Article

# Oxygen Tuned Local Structure and Phase-Change Performance of Germanium Telluride

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... For instance, N-and O-doped GeTe materials surprisingly exhibit higher crystalline resistance, although they undergo instability during the crystallization process. [6][7][8] Sn and Bi doping can effectively increase the crystallization speed of Ge 2 Sb 2 Te5 and reduce the melting temperature. 9,10 In addition, a bottom-up approach integrated with nanowires (NWs) has been emerging for scaling down the size of devices and assembly at the nanoscale, which can enable fast phase switching at very low power consumption. ...
... The spectra were deconvolved into fitted Gaussian bands to quantitatively analyze the fine structure, denoted as peaks A (A1 and A2), B, and C within the frequency range of 50 cm −1 -250 cm −1 , which showed good consistency with previous reports. 7,12,13 The Gaussian fitting results of all the Raman spectra are illustrated in Table I. Peaks A and B were assigned to scattering at the transverse optical (TO) phonons of symmetry Eg and the longitudinal optical (LO) phonons of symmetry A1g, respectively. ...
... Peak A1 was due to Te atom vibrations and peak A2 corresponded to the vibrations of defective octahedral cation sites. 7 Peak B (A1g mode) represented the Peierls distortion mode, indicating a spontaneous symmetrybreaking distortion into the R structure from the C structure. 14 The broad peak C with weak intensity was uncertain in our results, but it did not impact the identification of the two main peaks. ...
Article
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Sn-doped GeTe (SGT) nanowires (NWs) were investigated systematically for use in phase-change memory (PCM) applications. Composition and microstructure characterizations indicate that SGT with ∼3.0% Sn (SGT_3.0) NWs preserves the GeTe rhombohedral (R) structure, whereas SGT with a Sn content of ∼25.0% (SGT_25.0) NWs exhibits a cubic (C) structure. R–C structural conversion of SGT NWs is revealed with increasing Sn content. According to ab initio calculations, optimizing doping leads to a decrease in density of states near the Fermi level and reduces electrical conductivity, and thereby, SGT_3.0 is more applicable for PCM than SGT_25.0, which is attributed to Sn-induced structural change that brings about a diversity in the electrical properties. Experimentally, SGT_3.0 NWs have two significant threshold switchings and ideal high/low resistance ratio (∼105). Compared with undoped GeTe, SGT_3.0 NWs experience an increase in crystalline resistance, in agreement with our theoretical calculations, perfectly satisfying the requirement of low programming currents for PCM.
... This is attributed to the phase transition of amorphous state to crystalline state, and results in the PCM cells switching into a highly conductive state by a well-known field-induced voltage snap-back process. 21 The V th of ultrathin GeTe based PCM is only 1.33 V, which is far lower than that of GST based PCM studied here (V th = 4.02 V) as well as lower than that of conventional GeTe based PCM (V th = 1.8 V). 21 To obtain the performances of PCM, including the SET and RESET operations, the resistance-voltage (R-V) characteristics of GST and ultrathin GeTe based cells were measured by using electrical pulses. ...
... This is attributed to the phase transition of amorphous state to crystalline state, and results in the PCM cells switching into a highly conductive state by a well-known field-induced voltage snap-back process. 21 The V th of ultrathin GeTe based PCM is only 1.33 V, which is far lower than that of GST based PCM studied here (V th = 4.02 V) as well as lower than that of conventional GeTe based PCM (V th = 1.8 V). 21 To obtain the performances of PCM, including the SET and RESET operations, the resistance-voltage (R-V) characteristics of GST and ultrathin GeTe based cells were measured by using electrical pulses. The mushroom-type PCM device has a contact area of ∼100 × 100 nm 2 between the phase-change layer and the bottom electrode. ...
Article
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We designed the phase-change memory (PCM) cell based on ultrathin GeTe film (∼10 nm) and homemade nanoscale electrode filling craft to improve data retention ability and reduce programming energy, respectively. It was found that the temperature for ten years’ data retention of this ultrathin GeTe film is 160 ± 32.8 °C, which is much higher than that of conventional Ge 2 Sb 2 Te 5 (GST, 83 ± 20.6 °C) film. Benefit to the nature of fragile-to-strong crossover behavior in GeTe supercooled liquids that was confined in a two-dimension structure, a fast SET speed of 6 ns is also detected in this ultrathin GeTe PCM. Moreover, the RESET power consumption of this ultrathin GeTe PCM is measured as 1.8 ± 0.5 nJ, and it is much lower than that of GST PCM (16.5 ± 1.5 nJ), which is attributed to the nanoscale electrode of the devices. The above-mentioned improvements enable the application of ultrathin GeTe PCM in neuromorphic computing.
... The crystallization temperature was found by differentiating the reflectivity curve, which was recorded while heating the samples from room temperature to 300°C with a 4°C/min ramp rate (Linkam Scientific Instruments Ltd.). To protect the film from oxidation, which is known to influence its phase transitions, 43 the anneal was performed in an Ar atmosphere flowing at 4 SCCM. Raman spectra were collected at room temperature using a WITec Alpha300R system equipped with a 633 nm wavelength excitation; the incident laser intensity was kept low to minimize irradiationinduced heating of the probed region. ...
... The asdeposited amorphous GeTe peaks occur at A (92 cm −1 ), B (123 cm −1 ), C (162 cm −1 ), and D (218 cm −1 ) in the frequency range 50−250 cm −1 . 43,59 Upon crystallization, we observe a weaker signal in bands C (162 cm −1 ) and D (218 cm −1 ). This indicates a local structural change of Ge from a lower tetrahedral coordination to an octahedral coordination, thus confirming that GeTe crystallization has occurred. ...
Article
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The switchable optical and electrical properties of phase change materials (PCMs) are finding new applications beyond data storage in reconfigurable photonic devices. However, high power heat pulses are needed to melt-quench the material from crystalline to amorphous. This is especially true in silicon photonics, where the high thermal conductivity of the waveguide material makes heating the PCM energy inefficient. Here, we improve the energy efficiency of the laser-induced phase transitions by inserting a layer of two-dimensional (2D) material, either MoS2 or WS2, between the silica or silicon substrate and the PCM. The 2D material reduces the required laser power by at least 40% during the amorphization (RESET) process, depending on the substrate. Thermal simulations confirm that both MoS2 and WS2 2D layers act as a thermal barrier, which efficiently confines energy within the PCM layer. Remarkably, the thermal insulation effect of the 2D layer is equivalent to a ∼100 nm layer of SiO2. The high thermal boundary resistance induced by the van der Waals (vdW)-bonded layers limits the thermal diffusion through the layer interface. Hence, 2D materials with stable vdW interfaces can be used to improve the thermal efficiency of PCM-tuned Si photonic devices. Furthermore, our waveguide simulations show that the 2D layer does not affect the propagating mode in the Si waveguide; thus, this simple additional thin film produces a substantial energy efficiency improvement without degrading the optical performance of the waveguide. Our findings pave the way for energy-efficient laser-induced structural phase transitions in PCM-based reconfigurable photonic devices.
... Topological defects, such as defective octahedra, are thought to play a key role in Ge 2 Sb 2 Te 5 phase transitions [21][22][23][24][25] and arguably there is an optimum vacancy concentration that stabilizes the cubic phase of Ge 2 Sb 2 Te 5 . 26 Therefore, in Figs. ...
... Ge defective octahedra are necessary for crystallization of amorphous Ge 2 Sb 2 Te 5 . 23,24 We have shown that it is more difficult to form defective octahedra when the material is crystallized at high pressure, and this explains the measured Ge 2 Sb 2 Te 5 crystallization temperature increase with Si 3 N 4 thickness. The analysis also suggests structural features that should be engineered into Sb 2 Te 3 -GeTe PCMs: (1) to increase the stability of the structure, the proportion of Ge atoms with defective octahedral symmetry should be reduced, which would lower the probability of tetrahedrally coordinated Ge atoms forming; (2) to increase the rate with which Ge 2 Sb 2 Te 5 crystallizes, Ge atoms at defective octahedral sites are necessary. ...
Article
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The role of stress on the crystallization process of the phase change data storage material, Ge2Sb2Te5, is studied. When thin Ge2Sb2Te5 films are capped with Si3N4, stress is generated in the Ge2Sb2Te5 layer which causes the crystallization temperature to increase. Si3N4 films of 25 nm thickness increase the crystallization temperature from 446 K to 464 K. We show that stress predominantly destabilizes voids and increases the number of Ge-Sb and homopolar bonds in the vicinity of Ge atoms, and this makes the crystallization less probable, thus resulting in the increase in the measured temperature.
... Chalcogenides contain substantial concentrations of the elements in group 16 sometimes referred to as the oxygen group, oxide materials are generally not considered as chalcogenides since their properties tend to be very different to materials composed of S, Se, and Te. ...
... After all, billions of rewriteable optical digital versatile discs (DVDs), which are usually based on Ge 2 Sb 2 Te 5 or a similar phase change material, have been produced and sold around the globe since the 1990s. Phase change material research has since turned toward tailoring the materials properties for next generation FLASH-like solid state storage, [10][11][12][13][14][15][16] where the large change in electrical resistivity is exploited rather than the optical contrast. ...
Conference Paper
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... Extensive works have been done to explore the effects of oxygen or nitrogen on the properties of phase change material Ge 2 Sb 2 Te 5 (GST) and GeTe. [19][20][21][22][23][24][25] Some general conclusions have been made including that oxygen/nitrogen will modify the local structure of amorphous GST/GeTe due to the strong Ge-O/Ge-N bonding. Most importantly, the doping effects are about the crystallization kinetics, for example, it is found that N can slow down the crystal growth of GST, [26][27][28] which will improve the data-retention property of the amorphous GST. ...
