New insight into phase change memories

https://doi.org/10.1016/j.jnoncrysol.2011.02.014Get rights and content

Abstract

The switching mechanism in phase change memories was described on the basis of minimum switching unit: the commuton. A commuton is a minimum cluster of atoms that supports a reversible phase change from high to low electrical conduction state and back under the influence of an external signal. The switching process in a phase change chalcogenide film was modeled using two dimensional cellular automata approach. A system of 50 × 50 cells, each cell containing a commuton, was simulated. In the particular case of Ge2Sb2Te5 (investigated here) this system corresponds to a 30 × 30 nm area. The formation of the percolation path as a function of phase change induced in commutons explains the switching phenomenon. The influence of the percent of defects in the material on the percolation threshold has been studied.

Research Highlights

► The switching mechanism was described using the concept of commuton. ► The Ge2Sb2Te2 system was modeled using the cellular automata approach. ► Percolation as a function of phase change in the commutons explains the switching. ► A maximum of 20% defects in the system enhances the rapidity of commutation. ► Above this value an opposite effect appears and at 40% the commutation is suppressed.

Introduction

Memory technologies are approaching miniaturization limits because they are based on charge storage and it becomes very difficult to reliably retain sufficient electrons in these shrinking cells. Magnetic and ferroelectric random-access memories, share this struggle with scaling. Nonvolatile memory concepts [1] are based on resistance change rather than charge storage. They include phase-change memory in chalcogenides, programmable-metallization-cell memory in solid electrolytes, and resistance-change memory in transition-metal oxides.

In 1968 Stanford R. Ovshinsky [2] had demonstrated the possibility to get an electrical switching (from high to low resistivity) across a chalcogenide amorphous film by applying an appropriate voltage. The switching can be reversible or irreversible depending on the type of materials used. Later, another effect called optical switching [3] was observed. The main difference between these two effects is that instead of an electrical pulse a laser pulse is used. In rewriteable optical data storage a short pulse of a focused, high intensity laser beam locally heats the phase change material above the melting temperature. By rapid cooling the phase change alloy quenches the material into a disordered, amorphous state. This amorphous state has different optical properties than the surrounding crystalline state. The reading is made with a laser beam of low intensity, which detects the amorphous structure. To erase the stored information, a laser pulse with intermediate power is necessary. The laser locally heats the phase change film above the crystallization temperature and the material reverts to the crystalline state. In 2006 E. Mytilineou et al. proposed a hybrid electro-optical memory device [4] that can be programmed with an electric pulse and read optically or vice versa.

In spite of the development of knowledge in solid state physics the exact process that takes place in phase change materials during switching is not completely known. There are some papers which tried to elucidate this process. Kohara et al. [5] based on X-ray diffraction studies of the amorphous state concluded that amorphous Ge2Sb2Te5 is characterized by even-folded ring structures and amorphous GeTe has both even- and odd-numbered rings. Odd-numbered rings are evidence for homopolar bonds such as Ge―Ge bonds, the even numbered rings are consistent with heteropolar bonds that explains the absence of Ge―Ge bonds in Ge2Sb2Te5. Their investigations reveal that microscopically many relics of the crystal structure are preserved in the amorphous structure. As a consequence of these peculiar features the structure can be easily changed during switching by transformation of the homopolar bonds to heteropolar bonds and back.

Shportko et al. [6] have made measurements of the dielectric function and have observed that the optical dielectric constant is 70–200% larger for the crystalline than the amorphous phases. This difference is attributed to a significant change in bonding between the two phases. The situation in which a single, half-filled p-band forms two bonds to the left and right was called resonant bonding. Each Sb atom has six nearest neighbors, but only three p-electrons to facilitate covalent bonding between adjacent Sb atoms. Thus, the resonant bonds seem to be a key factor in the switching process.

