Reactions at a rare-earth-GaAs interface: Ce/GaAs(110).

Abstract

We present results of a detailed microscopic study of an evolving rare-earth--compound-semiconductor interface. High-resolution synchrotron-radiation photoemission results for Ce coverages in the range 0.08--30 monolayers (ML) (0.25--90 A\r{}) on cleaved GaAs(110) show that three new and distinct As environments are produced by disruption of the surface. The As 3d core lines in these Ce-As configurations are shifted to lower binding energy relative to GaAs. They reflect surface-bonded As and two different Ce-As bulk configurations. These As configurations have very different growth and attenuation behaviors, as revealed by line-shape decomposition at each coverage and component-specific attenuation studies. The attenuation of substrate As is very rapid because of semiconductor disruption and growth of the reacted phases. The first reacted phase reaches its maximum near 1.3 ML and is replaced by a second, fully reacted phase. Above 3.2 ML, this second reacted phase is also attenuated by a developing Ce overlayer. The final surface-coordinated As signal appears when the metallic Ce overlayer starts to form. Arsenic is then visible in small quantities to high coverage, representing 0.7% of the initial As concentration at CTHETA=30 ML. Ga 3d core studies indicate that Ga forms a Ce-Ga intermetallic of variable composition (total core shift 1.78 eV to lower binding energy). Likewise, Ga is much more mobile than As at the reacted interface (7% of initial signal at 30 ML). Valence-band studies at 30 and 60 eV show dominant As-derived p states for the intermixed region and provide evidence for strong ionic bonds. Comparison to single-crystal CeAs shows that the local environment of the bulk-coordinated As at the reacted interface is CeAs-like, having similar valence-band and 4f emission with differences which can be explained by disorder.