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It is well known that liquid sulfur undergoes a polymerization transition from S8 rings to the mixture of S8 rings and polymeric chains at Tp =159°C. In addition to the transition, it has been known that there is a transformation from S8 ring to a molecular species other than S8 ring and polymeric chain below Tp since 19th century. Such molecular species can also be obtained by the illumination of light. Several models of the molecular form have been presented. However, the conclusive model which explains all experimental results has not been settled yet. Recently we measured transient photo-induced optical absorption in liquid sulfur and Munejiri et al. performed first-principles molecular-dynamics simulations to clarify the microscopic origin of the experimental results. The results of the simulations presented a plausible molecular form as the molecular species, which is a tadpole-shaped S8 cluster. We show that the molecular form clearly explains the previous experimental results. A possibility of the formation of an intimate valence alternation pair in the small cluster is also discussed.
One of the recent applications of thin chalcogenide films is rewritable optical data recording. This technology is based on reversible phase transition between crystalline and amorphous state and vice versa. Dominant materials for rewritable optical recording are Ge Sb Te and Ag In Sb Te alloys. Material research still continues due to need of increasing storage capacity and data rates. Thin Ag Sb S films were prepared by RF magnetron sputtering as potential candidates for rewritable optical data storage films.
There were prepared polycrystalline bulks of AgSbS2. Composition and homogeneity of these bulks were checked by scanning electron microscopy with energy dispersive analysis (SEM EDX) structure and bonding relations were studied by Raman spectroscopy and × ray diffraction (XRD).
Targets for RF magnetron sputtering were prepared from pulverized bulks by hot pressing technique. Targets were characterized the same way as bulks.
Composition and homogeneity of prepared thin films were characterized by SEM EDX, character (amorphous/crystalline) was studied by XRD. Optical properties (spectral dependence of refractive index) were evaluated on basis of UV Vis NIR spectroscopy and variable angle spectral ellipsometry (VASE).
Crystallization abilities were traced by thermal dependence of optical transmission of prepared thin films.
One of the most difficult problems in condensed matter physics is to describe the microscopic liquid state. Owing to the dynamical nature of the liquid state, it is not possible to discuss specific microscopic structures, only ensemble averages can be specified. Such averages can be performed via molecular dynamics simulations. A problematic issue in performing such simulations is computing accurate interatomic forces. Although classical many-body potentials can be use for simulations of covalent materials, one must effectively map quantum phenomena such as hybridization onto such potentials. This mapping is complex and lacking a well-defined prescription. This step can be avoided by employing quantum forces from the pseudopotential-density functional method. Using molecular dynamics with quantum forces, we examine the local atomic order as well as some dynamic and electronic properties of the semiconducting liquid GeTe. Near the melting temperature, the Peierls distortion responsible for the lower temperature crystal phase of GeTe is shown to manifest itself within the liquid structure. At higher temperatures in the liquid, increasing disorder leads to an eventual semiconductor-metal transition. The calculated kinematic viscosity of the liquid is found to agree with the experimental value and is shown to arise from the small diffusion coefficient of the Te atoms. Using an ensemble average, we predict the dc conductivity of the melt to be consistent with recent measurements.
The novel phase change materials Si-Sb-Te films were prepared. The crystallization temperature of films increases with the increasing of Si concentration. Phase separation was observed in the Si-Sb-Te films, the dominant phase is Sb2Te3. The melting temperature of Si-Sb-Te decreased to ~550°C lower than 640°C of Ge2Sb2Te5. The decrease of film thickness of Si-Sb-Te films is less than 2% after annealing at 400°C, which is less than ~7% of the film thickness change of Ge2Sb2Te5 film. The crystalline resistivity of Si-Sb-Te films increased and the ratio of amorphous/crystalline resistivity of Si-Sb-Te films increased also comparing with Ge2Sb2Te5 film, which is benefit to reduce the writing current and keep higher on/off ratio of phase change memory. Reversible switch was performed in the devices with Si-Sb-Te films. The device with Si14.3Sb28.6Te57.2 film can be programmed with a 100 ns SET pulse and a 20 ns RESET pulse. The Reset current is only 1.37mA for a 10μm-sized device.
The high density optical and/or thermal, electrical memories based on recording into chalcogenide thin films are currently used in industrial dimension but search for new materials continues due to request of higher recording density and shorter lasers wavelength available. The thin films of amorphous chalcogenides e.g. of Ag-As-S(Se) systems making or ternary use different techniques i.e. optically-induced silver dissolution in the binary As-S or As-S-Se chalcogenides prepared by thermal evaporation or by spin coating techniques were prepared and studied. Ternary Ag-As-S and quarternary Ag-As-S(Se) chalcogenide films were also deposited by direct-pulsed laser deposition. Films with euthectic or stoichiometric compositions were prepared. Optically-induced phase-changes in films, process kinetics and analysis of the structural changes and their potential application are described.
