Amorphous Ge Sb Research Articles

Impact of interfaces on bipolar resistive switching behavior in amorphous Ge–Sb–Te thin films

Electrochemical metallization memories have received much attention as candidates for next generation non-volatile memory applications, due to their fast switching speed, simple structure, high scalability and low energy consumption. Chalcogenide compounds like Ge–Sb–Te-based materials are extensively studied solid electrolytes for such devices and considered within the most promising candidates. In this work, the influence of different electrode materials on bipolar resistive switching characteristics of amorphous Ge–Sb–Te thin films with close composition to 2:2:5, prepared by pulsed laser deposition, is studied. Detailed investigations by current-voltage measurements, secondary ion mass spectrometry and cross-sectional transmission electron microscopy in conjunction with energy-dispersive x-ray spectroscopy are presented. Depending on the utilized electrode material, analog switching (Cu, Ag, Ti), digital switching (Co, Cr, Ta, Al) or no switching (Pt, Au and Cr-CrOx) occurs. This work shows that the switching behavior of Ge–Sb–Te strongly depends on the solubility and reactivity of the electrode material.

Optical properties and thermal stability of amorphous Ge–Sb–Se films

Ge–Sb–Se chalcogenide films with mean coordination number (MCN) from 2.27 to 2.66 were deposited by magnetron sputtering method. Annealing time dependent physical properties, such as refractive index, optical band gap, and thickness were investigated. It was found that, the Ge15Sb9.6Se75.4 film with MCN is 2.4 exhibits the smallest change ratio after long time (35 h) heat annealing in refractive index (<1%), optical band gap (<1%), and thickness (<3%). Together with the analysis of topological model, the component of Ge15Sb9.6Se75.4 with MCN is 2.4 can be considered as the best thermal stable film in this Ge–Sb–Se system. The film with larger refractive index of ~2.7 is very helpful to the application like chalcogenide waveguide.

Optical and structure properties of amorphous Ge–Sb–Se films for ultrafast all-optical signal processing

In this paper, we deposited amorphous chalcogenide Ge–Sb–Se films using the RF sputtering method, then measured their optical and structural properties using various diagnosis tools. The linear refractive index and optical band-gap for as-deposited films were analyzed as a function of the chemical composition and the mean coordination number (MCN). The third-order optical nonlinearities were predicted by applying the Z-scan method combined with a model developed by Sheik-Bahae. The relationship between film compositions and the optical properties was analyzed by Raman spectra in terms of structural evolution. The data suggests that the “defect” gap states in the network play an important role in the third-order optical nonlinearity of these chalcogenide films. The Ge20Sb15Se65 films most suitable for all-optical signal processing applications had large nonlinear refractive indices (−8.735 × 10−15 m2/W), moderate nonlinear absorption (8.592 × 10−9m/W) and showed an ultrafast nonlinear response time (66 fs).

Optical properties and phase transition in photodoped amorphous Ge–Sb–Te:Ag thin films

Photodoping of Ag into amorphous Ge2Sb2Te5 thin films has been carried out by illuminating thermally evaporated Ge–Sb–Te:Ag bilayers with a halogen lamp in the inert atmosphere at room temperature. Amorphous nature of the as-deposited and photodoped films was confirmed by an X-ray diffraction study. The composition of Ge2Sb2Te5 thin films and the amount of Ag photodiffused have been gathered from electron probe micro-analyzer having a wavelength dispersive spectrometer. Amorphous-to-crystalline transition of the films was induced by thermal annealing and the structural phases were identified by X-ray diffraction. The crystalline temperature of the films was evaluated by temperature dependent sheet resistance measurements. The effects of photodoped Ag concentration on the optical properties of the Ge2Sb2Te5 films have been examined by transmission and reflection spectrometry. It was found that the optical band gap, the sheet resistance and the phase transition temperature increase while the absorption coefficient and extinction coefficient decrease upon incorporation of Ag in Ge2Sb2Te5 system. A correlation between the optical band gap and the electronegativity of the alloys indicates that the optical band gap increases with the decrease of electronegativity.

