Nowadays much emphasis is put on photovoltaics applications development. But in many cases basic silicon-based solar cells are not efficient enough. It was reported [1] that utilising multi-junction cells in tandem configuration it is possible to enhance light-to-electricity conversion efficiency over three times. One of the most promising materials in such devices are dilute nitrides, which can fill technology gap of 1 eV direct band gap semiconductors.
    Dilute nitrides are ternary or quaternary semiconductor alloys based on III-V elements, like Ga, As and In, containing low nitrogen content (up to few %). In contrast to a standard semiconductor alloy mixing law, incorporated small-sized nitrogen atoms do not cause increase of bulk material band gap increase, but its drop. On the other hand, addition of indium atoms cause increase of lattice parameter and further band gap bowing. Taking into account, that both mentioned atoms can compensate their influence on lattice parameter, the band gap engineering is possible for materials lattice matched to well-known and cheap GaAs substrates.
    Investigated devices were manufactured using Atmospheric-Pressure MOVPE system with AIXTRON AIX200 R&D horizontal reactor. Sub-cells as well as tunnel junctions were deposited onto GaAs (100)-oriented substrate. Organometallic materials were used as sources of Ga, In, N chemical elements, while As was taken from hydride. Deposited structures were measured by many techniques, including HRXRD, photoluminescence, CER spectroscopy, Raman spectroscopy and I-V setups. 

Acknowledgements
    This work was co-financed by Polish Ministry of Science and Higher Education under the grant no. N N515 607539 and N N507 552938, by the European Union within European Regional Development Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08-05), by Wroclaw University of Technology statutory grant. In addition, M.B. acknowledges the financial support from the European Union within European Social Fund.

References
[1] S. R. Kurtz, A. A. Allerman, E. D. Jones, J. M. Gee, J. J. Banas and B. E. Hammons, Appl. Phys. Lett. 74, 729

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