Sn-doped Ge-Sb-Te films on Si substrates were prepared by laser synthesis at the different growth temperatures. The compositions of Sn-doped Ge-Sb-Te films were analysized by X-ray photoelectron spectroscopy. The crystal structures of Sn-doped Ge-Sb-Te thin films with a Sn content of less than 30 at% are close to Ge2Sb2Te5. The crystallization behaviors of Sn-doped Ge-Sb-Te films were analyzed by self-developed phase change temperature tester. The crystallization temperatures of Sn4.3Ge32.9Sb28.1Te34.6, Sn9.8Ge20.3Sb28.4Te41.5 and Sn18.8Ge19.5Sb25.3Te36.4 are 141.5, 137.3 and 135.0 °C at a ramp rate of 20 °C/min, respectively. Doping Sn into Ge-Sb-Te will result in a decrease of crystallization temperature. It was also found that crystallization temperature increases with an increase of ramp rate for a phase change material. The activity energy Ea and frequency factor ¦Ô for Sn9.8Ge20.3Sb28.4Te41.5 thin films are 2.42 eV and 1.7 × 1026 Hz, respectively. The crystallization speed of Sn-doped Ge-Sb-Te is estimated to be faster than Ge2Sb2Te5.
We have demonstrated that certain chalcogenide layers within a spinning super-RENS optical disc allow to squeeze the 650 nm laser beam to a spot size as fine as 50 nm using a 15-nm chalcogenide film. The near-field light was focused at a depth of just over 30 nm after passing through a chalcogenide film. Finite-difference time-domain (FDTD) simulations also reproduced these results. We suggest that a conductive ring aperture generated in the chalcogenide layers plays an important role in the localized light focusing.
We present first principles electronic structure calculations of oxygen substitutional defects in the Sb2Te3 layered crystalline system and a model of amorphous Sb2Te3 using density functional theory (DFT). Our calculated formation energies for oxygen substitutional defects at Sb sites are above 2 eV, so most of our results are on the Sb2Te3-xOx [x = .0074 - .20] system, where one of two inequivalent Te sites are instead occupied by a single oxygen atom with formation energies between -1.2 eV and .2 eV. Defect formation energies for the system show a preference for oxygen atoms on the Te1 site at low concentrations that switches to the Te2 site at high concentrations at approximately 6 atomic percent. In agreement with experiment, we find that oxygen does widen the band gap, even at relatively low concentrations.
OUMTM (Ovonic Unified Memory), also called PCRAM (phase-change RAM) or CRAM (chalcogenide RAM) is a nonvolatile semiconductor memory technology being developed by Ovonyx, Inc. in a number of industrial joint development programs. OUM technology is based on an electrically initiated reversible amorphous to crystalline phase change process in multi-component chalcogenide alloy materials similar to those used in rewriteable optical disks. Fundamental processes in OUM devices, manufacturing technology, and progress towards commercialization of the technology will be reviewed.
Electronic structure calculations are presented for various model structures of the crystalline and amorphous phases of Ge2Sb2Te5 (GST). The structures are all found to possess a band gap of order 0.5 eV, indicating closed shell behaviour. It is pointed out that structural vacancies in A7-like GST are not electronically active. In addition, A7-like structures do not support valence alternation pair (VAP) defects, which are one model of the conduction processes in the glassy phase in non-volatile memories.
The impact of material crystallization characteristics on the switching behavior of phase change memory cells has been investigated using finite element simulation. Both a conventional vertical cell and a horizontal line cell have been analyzed, using the widely used Ge2Sb2Te5 (GST) which is a nucleation dominated material for the vertical cell, and Ag5.5In6.5Sb59Te29 (AIST) which is a growth dominated material for the horizontal cell. Nucleation and growth models were implemented for both materials. Both RESET and SET program cycles were simulated. From these simulations, it was shown that the crystallization models gave realistic results for switching voltages, currents and switching times for the two different cell types. It is found that for GST, both nucleation (at lower voltages) and growth (at higher voltages) can play an important role in the crystallization. However, for AIST, crystal growth from non-amorphized crystal regions dominated over nucleation for all program conditions. The high growth rate of AIST moreover is shown to allow much shorter SET times in the line cell compared to that of GST in the vertical cell.
Studies of amorphous (a-) semiconductors have been driven by technological advances as well as fundamental theories. Observation of electrical switching, for example, fueled early interest in a-chalcogenides. More recently a-chalcogenide switching has been applied quite successfully to DVD technology where the quest for the discovery of better-suited materials continues. Thus, switching provides researchers today with an active arena of technological as well as fundamental study. On the theoretical front, bond constraint theory and rigidity theory provide a powerful framework for understanding the structure and properties of a-materials. Applications of these theories to switching in a-chalcogenides holds the promise of finding the best composition suited for switching applications. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy is an ideally suited technique to investigate the switching properties of these materials. Results of previous EXAFS experiments will be presented and viewed through the lens of bond constraint theory and rigidity theory.
