Ge2Sb2Te5 Thin Films Research Articles

フェムト秒レーザー励起を用いたGeSbTeにおける超高速相変化の誘起

フェムト秒レーザー励起を用いたGeSbTeにおける超高速相変化の誘起

Polarity-dependent resistance switching in GeSbTe phase-change thin films: The importance of excess Sb in filament formation

We show that polarity-dependent resistance switching in GeSbTe thin films depends strongly on Sb composition by comparing current-voltage characteristics in Sb-excess Ge2Sb2+xTe5 and stoichiometric Ge2Sb2Te5 samples. This type of switching in Ge2Sb2+xTe5 films is reversible with both continuous and pulsed dc voltages less than 1.5 V. Low and high resistance states of this switching can be attributed to formation and rupture, respectively, of electrically conductive Sb-bridges between the Ge2Sb2Te5 crystals and electrodes through the resistive amorphous phase. The coexistence of polarity-dependent resistance switching with amorphous-crystalline phase-changes renders great opportunities to expand the applicability of GeSbTe films for data storage applications.

Comparison of Thermal Stability and Electrical Characterization between Ge2Sb2Te5 and Ge1Sb2Te4 Phase-Change Materials

In this paper, thermal stability and electrical characterization of Ge2Sb2Te5 and Ge1Sb2Te4 phase change materials were studied and compared. In-situ electrical resistance measurement at a constant heating rate of 2 K/min shows that Ge2Sb2Te5 takes 510 s for phase change while Ge1Sb2Te4 840 s. Under isothermal condition Ge2Sb2Te5 also crystallizes faster than Ge1Sb2Te4 does. Activation energy of 2.9 and 2.4 eV for Ge2Sb2Te5 and Ge1Sb2Te4 thin films with 30 nm thickness respectively indicates that Ge2Sb2Te5 is more thermally stable in the amorphous state than Ge1Sb2Te4. Ge1Sb2Te4 tends to be more conductive, which was confirmed by its larger carrier concentrations in the amorphous and crystalline states than that of Ge2Sb2Te5 from Hall effect measurement. These results are attributed to the fact that Ge2Sb2Te5 (or written as Ge1.6Sb1.6Te4) contains more Ge atoms substituting for Sb atoms and has less concentrations of vacancies (cavities) than Ge1Sb2Te4 does.

Layer‐by‐Layer Growth of GeSbTe Thin Films by Metal‐Organic CVD for Phase Change Memory Applications

AbstractGe, GeTe, and GeSbTe (GST) films are grown on both planar TiAlN/Si substrates and a trench structure (diameter 120 nm; depth 200 nm) using layer‐by‐layer, metal‐organic (MO) CVD. The GeTe and GeSbTe films completely fill the trench structure, and show well‐crystallized phases with a rhombohedral and hexagonal structure, respectively. The GeTe films grown on the trench structure exhibit stoichiometric variations along the depth of the trench. Stoichiometric Ge2Sb2Te5 compositions are observed only at the center of the trench structure. The concept of layer‐by‐layer MOCVD could be applied for depositing GST and GeTe films from constituent precursors with large differences in decomposition temperatures, provided the composition variation can be overcome.

Ge–Sb–Te thin films deposited by pulsed laser: An ellipsometry and Raman scattering spectroscopy study

Pulsed laser (532 nm) deposited Ge2Sb2Te5 thin films were investigated by means of spectroscopic ellipsometry and Raman scattering spectroscopy. Tauc–Lorentz and Cody–Lorentz models were employed for the evaluation of optical functions of thin films in as-deposited (amorphous) and crystalline (cubic) phases. The models’ parameters (Lorentz oscillator amplitude, resonance energy, oscillator width, optical band gap, and Urbach energy) calculated for amorphous and crystalline states are discussed. The vibrational modes observed in Raman spectra of amorphous layers are attributed to GeTe4−nGen (n=1, 2, eventually 0) tetrahedra connected by corners (partly by edges) and SbTe3 units. The Raman spectra of crystalline thin films suggest that the local bonding arrangement around Ge atoms changes; GeTe component is thus mainly responsible for the phase transition in Ge2Sb2Te5 alloys.

Nucleation, growth, and phase transformation mechanism of Ge2Sb2Te5 thin films

AbstractThe nucleation, growth, and metastable‐to‐stable phase transformation behavior of Ge2Sb2Te5 thin film were studied by means of transmission electron microscopy. The results confirm that nucleation begins at the surface of the film and proceeds by growth of grains through the thickness of film. Also, the result provides further grounds to the suggestion that some of the Ge2Sb2Te5 grains grow with an elongated grain shape to reduce interface energy between crystalline and amorphous phases at the initial stage of crystallization. We find edge dislocations in the metastable‐to‐stable phase transition region. We think these edge dislocations can explain the metastable‐to‐stable phase transformation mechanism as a result of vacancy diffusion.

