Investigation of strain release and morphology evolution in MOVPE-grownstep-graded III-V metamorphic buffers on Ge for MJ cells

In recent years, the growing energy demand, together with the increase in the efficiency needs, has boosted the research of PV technologies towards the realization of high efficiency MJ solar cells. However, the development and large-scale application of such devices is hampered by their high cost and complexity of the systems, and the identification of a sustainable approach is essential for a competitive solution in the solar energy market. One of the main strategies to tackle energy limits in traditional monolithic 3J InGaP/InGaAs/Ge cells lies in replacing the bottom cell with a heteroepitaxial III-V domain, whose band gap is engineered so that Ge substrate can be subsequently removed and re-used. In this work, MOVPE-grown step-graded III-V architectures with variable In content were obtained on Ge substrates and investigated, with the aim of clarifying the pathways and mechanisms underlying the realization high quality buffers. Crystalline, morphologic and optical properties were investigated after the progressive addition of layers with increasing In content. Eventually, in view of the realization of an effective MJ device, we explored the effects induced by the addition of a 2.5 μm thick layer with a lower In content, grown onto the overshoot layer (composed of In0.12Ga0.88As). The purpose lies in reducing defects and improving electronic properties. Results obtained confirm the high quality of the buffers and unveil the critical points, responsible for the most important changes in the architecture, and which should be considered in view of a future material engineering. The outcomes reached non only represent a contribution for the elucidation of III-V compound semiconductor properties but are also a step forward towards a sustainable and successful solution in advanced photovoltaic technologies.