... While for GeTe, oxygen was reported to increase its crystallization temperature, 21 thus enabling the application of GeTe at higher temperature. More recently, further experimental study 20 showed that oxygen can also increase the resistivity of GeTe, resulting in a substantial reduction of switching voltage for the devices based on GeTe-O films. Given the substantial influences of oxygen on the structure, crystallization kinetics as well as the electronic properties of GeTe, understanding the microscopic mechanism at atomic scale for these effects is imperative. ...
Article
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Oxygen is widely used to tune the performance of chalcogenide phase-change materials in the usage of phase-Change random access memory (PCRAM) which is considered as the most promising next-generation non-volatile memory. However, the microscopic role of oxygen in the write-erase process, i.e., the reversible phase transition between crystalline and amorphous state of phase-change materials is not clear yet. Using oxygen doped GeTe as an example, this work unravels the role of oxygen at the atomic scale by means of ab initio total energy calculations and ab initio molecular dynamics simulations. Our main finding is that after the amorphization and the subsequent re-crystallization process simulated by ab initio molecular dynamics, oxygen will drag one Ge atom out of its lattice site and both atoms stay in the interstitial region near the Te vacancy that was originally occupied by the oxygen, forming a "dumbbell-like" defect (O-VTe-Ge), which is in sharp contrast to the results of ab initio total energy calculations at 0 K showing that the oxygen prefers to substitute Te in crystalline GeTe. This specific defect configuration is found to be responsible for the slower crystallization speed and hence the improved data retention of oxygen doped GeTe as reported in recent experimental work. Moreover, we find that the oxygen will increase the effective mass of the carrier and thus increases the resistivity of GeTe. Our results unravel the microscopic mechanism of the oxygen-doping optimization of phase-change material GeTe, and the present reported mechanism can be applied to other oxygen doped ternary chalcogenide phase-change materials.
... In addition, there is another peak at around 125 cm −1 for these as-grown GST films. With an increase in nitrogen content, the peak at about 125 cm −1 is more obvious due to the distorted octahedral vibration mode [30]. After laser beam irradiation, the peak of 150 cm −1 becomes wider, even not obvious in Zone B' for pure GST films, as shown in Figure 6a, and there is a broad peak of around 125 cm −1 , which is attributed to the A1 mode of GeTe4-nGen (n = 0, 1, 2) corner-sharing tetrahedral units [31]. ...
Article
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Element-doped phase change material (PCM) could improve the performances, e.g., better thermal stability, higher electrical resistance, and faster crystallization speed; thus, the influence of the doping element needs to be further investigated. In this paper, a femtosecond laser, which could realize the ultrafast phase transition rate of PCM between amorphization and crystallization, was used to explore the properties of nitrogen-doped Ge2Sb2Te5 (GST), and a bond effect was proposed. The pure GST and different nitrogen contents of doped GST films were investigated by femtosecond laser pulse excitation through a pump–probe shadowgraph imaging technique. The results showed that the element-doped films could change photon absorption because of the increase in free carriers. This caused the faster rate of reflectivity to change in the irradiated area by the laser beam as the more nitrogen doped. When the nitrogen content increased, the crystallization evolution became harder because it enhanced the bond effect, which suppressed crystalline grain growth and improved the thermal stability. Based on the analysis in the paper, the desired performances of PCMs, e.g., ultrafast dynamics, crystallization evolution, and thermal stability, could be controlled according to the demands by modifying the bond effect.
... 144 Indeed, phase change compositions that do not exhibit a change in volume during the phase transition require less energy to switch because no work is done on the surrounding structures. 145 Finally, limiting configurational entropy change during a phase transition can be used to substantially increase phase transition efficiency. If one considers that GST consists of a spectrum of different atomic configurations 128,132 or microstates, then limiting the number of microstates that can exist will increase the efficiency of the phase transition. ...
Article
Phase transitions can occur in certain materials such as transition metal oxides (TMOs) and chalcogenides when there is a change in external conditions such as temperature and pressure. Along with phase transitions in these phase change materials (PCMs) come dramatic contrasts in various physical properties, which can be engineered to manipulate electrons, photons, polaritons, and phonons at the nanoscale, offering new opportunities for reconfigurable, active nanodevices. In this review, we particularly discuss phase-transition-enabled active nanotechnologies in nonvolatile electrical memory, tunable metamaterials, and metasurfaces for manipulation of both free-space photons and in-plane polaritons, and multifunctional emissivity control in the infrared (IR) spectrum. The fundamentals of PCMs are first introduced to explain the origins and principles of phase transitions. Thereafter, we discuss multiphysical nanodevices for electronic, photonic, and thermal management, attesting to the broad applications and exciting promises of PCMs. Emerging trends and valuable applications in all-optical neuromorphic devices, thermal data storage, and encryption are outlined in the end.
... 9292 It should be noted that this peak may also have contributions due to a defective octahedral GeTe 4 structural unit. 93 Previous experiments on various related material systems assign peaks near 200 and 240 cm −1 to GeSe 4 , AsSe 3 , and AsTe 3 structural units. 15,50,52,53,89,92,94 Our simulations indicate that the small peak at 280 cm −1 may be due to vibrations of Se atoms linking two GeSe 4 tetrahedra. ...
... Alloying of GST materials with widely used chemical elements like oxygen, carbon, nitrogen is a promising approach to enhance material properties and thus, to improve device characteristics, such as cycling endurance, higher temperature retention, writing speed and power consumption [10][11][12][13]. Particularly, the crystallization activation energy of the alloyed GST materials increases, which enhances crystallization temperature and thus, improves thermal stability of the amorphous phase [14][15][16][17]. In addition, the alloying supresses grain growth, which enhances device endurance [13]. ...
Article
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Ge-Sb-Te alloys are promising materials for non-volatile memory applications. Alloying of the materials with various elements is considered as prospective approach to enhance material properties. This work reports on the preparation and characterization of pure Ge-Sb-Te-O (GSTO) and alloyed with La-Sr-Mn-O (LSMO) thin films. Thermal heating of amorphous thin films to different temperatures show distinct crystallization behaviour. A general trend is the decrease in the size of GSTO crystallites and the suppression in the formation of stable trigonal GSTO phase with increasing content of LSMO. Microstructural studies by transmission electron microscopy show the formation of metastable GSTO nanocrystallites dispersed in the amorphous matrix. Analysis of local chemical bonding by X-ray spectroscopy reveal the presence of different oxides in the GSTO-LSMO composites. Moreover, the composites with a high LSMO content exhibit higher crystallization temperature and significant larger sheet resistance in amorphous and crystalline phase, while a memory device made of GSTO-LSMO alloy reveals bipolar switching and synaptic behaviour. In addition, the amount of LSMO in GSTO-LSMO thin films influences their optical properties and band gap. Overall, the results of this work reveal the highly promising potential of GSTO-LSMO nanocomposites for data storage and reconfigurable photonic applications as well as neuro-inspired computing.
... Moreover, for Ar-GeTe, the presence of O caused a slight increase and offset in the A 1g peak, which was in accordance with previous reports. 41 XPS was performed to confirm the chemical composition of the samples. Figure 3b shows that bulk GeTe, Ar-GeTe, and O-GeTe contained Ge, Te, and O. ...
Article
As a key method to convert solar into chemical energy, photocatalytic water decomposition has garnered attention. Moreover, the development of graphene and graphene-like 2D materials has brought fresh vitality in the field of photocatalysis. Here, we prepared 2-4 layers of GeTe nanosheets by ultrasonic-assisted liquid phase exfoliation in Argon and Air, which we referred to as Ar-GeTe and O-GeTe, respectively. The photocatalytic hydrogen production potential of 2D GeTe was experimentally investigated for the first time. The results indicated that minimally layered GeTe samples are indirect-gap semiconductors with the GeTe bandgap increasing after oxidation. All samples have suitable band positions that can drive photocatalytic water splitting into H2 under mild conditions, providing maximum hydrogen evolution rates of 1.13 (Ar-GeTe) and 0.54 mmol∙g-1∙h-1 (O-GeTe). With density functional theory computations, the structural stability of GeTe in air was discussed, revealing that oxygen atoms could easily combine with Ge to form a more stable structure, thus impacting the photocatalytic performance of 2D-GeTe. Therefore, the light requirement and oxygen deficiency of the material gives an advantage in the field of energy supply in space.
... We add that the spurious peak is insensitive to polarization analysis. Another indication of oxidation is that it leads to narrowing of the Raman peaks [31]. Intrinsic Raman peaks at ambient and low pressure in GeTe are considerably broadened, with a full width at half maximum beyond 20 cm −1 . ...
Article
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Pressure-induced phase transitions in GeTe, a prototype phase change material, have been studied to date with diffraction which is not sensitive to anharmonicity-induced dynamical effects. GeTe is also prone to surface oxidation which may compromise surface sensitive measurements. These factors could be responsible for the lack of clarity about the phases and transitions intervening in the phase diagram of GeTe. We have used high-pressure Raman scattering and ab initio pseudopotential density functional calculations to unambiguously establish the high-pressure phase diagram and identify three phases up to 57 GPa, a low-pressure rhombohedral phase, an intermediate pressure cubic phase, and a high-pressure orthorhombic phase. We detect substantial broadening and softening of Raman modes at low pressure and identify the transition regions and possible intermediate phases.
