Zbigniew Szczudło

Interaction of hydrogen with ultrathin titanium layers on tungsten

Abstract Hydrogen adsorption on titanium layers of an average thickness from 0.03 to 50 monolayers deposited on a tungsten substrate was investigated at liquid nitrogen temperature using the field electron emission microscopy techniques. It has been found that the interaction between titanium and hydrogen depends on the thickness and atomic structure of the Ti/W layer. For tightly arranged titanium layers isomorphous with the (011)W face the H–Ti interaction is weak and does not change the morphology and atomic structure of the layer. In the case of titanium layers isomorphous with the (001)W the H–Ti interaction dominates over the Ti–W interaction, beginning from the thinnest titanium layers deposited on the tungsten substrate. This results in a 3-dimensional nucleation of TiHx at the steps of the terraces of the (001)W face. For the titanium layers isomorphous with the (111)W and (112)W faces, the Ti–W interaction is stronger than the H–Ti interaction up to the thickness of 3 to 4 monolayers of titanium. Measurements of the average work function changes as a function of the H2 exposure indicate that, apart from the hydrogen adsorption state which raises the work function, there exists another one which decreases the work function. The latter state occurs in titanium layers isomorphous with the (111)W face for the coverage in the range from 0.65 to 2.7 titanium monolayers. Measurements of the local work function changes carried out for H2 adsorption on Ti/(111)W and Ti/(016)W indicate that this adsorption state cannot be recognized as the unstable β+ adsorption state observed during the hydrogen adsorption on a surface of bulk titanium [R. Duś, E. Nowicka, Z. Wolfram, Surf. Sci. 269/270 (1992) 545]. The one observed in this experiment is stable and is believed to result from the immersion of the adsorbed hydrogen atoms into the electron gas of the Ti–W interface.

Adsorption and nucleation of hafnium on facets and stepped surface areas of tungsten

Abstract The adsorption of Hf deposited from vapor in ultra-high vacuum onto thermally cleaned surfaces of W has been studied using field electron emission microscopy. Three-dimensional nucleation and two-dimensional layer formation on structurally and topographically different areas of the substrate were observed for various annealing temperatures and various amounts of the deposited adsorbate. The dependence of the average work function on the adsorbate coverage as well as on the structural and morphological changes of the adsorption layer, involved by annealing at various temperatures, has been measured.

STM observation of steps and terraces on tungsten (2 1 1) surface

Thermally cleaned W(211) surface with 0.7 degrees miscut consists of (211) terraces separated by monoatomic steps. When the surface is exposed to oxygen and subsequently annealed at 1100-1900 K, the width of (211) terraces increases and multilayer steps are formed. Similar step bunching is observed during routine cleaning of the sample by annealing in oxygen and thermal flashing in ultra high vacuum. During such cleaning procedure islands of c(6 x 4) reconstruction are observed.

Hf dissolution and surface alloy formation in Hf/W(011) adsorption system

The system Hf/W(011) was investigated using the STM, LEED, AES and CPD techniques. For low coverages (about 1 ML), dissolution of Hf in W and alloying on the W(011) surface was observed. For temperatures above 1200 K, mixing of the adsorbate and substrate atoms was observed. This process occurred at the edges of atomic steps. A surface alloy appeared at a temperature in excess of 1500 K. The alloy was being formed as large islands occupying one terrace each, and separated by atomic step bunchings of the substrate. At 1850 K the surface alloy decayed. The decay was followed by solution of the Hf in the subsurface layer of substrate.

Intermixing and surface alloying in Hf and Zr ad-layers on W(100)

Hafnium or zirconium layers of thickness from a fraction of geometrical monolayer up to 2 monolayers were vapor-deposited under ultra high vacuum on the surface of the (100)-oriented W single crystal and gradually annealed at various temperatures from 300 to 2100 K. Intermixing, interdiffusion, and alloying of Hf or Zr adlayers on W(100) were observed and studied by scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). Two surface alloys of different stoichometry have been identified for both Hf/W(100) and Zr/W(100) adsorption systems. Influence of oxygen adsorption on topography and morphology of Zr adlayers deposited on stepped W(100) surface was studied by STM. Formation of islands of Zr3W surface alloy surrounded by bunched steps was observed in this case.