AMMONIA DECOMPOSITION ON SILICON SURFACES STUDIED USING TRANSMISSION FOURIER-TRANSFORM INFRARED-SPECTROSCOPY

Fourier transform infrared (FTIR) transmission spectroscopy was used to monitor the decomposition of NH3 and ND3 on silicon surfaces. Experiments were performed in situ in an ultrahigh vacuum (UHV) chamber using high surface area porous silicon samples. The FTIR spectra revealed that NH3 (ND3) dissociatively adsorbs at 300 K to form SiH (SiD) and SiNH2 (SiND2) surface species. A comparison of the vibrational absorbances for the SiNH2 and Si2NH surface species indicated that the Si2NH species could account for less-than-or-equal-to 8% of the surface coverage at 300 K. The infrared absorbances of the SiN-H-2 (SiN-D2) scissors mode at 1534 cm-1 (1157 cm-1), the Si-H (Si-D) stretch at 2077 cm-1 (1510 cm-1) and the Si3-N vibrational modes at 930 and 750 cm-1 were employed to monitor the decomposition of the SiNH2 (SiND2) surface species. As the silicon surface was annealed to 700 K, the FTIR spectra revealed that the SiNH2 (SiND2) surface species gradually decomposed to produce Si3N species and additional SiH species. Above 680 K, the SiH surface species decreased concurrently with the desorption of H-2 from the porous silicon surface. The uptake of surface species at various adsorption temperatures was also monitored as a function of NH3 (ND3) exposure time. These studies revealed that similar results were obtained after NH3 (ND3) was adsorbed to saturation coverage at a particular adsorption temperature and after the surface was annealed to the same temperature following a saturation NH3 (ND3) exposure at 300 K.