Initiation strategies for simultaneous control of antiphase domains and stacking faults in GaAs solar cells on Ge

Incorporating a Ge junction into a lattice-matched GaInP/GaAs/GaInNAsSb triple-junction cell grown by molecular beam epitaxy (MBE) could enable concentrated efficiencies of ∼50%. Epitaxial integration allows lift-off and wafer bonding steps to be avoided, but growth of III-Vs on Ge by MBE can lead to antiphase domains (APD) and stacking fault pyramids (SFP), both of which diminish solar cell performance. Initiating growth by migration-enhanced epitaxy (MEE) is typically cited as necessary to obtain high-quality III-Vs on Ge. In this work, the authors show that typical MEE growth conditions force a compromise between APD height (h_APD) and SFP density (ρ_SFP). As APDs can readily self-terminate while SFPs cannot, a two-step initiation strategy was employed, where MEE is performed under conditions that minimize ρ_SFP followed by low-temperature MBE conditions that encourage APD termination. By doing so, the authors obtained ρ_SFP < 10⁴cm^-2 with h_APD ≤ 57 nm. The authors also demonstrated that high-quality GaAs on Ge can be grown without MEE initiation using conventional MBE conditions, though with taller APDs. Both the two-step initiation and conventional MBE initiation yield GaAs cells with high open-circuit voltage and internal quantum efficiency, demonstrating promising paths toward epitaxial integration of high-efficiency solar cells on Ge.