... V under current sweep, as shown in Figure 5c. The lower threshold voltage of SGT-2 than that of GT (1.8 V) 31 is suggested to result from the increase of the density of charge trapping defects due to Sc doping. 42 Reset operation under a 1000 ns current pulse has been monitored to calculate the power consumption of an SGT-2-based cell, as shown in Figure 5d. ...
Article
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Phase change memory (PCM) with advantages of high operation speed, multilevel storage capability, spiking-time-dependent plasticity, etc., has wide application scenarios in both Von Neumann systems and neuromorphic systems. In the automotive application, intelligent system not only needs high efficiency to handle massive data processing, but also good robustness to retain the existing data against high working temperature. In this work, Sc doped GeTe is developed for PCM, which has achieved 120 oC data retention for 10 years, 6 ns operation speed and 7 nJ low power-consumption. The high data retention is attributed to the high coordination number of Sc and its strong bonds with Te atoms in the amorphous phase, which enhances the robustness of the atomic matrices. Sc centered octahedrons in amorphous state provide nucleation center, leading to the fast crystallization. In crystalline phase, Sc atoms occupy Ge vacancies to form a homogenous GeTe-like rhombohedral phase. The strong covalent-like Sc-Te bonds weaken the neighbor Ge-Te bonds, lowering energy for melting. Together with the increased energy efficiency originated from confined grain size, the reduced power consumption has been achieved. The improvements in data retention, speed, and power efficiency have made Sc doped GeTe a promising candidate for high performance automobile electronics application.
... Chalcogenide-based thin films are important for modern electro-optic devices such as thin-film infrared transistors, 1 optical waveguides, 2 or phase-recording media. 3,4 Especially, Ge−Sb−Te films with a typical composition of Ge 2 Sb 2 Te 5 (GST) located at the pseudo-binary line connecting GeTe and Sb 2 Te 3 in the phase diagram have been applied as phasechange materials. 5−7 However, its nucleation-dominated crystallization has led to slow phase-transition speed. ...
... If this had happened, we would have expected the GST crystallization temperature to increase. 35 However, the GST crystallization temperature is 158 C which is the same as the isolated GST film. This implies that the Ge atoms are unoxidized. ...
Article
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The objective of this work is to design and demonstrate multilevel optical switches by combining different phase change materials. Ge2Sb2Te5 and VO2 nanolayer structures were designed to maximize the optical contrast between four different reflective states. These different optical states arise due to the independent structural phase transitions of VO2 and Ge2Sb2Te5 at different temperatures. The transfer matrix method was used to model Fresnel reflection for each structural phase combination and then to optimize the VO2 and Ge2Sb2Te5 layer thicknesses, which were found to be 70 nm and 50 nm. These multilevel optical switching results provide further possibilities to design composite materials for applications in active and programmable photonics.
... It is widely accepted that the enhanced optical and electrical contrast between the amorphous and crystalline phase of such materials results from an important difference in structure at the local scale, and particularly regarding the germanium atoms [6,8]. Early studies have suggested that this PC mechanism is, indeed, driven by a change in short-range order since x-ray absorption fine structure (EXAFS) revealed that the Ge-Te bond length exhibits a significant reduction by about 0.22Å upon amorphization [9]. ...
Article
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The effect of van der Waals dispersion correction in combination with density functional theory is investigated on a canonical amorphous phase-change material. Density functional theory (DFT), using the generalized gradient approximation, usually fails to reproduce the structure of amorphous tellurides, which manifests by an overestimation of the interatomic bond distances, and particularly the Ge-Te one involved in local geometries (tetrahedral or defect octahedral). Here, we take into account dispersion forces in a semiempirical way and apply such DFT simulations to amorphous GeTe. We obtain a substantial improvement of the simulated structure factor and pair-correlation function, which now reproduce the experimental counterparts with an unprecedented accuracy, including on a recent partial contribution from anomalous x-ray scattering and from x-ray absorption. A detailed analysis of the corresponding structures indicates that the dispersion correction reduces the Ge-Te bond length, increases the fraction of tetrahedral germanium, and reduces the presence of heteropolar so-called fourfold ABAB rings. Given that these structural features have been stressed to be central for the understanding of the phase-change mechanism, the present results challenge our current understanding of the crystal to amorphous transformation at play.
... A dopant effect of oxygen can be excluded since oxygen, like other dopants, is known to increase the crystallization temperature of GeTe or GST. [19][20][21] Analyses of the literature devoted to the crystallization of Ge x Te 100-x amorphous alloys 4,6,22,23 for x values between 30 and 70 show that the temperature of the onset of crystallization is minimum for x close to 50. These results were obtained on oxidized samples, the crystallization temperature being close to 180 C for x close to 50 in all these studies. ...
Article
The crystallization mechanisms of prototypical GeTe phase-change material thin films have been investigated by in situ scanning transmission electron microscopy annealing experiments. A novel sample preparation method has been developed to improve sample quality and stability during in situ annealing, enabling quantitative analysis and live recording of phase change events. Results show that for an uncapped 100 nm thick GeTe layer, exposure to air after fabrication leads to composition changes which promote heterogeneous nucleation at the oxidized surface. We also demonstrate that protecting the GeTe layer with a 10 nm SiN capping layer prevents nucleation at the surface and allows volume nucleation at a temperature 50 °C higher than the onset of crystallization in the oxidized sample. Our results have important implications regarding the integration of these materials in confined memory cells.
... The reference Raman spectrum acquired on highly ( l 0 0 ) oriented Sb Te 2 3 crystal presents the characteristic A 1g 1 , E g 2 , and A 1g 2 Raman modes of Sb Te 2 3 [33]. The two most prominent bands near 85 and 120cm −1 in the GeTe spectrum were assigned primarily to the E and A 1 phonon modes of crystalline GeTe [34], while the relatively weak C and D bands were identified as the phonons of the octahedral and tetrahedral Ge units [35], respectively. The A 1g 1 , A 1g 2 , and A 1 modes vibrate out-of-plane along the c axis of the hexagonal structures, while the E g 2 and E modes represent in-plane phonons in the crystals. ...
Article
By confining phase transitions to the nanoscale interface between two different crystals, interfacial phase change memory heterostructures represent the state of the art for energy efficient data storage. We present the effect of strain engineering on the electrical switching performance of the –GeTe superlattice van der Waals devices. Multiple Ge atoms switching through a two-dimensional Te layer reduces the activation barrier for further atoms to switch; an effect that can be enhanced by biaxial strain. The out-of-plane phonon mode of the GeTe crystal remains active in the superlattice heterostructures. The large in-plane biaxial strain imposed by the layers on the GeTe layers substantially improves the switching speed, reset energy, and cyclability of the superlattice memory devices. Moreover, carefully controlling residual stress in the layers of –GeTe interfacial phase change memories provides a new degree of freedom to design the properties of functional superlattice structures for memory and photonics applications.
... Diffraction peaks belonging to rhombohedral structure emerge as the annealing temperature increases to 300°C for the AgGeTe films, consistent with previous studies. 46,47 Crossbar structural Ag−AgGeTe−Ta memristors were fabricated using standard ultraviolet lithography and lift-off processes and then used for electronic characterization and logic computing. Figure 2a shows a top scanning electronic microscopy (SEM) view of the crossbar structure of 8 × 8 devices with Ag as the top electrodes. ...
Article
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Nonvolatile stateful logic computing in memristor is a promising paradigm to realize the unity of information storage and processing in the same physical location, which has shown great feasibility to break the von Neumann bottleneck in traditional computing architecture. How to reduce the computational complexity of memristor based logic function is a matter of concern. Here, based on a general logic expression, we proposed a method to implement arbitrary logic of complete 16 Boolean logic in two steps with one memristor in the crossbar architecture. A representative functional complete NAND logic is successfully experimentally demonstrated in the filamentary Ag/AgGeTe/Ta memristors to prove the validity of our method. We believe our work may promote the development of the revolutionary logic applications.
Article
In the new computing world, phase-change memory (PCM) has recently evolved as a non-volatile key-enabling technology that has been used as memory storage. PCM has also been explored as non-von Neumann computing for neuromorphic computing applications. It was discovered in the 1960s, still, there are many questions related to its thermal stability, endurance, electrical and structural dynamics. To enhance thermal stability and operation speed, many materials have been explored yet, some rare-earth elements prove to be suitable materials for improving the performance of the device. The article describes the applications of the PCM and the basic processes involved in the working of the device. READ and WRITE processes, material exploration, and cell designs have been discussed, concluding that mushroom-type cell design is good for fabricating PCM devices. Various performance-related properties of the device have been discussed including scalability, reliability, and variability. Finally, an outlook and future scope have been discussed.
Article
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Chalcogenides such as GeTe, PbTe, Sb2Te3, and Bi2Se3 are characterized by an unconventional combination of properties enabling a plethora of applications ranging from thermo‐electrics to phase change materials, topological insulators, and photonic switches. Chalcogenides possess pronounced optical absorption, relatively low effective masses, reasonably high electron mobilities, soft bonds, large bond polarizabilities, and low thermal conductivities. These remarkable characteristics are linked to an unconventional bonding mechanism characterized by a competition between electron delocalization and electron localization. Confinement, that is, the reduction of the sample dimension as realized in thin films should alter this competition and modify chemical bonds and the resulting properties. Here, pronounced changes of optical and vibrational properties are demonstrated for crystalline films of GeTe, while amorphous films of GeTe show no similar thickness dependence. For crystalline films, this thickness dependence persists up to remarkably large thicknesses above 15 nm. X‐ray diffraction and accompanying simulations employing density functional theory relate these changes to thickness dependent structural (Peierls) distortions, due to an increased electron localization between adjacent atoms upon reducing the film thickness. A thickness dependence and hence potential to modify film properties for all chalcogenide films with a similar bonding mechanism is expected. In GeTe and related chalcogenides, which all show a unique property portfolio in the crystalline state including strong optical absorption and soft bonds, the thickness dependent properties are explored. A pronounced thickness dependence of the optical and vibrational properties of crystalline GeTe is demonstrated, which is linked to a thickness dependent Peierls distortion, due to an increased electron localization.