Amorphous and crystallized Ge–Sb–Te thin films deposited by pulsed laser: Local structure using Raman scattering spectroscopy

UV pulsed laser deposition was employed for the fabrication of amorphous (GeTe)x(Sb2Te3)1−x, (x = 1, 0.66, 0.5, 0.33, and 0) thin films. The local structure of as-deposited (amorphous) as well as annealed (crystallized) layers was studied using Raman scattering spectroscopy. Raman spectra were recorded on bulk materials used for the deposition as well. The vibrational modes observed in amorphous Ge–Sb–Te films are attributed, apart from defective octahedral coordination, to GeTe4−nGen (n = 1, 2, eventually 0) corner- and/or edge-sharing tetrahedra and SbTe3 pyramidal entities, the latter dominated Raman spectra of all the films containing Sb2Te3. The structure of crystallized GeTe and Sb2Te3 resembled that of starting bulk materials which is evidenced by the presence of Γ3 (E) and Γ1 (A1) modes of rhombohedrally deformed rocksalt GeTe and Eg (2) and A1g (2) modes of hexagonal Sb2Te3. We propose that in case of crystallized Ge2Sb2Te5, Ge1Sb2Te4, and Ge1Sb4Te7 films, Raman active features might be attributed to defective octahedral local structure and/or softened A1 mode of corner-sharing GeTe4 tetrahedra, A1 mode of corner-sharing GeTe4−nGen (n = 1, 2) tetrahedra, GeTe4 F2 mode, Sb–Te mode in SbTe3 entities, eventually Eg (2) and A1g (2) modes of hexagonal Sb2Te3.

Simultaneous calorimetric and quick-EXAFS measurements to study the crystallization process in phase-change materials

A Calvet-type differential scanning calorimeter has been implemented on a synchrotron beamline devoted to X-ray absorption spectroscopy. As a case study, the complex crystallization process in amorphous Ge(15)Sb(85) phase-change material is followed by simultaneous calorimetric and quick-EXAFS measurements. A first crystallization at 514(1) K is related to the crystallization of an Sb-rich phase accompanied by segregation of Ge atoms. Upon further heating, the as-formed amorphous Ge regions crystallize at 604(1) K. A quantitative analysis of the latent heat allows a Ge(11)Sb(89) stoichiometry to be proposed for the first crystallized phase.

Pressure tunes electrical resistivity by four orders of magnitude in amorphous Ge2Sb2Te5 phase-change memory alloy.

Ge-Sb-Te-based phase-change memory is one of the most promising candidates to succeed the current flash memories. The application of phase-change materials for data storage and memory devices takes advantage of the fast phase transition (on the order of nanoseconds) and the large property contrasts (e.g., several orders of magnitude difference in electrical resistivity) between the amorphous and the crystalline states. Despite the importance of Ge-Sb-Te alloys and the intense research they have received, the possible phases in the temperature-pressure diagram, as well as the corresponding structure-property correlations, remain to be systematically explored. In this study, by subjecting the amorphous Ge(2)Sb(2)Te(5) (a-GST) to hydrostatic-like pressure (P), the thermodynamic variable alternative to temperature, we are able to tune its electrical resistivity by several orders of magnitude, similar to the resistivity contrast corresponding to the usually investigated amorphous-to-crystalline (a-GST to rock-salt GST) transition used in current phase-change memories. In particular, the electrical resistivity drops precipitously in the P = 0 to 8 GPa regime. A prominent structural signature representing the underlying evolution in atomic arrangements and bonding in this pressure regime, as revealed by the ab initio molecular dynamics simulations, is the reduction of low-electron-density regions, which contributes to the narrowing of band gap and delocalization of trapped electrons. At P > 8 GPa, we have observed major changes of the average local structures (bond angle and coordination numbers), gradually transforming the a-GST into a high-density, metallic-like state. This high-pressure glass is characterized by local motifs that bear similarities to the body-centered-cubic GST (bcc-GST) it eventually crystallizes into at 28 GPa, and hence represents a bcc-type polyamorph of a-GST.

Nature of phase transitions in crystalline and amorphous GeTe-Sb2Te3 phase change materials

The thermodynamic nature of phase stabilities and transformations are investigated in crystalline and amorphous Ge(1)Sb(2)Te(4) (GST124) phase change materials as a function of pressure and temperature using high-resolution synchrotron x-ray diffraction in a diamond anvil cell. The phase transformation sequences upon compression, for cubic and hexagonal GST124 phases are found to be: cubic → amorphous → orthorhombic → bcc and hexagonal → orthorhombic → bcc. The Clapeyron slopes for melting of the hexagonal and bcc phases are negative and positive, respectively, resulting in a pressure dependent minimum in the liquidus. When taken together, the phase equilibria relations are consistent with the presence of polyamorphism in this system with the as-deposited amorphous GST phase being the low entropy low-density amorphous phase and the laser melt-quenched and high-pressure amorphized GST being the high entropy high-density amorphous phase. The metastable phase boundary between these two polyamorphic phases is expected to have a negative Clapeyron slope.

