Effect of sodium diffused into Bridgman CuInSe2+x

The study describes diffusion of elemental sodium into p-type samples of Bridgman-grown CuInSe2+x, causing a change from p- to n-type behavior at the surface and deep into the material. This resulted in the creation of Na 1s and Se 3d XPS peaks at binding energies of 1071 and 54.5 eV respectively, where the latter has been attributed to CuInSe2 itself. Subsequent annealing of the samples in air gave rise to a second Se 3d peak at 59 eV, attributed to Na2SeO3 by oxidation of Na2Se. However, SeO2 also gives a peak at about the same binding energy. The presence of the diffused-in Na up to about 10% at the surface appears to promote the creation of a high ratio [Se]/[Cu] compound at the expense of the chalcopyrite at the sample surface. The diffused-in samples showed n/p homojunctions at depths of tens of microns for stoichiometric CuInSe2 and at smaller depths for CuInSe2.2, with excess Se content. Estimates of the diffusion coefficient for Na into the Bridgman samples at 250 °C were in the range of 10−10-10−8 cm2/s.