THE ADSORPTION AND THERMAL-DECOMPOSITION OF HYDROGEN-SULFIDE ON GAAS(100)

The adsorption of H2S on the Ga-rich GaAs(100)(4 x 1) surface has been investigated using HREELS, TDS and AES techniques. At substrate temperatures of 190 K, it is observed that dissociative adsorption occurs initially to produce SH and H species, with the latter bonded to both Ga and As surface sites. As the surface coverage of these species rises, molecular adsorption is also observed. The molecularly adsorbed phase desorbs at temperatures of 240 K when the surface is heated. The SH and H species formed on the surface undergo recombinative desorption at 320 K. However, the dissociation of SH to produce adsorbed S and H atoms also competes with this desorption process, with the H atoms thereby produced desorbing in the temperature range 500-600 K. Only very limited amounts of free S can be deposited for H2S adsorption at room temperature or below, since molecular H2S adsorption, as well as the formation of H atoms, inhibits the formation of the SH species which acts as the precursor for the deposition of surface S. The extent of S deposited at 550 K is substantially higher since both H2S and H are unstable at this temperature. AES indicates however that the surface concentration of S created by H2S exposure at even higher temperatures drops off, probably due to the diffusion of S into the bulk GaAs lattice. The amount of S deposited can be increased by electron-stimulated processes, and surface S species are found to be very effective in selectively inhibiting the molecular adsorption of H2S.