Low Growth Temperature MOCVD InGaP for Multi-junction Solar Cells

In view of the realization of high efficiency four-junction solar cells, InGaP layers, lattice matched to InGaAs, and (001) 6° off Ge substrate are grown by low pressure MOCVD at growth temperatures as low as 500 °C. The grown samples are undoped, p-type (doped by Zn) and n-type (doped by Te) materials with thickness around 1 μm. The ternary compound composition and structural properties are analysed by High Resolution X-Ray Diffraction and Transmission Electron Microscopy (TEM). Completely disordered InGaP layers are obtained with a target energy gap above 1.88 eV and a controlled Zn concentration around 1017 cm-3. The interface properties are studied by High Resolution TEM. A nanometric scale waviness is observed at the interface between InGaP and InGaAs and it is correlated to the step bunching of the substrate offcut. In addition to this, HRTEM shows a 2-3 nanometer thin layer originated by atomic interdiffusion between the As- and the P- based compounds. The difference in composition of this interdiffusion layer is demonstrated by depth resolved Cathodoluminescence (CL), which reveals - approaching the InGaP/InGaAs interface, a blue shift of the InGaP related peak and the appearance of a new CL emission band ascribed to a quaternary InGaAsP compound.