InGaP Layers Grown on Different GaAs Surfaces for High Efficiency Solar Cells
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1167-O03-04
InGaP Layers Grown on Different GaAs Surfaces for High Efficiency Solar Cells O. Martínez1, V. Hortelano1, V. Parra2, J. Jiménez1, T.Prutskij3, C. Pelosi4 1
GdS-Optronlab, Dpto. Física de la Materia Condensada, Edificio I+D, Paseo de Belén 1, 47011 Valladolid, Spain 2 Grupo Pevafersa, Av. Vicente Fernández Manso, 49800 Toro (Zamora), Spain 3 Instituto de Ciencias, BUAP, Apartado Postal 207, 72000 Puebla, México 4 IMEM-CNR, viale Usberti 37/A, 43100 Parma, Italy ABSTRACT InGaP layers grown on (111)Ga and (111)As GaAs substrate faces are investigated by microRaman spectroscopy, microphotoluminescence and cathodoluminescence. The growth on these polar faces benefits disorder with respect to the layers grown on (001) faces. It is shown that both (111)Ga and (111)As faces result in disordered InGaP layers. While the layers grown on (111)As faces present inhomogeneous compositions, the layers grown on (111)Ga faces present homogeneous compositions close to lattice matching and are almost disordered. INTRODUCTION Nowadays, there is a renewed interest on high efficiency solar cells, based on multijunction structures of III-V compounds [1]. The ternary alloy InGaP is an essential material for multijunction cells. InGaP lattice matched to GaAs presents very interesting properties, as its direct bandgap of 1.9 eV. On the other hand, it presents some advantages respect to AlGaAs, as the absence of DX centers - which are responsible for a drastic reduction of the free carrier concentration in AlGaAs [2] -, larger band gap, and a more favourable band gap alignment with GaAs. It has been suggested that it might be useful for solar cells to make use of the growth on (111) GaAs faces under mismatched conditions, where large internal electric fields are generated by the off-diagonal strain [3]. In fact, the electronic band structure of QWs on (111) faces is substantially modified. The piezoelectric fields could allow for a better extraction of minority carriers and for a larger absorption coefficient [4]. Also, one might expect suppression of long range order, which is responsible for the band gap shrinkage and the change of the band alignment between InGaP and GaAs [5]; the presence of order has adverse consequences for the device performance. The growth modes on (111) faces are different from those on (001) faces, because of the different dangling bonds exposed and the different surface reconstructions. The optimization of the InGaP layers grown on (111) faces could provide a useful route to achieve better photovoltaic devices. This optimization demands a significant characterization effort, aiming to study the main structural and optical properties of the InGaP layers grown on (111) faces. This characterization needs to address problems as the control of the composition (short range order), spontaneous ordering (long range order), and the presence of defects. We explore here the preliminary growth by metal-organic chemical vapour deposition (MOCVD) of InGaP layers on (111)Ga and (111)As - GaAs substrates, and compare
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