XPS Se 3d PEAK AT 59 eV FOR Na DIFFUSED IN BRIDGMAN CuInSe_(2+x)

摘要

Using stoichiometric p-type samples of Bridgman-grown CuInSe_2, elemental sodium was diffused into the surface, within an ampoule under vacuum, causing conversion of the samples to n-type to a depth of more than 100 microns from the surface. The amount of Na diffused in was monitored by the intensity of the XPS Na 1s peak at a binding energy of 1,071 eV. As a result, a single XPS Se 3d peak was observed at 54.5 eV but, after subsequent annealing the same samples in air in a sealed ampoule at 200 °C for 1 hour, a second Se 3d peak appeared at a binding energy of 59 eV. This peak was also seen earlier in Bridgman CuInSe_2, where Na was introduced into the melt prior to compound synthesis, for ingots stored for a long time in air. The 59 eV peak has been assigned to Na2SeO_3, created by oxidation of Na_2Se, itself formed by the combination of sodium with selenium destined for the creation of the CuInSe_2. As a consequence, the resulting chalcopyrite becomes stoichiometrically selenium deficient and hence n-type. From the measured n/p junction depths, an estimate of the diffusion coefficient was made in the range of 10~(-10) to 10~(-8) cm~2/sec for sodium in multi-crystalline CuInSe_2 at 250 °C.