In this Letter we report on the exploration of axial metal/semiconductor (Al/Ge) nanowire heterostructures with abrupt interfaces. The formation process is enabled by a thermal induced exchange reaction between the vapor–liquid–solid grown Ge nanowire and Al contact pads due to the substantially different diffusion behavior of Ge in Al and vice versa. Temperature-dependent I–V measurements revealed the metallic properties of the crystalline Al nanowire segments with a maximum current carrying capacity of about 0.8 MA/cm2. Transmission electron microscopy (TEM) characterization has confirmed both the composition and crystalline nature of the pure Al nanowire segments. A very sharp interface between the ⟨111⟩ oriented Ge nanowire and the reacted Al part was observed with a Schottky barrier height of 361 meV. To demonstrate the potential of this approach, a monolithic Al/Ge/Al heterostructure was used to fabricate a novel impact ionization device.

To fully exploit the potential of semiconducting nanowires for devices, high quality electrical contacts are of paramount importance. This work presents a detailed in situ transmission electron microscopy (TEM) study of a very promising type of NW contact where aluminum metal enters the germanium semiconducting nanowire to form an extremely abrupt and clean axial metal–semiconductor interface. We study this solid-state reaction between the aluminum contact and germanium nanowire in situ in the TEM using two different local heating methods. Following the reaction interface of the intrusion of Al in the Ge nanowire shows that at temperatures between 250 and 330 °C the position of the interface as a function of time is well fitted by a square root function, indicating that the reaction rate is limited by a diffusion process. Combining both chemical analysis and electron diffraction we find that the Ge of the nanowire core is completely exchanged by the entering Al atoms that form a monocrystalline nanowire with the usual face-centered cubic structure of Al, where the nanowire dimensions are inherited from the initial Ge nanowire. Model-based chemical mapping by energy dispersive X-ray spectroscopy (EDX) characterization reveals the three-dimensional chemical cross-section of the transformed nanowire with an Al core, surrounded by a thin pure Ge (∼2 nm), Al2O3 (∼3 nm), and Ge containing Al2O3 (∼1 nm) layer, respectively. The presence of Ge containing shells around the Al core indicates that Ge diffuses back into the metal reservoir by surface diffusion, which was confirmed by the detection of Ge atoms in the Al metal line by EDX analysis. Fitting a diffusion equation to the kinetic data allows the extraction of the diffusion coefficient at two different temperatures, which shows a good agreement with diffusion coefficients from literature for self-diffusion of Al.

A promising approach of making high quality contacts on semiconductors is a silicidation (for silicon) or germanidation (for germanium) annealing process, where the metal enters the semiconductor and creates a low resistance inter-metallic phase. In a nanowire, this process allows to fabricate axial heterostructures with dimensions depending only on the control and understanding of the thermally induced solid-state reaction. In this work, we present the first observation of both germanium and copper diffusion in opposite directions during the solid-state reaction of Cu contacts on Ge nanowires using in-situ Joule heating in a transmission electron microscope. The insitu observations allow us to follow the reaction in real time with nm spatial resolution. We follow the advancement of the reaction interface over time, which gives precious 2 information on the kinetics of this reaction. We combine the kinetic study with ex-situ characterization using model based energy dispersive X-ray spectroscopy (EDX) indicating that both Ge and Cu diffuse at the surface of the created Cu 3 Ge segment and the reaction rate is limited by Ge surface diffusion at temperatures between 360 and 600 • C. During the reaction, germanide crystals typically protrude from the reacted NW part. Their formation can however be avoided using a shell around the initial Ge NW. H a direct Joule heating experiments show slower reaction speeds indicating that the reaction can be initiated at lower temperatures. Moreover, they allow combining electrical measurements and heating in a single contacting scheme, rendering the Cu-Ge NW system promising for applications where very abrupt contacts and a perfectly controlled size of semiconducting region is required. Clearly in-situ TEM is a powerful technique to better understand the reaction kinetics and mechanism of metal-semiconductor phase formation.

Barium titanate–cobalt ferrite nanocomposite film showing room-temperature magnetoelectric multiferroics was fabricated only via affordable liquid phase processes.

Micro-concentrator solar cells enable higher power conversion efficiencies and material savings when compared to large-area non-concentrated solar cells. In this study, we use materials-efficient area-selective electrodeposition of the metallic elements, coupled with selenium reactive annealing, to form Cu(In,Ga)Se2 semiconductor absorber layers in patterned microelectrode arrays. This process achieves significant material savings of the low-abundance elements. The resulting copper-poor micro-absorber layers’ composition and homogeneity depend on the deposition charge, where higher charge leads to greater inhomogeneity in the Cu/In ratio and to a patchy presence of a CuIn5Se8 OVC phase. Photovoltaic devices show open-circuit voltages of up to 525 mV under a concentration factor of 18 ×, which is larger than other reported Cu(In,Ga)Se2 micro-solar cells fabricated by materials-efficient methods. Furthermore, a single micro-solar cell device, measured under light concentration, displayed a power conversion efficiency of 5% under a concentration factor of 33 ×. These results show the potential of the presented method to assemble micro-concentrator photovoltaic devices, which operate at higher efficiencies while using light concentration.

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Cadmium-free buffer layers deposited by a dry vacuum process are mandatory for low-cost and environmentally friendly Cu(In 1−x Ga x )Se 2 (CIGS) photovoltaic in-line production. Zn(O,S) has been identified as an alternative to the chemical bath deposited CdS buffer layer, providing comparable power conversion efficiencies. Recently, a significant efficiency enhancement has been reported for sputtered Zn(O,S) buffers after an annealing treatment of the complete solar cell stack; the enhancement was attributed to interdiffusion at the CIGS/ Zn(O,S) interface, resulting in wide-gap ZnSO 4 islands formation and reduced interface defects. Here, we exclude interdiffusion or island formation at the absorber/buffer interface after annealing up to 200 °C using high-resolution scanning transmission electron microscopy (HR-STEM) and energy-dispersive X-ray spectroscopy (EDX). Interestingly, HR-STEM imaging reveals an epitaxial relationship between a part of the Zn(O,S) buffer layer grains and the CIGS grains induced by annealing at such a low temperature. This alteration of the CIGS/buffer interface is expected to lead to a lower density of interface defects, and could explain the efficiency enhancement observed upon annealing the solar cell stack, although other causes cannot be excluded.

A high-throughput flame spray pyrolysis directly affords low-loading Pt catalyst supported on cerium oxide, which is an excellent material for selective semihydrogenation of C2H2 at 180 C, exhibiting nearly complete...