As-deposited Amorphous Ge2Sb2Te5 Research Articles

Medium-range structural order in amorphous Ge2Sb2Te5 phase change material

The crystallization rate is an important factor for the application of phase change materials as memory devices, which, thus, motivates a strong interest in the underlying crystallization processes. We analyzed the crystallization kinetics of as-deposited amorphous Ge2Sb2Te5 by differential scanning calorimetry, revealing that the thermal treatment below the crystallization onset temperature effectively enhances the crystallization rate compared to the as-deposited samples. Variable resolution fluctuation electron microscopy was carried out to characterize the corresponding amorphous structure of the as-deposited films and the differently annealed states regarding their medium-range order (MRO). As a result, relatively large MRO correlation lengths in the range of 3–4 nm were observed, whereas the MRO volume fraction doubled after longer annealing treatments, demonstrating the importance of MRO and MRO tuning for optimizing the properties of amorphous phase change materials. The increased volume fraction of the MRO seems to also favor an increased nanoindentation hardness.

Two-stage conductivity switching of GST thin films induced by femtosecond laser radiation

Crystallization of as-deposited amorphous Ge2Sb2Te5 films (180 nm) by femtosecond pulsed laser radiation have been studied using in-situ electrical resistance measurements with temporal resolution. The results show that a double pulses impact is optimal for the controllability of the crystallization process. In this case, the switching time of the film resistance is less than 100 ns and is associated with the formation of conducting crystalline paths in the material after the pulse laser impact. We have proposed a model of the electrical conductivity dynamics caused by impact of the ultrashort laser radiation, which is well related to experimental data.

(Digital Presentation) Calculation of the Energy Band Diagram and Estimation of Electronic Transport Parameters of Metastable Amorphous Ge2Sb2Te5

Phase change memory (PCM) is a high speed, high density non-volatile resistive memory technology that utilizes different phases (crystalline and amorphous) of phase change materials such as Ge2Sb2Te5 (GST) to store information [1]. Here, the material undergoes two types of reversible switching phenomena: (i) Ovonic Threshold Switching (OTS), which causes the amorphous phase of the material to switch from a highly resistive state to conductive state with application of high electric fields, resulting in current flow and (ii) Ovonic Memory Switching (OMS), which is due to the change of the phase of the material between amorphous and crystalline phases induced by heating [2]. Even though PCM entered high volume manufacturing, the electronic properties of the phase change materials are still not well understood [3,4]. In this work, we construct the energy band diagram of amorphous GST as a function of temperature using a temperature dependent model of effective activation energy of conduction in metastable amorphous GST [5], which we obtained from high-speed experiments on GST line-cells [6]. Assuming the bandgap energy to linearly decrease with temperature [7,8] and p-type conduction (based on positive Seebeck coefficients measured in a wide temperature range [9]) , we determine a temperature dependent Fermi energy level from 0K to melting temperature, Tmelt ~ 858 K. Liquid GST is expected to become metallic (bandgap collapsing to 0) at ~ 894K, based on the experimental results. We also estimate the carrier concentration at Tmelt utilizing the latent heat of fusion (126 kJ/kg) [10] to be pmelt = 1.47 x 1022 cm-3. Using the melt resistivity measured on GST thin film, we calculate the carrier mobility at melting point as µmelt ~0.187 cm2/V-s, close to the previously reported value of 0.15 cm2/V-s based on crystalline state mobility and a density of states calculation [2]. Assuming a weak temperature dependence of the mobility [5], we obtain the carrier concentration of ~3.37× 1017 cm-3 at room temperature which lies within the ~1017-1018 cm-3 range estimated in a former study [3]. Finally, we calculate conduction activation energy of as-deposited amorphous GST from temperature dependent Seebeck coefficient measured simultaneously with the resistance [9]. The activation energy varies as a parabolic function of temperature where it starts from 0 eV at 0 K, reaches a peak of ~0.257 eV near glass transition temperature (~400 K) with the room temperature value of ~0.24 eV and becomes 0 eV again at ~810 K. We also utilize the Seebeck coefficient measurements along with the band edges and Fermi energy level information from the energy band diagram to calculate the ratio of minority carrier concentration to the total carrier concentration as a function of temperature; this is useful to predict the temperature beyond which bipolar conduction becomes significant.Acknowledgment: Analysis is performed with the support of US National Science Foundation (NSF) award ECCS 1711626. The experimental data used for this analysis were collected with the support of US NSF DMR-1710468 on devices fabricated with the support of US Department of Energy Office of Basic Energy Sciences.

