Characteristics of dielectric layers formed by low-temperature vacuum ultraviolet-assisted oxidation of SiGe layers
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Characteristics of dielectric layers formed by low-temperature vacuum ultraviolet-assisted oxidation of SiGe layers Valentin Craciuna) and Ian W. Boyd Electronic and Electrical Engineering, University College London, Torrington Place, London WC1E 7JE, United Kingdom
Jacques Perriere Groupe de Physique des Solides, Universities Paris VII et VI, 2, place Jussieu, 75251 Paris Cedex 05, France
Bernie Hutton Chemistry Department, University College London, London, United Kingdom
Edward J. Nicholls Department of Applied Physics, University of Hull, Hull HU6 7RX, United Kingdom (Received 25 January 1999; accepted 7 June 1999)
Thin Si0.8Ge0.2 layers epitaxially grown on (100) Si substrates were oxidized at temperatures from 150 to 450 °C under vacuum ultraviolet (VUV) radiation emitted by an excimer lamp working with Xe ( ⳱ 172 nm). The structure and composition of the grown dielectric layers were investigated by Rutherford backscattering spectrometry, nuclear reactions analysis, ellipsometry, Fourier transform infrared spectroscopy, and x-ray photoelectron spectroscopy. These investigations have shown that, during the VUV-assisted oxidation process, Ge atoms were initially rejected from the grown SiO2 layer even at temperatures as low as those employed here. After a certain quantity of Ge accumulated at the interface, nanocrystalline Ge regions were directly excised from the remaining SiGe layer becoming embedded within the advancing SiO2 layer. The layers containing these nanocrystalline Ge particles exhibited the same visible photoluminescence spectra as those recorded from layers already known to contain nanocrystalline Ge or GeO2 particles, porous Ge, or nanocrystalline Ge particles exhibiting a different crystalline structure. This seems to indicate that the shell region of the nanocrystalline particle, and not its crystalline core, is the source of the photoluminescence.
I. INTRODUCTION
Si1–xGex layers have been successfully employed for the production of high-speed heterojunction bipolar transistors,1,2 superlattices, and quantum wells.3 Despite several advantages such as higher hole mobility or modulation doping, the development of a highperformance metal oxide semiconductor (MOS) transistor using a SiGe channel has been hampered by the lack of a suitable passivation layer.4–6 When a Si1–xGex layer is thermally oxidized, the kinetics of the process and the composition of the grown layer have been found to be a function of initial Ge concentration, x, and temperature, and oxidation conditions, i.e., dry or wet. Many investi-
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Permanent address: NILPRP, Bucharest V, PO Box MG-36, Romania. J. Mater. Res., Vol. 14, No. 9, Sep 1999
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gations have shown that the grown layers have poorer electrical characteristics than those grown by oxidizing pure Si, because the preferential oxidation of Si atoms and the rejection of Ge atoms for x < 0.5 alter the interface composition, planarity, and electrical properties.7–9
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