C.D. Wright

Models for phase-change of Ge2Sb2Te5 in optical and electrical memory devices

We investigated three different modeling approaches to simulate the crystallization behavior of Ge2Sb2Te5 in optical and, very recently, electrical phase-change memories. The first of these models is based on the Johnson–Mehl–Avrami–Kolmogorov (JMAK) formalism to calculate the fraction of crystallized material during isothermal anneals. In the literature, this model is widely used, but parameters of the model reported by different investigators vary considerably. We have shown that these discrepancies can be attributed to the inappropriate use of the JMAK approach. In order to overcome the restrictions imposed by JMAK theory, generalizations based on classical nucleation theory have been suggested. Material parameters required by the theory, such as viscosity, diffusivity, and fusion enthalpy of Ge2Sb2Te5, have been deduced from published experiments. Uncertainty in the material parameters in combination with approximate expressions used by the classical nucleation theory, however, lead us to suggest a co...

Terabit-per-square-inch data storage using phase-change media and scanning electrical nanoprobes

A theoretical study of the write, read, and erase processes in electrical scanning probe storage on phase-change media is presented. Electrical, thermal, and phase-transformation mechanisms are considered to produce a physically realistic description of this new approach to ultrahigh-density data storage. Models developed are applied to the design of a suitable storage layer stack with the necessary electrical, thermal, and tribological properties to support recorded bits of nanometric scale. The detailed structure of nanoscale crystalline and amorphous bits is also predicted. For an optimized trilayer stack comprising Ge/sub 2/Sb/sub 2/Te/sub 5/ sandwiched by amorphous or diamond-like carbon layers, crystalline bits were roughly trapezoidal in shape while amorphous bits were semi-ellipsoidal. In both cases, the energy required to write individual bits was very low (of the order of a few hundred picoJoules). Amorphous marks could be directly overwritten (erased), but crystalline bits could not. Readout performance was investigated by calculating the readout current as the tip scanned over isolated bits and bit patterns of increasing density. The highest readout contrast was generated by isolated crystalline bits in an amorphous matrix, but the narrowest readout pulses arose from isolated amorphous marks in a crystalline background. To assess the ultimate density capability of electrical probe recording the role of write-induced intersymbol interference and the thermodynamic stability of nanoscale marks were also studied.

Reciprocity in magnetic force microscopy

A theoretical treatment for contrast formation in the magnetic force microscope is given which relies on calculation of the force acting on the sample, rather than the more usual method which calculates the force acting on the microscope tip. The equivalence of this reciprocal force method is demonstrated by calculating the theoretical image for longitudinal step and arctangent magnetization transitions in thin‐film recording media. The reciprocal force approach leads naturally to unambiguous definitions for the resolution of the magnetic force microscope and it is shown that conventional resolution measures, such as impulse response, line‐spread function and step response, used in many other forms of microscopy, may be readily applied to the magnetic force microscope.

Development of a scanning laser microscope for magneto-optic studies of thin magnetic films

Abstract A magneto-optic scanning laser microscope has been built which is capable of observing the magnetic domain structure present in materials which exhibit the Kerr (polar and longitudinal) and Faraday magneto-optical effects. The samples are not specially coated to improve the Kerr contrast, images being formed of the domain structures in the “as produced” samples. The instrument is also capable of observing nonmagnetic contrast. In addition the microscope may be used, together with a high-power Ar ion laser, as a thermo-magneto-optic bit writer

Crystallization of Ge2Sb2Te5 films by amplified femtosecond optical pulses

The phase transition between the amorphous and crystalline states of Ge2Sb2Te5 has been studied by exposure of thin films to series of 60 femtosecond (fs) amplified laser pulses. The analysis of microscope images of marks of tens of microns in size provide an opportunity to examine the effect of a continuous range of optical fluence. For a fixed number of pulses, the dependence of the area of the crystalline mark upon the fluence is well described by simple algebraic results that provide strong evidence that thermal transport within the sample is one-dimensional (vertical). The crystalline mark area was thus defined by the incident fs laser beam profile rather than by lateral heat diffusion, with a sharp transition between the crystalline and amorphous materials as confirmed from line scans of the microscope images. A simplified, one-dimensional model that accounts for optical absorption, thermal transport and thermally activated crystallization provides values of the optical reflectivity and mark area th...

Write strategies for multiterabit per square inch scanned-probe phase-change memories

A mark-length write strategy for multiterabit per square inch scanned-probe memories is described that promises to increase the achievable user density by at least 50%, and potentially up to 100% or more, over conventional approaches. The viability of the write strategy has been demonstrated by experimental scanning probe write/read measurements on phase-change (GeSbTe) media. The advantages offered by adopting mark-length recording are likely to be equally applicable to other forms of scanned probe storage.

The Design of Rewritable Ultrahigh Density Scanning-Probe Phase-Change Memories

A systematic design of practicable media suitable for rewritable, ultrahigh density (>;1Tbit/sq.in.), high data rate (>;1Mbit/s/tip) scanning-probe phase-change memories is presented. The basic design requirements were met by a Si/TiN/Ge2Sb2Te5 (GST)/diamond-like carbon structure, with properly tailored electrical and thermal conductivities. Various alternatives for providing rewritability were investigated. In the first case, amorphous marks were written into a crystalline starting phase and subsequently erased by recrystallization, as in other already established phase-change memory technologies. Results imply that this approach is also appropriate for probe-based memories. However, experimentally, the successful writing of amorphous bits using scanning electrical probes has not been widely reported. In light of this, a second approach has been studied, that of writing crystalline bits in an amorphous starting matrix, with subsequent erasure by reamorphization. With conventional phase-change materials, such as continuous films of GST, this approach invariably leads to the formation of a crystalline “halo” surrounding the erased (reamorphized) region, with severe adverse consequences on the achievable density. Suppression of the “halo” was achieved using patterned media or slow-growth phase-change media, with the latter seemingly more viable.

Parameterized SPICE model for a phase-change RAM device

A simple form of a SPICE macro model for a generic phase-change random access memory device is presented. The approach is based upon lumped parameter multiple level models. The SPICE implementation is described using a series of increasingly complex modeling blocks for dc to transient analysis. The effect of nonlinear phase switching during the programming cycle is demonstrated in a SPICE simulation and compared to experimental data.

Temperature distributions in semi-infinite and finite-thickness media as a result of absorption of laser light.

A time-domain method is used to derive simple expressions for temperature distributions within media heated by a moving laser beam with a Gaussian power density profile. Various medium-absorption functions are considered. The solutions are given as single integrals with respect to time of simple functions. The expressions can be applied to multilayer media consisting of layers with different optical (light-absorbing) properties, provided that the layers have similar thermal properties. A number of optical recording examples have been examined in detail, and the results compared with those obtained by use of a more general (fast-Fourier-transform-based) approach.

Reciprocity based transfer function analysis in magnetic force microscopy

The transfer function of the imaging process in magnetic force microscopy is calculated theoretically using a reciprocal force based approach. The reciprocal force is that exerted on the sample by the tip, and according to Newton’s third law, is simply equal but opposite to the force exerted on the tip by the sample. The method allows the role of the tip in the imaging process to be represented in a particularly simple fashion, and the system transfer function is shown to depend on the Fourier transform of the field distribution produced by the microscope tip at the surface of that tip, an exponential spacing loss term reflecting the separation of the tip and the sample and finally a loss term dependent on the sample thickness.