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Translational energy (E T ) dependence of oxygen (O 2 ) adsorption on clean and modified Cu{100} has been studied by molecular beam surface scattering (MBSS) experiments. The role of surface defects on the adsorption dynamics has been investigated by blocking the defects with Ag and oxygen atoms, and by increasing the defect density with Ar + -ion bombardment. At low E T the adsorption of O 2 is shown to be indirect and facilitated either by a dynamical steering mechanism or a precursor state. At high E T the defects have only marginal effect on the adsorption. Additionally, the modification of the surface electronic structure by preadsorbed oxygen is shown to have a pronounced effect on the adsorption dynamics, whereas the electronic structure modification by preadsorbed Ag is shown to be relatively inefficient in changing the reactivity of Cu{100} towards O 2 adsorption.

Adsorption dynamics of O 2 on Cu͑100͒ and on reconstructed Cu͑100͒-͑2 ͱ 2 ϫ ͱ 2͒R45°-O at 300 and 553 K have been investigated by employing a supersonic molecular-beam surface-scattering technique. Experimental results suggest that an activated direct adsorption channel is operative on the clean Cu͑100͒, whereas the adsorption of O 2 on the reconstructed Cu͑100͒ is mediated either by a precursor state or by steering effects. First-principles molecular-dynamics simulations and potential-energy surface calculations show that the nature of the adsorption dynamics of O 2 is different between the clean and reconstructed Cu͑100͒ surfaces. The O 2 molecule is likely to diffuse away from the reconstructed area or to completely desorb from the surface, while in the case of the clean Cu͑100͒ surface, the adsorption occurs through a direct dissociative trajectory. We also find that in the case of the reconstructed surface, the steering occurs higher over the surface and that the recoil effect does not modify the surface as much as in the case of the clean surface. Moreover, the mobility of O and Cu adatoms on the reconstructed Cu surface is significantly lower than that on the clean surface both in the direction of the missing rows and in the direction perpendicular to them.

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A new ohmic contact scheme for gallium nitride is presented. The use of Nitride-forming metal Over Gallide-forming metal, “NOG”, can modify the thermodynamic activity of N and Ga near the interface. This in turn can modify the near-surface point defect concentrations, particularly the vacancies of Ga and N. The principle of this contact scheme was shown to be consistent with results from Ni/Au, Ni/Zn-Au, Ta/Ti, and Ni/Mg/Ni/Si contacts. In the present study, the “NOG” scheme was used to design Ni/Ti/Au and Ni/Al/Au metallization, and addition of Ti and Al nitride-forming metals to the Ni gallide-forming metal led to lower but still high contact resistance. Ti was shown to be better than Al as the nitride-forming metal based on the decrease of resistance in as deposited contacts. Compared to Ni/Au, four times more current was measured in Ni/Ti/Au contacts to p-GaN after anneal at 300°C for 5min. However the addition of the Ti nitride-forming metal led to lower stability at 500°C.