Letter Characterization of antiphase domains on GaAs grown on Ge substrates by conductive atomic force microscopy for photovoltaic applications B. Galiana a,n , I. Rey-Stolle c , I. Beinik b , C. Algora c , C. Teichert b , J.M. Molina-Aldareguia d , P. Tejedor a a Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), 28049 Madrid, Spain b Institute of Physics, University of Leoben, A-8700 Leoben, Austria c Instituto de Energı ´a Solar (IES-UPM), ETSI. Telecomunicacio´n, 28040 Madrid, Spain d Instituto Madrilen˜o de Estudios Avanzados de Materiales (Instituto IMDEA Materiales) c/Profesor Aranguren s/n, 28040 Madrid, Spain article info Article history: Received 7 July 2010 Received in revised form 8 November 2010 Accepted 8 December 2010 Available online 17 February 2011 Keywords: Conductive atomic force microscopy III–V solar cells Antiphase domains GaAs on Ge abstract The role of antiphase domains formed on GaAs grown on Ge is analyzed by means of conductive atomic force microscopy. The correlation of the derivative topography scans with the conductive scans shows a constant current value in most of the surface under study; although at certain locations high current leaks occur causing an inhomogeneous conductivity through the GaAs layer as the density of antiphase domains increases. This result implies that the existence of antiphase domains decreases the parallel resistance of solar cells, helping to understand the impact of these defects on the electrical behavior of these devices & 2011 Elsevier B.V. All rights reserved. 1. Introduction The growth of III–V semiconductor devices on Ge substrates has received considerable attention, particularly in the field of photovoltaic cells [1], initially for space applications and more recently for terrestrial high concentrator applications [2]. More concretely, GaInP/GaAs/Ge triple-junction solar cells have been demonstrated to attain record efficiencies of 41.6% at 364 suns [3]. Unfortunately, the GaAs/Ge material system has inherent problems that strongly impact the quality of the devices. The growth of polar GaAs on non-polar Ge substrates often leads to the formation of structural defects known as antiphase domains (APDs), which are bounded by antiphase boundaries (APBs). These features are one of the most common defects found when growing GaAs on Ge [4–13]. Most of the reports dealing with the suppression of APDs when growing GaAs on Ge by metalor- ganic vapor phase epitaxy (MOVPE) suggest to (i) use a substrate off-cut of 61 or even larger [4–6]; (ii) use an initial As monolayer coverage before growing the nucleation layer to form a single- domain surface [7,8] and (iii) create a GaAs nucleation layer at a growth temperature a few degrees either below or above the GaAs buffer layer growth temperature [9–11]. Consequently, by omitting any of these steps from an optimized procedure based on the previous assumptions, the density of APDs in the epitaxial layer can be increased. In terms of their effects, APDs essentially act as non-radiative recombination centers providing deep levels in the forbidden gap [4,12]. They also enhance the propagation of misfit dislocations, resulting from the slight lattice mismatch between GaAs and Ge [13] and they cause a significant surface roughness. Till now, these defects have been evaluated by means of different techniques, such as transmission electron microscopy (TEM) [5], atomic force micro- scopy (AFM) [14] and time-resolved photoluminescence (TRPL) [15]. These previous works have mainly focused on the influence of APDs on the dislocation density, roughness and minority carrier life-times of the resultant layer, but no details relative to their effect on the layer conductivity have been presented despite the great impact that APDs can have on solar cell applications. It is well known that the presence of APDs in active layers causes a degradation of the solar cell performance. Most of the authors have explained this result in terms of minority carrier lifetime degradation (i.e. photocurrent degradation), although a study of the electrical impact of APDs on GaAs solar cells is still lacking. Consequently, in this paper, APDs appearing on GaAs on Ge have been evaluated by means of conductive atomic force microscopy (C-AFM). This scanning technique has been proven to be an efficient current sensing technique, which allows performing simultaneously high-resolution topography profiling and accurate measurements of local conductivity variations at the nanometer Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.12.021 n Corresponding author. Tel.: +34 91 334 9000; fax: +34 91 372 0623. E-mail address: beatriz.galiana@icmm.csic.es (B. Galiana). Solar Energy Materials & Solar Cells 95 (2011) 1949–1954