GeTe Layer Research Articles

Thermal analysis of an indirectly heat pulsed non-volatile phase change material microwave switch

We show the finite element simulation of the melt/quench process in a phase change material (GeTe, germanium telluride) used for a radio frequency switch. The device is thermally activated by an independent NiCrSi (nickel chrome silicon) thin film heating element beneath a dielectric separating it electrically from the phase change layer. A comparison is made between the predicted and experimental minimum power to amorphize (MPA) for various thermal pulse powers and pulse time lengths. By including both the specific heat and latent heat of fusion for GeTe, we find that the MPA and the minimum power to crystallize follow the form of a hyperbola on the power time effect plot. We also find that the simulated time at which the entire center GeTe layer achieves melting accurately matches the MPA curve for pulse durations ranging from 75–1500 ns and pulse powers from 1.6–4 W.

Mirror-symmetric magneto-optical Kerr rotation using visible light in [(GeTe)2(Sb2Te3)1]n topological superlattices.

Interfacial phase change memory (iPCM), that has a structure of a superlattice made of alternating atomically thin GeTe and Sb2Te3 layers, has recently attracted attention not only due to its superior performance compared to the alloy of the same average composition in terms of energy consumption but also due to its strong response to an external magnetic field (giant magnetoresistance) that has been speculated to arise from switching between topological insulator (RESET) and normal insulator (SET) phases. Here we report magneto-optical Kerr rotation loops in the visible range, that have mirror symmetric resonances with respect to the magnetic field polarity at temperatures above 380 K when the material is in the SET phase that has Kramers-pairs in spin-split bands. We further found that this threshold temperature may be controlled if the sample was cooled in a magnetic field. The observed results open new possibilities for use of iPCM beyond phase-change memory applications.

Ferroelectric switching in epitaxial GeTe films

In this paper, using a resonance-enhanced piezoresponse force microscopy approach supported by density functional theory computer simulations, we have demonstrated the ferroelectric switching in epitaxial GeTe films. It has been shown that in films with thickness on the order of several nanometers reversible reorientation of polarization occurs due to swapping of the shorter and longer Ge-Te bonds in the interior of the material. It is also hinted that for ultra thin films consisting of just several atomic layers weakly bonded to the substrate, ferroelectric switching may proceed through exchange of Ge and Te planes within individual GeTe layers.

Influence of the Kinetic Adsorption Process on the Atomic Layer Deposition Process of (GeTe2)(1–x)(Sb2Te3)x Layers Using Ge4+–Alkoxide Precursors

(GeTe2)(1–x)(Sb2Te3)x (GST) layers were deposited via atomic layer deposition (ALD) at growth temperatures ranging from 50 to 120 °C using Ge(OCH3)4 or Ge(OC2H5)4, Sb(OC2H5)3, and [(CH3)3Si]2Te as the metal–organic precursors of the Ge, Sb, and Te elements, respectively. The GST layers with compositions lying on the GeTe2–Sb2Te3 tie lines could be obtained by varying the ratio of the Ge–Te and Sb–Te ALD cycles. Although the incorporation of an Sb–Te layer into the GST film occurred in a genuine ALD manner, that of the Ge–Te layer was governed by the kinetically limited physisorption of Ge precursors. The incorporation behavior of the Ge precursor with different ligands was explained by the adsorption and desorption kinetics based on the Brunauer–Emmett–Teller isotherm. The ALD-like film growth behavior could be well-explained by the kinetically limited incorporation of Ge atoms.

Emerging topological insulating phase in GeSbTe compounds

AbstractThe electronic property of GeSbTe compounds is studied within the notion of their topological insulating phase. We show that these compounds, as pseudo‐binary alloys of GeTe and Sb2Te3, in their crystalline phase with well‐ordered layer stacking have imbedded surface‐like states originating from topological insulating Sb2Te3 at interfaces between GeTe and Sb2Te3 layers. We demonstrate that these conducting states can significantly affect the electrical conduction of the compounds and are sensitive to Ge migration. Using first‐principles methods, we show that the topological insulating phase of GeSbTe compounds can be manipulated by controlling the composition and/or layer thickness. It is found that the interaction between the surface‐like states intervened by insulating GeTe layers creates an inversion gap to turn the compounds into topological insulating phase. We anticipate that the topological insulating property of GeSbTe can provide new insight into understanding the underlying physics in structural transitions of the compounds and a perspective of utilizing phase change materials as spin‐control devices as well as non‐volatile memory devices.

