Hannappel, Th.; May, M.M.; Lewerenz, H.-J.: Epitaxial III-V Thin Film Absorbers: Preparation, Efficient InP Photocathodes and Routes to High Efficiency Tandem Structures. In: Lewerenz, H.-J.; Laurence, P. [Eds.] : Photoelectrochemical Water Splitting : Materials, Processes and Architectures. London: RSC Publ., 2013. - ISBN 978-1-84973-647-3, p. 223-265
The material class of III-V semiconductors allows the design of highly efficient absorbers for light-induced water splitting due to its outstanding flexibility. This includes the possibility to tune magnitude and nature of band gaps as well as lattice constants enabling well-defined homo- and heteroepitaxial growth by metal-organic vapor phase epitaxy. Reflection anisotropy spectroscopy enables optical in-situ growth control facilitating the preparation of abrupt interfaces, which are essential for high-efficiency applications. Following this approach, InP(100) absorbers were prepared featuring a photocathode, that was both stable and efficient. Surface functionalization could be achieved by in-situ photoelectrochemical preparation of an n-type indium oxide layer, which contains phosphates and phosphites, creating a buried p-n+ junction with a favorable band alignment. In addition to stabilizing the cathode against corrosion, the junction enables efficient charge transfer to the Rh electrocatalyst at the electrolyte interface. Our recent progress in III-V heteroepitaxy on Si substrates extends the parameter space for heterostructures aiming for a water splitting tandem. With Si as bottom cell, the top cell should exhibit an energy gap in the order of 1.7 to 2.0 eV, which makes the lattice-matched dilute nitride GaP1-xNx a promising candidate. To gain a better understanding of initial surface transformations induced by the electrolyte, we employ model adsorption experiments with GaP(100), the material basis for the dilute nitride. Furthermore, we describe the concept for quantum-well, nanostructured absorbers, which are the subject of ongoing research.