Hexagonal Ge2Sb2Te5 Research Articles

Temperature-dependent thermal conductivity of Ge2Sb2Te5 polymorphs from 80 to 500 K

We report the thermal conductivity of amorphous, cubic, and hexagonal Ge2Sb2Te5 using time-domain thermoreflectance from 80 to 500 K. The measured thermal conductivities are 0.20 W m−1 K−1 for amorphous Ge2Sb2Te5, 0.63 W m−1 K−1 for the cubic phase, and 1.45 W m−1 K−1 for the hexagonal phase at room temperature. For amorphous Ge2Sb2Te5, the thermal conductivity increases monotonically with temperature when T < 300 K, showing a typical glass-like temperature dependence, and increases dramatically after heating up to 435 K due to partial crystallization to the cubic phase. For hexagonal Ge2Sb2Te5, electronic contribution to thermal conductivity is significant. The lattice thermal conductivity of the hexagonal phase shows a relatively low value of 0.47 W m−1 K−1 at room temperature and has a temperature dependence of T−1 when T > 100 K, suggesting that phonon–phonon scattering dominates its lattice thermal conductivity. Although cubic Ge2Sb2Te5 has a similar grain size to hexagonal Ge2Sb2Te5, its thermal conductivity shows a glass-like trend like that of the amorphous phase, indicating a high concentration of vacancies that strongly scatter heat-carrying phonons. These thermal transport mechanisms of Ge2Sb2Te5 polymorphs help improve the thermal design of phase change memory devices for more energy-efficient non-volatile memory.

Decoupling opposed thermoelectric properties

We create GeTe/Sb2Te3 multilayer thin films by alternating GeTe and Sb2Te3 to enhance thermoelectric properties. Crystal structure of the sample was characterized from amorphous to crystalline via annealing. The GeTe/Sb2Te3 multilayer thin film can significantly improve thermopower factor from ∼0.1 μW/mK2 to 815.1 μW/mK2 because the interfacial effect breaks down the trade-off between Seebeck coefficient and conductivity. GeTe/Sb2Te3 exhibits high Seebeck coefficient while maintaining large electrical conductivity under proper annealing at 575 K. Thus, a high power factor of 815.1 μW/mK2 obtained by the multilayer structure, which blocks carriers and rationally tunes the carrier density, leads to an enhanced Seebeck coefficient and power factor for thermoelectric devices. Improvement of electrical conduction originates from interfacial bonding and intermixing at a particular thickness ratio during element diffusion process. Results of microstructure characteristics revealed that the existence of hexagonal Ge2Sb2Te5 phases may drive the improvement of thermoelectric performance and provide us with a new way of enhancing thermoelectric devices.

Fast recovery of ion-irradiation-induced defects in Ge2Sb2Te5 thin films at room temperature

Phase-change materials serve a broad field of applications ranging from non-volatile electronic memory to optical data storage by providing reversible, repeatable, and rapid switching between amorphous and crystalline states accompanied by large changes in the electrical and optical properties. Here, we demonstrate how ion irradiation can be used to tailor disorder in initially crystalline Ge2Sb2Te5 (GST) thin films via the intentional creation of lattice defects. We found that continuous Ar+-ion irradiation at room temperature of GST films causes complete amorphization of GST when exceeding 0.6 (for rock-salt GST) and 3 (for hexagonal GST) displacements per atom (ndpa). While the transition from rock-salt to amorphous GST is caused by progressive amorphization via the accumulation of lattice defects, several transitions occur in hexagonal GST upon ion irradiation. In hexagonal GST, the creation of point defects and small defect clusters leads to the disordering of intrinsic vacancy layers (van der Waals gaps) that drives the electronic metal–insulator transition. Increasing disorder then induces a structural transition from hexagonal to rock-salt and then leads to amorphization. Furthermore, we observed different annealing behavior of defects for rock-salt and hexagonal GST. The higher amorphization threshold in hexagonal GST compared to rock-salt GST is caused by an increased defect-annealing rate, i.e., a higher resistance against ion-beam-induced disorder. Moreover, we observed that the recovery of defects in GST is on the time scale of seconds or less at room temperature.

