Thermally stimulated H emission and diffusion in hydrogenated amorphous silicon

We report first principles ab initio density functional calculations of hydrogen dynamics in hydrogenated amorphous silicon. Thermal motion of the host Si atoms drives H diffusion, as we demonstrate by direct simulation and explain with simple models. Si-Si bond centers and Si ring centers are local energy minima as expected. We also describe a new mechanism for breaking Si-H bonds to release free atomic H into the network: fluctuating bond center detachment (FBCD)-assisted diffusion, in which a bond center H may be liberated by intercession of a third nearby Si. H dynamics in a-Si:H is dominated by structural fluctuations intrinsic to the amorphous phase not present in the crystal. Copyright c © EPLA, 2007 Introduction. – Hydrogenated amorphous silicon is one of the most important electronic materials, and is used in applications ranging from TFTs in laptop displays to solar photovoltaics and IR imaging/detection. Hydrogen dynamics is key to creation and annihilation of defects and gap states, and is also linked to lightinduced degradation [1–4]. Although there have been several studies, a complete picture of H dynamics is not yet available. A widely held picture posits that unbonded H hops among various attractive sites before capture at a dangling bond [5–10]. An intermediate low-energy pathway involving a metastable dihydride structure has also been reported [11]. However, by calculating hopping rates for different trap sites, Fedders argued that thermal motion of hydrogen does not proceed from dangling bond to dangling bond via bond center (BC) sites and showed diffusion through intermediate levels to be insignificant [12]. In addition, Su et al. [13] proposed that Si-H bonds do not spontaneously release H, but rather require the mediation of an external agency: in their case a five-fold or “floating” bond. In crystalline Si, the importance of lattice dynamic activated diffusion has been reported [14,15]. Buda et al. [16] has shown diffusion of H in the form of jumps from BC site to another BC via intermediate hexagonal or tetrahedral sites. Previous work on a-Si and a-Si:H showed that the network dynamics is in some ways quite different from the crystal. In particular, the disorder of the network allows fluctuations in the positions of atoms leading to the interesting observation of “coordination fluctuation”. It has been observed in an early first principles simulation that even at T = 300K, the number of floating (fivefold) bonds fluctuated between zero and ten in a 216 atom cell, in a 1.8 ps simulation [17]. This work has been updated, and similar effects have been observed in networks including H. It was also found that most of the atoms in the lattice eventually participated in these fluctuations [18]. In this letter, we report an ab initio simulation which reveals the role of thermal motion of Si atoms in driving H diffusion. We have undertaken accurate simulations including static lattice simulation (in which Si atoms were frozen) and extended thermal simulation. The static Si lattice simulations shows no H diffusion as compared with the dynamic lattice case, suggesting that the motion of the “Si-sublattice” is important to the H dynamics. A feature of our work is that we determine diffusion mechanisms directly from thermal MD simulation, not by imposing a conventional hopping picture among wells (traps) with varying depths. The principal conclusion of the paper is that the dynamic lattice (particularly the motion of pairs or triplets of Si atoms with a BC H present) is a primary means for ejecting atomic H into the network. This mechanism could not be easily inferred from phenomenological kinetic equation models of H transport [12], though it should readily emerge using