Ultrafast temporal-spatial dynamics of amorphous-to-crystalline phase transition in Ge2Sb2Te5 thin film triggered by multiple femtosecond laser pulses irradiation

Studies have shown that the crystallization phase state of Ge2Sb2Te5 (GST) can be reversibly modulated by femtosecond (fs) laser multiple pulses, which have excellent applications in reconfigurable multi-level operation fields. In this study, the temporal-spatial crystalline evolution dynamics of amorphous GST film is investigated during two fs laser pulses excitation through a pump-probe shadowgraph imaging technique. A quasi-amorphous phase state, which is different from that in the initial as-deposited amorphous GST, is emerged through the first fs laser pulse excitation with a pulse energy lower than crystallization threshold. The experimental results reveal that a crystallization enhancement effect can be induced through the second pulse excitation based on this quasi-amorphous surface structure. The stimulative cluster generated in the quasi-amorphous reduces the amorphous-to-crystalline phase transition threshold for the second fs laser pulse irradiation. The spatially-resolved phase-transition threshold extension effect in a horizontal direction is proposed with the increasing pulse number to summarize the mechanism of the crystallization enhancement effect. The specific-grain-appearance (coarse grains and fine grains representing different phase transition approach) distributed area induced by single and double fs laser pulses irradiation are experimentally demonstrated corresponding to threshold extension theory.

Isothermal and CW laser crystallization of amorphous Ge2Sb2Te5 thin films

Transmission electron microscopy results are presented for as-deposited amorphous Ge2Sb2Te5 thin films after theirs isothermal annealing and CW laser illumination. Obtained microphotographs suggested that the crystallization process was driven solely by heterogeneous nucleation on the film boundary. The simplified model of steady-state crystallization process was developed for the case of heterogeneous nucleation mechanism. The values of nucleation rate and growth rate for isothermal and CW laser crystallization are calculated. The calculated values are in reasonable agreement with both our experimental data and with results previously published by the other authors.

Crystallization Kinetics of GeSbTe Phase-Change Nanoparticles Resolved by Ultrafast Calorimetry.

Although nanostructured phase-change materials (PCMs) are considered as the building blocks of next-generation phase-change memory and other emerging optoelectronic applications, the kinetics of the crystallization, the central property in switching, remains ambiguous in the high-temperature regime. Therefore, we present here an innovative exploration of the crystallization kinetics of Ge2Sb2Te5 (GST) nanoparticles (NPs) exploiting differential scanning calorimetry with ultrafast heating up to 40 000 K s–1. Our results demonstrate that the non-Arrhenius thermal dependence of viscosity at high temperature becomes an Arrhenius-like behavior when the glass transition is approached, indicating a fragile-to-strong (FS) crossover in the as-deposited amorphous GST NPs. The overall crystal growth rate of the GST NPs is unraveled as well. This unique feature of the FS crossover is favorable for memory applications as it is correlated to improved data retention. Furthermore, we show that methane incorporation during NP production enhances the stability of the amorphous NP phase (and thereby data retention), while a comparable maximum crystal growth rate is still observed. These results offer deep insight into the crystallization kinetics of nanostructured GST, paving the way for designing nonvolatile memories with PCM dimensions smaller than 20 nm.

