Ion beam effect on Ge-Se chalcogenide glass films: Non-volatile memory array formation, structural changes and device performance

The conductive bridge non-volatile memory technology is an emerging way to replace traditional charge based memory devices for future neural networks and configurable logic applications. An array of the memory devices that fulfills logic operations must be developed for implementing such architectures. A scheme to fabricate these arrays, using ion bombardment through a mask, has been suggested and advanced by us. Performance of the memory devices is studied, based on the formation of vias and damage accumulation due to the interactions of Ar+ ions with GexSe1-x (x=0.2, 0.3 and 0.4) chalcogenide glasses as a function of the ion energy and dose dependence. Blanket films and devices were created to study the structural changes, surface roughness, and device performance. Raman Spectroscopy, Atomic Force Microscopy (AFM), Energy Dispersive X-Ray Spectroscopy (EDS) and electrical measurements expound the Ar+ ions behavior on thin films of GexSe1-x system. Raman studies show that there is a decrease in area ratio between edge-shared to corner-shared structural units, revealing occurrence of structural reorganization within the system as a result of ion/film interaction. AFM results demonstrate a tendency in surface roughness improvement with increased Ge concentration, after ion bombardment. EDS results reveal a compositional change in the vias, with a clear tendency of greater interaction between ions and the Ge atoms, as evidenced by greater compositional changes in the Ge rich films

Evolution of Chemical Structure During Silver Photodiffusion into Chalcogenide Glass Thin Films

The change of chemical structure resulting after X–ray and photo-induced silver diffusion into chalcogenide glass (ChG) thin films is monitored by high resolution X-ray photoelectron spectroscopy (XPS). As40S60 and Ge30Se70 thin films, which are based on pyramids and tetrahedral structural units, are investigated as model materials. Survey, core level (As 3d, S 2p, Ge 3d, Ge 2p, Se 3d, Ag 3d5/2, O 1s, C 1s) and valence band spectra have been recorded and analyzed. Reference point for the binding energy is established by the subsequent deposition of thin gold film on top of the measured samples. The chemical structure gradually changes during diffusion of silver in all the samples. The mechanism of change depends on the chemical composition, thickness of the diffused silver layer and conditions of irradiation. It is revealed that surface oxygen can play important role in the Ag photodiffusion process, leading to phase separation on the surface of the films. Photodiffusion of Ag into As40S60 film leads to the formation of a uniform ternary phase and arsenic oxides on the surface. The formation of ethane-like Ge2(S1/2)6 units together with germanium oxidation are the main outcomes of X-ray induced Ag diffusion into Ge30Se70 film

Structural Transformation in Ge\u3csub\u3e\u3cem\u3ex\u3c/em\u3e\u3c/sub\u3eS\u3csub\u3e100−x\u3c/sub\u3e (10 ≤\u3cem\u3e x \u3c/em\u3e≤ 40) Network Glasses: Structural Varieties in Short-Range, Medium-Range, and Nanoscopic Scale

Precise x-ray diffraction measurements using high-energy x rays of synchrotron radiation and systematic Raman scattering measurements were carried out for GexS100−x (10 ⩽ x ⩽ 40) network glasses. The structural models of the network glasses were proposed based on the results. In the stoichiometric composition Ge33S67, GeS4 tetrahedral units are connected forming either corner-sharing or edge-sharing structures. In the S-rich glasses, S atoms are inserted between two neighboring GeS4 tetrahedra, resulting in a flexible floppy network. In a much more S-rich region, some S8 ring molecules are isolated from the network, and assemble to form a crystal in nanoscopic scale. In this respect, Ge10S90 samples are regarded as crystallized glasses. In the Ge-rich region, the GeS4 tetrahedra are connected with bridging Ge atoms. The connection makes a new rigid network. The bridging Ge-S bond is weaker than the intratetrahedron bond, and this leads to drastic changes in the optical properties

Study of the Sorption Properties of Ge20Se80 Thin Films for NO2 Gas Sensing

In this study we investigated the sorption ability of Ge20Se80 thin films applied as active layers of quartz crystal microbalance (QCM) for NO2 gas sensing. To identify the chalcogenide system appropriate for gas sensing, we provided data for the packing fraction of a number of chalcogenide systems and discussed their suitability. We performed Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and atom force microscopy (AFM) measurements on the thin films both before and after gas absorption, which showed that the introduced gas molecules interact electrostatically with the chalcogen atoms of the host material and initiate some degree of structural changes in it. The weight change due to NO2 gas absorption was measured by frequency change of the resonator. The absorbed mass increased monotonically with the thickness of chalcogenide films and the NO2 gas concentration. At the conditions of our experiment, up to 11.4 ng of the gas was absorbed into 200nm thick Ge20Se80 film at 5000 ppm NO2 concentration. The process of gas molecules absorption is found irreversible at the purging conditions.Comment: arXiv admin note: substantial text overlap with arXiv:1204.044

