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Content archived on 2022-12-23

Photochemical reactions at solid surfaces

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In a joint effort of the groups in Kharkov, Kiev, Paris, and Berlin a significant improvement in the performance and interpretation of light-induced etching of the main semiconducting materials Si and GaAs was obtained. It turned out that the efficiency for etching of Si with XeF2 as well as for GaAs with Cl2 increases to values between 10 and 100 removed atoms per photon for wavelengths around 120 nm. It was shown that the spatial resolution remains diffraction limited for Si and GaAs and that it is of the order of 100 nm. For future applications around the year 2005 to 2010, this will be the resolution required for the projected increase in packing densities for the chip technology. The high efficiency provides cost effectiveness and large throughput. The wavelength range around 120 nm is accessible with improved conventional light sources and optics. For Si/XeF2 surface excitation of a fluorosilyl layer was identified as the initiating process for the etching reaction, and an amplification by a chain reacting provides the high efficiency. A similar chain reaction appears for GaAs/Cl2 and Cu/Cl2 and the reaction products from this amplification process were observed in depth profiles of the films. From thermal desorption experiments a weakly bound adsorption state of Cl2 or GaAs was observed which represents the precursor for the etching reaction. A broad spectrum of adsorbates on well characterized substrates was studied to clarify the adsorption kinetics and surface reactions. In the case of O2 on W(111) multilayer condensation, physisorption and weak chemisorption were detected. Molecularly adsorbed O2 serves as a precursor to dissociative adsorption and the substrate structure influences the weakly chemisorbed species. The growth of O2 adlayers on W(110) follows the island mechanism. The atomic phase formation is determined by two different processes involving an extrinsic precursor and a Langmuir mechanism. For O2 adsorbed an Si(111) 7x7 selective photo-fragmentation by resonant and non-resonant core excitation of O (1s) was achieved. In addition it was observed that selective light-induced desorption of hydrogen from an Si(100) surface leads to a high reactivity with oxygen in the irradiated parts allowing for a lateral structuring of the oxidation. A new selfpropagation of a molecular reaction was explored on an atomic scale for the O2/ß-SiC(100) system.

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