Preprint
The switchable optical and electrical properties of phase change materials (PCMs) are finding new applications beyond data storage in reconfigurable photonic devices. However, high power heat pulses are needed to melt-quench the material from crystalline to amorphous. This is especially true in silicon photonics, where the high thermal conductivity of the waveguide material makes heating the PCM energy inefficient. Here, we improve the energy efficiency of the laser-induced phase transitions by inserting a layer of two-dimensional (2D) material, either MoS2 or WS2, between the silica or silicon and the PCM. The 2D material reduces the required laser power by at least 40% during the amorphization (RESET) process, depending on the substrate. Thermal simulations confirm that both MoS2 and WS2 2D layers act as a thermal barrier, which efficiently confines energy within the PCM layer. Remarkably, the thermal insulation effect of the 2D layer is equivalent to a ~100 nm layer of SiO2. The high thermal boundary resistance induced by the van der Waals (vdW)-bonded layers limits the thermal diffusion through the layer interfaces. Hence, 2D materials with stable vdW interfaces can be used to improve the thermal efficiency of PCM-tuned Si photonic devices. Furthermore, our waveguide simulations show that the 2D layer does not affect the propagating mode in the Si waveguide, thus this simple additional thin film produces a substantial energy efficiency improvement without degrading the optical performance of the waveguide. Our findings pave the way for energy-efficient laser-induced structural phase transitions in PCM-based reconfigurable photonic devices.
Article
We designed and demonstrated a four-state programmable mid-infrared band-stop absorber that exploits two different phase-change materials. This programmability is possible by exploiting Fabry–Pérot resonances in a Ge2Sb2Te5 film and vanadium dioxide nanoparticles' (VO2 NPs) dual layer. The reflectivity trough can be tuned to four different infrared (IR) wavelengths from 1906 to 2960 nm by heating the structure to different temperatures. The near-perfect absorber is reconfigurable, lithography-free, industrially scalable, polarization-insensitive, and omnidirectional. Our strategy opens a path for programmable infrared photonics.
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The measurement of the Seebeck coefficient of thin film (100 nm) amorphous germanium telluride containing ~ 31% oxygen under dark conditions and when exposed to monochromatic light in the 400 nm to 1800 nm wavelength region is reported. Exposure of the films to light is found to reduce the absolute value of the Seebeck coefficient compared to that measured in the dark. Furthermore, the magnitude of this reduction displays a distinctive spectral dependence over the wavelength range covered. The observed behaviour suggests that these measurements provide a method determining the optical bandgap of thin amorphous chalcogenide films. Further analysis of the data, along with that of X-ray photoelectron spectroscopy and photoconductivity studies, is used to determine the presence of sub-bandgap defect states and their role in determining the optical response of the Seebeck coefficient.
Article
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We investigated the crystallization behavior of Cr2Ge2Te6 (CrGT) and the effect of nitrogen (N) doping on it. We revealed that Ge or Cr-centered defective octahedral structure dominated in the amorphous phase of CrGT and the evolution of the c-axis-aligned Ge–Ge dimer and Cr-centered octahedral structure marked the beginning of crystallization. The further formation of new Cr–Te bonds resulted in a dramatic decline of carrier density in the crystalline phase, which explained the inverse resistance change between the amorphous and crystalline phases of CrGT. N atom can form stronger bonds with Ge or Cr than Cr–Te, restraining the shift of Cr to the center of the octahedron to form the new Cr–Te bonds, resulting in a constant carrier density change during crystallization. The thermal stability and data retention property have also shown an improvement by N doping
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Phase-change (PC) materials offer the property of thermally induced reversible change and exhibit different resistance values due to the structure changes between crystalline and amorphous phases. In this work, GeTe films with a thickness of 100 nm were deposited on the glass substrates by radio-frequency magnetron sputtering and annealed at different temperatures. The XRD analysis showed diffraction peaks corresponding to the rhombohedral structure of c-GeTe (021), (200), (202), and (220), respectively. Upon annealing at 300 and 340°C, the rhombohedral structure would transform into a rock salt structure. The Raman analysis revealed that a-Ge-Te covalent bond is the mixed structure of tetrahedron and defective octahedron changing into tetrahedral cells. The resistivity declined sharply from 0.72 to 3.8× 10 ⁻⁶ Ω-m, whereas the Hall mobility of holes sharply rose to 300 m ² /Vs. Based on the above films, a two-port directly heated switch device was designed and fabricated in sandwich structure to verify its phase-change characteristics. Low-resistance state was converted from crystallization with a 20ns 9V voltage pulse, while high-resistance state was by passing a 2 μs 2V voltage pulse. The on/off resistance ratio of switch could reach five orders of magnitude, while the switch state remained stable after the process termination. The results obtained strongly suggest that GeTe is a promising material for RF switches.
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Chalcogenide based phase change random access memory (PCRAM) holds great promise for high speed and large data storage applications. This memory is scalable, requires a low switching energy, has a high endurance, has fast switching speed, and is nonvolatile. However, decreasing the switching time whilst increasing the cycle endurance is a key challenge for this technology to replace dynamic random access memory. Here we demonstrate high speed and high endurance ultrafast transient switching in the SET state of a prototypical phase change memory cell. Volatile switching is modeled by electron-phonon and lattice scattering on short timescales and charge carrier excitation on long timescales. This volatile switching in phase change materials enables the design of hybrid memory modulators and ultrafast logic circuits.
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Phase change memory has high potential for next-generation nonvolatile memory technology. The effect of lower layer Sb2Te (ST) on the crystallization process of Ge2Sb2Te5 (GST) is investigated by observing the change of the microstructure. Compared to the GST-based device, this paper achieved a faster operation speed (20 ns) with stable SET and RESET resistances via the addition of ST under the GST. The crystallization of lower layer ST occurs prior to that in the upper layer GST due to lower crystallization temperature, and the crystal grains of ST can supply the heterogeneous nuclei to reduce the nucleation time and enhance the crystallization speed of GST, which accounts for the improved operation speed. In addition, the effect of atomic migration on the GST/ST interface and the device performance is also investigated.
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In the past several years, phase change materials (PCMs) have been widely applied in energy-saving non-volatile photonic devices, such as active perfect absorbers, nanopixel displays and all-photonic memories. To achieve high signal-to-noise ratio optical performance and high suitability for application in high temperature environments, PCMs should have both high optical contrast and thermal stability. However, the microscopic origins of these two properties have not been well explored. Integrating both properties into one PCM remains an open challenge. Here we show that structural disorder and structural stiffness play key roles in improving the optical contrast and thermal stability of PCMs through a combination of experiments, theoretical calculations and spectral fittings. We obtain high optical contrast and thermal stability in one PCM by analyzing the differences in optical properties and electronic structures among three most typical PCMs (Ge-Te, Sb-Te, and Ge-Sb-Te). Therefore, these results elucidate the dominant factors and physical mechanisms influencing the optical contrast and thermal stability of PCMs, which sheds new light on designing high-performance non-volatile optics based on PCMs.
Thesis
In this work, we address the current challenges encountered during in situ Transmission Electron Microscopy characterization of emerging non volatile data storage technologies. Recent innovation on in situ TEM holders based on silicon micro chips have led to great improvements compared to previous technologies. Still, in situ is a particularly complicated technique and experiments are extremely difficult to implement. This work provides new solutions to perform live observations at the atomic scale during both heating and biasing of a specimen inside the TEM. This was made possible through several improvements performed at different stages of the in situ TEM experiments. The main focus of this PhD concerned the issues faced during in situ biasing of a nanometer size resistive memory device. This was made possible through hardware investigation, sample preparation method developments, and in situ biasing TEM experiments.First, a new sample preparation method has been developed specifically to perform in situ heating experiments. Through this work, live crystallization of a GeTe phase change Memory Material is observed in the TEM. This allowed to obtain valuable information for the development of chalcogenide based Phase Change Resistive Memories. Then, new chips dedicated to in situ biasing experiments have been developed and manufactured. The FIB sample preparation is studied in order to improve electrical operation in the TEM. Quantitative TEM measurements are then performed on a reference PN junction to demonstrate the capabilities of this new in situ biasing experimental setup. By implementing these improvements performed on the TEM in situ biasing technique, results are obtained during live operation of a Conductive Bridge Resistive Memory device. This allowed to present new information on the resistive memories functioning mechanisms, as well as the in situ TEM characterization technique itself.
