Krisztian Kohary

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.

Threshold switching via electric field induced crystallization in phase-change memory devices

Phase-change devices exhibit characteristic threshold switching from the reset (off) to the set (on) state. Mainstream understanding of this electrical switching phenomenon is that it is initiated electronically via the influence of high electric fields on inter-band trap states in the amorphous phase. However, recent work has suggested that field induced (crystal) nucleation could instead be responsible. We compare and contrast these alternative switching “theories” via realistic simulations of device switching both with and without electric field dependent contributions to the system free energy. Results show that although threshold switching can indeed be obtained purely by electric field induced nucleation, the fields required are significantly larger than experimentally measured values.

Electric field induced crystallization in phase-change materials for memory applications

Emerging electrical memory technologies based on phase-change materials capitalize on a fast amorphous-to-crystalline transition. Recent evidence from measurements of relaxation oscillations and switching statistics in phase-change memory devices indicates the possibility that electric field induced crystal nucleation plays a dominant role in defining the characteristic electrical switching behavior. Here we present a detailed kinetics study of crystallization in the presence of an electric field for the phase-change material Ge2Sb2Te5. We derive quantitative crystallization maps to show the effects of both temperature and electric field on crystallization and we identify field ranges and parameter values where the electric field effects might play a significant role.

Determination of the anisotropic elastic properties of Ge1Sb2Te4

The elastic properties of Ge–Sb–Te (GST) alloys are important for phase-change devices (such as CD-RW, DVD-RW, Blu-ray, or phase-change random access memory) because the transition between the crystalline and amorphous phases involves a volume change accommodated by a strain estimated to be between 150 MPa and 10 GPa. However, the elastic properties of GST alloys are poorly characterized and the experimental and theoretical values show large discrepancies. We carry out a careful analysis of the elastic properties of a model system, crystalline Ge1Sb2Te4, using density functional theory and elastic anisotropy considerations. We show that Ge1Sb2Te4 exhibits significant anisotropy in its elastic properties.

Simulation of ultrahigh storage densities in nanoscale patterned probe phase change memories

Phase Change Memory (PCM), based on the reversible transitions of chalcogenides such as Ge 2 Sb 2 Te 5 (GST) between a high resistance amorphous state (binary ‘0’) and low resistance crystalline state (binary ‘1’), is one of the leading contenders to complement or even replace existing technologies such as Flash, DRAM and HDD [1]. This is attributed to its size scalability (sub-10nm [2-4]), ultrahigh storage densities (Tb/in2 [5-6]), fast programming speeds (picoseconds [7]), and potentially low programming currents (∼μA [3]).

The Scaling of Phase-Change Memory Materials and Devices

In this chapter we address the basic question of the extent to which phase-change memories can be scaled down in size. Fundamental physical limits, along with material properties and device design considerations, all affect the smallest phase-change devices that might be achieved; we explore each of these issues in turn. We also examine the effects of scaling on key performance parameters, in particular on device switching currents and energies and device switching speeds. Finally, we summarize device performance attributes for production-oriented and research-oriented cell designs and provide some perspectives for possible future developments.

Emerging Nanoscale Phase-Change Memories: A Summary of Device Scaling Studies

Phase-change materials have generated very significant interest in recent years due to their potential for data storage and memory applications. For the exploration of phase-change memory many different types of device structures and materials have been presented and investigated by numerous researchers, academic and industrial, using a variety of experimental and theoretical approaches. Among all the properties, size-scaling, reductions in power consumption and switching speeds, improvements in endurance and possibilities for multilevel storage have been the most prominent topics of research. Key approaches to achieving such properties, with a particular focus on size-scaling, ie, how small phase-change devices can be shrunk while still working effectively, are here presented in detail.

Phase-change RAM modelling and design via a Gillespie-type cellular automata approach

A Gillespie type cellular automata (GCA) simulator capable of spatio-temporal modelling of the amorphization and crystallization behaviour in phase change devices, such as random access memory cells (PCRAM), during complex annealing cycles is presented. The model is based on the bulk, electrostatic and surfaces energies to produce rates of nucleation, growth and dissociation of crystallites made of “monomers”. To deal with the events during the phase change transformation a stochastic Gillespie type algorithm is used. The phase change dynamics are coupled with the electrical and thermal fluxes to study the switching dynamics associated with the reset and set operations. The potential role of electric field induced nucleation is also investigated briefly.

Kinetics of electric field enhanced crystallisation in phase-change materials

We study the effect of electric field on crystallisation kinetics in phase change materials within the framework of classical nucleation theory. We identify the parameter set where the electric field has a pronounced effect on the crystallisation of phase-change materials.