Molecular beam epitaxy of metamorphic In_yGa_1-yP solar cells on mixed anion GaAs_x P_1-x/GaAs graded buffers

The authors have grown metamorphic In_yGa_1-yP on optimized GaAs_x P_1-x/GaAs graded buffers via solid source molecular beam epitaxy (MBE) for multijunction solar cell applications. In this work, the authors show that a previously developed kinetic growth model can be used to predict the composition of mixed anion GaAs_x P_1-x alloys on GaAs as a function of substrate temperature and group-V flux. The advantages of using a high growth temperature of 700°C are then described, including the minimized dependence of composition on small temperature variations, a linear dependence of film composition on incident group-V flux ratio, and the ability to attain low threading dislocation densities of ≤ 10⁶ cm^-2. The authors then discuss the effect of faceted trenches, a morphological defect specific to tensile strain relaxation, on minority carrier properties, as well as strategies to eliminate them. Growth temperature effects, phase separation, and difficulties encountered in n -type doping of InAlP:Si are then described in the context of In_y Ga _1-yP solar cell growth. The MBE growth techniques presented here have enabled the demonstration of 2.00 eV band gap metamorphic In_0.39 Ga_0.61P solar cells, exhibiting open-circuit voltages as high as 1.42 V. These results indicate that metamorphic In_y Ga_1-yP is a promising material for future multijunction solar cells.