Investigation of inter-diffusion in bilayer GeTe/SnSe phase change memory films

A metal-chalcogenide layer, SnSe, is inserted between the memory layer GeTe and the top electrode to form a phase change memory cell. The GeTe layer exhibits ovonic threshold switching at a threshold field of ~110V/μm. For subsequent implementation into applications and reliability, material inter-diffusion and sublimation are examined in bilayer phase change films of GeTe/SnSe. Transmission electron microscopy and parallel electron energy loss spectroscopy analyses reveal Sn migration to the GeTe layer, which is responsible for lowering the rhombohedral to cubic structural transformation temperature in GeTe. Incongruent sublimation of SnSe and GeTe is observed at temperatures higher than 500°C. Severe volatilization of Se results in the separation of a metallic Sn phase. The use of Al2O3 as a capping layer has been found to mitigate these effects.

Plasma Enhanced Chemical Vapor Deposition of Conformal GeTe Layer for Phase Change Memory Applications

Phase-change memory is one of the most promising technologies for the future non-volatile memories generation and the synthesis of highly conformal active material is one of the main challenges. Herein, we report the successful chemical vapor deposition of phase change materials conducted using original plasma enhanced pulsed liquid injection system. Smooth, conformal and amorphous as well crystalline GeTe layers were grown on large surfaces. Their phase-change characteristics present fast switch from amorphous to crystalline phases with high optical contrast between the two states. These properties are similar to those of sputtered films used in electronic devices. Moreover, thanks to appropriate process tuning, material structure and phase change properties can be tuned.

Phase-Change Characteristics and Crystal Structure in Multi Stacked GeTe/InTe Films

The changes in the microstructural and electrical properties of multi stacked GeTe/InTe layers were investigated. When the GeTe layer was thicker than the InTe layer, a Ge(In)Te phase occurred only after annealing. Although crystallization should originate from the GeTe layers, there was a uniform distribution of Ge and In atoms in the Ge(In)Te phase after 400 °C annealing. The dropping temperature of sheet resistance was slightly increased with a decrease in GeTe thickness. If the GeTe layer was thinner than the InTe layer, the phases observed after annealing were not only the Ge(In)Te phase, but also the In7Te10 phase. The formation of the In7Te10 phase was the primary cause of the increasing sheet resistance after crystallization by subsequent annealing.

Interfacial phase-change memory

Phase-change memory technology relies on the electrical and optical properties of certain materials changing substantially when the atomic structure of the material is altered by heating or some other excitation process. For example, switching the composite Ge(2)Sb(2)Te(5) (GST) alloy from its covalently bonded amorphous phase to its resonantly bonded metastable cubic crystalline phase decreases the resistivity by three orders of magnitude, and also increases reflectivity across the visible spectrum. Moreover, phase-change memory based on GST is scalable, and is therefore a candidate to replace Flash memory for non-volatile data storage applications. The energy needed to switch between the two phases depends on the intrinsic properties of the phase-change material and the device architecture; this energy is usually supplied by laser or electrical pulses. The switching energy for GST can be reduced by limiting the movement of the atoms to a single dimension, thus substantially reducing the entropic losses associated with the phase-change process. In particular, aligning the c-axis of a hexagonal Sb(2)Te(3) layer and the 〈111〉 direction of a cubic GeTe layer in a superlattice structure creates a material in which Ge atoms can switch between octahedral sites and lower-coordination sites at the interface of the superlattice layers. Here we demonstrate GeTe/Sb(2)Te(3) interfacial phase-change memory (IPCM) data storage devices with reduced switching energies, improved write-erase cycle lifetimes and faster switching speeds.

Investigation of Phase Transitions in Stacked GeTe/SnTe and Ge<sub>2</sub>Se<sub>3</sub>/SnTe Chalcogenide Films

Phase transitions in stacked GeTe/SnTe and Ge2Se3/SnTe thin layers for potential phase-change memory applications have been investigated by X-ray diffraction using an area detector system and by scanning electron microscopy. The as-deposited underlying GeTe or Ge2Se3 layer is amorphous, whereas the top SnTe layer is crystalline. In GeTe/SnTe stack, the crystallization of GeTe phase occurs near 170°C, and upon further heating, GeTe phase disappears, followed by the formation of rocksalt-structured GexSn1-xTe solid solution. In Ge2Se3/SnTe stack, the phase transition starts with the separation of SnSe phase due to the migration of Sn ions into the Ge2Se3 layer. The migration of Sn ions and the formation of SnSe are believed to facilitate the crystallization of Ge2Se3 solid solution at ~360°C, which is much lower than the crystallization temperature of Ge2Se3, therefore consuming less power during the phase transition.

