in crystalline GeTe and GeSb2Te4. By curing the problem of local density functional (LDA) such as over delocalization of the 3d states, we find that Fe on Ge/Sb site has its majority d states fully occupied while its minority d states are empty, which is different than previous predicted electronic configuration by LDA. From early transition metal Cr to heavier Ni, the majority 3d states are gradually populated until fully occupied and then the minority 3d states begin to be filled. In order to study the magnetic contrast, we use lower symmetry crystalline GeTe and GeSb2Te4 as the amorphous phases, respectively, which has been proposed to model the medium range disordering. We find that only Co substitution in r-GeSb2Te4 and s-GeSb2Te4 shows magnetic contrast. The experimental magnetic contrast for Fe doped GST may be due to additional TM-TM interaction, which is not included in our model. It can also be possible that these lower symmetry crystalline models are not sufficient to characterize the magnetic properties of real 3d TM doped amorphous GST.

Ab initio density functional theory study of the electronic, dynamic, and thermoelectric properties of the crystalline pseudobinary chalcogenide (GeTe)x/(Sb2Te3)Ibarra-Hernández1, Raty2 2018Phys. Rev. B

“…3). Our band-gap value is half of that measured experimentally (0.55 eV) [32]. We find that this forbidden gap is direct and located on the high-symmetry line Z-, where Z is located at the top of the Brillouin zone [see Fig.…”

Electronic Structure and Spin Configuration Trends of Single Transition Metal Impurity in Phase Change Material

“…Previous tunneling spectroscopy [38] and concentration dependence of susceptibility mass study [39] suggested a smaller energy gap around 0.1-0.2 eV. The lowest band gap energy values of 0.8 eV for the amorphous GeTe by the extrapolation method [37] and tunneling spectroscopy [40] agreed with each other. Note that o-GeTe is proposed as a lower symmetry crystalline approximation to the real amorphous GeTe and its sX band gap coincides with the experimental value.…”

“…Different experimental lowest band gap energy values have been reported for r-GeTe. Park et al [37] found this value to be 0.61 eV by linear extrapolation of the adsorption coefficient. Previous tunneling spectroscopy [38] and concentration dependence of susceptibility mass study [39] suggested a smaller energy gap around 0.1-0.2 eV.…”

“…For r-GeSb2Te4 and s-GeSb2Te4, the sX band gaps are 0.63 eV and 0.67 eV, respectively. The experimental extrapolation gave 0.49 eV and 0.71 eV for the crystalline and amorphous phases, respectively [37] and the LDA band gap for the crystalline phase is 0.55 eV [37]. The LDOSs (not shown) for both GeTe and GeSb2Te4 system show that the p states of the constituent atoms lie well above the s states, indicating weak s and p hybridization.…”

“…Note that o-GeTe is proposed as a lower symmetry crystalline approximation to the real amorphous GeTe and its sX band gap coincides with the experimental value. Theoretically, the calculated band gap values of r-GeTe varied from 0.367 eV to 0.66 eV based on the LDA functional [23,37,41], quantum mote carlo method gave even larger value of 0.7 eV [41]. Previous relativistic band structure calculations showed smaller band gap values, assuming the GeTe in the fcc structure [42,43].…”