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Si-based resistive random access memory (RRAM) device at the nanoscale with high uniformity has great potential application in the future. We demonstrate that the uniformity evolution of the a-SiNx:H RRAM at the low resistance state (LRS) and the high resistance state (HRS) can be clearly monitored by presetting Si dangling bond conductive pathway through thermal energy. It is found that the increased magnitude of uniformity for the LRS and the HRS are determined by the number of the preset Si dangling bonds (Si-DBs), which can be controlled by tuning thermal energy. As for LRS, the Si-DBs produced under the electric field along with the preset Si-DBs conductive pathways form the main conductive pathway. Theoretically calculation of current-voltage (I-V) curves indicates that the Si-DB conductive pathways obey the trap-assisted tunneling (TAT) model. In the HRS, the preset Si-DBs induced by thermal energy is the unique source of conductive pathway. The transmission mechanism is trap to trap process by the hopping of electrons under the low electric field, Poole-Frenkel (P-F) emission in the main region under the medium electric field and Fowler-Nordheim (F-N) tunneling under the high electric field. Our discovery of the uniformity evolution for a-SiNX:H RRAM device through presetting Si dangling bond conductive pathway supplies a new way to insight the resistive switching mechanism of next generation Si-based RRAM device.
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We describe sonication-assisted liquid-phase exfoliation of rhombohedral germanium telluride (α-GeTe) to obtain a good dispersion of α-GeTe nanosheets in ethanol. The thickness of the α-GeTe nanosheets is dependent on the exfoliation conditions, and few-layer α-GeTe nanosheets of 2-4 layers even monolayer α-GeTe were obtained. We use first-principle calculations to investigate the structural, electronic, and optical properties of monolayer and bulk α-GeTe and compare the optical band gap of centrifugally fractionated α-GeTe nanosheets dispersions to the computational predictions. We demonstrate that few layer α-GeTe nanosheets are purified selectively through centrifugation, and exhibit high sensitivity to Fe3+. The scalable production of two-dimensional α-GeTe nanosheets can be used in the future optoelectronic industry.
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The objective of this work is to demonstrate the usefulness of fractional factorial design for optimising the crystal quality of chalcogenide van der Waals (vdW) crystals. We statistically analyse the growth parameters of highly c-axis oriented Sb2Te3 crystals and Sb2Te3-GeTe phase change vdW heterostructure superlattices. The statistical significance of the growth parameters of temperature, pressure, power, buffer materials, and buffer layer thickness was found by fractional factorial design and response surface analysis. Temperature, pressure, power, and their second order interactions are the major factors that significantly influence the quality of the crystals. Additionally, using tungsten rather than molybdenum as a buffer layer, significantly enhances the crystal quality. Fractional factorial design minimises the number of experiments that are necessary to find the optimal growth conditions, resulting an order of magnitude improvement in the crystal quality. We highlight that statistical design of experiment methods, which is more commonly used in product design, should be considered more broadly by those designing and optimising materials.
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An a-SiNx-based resistive random access memory (RRAM) device with a forming-free characteristic has significant potentials for the industrialization of the next-generation memories. We demonstrate that a forming-free a-SiNxOy RRAM device can be achieved by an oxygen plasma treatment of ultra-thin a-SiNx:H films. Electron spin resonance spectroscopy reveals that Si dangling bonds with a high density (1019 cm-3) are distributed in the initial state, which exist in the forms of 〖"Si" 〗_"2" "N≡Si∙" , "Si" "O" _"2" "≡Si∙" , "O" _"3" "≡Si∙" , and "N" _"3" "≡Si∙" . X-ray photoelectron spectroscopy and temperature-dependent current analyses reveal that the silicon dangling bonds induced by the oxygen plasma treatment and external electric field contribute to the low resistance state (LRS). For the high resistance state (HRS), the rupture of the silicon dangling bond pathway is attributed to the partial passivation of Si dangling bonds by H+ and O2-. Both LRS and HRS transmissions obey the hopping conduction model. The proposed oxygen plasma treatment, introduced to generate a high density of Si dangling bonds in the SiNxOy:H films, provides a new approach to forming-free RRAM devices.
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The local structure change of Ge induced by carbon doping in as-deposited Ge2Sb2Te5 films were studied by extended X-ray absorption fine structure and Raman spectrum. Ge-C bonds are formed at the expense of reducing the coordination of Ge-Ge and Ge-Te bonds, and make the local structure of Ge to be a well-defined tetrahedral geometry, which increases the rigidity of amorphous network and reduces the number of ABAB rings, thus the crystallization temperature of carbon-doped Ge2Sb2Te5 (CGST) films are enhanced. The reduced proportion of the tetrahedral units GeTe4−nGen (n = 1, 2) caused by carbon doping accounts for the weaker Raman peak intensity at ∼124 cm⁻¹ in CGST films. Meanwhile, the impact of doping carbon on the crystalline structure of CGST films were investigated by high resolution transmission electron microscope.
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Phase-change materials (PCMs) that can reversibly transit between crystalline and amorphous phases have been widely used for data-storage and other functional devices. As PCMs scale down to nanoscale, the properties and transition procedures can vary, bringing both challenges and opportunities in scalability. This article describes the physical structures, properties and applications of nanoscale phase-change materials and devices. The limitations and performance of scaling properties in phase-change materials and the recent progress and challenges in phase-change devices are presented. At the end, some emerging applications related to phase-change materials are also introduced.
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Although the structural, mechanical and optical properties of amorphous Ge2Sb2Te5 films are very important for many practical applications, how to control and improve these properties has not yet been well explored. Here, we find that ion bombardment and nitrogen incorporation exert significant impacts on the structure, hardness and Urbach tail width (E0) of amorphous Ge2Sb2Te5 films. The underlying physical mechanisms are revealed through combinations of experiments and theoretical calculations. Ion bombardment causes crystallization of amorphous Ge2Sb2Te5 films, while nitrogen incorporation induces the transition from the crystalline film to amorphous film again, because nitrogen incorporation renders the formation of strong Ge-N bond, which increases the crystallization activation energy. Ion bombardment improves the hardness of the films by 18.5% and nitrogen incorporation can further improve the hardness by 39.2%. Both the ion bombardment and nitrogen incorporation significantly change E0 of films, which has not been previously reported. The underlying mechanism is that ion bombardment causes variation in disorderliness of film and nitrogen incorporation causes decrease in dielectric coefficient. Therefore, the combination of ion bombardment and nitrogen incorporation can finely control the crystallization, hardness and E0 of amorphous Ge2Sb2Te5 films, which provides a new research strategy to improve the structure and properties.
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Using N K-edge XANES studies, we demonstrate a noticeable difference in local structure around the nitrogen atoms in as-deposited amorphous and annealed N-doped GeTe-based phase change alloys. The pronounced changes appear as a ≈ 2 eV shift in the absorption edge to higher photon energies and the overall shape of the XANES spectrum. Comparison of the experimental XANES spectrum of the as-deposited amorphous phase with ab-initio XANES simulations discloses that the as-deposited phase mainly consists of the NGe3 and the NTe3 pyramidal units in approximately equal concentration. When annealed, NTe3 units gradually rebond to the NGe3 units and at the same time N atoms diffuse through the amorphous phase to form the GexNy aggregates. Upon long-standing annealing at 400∘C a compact interlayer of Ge3N4 is formed in the crystalline phase.
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Strain engineering is an emerging route for tuning the bandgap, carrier mobility, chemical reactivity and diffusivity of materials. Here we show how strain can be used to control atomic diffusion in van der Waals heterostructures of two-dimensional (2D) crystals. We use strain to increase the diffusivity of Ge and Te atoms that are confined to 5 Å thick 2D planes within an Sb2 Te3-GeTe van der Waals superlattice. The number of quintuple Sb2 Te3 2D crystal layers dictates the strain in the GeTe layers and consequently its diffusive atomic disordering. By identifying four critical rules for the superlattice configuration we lay the foundation for a generalizable approach to the design of switchable van der Waals heterostructures. As Sb2 Te3-GeTe is a topological insulator, we envision these rules enabling methods to control spin and topological properties of materials in reversible and energy efficient ways.
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Van der Waals heterostructure superlattices of Sb2 Te1 and GeTe are strain engineered to promote switchable atomic disordering, which is confined to the GeTe layer. Careful control of the strain in the structures presents a new degree of freedom to design the properties of functional superlattice structures for data storage and photonics applications.
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Ni-Ge-Te phase change material is proposed and investigated for phase change memory (PCM) applications. With Ni addition, the crystallization temperature, the data retention ability, and the crystallization speed are remarkably improved. The Ni-Ge-Te material has a high crystallization temperature (250 °C) and good data retention ability (149 °C). A reversible switching between SET and RESET state can be achieved by an electrical pulse as short as 6 ns. Up to ∼3 × 104 SET/RESET cycles are obtained with a resistance ratio of about two orders of magnitude. All of these demonstrate that Ni-Ge-Te alloy is a promising material for high speed and high temperature PCM applications.
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Active and durable electrocatalysts for methanol oxidation reaction are of critical importance to the commercial viability of direct methanol fuel cell technology. Unfortunately, current methanol oxidation electrocatalysts fall far short of expectations and suffer from rapid activity degradation. Here we report platinum-nickel hydroxide-graphene ternary hybrids as a possible solution to this long-standing issue. The incorporation of highly defective nickel hydroxide nanostructures is believed to play the decisive role in promoting the dissociative adsorption of water molecules and subsequent oxidative removal of carbonaceous poison on neighbouring platinum sites. As a result, the ternary hybrids exhibit exceptional activity and durability towards efficient methanol oxidation reaction. Under periodic reactivations, the hybrids can endure at least 500,000 s with negligible activity loss, which is, to the best of our knowledge, two to three orders of magnitude longer than all available electrocatalysts.