Selective unequal-thickness thin-film filters for IR spectral region

Abstract. A new five-layer structure of the dielectric interference filter for the infra-red region of spectrum is investigated. The main spectral parameters of such a narrow-band filter are determined. The dependence of the transmission band half-width on the thickness of a dividing layer in this system is investigated. It is shown that, in the case of the application of PbTe, GeTe, and SnTe layers with a given thickness; their number can be limited to five at providing the necessary selectivity of the filter: practically the 100-% transmission at the resonance frequency and 0.005 % outside the transmission band. Keywords: interference filter, narrow-band filter. Manuscript received 05.06.07; accepted for publication 07.02.08; published online 31.03.08. 1. Introduction It is known that the interference filters are well perspective as for their application in optical devices and actually determine the efficiency and reliability of an apparatus [1, 2]. Therefore, these filters must meet such requirements as the maximum passing energy in the transmission band and the minimum passing energy outside it. This characteristic and the transmission band width determine the least number of layers formed by a simple and known technological procedure that must provide a high reliability of filters. It should be noted that the primary purpose of developers is to provide the maximal selectivity and operational stability of interference filters. As is well known, the multilayer thin-film dielectric systems including the layers with optical thickness multiple to λ

First-Principles Study of Equilibrium Properties and Electronic Structures of GeTe–Sb2Te3 Pseudobinary Crystalline Films

On the basis of first-principles pseudopotential total energy calculations, Ge1Sb2Te4, Ge2Sb2Te5, and Ge1Sb4Te7 of GeTe–Sb2Te3 pseudobinary systems were studied. First, the most possible models for Ge1Sb2Te4 were compared and investigated in detail to identify the most favorable one by examining the lattice configurations, total energy, cohesive energy, and X-ray diffraction powder curves. Then, we discussed the density of states and band structures of selected models of the GeTe–Sb2Te3 pseudobinary system and compared the corresponding values with each other to find similar electronic structures that are due to the similar structures with repeating units of Sb2Te3 and GeTe layers. Furthermore, we pointed out why Ge2Sb2Te5 has the most stable structure among the three compounds by referring to a “Te–Te layer” model.

Investigation of Phase Transition in Stacked Ge-Chalcogenide/SnTe Phase-change Memory Films

ABSTRACTPhase transitions in stacked GeTe/SnTe and Ge2Se3/SnTe thin layers for potential phase-change memory applications have been investigated by X-ray diffraction using a two-dimensional area detector system. The as-deposited underlying GeTe or Ge2Se3 layer is amorphous, whereas the top SnTe layer is crystalline. In the GeTe/SnTe stack, the crystallization of GeTe phase occurs near 170°C, and upon further heating, the GeTe phase disappears, followed by the formation of rocksalt-structured GexSn1−xTe solid solution. In the Ge2Se3/SnTe stack, the phase transition starts with the separation of a SnSe phase due to the migration of Sn ions into the Ge2Se3 layer. SnSe is believed to facilitate the crystallization of Ge2Se3-SnTe solid solution at ∼360°C, which is much lower than the crystallization temperature of Ge2Se3, therefore consuming less power during the phase transition.

Study of the surface local hardness and tip-force induced surface corrugation in layered tellurides [formula omitted] and TaSi 0.414Te 2 by scanning tunneling microscopy

Commensurate layered tellurides NbGe xTe 2 ( x = 1 3 , 2 5 , 3 7 ) and an incommensurate layered telluride TaSi 0.414Te 2 were examined by scanning tunneling microscopy (STM). The STM images of the commensurate tellurides were simulated by calculating the partial electron density plots and density of states for their TeNb GeTe sandwich layers. In this image simulation, the tip-force induced depression of the surface Te atoms was taken into consideration. The surface of each TeNb GeTe layer consists of isolated and paired Te atoms, and during scanning the isolated Te atoms are depressed more than are the paired Te atoms. Though counterintuitive, the latter causes the isolated Te atoms to contribute more strongly to the STM images than do the paired Te atoms. This finding was used to characterize the incommensurate modulation and the chain faults in TaSi 0.414Te 2 by STM.

ChemInform Abstract: Electron Microscopy Investigation of the Homologous Series of the Mixed‐Layer Compounds As2Te3(GeTe)n.

AbstractThe structure of the title compounds consist of lamellae of close‐packed layers containing n GeTe layers alternating with single‐layer lamellae of As2Te3 (XRD, electron diffraction, and high resolution electron microscopy for the whole series of up to n = 9).