Thermal stability of GeSbTe thin films deposited by layer-by-layer metalorganic chemical vapor deposition

The thermal stability of the GeSbTe (GST) films deposited by layer-by-layer metalorganic chemical vapor deposition on the planar TiAlN∕Si and on trenches 120nm in diameter and 500nm deep (with aspect ratio of 4:1) was investigated under various annealing temperatures and durations in a nitrogen ambient atmosphere. The inhomogeneous distribution of the Ge and Sb elements in the as-grown GST layer was improved by a thermal treatment at 500°C for 60min in nitrogen ambient. The GST films annealed above 500°C for 30 and 60min were delaminated from the TiAlN electrode. Samples annealed at 700°C for 1min, above the melting temperature of the GST, show a decreased thickness, indicating that the GST films were thermally unstable. The as-grown films on trenches did not show a complete fill of the structure, whereas the trenches were more fully filled after a thermal treatment at 500°C for 60min.

The reason for the increased threshold switching voltage of SiO2 doped Ge2Sb2Te5 thin films for phase change random access memory

This study examined the threshold switching voltage (VT) of 150 nm thick SiO2 doped Ge2Sb2Te5 (SGST) films for phase change random access memory applications. The VT of the SGST films increased from ∼0.9 V (for GST) to ∼1.5 V with increasing SiO2 content. The optical band gap and Urbach edge of the SGST films were similar regardless of the SiO2 concentration. The dielectric constant decreased by ∼37% and the electrical resistivity increased by ∼19%. The increase in VT of SGST films is associated with an effective increase in electric field and the decreased generation rate caused by impact ionization.

Phase transition behaviors of Mo- and nitrogen-doped Ge2Sb2Te5 thin films investigated by in situ electrical measurements

Phase-change behaviors of Ge2Sb2Te5 (GST) thin films doped with molybdenum (Mo) or nitrogen (N) were investigated by utilizing in situ electrical property measurement, x-ray diffraction (XRD), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy. It was found that the Mo doping mainly reduces the resistivity level of amorphous GST while the N-doping raises both the resistivity levels of amorphous and crystalline GSTs. XRD and TEM analyses revealed that the element doping stabilizes the amorphous state of GST and suppresses the grain growth in GST films. This resulted in the increase in recrystallization temperature (Tm) and activation energy (Ea) of amorphous-to-crystalline phase transition in GST layers, as revealed by the calculation in terms of Kissinger’s theory. The results of data fitting into various percolation models and Johnson–Mehl–Avrami analysis indicated the heterogeneous feature of phase transition process in GST layers that the nucleation first occurs at the atmosphere∕sample interface and the recrystallization front advances into the interior of sample in a layer-by-layer manner along the direction of surface normal.

On angle resolved RF magnetron sputtering of Ge–Sb–Te thin films

Thin amorphous films of Ge–Sb–Te were deposited from Ge 2Sb 2Te 5 target by RF ( f = 13.56 MHz) magnetron sputtering in argon plasma. Composition and chemical homogeneity of target and prepared thin films were traced by Energy Dispersive X-ray Analysis coupled with Scanning Electron Microscope (SEM-EDX). SEM technique was also used for surface morphology observation. Crystallinity of target and prepared thin films was studied by X-ray Diffraction (XRD). Optical parameters of prepared thin films (spectral dependence of refractive index, optical band gap energy E g opt ) and film thicknesses were established via Variable Angle Spectroscopic Ellipsometry (VASE) supported by UV–Vis–NIR spectroscopy. Influence of deposition conditions (RF power, Ar pressure, angle divergency from normal direction) to composition, crystallinity, optical properties and deposition rate was established.

