Growth of an InGaAs/GaAsSb tunnel junction for an InP-based low band gap tandem solar cell

The hetero-interfaces of an InGaAs/GaAsSb tunnel junction, embedded into two InP barrier layers, as used in a tandem solar cell, were studied. With regard to the sharpness of these interfaces, different surface preparations and growth procedures were tested. For that, several surface reconstructions of In0.53Ga0.47As grown by metal organic chemical vapor deposition (MOCVD) were investigated in-situ with reflectance difference spectroscopy (RDS) and analyzed in ultrahigh vacuum (UHV) with low energy electron diffraction (LEED) and X-ray photoemission spectroscopy (XPS). Depending on the annealing temperature, three different surface reconstructions of MOCVD prepared InGaAs were found: As-rich (4 x 3), (2 x 4) and III-rich (4 x 2)/c(8 x 2). Next, thin GaAs0.51Sb0.49 layers were grown on both As-rich and III-rich InGaAs surfaces. LEED patterns showed the expected c(4 x 4) reconstruction of the As-terminated GaAsSb surface for both preparations, also of the same quality. A significant difference for the Sb to As ratios of thin GaAsSb layers were measured by XPS, indicating that the unusual growth of GaAsSb on a III-rich InGaAs surface results in a sharper InGaAs/GaAsSb hetero-interface. Moreover, we searched for the best preparation procedure of the InP barrier layer on GaAsSb. RDS and a stable growth of the top cell suggested a favorable growth procedure by ramping the growth temperature from 500 to 600 degrees C during the growth of the InP layer. The challenge here was to prepare GaAsSb at the optimum temperature of 500 degrees C, to switch then to the optimum growth temperature of the subsequent layers of 600 degrees C and to produce a well-defined and sharp interface. (c) 2006 Elsevier B.V. All rights reserved.