Reactive diffusion and electronic structure at the nickel/selenium interface

摘要

Ultraviolet photoemission spectroscopy has been used to study reactive diffusion at the interface between clean nickel and evaporated selenium films in ultrahigh vacuum. Photoelectronhyphen;energyhyphen;distribution curves were measured at 10.2 eV for sputterhyphen;cleaned nickel, evaporated selenium and nickel, and the reaction product nickel selenide, which is metallic. By depositing thick selenium films and monitoring the photocurrent as a function of time and temperature, the growth of nickel selenide in the interface region was measured. In agreement with the theory of reactive diffusion, the nickel selenide layer increased in thickness as the square root of the product of the growth constantkand time, wherek=1times;108thinsp;exp(minus;1.35eV/kBT) cm2thinsp;secminus;1. This rate is independent of impurity concentrations in both the nickel and selenium up to several atomic percent. A thin (sim;7 Aring;) nickel oxide layer prevented nickel selenide formation. The electronic structures of selenides prepared with various compositions were determined, and it was found that the interfacial selenide is the Ni1minus;xSe phase withxless than 0.23.