Mechanism of phase separation generation in Ge-based solar cell tunnel junctions.

Enhanced performance, reduced cost and compact dimensions are a prerequisite for novel products in the solar industry. III–V based multi-junction heterostructures are promising devices to give an answer to make a substancial step in the photovoltaic technology. However, such heterostructures need tunnel junctions to connect the different active layers and so ternary alloys are needed to tune the energy gap. In the present contribution, artefacts generated by ternary alloying are investigated by transmission electron microscopy (TEM). Composition modulation in In0 015Ga0 985As/In0 3Ga0 7P/Al0 3Ga0 7As/In0 3Ga0 6Al0 1P/In0 3Al0 7P/In0 015Ga0 985As heterostructures grown on Ge subtrates is observed. This should affect the spectral sensitivity of the active layer. But, as a probably more important consequence, this observed modulation is shown to also generate modulation in the Al0 3Ga0 7As tunnel junction. This behaviour is anomalous in standard AlGaAs thick epilayers grown on GaAs substrates. In the present case, it could strongly affect the carrier transport imposing an important handicap to their potential application within electro-optical components.