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GeTe-Sb2Te3 superlattices are nanostructured phase-change materials which are under intense investigation for non-volatile memory applications. They show superior properties compared to their bulk counterparts and significant efforts exist to explain the atomistic nature of their functionality. The present work sheds new light on the interface formation between GeTe and Sb2Te3, contradicting previously proposed models in the literature. For this purpose [GeTe(1 nm)-Sb2Te3(3 nm)]15 superlattices were grown on passivated Si(111) at 230 °C using molecular beam epitaxy and they have been characterized particularly with cross-sectional HAADF scanning transmission electron microscopy. Contrary to the previously proposed models, it is found that the ground state of the film actually consists of van der Waals bonded layers (i.e. a van der Waals heterostructure) of Sb2Te3 and rhombohedral GeSbTe. Moreover, it is shown by annealing the film at 400 °C, which reconfigures the superlattice into bulk rhombohedral GeSbTe, that this van der Waals layer is thermodynamically favored. These results are explained in terms of the bonding dimensionality of GeTe and Sb2Te3 and the strong tendency of these materials to intermix. The findings debate the previously proposed switching mechanisms of superlattice phase-change materials and give new insights in their possible memory application.
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Modified amorphous GeTe, formed by the pulsed laser irradiation of as-grown GeTe, was analyzed in terms of variations in the local bonding structure using Raman spectroscopy and X-ray absorption fine structure in tandem with first-principles density functional theory. Amorphized GeTe (acquired from the crystalline phase) was compared with the modified amorphous GeTe to investigate the similarities and discrepancies between these two amorphous phases. Raman spectroscopy showed that these materials have a similar distribution of Ge-centered local structure in both phases, which is mainly composed of an octahedral-like structure. However, extended X-ray absorption fine structure results show the presence of a unique second type of Ge-Te bonding in the amorphized GeTe, which can effectively reduce the energy required for recrystallization. A computational study based on molecular dynamics simulations verified our experimental observations, including the existence of a second type of Ge-Te bonding in the amorphized phase. Moreover we distinguished the structural characteristics underlying the different amorphous phases, such as local atomic configurations and structural symmetries. This journal is
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We computed the Raman spectrum of amorphous GeTe by ab initio simulations and empirical bond polarizability models. The calculated spectrum is in very good agreement with experimental data and contains the signatures of all the peculiar local structures of the amorphous phase revealed by recent ab initio simulations, namely, tetrahedral Ge and defective octahedral sites for a fraction of Ge (mostly 4-coordinated) and for all Te (mostly 3-coordinated) atoms. In particular, the spectrum above $190\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$ is dominated by tetrahedral structures, while the most prominent peaks around 120 and $165\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$ are mainly due to vibrations of atoms in defective octahedral sites. Finally, the peak around $75\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$, which dominates the spectrum in $HV$ scattering geometry, is mostly due to vibrational modes involving threefold coordinated Te atoms.
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Oxygen-doped germanium telluride phase change materials are proposed for high temperature applications. Up to 8 at.% oxygen is readily incorporated into GeTe, causing an increased crystallisation temperature and activation energy. The rhombohedral structure of the GeTe crystal is preserved in the oxygen doped films. For higher oxygen concentrations the material is found to phase separate into GeO2 and TeO2, which inhibits the technologically useful abrupt change in properties. Increasing the oxygen content in GeTe-O reduces the difference in film thickness and mass density between the amorphous and crystalline states. For oxygen concentrations between 5 and 6 at.%, the amorphous material and the crystalline material have the same density. Above 6 at.% O doping, crystallisation exhibits an anomalous density change, where the volume of the crystalline state is larger than that of the amorphous. The high thermal stability and zero-density change characteristic of Oxygen-incorporated GeTe, is recommended for efficient and low stress phase change memory devices that may operate at elevated temperatures.
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A new X-ray absorption fine-structure (XAFS) spectroscopy beamline for fundamental and applied catalysis research, called XAFCA, has been built by the Institute of Chemical and Engineering Sciences, and the Singapore Synchrotron Light Source. XAFCA covers the photon energy range from 1.2 to 12.8 keV, making use of two sets of monochromator crystals, an Si (111) crystal for the range from 2.1 to 12.8 keV and a KTiOPO 4 crystal [KTP (011)] for the range between 1.2 and 2.8 keV. Experiments can be carried out in the temperature range from 4.2 to 1000 K and pressures up to 30 bar for catalysis research. A safety system has been incorporated, allowing the use of flammable and toxic gases such as H 2 and CO.
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Carrier-type reversal to enable the formation of semiconductor p-n junctions is a prerequisite for many electronic applications. Chalcogenide glasses are p-type semiconductors and their applications have been limited by the extraordinary difficulty in obtaining n-type conductivity. The ability to form chalcogenide glass p-n junctions could improve the performance of phase-change memory and thermoelectric devices and allow the direct electronic control of nonlinear optical devices. Previously, carrier-type reversal has been restricted to the GeCh (Ch ¼ S, Se, Te) family of glasses, with very high Bi or Pb 'doping' concentrations (B5–11 at.%), incorporated during high-temperature glass melting. Here we report the first n-type doping of chalcogenide glasses by ion implantation of Bi into GeTe and GaLaSO amorphous films, demonstrating rectification and photocurrent in a Bi-implanted GaLaSO device. The electrical doping effect of Bi is observed at a 100 times lower concentration than for Bi melt-doped GeCh glasses.
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This paper investigates Carbon-doped GeTe (GeTeC) as novel material for Phase-Change Memories (PCM). In the first part of the manuscript, a study of GeTeC blanket layers is presented. Focus is on GeTeC amorphous phase stability, which has been studied by means of optical reflectivity and electrical resistivity measurements, and on GeTeC structure and composition, analyzed by XRD and Raman spectroscopy. Then, electrical characterization of GeTeC-based PCM devices is reported: resistance drift, data retention performances, RESET current and power, and SET time have been investigated. Very good data retention properties and reduction of RESET current make GeTeC suitable for both embedded and stand-alone PCM applications, thus suggesting GeTeC as promising candidate to address some of the major issues of today’s PCM technology.
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Present-day multimedia strongly relies on re-writable phase-change optical memories. We find that, different from current consensus Ge2Sb2Te5 (GST), the material of choice in digital versatile discs---random access memory (DVD-RAM), possesses a structure similar to ferroelectric GeTe, namely that Ge and Sb atoms are located off-center giving rise to a net dipole moment. Amorphisation of both GeTe and GST results in a significant shortening of covalent bonds and a decrease in the mean-square relative displacement concomitant with a drastic change in the short-range order. We demonstrate that the order-disorder transition in GeTe and GST is primarily due to a flip of Ge atoms from an octahedral position into a tetrahedral position without rupture of strong covalent bonds. It is this nature of the transformation that ensures large changes in reflectivity, fast disk performance and repeatable switching over millions cycles.
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X-ray absorption spectra near the K/sub ..cap alpha../ edge have been measured in various iron group compounds using the intense synchrotron radiation available at the Stanford Synchrotron Research Project. In the cubic compounds KMFâ where M = Mn/sup +2/, Fe/sup +2/, Co/sup +2/, Ni/sup +2/, and Zn/sup +2/, well resolved lines were observed and assigned to the 1s ..-->.. 3d, 1s ..-->.. 4s, and 1s ..-->.. 4p transitions. The observed energies agreed rather well with the spectroscopic energy levels of the Z + 1 ion and the intensities are shown to agree with those expected on the basis of one electron transition of the form Z 1sÂ²d/sup n/(L,S) ..-->.. (Z + 1)1sÂ²d/sup n/n'l'(L'',S). The energies of the intense 1s ..-->.. 4p transition increase by about 5 V going from KFeFâ to KâNaFeFâ, but only by about 1 V from KâFe(CN)â to KâFe(CN)â. The transitions confirm that upon oxidation of the hexacyanides the iron electronic structure barely changes. In the iron sulfur protein rubredoxin, where the iron is bound to a tetrahedron of sulfurs, the 1s ..-->.. 3d transition was about seven times more intense than the same transition in an octahedrally coordinated compound. These intensities parallel those observed in the d-d transitions of optical spectra, because in both types of spectra the intensities depend upon 4p admixture. In the heme protein cytochrome c, upon oxidation the 1s ..-->.. 4p transition shifts only about 1 V to higher energies, similar to the iron hexacyanides. These results are discussed in terms of covalent bonding.
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The electrical switching behavior of GeTe-based phase change memory devices is characterized by time resolved experiments. SET pulses with a duration of less than 16 ns are shown to crystallize the material. Depending on the resistance of the RESET state, the minimum SET pulse duration can even be reduced down to 1 ns. This finding is attributed to the increasing impact of crystal growth upon decreasing switchable volume. Using GeTe or materials with similar crystal growth velocities, hence promises nonvolatile phase change memories with dynamic random access memorylike switching speeds.
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Carbon-doped GeTe is a promising material for use in phase change memories since the addition of C increases the stability of the amorphous phase. By combining x-ray total scattering experiments and ab initio molecular dynamics, we show that carbon deeply modifies the structure of the amorphous phase through long carbon chains and tetrahedral and triangular units centered on carbon. A clear signature of these units is the appearance of an additional interatomic distance, around 3.3 Å in the measured pair distribution function. Besides, the first Ge-Ge and Ge-Te distances are almost not affected by doping.
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A comprehensive and thorough review of PCM technologies, including a discussion of material and device issues, is provided in this paper. ABSTRACT | In this paper, recent progress of phase change memory (PCM) is reviewed. The electrical and thermal proper-ties of phase change materials are surveyed with a focus on the scalability of the materials and their impact on device design. Innovations in the device structure, memory cell selector, and strategies for achieving multibit operation and 3-D, multilayer high-density memory arrays are described. The scaling prop-erties of PCM are illustrated with recent experimental results using special device test structures and novel material synthe-sis. Factors affecting the reliability of PCM are discussed.
