Comparison of Low Intensity Laser Enhancement of Oxygen Chemisorption on GaAs Using O 2 and N 2 O
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COMPARISON OF LOW INTENSITY LASER ENHANCEMENT OF OXYGEN CHEMISORPTION ON GaAs USING 02 AND N20 K.A.BERTNESS, C. E. McCANTS, T. T. CHIANG, P. H. MAHOWALD, A.K.WAHI, T. KENDELEWICZ, I. LINDAU, AND W. E. SPICER Stanford Electronics Laboratories, Stanford University, Stanford, CA 94305 ABSTRACT The sticking coefficient for molecular oxygen on GaAs(1 10) up to about one-halft monolayer coverage can be enhanced by two or three orders of magnitude when the semiconductor surface is illuminated by an argon ion laser with intensities of 5 W/crn or less. Previous work has shown that this effect is nonthermal and roughly independent of wavelength for above-bandgap radiation, implying that photogenerated electrons and/or holes are responsible in some way. In this paper we present recent work which shows that transfer of energy from the GaAs to oxygen molecules physisorbed on the surface causes the breakup of the oxygen and so enhances the reaction. This conclusion comes from comparison of our photoemission results for 0z and N2 0 exposures made under the same conditions, N2 0 being chosen since only 1.7 eV is needed to remove the oxygen atom as opposed to the 5.1eV required to break an 0-0 bond. Nitrous oxide, in contrast to 02, shows only slight photoenhancement and has a much lower activation energy for the dissociative sticking coefficient, demonstrating that removal of the dissociation of 02 as a reaction step also removes the major part of the activation energy barrier to oxygen chemisorption. Since N2 0 and photoenhanced 02 have similar kinetics, we conclude that the breakup of 02 is the reaction step accelerated by illumination. Another clue to the nature of the photoenhancement comes from our finding that both n- and p-type GaAs display similar 02 laser enhancement ratios. An estimate of the carrier concentrations at the surface predicts that electrons and holes are present in equal amounts at the surfaces of both doping types, however, so this result does not distinguish between single-carrier and recombination-related enhancement mechanisms. INTRODUCTION The use of visible light to enhance chemical reactions with semiconductors has potentially many applications, quite a few of which have already been realized ( see, for example, Reference 1). Device fabrication problems with Ill-V and II-VI semiconductors often point to development of processes which accelerate chemical reactions while keeping the semiconductor near room temperature. In addition, an understanding of the role which photons can play in enhancing surface reactions may prove useful in reducing defect formation in optically active semiconductor devices, such as lasers, photodetectors and solar cells. This paper deals with the specific system of oxygen chemisorption on GaAs(1 10), but we expect that the mechanisms described could be important in any fluid-solid reaction which has a large activation energy barrier due to the breakup of the fluid molecules. Previous work [2,3,4 ] on laser-enhanced oxidation of GaAs has eliminated some of the more mundane expl
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