Thin, High Quality GaInP Compositionally Graded Buffer Layers Grown at High Growth Rates for Metamorphic III-V Solar Cell Applications

The metamorphic growth of lattice-mismatched materials has allowed optimizing the bandgap combination in multijunction solar cells for the solar spectrum under consideration. Buffer structures are used to accommodate the lattice-mismatch by introducing dislocations and relaxing the material in a controlled way. However, the metamorphic buffers typically involve significant growth time and material usage, which increases the cost of these solar cells. In this work, the thinning of buffer structures with continuously, linearly graded misfit is addressed with the goal of increasing the cost-effectiveness of metamorphic multijunction solar cells. The relaxation dynamics and quality of the buffer layers analyzed were assessed by in-situ stress measurements and ex-situ measurements of residual strain, threading dislocation density and surface roughness. Their ultimate quality has been tested using these buffers as templates for the growth of 1 eV Ga_0.73In_0.27As solar cells. The deleterious effect of thinning the grade layer of these buffer structures from 2 to 1 μm was investigated. It is shown that prompting the relaxation of the buffer by using a stepwise misfit jump at the beginning of the grade layer improves the quality of the thinned buffer structure. The residual threading dislocation density of the optimized thin buffers, grown at a high growth rate of 7 μm/h, is 3×10⁶ cm^-2, and solar cells on these buffers exhibit near-ideal carrier collection efficiency and a V_oc of 0.62 V at 1-sun direct terrestrial spectrum.