Structure, electrical, optical and thermal properties of Ge4Sb4Te (x= 8, 9 and 10) thin films (original) (raw)
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Effect of Ag doping on electrical properties Ge2Sb2Te5 thin films
DAE SOLID STATE PHYSICS SYMPOSIUM 2018
In this work thin films of the phase change material Ge 2 Sb 2 Te 5 (GST), pure and Ag-doped, were studied. These films were prepared by thermal evaporation method. Amorphous nature of both pure and Ag-doped GST thin films has been confirmed from X-ray diffraction analysis. Raman spectra confirms the host structure of GST which is confirmed by the two sharp peaks at 126.4 cm-1 and 144.9 cm-1 for GST thin films. The hole concentration was found to increase by three orders of magnitude due to Ag doping, as measured by Hall measurements. I-V measurements of the samples show thermal switching at moderate voltage as large current flows through Ag-doped GST. The increase in conductivity was attributed to the crystallization of the films due to heating caused by the large electric current for I-V measurements.
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.
Crystallization kinetics of Ge4Sb1Te5 films
Thin Solid Films, 2002
The crystallization kinetics of sputtered Ge Sb Te films has been investigated using temperature-dependent sheet resistance 4 1 5 measurements in conjunction with structural investigations employing X-ray diffraction (XRD) and X-ray reflectometry (XRR). This work correlates the kinetics of crystallization of thin Ge Sb Te films to the structural and density changes. The resistivity 4 1 5 of the Ge Sb Te samples decreases from 2 to 3.1=10 V m upon annealing above the phase transition temperature. The y5 4 1 5
Chalcogenide Letters
In this work we will focus first on obtain the sheet resistance, Rs of thin film whose surface thickness is equal to 1000 nm for the chalcogine Ge20Sb20Te60 at different heating rates , this is done against temperature in the range from 70 to 370 °C. The sheet resistance curve and through derivation of sheet resistance as a function of temperature there was clearly evidence of two crystallization regions for the studied sample. Second, the thermal data we obtained was used to complete the thermal calculations and then the electrical calculations thereafter. The activation energies of crystallization were evaluated by kinetic criteria. The activation energy, Ec , and Avrami index, n, were obtained by analyzing the data via JMA methods. The results indicated that the transformation from amorphous to crystalline phases. The crystalline phases for the as-deposited and annealed film were identified via x-ray diffraction (XRD).
Structural change of laser-irradiated Ge 2Sb 2Te 5 films studied by electrical property measurement
J Non Cryst Solids, 2008
Phases and equilibria Structure Long range order Short-range order Structural relaxation X-ray diffraction a b s t r a c t Sheet resistance of laser-irradiated Ge 2 Sb 2 Te 5 thin films prepared by magnetron sputtering was measured by the four-point probe method. With increasing laser power the sheet resistance undergoes an abrupt drop from 10 7 to 10 3 X/h at about 580 mW. The abrupt drop in resistance is due to the structural change from amorphous to crystalline state as revealed by X-ray diffraction (XRD) study of the samples around the abrupt change point. Crystallized dots were also formed in the amorphous Ge 2 Sb 2 Te 5 films by focused short pulse laser-irradiated, the resistivities at the crystallized dots and the non-crystallized area are 3.375 Â 10 À3 and 2.725 X m, sheet resistance is 3.37 Â 10 4 and 2.725 Â 10 7 X/h respectively, deduced from the I-V curves that is obtained by conductive atomic force microscope (C-AFM).
DC AND AC CONDUCTIVITY MEASUREMENTS ON Ge2Sb2Te5 FILMS
Thin films of Ge2Sb2Te5 were prepared using thermal evaporation technique. The amorphous structure and stoichiometery of the films were identified by X-ray diffraction and Energy Dispersive X-ray analysis, EDX. The conductivity measurements were carried out using DC I-V curves and AC Impedance spectroscopy techniques. The measured activation energy (Ev) is found to be about 0.36-0.39 eV, respectively, which is approximately half the energy gap (Eg= 0.72-0.78 eV). The amorphous-crystalline transition temperature (Tc) of the films was also estimated to be 135 o C. The results can be Phase change materials based on chalcogenide alloys are found to be suitable for optical and electrical memories due to its fast crystallization. The operation principle of these devices is based on the ability of the active materials to reversibly transform between amorphous and crystalline phases. Among these alloys, Ge2Sb2Te5 exhibits the best performance when used in DVD-RAM in terms of speed and stabi...
Journal of Alloys and Compounds, 2015
a b s t r a c t GermaniumeAntimonyeTellurium (GeeSbeTe) thin films were deposited on silicon wafers with 1-mm silicon dioxide (SiO 2 /Si) by pulsed dc magnetron sputtering from a 99.99% GeSbTe target of 1:1:1 ratio at ambient temperature. The samples were annealed at 573, 623, 673, and 723 K for 3600 s in a vacuum state. The effects of the annealing treatment on phase identification, atomic composition, morphology and film thickness, carrier concentration, mobility, and Seebeck coefficient of the GeeSbeTe samples have been investigated by grazing-incidence X-ray diffraction, auger electron spectroscopy, field-emission scanning electron microscopy, Hall-effect measurements, and steady state method, respectively. The results demonstrated that the as-deposited GeeSbeTe sample was amorphous. Atomic composition of as-deposited and annealed films at 573 K and 623 K were GeSb 0.75 Te 0.5 while annealed films at 673 K and 723 K were GeSbTe 0.5 due to Sb-rich GeSb 0.75 Te 0.5 . The samples annealed at 573 K and 623 K showed the crystal phases of cubic structure (c-GeSb 0.75 Te 0.5 ) into hexagonal structure (h-GeSbTe 0.5 ) after annealing at 673 K and 723 K. The study demonstrated the insulating condition from the as-deposited GeSbTe film, and the changes towards the thermoelectric properties from the annealing treatments. The GeSbTe films annealed at 673 K yielded excellent thermoelectric properties with the electrical resistivity, Seebeck coefficient, and power factor at approximately 1.45 Â 10 À5 Um, 71.07 mV K À1 , and 3.48 Â 10 À4 W m À1 K À2 , respectively.
Crystallization process in Ge2Sb2Te5 amorphous films
Vacuum, 2010
The aim of this work is to investigate the isokinetic and isothermal amorphous-to-crystalline phase transformation process in Ge 2 Sb 2 Te 5 ternary alloys. The experiments were carried out using electrical impedance, X-ray diffraction and reflection measurements. The results have shown that, upon annealing, the crystallization process in amorphous Ge 2 Sb 2 Te 5 films starts with nuclei which were identified as the Ge 1 Sb 4 Te 7 crystalline phase. As temperature increases (or time of isothermal annealing) these nuclei are transformed into the fcc-Ge 2 Sb 2 Te 5 phase. In order to establish the mechanism of crystallization for this system, a stochastic lattice model was implemented to analyze nucleation and growth of the two phases involved (i.e., the metastable Ge 1 Sb 4 Te 7 nuclei followed by the stable fcc-Ge 2 Sb 2 Te 5). The results of the simulations demonstrate close agreement with experimental results. Furthermore, the crystallization process in amorphous films with the Ge 1 Sb 4 Te 7 composition shows the existence of only one phase during the whole process and can be described by the classical Johnson-Mehl-Avrami-Kolmogorov model.