Vaughn R. Deline

Mechanism for bistability in organic memory elements

We demonstrate that the resistive switching phenomenon observed in organic semiconductor layers containing granular metal particles conforms to a charge storage mechanism described by Simmons and Verderber [Proc. R. Soc. A 391, 77 (1967)]. The space-charge field due to the stored charge inhibits further charge injection from the electrodes. The equilibrium current–voltage curve is N shaped and the low and high resistance states are obtained by applying voltage close to the local maximum and minimum, respectively.

Evidence for segregation of Te in Ge2Sb2Te5 films: Effect on the “phase-change” stress

The authors present direct evidence for Te segregation to the grain boundaries in chalcogenide Ge2Sb2Te5 films by using transmission electron microscopy scans with a 0.5nm diameter focused probe. This finding is consistent with the observed impeded grain growth and with the post-transition relief of a “spikelike” stress, fully to the pretransition level. Te motion shows up in void formation below 200°C, a pileup of Te at the surface and its loss at higher (above 400°C) temperatures. Tuning the driving force for this segregation may be key for the optimal phase-change material design.

Irreversible modification of Ge2Sb2Te5 phase change material by nanometer-thin Ti adhesion layers in a device-compatible stack

The authors report on an interaction between chalcogenide phase-change material Ge2Sb2Te5 and a thin Ti adhesion layer considered for integration into a structure of a memory cell. Segregation of Te atoms in the chalcogenide film to the interface drives an interaction between Ti and Te atoms and formation of Ti–Te binary phases. This reaction has distinct signatures in the film-stack stress even for a nanometer-thin Ti layer. The irreversible Te segregation and modification of Ge2Sb2Te5 change the crystallization process, completely suppressing the final transformation into an otherwise stable hcp phase, and thus impacts the ultimate life cycle of such a phase-change based memory cell.

High Tc YBa2Cu3O7−x thin films on Si substrates by dc magnetron sputtering from a stoichiometric oxide target

Thin films of YBa2Cu3O7−x were deposited on Si substrates at 600–700 °C by dc magnetron sputtering from a stoichiometric oxide target. Resistivity measurement results indicate that these films are superconducting with a zero resistance Tc as high as 76 K, without further high‐temperature post‐annealing treatments. These films give both core and valence‐band x‐ray photoemission, and x‐ray diffraction spectra similar to those for superconducting films prepared with a high‐temperature post‐annealing step. No significant diffusion of Si from the substrate into the film was detected for the films deposited at 650 °C or lower, according to depth profiles obtained using secondary ion mass spectrometry.

Reaction of silicon with films of CoNi alloys: Phase separation of the monosilicides and nucleation of the disilicides

Abstract CoNi alloy films with compositions varying from 5 to 95 at.% Co were deposited onto silicon substrates by evaporation from electron beam sources. After annealing, the reaction products were analyzed by Rutherford backscattering spectrometry, Auger electron spectroscopy, secondary ion mass spectrometry and X-ray diffraction as a function of the annealing temperature (from 450 to 750 °C). The silicide formation is influenced by the isomorphism of the compounds Co2Si and Ni2Si, and CoSi2 and NiSi2, and the lack of isomorphism of CoSi and NiSi. It is observed that the formation of the monosilicides is accompanied by a non-uniform distribution of the metallic elements as a function of depth. The formation of the alloyed disilicides always occurs at temperatures much lower than those required to form NiSi2 from pure nickel. For most cases the alloyed disilicides form at temperatures even lower than that required for the formation of CoSi2 (pure). The position where the formation of the disilicides begins varies with the composition of the alloys. In some cases the initial formation of the disilicide occurs at the free surface and is followed by inward growth; in other cases this occurs inside the complex monosilicide layer. Such effects illustrate quite dramatically the importance of nucleation phenomena in the formation of these disilicides. Nucleation of the disilicide phase at the interface between CoSi and NiSi is attributed to the contribution of the entropy of mixing to the driving force for the transformation, clearly illustrating the importance of entropy in some classes of solid state reactions.

Direct evidence for abrupt postcrystallization germanium precipitation in thin phase-change films of Sb–15at.% Ge

We present evidence for the instability in the crystalline (metallic) state of binary Te-free phase-change Ge–Sb thin films considered for integration into nonvolatile nanosized memory cells. We find that while the amorphous (semiconducting) phase of eutectic Sb–15at.% Ge is very robust until Sb crystallization at 240°C, at about 350°C, germanium rapidly precipitates out. Ge precipitation, visualized directly with transmission electron microscopy, is exothermic and is found to affect the films’ reflectivity, resistance, and stress. It converts melting into a two-step process, which may seriously impact the switching reliability of a device.

