The electrical and optoelectronic properties of materials are determined by the chemical potentials of their constituents. The relative density of point defects is thus controlled, allowing to craft microstructure, trap densities and doping levels. Here, we show that the chemical potentials of chalcogenide materials near the edge of their existence region are not only determined during growth but also at room temperature by post-processing. In particular, we study the generation of anion vacancies, which are critical defects in chalcogenide semiconductors and topological insulators. The example of CuInSe 2 photovoltaic semiconductor reveals that single phase material crosses the phase boundary and forms surface secondary phases upon oxidation, thereby creating anion vacancies. The arising metastable point defect population explains a common root cause of performance losses. This study shows how selective defect annihilation is attained with tailored chemical treatments that mitigate anion vacancy formation and improve the performance of CuInSe 2 solar cells.

2D GaSe is a semiconductor belonging to the group of post‐transition metal chalcogenides with great potential for advanced optoelectronic applications. The weak interlayer interaction in multilayer 2D materials allows the formation of several polymorphs. Here, the first structural observation of a new GaSe polymorph is reported, characterized by a distinct atomic configuration with a centrosymmetric monolayer (D3d point group). The atomic structure of this new GaSe polymorph is determined by aberration‐corrected scanning transmission electron microscopy. Density‐functional theory calculations verify the structural stability of this polymorph. Furthermore, the band structure and Raman intensities are calculated, predicting slight differences to the currently known polymorphs. In addition, the occurrence of layer rotations, interlayer relative orientations, as well as translation shear faults is discussed. The experimental confirmation of the new GaSe polymorph indicates the importance of investigating changes in the crystal structure, which can further impact the properties of this family of compounds.

Nevertheless, the research of 2D materials is still in its early stages. The field started with graphene several years ago, [3] but more materials have been added continuously to the list. Among these, transition metal dichalcogenides (TMDC), [4] which have impressive optical properties in the visible range, are currently one of the most studied 2D materials. Devices made of these materials have shown remarkable performance as photodetectors, [5,6] nonvolatile random access memory, [7] or photovoltaics, [8] even when using simple manual exfoliation methods. More recently, 2D indium selenides (In x Se y) have gained attention, [9] due to their bandgap in the visible spectral region, comparable to TMDCs, but also due to several novel properties: InSe exhibits one of the largest mobilities of 2D semiconductor materials, [10] and β-In 2 Se 3 shows good mobility, [11] excellent photoresponsivity, [12] and exotic ferroelectricity [13] (the ordered ferroelectric phase is also called β′-In 2 Se 3 by some authors). [11,13] Transistors with high mobility and on/off ratio have already been realized. [14] Therefore, 2D In 2 Se 3 materials have the potential to address several limitations of the current silicon (Si) and III-V technologies, such as improved mobility and overall performance of transistors for electronics, as well as integrated photodetectors and light emitters in the same material system (same die), all possible on virtually any substrate, including transparent and flexible substrates. [15,16] Yet, the majority of reported devices from 2D materials rely on fabrication methods based on exfoliation and transfer of layers onto other substrates or other 2D materials. While this device fabrication process allows unprecedented flexibility in the combination of materials and therefore has nearly unlimited device design possibilities, [1] it leads typically to individual or few devices and is a slow and tedious process. On the other hand, due to their layered nature, instead of strong covalent bonds (as in Si and compound semiconductors) van der Waals forces exist between the layers. Hence, 2D materials can be grown epitaxially on substrates with completely different lattice constants without creating the strain and the defects commonly found in mismatched heteroepitaxy. This method is called van der Waals (vdW) epitaxy. [17] Nevertheless, the substrate does influence the growth, [18] as it modifies the nucleation of the first layer. Some substrates provide growth with superior quality (amount of defects, 2D materials are considered the future of electronics and photonics, stimulated by their remarkable performance. Among the 2D materials family, β-In 2 Se 3 shows good mobility, excellent photoresponsivity, and exotic ferroelectricity, making it suitable for a wide variety of applications. To date, most reported devices from 2D materials in general, and β-In 2 Se 3 in specific, rely on cumbersome fabrication methods using mechanical exfoliation and transfer of layers onto other substrates. However, for a successful ado...

No abstract