Kolobov et al. [7] have shown that the germanium atoms, in the f.c.c. structure formed by tellurium atoms, occupy the octahedral and tetrahedral symmetry positions in the crystalline and amorphous state, respectively. An intense laser pulse induces the rupture of the weak bonds and the germanium atom flips into the tetrahedral position. It is remarkable that the covalent bonds remain intact. The authors conclude that Ge―Sb―Te can be viewed as being built from well-defined rigid building blocks of composition Ge2Sb2Te5. In the crystalline state, the constraint of the mutual arrangement of the building blocks in space is such that tellurium atoms form an f.c.c. lattice. Inter-block interaction and long-range ordering cause the resulting structure to resemble the rock-salt structure. In the amorphous state, inter-block interaction is weakened, which allows the block structure to relax so that the bonds shrink and germanium umbrella flips into its preferred tetrahedral coordination.

Another idea suggested by Popescu et al. [9] was that, a transition from an amorphous state to another metastable amorphous phase could be possible. Their calculation evidenced that there are a lot of amorphous states energetically possible.

Hegedüs et al. [8] have simulated the phase change switching using ab initio molecular-dynamics simulations and have described the entire write/erase cycle for the Ge2Sb2Te5 composition. Deep insight is gained into the phase-change process; very high densities of connected square rings, characteristic of the metastable rock-salt structure, form during melt–cooling and are also quenched into the amorphous phase. Their presence strongly facilitates the homogeneous crystal nucleation of Ge2Sb2Te5.

Molecular dynamics modeling leads to improved accuracy in the simulations, but suffer from important technical limitations in the time and size of the computation.

Cellular automata models [10], could be very drastic in their approximations, but are able to offer a general view on global issues such as thermodynamics and dynamics. They offer a complementary approach to the direct simulation of detailed models. Their characterization in terms of a few internal parameters makes them more flexible and versatile, offering unifying views of different cases. There is a strong need for simplified models that can keep the physical information and provide a more efficient simulation tool.

Yu and Wright [11] used cellular automata for modeling nucleation and growth in phase change materials. They were able to predict the dynamic phase-transition behavior on the nanosecond and nanometer time and length scale.

Section snippets

The cellular automata model

Popescu et al. defined in [12] the smallest unit that preserves the property of switching in memory materials called commuton (Fig. 1). This concept of commuton was used to build a cellular automata model for simulating the percolation process that appears in phase change materials.

Results and discussion

The size of the grid used in the simulation was 50 × 50 commutons in a closed box. The interaction with the walls of the box was neglected. The particular case of Ge2Sb2Te5 was studied. If we consider that the dimension of a commuton is comparable with the cell of a rock-salt lattice of 6 Å, then the simulated system corresponds to a 30 × 30 nm area of Ge2Sb2Te5. All the commutons in the simulation have the same threshold energy. In Fig. 3 one can see the evolution of the system during the

Conclusions

It was demonstrated that the complex behavior of the phase change materials can be simulated using simple, local rules that evidence the emergent properties of the commutons which cannot be revealed by molecular dynamics.

The percolation threshold was determined for a 30 × 30 nm area of Ge2Sb2Te5. Also, the abrupt decrease of the resistance of the material was successfully reproduced and the activation energy was determined.

The influence of the defects in the material was investigated. It was found

Acknowledgment

The author is grateful to Prof. Dr. Mihai Popescu from National Institute of Materials Physics, who is also my Ph.D. supervisor, for his useful advices and for the fruitful discussions and also to the Ph.D. Program POSDRU/6/1.5/S/10 of the Faculty of Physics, University of Bucharest for financing this study.

References (16)

  • E. Mytilineou et al.

    J. Non-Cryst. Sol.

    (2006)
  • M. Popescu et al.

    J. Non-Cryst. Sol.

    (2009)
  • W. Yu et al.

    J. Univ. Sci. Tech. Beijing

    (2008)
  • G.I. Meijer

    Science

    (2008)
  • S.R. Ovshinsky

    Phys. Rev. Lett.

    (1968)
  • M. Wuttig et al.

    Nat. Mater.

    (2007)
  • S. Kohara et al.

    Appl. Phys. Lett.

    (2006)
  • K. Shportko et al.

    Nat. Mater.

    (2008)
There are more references available in the full text version of this article.
View full text