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ABSTRACT Despite its importance for technological applications, the behavior of hydrogen in amorphous silicon is not fully understood. In particular, the anomalously low activation energy (1.5 eV) for hydrogen diffusion has remained unexplained. We investigate the interaction of hydrogen with dangling bonds using first-principles pseudopotential-density-functional calculations. Our analysis shows that the diffusion activation energy should be measured from the hydrogen chemical potential, and that this level should be identified with the formation energy of Si-H bonds. A quantitative identification of the energy levels with experimental observables is then possible.

INTRODUCTION Hydrogen passivation of dangling-bond defects leads to a significant improvement in the electronic properties of amorphous Si (a-Si). Hydrogen has also been suggested to play a role in the formation of defects in the material.' Despite the importance of these processes, the behavior of hydrogen in a-Si is still not fully understood. In particular, the anomalously low activation energy (1.5 eV) for hydrogen diffusion has remained unexplained. The energy required to break an Si-H bond in an SiH 4 molecule is between 3.5 and 4 eV; even taking into account that the energy of a mobile H atom inside the a-Si network is lower than in free space, an activation energy of 1.5 eV seems insufficient to excite H atoms out of Si-H bonds. We address these and other aspects of the interactions between hydrogen and amorphous silicon in a theoretical framework based on state-of-the-art first-principles calculations. The computational approach is founded on density-functional theory in the local-density approximation 2 and first-principles pseudopotentials. 3 We use a supercell geometry and plane-wave basis set, and zero-point energies are added to our computed energies. The calculations are carried out for a crystalline system; the relevant bonding structures depend mostly on the local environment, and are insensitive to the large-scale structure of the network. Details about the computations can be found in Refs. [4] and [5].

CONFIGURATIONS OF HYDROGEN IN SILICON The configurations that hydrogen assumes in its interactions with bulk crystalline (c-Si) or amorphous silicon include isolated hydrogen in an interstitial position, and hydrogen at an Si-H bond (effectively tying off a dangling bond). Our first-principles technique allows us to derive the atomic and electronic structure, as well as the energy of such configurations. For hydrogen in the neutral charge state in c-Si, the stable site for an isolated interstitial is at the bond-center (BC) site. The formation energy of a configuration is defined as:

Efor

= Et=t - n5siEs - EH,

(1)

389 Mat. Res. Soc. Symp. Proc. Vol. 377 ©1995 Materials Research Society Downloaded from http:/www.cambridge.org/core. University of Warwick, on 11 Dec 2016 at 11:46:04, subject to the Cambridge Core terms of use, available at http:/www.cambridge.org/core/terms. http://dx.doi.org/10.1557/PROC-377-389

Energy per H atom