A LEED study of the epitaxial growth of copper on the (110) surface of tungsten

Low-energy electron-diffraction (LEED) has been employed in a study of the epitaxial deposition of copper on a single crystal (110) face of tungsten under ultra-high vacuum conditions. Copper was evaporated at 10−9 Torr from a Knudsen cell placed in the LEED chamber. The flux of copper at the sample could be determined to an accuracy of ± 10%. Using a fluorescent-display, LEED apparatus, the structure of the deposit was continuously monitored and recorded. Evaporation onto clean tungsten at room temperature resulted in partial alloying and then the formation of well oriented, uniformly thin copper (111) layers. Strong multiple scattering was observed at coverages of between one and two atomic layers. Moderate heating resulted in diffusion into the tungsten and alloy formation. When a half monolayer of oxygen was first chemisorbed on the tungsten, epitaxy was severely inhibited. Very little sign of any ordered copper was apparent after a total flux corresponding to 20 atomic layers. Contributing to this was the apparent very low condensation coefficient for the copper on such a surface. If some physisorbed oxygen was present, in addition to the chemisorbed, there was a marked improvement in epitaxy, though it was still considerably poorer than that on the clean tungsten surface, since a spread in orientation of about ± 2° was observed. New structures involving only the tungsten and oxygen were observed on heating at this stage. Oxygen, when introduced at less than a monolayer coverage of copper, or at any time when the alloy was observed, displaced the copper from the tungsten-copper matrix and left it in well oriented form as on the clean tungsten surface. When the oxygen was introduced into the system after depositing about six atomic layers of the copper onto clean tungsten, no change in the diffraction pattern was observed, indicating that the oxygen was not readily chemisorbed on the copper film at room temperature. On heating, however, it was observed to be bound to the tungsten, while the surface of the copper remained unaffected in structure.