Local electrical characterization of laser-recorded phase-change marks on amorphous Ge_2Sb_2Te_5 thin films

Amorphous thin films of Ge(2)Sb(2)Te(5), sputter-deposited on a thin-film gold electrode, are investigated for the purpose of understanding the local electrical conductivity of recorded marks under the influence of focused laser beam. Being amorphous, the as-deposited chalcogenide films have negligible electrical conductivity. With the aid of a focused laser beam, however, we have written on these films micron-sized crystalline marks, ablated holes surrounded by crystalline rings, and other multi-ring structures containing both amorphous and crystalline zones. Within these structures, nano-scale regions of superior local conductivity have been mapped and probed using our high-resolution, high-sensitivity conductive-tip atomic force microscope (C-AFM). Scanning electron microscopy and energy-dispersive spectrometry have also been used to clarify the origins of high conductivity in and around the recorded marks. When the Ge(2)Sb(2)Te(5) layer is sufficiently thin, and when laser crystallization/ablation is used to define long isolated crystalline stripes on the samples, we find the C-AFM-based method of extracting information from the recorded marks to be superior to other forms of microscopy for this particular class of materials. Given the tremendous potential of chalcogenides as the leading media candidates for high-density memories, local electrical characterization of marks recorded on as-deposited amorphous Ge(2)Sb(2)Te(5) films provides useful information for furthering research and development efforts in this important area of modern technology.

Optical properties of CVD grown amorphous Ge–Sb–S thin films

Germanium antimony sulphide (Ge–Sb–S) amorphous thin films have been fabricated on commercial glass substrates by chemical vapour deposition (CVD). The compositions and thicknesses of these thin films have been characterized by micro-Raman, scanning electron microscope (SEM) and energy dispersive X-ray analysis (EDX) techniques. Optical transmission spectra measured by UV–VIS-NIR spectrometer have been used to determine the optical properties of these CVD-grown Ge–Sb–S amorphous thin films with Swanepoel’s methods. Optical band gaps of these films were calculated from Tauc’s extrapolation procedure. The dispersions of the refractive indices of these films have also been determined using the Wemple–DiDomenico method.

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.

Phase change nanodot arrays fabricated using a self-assembly diblock copolymer approach

Self-assembling diblock copolymer, polystyrene-b-poly-4-vinylpyridine (PS-b-P4VP), was used as the template for fabricating phase change nanostructures. The high density GeSb nanodots were formed by etching into an amorphous GeSb thin film using silica hard mask which was patterned on top of polymer. The nanodot arrays are 15nm in diameter with 30nm spacing. This is smaller than most structures obtained by e-beam lithography. Time-resolved x-ray diffraction studies showed that the phase transition occurred at 235°C, which is 5°C lower than blanket GeSb film but higher than that of Ge2Sb2Te5 (150°C). GeSb showed good temperature stability for fabrication of small memory devices.

Local structural change of amorphous Ge–Sb–Te thin film on annealing

Structures of amorphous Ge–Sb–Te film included in phase change optical disk have been examined using transmission electron microscopy. Cross-sectional high-resolution electron microscopy (HREM) observation indicated that locally ordered regions as small as ∼2nm exist in the amorphous Ge–Sb–Te thin film. These ordered regions grew in size after annealing at 160°C for 120min. On the basis of the HREM and nano-beam electron diffraction, it was concluded that the atomic structure of the locally ordered regions is similar to that of crystalline Sb.

Structural phase transition and fluorescence lifetime of amorphous chalcogenide alloys doped with rare-earths

Lifetime of excited electronic states inside the 4f configuration of rare-earth elements embedded in covalent chalcogenide glasses turns out to be sensitive to medium-range structural phase transition of the host glasses. We have measured lifetimes of 1.6 μm emission originating from Pr 3+: ( 3F 3, 3F 4) → 3H 4 transition in amorphous Ge–Sb–Se alloys. The longest lifetime is measured at around the mean coordination number of ∼2.67, which arises coincidently from structural changes of the host glasses from two-dimensional layers to three-dimensional networks.

