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PRECURSORS

The energy states and local geometries of Si-atom dangling bonds have been understood for a number of years [11]. Si-atom dangling bonds are stable in all three charge states: i) negatively charged, ii) positively charged and iii) neutral, and in addition are a positive U-system. In devicequality a-Si:H, the hydrogen content is typically 10 at. % or less, and the bonding of H-atoms is 715

Mat. Res. Soc. Symp. Proc. Vol. 507 © 1998 Materials Research Society

primarily in Si-H arrangements. The focus here is on Si-H groups and their interactions with nearneighbor Si atoms. Figure 1 includes the bonding arrangements of H including the three center bonds addressed in this paper. The calculations of this paper, as well as those of Van de Walle and coworkers [12], indicate that the symmetric neutral three center - three electron (3c-3e) bond between one H-atom and two Si-atoms is destabilized by having one electron in an anti-bonding orbital. On the other hand asymmetric neutral bonding interactions between the H-atom of an Si-H group and nearneighbor 0- or N-atoms and/or NH or OH groups represent conventional H-bonding configurations and are stable with H-bond energies in the fractions of eV's [13]. Finally, the positively charged 3c2e bond is stable. Negative 3c-4e bonds are even more unstable than neutral 3c-3e bonds since two electrons occupy anti-bonding states. ÷.

Eb = 6.81 eV

S -H

Si-H

Eb 6.90 eV

partially covalent bonds (ýýISiO-

ý

[o2.35 Ai-i-T

Si-O-Si

1. A

Eb = 5.90 eV

0 H 0S1 Fig. 2. Schematic representation of bonding geometries of the (Si-H-Si)+ defect center.

H-Bond -- OH----O

7

Two Electron - Three Center Bond (Si-H-Si)+ 00

®Si OH

Fig. 1. Schematic representation of bonding arrangements of H a-Si:H and a-Si:H alloys. Fig. 3. Bond energy of (Si-H-Si)+ versus Si-Si distance.

6.8 S

6.6

w 6.4 C ,LI

E 6.2 0 6 5.8 2.2

2.4

2.6 2.8 Si-Si Distance

3

3.2

3.4

(A)

Figure 2 includes three different bonding environments of (Si-H-Si)÷, the 3c-2e symmetric bonding state of H-atoms in Si. The calculations of this paper, as well as those of Jones [14] and Van de Walle and coworkers [12] agree that the linear bonding arrangement is stable. In addition, the ab initio calculations of this paper indicate that this bonding group is stable in both linear and bent arrangements with the H-atom being equidistant from its two Si-atom neighbors. The Si-Si distance for stable bonding varies from 2.35 A, the normal Si-Si-atom spacing in crystalline and amorphous Si up to almost 3 A. Figure 3 gives the (Si-H-Si)+ bond energy as a function of the SiSi spacing. As indicated in Fig. 3 the most stable arrangement occurs at the transition between the bent and linear bonding arrangements. In addition, our calculations, as well as those of Van de Walle et al. have shown that the neutral (Si-H-Si)o group is significantly less stable the positivelycharged center. The ab initio calculations indicate if neutral (Si-H-Si)o groups are formed 716

during a reaction such as the one in Eqn. 2(a), they would deca