Single walled carbon nanotubes (SWNTs) are highly desired for future electronic applications due to the excellent electrical, mechanical and thermal properties. However, the density and the selectivity in the growth of aligned semiconducting nanotubes do not coexist previously: when the selectivity is high, the density is low and vice versa. In the present work, we found that the random carbon nanotubes (CNTs) in the catalyst area block the growth of aligned SWNTs along the lattice structure on quartz surface, thus significantly reduced the density of nanotubes during growth. More interestingly, it was shown that the random CNTs can be selectively removed through appropriate treatments using water vapor as an in situ etchant while the aligned SWNTs survive even after long time water vapor treatment. To obtain high-density semiconducting SWNT arrays, we designed an improved multiple-cycle growth method, which included the treatment of SWNTs with water vapor after each growth cycle without cooling down the system. Using this method, we have successfully obtained dense semiconducting SWNTs (~10 SWNTs/μm) over large areas and with high uniformity.