Multijunction solar cells that are based on MOVPE-grown III-V compound semiconductor layers achieve the highest solar cell efficiencies. However, major challenges in epitaxial growth procedures need to be overcome to develop devices that contain III-V multinary compound interfaces, as the electrical junction and IV characteristics are highly sensitive to interface quality. In an attempt to perform comprehensive structural and electrical characterization of working photovoltaic devices with high spatial resolution, we have combined in situ electrical biasing in the transmission electron microscope (TEM) with optimized TEM sample preparation and contacting. Electrostatic potentials at p-n junctions can be measured with nm spatial resolution using off-axis electron holography (EH), while electron beam induced current (EBIC) measurements in scanning TEM (STEM) can be used to map the generation and collection of charge carriers in direct bandgap materials. These complementary methods can be used to detect p-n junctions, non-uniform electrical properties and defects acting as recombination centres.

Here, we present in situ EH and STEM-EBIC measurements of electrically-conatcted GaAs, InP and GaInP p-n junctions with different doping concentrations. Electron-transparent specimens of different thickness (Fig. 1A) were studied to assess the influence on the measurements of electrically inactive specimen surface layers introduced during focused ion beam milling. The TEM specimens were connected to dedicated electrical biasing chips and studied in a biasing TEM specimen holder (Protochips Aduro 500). Different contacting materials, such as W, Au and Pt, were applied to the top and bottom lamella surfaces to assess the reliability of Ohmic contacting. Fig. 1B shows a STEM-generated EBIC map, which reveals the spatial distribution of charge generation near the electrical junctions. Fig. 1C shows a map of the electron optical phase shift (proportional to projected electrostatic potential) reconstructed from an off-axis electron hologram stack. A line profile of the phase distribution across the junctions, extracted from the phase image, is shown in Fig. 1D. The results demonstrate the successful combination of these methods for the in situ characterization of the electrostatic potential and current distributions near p-n junctions.