Kinetics of the Interaction of Atomic Species With (100) Gallium Arsenide Surfaces
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The Institute for Microstructural Sciences (NRC, Ottawa) provided the silicon samples used in this work. Intrinsic Si was grown on top of either n or p-type (100) Czochralski Si substrates by chemical vapor deposition. The 5x5 mm GaAs samples and the 5x10 mm exposed surface Si samples, respectively, cut from the wafers were dipped in a 10% HF solution for about 60 seconds, rinsed in deionized water, and quickly inserted into the reactor and dried under a flow of argon. The photoluminescence intensity (PLI) was used to measure the steady state carrier concentration that is inversely proportional to the concentration of surface states. The GaAs band gap fluorescence at 865 nm was excited by a 10 mW red (632.8 nm) HeNe laser, focused onto the entrance slit of a scanning monochromator, dispersed and detected by an infrared sensitive photomultiplier tube, connected to a photon counter and a PC. Thus the PLI could be measured as a function of real time. A 200 W 2.45 GHz microwave generator attached to a quarter wave cavity, was used to create the hydrogen or deuterium plasma inside a quartz discharge tube located about 20 cm upstream from the reaction chamber. Molecular hydrogen or deuterium (0.1 Torr) diluted with up to 0.3 Torr of argon flowed through the quartz discharge tube. The GaAs wafer was exposed to the atomic hydrogen for various periods (from a few seconds up to 5 minutes). All the GaAs exposures to atomic hydrogen were performed at room temperature. Sulphur atoms were produced in situ with the two step rapid reaction of hydrogen sulphide with atomic hydrogen, yielding one S atom for each H atom consumed. The technique7 involved adding a very small amount of H 2S (1-2%) to a stream of H atoms. A 20 A layer of Si0 2 was grown at 200'C on Si by exposing a clean surface to atomic oxygen from an upstream microwave discharge in 0.5 Torr of 02. Because silicon luminescence is extremely weak, its PLI could not be used. We therefore monitored the changing surface carrier recombination sites by following the laser induced (633 nm HeNe laser) conductivity of an Si0 2 covered Si wafer during the exposure to H atoms. This "contactless" technique, described in detail elsewhere8 , employed a radio frequency signal to measure the change in the conductivity of the Si chip. The Si/Si0 2 interfaces were exposed to hydrogen or deuterium atoms in a manner similar to that used with GaAs. The only difference was that the working temperature of the Si was typically about 200'C. RESULTS Changes in the "surface recombination site" density (i.e. the inverse of the 865 nm PLI signal) on GaAs exposed to hydrogen atoms are shown in Figure l(a). An irreversible process occurred initially, i.e. the trap density decreased by about a factor of 3 when the discharge was on first turned on and decreased even further (up to 10-100 times the initial trap density, depending on the sample) when the discharge was shut off. Subsequently the trap density reversibly increased when the plasma was switched on, and decreased when it was switched off. The
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