General comparison of the surface processes involved in nitridation of Si(100)-2 x 1 by NH3 and in SiNx film deposition: a photoemission study.

Abstract

With use of x-ray photoelectron spectroscopy (XPS), the adsorption of ${\mathrm{NH}}_{3}$, both on c-Si(100) and during silicon nitride film growth, is comparatively examined over a wide range of substrate temperature ${T}_{S}$ between room temperature and 800 \ifmmode^\circ\else\textdegree\fi{}C, gas pressure P(${\mathrm{NH}}_{3}$), and exposure E. For sake of comparison with the surface parameters, dynamical exposure ${E}_{D}$ or exposure times ${t}_{D}$ are defined for the growth process. Both in surface adsorption and during film growth strongly correlated behaviors can be observed. They concern the following: (i) the general ${T}_{S}$-dependent evolution of the chemisorbed species, i.e., ${\mathrm{NH}}_{\mathrm{x}}$, H, and finally N, and (ii) the particular exposure and ${T}_{S}$ behaviors of the latter nitridation regime, occurring in the 350--800 \ifmmode^\circ\else\textdegree\fi{}C range.At high ${T}_{S}$ (600--800 \ifmmode^\circ\else\textdegree\fi{}C) and low ${\mathrm{NH}}_{3}$ pressure, growth of ${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$ islands on the c-Si(100) surface is related to heterogeneous ${\mathrm{SiN}}_{\mathrm{x}}$-film formation presenting strong phase separation (Si and ${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$ cluster mixture). Very low nitrogen uptake, governed by N thermal desorption increasing with ${T}_{S}$, is found in this domain. For higher ${\mathrm{NH}}_{3}$ pressures or exposures (${E}_{D}$ or Eg200 L, where L denotes langmuirs; 1 L${=10}^{\mathrm{\ensuremath{-}}6}$ Torr s) complete nitridation in thick ${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$ films when Si is deposited or in a thin homogeneous ${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$ overlayer on the c-Si surface, is achieved. At lower ${T}_{S}$ (350--600 \ifmmode^\circ\else\textdegree\fi{}C) the nitrogen is chemisorbed at the surface in a more homogeneous way, the deficient bulk diffusion limiting interaction with the top layer and leading to a more rapid saturation of the nitrogen uptake with exposure. In this regime intermediate silicon subnitride environments Si-${\mathrm{N}}_{3}$Si are essentially found, whose counterpart for film deposition is nitride formation with a microstructure nearer to the random bonding model. Despite a ${T}_{S}$ shift, due to the room-temperature dissociative chemisorption of ${\mathrm{O}}_{2}$ on Si (while for atomic nitrogen it starts only above 350 \ifmmode^\circ\else\textdegree\fi{}C), we note a qualitative similarity between the main features involved in the initial, fast silicon nitridation stage and those occurring during oxidation.