Carrier Dynamics and Defects in Bulk 1eV InGaAsNSb Materials and InGaAs Layers with MBL Grown by MOVPE for Multi-junctio
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Carrier Dynamics and Defects in Bulk 1eV InGaAsNSb Materials and InGaAs Layers with MBL Grown by MOVPE for Multi-junction Solar Cells Yongkun Sina, Stephen LaLumondierea, Brendan Forana, William Lotshawa, Steven C. Mossa Tae Wan Kimb, Steven Ruderc, Luke J. Mawstb, and Thomas F. Kuechc a
Electronics and Photonics Lab, The Aerospace Corporation, El Segundo, CA 90245 Electrical and Computer Engineering Dept, c Chemical and Biological Engineering Dept, University of Wisconsin – Madison, Madison, WI 53706 b
ABSTRACT Multi-junction III-V solar cells are based on a triple-junction design that employs a 1eV bottom junction grown on the GaAs substrate with a GaAs middle junction and a lattice-matched InGaP top junction. There are two possible approaches implementing the triple-junction design. The first approach is to utilize lattice-matched dilute nitride materials such as InGaAsN(Sb) and the second approach is to utilize lattice-mismatched InGaAs employing a metamorphic buffer layer (MBL). Both approaches have a potential to achieve high performance triple-junction solar cells. A record efficiency of 43.5% was achieved from multi-junction solar cells using the first approach [1] and the solar cells using the second approach yielded an efficiency of 41.1% [2]. We studied carrier dynamics and defects in bulk 1eV InGaAsNSb materials and InGaAs layers with MBL grown by MOVPE for multi-junction solar cells. INTRODUCTION Dilute nitride materials with a 1eV band-gap that are lattice matched to GaAs substrates are attractive for high efficiency multi-junction solar cells. It has recently been shown that the addition of Sb in dilute amounts to InGaNAs can increase the internal quantum efficiency (IQE) of bulk, lattice-matched MBE-grown material used in solar cells [3], where carrier lifetime measurements are crucial in optimizing material growth. A few research groups have reported carrier lifetimes of MBE-grown bulk InGaNAsSb dilute nitride materials [3] and our group has recently reported carrier lifetimes of MOCVD-grown bulk InGaNAsSb materials lattice-matched to GaAs with band gap energies in the range of 1.0 eV [4]. Performance-related issues, such as low IQEs and photocurrent mismatch between individual cells have been correlated to the N content (~2-3%) required to achieve a 1.0 eV band gap in InGaNAs material [3]. Post-growth thermal treatments improve the IQEs of these devices. A metamorphic buffer layer (MBL) is employed to produce a virtual substrate with a larger lattice constant, expanding the compositional design space for solar cells grown on conventional substrates. Growth optimization studies have been performed to reduce threading dislocations that propagate into device active layers grown on top of the MBLs [5], but the cross-hatched surface morphology typically observed with MBLs can negatively impact qualities of device structures grown on top of the MBL. We investigated a novel technique to improve the surface morphology of MBLs on GaAs substrates while maintaining an epi-appropriate surface chemical compos
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