Metamorphic solar cells employing chemical mechanical polishing and MOVPE regrowth

We have investigated the MOVPE growth of In_xGa_1-xAs metamorphic buffer layer (MBL) structures with a focus on techniques to improve the surface morphology and determine the influence of morphology on subsequently grown single-junction device structures. To improve the surface roughness of the underlying MBL, Chemical-Mechanical Polishing (CMP) is employed and MOVPE regrowth of single-junction (SJ) solar cells on top of the polished surface is performed. AFM image analysis indicates the CMP process is effective in reducing the step-graded In_xGa_1-xAs MBL surface roughness from ~7-10 nm (as-grown) to 2.3 nm post CMP. A post-CMP ozone/HF treatment was found to be effective in removing silica residue remaining on the surface from the CMP process, allowing for the growth of films on top of the MBL surface subjected to CMP. Improved short circuit current density (J_sc) and external quantum efficiency (QE) were obtained from SJ (1eV) devices which employ the CMP process compared with those on as-grown MBLs.