Effects of Defect Density and Compositional Grading on GaAsP Photovoltaic Performance

The photovoltaic performance of n** plus p/p** plus GaAs//0//. //7//4P//0//. //2//6 shallow homojunctions is studied as a function of the dislocation density in the vicinity of the n** plus /p junction. Compositionally graded GaAsP/GaAs heterostructures grown by atmospheric-pressure metalorganic vapor phase epitaxy (APMOVPE) are used to produce shallow-homojunction devices with dislocation densities that vary over a wide range. The device structures are characterized and compared using optical microscopy, electron probe microanalysis, electron beam induced current, transmission electron microscopy, light J-V, dark J-V, and external quantum efficiency. In general, fewer compositional steps and thinner grades resulted in rougher surfaces, higher defect densities, and lower conversion efficiencies. Strong correlations are seen among the grading structure, dislocation configuration, surface morphology and solar cell characteristics. Thin compositional grades of the proper configuration are found to reduce the threading dislocation density to less than 10**6 cm**-**2.