Our group focuses on the development of stable and efficient metal oxide passivation protection layers with an appropriate energetic alignment (in order to minimize the efficiency losses) to (i) multiabsorber photoelectrochemical cells consisting of a combination of III-V semiconductors and Si photoabsorbers and to (ii) selective catalysts for a direct, solar-driven sustainable fuel production. The group combines fundamental study with applied research.

Recommended selected publication: key challenges of integrated devices for a direct and efficient water splitting.

Solar fuels produced using renewable energy allow the solar energy to be stored in the form of chemical bonds, such as for example in hydrogen molecules. So-called photoelectrochemical cells are considered the foremost route toward decarbonized solar fuel production. These systems use semiconductor-based materials to drive, in the case of hydrogen production, the splitting of water molecules into hydrogen and oxygen. A very good photoabsorber for such cells is based on III-V semiconductors. However, they are known to be liable to damage or even to dissolve in the electrolyte. To ensure the durability and reliability of the system, a protection layer must be integrated. To drive the desired chemical reaction, an appropriate and highly selective catalyst must be added as well. Such systems already exist, but they still suffer from low stability, which is defined by the material composition of both the protection layer and the catalyst.

Metal Organic Chemical Vapor Deposition (MOCVD) is a widely used method for preparing epitaxial structures with high purity and structural order by depositing atoms on a wafer substrate. It is utilized for a broad variety of applications in industry at large scale and with high throughput. The MOCVD growth is complex and the surface and interface preparation will affect the quality of the subsequently grown layer and electronic states in the band gap at the heterointerfaces.

Near ambient pressure limits the application of surface sensitive methods, however an optical method, reflection anisotropy spectroscopy (RAS), which is highly surface-sensitive, can be used for example to analyze the composition and the surface reconstruction of surfaces.

Samples can be transferred through ultra-high vacuum into atomic layer deposition chamber or ultra-high vacuum-based, surface sensitive measurement chambers via a dedicated transfer shuttle. For the characterization we have available a variety of methods.

Recommended selected publication: the use of the optical in situ spectroscopy which enables precise (in atomic-scale) and sophisticated in situ control over surfaces preparation.

Atomic layer deposition (ALD) is a modified process of chemical vapor deposition. During that process self-limiting surface reactions allow to deposit extremely thin and homogeneous layers with low-defect density. Metal oxide layers, such as TiO2 has been shown to exhibit much higher stability than III-V semiconductors when in contact with the electrolyte under operation. Despite efforts to optimize thin film depositions, there is still ambiguity regarding the reaction mechanisms of the precursor with the substrate (interface formation) and the impact of the process parameters on the properties of the grown layer, such as structure, purity and uniformity, and more importantly, its impact on the chemical stability.

In particular, defects, which originate at the III-V semiconductor/metal-oxide heterointerface will affect the film stability once exposed to the electrolyte under light illumination. Choice of the precursors for the metal and oxide, growth temperature and the substrate preparation will strongly influence the chemical and electronic structure at the heterointerface, and the structure, surface morphology and defects of the grown film.

ALD process for Ti-based and (Al)Ga-based oxide passivation and protection layers with low defect density and high stability against photocorrosion will be developed. Optical in situ spectroscopy (RAS) and in line X-ray photoelectronspectroscopy (XPS) will be used to understand the chemical composition at the heterointerface of the film at the initial stage of the growth.

Interfaces are key elements that define electronic properties of the final device. Inevitably, however, most of the active interfaces of III–V semiconductor devices are buried and it is therefore not straightforward to characterize them. The interface between the solid and the electrolyte is decisive for the performance of the application. Both, the solid/solid and solid/liquid interfaces should be understood in terms of chemical composition, crystalline structure, defects and electronic structure.

Researching these cells in contact with an electrolyte under realistic working conditions is challenging due to ambient pressure, but necessary to thoroughly investigate how the surface electronic surface relates to interfacial chemistry and how does it change over the time.

X-ray photoelectron spectroscopy (XPS) is an excellent technique to explore chemically complex heterointerfaces. Enhanced information depth is achievable using increased photon energy to resolve core-levels regime, and soft X-ray can be used to probe the valance band region. Ambient pressure photoelectron spectroscopy will be used to probe samples in contact with an electrolyte. The results will be correlated with photoelectrochemical measurements. Well-defined III-V heterointerfaces will be prepared with and without catalysts and passivation protection layers.

Recommended selected publication: the use of the hard X-ray photoelectron spectroscopy to resolve the atomic and electronic structure of buried heterointerfaces.