Adsorption kinetics for ethylsilane, diethylsilane, and diethylgermane on Si(111) 7×7

Abstract

The adsorption kinetics for ethylsilane (ES), diethylsilane (DES), and diethylgermane (DEG) on Si(111) 7×7 were studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. The initial reactive sticking coefficients were determined as a function of surface temperature using LITD measurements. In these experiments, the ethyl coverage vs adsorption time was monitored using CH2=CH2 (ethylene) LITD signals that were produced by the β-hydride elimination of the surface ethyl groups, e.g. Si–CH2CH3(ad)→Si–H(ad)+CH2=CH2(g). The initial reactive sticking coefficients were S0≊2×10−3, 4×10−3, and 5×10−2 for DES, ES, and DEG, respectively, at 200 K. As expected from a precursor-mediated adsorption model, the initial reactive sticking coefficients were observed to decrease with increasing surface temperature. Experiments with preadsorbed hydrogen also demonstrated that the initial reactive sticking coefficients of DES and DEG were reduced as a function of hydrogen coverage. This behavior indicated that alkylsilane and alkylgermane adsorption on Si(111) 7×7 requires free dangling bond sites. LITD experiments revealed that the ethyl surface coverage saturated after large exposures. The saturation coverage corresponded to a deposited Si or Ge coverage of Θ=0.13 monolayer for DES and DEG and was independent of surface temperature between 200–400 K. DEG adsorption cycles were used to deposit increasing amounts of Ge on Si(111) 7×7. The deposited coverage was examined using H2 TPD studies which indicated that the germanium may be forming islands. LITD experiments were also used to monitor ethyl surface diffusion on Si(111) 7×7 after DES adsorption. No evidence of significant ethyl surface mobility (D≤1.0×10−10 cm2/s) was found for surface temperatures as high as 600 K.