Review of electrical contacts to phase change materials and an unexpected trend between metal work function and contact resistance to germanium telluride

Devices based on the unique phase transitions of phase change materials (PCMs) like GeTe and Ge2Sb2Te5 (GST) require low-resistance and thermally stable Ohmic contacts. This work reviews the literature on electrical contacts to GeTe, GST, GeCu2Te3 (GCuT), and Ge2Cr2Te6 (GCrT), especially GeTe due to the greater number of studies. We briefly review how the method used to measure the contact resistance (Rc) and specific contact resistance (ρc) can influence the values extracted, since measurements of low contact resistances are susceptible to artifacts, and we include a direct comparison of Au-, Pt-, Ni-, Mo-, Cr-, Sn-, and Ti-based contacts using a systematic approach. Premetallization surface treatment of GeTe, using ex situ or in situ approaches, is critical for minimizing contact resistance (Rc). Transmission electron microscopy reveals that interfacial reactions often occur and also clearly influence Rc. The lowest Rc values (∼0.004 ± 0.001 Ω mm) from the direct comparison were achieved with as-deposited Mo/Ti/Pt/Au (Ar+ plasma treatment) contacts and annealed Sn/Fe/Au (de-ionized H2O premetallization treatment). In the case of Sn-based contacts, low Rc was attributed, in part, to the formation of SnTe at the contact interface; however, for Mo-based contacts, no such interfacial reaction was observed. Comparing all contact metals tested beneath a cap of at least 100 nm of Au, Mo/Ti/Pt/Au offered the lowest contact resistance as-deposited, even though the work function of Mo is only 4.6 eV, and the low contact resistance remained stable even after annealing at 200 °C for 30 min. This trend is surprising, as high work function metals, like Ni and Pt, would be expected to provide lower Rc values when they are in contact with a p-type semiconductor like GeTe. Through materials’ characterization, an inverse relationship between the metal work function and Rc for higher work function metals can be attributed to the reactivity of many of the metals with GeTe. Studies of contacts to GST in the literature involve only a small number of contact materials (Ti, TiN, TiW, W, Pt, and graphene) and employ varied geometries for extracting contact resistance. For hexagonal GST, TiW is reported to provide the lowest ρc of ∼2 × 10−7 Ω cm2, while TiN provided the lowest reported ρc of ∼3 × 10−7 Ω cm2 to cubic GST. For the ternary PCMs GCuT and GCrT, contact resistance studies in the literature are also limited, with W being the only metal studied. While more extensive work is necessary to draw wider conclusions about trends in current transport at metal/GST, metal/GCuT, and metal/GCrT interfaces, reduction of Rc and high thermal stability are critical to engineering more efficient and reliable devices based on these materials.

Temperature-Dependent Contact Resistance to Nonvolatile Memory Materials

Emerging nonvolatile memories store data by reversible resistive switching in phase-change materials or metal oxides. As memory cell dimensions are reduced to ~10-nm scale or below, electrical contacts can dominate the device behavior, yet are often poorly understood. Here, we study the contact resistance to memory materials Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Sb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> (GST), TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> with low-current and temperature-dependent measurements. We find that the contact resistivity varies over ten orders of magnitude depending on the material; contact resistivity to cubic GST is near 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> Ω·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (~1000 times greater than the hexagonal GST) while that to HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is as high as 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> Ω·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at room temperature. Contact resistivity decreases with increasing temperature and with increasing current density, the latter due to the non-Ohmic nature of the contacts. These results are important to understand the design, scaling, and behavior of nanoscale data storage devices.

Optic phonons and anisotropic thermal conductivity in hexagonal Ge2Sb2Te5.

