Strain-induced surface reactivity: Low temperature Cr/W(110) nitridation

Synchrotron photoelectron and Auger electron spectroscopies have been used to study the reaction of nitrogen with ultrathin chromium overlayers vapor deposited on W(110). Molecular nitrogen dissociates via a metastable π-bonded intermediate on Cr(110), the strained (1×1) Cr/W(110) monolayer and the (2×2) Cr/W(110) bilayer surfaces. However, the activation energy for dissociation of the π-bonded intermediate is lower on the strained overlayers than on Cr(110). This has been exploited to grow ultrathin CrN layers on W(110), which are spectroscopically indistinguishable from chromium nitride formed by reacting atomic Cr and N2 simultaneously on W(110) at 300 K. This demonstrates that nitridation proceeds via energetically equivalent pathways at isolated Cr adatoms on W(110) and at majority sites on the strained Cr/W(110) overlayers. In addition, an anomalous narrow band of states near the Fermi level is found for these surface nitrides and, tentatively, is attributed to unique surface or interface states of CrN:W(110). These findings suggest that strained pseudomorphic overlayers may be useful templates for low temperature nitride film growth.