We introduce a technique to permit x-ray absorption spectroscopy studies focusing on individual phase-change (Ge2Sb2Te5) memory cells in fully integrated PC-RAM structures. Devices were investigated employing an x-ray nanobeam of only about 300 nm diameter, which could be fully contained within the spatial extent of the active area within a single device cell and enabled us to investigate individual devices without interference from non-switching material surrounding the area of interest. By monitoring the fluorescence signals of tungsten and germanium at a photon energy corresponding to the Ge K-edge absorption edge white line position, we were successful in producing 2D area maps of the active cell region, which clearly show the imbedded tungsten heater element and the switched region of the phase change material. Additionally, position dependent changes in the phase change material could be traced by taking an array of XANES spectra at the Ge K-edge on and in the vicinity of individual devices.

The elastic properties of chalcogenide materials used for phase change applications in rewritable optical media (such as CD-RW, DVD-RW, etc) are still poorly characterized and the previously published experimental and theoretical values show large discrepancies. In this manuscript, we review these results and 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.

The reversible switching between the amorphous and crystalline phases of Ge2Sb2Te5 (GST) is investigated with ab initio molecular dynamics. We apply different quench rates (-16 K/ps, -5 K/ps, -2 K/ps, and -0.45 K/ps) and different annealing temperatures (500 K, 600 K, 700 K, and 800 K) to amorphize and crystallize GST respectively. Results show that the generated amorphous is strongly dependent on the quench rate. For -16 K/ps and -5 K/ps, generated amorphous samples have different density of crystal seeds, higher in the later. The amorphous structure formed at -2 K/ps contains a single crystalline cluster, while that formed at the quench rate of -0.45 K/ps had sufficient time to completely crystallize the amorphous phase. Annealing the amorphous systems formed at different rates shows that crystallization depends both on the annealing temperature and on the structure of the initial system (i.e., whether or not it contains crystalline clusters or crystal seeds). At 500 K, formation of crystalline clusters occurs readily within a few ps while the rate at which they grow is slow, taking 0.9 ns to complete the crystallization. In contrast, crystalline cluster formation is inhibited at 800 K. In the intermediate temperature range, both crystalline cluster formation and growth occur within a few hundred ps indicating that these temperatures leads to the fastest crystallization. The crystallization of a 63-atom at ∼900 K resulted in a highly relaxed crystal structure showing a clear tendency for separation of Ge and Sb species in layers. This model also indicates a tendency of segregation of vacancies, suggesting that vacancy layering may play a key role in the crystallization process.

Emerging phase-change electrical memory technologies rely on the fast amorphous to crystalline transition, which is usually characterized by an ‘S-shape’ current-voltage curve. We investigate the possibility that electric field induced nucleation may play a dominant role in defining this characteristic electrical switching behavior. We derive quantitative crystallization maps to study the kinetics of the amorphous to crystalline transition in the presence of electric field contribution to the free energy and we investigate how the prediction of the electric field induced nucleation model is affected by material properties such as enthalpy and relative electric permittivity.

In this work we present preliminary calculations and simulations to demonstrate feasibility of programming a nanoscale Phase Change Random Access Memory (PCRAM) cell by means of a silicon nanowire ballistic transistor (SNWBT). Memory cells based on ballistic transistors bear the advantage of having a small size and high-speed operation with low power requirements. A one-dimensional MOSFET model (FETToy) was used to estimate the output current of the nanowire as a function of its diameter. The gate oxide thickness was 1.5 nm, and the Fermi level at source was set to -0.32 eV. For the case of VDS = VGS = 1 V, when the nanowire diameter was increased from 1 to 60 nm, the output power density dropped from 109 to 106 W cm-2 , while the current increased from 20 to 90 μA. Finite element electro-thermal analysis were carried out on a segmented cylindrical phase-change memory cell made of Ge2Sb2Te5 (GST) chalcogenide, connected in series to the SNWBT. The diameter of the combined device, d, and the aspect ratio of the GST region were selected so as to achieve optimum heating of the GST. With the assumption that the bulk thermal conductivity of GST does not change significantly at the nanoscale, it was shown that for d = 24 nm, a ‘reset’ programming current of ID = 80 μA can heat the GST up to its melting point. The results presented herein can help in the design of low cost, high speed, and radiation tolerant nanoscale PCRAM devices.

We present a set of density functional theory (DFT) calculations on the electronic structure of Ag and Sn in Ge2 Se3 in a periodic model. We show that electron self-trapping is a persistent feature in the presence of many defects. Ag and Sn autoionize upon entering Ge2 Se3 becoming Ag+ and Sn2+ , respectively, and the freed electrons self trap at the lowest energy site. Both Ag and Sn can substitute for Ge, and we present formation energies as a function of Fermi level that show that Sn can substantially alter the incorporation of Ag into the Ge2Se3 network.

The phase-change-memory (PCM) relies on the ability of a chalcogenide material, usually Ge2Sb2Te5 (GST), to switch from the amorphous phase with a high electrical resistance (~MΩ) to a crystalline phase with a low electrical resistance (∼kΩ). The structural stability of the amorphous phase is critically affected by temperature-activated crystallization and resistance drift due to structural relaxation (SR) [1]. While amorphous chalcogenides are relatively stable with respect to crystallization, thanks to a high activation energy above 2 eV [2], the lower activation energy of SR can strongly affect the PCM electrical properties of the amorphous phase, including resistance, activation energy for conduction and, most importantly, the threshold voltage VT. The latter marks the boundary between read and programming operations in PCM, thus VT instability must be carefully predicted to minimize read disturbs within the array. To this purpose, understanding the switching mechanisms and modeling of VT is of utmost importance for PCM device development and scaling.

Phase-change memory materials (PCMMs) are semiconductors that exhibit rapid order-disorder transition under electrical or optical pulse excitation. Currently thin-film-based PCMMs play a dominant role in fabrication of non-volatile memory devices. In contrast, phase-change nanowires (PCNWs) have the potential to overcome future challenges such as high data density and low power consumption. Among the various methods to synthesize PCNWs, the vapor-liquid-solid method has been reported previously. In this paper, we report synthesis of Sb-Te PCNWs using a templated electrochemical method. Nanoporous anodic aluminum oxide (AAO) was used as a template for the growth of nanowires. Sb-Te PCNWs with different compositions, diameters and aspect ratios were grown inside the AAO template by electrodeposition. Composition and structure of these nanowires were characterized by energy dispersive X-ray spectroscopy, X-ray diffraction, and scanning and transmission electron microscopy. It is found that electrodeposition through nanosized channels results in materials that are quite different from those electrodeposited on unrestricted surface. The mechanism of nanowire formation inside the channels of AAO template is discussed.