M. Krbal

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.

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.

Enhanced crystallization of GeTe from an Sb2Te3 template

Crystalline Sb2Te3 templates reduce the crystallization time of the phase change material GeTe by four orders of magnitude to 20 ns. Structural measurements and density functional theory molecular dynamics atomistic modeling show that this reduction is a direct consequence of textured crystal growth from a plane of octahedral crystal nucleation centers. The nucleation template serves to reduce the crystallization activation energy by 2.6 eV allowing crystallization to proceed at a temperature 95 °C lower than that of the untemplated GeTe film.

Temperature independence of pressure-induced amorphization of the phase-change memory alloy Ge2Sb2Te5

In the temperature range from room temperature to about 150°C, the prototypic phase-change material Ge2Sb2Te5 becomes amorphous upon hydrostatic compression. In the studied temperature range, the onset of amorphization is at about 15GPa and the material completely amorphizes at 25GPa; these values do not depend on temperature. Upon decompression, the amorphous phase is stable at lower temperatures, yet at higher temperatures (145°C), the initial fcc phase is recovered upon decompression. A possible mechanism of pressure-induced amorphization and its implications for phase-change memories are discussed.

Liquid Ge2Sb2Te5 studied by extended x-ray absorption

We report on x-ray absorption studies of the structure of the liquid phase of a prototypical phase-change material Ge2Sb2Te5. We demonstrate that the local structure of liquid Ge2Sb2Te5 is very similar to that of amorphous Ge2Sb2Te5. Ge atoms in the liquid phase are found to be covalently bonded suggesting a semiconducting nature of the melt.

Selective detection of tetrahedral units in amorphous GeTe-based phase change alloys using Ge L3-edge x-ray absorption near-edge structure spectroscopy

Using Ge L3-edge x-ray absorption near-edge structure (XANES) studies, we demonstrate a noticeable difference in local structure between amorphous and thermally crystallized GeTe-based phase change alloys. The pronounced change appears as a step-like feature at the absorption edge corresponding to a 2p → 5s (4d) electron transition. Comparison with ab initio XANES simulations suggest that the step-like feature is due to the presence of tetrahedrally coordinated Ge atoms in the as-deposited samples. The obtained results demonstrate that Ge L3-edge XANES can be used as a structural probe for the existence of tetrahedral Ge sites in GeTe-based phase change alloys.

Effect of doping on global and local order in crystalline GeTe

Effect of nitrogen and carbon doping on the structure of GeTe has been investigated using x-ray diffraction and extended x-ray absorption fine structure (EXAFS) spectroscopies. While Bragg diffraction which probes the global structure exhibits a clear transition upon doping from the rhombohedral phase to the cubic (rocksalt) phase, the local structure probed by EXAFS remains rhombohedrally distorted across the compositions studied. The apparent inconsistency between the results of the two techniques used is attributed to disordering upon doping and the resulting order-disorder transition that is “seen” by site-averaging diffraction as a displacive rhombohedral-to-cubic transition.

Amorphous InSb: Longer bonds yet higher density

Results of x-ray absorption studies of the structure of amorphous InSb are reported. We demonstrate that approximately 1% bond elongation in the amorphous phase (as compared to the crystalline phase) is accompanied by a counterintuitive increase (approximately 5%) in density. We argue that this controversy is due to the formation of wrong bonds in the amorphous phase with both Sb and In atoms effectively preserving their tetrahedral coordination. Our results additionally offer an alternative interpretation of the semiconductor-metal transition observed upon melting of InSb.

Stress Limited Scaling of Ge 2 Sb 2 Te 5

The influence of stress on the phase change behaviour of Ge2Sb2Te5 encapsulated by ZnSSiO2 and TiN is investigated using temperature dependent Extended X-ray Asbsorption Fines Structure and Ellipsometry to determine the crystallisation temperature. The encapsulation material surrounding the Ge2Sb2Te5 has an increasingly dominant effect on the material’s ability to change phase and can cause a profound increase in its crystallization temperature. We have experimentally shown 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 Ge2Sb2Te5 films just 2 nm thick.