Structure and composition of lattice-mismatched III–V epilayers for high-efficiency photovoltaics

Lattice mis-matched (LMM) multilayered structures were grown using metalorganic chemical vapor deposition (MOCVD) at NREL. Energy dispersive x-ray (EDX) spectrometric mapping was conducted on transmission electron microscope (TEM) sample cross-sections of GaAs_1-yP_y grades on GaAs substrates, with known layer compositions. The compositions were determined by x-ray diffraction, which serve as standards for EDX. Data were acquired by both TEM spot analysis and 2-D maps in scanning TEM mode. The I_P/I_As intensity ratio obtained from EDX, plotted against y/(1-y) obtained from XRD shows a linear slope. We seek to parameterize the variations from these standard samples for accurate composition determination of unknown samples. The quantitative method we are developing is based on principle component analysis (PCA). We anticipate our PCA algorithm will have applications in future growth designs, including heterostructures with multiple quaternary steps, or even continuous grades.