Profiling the Built-in Electrical Potential in III-V Multijunction Solar Cells: Preprint

We report on a direct measurement of the electrical potential on cross-sections of GaInP2/GaAs multiple-junction solar cells by using an ultrahigh-vacuum scanning Kelvin probe microscope (UHV-SKPM). The UHV-SKPM allows us to measure the potential without air molecules being adsorbed on the cross-sectional surface. Moreover, it uses a GaAs laser with photon energy of 1.4 eV for the atomic forcemicroscope (AFM) operation. This eliminated the light-absorption-induced bottom-junction flattening and top-junction enhancement, which happened in our previous potential measurement using a 1.85-eV laser for the AFM operation. Three potentials were measured at the top, tunneling, and bottom junctions. Values of the potentials are smaller than the potentials in the bulk. This indicates that theFermi level on the UHV-cleaved (110) surface was pinned, presumably due to defects upon cleaving. We also observed higher potentials at atomic steps than on the terraces for both GaInP2 epitaxial layer and GaAs substrate. Combining scanning tunneling microscopy (STM) and SKPM measurements, we found that the potential height at steps of the GaAs substrate depends on the step direction, which isprobably a direct result of unbalanced cations and anions at the steps.