A detailed mechanism of degradation behaviour of biodegradable as-ECAPed Zn-0.8Mg-0.2Sr with emphasis on localized corrosion attack.

In this study, advanced techniques such as atom probe tomography, atomic force microscopy, X-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy were used to determine the corrosion mechanism of the as-ECAPed Zn-0.8Mg-0.2Sr alloy. The influence of microstructural and surface features on the corrosion mechanism was investigated. Despite its significance, the surface composition before exposure is often neglected by the scientific community. The analyses revealed the formation of thin ZnO, MgO, and MgCO3 layers on the surface of the material before exposure. These layers participated in the formation of corrosion products, leading to the predominant occurrence of hydrozincite. In addition, the layers possessed different resistance to the environment, resulting in localized corrosion attacks. The segregation of Mg on the Zn grain boundaries with lower potential compared with the Zn-matrix was revealed by atom probe tomography and atomic force microscopy. The degradation process was initiated by the activity of micro-galvanic cells, specifically Zn - Mg2Zn11/SrZn13. This process led to the activity of the crevice corrosion mechanism and subsequent attack to a depth of 250 µm. The corrosion rate of the alloy determined by the weight loss method was 0.36 mm·a-1. Based on this detailed study, the degradation mechanism of the Zn-0.8Mg-0.2Sr alloy is proposed.

Electron Transport Properties in High Electron Mobility Transistor Structures Improved by V-Pit Formation on the AlGaN/GaN Interface.

This work suggests new morphology for the AlGaN/GaN interface which enhances electron mobility in two-dimensional electron gas (2DEG) of high-electron mobility transistor (HEMT) structures. The widely used technology for the preparation of GaN channels in AlGaN/GaN HEMT transistors is growth at a high temperature of around 1000 °C in an H2 atmosphere. The main reason for these conditions is the aim to prepare an atomically flat epitaxial surface for the AlGaN/GaN interface and to achieve a layer with the lowest possible carbon concentration. In this work, we show that a smooth AlGaN/GaN interface is not necessary for high electron mobility in 2DEG. Surprisingly, when the high-temperature GaN channel layer is replaced by the layer grown at a temperature of 870 °C in an N2 atmosphere using TEGa as a precursor, the electron Hall mobility increases significantly. This unexpected behavior can be explained by a spatial separation of electrons by V-pits from the regions surrounding dislocation which contain increased concentration of point defects and impurities.

Nanoscale Study of the Hole-Selective Passivating Contacts with High Thermal Budget Using C-AFM Tomography.

We investigate hole-selective passivating contacts that consist of an interfacial layer of silicon oxide (SiOx) and a layer of boron-doped SiCx(p). The fabrication process of these contacts involves an annealing step at temperatures above 750 °C which crystallizes the initially amorphous layer and diffuses dopants across the interfacial oxide into the wafer to facilitate charge transport, but it can also disrupt the SiOx layer necessary for wafer-surface passivation. To investigate the transport mechanism of the charge carriers through the selective contact and its changes during the annealing process, we utilize various characterization methods, such as transmission electron microscopy, micro Raman spectroscopy, and conductive atomic force microscopy. Combining the latter with a sequential removal of material, we assemble a tomographic reconstruction of the crystallized layer that reveals the presence of preferential vertical transport channels.

Nucleation and growth of metal-catalyzed silicon nanowires under plasma.

We report the results of a microscopic study of the nucleation and early growth stages of metal-catalyzed silicon nanowires in plasma-enhanced chemical vapor deposition. The nucleation of silicon nanowires is investigated as a function of different deposition conditions and metal catalysts (Sn, In and Au) using correlation of atomic force microscopy and scanning electron microscopy. This correlation method enabled us to visualize individual catalytic nanoparticles before and after the nanowire growth and identify the key parameters influencing the nanowire nucleation under plasma. The size and position of catalytic nanoparticles are found to play a significant role in the nucleation. We demonstrate that only small isolated nanoparticles in the range of 10-20 nm contribute to the nanowire growth under plasma, while larger nanoparticles are inactive because they get buried under a layer of a-Si:H before reaching supersaturation. Systematic analysis of different growth parameters reveals that the nanowire growth in plasma contradicts the vapor-liquid-solid mechanism at thermal equilibrium in many ways. The nanowire growth is much faster and proceeds even at negligible silicon solubility and bellow the eutectic temperature of the metal-silicon alloy. Based on the observations, we propose the nanowire growth under plasma to be characterized by the rapid solidification mechanism, where a crystalline silicon phase emerges from a metastable supersaturated liquid metal-silicon phase in local nonequilibrium.