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Amorphous films having a component of the stoichiometric GeTe‐Sb 2 Te 3 pseudobinary alloy system, GeSb 2 Te 4 or Ge 2 Sb 2 Te 5 representatively, were found to have featuring characteristics for optical memory material presenting a large optical change and enabling high‐speed one‐beam data rewriting. The material films being sandwiched by heat‐conductive ZnS layers can be crystallized (low power) or reamorphized (high power) by laser irradiation of very short duration, less than 50 ns. The cooling speed of the sandwiched film is extremely high: more than 10<sup>10</sup> deg/s, which permits the molten material to convert to the amorphous state spontaneously; whereas, a low‐power pulse irradiation of the same duration changed the exposed portion into the crystalline state. The optical constant changes between the amorphous state and the crystalline state of them were measured to be large: from 4.7+i1.3 to 6.9+i2.6 and from 5.0+i1.3 to 6.5+i3.5, respectively. The crystallized portion was known to have a GeTe‐like fcc structure by an analytical experiment using transmission electron microscopy, differential scanning calorimetry, and x‐ray and electron diffraction methods. The high crystallization speed is ascribed to (1) the pseudobinary system which can form crystalline compositions without any phase separation, (2) the high symmetry of the fcc structure which is the nearest to the random amorphous structure, (3) the high‐energy difference between the amorphous state and the fcc crystal state.
Conference Paper
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The commercialization of Phase-Change Memories (PCM), based on the well-known GST compound, have been recently started, tailored for consumer applications. Despite other excellent performances (i.e. low-power, scalability,...), data retention is assured up to 85°C, still limited for the automotive market segment. Alternative active material able to comply with the stringent requirements of automotive applications should possibly exhibit higher crystallization temperature (T<sub>C</sub>) as well as higher Activation Energy (E<sub>A</sub>) with respect to GST. Recent literature shows that GeTe provides better retention, while several works put in evidence how data retention is enhanced by inclusions in pure host alloys.
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In this letter, we present a study on the electrical behavior of phase-change memories (PCMs) based on a GeTe active material. GeTe PCMs show, first, extremely rapid SET operation (yielding a gain of more than one decade in energy per bit with respect to standard GST PCMs), second, robust cycling, up to 1 ?? 10<sup>5</sup>, with 30-ns SET and RESET stress time, and third, a better retention behavior at high temperature with respect to GST PCMs. These results, obtained on single cells, suggest GeTe as a promising alternative material to standard GST to improve PCM performance and reliability.
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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.
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The liquid and amorphous structures of Ge0.15Te0.85 and GeTe alloys are characterized using combined density functional/molecular dynamics simulations. Te is threefold coordinated, in contrast with predictions of the "8-N rule," and Ge atoms (fourfold coordinated) show octahedral and tetrahedral bonding angles. Cubic local environment occurs in both materials, and GeTe shows a pronounced alternation of atomic types. Tetrahedral Ge coordination is more common in the eutectic Ge0.15Te0.85, which comprises corner- and edge-sharing GeTe4 units surrounded by Te. There is no Te segregation, and the material resembles neither GeTe nor Te. The ubiquitous cavities (voids) have been overlooked in Ge0.15Te0.85, where they comprise over 25% of the volume.
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Ageing of the amorphous phase of chalcogenide phase change materials is characterized by a large increase of their resistivity with time. This phenomenon, known as resistance drift and commonly attributed to structural relaxation, the nature of which remains unknown, has until now hindered the development of ultra-high multilevel storage devices. The origin of the resistance drift of amorphous GeTe thin films is studied here by resistivity measurements and grazing incidence x-ray absorption spectroscopy (GIXAS). The local order around Ge atoms is investigated at the Ge K-edge on a-GeTe samples previously set at different resistance drift levels by thermal annealing. In all samples, Ge–Ge and Ge–Te bonds coexist. This study demonstrates that the drift phenomenon is concomitant with structural changes linked to Ge–Ge homopolar bonds.
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Implementing on-chip non-volatile photonic memories has been a long-term, yet elusive goal. Photonic data storage would dramatically improve performance in existing computing architectures by reducing the latencies associated with electrical memories and potentially eliminating optoelectronic conversions. Furthermore, multi-level photonic memories with random access would allow for leveraging even greater computational capability. However, photonic memories have thus far been volatile. Here, we demonstrate a robust, non-volatile, all-photonic memory based on phase-change materials. By using optical near-field effects, we realize bit storage of up to eight levels in a single device that readily switches between intermediate states. Our on-chip memory cells feature single-shot readout and switching energies as low as 13.4 pJ at speeds approaching 1 GHz. We show that individual memory elements can be addressed using a wavelength multiplexing scheme. Our multi-level, multi-bit devices provide a pathway towards eliminating the von Neumann bottleneck and portend a new paradigm in all-photonic memory and non-conventional computing.
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GeTe is an end-point of the GeTe-Sb2Te3 quasibinary alloys often referred to as phase-change memory materials. The polycrystalline nature of the crystalline films used in devices and the concomitant presence of grain boundaries complicate detailed structural studies of the local structure. Recent progress in the epitaxial growth of phase-change materials offers unique possibilities for precise structural investigations. In this work, we report on results of x-ray absorption near-edge structure (XANES) studies of GeTe and Ge 2Sb2Te5 epitaxial films grown on Si and InAs substrates with (100) and (111) orientations. The results show a strong dependence of the local structure on the substrate material and especially orientation and are discussed in conjunction with polycrystalline samples and ab-initio XANES simulations.
Article
Phase-change materials are highly promising for next-generation non-volatile data storage technology. The pronounced effects of C doping on structural and electrical phase-change behaviors of Ge2Sb2Te5 material are investigated at the atomic level by combining experiments and ab initio molecular dynamics. C dopants are found to fundamentally affect the amorphous structure of Ge2Sb2Te5 by altering the local environments of GeTe tetrahedral units with stable C-C chains. The incorporated C increases the amorphous stability due to the enhanced covalent nature of the material with larger tetrahedral Ge sites. The four-membered rings with alternating atoms are reduced greatly with carbon addition, leading to sluggish phase transition and confined crystal grains. The lower RESET power is presented in the PCM cells with carbon-doped material, benefiting from its high resistivity and low thermal conductivity.
Article
Despite its simple chemical constitution and unparalleled technological importance, the phase-change material germanium telluride (GeTe) still poses fundamental questions. In particular, the bonding mechanisms in amorphous GeTe have remained elusive to date, owing to the lack of suitable bond-analysis tools. Herein, we introduce a bonding indicator for amorphous structures, dubbed “bond-weighted distribution function” (BWDF), and we apply this method to amorphous GeTe. The results underline a peculiar role of homopolar GeGe bonds, which locally stabilize tetrahedral fragments but not the global network. This atom-resolved (i.e., chemical) perspective has implications for the stability of amorphous “zero bits” and thus for the technologically relevant resistance-drift phenomenon.
Article
Phase change materials are of great interest as active layers in rewritable optical disks and novel electronic nonvolatile memories. These applications rest on a fast and reversible transformation between the amorphous and crystalline phases upon heating, taking place on the nanosecond time scale. In this work, we investigate the microscopic origin of the fast crystallization process by means of large-scale molecular dynamics simulations of the phase change compound GeTe. To this end, we use an interatomic potential generated from a Neural Network fitting of a large database of ab initio energies. We demonstrate that in the temperature range of the programming protocols of the electronic memories (500–700 K), nucleation of the crystal in the supercooled liquid is not rate-limiting. In this temperature range, the growth of supercritical nuclei is very fast because of a large atomic mobility, which is, in turn, the consequence of the high fragility of the supercooled liquid and the associated breakdown of the Stokes–Einstein relation between viscosity and diffusivity.
Article
In this paper, Cu-doped GeTe materials have been investigated for high-temperature phase change memory (PCM) application. Cu incorporated in GeTe increases the crystallization temperature, crystallization active energy, and band gap significantly. The incorporated Cu has effect on refining the crystal growth and brings more number of bonds in GeTe, and has remarkable effect on improving the thermal stability of GeTe. The 10-year data retention of Cu0.06(GeTe)0.94 and Cu0.13(GeTe)0.87 are 151 °C and 185 °C, which is higher than that of GeTe. PCM device based on Cu0.06(GeTe)0.94 exhibits faster switching speed than that of GeTe. In addition, Cu0.06(GeTe)0.94 shows endurance up to 2.8 × 103 cycles with an on/off ratio of two orders of magnitude.
Article
We discuss various mechanisms that lead to a high crystallization rate of the recording layer in a phase change disk. The role of interface layers, recording layer composition and its thickness is especially emphasized. Disks with a data bit rate beyond 40 Mbit/s have been realized.
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Bonding in the crystalline phase of GeTe-based phase-change alloys is believed to be purely p type. In this work we demonstrate that in the absence of long-range order, despite the preserved bonding angles of ˜90∘, sp3 hybridization is more favorable with a pair of nonbonding electrons localized on a Ge sp3 orbital, i.e., in contrast to the crystalline phase, amorphous GeTe is a lone-pair (LP) semiconductor. Upon disordering, tetrahedral Ge sites are formed due to unpairing of LP electrons and subsequent formation of additional Ge-Ge bonds, a process generating additional free electrons. Recombination of these electrons during the structure relaxation may be the underlying reason for the conductivity drift in the amorphous phase making the latter process analogous to persistent photoconductivity. Implications for the stability of the amorphous phase are also discussed.
