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Electron dynamics at surface-modified photocathodes

Subject Area Synthesis and Properties of Functional Materials
Term from 2019 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 424936963
 
Final Report Year 2024

Final Report Abstract

At TUIL atomically well-ordered InP(001) and AlInP(001) surfaces were prepared in metalorganic vapor-phase epitaxy reactor and studied by ultra-high vacuum (UHV) surface sensitive techniques. The surface structure and the surface states of these samples prior to and after exposure to H2O vapor and O2 in UHV were investigated. Selected samples were transferred, in UHV to preserve their surface reconstruction, to TUDa (PA1) to study surface states, to HZB to study near-surface electronic structure (see below) and for a subsequent deposition of TiO2 (see PA1). We found that on phosphorus-terminated (P-rich) InP(100) and AlInP(100) surfaces, the P-P dimers are not a favorable bonding site for oxygen adsorption and are much more stable upon O2 or H2O exposure compared to the indium-terminated surfaces. The near surface region at the oxidized n-AlInP(001) samples can be modified by so called surface functionalization, which leads to a partial transformation of the surface into a mixture phosphates and phosphites. Such procedure leads to lowering the conduction band maximum, which facilitates the transport of electrons from the bulk towards the surface. At HZB the near-surface electronic structure of P-rich, p-doped InP(100) (p-InP) and n-doped AlInP(100) surfaces were studied using tr-2PPE, revealing distinct surface resonances and -states. Pathways for migration of photoexcited electron bulk electrons to the surface were identified, mainly driven by the downward band bending of the p-doped sampled due to surface Fermi level pinning. Decay constants for conduction band states were obtained completing the understanding of nearsurface dynamics in p-InP. The effects of TiO2 deposition on near-surface electron dynamics were studied and found to preserve the discrete states within the P-rich InP/TiO2 interface. For the n-AlInP a bulk-to-surface transition cluster of peaks was found, similarly to p-InP, exhibiting fast thermalization and at least in part explaining its high performance as a window layer. This understanding of p-InP as a P-containing III-V reference surface and as a PEC/PV photoabsorber, as well as the observations on n-AlInP as a window layer reported here are the first such detailed data sets on near-surface electron dynamics on III-V semiconductors.The DFT calculations in Paderborn provided much new, and sometimes surprising insight into III-V(001) surfaces and interfaces and contributed substantially to a better understanding of experimental data. The most interesting results are (i) a comprehensive characterization of the GaInP/AlInP interface including band alignments for the complete stoichiometry range, (ii) the explanation of the Fermi level pinning at gas-phase epitaxy grown InP(001) surfaces in terms of H desorption related surface defects, (iii) the calculation of an AlInP(001) surface phase diagram which allows for the identification of stable surface structures in dependence on pressure and temperature and (iv) the interpretation of AlInP(001) surface optical anisotropies in terms of surface structural motifs as well as subsurface ordering effects.

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