Hi all,
sorry for double-posting, but the "discussion topic" doesn't seem to get enough attention. We have recently started to work with Al single crystals for surface science experiments. Unfortunately, after a few cleaning cycles the crystal surface gets cloudy or "milky". Our typical cleaning cycle involves 1 kV Ar sputtering (ion current around 3-5 uA) x 30-60 min followed by annealing to 600-700 K (time varies, but typically not less than 15 min with somewhat fast ramping up and down, but no quenching). A high resolution XPS does not show any considerable amount of adsorbates or anything unusual whatsoever (some traces of oxygen which were there even for the mirror-like surface, some traces of carbon). Therefore, I suspect the crystal experienced some faceting/graining of the surface which resulted in that the surface has become mesoscopically rough. LEED spots have possibly become somewhat broader but this is really hard to estimate. Unfortunately, we had no time to perform AFM measurements on this surface. But it looks indeed as an annealing protocol is vitally important to keep the surface nice and well-defined. Therefore, I wonder if anyone else has experienced the same problem with Al (or maybe other crystals, too?) and/or knows how to overcome this issue? Maybe someone could share an annealing protocol or give a link to such a protocol if published? Tips and tricks? All meaningful opinions are welcome!
Dear Nikolay A Vinogradov
Ion sputtering is non-equilibrium process involves two competitive mechanisms, surface roughening caused by erosion and smoothing due to surface diffusion processes. Therefore the final surface structure and its morphology are highly depends on the ion flux and substrate temperature as parameters able to change the kinetics of the ion sputtering process. Now, the integrity of final surface structure, away from cloudy or "milky "structure, may obtain through the diffusion of adatoms and vacancies. Therefore, there is optimum substrate temperature at constant ion flux and optimum ion flux at constant substrate temperature. Now if you got these non liner functions experimentally you will no need to include annealing step as part of cleaning process.
Best regards
I would just add that annealing at a temperature larger than 500 degC would cause a very large surface rearrangement.. you start approaching the melting point that is 660 degC for bulk material. Just to make an example, in semiconductor device fabrication, where Al or some of its alloys are used as top metallisation, the thermal process in the flow after the Al deposition are performed at temperatures well below 600 degC.
From what I recall for Aluminium that is quite common (its been about 20 years since I studied Al and Mg single crystals). I would recommend using a lower temperature for annealing if its of great concern.
A useful reference for single crystal prepartion in UHV is Appl. Surf. Sci 10 (1982) 143-201. There may be sometthing useful in that reference
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