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Raman scattering data is used to determine the lowering of the barrier height resulting from a chemical passivation treatment which has previously been shown to reduce surface recombination velocity. Using a GaAs sample doped so that free electron carrier density in the bulk is 1018cm-3, the barrier height is shown to drop from .78 ± .02 ev to .51 ± .16 ev. Recently Sandroff et. al. 1 have shown that GaAs may be chemically passivated so as to increase dramatically the gain of a bipolar transistor. The passivation treatment con-sists of spinning a thin coat of one of a class of inorganic sulfides 2 onto the GaAs surface in air; in particular an aqueous solution of Na2S reduces the surface recombination velocity to nearly that of an AlGaAs/GaAs heterostructure interface2. Clearly the barrier height3 of the GaAs surface has been lowered, and a direct contactless measurement of this change can be made using Raman scattering tech-niques.4-6 Whereas previous work on band bending effects in GaAs used either <110> surfaces and resonance excita-tion5'6 or <111> surfaces,4 experiments to be described in this paper were performed on <100> surfaces, since this is the surface of choice. in fabricating devices. Excitation was supplied by the 5145A line of an Ar+ ion laser. Backscatter-ing geometry was used in a microprobe apparatus which has been described elsewhere.7 The E vector of the incoming radiation was always fixed parallel to the {100} direction in the plane of the sample and the scattered radiation was analyzed in the perpendicular direction, i.e. z(xy)i. Thus only LO phonons are allowed according to the selection rules.8 Samples were prepared by MBE. For n-type doping with n<1017cm-3, the LO phonon was observed at 291.5 cm-1. Some slight polarization leakage enabled obser-vation of the TO phonon at 268 cm-1; the LO/TO ratio was in excess of 16. At sufficiently high doping levels, the sur-face depletion layer will become thinner 5 than the 1053A penetration depth9 at 5145A. In the bulk doped material, the LO phonon couples with the free electron plasmon1°-12 and splits into two peaks whose frequencies are a function of free carrier density and excitation wavelength.13,14
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