Electron Beam Effects in Ge–Se Thin Films and Resistance Change Memory Devices

Chalcogenide glasses are the advanced materials of choice for the emerging nanoionic memory devices – conductive bridge random access memory (CBRAM). To understand the nature of the effects occurring in these devices under influence of electron-beam radiation, the interaction of blanked chalcogenide films and nanostructured films containing chalcogenide glass and silver (Ag) source are studied. Raman spectroscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction are used for establishing the structural and compositional effects occurring under radiation. They have strong compositional dependence with the stoichiometric compositions being most stable showing less structural changes after radiation. These effects are associated with the availability of lone-pair electrons, their participation in the bonding configurations and the coupling of electron states in the bandgap. They are further enhanced in the bilayers by silver diffusion in the chalcogenide matrix, as a result of interaction with electrons. These effects are used to interpret the electrical performance of CBRAM devices after radiation. The devices are characterized by their resistance states, threshold voltage and endurance. Those based on selenium-rich and stoichiometric composition undergo continuous parameters changes with increase in the radiation dose while in the devices based on germanium-rich composition a counter play of the structural changes and expulsion of silver occur

Structural Study of Ag-Ge-S Solid Electrolyte Glass System for Resistive Radiation Sensing

Solid electrolytes based on chalcogenide glasses have been one of the most promising candidates for the next generation non-volatile memories. Here we propose a new application of chalcogenide solid electrolytes for low cost, high performance microelectronic radiation sensor that reacts to γ-radiation to produce an easily measurable change in electrical resistance. The active layer material is Ag-doped GeS thin film glasses and several compositions of GeS base glasses were tested for best resistive sensing capability. Energy-dispersive X-ray spectroscopy (EDS) was used for elemental analysis and Raman scattering spectroscopy was measured to determine the structural details and radiation induced structural changes. We also present initial electrical measurement results with test devices

Class-specific restrictions define primase interactions with DNA template and replicative helicase

Bacterial primase is stimulated by replicative helicase to produce RNA primers that are essential for DNA replication. To identify mechanisms regulating primase activity, we characterized primase initiation specificity and interactions with the replicative helicase for gram-positive Firmicutes (Staphylococcus, Bacillus and Geobacillus) and gram-negative Proteobacteria (Escherichia, Yersinia and Pseudomonas). Contributions of the primase zinc-binding domain, RNA polymerase domain and helicase-binding domain on de novo primer synthesis were determined using mutated, truncated, chimeric and wild-type primases. Key residues in the β4 strand of the primase zinc-binding domain defined class-associated trinucleotide recognition and substitution of these amino acids transferred specificity across classes. A change in template recognition provided functional evidence for interaction in trans between the zinc-binding domain and RNA polymerase domain of two separate primases. Helicase binding to the primase C-terminal helicase-binding domain modulated RNA primer length in a species-specific manner and productive interactions paralleled genetic relatedness. Results demonstrated that primase template specificity is conserved within a bacterial class, whereas the primase–helicase interaction has co-evolved within each species

Three-Dimensional Structure of N-Terminal Domain of DnaB Helicase and Helicase-Primase Interactions in Helicobacter pylori

Replication initiation is a crucial step in genome duplication and homohexameric DnaB helicase plays a central role in the replication initiation process by unwinding the duplex DNA and interacting with several other proteins during the process of replication. N-terminal domain of DnaB is critical for helicase activity and for DnaG primase interactions. We present here the crystal structure of the N-terminal domain (NTD) of H. pylori DnaB (HpDnaB) helicase at 2.2 Å resolution and compare the structural differences among helicases and correlate with the functional differences. The structural details of NTD suggest that the linker region between NTD and C-terminal helicase domain plays a vital role in accurate assembly of NTD dimers. The sequence analysis of the linker regions from several helicases reveals that they should form four helix bundles. We also report the characterization of H. pylori DnaG primase and study the helicase-primase interactions, where HpDnaG primase stimulates DNA unwinding activity of HpDnaB suggesting presence of helicase-primase cohort at the replication fork. The protein-protein interaction study of C-terminal domain of primase and different deletion constructs of helicase suggests that linker is essential for proper conformation of NTD to interact strongly with HpDnaG. The surface charge distribution on the primase binding surface of NTDs of various helicases suggests that DnaB-DnaG interaction and stability of the complex is most probably charge dependent. Structure of the linker and helicase-primase interactions indicate that HpDnaB differs greatly from E.coli DnaB despite both belong to gram negative bacteria