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We present a treatment of the defect-pool model, for the calculation of the density of electronic gap states in hydrogenated amorphous silicon, based on the equilibration of elemental chemical reactions involving the separate release and capture of hydrogen. We derive the corresponding hydrogen density of states, describing the distribution of hydrogen binding energies, and show that the two densities of states are completely consistent. Hydrogen can be captured into weak SiSi bonds, which can be occupied by one or two hydrogen atoms. These are the dominant chemical reactions controlling the defect density. The effective hydrogen correlation energy is variable, being negative for most sites but positive where most defects occur. We show that the electronic density of states reproduces the main features of our earlier defect-pool model, with more charged defects than neutral defects for intrinsic amorphous silicon. The electronic density of states and the corresponding hydrogen density of states are consistent with a wide range of experimental results, including hydrogenation-dehydrogenation and hydrogen diffusion.

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We investigate the mechanism for Si dangling bond defect creation in amorphous silicon thin film transistors as a result of bias stress. We show that the rate of defect creation does not depend on the total hydrogen content or the type of hydrogen bonding in the amorphous silicon. However, the rate of defect creation does show a clear correlation with the Urbach energy and the intrinsic stress in the film. These important results support a localized model for defect creation, i.e., where a Si–Si bond breaks and a nearby H atom switches to stabilize the broken bond, as opposed to models involving the long-range diffusion of hydrogen. Our experimental results demonstrate the importance of optimizing the intrinsic stress in the films to obtain maximum stability and mobility. An important implication is that a deposition process where intrinsic stress can be independently controlled, such as an ion-energy controlled deposition should be beneficial, particularly for deposition temperatures below 300 °C.