The lattice thermal conductivity (κ) of hexagonal Ge2Sb2Te5 (h-GST) is studied via direct first-principles calculations. We find significant intrinsic anisotropy (κa/κc~2) of κ in bulk h-GST, with the dominant contribution to κ from optic phonons, ~75%. This is extremely unusual as the acoustic phonon modes are the majority heat carriers in typical semiconductors and insulators. The anisotropy derives from varying bonding along different crystal directions, specifically from weak interlayer bonding along the c-axis, which gives anisotropic phonon dispersions. The phonon spectrum of h-GST has very dispersive optic branches with higher group velocities along the a-axis as compared to flat optic bands along the c-axis. The large optic mode contributions to the thermal conductivity in low-κ h-GST is unusual, and development of fundamental physical understanding of these contributions may be critical to better understanding of thermal conduction in other complex layered materials.

Failure Analysis of Nitrogen-Doped Ge2Sb2Te5Phase Change Memory

Phase change memory (PCM) devices, consisting of nitrogen-doped Ge2Sb2Te5 (NGST) chalcogenide, are fully integrated in the 110-nm complementary metal–oxide–semiconductor technology. By combining high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDS), we thoroughly analyze the original fail cells. The direct results of structural observation and compositional analysis of the phase change cells are displayed. The original fail cells, which are initially at the extra low-resistance state, cannot be operated to high-resistance state when applying the normal reset condition as most of the successfully reset cells. Hexagonal Ge2Sb2Te5 (GST) and TiTe2 phases are observed in the microstructure of the original fail cells. The element analysis indicates that the atomic composition of the fail cells changes to nonstoichiometric phase. Titanium atoms incorporate into the GST-based film, whereas tellurium and antimony atoms diffuse into the titanium nitride adhesive layer. The germanium atoms accumulate at the bottom electrode. It is believed that phase change materials, which undergo very high thermal budget during back-end processing at 350 °C or even high at 400 °C during the fabrication, can inevitably cause intrinsic problems in stability of the PCM devices. The irreversible modification of the NGST composition and these hexagonal GST and TiTe2 phases will directly result in the early failure of the PCM.

Evolutionary design of interfacial phase change van der Waals heterostructures.

We use an evolutionary algorithm to explore the design space of hexagonal Ge2Sb2Te5; a van der Waals layered two dimensional crystal heterostructure. The Ge2Sb2Te5 structure is more complicated than previously thought. Predominant features include layers of Ge3Sb2Te6 and Ge1Sb2Te4 two dimensional crystals that interact through Te-Te van der Waals bonds. Interestingly, (Ge/Sb)-Te-(Ge/Sb)-Te alternation is a common feature for the most stable structures of each generation's evolution. This emergent rule provides an important structural motif that must be included in the design of high performance Sb2Te3-GeTe van der Waals heterostructure superlattices with interfacial atomic switching capability. The structures predicted by the algorithm agree well with experimental measurements on highly oriented, and single crystal Ge2Sb2Te5 samples. By analysing the evolutionary algorithm optimised structures, we show that diffusive atomic switching is probable by Ge atoms undergoing a transition at the van der Waals interface from layers of Ge3Sb2Te6 to Ge1Sb2Te4 thus producing two blocks of Ge2Sb2Te5. Evolutionary methods present an efficient approach to explore the enormous multi-dimensional design parameter space of van der Waals bonded heterostructure superlattices.

Electron–phonon interaction and thermal boundary resistance at the interfaces of Ge2Sb2Te5 with metals and dielectrics

The Ge2Sb2Te5 compound is of interest for applications in phase change non-volatile memories. First-principles calculations of phonon dispersion relations and electron–phonon coupling constant provide an estimate of the electron–phonon contribution to the thermal boundary resistance at the interfaces of Ge2Sb2Te5 with dielectrics (silica) and metal electrodes (Al and TiN). The diffuse mismatch model including full phononic dispersion has been used to compute the phononic contribution to the thermal boundary resistance. The calculated value of the electron–phonon contribution to the TBR at 300 K of about 14 m2K GW−1 would dominate the TBR at the interfaces of hexagonal Ge2Sb2Te5 with the surrounding dielectrics and metals considered here once interdiffusion at the boundaries could be minimized.

