Lattice-Mismatched 0.7-eV GaInAs Solar Cells Grown on GaAs Using GaInP Compositionally Graded Buffers

The three-junction 1.8/1.4/1.0-eV inverted metamorphic multijunction solar cell can be extended to four junctions by adding another lattice-mismatched GaInAs junction with a bandgap of 0.7 eV. However, this requires a significant amount of mismatch to GaAs substrates, i.e., 3.8%, which is difficult to obtain while maintaining high-quality material. In this paper, we perform an in-depth investigation of a GaInP compositionally graded buffer varying in composition between Ga 0.5In 0.5P and InP in order to identify limitations to dislocation glide and sources of excess dislocation formation. In situ wafer curvature, cathodoluminescence, and X-ray diffraction (XRD) are used to analyze dislocation glide; transmission electron microscopy, atomic force microscope, and XRD are used to analyze material structural properties. Composition nonuniformities and roughness are observed, and a region in the compositionally graded buffer where a significant number of excess dislocations are formed is identified. The formation of these dislocations is related to atomic ordering, which has a large influence on the dislocation behavior. Adding thickness to the region in the buffer where dislocations are formed reduces the threading dislocation density an order of magnitude. Metamorphic 0.74 eV solar cells grown on this template have internal quantum efficiency > 90% and Voc > 0.3 V with Jsc set to 13 mA/cm2, which is the expected current in a multijunction device. These results are compared with lattice-matched GaInAs/InP solar cells to evaluate the loss associated with the lattice-mismatch.