Article
The effects of the addition of Si on the temperature dependence of the electrical resistance and crystallization temperature, Tx, of GeTe films were investigated by a two-point probe measurement. Tx of the (GeTe)100−xSix films increased with increasing Si content from x = 0 (188 °C) to x = 12.6 at% (297 °C). It was confirmed from XPS measurements that Si–Te bonds were formed in the (GeTe)100−xSix amorphous film by the addition of Si. The dependence of Si content on the Tx of the (GeTe)100−xSix film was calculated based on Lankhorst's model, which showed a linear relationship between Tx and the Si content. In the region of low Si content (x ≤ 3.8 at%), the obtained result was in good agreement with the calculated result. Meanwhile, in the region of high Si content (x ≥ 5.4 at%), the obtained result considerably deviated from the calculated line. This may be due to the formation of the strongest Si–Si bonds in the amorphous (GeTe)100−xSix film. It was found from XRD measurement that the (GeTe)87.4Si12.6 amorphous film showed crystallization with phase separation into α-GeTe and Si. The obtained results suggested that suitable Si content of the (GeTe)100−xSix film is in the range 3–7 at% because of the high Tx, a small temperature interval of crystallization and the absence of phase separation during crystallization.
Article
The local structure around nitrogen species in N-doped GeTe has been investigated using (soft) x-ray absorption spectroscopy and high-resolution x-ray photoemission spectroscopy. The obtained results demonstrate that while in as-deposited amorphous GeTe nitrogen is predominantly bonded to Ge atoms, upon crystallization the majority of nitrogen forms N-2 molecules that are likely to be located in the grain boundaries, with only a small fraction of nitrogen species remaining bonded to Ge. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3683522]
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Structural details of the amorphous binary GeTe and ternary Ge2Sb2Te5 (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 GeTe4−nGen (n=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 Sb2T3 component in Raman scattering; reasons about this explanation, as well as for the lack of any Te–Te bonds are briefly described.
Article
Measurements of light scattering as a function of temperature are reported for single crystalline α-GeTe. A soft mode is observed at 140 cm−2, assigned as Γ1 phonon mode, which is responsible for the α-β phase transformation. The eigenvectors of the phonon modes Γ1 and Γ3 are determined. The results provide first direct evidence that α-GeTe is a displacive ferroelectric in which the longitudinal optical mode is perfectly screened by the conduction electrons.ZusammenfassungEs wird berichtet über Messungen der Lichtstreuung an einkristallinem α-GeTe in Abhängigkeit von der Temperatur. Bei 140 cm−1 wird eine weiche Gitterschwingung der Symmetrie Γ1 beobachtet, welche verantwortlich ist für die α-β Phasenumwandlung. Die Eigenvektoren der Gitterschwingungen von Symmetrie Γ1 und Γ3 werden bestimmt. Die Ergebnisse zeigen in direkter Weise, dass α-GeTe ein Ferroelektrikum displaziver Art ist, bei dem die longitudinal optische Schwingung perfekt abgeschirmt ist durch die Leitungselektronen.
Article
Germanium-antimony-tellurite (GST) is a very attractive material not only for rewritable optical media but also for realizing solid state devices. Recently, the study of the switching mechanism between the amorphous and crystal states has actively been carried out experimentally and theoretically. Now, the role of the flip-flop transition of a Ge atom in a distorted simple-cubic unit cell is the center of discussion. Turning our viewpoint towards a much wider region beyond a unit cell, we can understand that GeSbTe consists of two units: one is a Sb2Te3 layer and the other is a Ge2Te2 layer. On the based of this simple model, we fabricated the superlattice of GST alloys and estimated their thermal properties by differential scanning calorimetry (DSC). In this paper, we discuss the proof of the Ge switch on the basis of thermo-histories.
Article
The local structure and crystallization of amorphous GeTe (a-GeTe) were examined by means of Ge K-edge EXAFS. In a-GeTe, both Ge-Ge and Ge-Te bonds were observed to exist in nearest neighbors of Ge. The average coordination number around Ge is 3.7, which is close to the tetrahedral structure. A random covalent network (RCN) model seems to be suitable for the local Structure. After a-GeTe crystallizes at 129°C, the Ge-Ge bond disappears and the Ge-Te bond length increases considerably. As temperature rises, in a-GeTe the Debye-Waller factor of the Ge-Te bond increases greatly, while that of the Ge-Ge bond increases only slightly. At the crystallization, it is found that the fluctuation of the Ge-Te bond length plays a major role in the change of the local structure and bonding state around Ge.
Article
Chalcogenide glasses are widely used in phase-change nonvolatile memories and in optical recording media for their ability to rapidly change their structure to crystalline, thus obtaining different electrical resistance and optical reflectivity. Chalcogenide glasses universally display threshold switching, that is a sudden, reversible transition from a high-resistivity state to a low-resistivity state observed in the current-voltage (I-V) characteristic. Since threshold switching controls the operating voltage and speed of phase-change memories, the predictability of the switching voltage, current, and speed is of critical importance for selecting the proper chalcogenide material for memory applications. Although threshold switching has long been recognized to be an electronic process with an intimate relation to localized states, its detailed physical mechanism is still not clear. In this work, threshold switching is explained by the field-induced energy increase in electrons in their hopping transport, moderated by the energy relaxation due to phonon-electron interaction. The energy increase leads to an enhancement of conductivity and a collapse of the electric field within the amorphous chalcogenide layer, accounting for the observed negative differential resistance at switching. Threshold switching is found to obey to a constant electrical-power condition. The proposed model generally applies to low-mobility semiconductors featuring a deep Fermi level and hopping-type conduction, and can predict the thickness, temperature, and material dependence of threshold voltage and current.
Article
The change in the local order of amorphous sputter deposited GeTe thin films irradiated with Ge+ ion and its influence on the subsequent thermal induced crystallization has been investigated by means of micro-Raman spectroscopy and in situ time-resolved reflectivity. A reduction in the Ge-rich tetrahedral species and an enhancement of the crystallization kinetics occurred in the irradiated amorphous samples. The rearrangement of the amorphous network is suggested to be related to thermal spikes effects rather than to the defects produced by the ions in the collision cascade.
Article
Through the use of first-principles Ge K-edge XANES simulations we demonstrate that the structure of melt-quenched amorphous Ge-Sb-Te is intrinsically complex and is a mixture of Ge(3):Te(3) and Ge(4):Te(2) configurations in comparable concentrations, in contrast to the as-deposited amorphous phase that is dominated by the Ge(4):Te(2) configurations. The reasons for Ge-Te polyvalency are discussed and it is argued that both configurations are compatible with the Mott 8–N rule and the definition of an ideal amorphous solid. The near-perfect Te-Te distance match between the two major configurations accounts for the high cyclability of phase-change materials. Stable compositions in the Ge-Sb-Te system are suggested.
Article
The metal K-edge absorption spectra of 3d transition metals and oxides are measured by x-ray-absorption spectroscopy. Experiments are performed on metal foils or fine powders dusted onto Scotch tape, at the Cornell High Energy Synchrotron Source. Comparisons to theory are made for both the metal and oxide data. We concentrate on the spectral nearedge structure (NES) and attempt to interpret its origins. For the metals, we find excellent agreement between the data and one-electron calculations. This agreement is much closer than previously found for the L23 edges in the same materials. The picture for the oxide near-edge structure is not so auspicious. Although symmetry-based molecular-orbital (MO) theory has been widely invoked, it is probably inadequate. Recent calculations indicate that while some of the near-edge peaks are indeed attributable to one-electron MO-type transitions whose energies are modified by the presence of the core hole, other features have no such simple origin. Finally we compare the oxide NES of the metal L23,K edges and the oxygen K edge altogether. These are found to agree with each other and to MO theory only for TiO2. The mismatch of the three NES's in the other oxides indicates the failure of one-electron theory for insulators. Instead, the near-edge structure must be dominated by core excitons, as previously shown for NiO. The presence of excitons explains the lack of agreement between the NES for core excitations of different atoms in the same solid.
Article
Effect of nitrogen and carbon doping on the structure of GeTe has been investigated using x-ray diffraction and extended x-ray absorption fine structure (EXAFS) spectroscopies. While Bragg diffraction which probes the global structure exhibits a clear transition upon doping from the rhombohedral phase to the cubic (rocksalt) phase, the local structure probed by EXAFS remains rhombohedrally distorted across the compositions studied. The apparent inconsistency between the results of the two techniques used is attributed to disordering upon doping and the resulting order-disorder transition that is “seen” by site-averaging diffraction as a displacive rhombohedral-to-cubic transition.
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We report an inelastic (Raman) light scattering study on bulk crystalline GeTe (c-GeTe) and amorphous GeTe (a-GeTe) thin films and found to show pronounced similarities in local structure between the two states. In c-GeTe, the observed Raman modes represent the Ge atoms are in three different environments, namely, tetrahedral, distorted, and defective octahedral sites. On the other hand, in a-GeTe, Raman spectrum reveals Ge sites in tetrahedral and defective octahedral environment. We suggest that the structure of c-GeTe consists of highly distorted as well as defective Ge sites, which leads to the large concentration of intrinsic defects (vacancies). These random defects would act as topological disorder in the lattice and cause the bands to develop tails at the band edges, a continuum of localized levels appearing in the gap. The present study deepens the understanding of the local atomic structure, influence of defects and its close relation to the phase-change mechanism.