III-V compound semiconductors are important for various electronic and optoelectronic applications, e.g., high-electron-mobility transistors, light-emitting diodes, photodetectors, electro-optic modulators, and frequencymixing components. Solar cells made of III-V semiconductors reach the highest efficiencies of any photovoltaic technology so far . The Al x In 1-x P (AlInP) material system is frequently used as window layer in solar cells, due to its favorable combination of chemical stability, sufficiently wide bandgap, and high quality heteroepitaxial interfaces with many absorbers . In this context, the Fermi level pinning due to AlInP surface states is highly relevant for the device performance . This holds also for the usage of AlInP as intermediate layer to form low resistance ohmic contacts .