Development of metamorphic buffer structures for inverted metamorphic solar cells

Lattice-mismatched materials can help to improve the theoretical efficiency potential of today's III-V multi-junction solar cells. Carefully designed buffer structures are incorporated in these solar cells to grade the lattice constant and relax the crystal. These buffer layers need to result in low surface roughness, low threading dislocation density and high relaxation. Ga
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P buffer structures grown on GaAs substrates with different orientation were investigated regarding surface roughness, lattice tilt and relaxation. The buffer structures were varied in stress and strain by a tuning layer with altering composition. Close to 100 % relaxation of the crystal lattice could be reached for tuning layers with a 0.3 % larger lattice-constant compared to the target value. From the AFM data, a preference for (111)B misoriented substrates was found, since these buffer structures show the smallest RMS roughness.