Hydrogen-mediated creation and annihilation of strain in amorphous silicon

The influence of an increasing hydrogen concentration on the properties of hydrogenated amorphous silicon (a-Si:H) was investigated. An increase of the Si-H bond concentration by as much as 3×${10}^{21}$ ${\mathrm{cm}}^{\mathrm{-}3}$ changes neither the defect density, the weak-bond density, nor the metastability. These results suggest that hydrogen is accommodated in pairs pinning the hydrogen chemical potential, which is indicative of a negative correlation energy. Data on annealing of a-Si:H at high temperatures show that the exponential band tails do not broaden as a function of the temperature. These experiments suggest that the random-network strain energy in device-quality a-Si:H is in metastable equilibrium. Based on our experimental results, we propose that internal strain propagates within the network and can be generated or reduced by annealing and/or the incorporation of hydrogen. According to maximum entropy the slope of the exponential band tails represents the average strain energy per lattice bond.