The formation of disilicides from bilayers of Ni/Co and Co/Ni on silicon: Phase separation and solid solution

Abstract Bilayers of Ni/Co and Co/Ni were deposited onto silicon by means of electron beam evaporation. The annealing behavior was investigated as a function of time in the temperature range 475–550 °C by Rutherford backscattering spectrometry, Auger electron spectroscopy, scanning electron microscopy and X-ray analysis. In this temperature range it is observed that the disilicide (solid solution) nucleates and grows from a film containing both the monosilicide of cobalt and that of nickel. These nearly insoluble monosilicides are separated into layers; the disilicide grows from the interface between the NiSi and the CoSi phase. If there are equal amounts of the two metals this interface is located approximately midway between the upper surface and the silicon substrate. The disilicide grows simultaneously both toward the free surface and toward the silicon substrate. The growth kinetics of the disilicide have been analyzed and are found to be complex. The general behavior of the disilicide formation is found to be similar to that found for Co-Ni alloy films and as for the latter case indicates the primacy of nucleation over diffusion as the controlling mechanism in the disilicide formation. The site of nucleation of the disilicide at the NiSi-CoSi interface is explained by a model based on (a) the phase separation of the two monosilicides and (b) the high entropy of mixing of the mixed disilicide. In agreement with this model the nucleation of the mixed disilicides is shown to occur at temperatures much lower than those for either CoSi 2 or NiSi 2 . Given the common crystal structure and the almost identical lattice parameters, the mutual solubilities of CoSi 2 and NiSi 2 follow from elementary concepts of alloy phase theory. However, the behavior of the monosilicides is more unexpected. The limited solubilities of the monosilicides, CoSi and NiSi, in each other are shown to result from specific features of the electronic band structure of these two compounds, features which are not found for example in the isomorphous disilicides CoSi 2 and NiSi 2 .

Liquid immersion lithography: evaluation of resist issues

We address in this report a set of key questions tied to the implementation of liquid immersion lithography, from the perspective of the resist materials. We discuss the broad question of whether chemically amplified resists are capable of achieving the spatial resolution that ultimately will be required for the most advanced immersion scenario. Initial studies undertaken using model 193 nm resist materials provide some insight into how an aqueous liquid immersion process can affect the resist material.

Vanishing interfacial tension in an immiscible polymer blend

The properties of the interface between polystyrene (PS) and a random copolymer of polystyrene and poly(parahydroxystyrene) (PPHS) were modified by addition of a diblock copolymer of deuterated polystyrene (dPS) and poly(2‐vinylpyridine) (PVP). Segregation of diblock copolymer chains to the interface between the immiscible blend components is driven by the compatibility of the dPS block with the PS phase and by a strong favorable interaction between the PVP block and the PPHS units of the random copolymer. Forward recoil spectrometry was used to measure the equilibrium excess of copolymer chains at the interface as a function of the copolymer volume fraction in the bulk PS phase. A dramatic increase in the interfacial adsorption is identified with the formation of emulsified ‘‘droplet’’ phases which appear when the interfacial free energy between the immiscible blends becomes vanishingly small. Dynamic secondary‐ion mass spectrometry was used to characterize these emulsified phases, and to verify our pict...

Torwards marketable efficiency solution-processed kesterite and chalcopyrite photovoltaic devices

Although CuIn1−xGaxSe2−ySy (CIGS) chalcopyrite and Cu2ZnSn(S,Se)4 (CZTSSe) kesterite-related films offer significant potential for low-cost high-efficiency photovoltaic (PV) devices, the complicated multi-element nature of these materials generally leads to the requirement of more complex and costly deposition processes. This talk focuses on employing the unique solvent properties of hydrazine to solution-deposit CIGS and CZTSSe films for high-performance solar cells. CIGS films are deposited by completely dissolving all elements in hydrazine, solution-depositing a molecular precursor film, and heat treating in an inert atmosphere, to yield a single-phase chalcopyrite film (no post-deposition selenization required). Trace additions of Sb improve grain structure in the resulting film and enhance device performance. Devices based on a glass/Mo/spin-coated CIGS/CdS/i-ZnO/ITO structure yield power conversion efficiencies of as high as 13.6% (AM1.5 illumination; NREL certified). Analogous CZTSSe absorber layers have been processed using a hybrid hydrazine-based slurry approach, enabling liquid-based deposition of kesterite-type films and resulting device efficiencies of as high as 9.6% (AM1.5 illumination; NREL certified)—exceeding the previous kesterite performance record by ∼40%. The combination of improved efficiency, In-free absorber and solution-based processing opens opportunities for development of a low-cost and pervasive technology.