Growth of Lattice-Matched GaInAsP Grown on Vicinal GaAs(001) Substrates within the Miscibility Gap for Solar Cells

The growth of quaternary Ga_0.68In_0.32As_0.35P_0.65 by metal-organic vapor phase epitaxy is very sensitive to growth conditions because the composition is within a miscibility gap. In this investigation, we fabricated 1 µm-thick lattice-matched GaInAsP films grown on GaAs(001) for application to solar cells. The growth temperature and substrate miscut are varied in order to characterize the effect of the surface diffusion of adatoms on the material quality of alloys. Transmission electron microscopy and two-dimensional in-situ multi-beam optical stress determine that growth temperatures of 650 °C and below enhance the formation of the CuPt_B atomic ordering and suppress material decomposition, which is found to occur at the growth surface. The root-mean-square (RMS) roughness is reduced from 33.6 nm for 750 °C to 1.62 nm for 650 °C, determined by atomic force microscopy. Initial investigations show that the RMS roughness can be further reduced using increased miscut angle, and substrates miscut toward (111)A, leading to an RMS roughness of 0.56 nm for the sample grown at 600 °C on GaAs miscut 6° toward (111)A. Using these conditions, we fabricate an inverted hetero-junction 1.62 eV Ga_0.68In_0.32As_0.35P_0.65 solar cell without an anti-reflection coating with a short-circuit current density, open-circuit voltage, fill factor, and efficiency of 12.23 mA/cm², 1.12 V, 86.18%, and 11.80%, respectively.