The generation kinetics and origins of pyramidal hillocks grown on Si(111) surfaces with an off angle of 0.55-degrees from [111] were investigated for the chemical vapor deposition of Si with SiH2Cl2. According to the top shape of the hillock, most hillocks can be classified to three kinds : bright-, sharp-, and flat-top hillocks. It was shown that the flat-top hillock is grown with the stacking fault and the sharp-top and bright-top hillock result from the hydrocarbon and oxide (Si particle) contaminants on the substrate, respectively. The sharp-top hillock was observed to develop along the dislocation. The generation kinetics of the stacking fault was suggested to be involved with chlorine adsorbate on the surface. Because of the large hillock size, the existence of growth defects due to the stacking fault and dislocation can be detected easily for very thin films with no etching. Furthermore, we found that the three kinds of hillocks can be drastically reduced in density by using carrier hydrogen gas excited with ultraviolet light from low-pressure Hg lamps during growth as well as substrate cleaning at atmospheric pressure. This indicates that the photoexcited hydrogen has a cleaning effect for the hydrocarbon, oxide, and chlorine on the surface during Si growth.