Optimization of MOCVD grown InGaP metamorphic buffers with MOCVD growth conditions and surfactant

Abstract

InGaP metamorphic (MM) buffers are utilized in inverted metamorphic solar cells which offer high efficiency and provide for low cost and weight flexible solar cells. Metamorphic buffers are typically produced by metal organic chemical vapor deposition (MOCVD) technology and allow for a change in InGaP lattice constant while managing dislocation density for the growth of subsequent high quality III-V layers. We report on the optimization of a nine step graded InGaP metamorphic buffer with final lattice constant of 5.785 Å with growth temperature and V/III ratio using AFM, photoluminescence, x-ray and cathodoluminescence. From this optimization we demonstrate final InGaP layers buffer layers can be produced with AFM determined rms roughness values of 1-2 nm, x-ray determined strain relaxation ~ 100% and CL determined threading dislocation densities of 1-5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Using these optimized growth conditions, we also show the InGaP growth rate used in the MM buffer can be increased to approximately 5.0 μm/hr with similar material characteristics. Finally, we also evaluated the use of the surfactant, trimethylbismuth, during the growth of the InGaP MM buffer and show improvement in SEM surface morphology and PL intensity without degradation in other material parameters. Since advanced multi-cell solar cell designs use multiple MM buffers, fully relaxed final InGaP buffers with low surface roughness and dislocation density are important in the cell performance. Our results show that InGaP MM buffers with final surface roughness of <;2 nm, strain relaxation > 95%, and dislocation density of <; 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> are achievable with the use of surfactant and optimized growth conditions.