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The motivation of this work is the tailored growth of Ge nanocrystals for photovoltaic applications. The use of superlattices provides a reliable method to control the Ge nanocrystal size after phase separation. In this paper, we report on the deposition of (GeOx–SiO2) superlattices via reactive dc magnetron sputtering and the self-ordered Ge nanocrystal formation during subsequent annealing. Attention is directed mainly to define proper deposition conditions for tuning the GeOx composition between elemental Ge (x=0) and GeO2 (x=2) by the variation in the deposition temperature and the oxygen partial pressure. A convenient process window has been found which allows sequential GeOx–SiO2 deposition without changing the oxygen partial pressure during deposition. The phase separation and Ge nanocrystal formation after subsequent annealing were investigated with in situ x-ray scattering, Raman spectroscopy, and electron microscopy. By these methods the existence of 2–5 nm Ge nanocrystals at annealing temperatures of 600–750 °C has been confirmed which is within the superlattice stability range. The technique used allows the fabrication of superlattice stacks with very smooth interfaces (roughness<1 nm); thus the Ge nanocrystal layers could be separated by very thin SiO2 films (d<3 nm) which offers interesting possibilities for charge transport via direct tunneling. [ABSTRACT FROM AUTHOR]

Au nanocrystals (NCs) were synthesized in a thin SiO2 layer by ion implantation and annealing in a tight distribution close to the Si/SiO2 interface. Between the NCs and the Si substrate a thin tunneling oxide forms self-organized during annealing totally depleted from Au NCs. Memory behavior is demonstrated by electron charging and discharging on metal–oxide–semiconductor capacitors. Lenticular liquid Au:Si droplets nucleate at the Si/SiO2 interface from silicon regions supersaturated by Au close to the oxide. Au NCs embedded in SiO2 above these droplets are stabilized during annealing due to a modified detailed balance of Au atom detachment and attachment. Capacitance-voltage and spreading resistance measurements reveal the impact of the Au contamination in the Si substrate. Structure and distribution of Au droplets and NCs are characterized by x-ray diffraction and transmission electron microscopy. [ABSTRACT FROM AUTHOR]

Nowadays ferromagnetism is often found in potential diluted magnetic semiconductor systems. However, many authors argue that the observed ferromagnetism stems from ferromagnetic precipitates or spinodal decomposition rather than from carrier-mediated magnetic impurities, as required for a diluted magnetic semiconductor. In the present article, we answer this question for Fe-implanted ZnO single crystals comprehensively. Different implantation fluences, temperatures, and post-implantation annealing temperatures have been chosen in order to evaluate the structural and magnetic properties over a wide range of parameters. Three different regimes with respect to Fe concentration and process temperature are found: (1) Disperse Fe2+ and Fe3+ at low Fe concentrations and low processing temperatures, (2) FeZn2O4 at very high processing temperatures, and (3) an intermediate regime with a coexistence of metallic Fe (Fe0) and ionic Fe (Fe2+ and Fe3+). Ferromagnetism is only observed in the latter two cases, where inverted spinel ZnFe2O4 and α-Fe nanocrystals are the origin of the observed ferromagnetic behavior, respectively. The ionic Fe in the last case could contribute to a carrier-mediated coupling. However, the separation between Fe ions is too large to couple ferromagnetically due to the lack of p-type carrier. For comparison, investigations of Fe-implanted epitaxial ZnO thin films are presented. [ABSTRACT FROM AUTHOR]

The write performance of a multidot-memory-like structure with respect to hole tunneling is investigated in metal-oxide-semiconductor (MOS) capacitors. The oxide of the MOS structure on p-type Si contains a layer of ion beam synthesized Ge nanocrystals (NC’s) very close to the Si/SiO2 interface. This structure is modeled in a floating gate-like approach, where the NC’s are considered as individual storage nodes and charged by direct tunneling of holes. Quantum confinement and Coulomb blockade effects of small Ge NC’s (1–6 nm) are discussed and found to be negligible for the present structure. A close agreement between the calculated write characteristics and experimental data clearly confirms the validity of the model. From the simulation results it is predicted that a flatband voltage shift of about ΔVFB=-1 V could be gathered with programming times tprog<1 μs. The write parameters (pulse voltage and duration) for a given ΔVFB value are mainly determined by the distance of the NC’s to the substrate. [ABSTRACT FROM AUTHOR]

Abstract: The formation of interface and border states in metal–oxide–semiconductor structures associated with the generation of embedded germanium nanocrystals in 20nm SiO2-layers by means of ion implantation and a subsequent annealing was examined. Deep level transient spectroscopy and related time-domain techniques were applied in order to study the charge trapping and emission at the Si–SiO2 interface. A significant dependence of the interface state density D it on the conditions of the cluster generation was found. Any Ge-implanted sample features a pronounced level at about 0.31eV above the valence band edge and a concentration above 1013 cm−2 eV−1, likely related to a Pb-center. A systematic variation of the filling pulse parameters was utilized in order to separate the response of fast and slow states, and to substantiate the existence of border traps located in the vicinity of the Si–SiO2 interface. The role of interface and border traps for the relaxation of the trapped charge in the nanocrystals is illustrated. [Copyright &y& Elsevier]

Abstract: The evolution of ion beam synthesized Co and Ge nanoclusters into a SiO2 matrix during annealing processes has been investigated by X-ray diffraction and transmission electron microscopy. Remarkable differences have been found between Ge and Co clusters behaviour. For Ge implanted SiO2 films, a clear influence of near-surface Ge oxidation and nanoclusters melting has been established. Annealing at temperatures around 1000°C leads to the formation of small (diameter ∼5nm) nanocrystals. Classical Ostwald ripening mainly drives the clusters thermodynamical growth. On the contrary, for Co-implanted SiO2 films, a jump-like transition in nanoclusters evolution has been established at about 800°C. A homogenous distribution of small (diameter ∼4nm) amorphous clusters is transformed into a bimodal clusters profile, characterised by large (diameter between 20 and 40nm) nanocrystals near the surface and a region of smaller clusters (diameter ∼7nm) in depth. During Co nanoclusters formation the influence of nanoclusters melting can be neglected. [Copyright &y& Elsevier]

A Ge nanocrystal layer embedded in a thin gate oxide was prepared by ion beam synthesis in direct-tunneling distance to the Si substrate. The write performance was investigated in metal-oxide-semiconductor capacitors by means of capacitance measurements. With the experimental data and calculations using a floating-gate-like approach, the distribution of the tunneling oxide thickness dtox can be determined in high precision confirmed by high-angle annular dark-field scanning transmission electron microscopy imaging. The evolution of dtox during heat treatment is discussed in terms of Ostwald ripening; i.e., dtox increases with annealing time. [ABSTRACT FROM AUTHOR]