The adsorption and decomposition of trimethylgallium （Ga（CH3）3, TMG） on Pd（111） and the effect of pre-covered H and O were studied by temperature programmed desorption spectroscopy and X-ray photoelectron spectroscopy. TMG adsorbs dissociatively at 140 K and the surface is covered by a mixture of Ga（CH3）x （x=1, 2 or 3） and CHx（a） （x=1, 2 or 3） species. During the heating process, the decomposition of Ga（CH3）3 on clean Pd（111） follows a progressive Ga-C bond cleavage process with CH4 and H2 as the desorption products. The desorption of Ga-containing molecules （probably GaCH3） is also identi ed in the temperature range of 275-325 K. At higher annealing temperature, carbon deposits and metallic Ga are left on the surface and start to di use into the bulk of the substrate. The presence of precovered H（a） and O（a） has a signi cant effect on the adsorption and decomposition behavior of TMG. When the surface is pre-covered by saturated H2, CH4, and H2 desorptions are mainly observed at 315 K, which is ascribed to the dissociation of GaCH3 intermediate. In the case of O-precovered surface, the dissociation mostly occurs at 258 K, of which a Pd-O-Ga（CH3）2 structure is assumed to be the precusor. The presented results may provide some insights into the mechanism of surface reaction during the lm deposition by using trimethylgallium as precursor.
