2262
J . Phys. Chem. 1993,97, 2262-2269
Surface Far-Ultraviolet Photochemistry of Ethyl Chloride on GaAs( 100) Vladimir Liberman, Matthew C. Nooney, Richard J. Amata, and Richard M. Martin' Department of Chemistry, University of California, Santa Barbara, California 93106 Received: October I, 1992; In Final Form: December 2, 1992
The photoinduced dissociation a t 193 nm of monolayer ethyl chloride (EtCl) was studied on the Ga-rich GaAs(100) (8X2)Ga surface a t 90 K. Photodepletion of EtCl is efficient, with a cross section of 6 X 10-19cm2. Sixty percent of the EtCl molecules depleted by irradiation give ethyl groups bonded to the surface. The ethyl groups are stable up to 4 0 0 K, which is the onset of thermal desorption of the products. The thermal desorption products are C2H4, C2H5, CzHb, H2, and GaCl. The hydrocarbon product distribution is approximately 70% C2H4, 20% C2H6,and 10% C2H5 radicals. Experiments with CD3CH2Cl provide evidence that ethylene is formed by 0-hydride elimination. The zero-coverage first-order kinetic parameters for this reaction are approximately E, = 100 kJ/mol and A = 4.4 X 107/s. Comparison with the gas-phase photodissociation cross section indicates that the photodepletion is primarily due to substrate-mediated processes rather than to direct photodissociation of EtCl. The most probable mechanism involves photogenerated electron transport to the surface followed by electron dissociative attachment to EtCl.
1. Introduction
2. Experimental Section
In a recent study of atomic layer etching chemistry of Cl2 on Ga-rich GaAs( loo), it was found that a t 85 K chlorination forms a GaCl layer, with the reaction saturating at a C12 exposure of about 1 langmuir.1 Thus, this reaction is facile even a t low temperature and cannot be controlled practically by thermal means. One approach to gaining more control of atomic layer etching is to use an adsorbate that does not react thermally and drive the reaction photochemically. Laser photochemical control of the reaction also has potential lithographic applications. The present work is a step in this direction: a study of the monolayer photochemistry of ethyl chloride on Ga-rich GaAs( 100). The number of previous studies of photoinduced reactions of halogen-containing molecules with GaAs surfaces is quite limited.2 Thermal and UV-laser-induced dissociation of CH3Cl on Garich GaAs( 100) has been studied previously.3 In the absence of irradiation, a small fraction of the CH3Cl (adsorbed on the surface at liquid nitrogen (LN2) temperature) decomposed upon heating to yield surface C and C1. Photodissociation of the molecule was found to occur through two main channels: direct photofragmentation (dominant a t 193nm) and dissociation through electron attachment (at both 193 and 248 nm). Light-induced reaction of CCld on G a A ~ ( 1 0 0 )and ~ (110)4 has also been studied previously. For the (100) surface, the dissociation cross section at 222 nm was found to be several times higher than the corresponding gas-phase value, suggesting that substrat,e excitation plays an important role in the photodi~sociation.~With the (1 10) surface two photodissociation channels were observed: direct photofragmentation (for 193-nm irradiation) and dissociation attributed to generation of electron-hole pairs in the GaAs ~ u b s t r a t e .The ~ latter process was observed for illumination with all above-bandgap light wavelengths, Le., X < 800 nm. In the present work ethyl chloride (EtCl), C2HsCl,was chosen as the adsorbate for several reasons. First, it was expected to produce C1 atoms on the surface, thus initiating etching on the atomic scale. Second, In recent studies of photoinduced reactions of EtCl on Ag( 111)5and Pt( 111),6 no thermal decomposition of the EtCl was observed, while UV photochemical decomposition was very efficient on both substrates. Finally, from previous studies of the decomposition of ethyl-group-containing arsenic and gallium compounds on GaAs, little, if any, carbon deposition is expected from ethyl group reactions on the surface.'
The experimental apparatus has been described in detail previously.*-'0 The experiments were performed in an ultrahighvacuum (UHV) chamber with a base pressure of
