NEWS & VIEWSNEWS & VIEWS

PHASE-CHANGE MATERIALS

Towards a universal memory?

MATTHIAS WUTTIG is at the I. Physikalisches Institut der RWTH Aachen, Lehrstuhl für Physik neuer Materialien, 52056 Aachen, Germany.e-mail: wuttig@physik.rwth-aachen.de

Just imagine you could start your laptop or personal computer and have it ready for you to work in less than a second, without having

to wait for its operating system and other software to load. Or that you were able to record and watch full-length movies on your mobile phone. Both applications and many more would become feasible if a universal memory were available. This memory should be non-volatile — that is, it should be able to retain information even when all power to it is removed — and it should be able to store large amounts of data in a very small space. It should be able to read, write and erase this data at high speed and — crucial for mobile applications — should do so with little consumption of power. Finally, the whole device should be inexpensive. On page 347 of this issue1, Martijn Lankhorst and colleagues demonstrate a memory that uses both a new device structure and a new storage material, which shows great promise for realizing a truly universal memory.

Currently, the most popular type of memory for mobile applications is known as ‘fl ash’ random access memory (fl ash-RAM)2, and can be found in devices ranging from digital cameras to personal music players. Although substantial improvements in the performance and cost of fl ash-RAM have been made over the past decade, a number of drawbacks remain, most notably, limited cyclability and scaling problems when moving to increasingly smaller dimensions. Consequently, there has been a fevered search by many companies to fi nd an alternative memory technology to overcome such issues and satisfy all the criteria that modern and mobile electronic applications demand.

The two most promising contenders being developed to succeed fl ash memory are magnetic RAM3 — based on storing information in the magnetization states of a ferromagnetic material, and

ferroelectric RAM4 — based on storing information in the polarization states of a ferroelectric material. Lately, however, a third memory concept has re-entered the scene. Commonly referred to as Ovonic memory, after Stanford Ovshinsky who fi rst proposed it in 19685, it exploits the fact that certain alloys of group V or group VI elements, known as phase-change materials, can be induced to switch reversibly between their amorphous and crystalline states with an electrical current, and that the electrical resistance of these two states is substantially different.

Materials that undergo a structural transformation between amorphous and crystalline phases under the infl uence of an electrical current have been known for decades. But with the discovery of new phase-change materials and new device confi gurations to make them switch, interest in them is being rejuvenated.

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Figure 1 The operation principle of a memory device based on phase changes. The logical state of the phase-change memory cell described by Lankhorst et al.1 is determined by the structural state of the material on which it is based — that is, whether it is in an ordered crystalline (‘set’ state equivalent to a binary ‘1’) or disordered amorphous state (‘reset’ state equivalent to a binary ‘0’). To switch the memory cell to the set state, a moderate magnitude voltage pulse (set pulse) is applied to the material with a cell to heat it to just above its glass-transition temperature. This increases the mobility of atoms in the material — but without melting it — and allows them to rearrange into a more energetically favourable crystalline state. Conversely, to switch the cell back to its reset state, a larger voltage pulse (reset pulse) is applied to heat the material above its melting temperature. Fast cooling by the material’s surroundings retains the disorder inherited from the liquid state, leaving the material in an amorphous state. Because the electrical resistivity (Ω) of the material in its amorphous state is substantially higher than that in its crystalline state, determining the state of a cell (‘read’ operation) is simply a matter of measuring its resistance.

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When a phase-change material in its amorphous high-resistance state is heated to just above its glass-transition temperature by an applied electrical pulse (set operation), increased mobility of its atoms allow them to rearrange into a more energetically favourable crystalline state (Fig. 1). To switch it back to the amorphous state, the material is heated to above its melting temperature with a voltage pulse of greater magnitude (reset operation) causing it to liquefy and then rapidly cool (by contact with its surroundings) to quench it in the amorphous state. Because the amorphous and crystalline states have very different electrical resistances, the information recorded can be read easily.