Ge2Sb2Te5 is under intense investigation for phase-change memory devices, including rewriteable DVDs where optical illumination is used to switch between the glassy and crystalline states. We investigate the influence of optical irradiation on amorphous phase. Many chalcogenides display photo-oxidation, photodarkening or photo-bleaching, but little has been reported on the Ge-Sb-Te system. Using spectroscopic ellipsometry (SE) and secondary ion mass spectrometry, we determine that the samples have a strong tendency to photo-oxidize; if this is not accounted for, then the analysis of SE data appears to show photodarkening. Other authors recently reported photodarkening in nonstoichiometric GexSb20-xTe80 [Pamukchieva et al., Proc. SPIE 5581, 608 (2004); Pamukchieva et al., J. Optoelectron. Adv. Mater 7, 1277 (2005)], but our analysis suggests that the changes were also the result of photo-oxidation. The oxide has lower value of (n, k) than Ge2Sb2Te5, and can be etched by hydrofluoric acid or water. The photo-oxidation is presumably the result of free carrier generation in the Ge2Sb2Te5. Our observation of negligible photodarkening is consistent with previous works that found less photodarkening in tellurides compared with selenides or sulfides, and that an increase in the mean coordination number, here by Ge addition, further reduces the photodarkening effect.
Initial 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 106 SET-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 1011 cycles) much higher than of CG1 cells?” We show that superior endurance is related to features of –U centers  creation in GST during RESET of PC-RAM cells. The native –U centers exist in g-Ch due to the softness of atomic potentials . They play a significant role in SET process of CG1 and GST [2,4]. The –U centers behavior in GST  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  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 , the creation and destruction of –U centers during RESET and SET processes in GST  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.
Transient photo-induced optical absorption in amorphous and liquid As2Se3 was investigated using a nanosecond pulsed laser and a specially designed optical cell. The measurements have been performed in the second and minute domain and the nanosecond and microsecond domain. From the measurements in the second and minute domain using repeated illumination, we observed the accumulation of photo-induced absorption and the decay after stopping the illumination. The durable photodarkening was also observed after the decay. The accumulated photoinduced change becomes smaller with increasing temperature and approaches to be zero around the glass transition temperature. From the measurements in the nanosecond and microsecond domain, the transient photodarkening in the time domain responded for each pulsed laser was observed. This fast photodarkening was observed even in the liquid state. This is contrast to the cases of the durable photodarkening and the transient photodarkening in the second and minute domain. The origin of the observed photoinduced changes is discussed.
Phase Change Random Access Memory [PRAM] is one of the candidate for next generation memory due to its non-volitality, high speed, high density and compatibility with Si-based semiconductor process. Ge2Sb2Te5 [GST] thin film , an active layer in this device, is utilized because it has the well-known property of rapid crystallization without phase separation in erasable compact discs industry.
We investigated the difference of the character of the GST thin film with various sputtering methods. 100nm thick GST films were prepared with DC magnetron sputtering and RF magnetron sputtering for this experiment. XRF, XRD,SEM and four point probe measurement are used to analyze the electrical properties of these films.
As for the composition of the DC sputtered GST films, Te was insufficient from target composition, while the composition of RF sputtered GST films were almost same as target composition. The RF sputtered GST films were composed of hcp by 400°C annealing. On the other hand, the DC sputtered films were mixed-phase of fcc and hcp. The resistivity of DC Sputtered GST films was higher than RF sputtered film cause of poor crystallinity. The uniformity of RF sputtered film was better than DC sputtered film.
The optical and structural properties of amorphous sputtered films of Ge2Sb2Te5 depend strongly on the preparation conditions. Films grown at higher growth rates exhibit greater local strains as indicated by the slope of the optical absorption in the exponential “band-tail” region, but these films also incorporate smaller densities of oxygen impurities. At slower growth rates the band-tail slopes are sharper (smaller local strains) but there is greater oxygen incorporation. We will discuss several experiments that suggest that the local strain relief in the films grown at slower growth rates is due to a greater ability of the atoms to rearrange on the growing surface and not to increased oxygen incorporation. Small angle x-ray scattering experiments show that the films exhibit small elliptical “voids” with long axes perpendicular to the growing surface. The approximate dimensions of these voids are 3 × 20 nm. These films can be switched optically with little change in surface topography as measured by atomic force microscopy. Electron spin resonance measurements indicate that paramagnetic defects exist in some films but are either absent or below the detection limit (~ 1018 cm-3) in most films. The implications of these results for the switching mechanisms will be discussed.
Annealing at 150 °C induces phase separation in amorphous (Ge2Se7)88Bi5Sb7 bulk samples. Spectrally resolved steady-state photoconductivity measurements indicate the presence of crystalline Bi2Se3 clusters in the annealed material, but also the subsequent gradual disappearance of this microstructure at room temperature. Similar annealing-induced metastable changes are observed in other elements of a (Ge2Se7)88BixSb12-x sample series.
This paper discusses the modeling of phase change, chalcogenide alloy, electrical memory devices. Optical disk modeling, which uses the same alloys has yielded a good understanding of how the material's structural change is related to temperature, time, nucleation of crystallites, and crystal growth. From this base, models of electrical memory behavior have been developed. Modeling the complex electronic nature of the amorphous phase is discussed and suggestions for improving device performance using these models are made.