Influence of doping on the structure and optical characteristics of Ge 2 Sb 2 Te 5 amorphous films (original) (raw)
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Raman scattering study of GeTe and Ge2Sb2Te5 phase-change materials
Journal of Physics and Chemistry of Solids, 2007
Structural details of the amorphous binary GeTe and ternary Ge 2 Sb 2 Te 5 (GST) phase-change materials are investigated with the aid of Raman scattering. In the case of the a-GeTe, a plethora of Raman bands have been recorded and assigned on the basis of a network structure consisting of corner-and edge-sharing tetrahedra of the type GeTe 4Àn Ge n (n ¼ 0, 1, 2, 3, 4). Significant temperature-induced structural changes take place in this material even at temperatures well below the crystallization temperature. These changes tend to organize the local structure, in particular the coordination number of Ge atoms, so as to facilitate the amorphous-to-crystal transformation. The much simpler Raman spectrum of GST, characterized by one vibrational band, is accounted for by the dominance of the Sb 2 T 3 component in Raman scattering; reasons about this explanation, as well as for the lack of any Te-Te bonds are briefly described.
Structural Changes in Doped Ge2Sb2Te5 Thin Films Studied by Raman Spectroscopy
Physics Procedia, 2013
In this study, we investigated Ge 2 Sb 2 Te 5 (GST225) amorphous thin films doped with Bi, Sn and In, using Raman scattering spectroscopy, to obtain information about structural changes after doping. Such impurities as Bi and Sn were chosen due to their isomorphism with one of the main components; indium is an active dopant for phase change materials. Two main, most intensive bands appeared at 125 and 153 cm 1 in the spectrum of undoped amorphous GST225 thin film. Additional small bands in the range of 80 cm-1 and near 300 cm-1 , which disappeared in Raman spectra of crystalline GST225 thin films, were also observed. The obtained peak parameters were found to correlate with the dopant type and concentration. The concentration dependencies are not monotonic, and this fact indicates different incorporation mechanisms for different dopant levels.
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.
Effect of indium doping on Ge 2 Sb 2 Te 5 thin films for phase-change optical storage
Applied Physics A-materials Science & Processing, 2005
The influence of In doping on the crystallization kinetics of Ge2Sb2Te5 has been investigated using four-point-probe electrical resistance measurements, grazing incidence X-ray diffraction (XRD), X-ray reflectometry (XRR), variable incident angle spectroscopic ellipsometry, a static tester, and atomic force microscopy. For a stoichiometric Ge2Sb2Te5 alloy doped with 3% In, the amorphous-to-crystalline transition is observed at 150 °C in the sheet resistance measurements. XRD reveals the formation of a predominant NaCl-type Ge2Sb2Te5 phase during the amorphous-to-crystalline transition together with small amounts of crystalline In2Te3. Density values of 5.88±0.05 g cm-3 and 6.22±0.05 g cm-3 are measured by XRR for the film in the amorphous and crystalline states, respectively. Perfect erasure can be achieved by laser pulses longer than 165 ns. The retarded crystallization, as compared with the undoped Ge2Sb2Te5 alloy, is attributed to the observed phase segregation. Sufficient optical contrast is exhibited and can be correlated with the large density change upon crystallization.
Evidence of Crystallization–Induced Segregation in the Phase Change Material Te-Rich GST
Journal of The Electrochemical Society, 2011
When the phase change material Ge 2 Sb 2 Te 5 melts during the RESET switching of a phase change memory (PCM) device, it is known that electromigration (elemental segregation under electric field or current) can cause the region near the positively-biased electrode to become rich in Tellurium. We show that the laser-induced crystallization of deposited films of similarly Telluriumrich GeSbTe (GST) material is 1000 Â slower than the crystallization of conventional Ge 2 Sb 2 Te 5 material, and that this material can readily flow and create voids at temperatures as low as 350 C. AFM and Auger analysis of Te-rich GST films reveals significant thermal-induced motion and elemental segregation, occurring in the absence of an electrical field and at similarly low temperatures. This phenomenon, termed crystallization-induced segregation, occurs when material rearrangement within the poorly-crystallizing, under-cooled-liquid Te-rich GST matrix forms a local stoichiometry which is capable of much more rapid crystallization. The process of crystallization itself then reinforces this stoichiometry, at the expense of the elements not needed, with a growth-rate limited by material diffusion. This crystallization-induced segregation -which can readily occur in GSTbased phase change memory (PCM) devices, especially after the onset of electromigration -is an important additional component to understanding both bias-polarity-based phenomena and endurance failure in GST-based PCM devices.