Theory of Hydrogen Complexes in Si
- PDF / 411,118 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 98 Downloads / 206 Views
THEORY OF HYDROGEN COMPLEXES IN Si S. B. ZHANG AND W. B. JACKSON Xerox Palo Alto Research Center, Palo Alto, CA 94304
ABSTRACT The predominance of Si-H bonding and the origin of {111} platelets in hydrogenated Si remain important unsolved problems in the study of H in Si. Recent theoretical and experimental results indicate that H predominately enters the Si network in pairs. A promising diatomic H configuration consists of a bond centered H closely associated with an antibonding centered H. In this work, we show that adjacent diatomic H pairs have a binding energy of 0.2 eV/2H. The binding originates from relaxation of strained Si-Si backbonds. Further clustering of the H pairs eliminates all strained bonds, forming a hydrogenated platelet oriented along the {111} plane.The binding energy of 3.95 eV/2H for the platelet is 0.15 eV lower than that for interstitial H 2 molecules in c-Si. Lattice expansion makes the platelets energetically more competitive with the lowest energy Si-H bonding confi gration at hydrogenated Si (111) surfaces. These higher level complexes explain the formation of platelets, Raman spectra, and absence of gap states in hydrogenated cSi as well as the clustered phase seen in NMR and of H evolution and diffusion in hydrogenated amorphous Si.
Introduction Hydrogen has been extensively used in Si based technology such as device processing, defect passivation and removal of unwanted Si dangling bonds. Despite the chemical simplicity of H, its bonding configurations in Si can be complex. Recent theories predict that an interstial H 2 molecule at the tetrahedral interstial site (Td) is the lowest energy configuration [1,21. In contrary, a number of measurements on H in amorphous (a-) and crystalline (c-) Si found that Si-H bonding is more prevalent than molecules particulary at high H concentration. These observations include (1) the development of {111} platelets in c-Si exposed to atomic H [31, (2) the presence of only around 1% H2 molecules in hydrogenated amorphous silicon (aSi:H) contianing over 10-30 at. % H as determined by H nuclear magnetic resonance (NMR) [4], (3) small angle neutron scattering indicating large H inhomogenieties [5,6], and (4) the presence of a clustered H phase in NMR of a-Si:H [7]. The Si platelets are visible in electron microscopy, have Si-H vibrations at 2140 cm- 1 that disappear upon H evolution [8,9]. With diameters up to 100nm, they do not form along preexisiting planar or line defects, and are consistent with expansion of the lattice by 20-30% [3].
Methods We used a self-consistent pseudopotential approach within the local density approximation (LDA) [10] with Wigner interpolation formula for the correlation energy [11]. The wave functions were expanded in a plane wave basis [12] with kinetic energies up to 22 Ry for the energy difference between H2 and H 2 * and 10 Ry otherwise. A sixteen and a six Si-atom supercell were employed for small H-clusters and for extended H-platelets, respectively, with five and ten k-points included in the Mat. Res. Soc. Symp. P
Data Loading...
