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.

We have investigated by atomic force microscopy and scanning tunneling microscopy subsequent stages of the heteroepitaxy of InAs on GaAs(001) from the initial formation of the strained two-dimensional wetting layer up to the development of three-dimensional quantum dots. We provide evidence of structural features that play a crucial role in the two- to three-dimensional transition and discuss their contribution to the final morphology of the self-assembled nanoparticles. A model is suggested for the strained phase at the critical thickness consisting of an intermixed InxGa1 - xAs surface layer of composition x = 0.82 and InAs "floating" on top. Such "floating" phase participate to the large mass transport along the surface during the two- to three-dimensional transition that accounts quantitatively for the total volume of dots

A combination of far-infrared and Raman spectroscopy is employed to investigate vibrational modes and the carrier behavior in amorphous and crystalline ordered GeTe-Sb₂Te₃ alloys (GST) epitaxially grown on Si(111). The infrared active GST mode is not observed in the Raman spectra and vice versa, indication of the fact that inversion symmetry is preserved in the metastable cubic phase in accordance with the F"Equation missing"<!-- image only, no MathML or LaTex -->m3 space group. For the trigonal phase, instead, a partial symmetry break due to Ge/Sb mixed anion layers is observed. By studying the crystallization process upon annealing with both the techniques, we identify temperature regions corresponding to the occurrence of different phases as well as the transition from one phase to the next. Activation energies of 0.43 eV and 0.08 eV for the electron conduction are obtained for both cubic and trigonal phases, respectively. In addition a metal-insulator transition is clearly identified to occur at the onset of the transition between the disordered and the ordered cubic phase.

We studied the temperature dependence of the two-dimensional to three-dimensional growth transition in InAs∕GaAs(001) heteroepitaxy by means of reflection high energy electron diffraction and atomic force microscopy. The observed shift of the transition to higher InAs deposition times, at temperatures above 500°C, is not a change of critical thickness for islanding, which instead, is constant in the 450–560°C range. Consequently, In-Ga intermixing and surface and interface strain have a negligible dependence on temperature in this range.

We have investigated, by means of atomic force microscopy, the complete evolution of $\mathrm{InAs}∕\mathrm{GaAs}\left(001\right)$ quantum dots as a function of deposited InAs. Direct evidence is found for step erosion by quantum dots nucleated onto the step edge and an estimate of the eroded volume is provided. By studying the quantum dots volume as a function of InAs coverage, we show that the wetting layer contribution is confined within a narrow range of coverage around the two- and three-dimensional transition.