Observation of the Amorphous-to-Crystalline Surface Transition in Al-AlxOyUsing Slow Positrons

Abstract

The amorphous-to-crystalline surface transition of k\ x O y on the Al(lll) surface is observed between 650 and 800 K with different 0 2 exposures by measuring the positronium (Ps) fraction produced by e* impinging on the surface. The data are interpreted in terms of vacancy-type defects in the film or at the metal-metal-oxide interface which as trapping sites for e + or Ps. As the ordering process proceeds to completion the trapping centers anneal out and the Ps fraction increases, showing an irreversible transition. This technique provides a new experimental method to study interfaces. PACS numbers: 68.20.+ t, 64.70.Kb, 78.70.Bj With the advent of efficient monoenergetic positron sources incorporated in ultrahigh-vacuum systems, it is now possible to study positron interactions with well-characterized surfaces. In a recent series of papers 1-5 the behavior of positrons at a number of metal surfaces has been reported. In some metals an implanted positron can (i) be reemitted into the vacuum, 2 (ii) escape with an electron and form positronium (Ps), 1,s or (iii) annihilate in the perfect lattice, in a lattice vacancy, or in the surface state. 4 ' 5 The positron can be thermally activated from this surface state and the depth of the potential well can be deduced. 4 " 6 In all clean metals studied the formation of Ps has been found to be the dominant mechanism for the escape of positrons from a metalvacuum interface.In the present study we have exposed an Al(lll) single-crystal surface to molecular oxygen (0 2 ). The fraction of Ps produced at room temperature increased initially with increasing 0 2 exposures saturating at less than 500 L 0 2 (1 langmuir = 1L = 10~6 Torr sec) and subsequently decreased at larger oxygen exposures. After separate total exposures of 500 L and 10 6 L 0 2 , the surface was examined with low-energy positrons as a function of sample temperature, at various incident positron energies, E. When the temperature at which the transition from amorphous to ordered Al x O y occurs at the surface the Ps fraction increases and is identified as the removal of trapping centers (vacancy-type defects) for positrons or Ps.The experimental details of the apparatus and the method of measurement have been described elsewhere. 3 In this experiment the existence of Ps (bound electron-positron state) is detected by observing changes in the energy spectrum produced by the three-photon emission that occurs upon the decay of orthopositronium ( 3 S 1 ). 7 The incident positron energy was varied by biasing the target at different voltages. The sample was cleaned by repeated cycles of Ar-ion bombardment and annealing (>400 C). Surface characterization was performed by low-energy electron diffraction (LEED) and Auger-electron spectroscopy (AES). Before the sample was exposed to 0 2 , no detectable impurities were observed by AES. The samples were maintained in a vacuum of ~1 xlO" 10 Torr during the experimental run (<8 h) except for those periods of 0 2 exposure. After each run AES spectra were taken again and no d...

Structural ordering of ultra-thin, amorphous aluminium-oxide films

“…Al cations in tetrahedral or octahedral interstices of the distorted, densely packed oxygen sublattice, respectively), which may correspond to the vacancy-type of defects as reported in Ref. [34].…”

Intrinsic stress evolution during amorphous oxide film growth on Al surfaces

“…The am-Al 2 O 3 films formed on Al(111) and Al(100) can be described as constituted of neighboring "building blocks" of edge-and corner-sharing [AlO 4 ], [AlO 5 ], and [AlO 6 ] polyhedra (with the Al cations in, respectively, tetrahedral, pentagonal, and octahedral interstices of the distorted, densely packed oxygen sublattice), [39][40][41] where the free volume is associated with vacancy-type defects. 42 Although the oxide films on Al(111) and Al(100) are both amorphous according to the HRTEM analysis, local chemical state analysis by XPS revealed strikingly different microstructural developments for the evolving oxide films on the two Al surfaces. 16 On Al(100), the electronic polarizabilities around the core-ionized Al and O ions remain practically constant with increasing thickness, which evidences that the local chemical environments of the constituent Al and O ions is practically constant during growth.…”

Field Ionization of Strongly Magnetized Rydberg Positronium: A New Physical Mechanism for Positron Accumulation

“…As the electric field in the trap well is increased farther than shown in the figure, the accumulation rate begins to drop slightly, presumably because the electric field starts to influence the tuning of the relative velocity previously optimized by changing V t and more field ionization takes place before the trapping well. The formation of the ground state and lowest excited states of positronium at the surface of crystal moderators is well known [12] and becomes more efficient when the moderator is heated [13]. Stabilization of largeorbit positronium in a strong magnetic field is also predicted [14].…”