Ultrathin phase-change coatings on metals for electrothermally tunable colors

Metal surfaces coated with ultrathin lossy dielectrics enable color generation through strong interferences in the visible spectrum. Using a phase-change thin film as the coating layer offers tuning the generated color by crystallization or re-amorphization. Here, we study the optical response of surfaces consisting of thin (5–40 nm) phase-changing Ge2Sb2Te5 (GST) films on metal, primarily Al, layers. A color scale ranging from yellow to red to blue that is obtained using different thicknesses of as-deposited amorphous GST layers turns dim gray upon annealing-induced crystallization of the GST. Moreover, when a relatively thick (>100 nm) and lossless dielectric film is introduced between the GST and Al layers, optical cavity modes are observed, offering a rich color gamut at the expense of the angle independent optical response. Finally, a color pixel structure is proposed for ultrahigh resolution (pixel size: 5 × 5 μm2), non-volatile displays, where the metal layer acting like a mirror is used as a heater element. The electrothermal simulations of such a pixel structure suggest that crystallization and re-amorphization of the GST layer using electrical pulses are possible for electrothermal color tuning.

Work function contrast and energy band modulation between amorphous and crystalline Ge2Sb2Te5 films

The work function (WF) is of crucial importance to dominate the carrier transport properties of the Ge-Sb-Te based interfaces. In this letter, the electrostatic force microscopy is proposed to extract the WF of Ge2Sb2Te5 (GST) films with high spatial and energy resolution. The measured WF of as-deposited amorphous GST is 5.34 eV and decreases drastically after the amorphous GST is crystallized by annealing or laser illumination. A 512 × 512 array 2D-WF map is designed to study the WF spatial distribution and shows a good consistency. The WF contrast between a-GST and c-GST is ascribed to band modulation, especially the modification of electron affinity including the contribution of charges or dipoles. Then, the band alignments of GST/n-Si heterostructures are obtained based on the Anderson's rule. Due to the band modulation, the I-V characteristics of a-GST/Si heterojunction and c-GST/Si heterojunction are very different from each other. The quantitative relationship is calculated by solving the Poisson's equation, which agrees well with the I-V measurements. Our findings not only suggest a way to further understand the electrical transport properties of Ge-Sb-Te based interfaces but also provide a non-touch method to distinguish crystalline area from amorphous matrix with high spatial resolution.

Controllable crystallization of Ge2Sb2Te5 phase-change memory thin films driven by multiple femtosecond laser pulses

In this study, controllable crystallization processes of as-deposited amorphous Ge2Sb2Te5 phase-change memory thin films driven by multiple femtosecond laser pulses were investigated in detail. The threshold effects of pulse fluence and the number of pulses were analyzed comprehensively using real-time reflectivity measurements and two-temperature model calculations. The different optical transients indicated three kinds of crystallization processes at low, medium, and high fluences. These results may provide further insights into the ultrafast phase-transition mechanics and are useful in the design of programmable multi-level logic devices based on phase change memory materials.

Preferred orientation of nanoscale order at the surface of amorphous Ge2Sb2Te5 films

We report evidence that as-deposited amorphous Ge2Sb2Te5 thin films contain nanoscale clusters that exhibit a preferred orientation, attributed to the earliest stages of heterogeneous nucleation. Fluctuation transmission electron microscopy reveals structural order in the samples, but (220)-related contributions are suppressed. When homogeneous nucleation is promoted via electron bombardment, the sample remains diffraction amorphous but the (220) contribution appears. We simulated data for randomly oriented nanoscale order using ab initio molecular-dynamics models of Ge2Sb2Te5. The simulated (220) contribution always has larger magnitude than higher-order signals; thus, the lack of the experimental signal indicates a significant preferred orientation.

