Flow units as dynamic defects in metallic glassy materials

In a crystalline material, structural defects such as dislocations or twins are well defined and largely determine the mechanical and other properties of the material. For metallic glass (MG) with unique properties in the absence of a long-range lattice, intensive efforts have focused on the search for similar ‘defects’. The primary objective has been the elucidation of the flow mechanism of MGs. However, their atomistic mechanism of mechanical deformation and atomic flow response to stress, temperature, and failure, have proven to be challenging. In this paper, we briefly review the state-of-the-art studies on the dynamic defects in metallic glasses from the perspective of flow units. The characteristics, activation and evolution processes of flow units as well as their correlation with mechanical properties, including plasticity, strength, fracture, and dynamic relaxation, are introduced. We show that flow units that are similar to structural defects such as dislocations are crucial in the optimization and design of metallic glassy materials via the thermal, mechanical and high-pressure tailoring of these units. In this report, the relevant issues and open questions with regard to the flow unit model are also introduced and discussed.

Advanced Real Time Metrology of AlGaN/GaN and InGaN/GaN Epitaxy

Nitride materials are critical for a range of applications, including UV-visible light emitting diodes (LEDs). Advancing the performance, reliability and synthesis of AlGaN/GaN and InGaN/GaN heterojunction devices requires a systematic methodology enabling characterization of key metric like alloy composition, thickness and quality possibly in real time. This contribution reports on the real time characterization of the plasma assisted molecular beam epitaxy of AlGaN/GaN and InGaN/GaN heterostructures. Spectroscopic ellipsometry real time monitoring has revealed a number of key process and material iusses, such as the roughening of the GaN templates depending on plasma exposure during the substrate cleaning step, the composition of the alloy and the growth mode. Parameters like the plasma conditions, the surface temperature and the atomic flow ratio are investigated to understand the interplay process-material composition-structure-optical properties.

In-Plane Texturing of Buffer Layers by Alternating Beam Assisted Deposition: Large Area and Small Area Applications

In standard ion beam assisted deposition (IBAD), the growing film is exposed to inclined ion etching in order to achieve a preferable in-plane orientation of the crystalline structure. Recently, we suggested exposing the film periodically to deposition pulses and to etching pulses, i.e. to assisted beam pulses. As a long sequence of alternations of these two pulses is needed, we named this method “alternating beam assisted deposition” (ABAD). In real application, the substrates exposed to the molecular/atomic flow originating from the sputter source acquire a few nanometer thick layer of yttria-stabilized zirconia (YSZ). In the next step, this layer undergoes ion etching with an Ar ion beam emitted from the source with a particle energy between 200 and 300 eV. Simultaneously with ion-beam exposition, an additional electron beam provides neutralizing of the electrical change in the substrate plane. The ion beam guided 55° at a 55° angle of incidence provides selective etching of the YSZ layer, leading finally after numerous deposition-etching cycles to a sufficiently high quality of in-plane texture in the YSZ layer with the best FWHM values of 8°-9°.