We investigate gas-phase chemistry, and the properties of SiO2 films and the Si/SiO2 interface formed by plasma chemical vapor deposition with tetraethylorthosilicate (TEOS)/O-2 and various dilution gases. The N-2 dilution gas produced a faster deposition rate than either He or Ar dilution gases. Excited nitrogen gas reacts with TEOS to form NH radicals, which enhance the dissociation of TEOS. Compared with He, the N-2 dilution gas suppressed the formation of OH radicals in the gas phase and reduced the concentration of SiH and SiOH in the SiO2 film. As a result, characteristics of the Si/SiO2 interface were improved. At the same deposition condition, the interface roughness was almost identical at approximately two to three atomic layers regardless of dilution gas. However, the increase in plasma power enhanced the interface roughness to three to five atomic layers, which possibly increased the interface traps density. The P-b center decreased abruptly down to 1.1 x 10(11)/eV cm(2), and the interface characteristic improved when using the N-2 dilution gas.