Understanding the early cycling evolution behaviors for phase change memory application

The RESET current of T-shaped phase change memory cells with 35 nm heating electrodes has been studied to understand the behavior of early cycling evolution. Results show that the RESET current has been significantly reduced after the early cycling evolution (1st RESET) operation. Compared the transmission electron microscope images, it is found that the hexagonal Ge2Sb2Te5 (GST) crystal grains are changed into the grains with face centered cubic structure after the early cycling evolution operation, which is taken as the major reason for the reduced RESET current, confirmed by a two-dimensional finite analysis and ab initio calculations.

Microstructural failure in Ge2Sb2Te5phase change memory cell

The microstructural behavior of a Ge2Sb2Te5 (GST) film in a phase change memory cell has been investigated by high-resolution transmission electron microscopy (HR-TEM). The HR-TEM results indicate that both hexagonal structured GeSbTe (GST) and metastable GeTe phases appeared in the amorphous GST film after about 103 cycles of set/reset operations. These hexagonal GST grains, although the grain size is very small, are relatively not easily changed into the amorphous phase compared with the fcc GST, which results in an imperfect phase transformation during the quenching process of the reset cycle. Voids are formed at the interface close to the top electrode and the formation of the voids widens the GST film during the multiple cycles of set/reset operations. These experimental results may explain the failure mechanism of the GST memory cell.

Erratum: “The origin of non-Drude terahertz conductivity in nanomaterials” [Appl. Phys. Lett. 100, 132102 (2012)

In a recent letter, we proposed a model for the nonDrude terahertz conductivity of nanosilicon and hexagonal Ge2Sb2Te5 (GST) based on a series sequence of two contributions: the conductivities of free carriers and tunneling carriers. The best fit of the proposed model to nanosilicon experimental data also used e1, where e1 is the background dielectric constant, as one of the parameters. As the experimental THz conductivity reported for nanosilicon has been obtained from optical-pumping THz spectroscopy (Ref. 7), the effect of e1 is subtracted from the experiment itself. Thus, we do not need to include the effect of e1 in fitting the model to experimental data. Figs. 1(a) and 1(b) show the corrected results of the real and imaginary part of conductivity in nanosilicon. Deduced physical parameters are also modified (f1⁄4 0.83, nf1⁄4 5.3 10 cm , s1⁄4 2.0 10 14 s, nt1⁄4 2.0 10 cm , and st1⁄4 2.0 10 14 s). Note that the results for GST are unaltered, because the experimental data are not obtained by optical-pumping. Finally, we should comment on the way in which we used Eq. (5) to calculate the overall conductivity. We described the tunneling conductivity as rt*(x)1⁄4 rtR(x) þ irtI(x) in Eq. (3). Since Eqs. (2) and (3) in the literature have been derived for e ixt and e, respectively, we had to change i to i in rt*(x) in the actual calculations in Eq. (5) to maintain consistency and causality. Also, Ref. 7 cited in the text in line 10, column 2 on page 2 should have been 9.

&lt;i&gt;Ab Initio&lt;/i&gt; Study on Hexagonal Ge&lt;sub&gt;2&lt;/sub&gt;Sb&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;5&lt;/sub&gt;-A Phase-Change Material for Nonvolatile Memories

On the basis of ab initio total energy calculations, we have performed an extensive study on the stacking sequence and random occupation of Ge and Sb to make the same layer in stable hexagonal Ge2Sb2Te5 (h-GST), an excellent candidate for phase change random memory applications. The results demonstrate that the atomic arrangements have great effects on lattice parameter c and electronic properties of h-GST. h-GST changes from semiconductor to metallic behavior as varying the atomic sequence.

