Wavy growth and related defects in strain-balanced multi-quantum wells for photovoltaic devices

Transmission electron microscopy, high resolution X-ray diffraction, cathodoluminescence were performed on strain balanced InGaAs based multi-quantum wells (MQWs) to be used as active layers in thermophotovoltaic devices operating at around 2 mum. The epitaxial growth front in the MQWs is flat tip to a certain thickness where a change in the growth mode generates MQW waviness propagating vertically through the structure with increasing amplitude. The local stress concentration in the MQW regions exhibiting significant local rippling results in the formation of clusters of extended defects whose properties vary according to the strain and the growth temperature conditions of the structures. Experimentally it has been found that the wavy growth onset occurs once a threshold value of the elastic energy density is achieved. The well/barrier thickness ratio were changed in order to minimize this threshold energy and thus to retain good material quality even at the longer wavelength. Forty repetitions defect free MQWs, emitting at 1.96 mum at room temperature were successfully grown in this way.