Marco Nardone

Electrical conduction in chalcogenide glasses of phase change memory

Amorphous chalcogenides have been extensively studied over the last half century due to their application in rewritable optical data storage and in non-volatile phase change memory devices. Yet, the nature of the observed non-ohmic conduction in these glasses is still under debate. In this review, we consolidate and expand the current state of knowledge related to dc conduction in these materials. An overview of the pertinent experimental data is followed by a review of the physics of localized states that are peculiar to chalcogenide glasses. We then describe and evaluate twelve relevant transport mechanisms with conductivities that depend exponentially on the electric field. The discussed mechanisms include various forms of Poole-Frenkel ionization, Schottky emission, hopping conduction, field-induced delocalization of tail states, space-charge-limited current, field emission, percolation band conduction, and transport through crystalline inclusions. Most of the candidates provide more or less satisfact...

A unified model of nucleation switching

A unified field-induced nucleation model provides a common mechanism for switching in chalcogenide phase change memory and related devices of arbitrary thickness. We employ the model to derive equations for the threshold and holding voltages in terms of material parameters and device thickness, which are in excellent agreement with previous measurements and our data.

Conductive path formation in glasses of phase change memory

We present a model of data retention for phase change memory devices in which the active medium is a thin layer of chalcogenide glass. Data retention capability is compromised when a crystalline path is spontaneously formed in the glassy host, essentially shunting the device. We determine the probability and statistics of device failure for systems in which the crystalline volume fraction is below the critical volume fraction of percolation theory. In that regime, we show that rectilinear crystalline path formation is favored and we determine the criteria for when such paths dominate over the typical percolation cluster scenario. Our analytical approach, based on modeling the formation of such paths in terms of a half-space random walk, leads to closed form expressions that relate data retention characteristics to device parameters. The model is used to examine the effects of device geometry, temperature, and external fields. The temporal statistics of device reliability are also considered for several fa...

Relaxation oscillations in chalcogenide phase change memory

The results of a comprehensive experimental study of relaxation oscillations in chalcogenide phase change memory are presented. Extending the previous work, voltage and current oscillations were measured over much longer periods of time and with a broad range of applied voltages, load resistances, and device thicknesses. The effects of various reset voltage levels and material types were also considered. Several types of oscillation patterns were observed; most were continuous through the measurement period while others exhibited few or no oscillations. Also observed were two distinct regimes of oscillations; one of stable amplitudes followed by one of decaying amplitudes. The duration of the stable regime and the total time for oscillation decay were found to be directly proportional to the device thickness. In addition, temporal drift of the threshold voltage was observed which provided a method for measuring the variation in the drift coefficient between different materials. A numerical model was devel...

Theory of electronic transport in noncrystalline junctions

A theory of electronic transport in noncrystalline junctions is developed and compared to the experimental data. Junction transport is represented as hopping in both real space and energy space, which is dominated by rare yet exponentially effective optimum channels representing favorable configurations of localized states. Our work correlates the current-voltage characteristics of noncrystalline, thin-film devices with material parameters and predicts large ideality factors that increase under light and depend on applied bias. Also, the frequently observed variations in efficiency and degradation between nominally identical devices are a natural consequence of the theory. The theory is shown to be in good qualitative agreement with our measurements extracted from a large set of experimental data on thin-film cadmium telluride/cadmium sulfide solar cells.

Degradation of CdTe Solar Cells: Simulation and Experiment

Time-dependent numerical modeling is employed in conjunction with experimental data to investigate degradation mechanisms in cadmium telluride (CdTe) solar cells. Two mechanisms are tested against the data: 1) defect generation in the junction region caused by excess charge carriers and reactant defects and 2) back barrier increase. Junction effects result in stable Jsc with significant losses in Voc and FF, in accordance with typical data for the type of light-soak stress conditions considered here. The back barrier increase causes additional FF loss. The results suggest that both mid-gap recombination centers and shallow acceptor-type defects form near the main junction as degradation proceeds. Our data reaffirm that the inclusion of copper in the back contact is associated with better initial performance and more rapid degradation. Correlations were observed between degradation rates and apparent doping hysteresis obtained from bidirectional capacitance-voltage (C-V) scans. Our time-resolved photoluminescence (PL) results show no correlation between light-soak-induced Voc loss and PL lifetime.

Towards understanding junction degradation in cadmium telluride solar cells

A degradation mechanism in cadmium telluride (CdTe/CdS) solar cells is investigated using time-dependent numerical modeling to simulate various temperature, bias, and illumination stress conditions. The physical mechanism is based on defect generation rates that are proportional to nonequilibrium charge carrier concentrations. It is found that a commonly observed degradation mode for CdTe/CdS solar cells can be reproduced only if defects are allowed to form in a narrow region of the absorber layer close to the CdTe/CdS junction. A key aspect of this junction degradation is that both mid-gap donor and shallow acceptor-type defects must be generated simultaneously in response to photo-excitation or applied bias. The numerical approach employed here can be extended to study other mechanisms for any photovoltaic technology.

Thermodynamics of second phase conductive filaments

We present a theory of second phase conductive filaments in phase transformable systems; applications include threshold switches, phase change memory, resistive memory, and shunting in thin film structures. We show that the average filament parameters can be described thermodynamically. In agreement with the published data, the predicted filament current-voltage characteristics exhibit negative differential resistance that vanishes at high currents where the current density becomes a bulk material property. Our description is extendible to filament transients and allows for efficient numerical simulation.

Shunting path formation in thin film structures

We present a model for shunt formation in thin films containing small volume fractions of conductive components, below the critical volume fraction of percolation theory. We show that in this regime shunting is due to almost rectilinear conductive paths, which is beyond the percolation theory framework. The criteria of rectilinear paths shunting versus the percolation cluster scenario are established. The time and temperature dependence of shunting statistics is predicted with possible applications in phase change memory and thin oxides.

Nucleation of plasmonic resonance nanoparticles

We predict the electromagnetic field driven nucleation of nanoparticles that provide plasmonic oscillations in resonance with the field frequency. The oscillations assume a phase that maximizes the particle polarization and energy gain due to nucleation. We derive closed form expressions for the corresponding nucleation barrier and particle shape vs. field frequency and strength, metal plasma frequency, conductivity, and the host dielectric permittivity. We show that the plasmonic polarization allows for nucleation of particles that would not be stable in zero field.