Comparison of optical transients during the picosecond laser pulse-induced crystallization of GeSbTe and AgInSbTe phase-change thin films: Nucleation-driven versus growth-driven processes

Direct comparison of the real-time in-situ crystallization behavior of as-deposited amorphous Ge2Sb2Te5 (GeSbTe) and Ag8In14Sb55Te23 (AgInSbTe) phase-change thin films driven by picosecond laser pulses was performed by a time-resolved optical pump-probe technique with nanosecond resolution. Different optical transients showed various crystallization processes because of the dissimilar nucleation- and growth-dominated mechanisms of the two materials. The effects of laser pulse fluence, thermal conductive structure, and successive pulse irradiation on their crystallization dynamics were also discussed. A schematic was then established to describe the different crystallization processes beginning from the as-deposited amorphous state. The results may provide further insight into the phase-change mechanism under extra-non-equilibrium conditions and aid the development of ultrafast phase-change memory materials.

Switching and memory effects in partly crystallized amorphous Ge2Sb2Te5 films in a current controlled mode

Switching and memory effects in as-deposited amorphous Ge2Sb2Te5 films with a considerable concentration of crystalline nuclei have been investigated. Variation of the phase composition of the sample has been confirmed by Raman spectroscopy data. The influence of nuclei on parameters of the current voltage characteristic has been studied. Significant variation of initial resistance and threshold voltage due to a different nuclei configuration has been observed. In some cases the current voltage characteristic was monotonous i.e. the intrinsic S-shape of the current–voltage characteristic disappeared and memory recording occurred without prior switching. The measurements were made in a current controlled mode which allowed conducting a thorough investigation of switching and current filament formation.

Different crystallization processes of as-deposited amorphous Ge2Sb2Te5 films on nano- and picosecond single laser pulse irradiation

The crystallization dynamics of as-deposited amorphous Ge2Sb2Te5 films induced by nano- and picosecond single laser pulse irradiation is studied using in situ reflectivity measurements. Compared with nanosecond laser pulse, the typical recalescence phenomenon did not appear during the picosecond laser pulse-induced crystallization processes when the pulse fluence gradually increased from crystallization to ablation threshold. The absence of melting and recalescence phenomenon significantly decreased the crystallization time from hundreds to a few tens of nanoseconds. The role of pulse duration time scale on the crystallization process is qualitatively analyzed.

Band gap widening with time induced by structural relaxation in amorphous Ge2Sb2Te5 films

In a phase change memory the device resistance corresponding to the amorphous phase monotonically increases with time after the reset programming operation. This phenomenon, called drift, affects the stability of the high resistive state, namely the reset state. In this work we investigate the resistance-drift process through ellipsometric measurements as a function of time in thin film of as-deposited amorphous Ge2Sb2Te5 alloy. We show a tight correlation between the resistance increase with time and the optical band gap widening extracted by ellipsometric measurements. This characterization supports the drift origin due to a structural atomic rearrangement of the amorphous network affecting the band structure that, in particular, promotes the increase of the energy gap and the reduction of localized states within the energy gap.

Evidence for correlated structural and electrical changes in a Ge2Sb2Te5thin film from combined synchrotron X-ray techniques and sheet resistance measurements duringin situthermal annealing

The temperature-induced phase transition in an as-deposited amorphous Ge2Sb2Te5(GST) thin film was studied by a unique combination ofin situsynchrotron techniques (diffraction and reflectivity of X-rays) and sheet resistance measurements. The combination of these characterization techniques allowed the simultaneous extraction of structural (density and film thickness) and electrical characteristics of the GST film during its thermal annealing. It is shown that, at 425 (3) K, the appearance of diffraction peaks associated with a metastable crystalline cubic phase is unambiguously correlated to a density increase in combination with a layer thickness reduction and a resistivity switch towards a lower-resistance state. Under the present annealing conditions, the Ge2Sb2Te5film consists of a polycrystalline layer capped by an amorphous layer that strongly degrades the electrical conductivity.

