Investigation of electronic structure of amorphous, metastable, and stable phases of Ge[sub 1]Sb[sub 2]Te[sub 4] film by high-resolution x-ray photoemission spectroscopy (original) (raw)
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Applied Physics Letters, 2006
Amorphous Ge 2 Sb 2 Te 5 ͑a-GST͒ film, 100 nm thick, was grown by cosputtering from GeTe and Sb 2 Te 3 targets on a silicon wafer at room temperature. The native oxidized layer, which was formed in air and about 20 nm thick measured by secondary ion mass spectroscopy, was removed by Ne + ion sputtering for 1 h with 0.6 kV beam energy. Core-level spectra of the Te 3d and 4d, Sb 3d and 4d, and Ge 3d of the oxygen-free a-GST were obtained by high-resolution x-ray photoelectron spectroscopy with synchrotron radiation and compared with those from Ge and GeTe. The analysis implies that the a-GST is composed on the base of chemical states of GeTe.
Journal of Applied Physics, 2005
Ge-Sb-Te alloys are widely used for data recording based on the rapid and reversible amorphous-to-crystalline phase transformation that is accompanied by increases in the optical reflectivity and the electrical conductivity. However, uncertainties about the optical band gaps and electronic transport properties of these phases have persisted because of inappropriate interpretation of reported data and the lack of definitive analytical studies. In this paper we characterize the most widely used composition, Ge 2 Sb 2 Te 5 , in its amorphous, face-centered-cubic, and hexagonal phases, and explain the origins of inconsistent or unphysical results in previous reports. The optical absorption in all of these phases follows the relationship ␣h ϰ ͑h − E g opt ͒ 2 , which corresponds to the optical transitions in most amorphous semiconductors as proposed by Tauc, Grigorovici, and Vancu ͓Tauc et al., Phys. Status Solidi 15, 627 ͑1966͔͒, and to those in indirect-gap crystalline semiconductors. The optical band gaps of the amorphous, face-centered-cubic, and hexagonal phases are 0.7, 0.5, and 0.5 eV, respectively. The subgap absorption in the amorphous phase shows an exponential decay with an Urbach slope of 81 meV. We measured the photoconductivity of amorphous Ge 2 Sb 2 Te 5 and determined a mobility-lifetime product of 3 ϫ 10 −9 cm 2 / V. The spectral photoconductivity shows a threshold at about 0.7 eV, in agreement with our analysis of the optical band gap. The face-centered-cubic and hexagonal phases are highly conductive and do not show freeze-out; even at 5 K the density of free carriers remains at 10 19 -10 20 cm −3 , so these are degenerate semiconductors in which the Fermi level resides inside a band. In the hexagonal phase, the effect of free electrons on the Hall coefficient is significant at high temperatures. When the Hall data are fitted using the two-carrier analysis, the hole mobility is found to decrease slowly with temperature, as expected. The considerations discussed in this paper can be readily applied to study related chalcogenide materials.
Journal of Applied Physics, 2002
The crystal structures of GeSb 2 Te 4 , Ge 2 Sb 2 Te 5 , and Ge 3 Sb 2 Te 6 were determined using electron diffraction and high-resolution transmission electron microscopy. The structure determined for the former two crystals deviates from the ones proposed in the literature. These crystal structures were developed jointly upon cooling of liquid Ge 2 Sb 2 Te 5. A stacking disorder parallel to the basal plane was observed that increases with increasing cooling rates. For the Ge x Sb 2 Te 3ϩx (xϭ1,2,3) crystals it is shown that an a,b,c stacking holds with an alternating stacking of x GeTe double layers identically present in binary GeTe and one Te-Sb-Te-Te-Sb-repeat unit also present in binary Sb 2 Te 3. A stacking disorder is a logical consequence of building crystals with these two principal units. On the other hand, it is likely that all stable crystals of the Ge-Sb-Te systems are an ordered sequence of these two units. Some of the implications of these findings of the stable and metastable crystal structures that develop from amorphous Ge 2 Sb 2 Te 5 are presented so as to understand the crucial crystallization process in Ge 2 Sb 2 Te 5 phase change material.
