Thin, high quality GaInP compositionally graded buffer layers grown at high growth rates for metamorphic III–V solar cell applications I. Garcia a,b,n , R.M. France a , J.F. Geisz a , J. Simon a a National Renewable Energy Laboratory, Golden, CO 80401, USA b Instituto de Energía Solar, Universidad Politécnica de Madrid, Avda Complutense s/n, 28040 Madrid, Spain article info Available online 31 October 2013 Keywords: A1. Stresses A3. Metalorganic vapor phase epitaxy B1. Phospides B2. Semiconducting III–V materials B3. Solar cells abstract The metamorphic growth of lattice-mismatched materials has allowed optimizing the bandgap combina- tion in multijunction solar cells for the solar spectrum under consideration. Buffer structures are used to accommodate the lattice-mismatch by introducing dislocations and relaxing the material in a controlled way. However, the metamorphic buffers typically involve significant growth time and material usage, which increases the cost of these solar cells. In this work, the thinning of buffer structures with continuously, linearly graded misfit is addressed with the goal of increasing the cost-effectiveness of metamorphic multijunction solar cells. The relaxation dynamics and quality of the buffer layers analyzed were assessed by in-situ stress measurements and ex-situ measurements of residual strain, threading dislocation density and surface roughness. Their ultimate quality has been tested using these buffers as templates for the growth of 1 eV Ga 0.73 In 0.27 As solar cells. The deleterious effect of thinning the grade layer of these buffer structures from 2 to 1 μm was investigated. It is shown that prompting the relaxation of the buffer by using a stepwise misfit jump at the beginning of the grade layer improves the quality of the thinned buffer structure. The residual threading dislocation density of the optimized thin buffers, grown at a high growth rate of 7 μm/h, is 3  10 6 cm 2 , and solar cells on these buffers exhibit near-ideal carrier collection efficiency and a V oc of 0.62 V at 1-sun direct terrestrial spectrum. & 2013 Elsevier B.V. All rights reserved. 1. Introduction The possibility of epitaxially growing materials lattice-mismatched to the substrate broadens the palette of material bandgaps available to the device engineer. In the pseudomorphic growth of lattice mis- matched structures, the different layers are grown strain-balanced, by growing them thin enough so that no relaxation occurs. This prevents the formation of dislocations so that the quality of the material is not affected by the lattice mismatch. In applications where thick lattice- mismatched layers are required, such as the case of solar cells, the use of pseudomorphic layers is not possible and metamorphic growth is used instead. In this case, the glide of dislocations relieves the strain to allow thick, strain-free lattice-mismatched material. In inverted meta- morphic triple-junction solar cells (3J-IMM), a metamorphic 1 eV Ga 0.73 In 0.27 As bottom cell is used in order to achieve a nearly optimal bandgap combination for the AM1.5 direct terrestrial spectrum [1].A compositionally graded Ga x In 1x P buffer structure is used to accom- modate the  1.9% lattice mismatch between the GaAs substrate and the 1 eV Ga 0.73 In 0.27 As bottom cell [1]. This buffer must exhibit a low threading dislocation density (TDD) to allow high performance lattice- mismatched solar cells, and also must be electrically conductive and optically transparent to the light it receives. The GaInP-based buffer used meets these requirements. Moreover, GaInP material has been found to exhibit beneficial dislocation formation and glide character- istics when grown highly ordered on (111)B miscut substrates. This substrate misorientation propitiates the formation of single-variant ordering [2]. High quality GaInP-based stepped-grade buffer structures for 1 eV Ga 0.73 In 0.27 As cells have been developed, with TDD as low as 1  10 6 cm 2 [3]. The solar cells grown on these buffers exhibit carrier collection efficiencies close to 1, as revealed by quantum efficiency measurements, and open circuit voltages as high as 0.63 V. According to a kinetic model developed to describe relaxation in graded buffers [4], high temperatures, low growth rates, and thick buffers are beneficial to minimize the TDD in the buffer. However, these growth conditions, particularly a low growth rate and a large thickness, are detrimental for the cost-effectiveness of the metamorphic semiconductor devices grown using these buf- fers. Thin buffers grown at high growth rates are preferable in this sense. For the high quality GaInP-based buffers used in this work, a high growth rate of 7 μm/h is used. Despite the kinetic model prediction, we found in a previous work that, for the set of growth conditions used, increasing the growth rate did not lead to an increased TDD [5]. A lower surface roughness was observed for Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth 0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.10.043 n Corresponding author at: National Renewable Energy Laboratory, Golden, CO 80401, USA. Tel.: þ1 303 384 7228. E-mail address: ivan.garcia@nrel.gov (I. Garcia). Journal of Crystal Growth 393 (2014) 64–69