Stefania Privitera

Metal - Insulator Transition Driven by Vacancy Ordering in GeSbTe Phase Change Materials

Phase Change Materials (PCMs) are unique compounds employed in non-volatile random access memory thanks to the rapid and reversible transformation between the amorphous and crystalline state that display large differences in electrical and optical properties. In addition to the amorphous-to-crystalline transition, experimental results on polycrystalline GeSbTe alloys (GST) films evidenced a Metal-Insulator Transition (MIT) attributed to disorder in the crystalline phase. Here we report on a fundamental advance in the fabrication of GST with out-of-plane stacking of ordered vacancy layers by means of three distinct methods: Molecular Beam Epitaxy, thermal annealing and application of femtosecond laser pulses. We assess the degree of vacancy ordering and explicitly correlate it with the MIT. We further tune the ordering in a controlled fashion attaining a large range of resistivity. Employing ordered GST might allow the realization of cells with larger programming windows.

Effect of the linewidth reduction on the characteristic time spread in C49–C54 phase transition

The kinetics of the C49–C54 phase transformation in TiSi2 narrow strips for width in the 0.5–1.3 μm range was investigated by sheet resistance measurements. The experimental data follow the Johnson–Mehl–Avrami equation for the fraction of the transformed material, with an exponent equal to 1 for all of the analyzed linewidths. Nucleation sites saturation occurs and the growth is one-dimensional along the length of the strip. The characteristic time, as obtained by the fit, increases as 1/W, W being the width of the strip, and a nucleation density of about 0.05 sites/μm2 has been obtained. The distribution of the characteristic times around the average value increases with decreasing the linewidth. The amplitude of the dispersion is in quantitative agreement with the statistical fluctuation of the number of nucleation sites.

Chemical and structural arrangement of the trigonal phase in GeSbTe thin films

The thermal and electrical properties of phase change materials, mainly GeSbTe alloys, in the crystalline state strongly depend on their phase and on the associated degree of order. The switching of Ge atoms in superlattice structures with trigonal phase has been recently proposed to develop memories with reduced switching energy, in which two differently ordered crystalline phases are the logic states. A detailed knowledge of the stacking plane sequence, of the local composition and of the vacancy distribution is therefore crucial in order to understand the underlying mechanism of phase transformations in the crystalline state and to evaluate the retention properties. This information is provided, as reported in this paper, by scanning transmission electron microscopy analysis of polycrystalline and epitaxial Ge2Sb2Te5 thin samples, using the Z-contrast high-angle annular dark field method. Electron diffraction clearly confirms the presence of compositional mixing with stacking blocks of 11, 9 or 7 planes corresponding to Ge3Sb2Te6, Ge2Sb2Te5, and GeSb2Te4, alloys respectively in the same trigonal phase. By increasing the degree of order (according to the annealing temperature, the growth condition, etc) the spread in the statistical distribution of the blocks reduces and the distribution of the atoms in the cation planes also changes from a homogenous Ge/Sb mixing towards a Sb-enrichment in the planes closest to the van der Waals gaps. Therefore we show that the trigonal phase of Ge2Sb2Te5, the most studied chalcogenide for phase-change memories, is actually obtained in different configurations depending on the distribution of the stacking blocks (7-9-11 planes) and on the atomic occupation (Ge/Sb) at the cation planes. These results give an insight in the factors determining the stability of the trigonal phase and suggest a dynamic path evolution that could have a key role in the switching mechanism of interfacial phase change memories and in their data retention.

Determination of C54 nucleation site density in narrow stripes by sheet resistance measurements and m-Raman spectroscopy

Abstract The kinetic of the C49–C54 phase transformation at 730°C in TiSi 2 narrow strips for width in the 0.5–1.3-μm range was investigated by resistance measurements and μ-Raman spectroscopy. With this last technique a growth rate of 0.15 μm/s and a nucleation density of about 0.035 sites/μm 2 were obtained. The fraction of the transformed material as measured by resistance follows the Johnson–Mehl–Avrami equation, with an exponent equal to 1 for all of the analysed linewidths. Nucleation site saturation occurs and the growth is one-dimensional along the length of the strip. The characteristic time increases as 1/ W , W being the width of the strip, and, taking into account the growth rate obtained by μ-Raman spectroscopy, the nucleation density resulted 0.034 sites/μm 2 in excellent agreement with the μ-Raman results.

