novel Semiconducting ALloys In ENergy Technology

Materials with radically enhanced performance for harvesting, conversion and transmission of energy are essential for a sustainable planet, where semiconducting oxides offer an attractive and wide span of functional properties. The project "novel Semiconducting ALloys In ENergy Technology" (Salient) explored new environmentally friendly materials for harvesting, conversion and transmission of energy. Oxide semiconductors are a vibrant field of research and development. However, for oxide semiconductors in general, and for ZnO in particular, it is challenging to exploit all their attractive inherent properties and full use in solid state devices. On the other hand, combined with a more mature technology, like gallium nitride (GaN), ZnO could potentially overcome several of its major challenges, like a stable p-type material. In fact, the tuning possibility of functional properties may exceed that of the parent semiconductors by unconventional alloying. Hence, the aim of the research project Salient was to investigate the physics of unconventional alloying, with alloys between ZnO and GaN as a main task. Salient started in 2015, and our efforts was in the beginning focused on developing ZnO-GaN alloys using magnetron sputtering and alloy compositions from pure ZnO up to about 20% GaN. In the project we have managed to grow high quality alloys with a strong band bowing, similar or even stronger than that previously reported for GaN with up to 20% ZnO. Further, we have studied the impact of post deposition annealing, and thus how the materials system responds when it is brought closer to thermal equilibrium. The activity has been supported by guest researchers working on first principle calculations, where we have shown that the local ordering of Ga an N in ZnO rich material has a strong effect on the band structure of the alloy. A key scientific question in Salient have been the origin of the band bowing effect in ZnO-GaN alloys. Thus we have had a strong focus on investigating this band bowing effect, and we have shown that this band bending can best be understood by a defect model when e.g. we introduce more and more GaN into ZnO. We also started to experiment with alloying different materials acting as absorbers in solar cells, where special emphasis was put on materials that are environmentally friendly, non-toxic and potentially cheap. In particular, we have developed a synthesis for ZnSnN2, a novel material with applications within solar cells. Indeed, we have managed to grow high quality films suitable to extract fundamental parameters such as lattice constants and band gap, and with mobility and dislocation density on par or better than that reported in the literature. The results were intriguing and has spurred interest and efforts beyond the Salient project, and will hopefully be continued in later projects.

An important aim of the project was to increase the competence of the research fellow as a young researcher. This goal was achieved through the research outcome as evidenced by the large number of papers, experience in PhD supervision, international collaboration, and peer-review work for high-impact journals. The Salient project was enabling in all these accomplishments. The project has enabled several collaborations and experiments that would otherwise not have been possible. Salient was a basic research project, contributing to understanding materials related to making renewable energy economically feasible, which is one of the main challenges of our society in the 21st century. The scientific goals of the project was achieved, although one conclusion of the project is that the ZnO-GaN is not optimal for solar cell applications due to the origin of the band bowing effect. Nonetheless, this has been one effort towards the larger goal of environmentally friendly energy harvesting.

With the urgent need of new and environmentally friendly materials for harvesting, conversion and transmission of energy, SALIENT addresses novel semiconducting materials related to thin films and nanostructures of advanced opto-electronic materials. SALIENT is a basic and cross-disciplinary research project combining synthesis, characterization and theoretical modeling within solid state physics and materials science. To expand and radically enhance the functionality of semiconducting oxides, the tuning possibility by unconventional alloying is pursued in order to exceed the performance of the parent semiconductors. Hence, the main scientific objective of SALIENT is to develop and understand the underlying physics of ZnO-X compounds, where X consists of Group III nitrides. The project is divided into two main tasks: I - Synthesis and structural characterization of novel semiconductor alloys based on ZnO and group III Nitrides, and II - Functional characterization and modeling of semiconductor ZnO-X alloys. The synthesis will follow two routes, a low X regime (concentration of X < 1%) utilizing ion implantation, and a high X regime (X > 5%) utilizing magnetron sputtering and atomic layer deposition. Through the development of this novel class of materials, three fundamental solid state physics issues will be explored: i) Controlling the structural properties of ZnO-X alloys, ii) Understanding and tailor the band bowing in ZnO-X alloys, iii) Tuning shallow acceptor level positions with respect to the valence band by ZnO-X alloying. SALIENT is coordinated by Lasse Vines and the LENS research group (Light and electricity from novel semiconductors) at UiO. The project is planned from May 2015 until November 2018 and involves 1 PhD student, 1 Post Doc (2 years), and 1 Guest Researcher (6 Months). SALIENT will further strongly interact with international partners and a recently started long-term and strategic research program entitled FOXHOUND.