Vertically aligned GaN nanorod arrays with nonpolar InGaN/GaN multi quantum wells (MQW) were grown by MOVPE on c-plane GaN-on-sapphire templates. The chemical and structural properties of single nanorods are optically investigated with a spatial resolution beyond the diffraction limit using tip-enhanced Raman spectroscopy (TERS). This enables the local mapping of variations in the chemical composition, charge distribution, and strain in the MQW region of the nanorods. Nanoscale fluctuations of the In content in the InGaN layer of a few percent can be identified and visualized with a lateral resolution below 35 nm. We obtain evidence for the presence of indium clustering and the formation of cubic inclusions in the wurtzite matrix near the QW layers. These results are directly confirmed by high-resolution TEM images, revealing the presence of stacking faults and different polymorphs close to the surface near the MQW region. The combination of TERS and HRTEM demonstrates the potential of this nanoscale near-field imaging technique, establishing TERS as a very potent, comprehensive, and nondestructive tool for the characterization and optimization of technologically relevant semiconductor nanostructures.
The block copolymer self-assembly approach has received great attention in recent years as a possible way to overcome the limits of conventional lithography and to fabricate sub-22 nm structures. At this level, precise nanometric control is crucial for technological applications and the search for a flexible and reproducible protocol is a great challenge. The polystyrene-b-poly(methylmethacrylate) (PS-b-PMMA) system, with a styrene fraction of 0.71, spontaneously separates into a periodic array of hexagonally packed PMMA cylinders embedded in a matrix of PS and, under suitable processing conditions, this is perpendicularly oriented with respect to the underlying substrate. The selective removal of the PMMA allows us to obtain a nanoporous PS matrix with well-defined pore dimensions. Perpendicular orientation of the PMMA cylinders requires surface neutralization by means of a suitable PS-r-PMMA random copolymer. The choice of the random copolymer is not trivial, because different PS-r-PMMA copolymers strongly affect the characteristics of the PS-b-PMMA film deposited on it. In this paper the effects of the selected PS-r-PMMA on the arrangement as well as on the peculiar dimensions (pore diameter, pore to pore distance) of the final nanoporous PS thin film are studied. Reliable protocols for the fabrication of a disposable polymeric mask are proposed in view of its application in advanced lithographic processes.
We predict theoretically and show experimentally the occurrence of quantum confinement in hydrogenated nanocrystalline silicon. We prove that only valence states (positively charged carriers) are confined effectively within the nanograins. The emission associated to confined states is verified by photoluminescence experiments on nanocrystalline samples with controlled grain size. According to the present study, we propose nanocrystalline silicon as a promising material for oxygen-free optoelectronics, silicon-based memories and photovoltaics.
The self-assembly of block-copolymer thin films in periodic nanostructures has received considerable attention during the last decade due to their potential applications in nanofabrication and nanolithography. We followed the morphologies developed in thin films of a cylinder-forming diblock copolymer polystyrene-b-poly(methylmethacrylate) ((PS-b-PMMA), PS 46.1 kg mol( - 1), PMMA 21.0 kg mol( - 1), lattice spacing L(0) = 36 nm), as a function of the film thickness (t), analyzing the effect of thickness commensurability on domain orientation in respect to the substrate. The study was circumscribed to the unexplored range of thickness below L(0). Two thickness windows with perpendicular orientation of the PMMA domains were identified: a well-known window at t approximately L(0) and a new window at t approximately L(0)/2. A half-parallel cylinder morphology was observed for [Formula: see text] with a progressive change in morphology [Formula: see text] when thickness increases from L(0)/2 to L(0). This experimental evidence provides new insights on the mechanism of block copolymers self-organization and indicates the possibility to tune the thickness of the nanostructured polymeric film below L(0), allowing the fabrication of ultrathin soft masks for advanced lithographic processes.
