Characterization of Metamorphic GaAsP/Si Materials and Devices for Photovoltaic Applications

GaAsyP_1-y anion-sublattice compositionally graded buffers and device structures were grown directly on Si(100) substrates by way of a high-quality GaP integration layer, yielding GaAsP target layers having band gaps of photovoltaic interest (1.65-1.8 eV), free of antiphase domains/borders, stacking faults, and microtwins. GaAsyP_1-y growths on both Si and GaP substrates were compared via high-resolution X-ray diffractometry of the metamorphic buffers and deep-level transient spectroscopy (DLTS) of p⁺-n diodes that are lattice matched to the final buffer layer. Structural analysis indicates highly efficient epitaxial relaxation throughout the entire growth structure for both types of samples and suggests no significant difference in physical behavior between the two types of samples. DLTS measurements performed on GaAsP diodes fabricated on both Si and GaP substrates reveal the existence of identical sets of traps residing in the n-type GaAsP layers in both types of samples: a single majority carrier (electron) trap, which is located at EC - 0.18 eV, and a single minority carrier (hole) trap, which is located at EV + 0.71 eV. Prototype 1.75-eV GaAsP solar cell test devices grown on GaAsyP_1-y/Si buffers show good preliminary performance characteristics and offer great promise for future high-efficiency III-V photovoltaics integrated with Si substrates and devices.