A Comparative Study of InGaN/GaN Multiple-Quantum-Well Solar Cells Grown on Sapphire and AlN Template by Metalorganic Chemical Vapor Deposition

Two kinds of substrates, sapphire and AlN/sapphire template (AlN template), are used for the growth of InGaN/GaN multi-quantum-well solar cell structures by metalorganic chemical vapor deposition, and their material and device properties are investigated. The results show that the samples grown on AlN template have a better crystal quality with a larger in-plane compressive strain than those on sapphire, and solar cells fabricated on sapphire mostly exhibit better performance than those on AlN template. An analysis of the photoluminescence measurements indicates that a critical InGaN well thickness related to the generation of nonradiative recombination centers, which affects the internal and external quantum efficiencies, is thinner in samples grown on AlN template than in samples on sapphire. The critical thickness is speculated to be related to the large in-plane compressive strain in the samples on AlN template. By contrast, in comparison between samples with a sufficiently thin InGaN well thickness of 1.0nm, the sample on AlN template exhibits better solar cell performance than on sapphire. This implies that the improved crystal quality contributes to the improvement of internal quantum efficiency as long as the well layer is thinner than the critical thickness.