Istituto dei materiali per l'elettronica ed il magnetismo     
Nasi L., Lazzarini L., Ferrari C. Strain balanced multi-quantum wells for photovoltaic applications: A way to extend the absorption edge to longer wavelengths. G. Salviati, T. Sekiguchi, S. Heun and A. Gustafsson, Eds (eds.). Trivandrum-695 023, Kerala, India: Research Signpost, T. C., 2008.
Novel structures based on strain balanced InxGa(l-x)As/InyGa(l-y)As multi-quantum wells were grown in order to atend the absorption edge of photovoltaic devices to longer wavelengths. The structures were balanced with respect to the InP substrates for termophotovoltaic cells and with respect to InGaAs/GaAs virtual substrates for solar cells. Transmission electron microscopy and highresolution X-ray diffraction were performed to study the structural problems arising from the strain balanced epitaxy in both systems. For the InP-based structures for thermophotovoltaic applications, the wavy growth mechanism is found to be the structural limitation in extending the absorption edge to very high wavelengths. An empirical model to predict the maximum number of layers that can be grown without modulations as a function of the strain energy stored in the multi-quantum wells and the growth temperature is presented and successfully applied. Concerning the GaAs-based structures grown on virtual substrates for solar applications, the structural restrictions in achieving higher wavelengths are due to the cross hatching of the buffer surface which causes lateral thickness modulation in the multi-quantum wells. Reducing the surface roughness induced by the cross hatching thus represents the key issue to be resolved before solar cells can beproduced with these structures.
Subject Multi-quantum wells, misfit dislocations

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