Mustafa M. Aziz

Arithmetic and Biologically‐Inspired Computing Using Phase‐Change Materials

Computers in which processing and memory functions are performed simultaneously and at the same location have long been a scientific “dream”, since they promise dramatic improvements in performance along with the opportunity to design and build ‘brain-like’ systems.1–3 This “dream” has moved a step closer following recent investigations of so-called memristor (memory resistor) devices.4–8 However, phase-change materials also offer a promising route to the practical realisation of new forms of general-purpose and biologically-inspired computing.9–11 Here we provide, for the first time, an experimental proof-of-principle of such a phase-change material-based “processor”. We demonstrate reliable experimental execution of the four basic arithmetic processes of addition, multiplication, division and subtraction, with simultaneous storage of the result. This arithmetic functionality is possible because phase-change materials exhibit a natural accumulation property, a property that can also be exploited to implement an “integrate and fire” neuron.12, 13 The ability of phase-change devices to ‘remember’ previous excitations also imbues them with memristor-type functionality,4, 8 meaning that they can also provide synaptic-like learning.6, 7, 13 Our results demonstrate convincingly these remarkable computing capabilities of phase-change materials. Our experiments are performed in the optical domain, but equivalent processing capabilities are also inherent to electrical phase-change devices.

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.

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.

An analytical model for nanoscale electrothermal probe recording on phase-change media

Scanning probe memories are now emerging as a means of achieving nanoscale resolution data storage. The use of microscopic conductive tips in contact with a phase-change material to record data as amorphous and crystalline marks is one such approach, making use of the large difference in electrical conductivity between the two phases to distinguish between two binary states on replay and hence provide a memory function. The writing process is complex and involves electronic, thermal, and phase-change processes that are difficult to model and study except using numerical techniques. A simplified analytical model of electrothermal writing by probe on a basic two-layer phase-change structure is developed here, and used to predict the required voltage levels for recording and the expected diameters of recorded crystalline and amorphous marks. A simplified model of cooling and solidification was also developed to study the cooling rates during amorphization. The predictions are shown to be in agreement with pu...

A slope-theory approach to electrical probe recording on phase-change media

A theoretical approach to predicting the spatial extent of the amorphous to crystalline transition region during the probe recording process on phase-change storage media is presented. The extent of this transition region determines the ultimate achievable linear density for data storage using phase-change materials. The approach has parallels with the slope theory used to find magnetic transition lengths in magnetic recording, and shows that the amorphous to crystalline transition length can be minimized by reducing the thickness of the phase-change layer, by minimizing lateral heat flow, and by maximizing the ratio of the activation energy for crystallization to the transition temperature Ec∕Tt.

Signal and noise characteristics of patterned media

Summary form only given. General analytical expressions for signal and noise were derived for magnetised rectangular islands in patterned media. These allow signal-to-noise ratios to be evaluated in terms of the optimum bit size and transducer geometry.

Accuracy of expressions for the magnetic field of a ring head

The Fourier method used by Fan has produced a representation of the field of a semi-infinite pole head consisting of a simple analytical approximation (Karlqvist field) plus an infinite series of correction terms. A method is developed in this paper by which other approximations to head fields have their infinite series of correction terms evaluated to make them exact. Expressions with only one correction term produced by Ruigrok and Szczech et al., are studied, and enhancement of them is shown to offer accurate approximations to the exact head fields.

Exact harmonic coefficients for a magnetic ring head

The magnetic field of a ring head has been analyzed by Westmijze [1953], using a conformal mapping, and by Fan [1961], using Fourier techniques. Here these methods are reexamined and combined to give, for the first time, an explicit analytic expression for the harmonic coefficients in the Fan solution.