Although the inherent simplicity of Ovonic memory and its potential for integration into semiconductor device manufacturing suggested great promise, diffi culties in reducing the long switching times required to write and erase prototype devices restricted their initial development as a competitive memory technology. The identifi cation of new and better phase-change materials in recent years, however, has led to substantial improvements in the speed of Ovonic devices, which has prompted renewed interest by semiconductor manufacturers. Yet, there are still many hurdles in terms of both speed and power consumption to be overcome before this technology can compete with the more mature MRAM and FRAM technologies. To this end, the work by Lankhorst et al.1 advances this technology towards the universal memory envisaged by Ovshinsky in two important respects.

The fi rst is that they demonstrate a novel cell architecture that enables a pronounced reduction in switching power. Conventional Ovonic memory cells consist of a thin layer of phase-change material sandwiched between two conducting contacts. In contrast, the authors’ device consists of an elongated thin line of phase-change material connecting two horizontally separated contacts. This architecture allows removal of the electrodes from the region where the phase-change material gets hot, hence reducing constraints on electrode materials in terms of temperature stability and reactivity. Moreover, it allows the phase-change material to be encased within a dielectric of low thermal conductivity, which reduces heat loss, and which in turn reduces the amount of power needed to crystallize or amorphize the material. As an added bonus, not only does the new device structure allow improvements in performance, its simplicity should enable it to be fabricated with only a few additional lithography steps.

The second advance, which is of equal importance, is the demonstration of a different

phase-change material to those used previously. This new material switches at lower threshold voltages so that the transformation between the amorphous state and the crystalline state is strongly facilitated. Threshold switching7 is a very important ingredient to the phase change. As depicted in Fig. 1, the resistance in the amorphous state is very high, and hence requires high voltages to dump suffi cient power into the active region. These voltages are not available in battery-operated devices. Fortunately though, threshold switching overcomes this obstacle by creating conductive, yet still amorphous, fi laments in the disordered state that dissipate the power required to crystallize the surroundings. The material identifi ed by the authors shows an especially low threshold voltage leading to even greater reductions in switching power. Moreover, the material exhibits crystallization behaviour that differs signifi cantly from previously used compounds, enabling switching at rather short times. These improvements of the memory concept should help pave the way to a truly universal memory.

Despite the renewed promise of the latest work, there are still a number of outstanding issues to be addressed before these devices become commercially viable. In terms of fundamental science, more work is needed to clarify how the threshold voltage depends on stoichiometry, so that materials with low switching powers can be identifi ed without resorting to purely empirical approaches. Once this is achieved, researchers will then need to demonstrate that these new materials are able to undergo the many read–write–erase cycles demanded of consumer devices. Finally, scaling issues still need to be addressed, so that as many digital bits can be crammed into as small a space as possible. The activation barrier for crystallization of submicrometre-size volumes of the amorphous region often exceeds several electron volts. This guarantees a good thermal stability against unwanted crystallization. Nevertheless, it still needs to be proven that even amorphous regions of the size of just several nanometres are stable as well. These are exciting questions and problems for materials scientists to tackle. If they can be solved the future is bright for phase-change-based memories.

REFERENCES1. Lankhorst, M. H. R., Ketelaars, B. W. S. M. M. & Wolters, R. A. M. Nature

Mater. 4, 347–352 (2005).2. Waser, R. (ed.) Nanoelectronics and Information Technology (Wiley-VCH,

Berlin 2003).3. Scott, J. F. & de Araujo, C. A. P. Science 246, 1400–1405 (1989).4. Butler, W. H. & Gupta, A. Nature Mater. 3, 845–847 (2004).5. Ovshinsky, S. R. Phys. Rev. Lett. 21, 1450–1453 (1968).6. Gill, M., Lowrey, T. & Park, J. IEEE International Solid-State Circuits

Conference. Digest of Technical Papers Vol. 1, 202–459 (San Francisco, California, 2002).

7. Adler, D., Henisch, H. K. & Mott, N. Rev. Mod. Phys. 50, 209–220 (1978).

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