Journal of Applied Physics, 2000
Temperature dependent measurements of the electrical resistance have been employed to study structural changes in sputtered Ge 2 Sb 2 Te 5 films. The pronounced changes of film resistance due to structural changes enable a precise determination of transition temperatures and activation energies. Furthermore the technique is sensitive enough to measure the influence of ultrathin capping layers on the transformation kinetics. With increasing temperature the Ge 2 Sb 2 Te 5 films undergo a structural change from an amorphous to rock salt structure (Fm3m) around 140°C and finally a hexagonal structure (p3m) around 310°C. Both structural changes are accompanied by a major drop of resistance. Applying the Kissinger method ͓Anal. Chem. 29, 1702 ͑1957͔͒ the activation energy for crystallization to the rock salt structure is determined to be 2.24Ϯ0.11 eV, and for the phase transformation to the hexagonal phase to be 3.64Ϯ0.19 eV, respectively. A thin capping layer of ZnS-SiO 2 leads to an increase of the first transition temperature as well as of the corresponding activation energy (2.7Ϯ0.2 eV͒.
Journal of Applied Physics, 2008
We present chemical state information on contamination-free Ge 2 Sb 2 Te 5 thin film using high-resolution x-ray photoelectron spectroscopy ͑HRXPS͒ and the corresponding theoretical understanding of the chemical states, on both amorphous and metastable phases, illuminating the phase-change mechanism of the system. HRXPS data revealed that the Sb 4d shallow core level was split into two components having different binding energies and that the spin-orbit splitting feature of the Ge 3d level was enhanced as the system became metastable. Negligible change was observed in the Te 4d shallow core level, and in contrary to the previous report's prediction less change in valance band spectra was observed. The results imply that Sb movement is also involved in the phase-change mechanism and that acquisition of shallow core-level spectra can be a useful measure for understanding phase-change mechanism. Hydrogenated SbTe 6 octahedral-like cluster model was introduced to schematically interpret the generation of the two components in the Sb 4d level in metastable state, having an isotropic six-bonds configuration, and an anistropic six-bonds ͑three-short and three-elongated bonds͒ configuration. The amorphous state was modeled to have three-short bonds configuration. Finally, Stibnite-like building block model was used to show that the existence of the above two configurations for Sb atoms is feasible in the Ge 2 Sb 2 Te 5 system.
Structure of oxygen-doped Ge:Sb:Te films
Thin Solid Films, 2006
In this study, the structure of amorphous and crystalline of Ge:Sb:Te:O films, with oxygen content from 0 to about 28 at.%, have been analyzed using X-ray diffraction, impedance measurements and X-ray photoelectron spectroscopy. From these results, the location of the oxygen atoms in the crystallographic structure has been deduced. The results have shown that, in films with oxygen content below 10 at.%, Te, Sb and most of Ge are in metallic state and the free oxygen is probably located at the tetrahedral interstitial sites. In amorphous films with higher contents of oxygen up to 28 at.%, tellurium is forming part of the Ge:Sb:Te alloy, while some of the germanium and antimony form amorphous oxides. This amorphous oxides segregate to the grains boundaries during crystallization. Both, the free oxygen and germanium oxide formed, act as nucleation centers for crystallization. Due to the deficit of germanium and antimony thus created, the amorphous films crystallized as Sb 2 Te 3 in the rhombohedral phase with segregation of the excess crystalline tellurium. Such diffusion-limited process increases the nucleation time in laserinduced crystallization.
Ab initio study of the structural, vibrational and thermal properties of Ge 2 Sb 2 Te 5
International Journal of Computational Materials Science and Engineering, 2015
The structural, vibrational and thermal properties of hexagonal as well as cubic Ge 2 Sb 2 Te 5 (GST) have been calculated from first principles. The relative stability of the possible stacking sequences of hexagonal GST has been confirmed to depend on the choice for the exchange-correlation (XC) energy functional. It is apparent that without the inclusion of the Te 4d orbitals in the valence states, the lattice parameters can be underestimated by as much as 3.9% compared to experiment and all-electron calculations. From phonon dispersion curves, it has been confirmed that the hexagonal phase is, indeed, stable whereas the cubic phase is metastable. In particular, calculations based on the quasi-harmonic approximation (QHA) reveal an extra heat capacity beyond the Dulong–Petit limit at high temperatures for both hexagonal and cubic GST. Moreover, cubic GST exhibits a residual entropy at 0 K, in agreement with experimental studies which attribute this phenomenon to substitutional dis...