Atomic-order thermal nitridation of silicon at low temperatures

Atomic-order nitridation of Si(100) in an NH, environment (124-1400 Pa) at 300-650°C has been investigated using an ultraclean low pressure hot-wall reactor system. At 500°C or higher, the N atom concentration (n_X) initially increases and tends to saturate to a certain value (d ∼ 5 Å, n_X ∼ 3 × 10¹⁵ cm^-2). At 400°C or lower, on the H-terminated Si surface, the Si-hydride decreases with increasing NH₃ exposure time and becomes hardly observed when n_X reaches nearly to the surface Si atom concentration (6.8 × 10¹⁴ cm^-2). On the H-free Si surface, n_X increases up to ∼2 × 10¹⁴ cm^-2 with the appearance of the Si-hydride instantly after NH₃ exposure. It is expected that NH₃ dissociatively adsorbs on the Si dangling bonds. It is found that n_X is well described by Langmuir-type physical adsorption and reaction of NH₃ on the Si surface. The ultrathin nitride film shows very good characteristics as a mask against oxidation.