Effects of hydrostatic pressure on the electrical properties of hexagonal Ge2Sb2Te5: Experimental and theoretical approaches

A combination of experiments and first-principles method calculations has been applied to investigate the influence of the hydrostatic pressure on the electrical properties of the phase-change material hexagonal Ge2Sb2Te5 (h-GST). Experimentally, it is found that the resistance of h-GST declines monotonically with increasing hydrostatic pressure up to 0.7 GPa. Theoretically, the band-structure calculations revealed that the electronic band gap also decreases with the pressure. The hydrostatic pressure increases the conductivity of h-GST by reducing the electronic band gap. The dEg/dP obtained from theoretical calculations and the d ln ρ/dP by experimental result are in the same order of magnitude.

First-principles calculations on the energetics of nitrogen-doped hexagonal Ge2Sb2Te5

The energetics of nitrogen-doped Ge2Sb2Te5 (GST225) with a hexagonal structure was investigated using first-principles calculations. Nitrogen was considered to be incorporated into the GST225 matrix as either N atoms (2.7 at. %) or N2 molecules (5.3 at. %) at four possible interstitial sites. The formation energies of nitrogen-doped GST225 were all positive for both the atomic state and molecular state nitrogen, implying that the incorporation of nitrogen is not a spontaneous reaction with respect to the N2 molecule. The formation energies of GST225 doped with N2 molecule are much lower than those of GST225 doped with nitrogen atom for all of the interstitial sites considered. The changes in the electron densities caused by the addition of nitrogen showed that atomic nitrogen forms bonds with neighboring constituent atoms of GST225 and prefers Ge as the first nearest atom. On the other hand, N2 stays almost still and remains in the molecular state at the interstitial sites. The calculation results support two different experimental reports on the state of nitrogen in GST225.

Theoretical and Experimental Investigations of the Optical Properties of Ge2Sb2Te5 for Multi-State Optical Data Storage

Ge2Sb2Te5 (GST) is widely used as an excellent data recording material based on its fast and reversible amorphous-to-crystalline phase transformation that is accompanied by changes in the optical re ectivity and the electrical conductivity. It was demonstrated that GST can be transformed reversibly between the amorphous and the cubic phases, and, under certain conditions, it can be transformed between the cubic and the hexagonal phases as well. Thus, if we can take advantage of the di erences in the optical and the electrical properties among the amorphous, the cubic, and the hexagonal states, we could use GST as a multi-state storage material, and its storage density would be improved remarkably. In this study, we investigate the electronic and the optical properties of crystalline GST by using both ab-initio and experimental analyses. The results show that the cubic GST is semi-conductive while the hexagonal GST may exhibit much higher conductivity. The re ectivity contrast of the two crystalline states is above 8 % in the range from 400 nm to 700 nm, indicating a potential for applications of GST in multi-state optical data storage.

High-Resolution Dynamic Analysis of the Phase Transformation in Ge&lt;sub&gt;2&lt;/sub&gt;Sb&lt;sub&gt;2&lt;/sub&gt;Te&lt;sub&gt;5&lt;/sub&gt; Alloy

Phase transformation and crystal growth behavior of Ge2Sb2Te5 were investigated systematically by means of in situ heating (from room temperature to 500 oC) of amorphous Ge2Sb2Te5 alloy in a high voltage electron microscope with real-time monitoring. Large-scale crystallization occurred to amorphous Ge2Sb2Te5 around 200 oC. Large crystal growth developed on heating from 200 oC to 400 oC, and single crystalline grains grew up to 150 nm. Eventually the onset of partial melting of thin Ge2Sb2Te5 foil was at 500 oC and liquid Ge2Sb2Te5 was observed for the first time by high-resolution transmission electron microscopy. Hexagonal Ge2Sb2Te5 phase remains after a subsequent cooling.

