V. G. Karpov

Fundamental drift of parameters in chalcogenide phase change memory

We present the data on temporal (t) drift of parameters in chalcogenide phase change memory that significantly complement the earlier published results. The threshold voltage Vth and the amorphous state resistance R are shown to drift as ΔVth∝v ln t and R∝tα in broad intervals spanning up to nine decades in time; the drift coefficient v depends on glass parameters and temperature, but does not depend on device thickness. We have demonstrated that drift saturates at long enough times that can be shorten with temperature increase. All available data on drift dynamics are fully consistent with the classical double-well-potential model, which gives simple analytical expressions for the observed temporal dependencies including numerical parameters.

Nucleation switching in phase change memory

The authors propose a simple physical model of threshold switching in phase change memory cells based on the field induced nucleation of conductive cylindrical crystallites. The model is solved analytically and leads to a number of predictions including correlations between the threshold voltage Vth and material parameters, such as the nucleation barrier and radius, amorphous layer thickness, as well as Vth versus temperature and switching delay time. The authors have carried out verifying experiments, and good agreement is achieved.

Evidence of field induced nucleation in phase change memory

Experimental data on switching in phase change memory testify in favor of its underlying nucleation mechanism with field dependent nucleation barrier. Similar to the standard nucleation, switching occurs after exponentially long delay time when the conditions are not favorable enough, e.g., the nucleation barrier not low or the temperature not high enough. The switching statistics is found to be consistent with the nucleation mechanism as well. A theoretical model of nucleation switching is outlined.

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.

Blocking thin-film nonuniformities: Photovoltaic self-healing

An approach is developed to block the effects of lateral nonuniformities in thin-film semiconductor structures. The nonuniformity modulates the surface photovoltage distribution. When exposed to light and immersed in a proper electrolyte, this distribution will generate laterally nonuniform electrochemical reactions. Such treatments result in a nonuniform interfacial layer that balances the original nonuniformity. This approach has been implemented for CdTe/CdS photovoltaic devices, in which it improved the device efficiency from 1%–3% to 11%–12%.

Back contact and reach-through diode effects in thin-film photovoltaics

The physics of back contact effects in photovoltaic devices is revisited. We show that the back contact Schottky barrier can act in either back-diode or reach-through diode regimes. This understanding predicts that rare local spots with low back barrier hole transparency and/or weak main junctions can shunt the photocurrent thus decreasing the measured open-circuit voltage and device efficiency. We derive several more specific predictions of our model and verify them experimentally for the case of thin-film CdTe photovoltaics. Our concept has practical implications: a simple recipe leading to an efficient (13%) copper-free CdTe solar cell.

Crystal nucleation in glasses of phase change memory

We propose a theory of field induced crystal nucleation in disordered glass structure applicable to chalcogenide phase change memory. In the region of symmetry breaking strong electric fields, the nucleation is dominated by cylinder shaped particles with bias dependent nucleation barriers. Statistical fluctuations in microscopic structure of a glass translate into probabilistic distributions of induction times and threshold voltages having respectively log-normal and normal shape. These distributions are exponentially sensitive to the applied voltage, temperature, and material parameters.

Piezo-effect and physics of CdS-based thin-film photovoltaics

We report a strong reversible piezo-effect in CdTe∕CdS photovoltaics consistent with the piezo parameters of CdS. Our finding suggests a different understanding of CdS-based solar cells including CdTe- and CuIn(Ga)Se-based devices. Because the CdS film is put into compression in the device, the piezo coupling generates surface charges and the electric field opposing that of the absorber layer. The corresponding potential barrier makes CdS insulating and the device operate in a metal-insulator-semiconductor mode. Our understanding introduces the concept of piezo-photovoltaics and suggests specific practical implications.

Shunt screening, size effects and I /V analysis in thin-film photovoltaics

We present an analytical model that quantitatively describes the physics behind shunting in thin film photovoltaics and predicts size-dependent effects in the I/V characteristics of solar cells. The model consists of an array of microdiodes and a shunt in parallel between the two electrodes, one of which mimics the transparent conductive oxide and has a finite resistance. We introduce the concept of the screening length L, over which the shunt affects the system electric potential. The nature of this screening is that the system generates currents in response to the point perturbation caused by the shunt. L is expressed explicitly in the terms of the system parameters. We find the spatial distribution of the electric potential in the system and its I/V characteristics. The measured I/V characteristics depend on the relationship between the cell size l and L, being markedly different for the cases of small (l≪L) and large (l≫L) cells. We introduce a new regime of the large photovoltaic cell where all the c...

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...