Scanning tunneling microscope modifications of amorphous Ge–Sb–Te films

Surface modifications produced in Ge–Sb–Te films by a scanning tunneling microscope have been studied. Three kinds of modifications appear to depend upon the applied voltages to the tip. Positive tip voltages of about 2 V produce depressions accompanying peripheral mounds, which can be accounted for as mechanical scratching by the tip. When the humidity is higher than ∼50%, tip voltages between 5 and 10 V produce depressions with no peripheral mounds, and negative tip voltages of about −10 V produce expansions. These humidity-assisted depression and expansion are assumed to be caused by electrochemical etchings and anodic oxidation processes.

Nanoscale electrical phase-change in GeSb 2Te 4 films with scanning probe microscopes

Nanometer-scale electrical recordings in amorphous chalcogenide films have been studied using an atomic force microscope (AFM) and a scanning tunneling microscope (STM). When electrical pulses are applied to amorphous GeSb 2Te 4 films through the conducting cantilevers of the AFM, depending upon the electric power, two kinds of conductive regions appear. Low-power pulses (∼1 fJ) produce medium resistance (∼10 M Ω) marks, and high-power pulses (∼10 pJ) give low-resistance (∼1 k Ω) marks with a minimal size of ∼50 nm. X-ray diffraction patterns of the low-resistance marks show crystalline peaks, which manifest an electrical phase-change from amorphous to crystalline states. Erasures by electrical pulses are possible. Lithography in GeSb 2Te 4 films with a STM is also demonstrated.

Crystallization studies on phase-change optical recording media by use of a two-dimensional periodic mark array

We present the results of crystallization studies in thin-film samples of amorphous and crystalline Ge(x)Sb(y)Te(z). The experiments, conducted at moderately elevated temperatures, are based on measurements of the first-order diffraction efficiency from a two-dimensional periodic array of recorded marks. When the samples are slowly heated above room temperature, changes in the efficiencies of various diffracted orders give information about the on-going crystallization process within the sample. Two different compositions of the GeSbTe alloy are used in these experiments. Measurements on Ge(2)Sb(2.3)Te(5) films show crystallization dominated by nucleation. For the Sb-rich eutectic composition Ge-(SbTe), crystallization is found to be dominated by growth from crystalline boundaries. We also show that crystalline marks written by relatively high-power laser pulses are different in their optical properties from the regions crystallized by slow heating of the sample to moderate temperatures.

Universal dynamics during and after ultrafast laser-induced semiconductor-to-metal transitions

We observe common features in semiconductor-to-metal transitions induced by femtosecond laser pulses in crystalline GaAs, amorphous GaAs, and Sb-rich films of amorphous GeSb, by tracking ultrafast changes in the spectral dielectric function. The dielectric function of the metal-like state reveals a decay in the plasma frequency with time after the transition. In addition, the plasma frequency roughly decreases with increasing excitation fluence.

Ultrafast laser-induced phase transitions in amorphous GeSb films.

Time-resolved measurements of the spectral dielectric function reveal new information about ultrafast phase transitions induced by femtosecond laser pulses in Sb-rich amorphous GeSb films. The excitation generates a nonthermal phase within 200 fs. The dielectric function of this phase differs from that of the crystalline phase, contrary to previous suggestions of a disorder-to-order transition. The observed dielectric function is close to that of the liquid phase, indicating an ultrafast transition from the amorphous phase to a different disordered state.

Study of Oxygen-Doped GeSbTe Film and Its Effect as an Interface Layer on the Recording Properties in the Blue Wavelength

An oxygen-doped GeSbTe interface layer improves the overwriting characteristics of the phase-change optical disk in the blue wavelength. The thermal and optical properties of oxygen-doped GeSbTe film and its crystal structure were investigated. Crystallization temperature and activation energy of the amorphous Ge–Sb–Te–O films are increased with the oxygen concentration while the melting point is decreased. The refractive index of the crystalline state monotonically increases with the oxygen concentration of the film, while its extinction coefficient monotonically decreases. In terms of the crystalline structure, fcc characteristic peaks disappear gradually with oxygen concentration, and above 35 at.% of oxygen, hexagonal peaks appear.