Laser-Induced Desorption of in and Ga from Si(100) and Adsorbate Enhanced Surface Damage
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LASER-INDUCED DESORPTION OF In AND Ga FROM Si(100) AND ADSORBATE ENHANCED SURFACE DAMAGE PAUL G. STRUPPt, APRIL L. ALSTRIN, BRENDA J. KORTE, AND STEPHEN R. LEONEt Joint Institute for Laboratory Astrophysics, National Institute of Standards and Technology and University of Colorado, and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0440 tNRC-NIST postdoctoral fellow :Staff member, Quantum Physics Division, National Institute of Standards and Technology ABSTRACT Laser-induced desorption (LID) of In and Ga from Si(100) under ultra-high vacuum conditions is investigated. The frequency doubled 532 nm, 2-6 ns output of a Nd:YAG laser is focussed to 0. 14+0.03 J/cm 2 on the Si surface to induce desorption. Desorbed In or Ga atoms are detected by laser-induced fluorescence initiated by a pulsed dye laser propagating in front of the surface. LID occurs by thermal desorption with approximate desorption energies and pre-exponential factors in agreement with literature values from previous isothermal desorption measurements. Experiments at higher coverages suggest that desorption occurs predominantly from the two-dimensional (21)) adsorbate layer with little desorption occurring directly from adsorbate islands. The 2D layer is resupplied by either diffusion out of adsorbate islands or by diffusion of incorporated adsorbate out of the bulk. Adsorbate-enhanced laser-induced surface damage is also observed; only 0.2 monolayer of In reduces the number of laser pulses required to observe damage by greater than a factor of 30. INTRODUCTION In the laser processing of semiconductor materials, the effects of laser irradiation on adsorbate covered surfaces is of particular interest. For example, laser irradiation of Ga and In dosed surfaces was used for the production of p-n junctions and Ohmic contacts [1-3]. Lasers were used to dissociate adsorbed organometallic molecules in order to deposit metal layers [4]. Laser enhanced etching of Si surfaces was studied for chlorine adsorbed on Si surfaces [5]. Because of the wide applicability of laser irradiation of adsorbate covered surfaces, effects such as laser-induced incorporation of adsorbates into the substrate and laser-induced desorption dynamics need to be understood. The results of a study of these effects are presented in this paper for a technologically important system of In and Ga adsorbed on Si(100). We find evidence for the diffusion of adsorbed In or Ga into the Si bulk due to laser irradiation. The incorporated In or Ga lead to a reduction in the damage threshold in terms of number of laser pulses at constant energy by a factor of greater than 30. The laser-induced desorption (LID) dynamics of In and Ga are also investigated. At coverages where adsorbate islands are known to exist on top of an adsorbate layer (Stranski-Krastanov (SK) growth), the results are consistent with a model in which desorption occurs primarily from the two-dimensional (2D) adsorbate phase which is bound directly to the
Mat. Res. Soc. Symp. Proc. Vol. 236.
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