Ageing mechanisms of highly active and stable nickel-coated silicon photoanodes for water splitting

The photoelectrochemical (PEC) water splitting cell, a device that uses sunlight to produce hydrogen, has garnered very much interest due to its simple structure, low fabrication cost and good performance. In these cells, a semiconductor photoelectrode is immersed in a liquid and, when illuminated, hydrogen and/or oxygen can be generated on its surface by electrolysis. Metal catalysts are often used to enhance the activity of the semiconductor, but the lifetime of the semiconductor photoelectrode is still the main bottleneck of this technology. In this manuscript we report the ageing mechanisms of silicon photoanodes coated with nickel films of different thicknesses (under the light-driven oxygen evolution reaction, OER). The n-Si photoanodes coated with 2 nm-thick, 5 nm-thick and 10 nm-thick nickel layers showed lifetimes of ∼18 h, ∼150 h and >260 h, respectively. While the 2 nm-thick sample degraded due to the formation of a thin SiOX layer at the metal–silicon interface, the performance of the thicker samples decreased due to the formation of holes. The 5 nm-thick and 10 nm-thick nickel films turned into homogeneous potassium-free NiOX films suitable for water splitting, and this conversion markedly enhanced the performance of the cells. The density/size of holes in the surface decreased/increased with the metal thickness. The potassium contamination in the 2 nm-thick Ni sample took place in the form of nanofilaments, and we demonstrated that the widely used X-ray photoelectron spectroscopy tests are blind to these features, which may have been ignored in all previous reports. These results could be useful for understanding the degradation and enhancing the yield of water-splitting solar cells.

Kinetics of the C49–C54 transformation by micro-Raman imaging

Abstract The kinetics of the C49–C54 phase transition was analysed in TiSi 2 thin films obtained by reacting a Ti layer deposited on amorphous silicon. The C54 fraction was determined in the temperature range 680–720°C by electrical measurements and by micro-Raman spectroscopy. The Raman spectra were acquired by scanning large silicide areas (100×50 μm) and images showing the evolution of the C54 grains at different times and temperatures were obtained. The transformed fraction, the density and size distribution of the C54 grains were measured, and a detailed discussion of the errors associated with the micro-Raman technique is presented. The data indicate that the nucleation rate is not constant and the Johnson–Mehl–Avrami model cannot describe this transition.

Role of interfaces on the stability and electrical properties of Ge2Sb2Te5 crystalline structures

GeSbTe-based materials exhibit multiple crystalline phases, from disordered rocksalt, to rocksalt with ordered vacancy layers, and to the stable trigonal phase. In this paper we investigate the role of the interfaces on the structural and electrical properties of Ge2Sb2Te5. We find that the site of nucleation of the metastable rocksalt phase is crucial in determining the evolution towards vacancy ordering and the stable phase. By properly choosing the substrate and the capping layers, nucleation sites engineering can be obtained, thus promoting or preventing the vacancy ordering in the rocksalt structure or the conversion into the trigonal phase. The vacancy ordering occurs at lower annealing temperatures (170 °C) for films deposited in the amorphous phase on silicon (111), compared to the case of SiO2 substrate (200 °C), or in presence of a capping layer (330 °C). The mechanisms governing the nucleation have been explained in terms of interfacial energies. Resistance variations of about one order of magnitude have been measured upon transition from the disordered to the ordered rocksalt structure and then to the trigonal phase. The possibility to control the formation of the crystalline phases characterized by marked resistivity contrast is of fundamental relevance for the development of multilevel phase change data storage.

Study of the Effects of Growth Rate, Miscut Direction and Postgrowth Argon Annealing on the Surface Morphology of Homoepitaxially Grown 4H Silicon Carbide Films

we study the surface morphology of homoepitaxially grown 4H silicon carbide in terms of growth rate, miscut direction of the substrate and post growth argon thermal annealings. All the results indicate that the final surface morphology is the result of a competition between energetic reorganization and kinetic randomness. Because in all observed conditions energetic reorganization favors surface ondulations (“step bunching”), out-of-equilibrium conditions are one of the keys to favor the reduction of the surface roughness to values below ~0.5 nm. We theoretically support these results using kinetics superlattice Monte Carlo simulations (KslMC)

C49-C54 phase transition in nanometric titanium disilicide grains

The formation of single C49-TiSi2 grains embedded in a polycrystalline Si matrix has been studied in detail by means of in-situ sheet resistance, x-ray diffraction measurements, and energy filtered transmission electron microscopy characterization. The C49 clusters were obtained by starting from a 3 nm-Ti/40 nm-Si multilayer structure. After the C49 clusters formation, increasing the temperature in the 800–1100 °C range, the first-order C49-C54 transition has been therefore investigated in a system without triple grains boundaries. At each temperature the C54 fraction initially increases with the annealing time until it reaches a maximum constant value within ∼80 s. The maximum value of the converted C54 fraction increases linearly with the annealing temperature in the studied range. The nucleation sites density in these samples is ∼2×1011 cm−2, several orders of magnitude higher than in continuous TiSi2 films, where this value is about 4×106 cm−2.

Electrical properties of extended defects in 4H-SiC investigated by photoinduced current measurements

We study the correlation between crystal quality and electrical transport in 4H-SiC by micro-photoluminescence and laser-beam-induced photocurrent measurements. A focused HeCd laser at 325 nm has been employed to simultaneously measure, with a spatial resolution of a few microns, both the photoluminescence and current–voltage characteristics of 4H-SiC Schottky diodes. We found that the laser-induced photocurrent acquired along a defect can give information on its spatial distribution in depth and that the local minority carrier lifetime and generation depend on the type of stacking fault, both decreasing for defects with deeper intragap levels.