Crystalline and amorphous structures of Ge–Sb–Te nanoparticles

We report effects of thermal annealing on the structures of Ge–Sb–Te (GST) nanoparticles synthesized by pulsed laser ablation deposition. The average diameter of the GST nanoparticles is an order of 10nm. The as-prepared sample contains nanocrystals surrounded by an amorphous phase. Further crystallization occurs during annealing. The structures of the nanocrystals and amorphous phase were studied by electron diffraction and radial distribution function analyses. The results show that the nanoparticles annealed at 100°C are crystalline, consisting of a mixture of face centered cubic (fcc) and hexagonal Ge2Sb2Te5 (dominant). In comparison, the nanoparticles annealed at 200°C are mostly fcc. The surrounding amorphous phase has similar atomic arrangements to the previously reported amorphous GST thin films.

Dielectric Constants and Endurance of Chalcogenide Phase-Change Non-Volatile Memory

AbstractInitial attempts to create memory from chalcogenide glasses (g-Ch) had limited success particularly because the first generation of these materials (labeled as CG1) has inferior endurance (about 106SET-RESET cycles). Recent progress in phase-change non-volatile memory (PC-RAM) related to superior properties of Ge2Sb2Te5(GST) alloy [1,2]. The paper answers the vital for PC-RAM development question: “Why is endurance of GST memory cells (about 1011cycles) much higher than of CG1 cells?” We show that superior endurance is related to features of –U centers [3] creation in GST during RESET of PC-RAM cells. The native –U centers exist in g-Ch due to the softness of atomic potentials [3]. They play a significant role in SET process of CG1 and GST [2,4]. The –U centers behavior in GST [5] is different compared with CG1 while other properties (threshold voltage, resistivity, etc.) are basically the same [1-4]. We found that dielectric permittivities e of CG1 and GST are also different.The e values in Ge-Sb-Te alloys films have been determined from impedance measurements in sandwich samples using method described in [6] and reported in the paper. Amorphous GST has relatively high and distinct static and optical dielectric permittivities eo=16.5 and e'=15.3 to compare with CG1 where e practically independent on frequency. Hence the second term in the effective polarization potential Ep = q^2[(1-1/e')/r + (1/e' - 1/eo)/L] is strong (here r and L are the average atomic radius and interatomic bond distance, q is the electron charge). It allows to screen the Coulomb repulsion at an –U center quite effectively in GST. Therefore polarization helps –U centers creation in amorphous GST and impedes these centers formation in crystalline hexagonal GST films where eo=38 and e'=61. In contrast to CG1 [3], the creation and destruction of –U centers during RESET and SET processes in GST [4] are not accompanied by strong plastic mechanical stresses in a memory cell. This feature (higher barrier between elastic and plastic deformations) predetermines possibility of numerous SET-RESET cycles in this alloy. Therefore, the deformation mechanism of –U centers formation in CG1 leads to inferior endurance while the polarization mechanism of their creation in GST ensures decent endurance. Second factor is hybritization of –U centers with extended states can also play role in good memory alloys. Obtained experimental e values in non-stochiometric films allow to conclude about expected endurance in the framework of the proposed model.

Electronic Properties of Amorphous and Crystalline Ge2Sb2Te5 Films

Optical and electrical properties of sputtered Ge2Sb2Te5 films in amorphous and crystalline states have been studied. The optical band-gaps of amorphous, cubic (NaCl-type), and hexagonal Ge2Sb2Te5 are 0.74, 0.5, and 0.5 eV, respectively. Electrically, the amorphous and cubic states behave as semiconductors with activation energies of 0.45 and 0.14 eV, while the hexagonal state is metallic. The resistivity decreases slightly at the melting point of ∼600°C. All the states show p-type thermoelectric power, in which the amorphous and the cubic state have the activation energies of 0.3 and 0.14 eV. Carrier parameters and electronic densities-of-states are estimated and considered.