Crystallization of sputtered-deposited and ion implanted amorphous Ge2Sb2Te5 thin films

X-ray diffraction and transmission electron microscopy have been utilized to measure the ion irradiation-induced modification in amorphous Ge2Sb2Te5 thin films. The isothermal crystallization of sputtered-deposited and Sb+ ion irradiated amorphous samples has been studied, focusing on the evolution of the microstructure during the initial stage of the transformation. In both samples, the amorphous to crystal transition occurs through the nucleation of face centered cubic (fcc) crystal domains at the film surface. A fast bidimensional growth of the crystalline nuclei in the sputtered-deposited films occurs by the generation of transrotational grains. The lattice parameter decreases as the crystalline fraction increases above 80%, and it approaches the fcc bulk value at the end of the transformation. Ion irradiation produces a densification of the deposited amorphous film (∼4% vertical shrinkage measured by atomic force microscopy) and an enhancement of the crystallization rate. Even in the irradiated amorphous, the nucleation occurs at the film surface and proceeds by the growth of the grains through the thickness of the layer although the density of the transrotational grains is strongly reduced with respect to the unirradiated amorphous. A link between the ion beam induced densification and the reduction of transrotational grains is proposed.

Influence of Substrates on the Nucleation and Growth Behaviors of Ge2Sb2Te5 Films by Combined Plasma-Enhanced Atomic Layer and Chemical Vapor Deposition

This study concerned the effect of the substrate on the nucleation and growth behavior of Ge2Sb2Te5 (GST) thin films deposited by a combination of plasma-enhanced chemical vapor deposition (for Sb and Te) and plasma-enhanced atomic layer deposition (for Ge) processes at wafer temperatures ranging from 100 to 200 °C using Ge(i-C4H9)4, Sb(i-C3H7)3, and Te(i-C3H7)2 as the Ge, Sb, and Te precursors, respectively. Several oxide and nitride layers that were formed on the Si substrate were concerned as substrates. The nucleation of the GST films on the SiO2, Si3N4, and ZrO2 substrates was seriously retarded (long incubation cycles) compared to those on the TiN and TiO2 substrates, where smooth film growth with negligible incubation cycles was achieved. The GST film did not grow at all on the HfO2 substrate. The reason for the enhanced nucleation and growth properties of GST on the TiO2 and TiN (partially oxidized) substrates was related to the formation of a GeO2 phase and the charge exchange effect of a partial...

Crystallization Behavior of Laser Induced Ge2Sb2Te5 and N-doped Ge2Sb2Te5 Thin Films

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Amorphous to fcc-polycrystal transition in Ge2Sb2Te5 thin films studied by electrical measurements: Data analysis and comparison with direct microscopy observations

We experimentally investigate the isothermal amorphous-to-fcc polycrystalline phase transition process in amorphous Ge2Sb2Te5 thin films prepared by sputtering. The amorphous layers were either as deposited or formed by Ar+ ion implantation after crystallization at 300 °C. The kinetics of the amorphous-to-polycrystal transition are analyzed through electrical measurements, in which the Johnson–Mehl–Avrami–Kolmogorov theory is employed. The procedure to extract the kinetics of the phase transition from conductivity versus time data is carefully discussed and compared to data of cross-sectional transmission electron microscopy images versus anneal time. By following this proposed procedure, the nucleation and growth parameters, and the activation energies have been determined. Results indicate that the process of isothermal crystallization in Ge2Sb2Te5 takes place in two stages, in which the Avrami exponent changes in the range from 3 to 1. These results are understood in terms of modifications in the kinetics of the phase transition.

Influence of Silicon Doping on the Properties of Sputtered Ge2Sb2Te5 Thin Film

Ge2Sb2Te5 is a promising candidate material for next-generation memory devices due to its fast phase-changing characteristics. The power consumption of an electronic device is a key factor in determining the working efficiency of next-generation memory devices. In this study, Si was doped on Ge2Sb2Te5 thin film to reduce the power consumption during device operation. Both the crystal fraction and the electrical properties of the Si-doped Ge2Sb2Te5 film were measured using in situ measurements. The analyses of the crystal structure by X-ray diffraction indicated that the doped Si and the Ge2Sb2Te5 formed a solid solution, where the doped Si occupied the interstitial sites of the Ge2Sb2Te5 crystal. Si doping was shown to elevate the crystallization temperature and enhance the thermal stability of Ge2Sb2Te5.