Properties of Phase Change Materials Modified by Ion Implantation

ABSTRACTIon implantation of germanium and carbon ions into thin films of Ge2Sb2Te5 (GST) and GeTe was applied to modify the properties of these phase change materials. It was found that it is possible to amorphize crystalline GST and GeTe using ion implantation for optimized ion doses and energies which depend on the film thickness, ion species and capping layer. A relatively low minimum dose is required for complete amorphization as judged by the absence of diffraction peaks in x-ray diffraction (XRD) scans. It is 4–5×1013 cm−2 for germanium implantation into GST, and slightly higher (1014 cm−2) for germanium implantation into GeTe. The properties of the re-amorphized films depend on ion species, dose and energy. The re-crystallization temperature of re-amorphized GST by ion implantation is comparable or higher than as-deposited amorphous GST. Carbon implantation in particular leads to a large increase in the crystallization temperature Tx. A carbon dose of 1016 cm−2 implanted into 20 nm amorphous GST yielded a crystallization temperature of 300 ºC, much higher than the crystallization temperature of 160 ºC we recorded for as-deposited, amorphous GST. Similarly, high dose carbon implantation into amorphous GeTe leads to large increase in Tx. We recorded a shift in Tx from 195 ºC for as-deposited GeTe to 400 ºC for C-implanted GeTe. Crystalline GeTe re-amorphized by a low dose germanium ion implantation exhibits a re-crystallization temperature below Tx of as-deposited amorphous GeTe and Tx increased with the implanted Ge dose to a crystallization temperature above that of unimplanted GeTe. Ion implantation can be regarded an additional tool to create phase change materials with different and improved switching properties that cannot be achieved by conventional sputter deposition.

Pressure-induced crystallization of amorphous Ge2Sb2Te5

Using in situ x-ray diffraction, we demonstrate a pressure-induced crystallization of as-deposited amorphous Ge2Sb2Te5 (a-GST) into a body-centered-cubic (bcc) solid solution at 28 GPa, and the back transformation from the bcc-GST to a-GST. A large hysteresis loop was observed, as the bcc-GST was retained until 15 GPa. Comparisons have been made, employing the x-ray data and the structural information obtained from ab initio molecular dynamics simulations, between the as-deposited a-GST and the a-GST obtained from the pressure-induced collapse of the rocksalt GST, both at a high hydrostatic pressure (20 GPa) prior to their crystallization to bcc. The results suggest that both routes have resulted in the same high-pressure amorphous state, which explains their crystallization into bcc-GST at similar pressures.

Direct observations of Ge2Sb2Te5 recording marks in the phase-change disk

Atomistic structures of the Ge2Sb2Te5 thin film in the real phase-change disk have been investigated using transmission electron microscopy (TEM). As-deposited amorphous Ge2Sb2Te5 films were laser-irradiated for initialization (crystallization) and recording. Cross-sectional TEM observations revealed that the recording mark was fully amorphized by laser irradiation. A slight difference between the as-deposited and the laser irradiation-induced amorphous Ge2Sb2Te5 was observed in the intensity profile of nanobeam electron diffraction patterns and atomic pair distribution functions. This difference was attributed to structural relaxation of amorphous Ge2Sb2Te5, which gives rise to the alteration of chemical order.

Characteristics at high electric fields in amorphous Ge2Sb2Te5 films

Current–voltage (I–V) characteristics have been studied in as-deposited amorphous Ge2Sb2Te5 films. At low electric fields, ohmic behavior is observed. On the other hand, at high electric fields (E>106V/m) the current becomes non-ohmic. Analysis of the data shows the existence of Poole–Frenkel conduction in which lnI vs V1/2 curves are found to be straight lines. Pre-switching behaviors are also discussed from electrode configuration.

Structural Evolution of Amorphous Ge2Sb2Te5 Thin Films in the Transition to the Crystalline Phase

ABSTRACTThe crystallization kinetics of as-deposited amorphous Ge2Sb2Te5 thin films has been measured by in situ time resolved reflectivity. X-ray diffraction and Raman scattering analyses of partially transformed samples allowed to correlate the evolution of the transition to the structural modification in the long and short range configuration. The experimental results evidenced that during the early stages of crystallization there is a reduction of Ge-Te tetrahedral bonds, characteristics of the Ge coordination in amorphous Ge2Sb2Te5 films.