Optimization of InGaAs metamorphic buffers for triple junction solar cells

Abstract

Triple junction solar cells (TJSC) using metamorphic buffers have been reported with over 40% efficiencies and offer higher theoretical efficiency than solar cells lattice matched to germanium. Metamorphic (MM) buffers composed of step graded InGaAs layers allow for alloys of InGaAs and InGaP to be grown with different band gap energies enabling higher efficiency solar cells. In this paper, we initially discuss MOCVD growth experiments to optimize the material properties of metamorphic InGaAs buffer layers grown at ∼ 14 μm/hour, namely; growth temperature and overshoot layer composition. InGaAs buffer layers were evaluated using x-ray reciprocal space mapping (RSM) to determine strain relaxation of the buffer layers, atomic forces microscopy (AFM) to optimize surface morphology and photoluminescence (PL) to assess material quality. Selected growth conditions were also evaluated by transmission electron microscopy (TEM) and cathodoluminescence (CL). After these layers were optimized for growth temperature and overshoot layer, we also evaluated layer thickness and the number of step grades in the MM buffer using the same characterization techniques. Our results show that growth of the step grade InGaAs layers at 620 °C and using an overshoot later of approximately 12% Indium in InGaAs provide for the optimum surface morphology and strain relaxation of > 90% for a final buffer layer InGaAs composition with ∼ 9% indium mole fraction. Cross section TEM images of the MM buffer also show dislocations starting at the step grade and being terminated at the layer interfaces. No threading dislocations were observed in the uppermost part of the epitaxial layer stack with TEM. CL data on MM buffer structures reveal the presence of crosshatching associated with misfit dislocations and threading dislocations densities were reduced by an order of magnitude to ∼105/cm2 for the optimized growth conditions. Our data also shows that seven layer grades between lattice matched InGaAs and the final layer of the grade provide for the best buffer material characteristics. This materials study demonstrates that MOCVD metamorphic buffers can be produced to enable optimization of the band gaps for the middle and top cells leading to enhanced solar cell performance of triple junction solar cells.