Comparison of arsenide and phosphide based graded buffer layers used in inverted metamorphic solar cells

The effect of graded buffer layer (GBL) composition on inverted metamorphic 1.1-eV In _0.24Ga _0.76As single-junction solar cells, with applications as a subcell in high-efficiency multijunction solar cells, is investigated. In experiment A, Al _xIn _yGa _(1-x-y)As was used as a GBL to transition from the GaAs substrate lattice constant to that of the In _0.24Ga _0.76As cell. In experiment B, In _1-xGa _xP was employed. Both GBLs were deposited using growth conditions optimized for lattice-matched growth. Reciprocal space maps showed that the InGaAs cell was fully relaxed in both experiments. They also revealed that the AlInGaAs GBL relaxed very quickly after the start of growth (0.1 μm). The InGaP GBL on the other hand remained partially strained throughout its structure and full relaxation was only achieved after growth of the InGaAs cell. Atomic force microscopy of the surface of the AlInGaAs GBL showed typical cross-hatch morphology with a roughness of 8.9 nm. The surface of the InGaP GBL was much rougher at 18.3 nm with unusual morphology, likely due to 3D island formation due to unrelieved strain. These findings were confirmed by transmission electron microscopy where the InGaAs cell of experiment A was largely free of imperfections with a defect density of 1.1 × 10 ⁶ cm ^-2. Experiment B had defects readily seen throughout the GBL and the InGaAs cell above had a defect density of 1.5 × 10 ⁹ cm ^-2. The hardness of the AlInGaAs GBL surface was measured to be 7.2 GPa and exhibited an indentation size effect. The hardness of the InGaP GBL surface was 10.2 GPa regardless of the depth of penetration of the indenter. The lack of indentation size effect in InGaP is due to the high density of dislocations already present in the material due to unrelieved strain. Solar cells fabricated from experiment A wafers exhibited excellent band gap-voltage offset W _oc (E _g/q) - V _oc of 0.414 V. Cells from experiment B exhibited a poor W _oc of 0.686 V, most likely due to the threading dislocations acting as non-radiative recombination centers.