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 (hAPD) and SFP density (ρSFP). As APDs can readily self-terminate while SFPs cannot, a two-step initiation strategy was employed, where MEE is performed under conditions that minimize ρSFP followed by low-temperature MBE conditions that encourage APD termination. By doing so, the authors obtained ρSFP < 104 cm−2 with hAPD ≤ 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.