BITS Faculty Publications
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Item Influence of substrate domain boundaries on surface reconstructions of Ga/Si(1 1 1)(Elsevier, 2004-03) Gangopadhyay, SubhashisThe temperature induced phase transition of the Ga/Si(1 1 1) surface for submonolayer Ga coverages has been monitored by variable temperature scanning tunneling microscopy. After room temperature deposition of about 1/3 monolayer of Ga on Si(1 1 1), the Si(1 1 1) surface is mostly covered with Ga-induced magic clusters in a 7 × 7 like arrangement whereas at the domain boundary regions of the former Si(1 1 1)-7 × 7 reconstruction an increased density of excess Ga islands is found. The magic clusters are stable against annealing up to 350 °C. At this temperature, however, the Ga-islands coalesce and a mixture of and 6.3 × 6.3 structures is formed at the domain boundary regions. At an annealing temperature of 400 °C a phase transformation of the structure to 6.3 × 6.3 is found at the domain boundary region opposite to the usual thermal phase sequence. This can be explained in terms of an interplay of surface kinetics and surface stress.Item Surface oxidation of GaN(0001): Nitrogen plasma-assisted cleaning for ultrahigh vacuum applications(AIP, 2014) Gangopadhyay, SubhashisThe cleaning of metal-organic vapor-phase epitaxial GaN(0001) template layers grown on sapphire has been investigated. Different procedures, performed under ultrahigh vacuum conditions, including degassing and exposure to active nitrogen from a radio frequency nitrogen plasma source have been compared. For this purpose, x-ray photoelectron spectroscopy, reflection high-energy electron diffraction, and scanning tunneling microscopy have been employed in order to assess chemical as well as structural and morphological surface properties. Initial degassing at 600 °C under ultrahigh vacuum conditions only partially eliminates the surface contaminants. In contrast to plasma assisted nitrogen cleaning at temperatures as low as 300 °C, active-nitrogen exposure at temperatures as high as 700 °C removes the majority of oxide species from the surface. However, extended high-temperature active-nitrogen cleaning leads to severe surface roughening. Optimum results regarding both the removal of surface oxides as well as the surface structural and morphological quality have been achieved for a combination of initial low-temperature plasma-assisted cleaning, followed by a rapid nitrogen plasma-assisted cleaning at high temperature.