Influence of the additive Ag for crystallization of amorphous Ge–Sb–Te thin films

We have investigated the optical and amorphous-to-crystalline transition properties in four-types of chalcogenide thin films; Ge 2Sb 2Te 5, Ge 8Sb 2Te 11, Ag–Ge 2Sb 2Te 5 and Ag–Ge 8Sb 2Te 11. Crystallization was caused by nano-pulse illumination ( λ = 658 nm) with power ( P) of 1–17 mW and pulse duration ( t) of 10–460 ns, and the morphologies of crystallized spots were observed by SEM and microscope. It was found that the crystallized spot nearby linearly increases in size with increasing the illuminating energy ( E = P ∙ t) and eventually ablated out by over illumination. Changes in the optical transmittance of as-deposited and annealed films were measured using a UV–vis–IR spectrophotometer. In addition, a speed of amorphous-to-crystalline transition was evaluated by detecting the reflection response signals for the nano-pulse scanning. Conclusively, the Ge 8Sb 2Te 11 film has a faster crystallization speed than the Ge 2Sb 2Te 5 film despite its higher crystallization temperature. The crystallization speed was largely improved by adding Ag in Ge 2Sb 2Te 5 film but not in Ge 8Sb 2Te 11 film. To explain these results, we considered a heat confinement by electron hopping.

Evolution of the Transrotational Structure During Crystallization of Amorphous Ge2Sb2Te5 Thin Films

AbstractThe crystallization of amorphous Ge2Sb2Te5 thin films has been studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The analysis has been performed on partially crystallized films, with a surface crystalline fraction (fS) ranging from 20% to 100%. XRD analysis indicates the presence, in the partially transformed layer, of grains with average lattice parameters higher than that of the equilibrium metastable cubic phase (from 6.06 Å at fS=20% to 6.01 Å at fS=100%). The amorphous to crystal transition, as shown by TEM analysis, occurs through the nucleation of face-centered-cubic crystal domains at the film surface. Local dimples appear in the crystallized areas, due to the higher atomic density of the crystal phase compared to the amorphous one. At the initial stage of the transformation, a fast bi-dimensional growth of such crystalline nucleus occurs by the generation of transrotational grains in which the lattice bending gives rise to an average lattice parameter significantly larger than that of the face-centered-cubic phase in good agreement with the XRD data. As the crystallized fraction increases above 80%, dimples and transrotational structures start to disappear and the lattice parameter approaches the bulk value.

The Influence of Nitrogen Doping on the Chemical and Local Bonding Environment of Amorphous and Crystalline Ge2Sb2Te5

AbstractRecent interest in phase change materials (PCMs) for non-volatile memory applications has been fueled by the promise of scalability beyond the limit of conventional DRAM and NAND flash memory [1]. However, for such solid state device applications, Ge2Sb2Te5 (GST), GeSb, and other chalcogenide PCMs require doping. Doping favorably modifies crystallization speed, crystallization temperature, and thermal stability but the chemical role of the dopant is not yet fully understood. In this work, X-ray Absorption Fine Spectroscopy (XAFS) is used to examine the chemical and structural role of nitrogen doping (N-) in as-deposited and crystalline GST thin films. The study focuses on the chemical and local bonding environment around each of the elements in the sample, in pre and post-anneal states, and at various doping concentrations. We conclude that the nitrogen dopant forms stable Ge-N bonds as deposited, which is distinct from GST bonds, and remain at the grain boundary of the crystallites such that the annealed film is comprised of crystallites with a dopant rich grain boundary.

Microstructural and optical analysis of superresolution phenomena due to Ge2Sb2Te5 thin films at blue light regime

Superresolution (SR) phenomena due to Ge2Sb2Te5 films were examined by combined analysis of the transmission electron microscopy (TEM) microstructures of the laser-irradiated films and the results from dynamic and static tests using blue lasers. A new finding was made that comprises a complementary case of the classical SR readout by Ge2Sb2Te5 film; an amorphous band instead of a closed aperture of melt in the crystalline background forms behind a moving laser but still produces a high SR signal. A complete carrier-to-noise-ratio curve of a SR-read-only memory employing Ge2Sb2Te5 may be derived from a nonlinear optical effect, specifically thermally assisted saturable absorption.

Influences of partial melting and overheating on amorphization of Ge2Sb2Te5 during the reset process

AbstractIt is very important to observe the change in the atomic structure of melt‐quenched amorphous Ge2Sb2Te5 (GST) to discover the failure mechanism of phase‐change random access memory (PRAM) devices during the reset process. We fabricated the phase‐change devices and measured the initial resistance changes with various reset pulse amplitudes. For the observation of the atomic structure changes of GST thin films, we designed the specimens for transmission electron microscopy (TEM) and annealed them below and above the melting temperature of GST (Tm = 650 °C) before quenching. The annealed‐and‐quenched GST at 630 °C has a hexagonal structure with a grain size of about 10 nm after the partial melting. The annealed‐and‐quenched GST at 750 °C also has a hexagonal structure because of the latent heat. The observed atomic structures of GST are in accordance with the resistance changes inferred from the fabricated edge‐contact type of PRAM cell. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)