Alexander V. Kolobov

Interfacial phase-change memory

Phase-change memory technology relies on the electrical and optical properties of certain materials changing substantially when the atomic structure of the material is altered by heating or some other excitation process. For example, switching the composite Ge(2)Sb(2)Te(5) (GST) alloy from its covalently bonded amorphous phase to its resonantly bonded metastable cubic crystalline phase decreases the resistivity by three orders of magnitude, and also increases reflectivity across the visible spectrum. Moreover, phase-change memory based on GST is scalable, and is therefore a candidate to replace Flash memory for non-volatile data storage applications. The energy needed to switch between the two phases depends on the intrinsic properties of the phase-change material and the device architecture; this energy is usually supplied by laser or electrical pulses. The switching energy for GST can be reduced by limiting the movement of the atoms to a single dimension, thus substantially reducing the entropic losses associated with the phase-change process. In particular, aligning the c-axis of a hexagonal Sb(2)Te(3) layer and the 〈111〉 direction of a cubic GeTe layer in a superlattice structure creates a material in which Ge atoms can switch between octahedral sites and lower-coordination sites at the interface of the superlattice layers. Here we demonstrate GeTe/Sb(2)Te(3) interfacial phase-change memory (IPCM) data storage devices with reduced switching energies, improved write-erase cycle lifetimes and faster switching speeds.

Toward the Ultimate Limit of Phase Change in Ge2Sb2Te5

The limit to which the phase change memory material Ge(2)Sb(2)Te(5) can be scaled toward the smallest possible memory cell is investigated using structural and optical methodologies. The encapsulation material surrounding the Ge(2)Sb(2)Te(5) has an increasingly dominant effect on the materials ability to change phase, and a profound increase in the crystallization temperature is observed when the Ge(2)Sb(2)Te(5) layer is less than 6 nm thick. We have found that the increased crystallization temperature originates from compressive stress exerted from the encapsulation material. By minimizing the stress, we have maintained the bulk crystallization temperature in Ge(2)Sb(2)Te(5) films just 2 nm thick.

Raman scattering study of the a-GeTe structure and possible mechanism for the amorphous to crystal transition

We report on an inelastic (Raman) light scattering study of the local structure of amorphous GeTe (a-GeTe) films. A detailed analysis of the temperature-reduced Raman spectra has shown that appreciable structural changes occur as a function of temperature. These changes involve modifications of atomic arrangements such as to facilitate the rapid amorphous to crystal transformation, which is the major advantage of phase-change materials used in optical data storage media. A particular structural model, supported by polarization analysis, is proposed which is compatible with the experimental data as regards both the structure of a-GeTe and the crystallization transition. The remarkable difference between the Raman spectrum of the crystal and the glass can thus naturally be accounted for.

Distortion-triggered loss of long-range order in solids with bonding energy hierarchy

An amorphous-to-crystal transition in phase-change materials like Ge-Sb-Te is widely used for data storage. The basic principle is to take advantage of the property contrast between the crystalline and amorphous states to encode information; amorphization is believed to be caused by melting the materials with an intense laser or electrical pulse and subsequently quenching the melt. Here, we demonstrate that distortions in the crystalline phase may trigger a collapse of long-range order, generating the amorphous phase without going through the liquid state. We further show that the principal change in optical properties occurs during the distortion of the still crystalline structure, upsetting yet another commonly held belief that attributes the change in properties to the loss of long-range order. Furthermore, our results suggest a way to lower energy consumption by condensing phase change inducing energy into shorter pulses or through the use of coherent phonon excitation.

Local structure of crystallized GeTe films

The structure of crystallized GeTe films has been studied by x-ray absorption fine structure spectroscopy. We find that in addition to Ge–Te bonds (2.20 and 3.13 A) ∼10% of Ge–Ge bonds are present. Our results indicate that the crystallized GeTe film consists of GeTe crystallites with 10% Ge vacancies, separated by a quasiamorphous-Ge phase.

Photomelting of selenium at low temperature

We report on a photoinduced phenomenon in solids, namely, photomelting at low temperature. We have found that both trigonal and amorphous selenium can be molten by illumination with light at a temperature of ∼77 K. This phenomenon is pure optical (athermal) and it is associated with light-induced breaking of the interchain (intermolecular) bonds in selenium. The photomelting is important for basic science (as an example of photoinduced phase transition in condensed matter and as a key photoinduced phenomenon in selenium and related materials) and for applications (as a tool for fine manipulation with shape of solids by light at low temperatures).

An in situ Raman study of polarization-dependent photocrystallization in amorphous selenium films

Photocrystallization of amorphous selenium (a-Se) films under illumination by polarized light with 632.8 or 647.1 nm wavelength has been studied by Raman spectroscopy. Preferential orientation of trigonal crystalline, selenium (t-Se) obtained as a result of photocrystallization has been observed, threefold c axis of t-Se being oriented perpendicular to the direction of the polarization of the illuminating light. Although the mechanism of polarization-dependent photocrystallization seems to be optical in origin, an alternative, essentially thermal, mechanism of the polarization-dependent photocrystallization of a-Se is discussed.

Ferroelectric catastrophe: beyond nanometre-scale optical resolution

The optical diffraction limit is rigidly determined as a simple equation of wavelength ? and lens numerical aperture NA (): ?/2/NA. In this paper, we report that Ag5.8In4.4Sb61.0Te28.8 and Ge2Sb2Te5 chalcogenide thin films, which are typical of optical recording materials used in digital versatile discs (DVDs), enable a resolution of under ?/10 due to their ferroelectric properties. In the Ag5.8In4.4Sb61.0Te28.8 film it was found that this optical super-resolution can be observed between?350 and 400??C, resulting in a second phase transition from a hexagonal (A7 belonging to ) to a rhombohedral structure of R32 or R3m. In Ge2Sb2Te5, on the other hand, the temperature range is much wider, between?250 and 450??C, which is also due to a second phase transition from a NaCl-type fcc to a hexagonal structure.

A model of photostructural changes in chalcogenide vitreous semiconductors: 1. Theoretical considerations

Abstract Light-induced processes in chalcogenide vitreous semiconductors are considered in the framework of a configurational model of two stable states (ground and metastable) of atomic units with optical and thermal transitions between these states. Probabilities of these transitions as functions of photon energy and temperature are calculated. an expression for the metastable state population kinetics is given.

Crystallization-induced short-range order changes in amorphous GeTe

By means of x-ray absorption fine structure and Raman scattering spectroscopies we demonstrate that the structure of amorphous GeTe is likely to be a mixture of 4(Ge):2(Te) and 3(Ge):3(Te)-coordinated structural units. Upon crystallization, a rhombohedral (distorted rocksalt) structure is established with about 10% of vacancies occurring on Ge sites. The vacancies are believed to play an important role in determining the ratio of 3(Ge):3(Te) and 4(Ge):2(Te) structural units.