Hard amorphous (diamond-like) carbons

The behaviour of defects in amorphous silicon nitride (a-Si3N4) and the hydrogenated amorphous silicon nitride (a-SiNx: H) alloys are described. The main defects in a-Si3N4 are the Si and N dangling bonds (DBs). The Si DB forms a sp3 state near midgap, while the N DB forms a highly localized pπ level just above the valence-band edge. The behaviour of the alloys changes near x ≍ 1.1, the percolation threshold of Si-Si bonds. In the Si-rich alloys (x 1.1 alloys behave largely like silicon nitride; the valence band changes towards N pπ like, both Si and N DBs can arise and the Si DB can have a high density and prefers to be in its charged diamagnetic configurations. The density of defects in a-SiN x : H, via the equilibration with weak Si-Si bonds, depends on the diffusion coefficient of hydrogen. A theory of H diffusion is developed which shows that the optimum depos...

Defects and hydrogen in amorphous silicon nitride

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.

Relative importance of the Si–Si bond and Si–H bond for the stability of amorphous silicon thin film transistors

▪ Abstract Hydrogenated amorphous silicon (a-Si:H) exhibits a metastable light-induced degradation of its optoelectronic properties that is called the Staebler-Wronski effect, after its discoverers. This degradation effect is associated with the relatively high diffusion coefficient of hydrogen and the changes in local bonding coordination promoted by hydrogen. Reviewed are the fundamental aspects of the interplay between hydrogen and electronic energy states that form the basis of competing microscopic models for explaining the degradation effect. These models are tested against the latest experimental observations, and material and preparation parameters that reduce the Staebler-Wronski effect are discussed.

Development in Understanding and Controlling the Staebler-Wronski Effect in a-Si:H

A physical model is presented which quantitatively describes the threshold voltage instability, commonly known as drift, in n-channel Si/sub 3/N/sub 4/-gate pH ISFET's. The origin of the so-called drift is postulated to be associated with the relatively slow conversion of the silicon nitride surface to a hydrated SiO/sub 2/ or oxynitride layer. The rate of hydration is modeled by a hopping and/or trap-limited transport mechanism known as dispersive transport. Hydration leads to a decrease in the overall insulator capacitance with time, which gives rise to a monotonic temporal increase in the threshold voltage.

A physical model for threshold voltage instability in Si/sub 3/N/sub 4/-gate H/sup +/-sensitive FET's (pH ISFET's)

Experimental and theoretical studies of the thermal equilibrium defect density in undoped a-Si:H are reported. The defect density measured by electron-spin resonance increases with temperature with an activation energy of 0.15\char21{}0.2 eV. The equilibration time is activated with an energy of about 1.5 eV, and the shape of the decay follows a stretched exponential, as in doped a-Si:H. The experiments confirm that defect equilibration occurs over a range of temperatures and sample deposition conditions. The relaxation time depends on the growth conditions, and the thermal defects are shown to anneal more slowly than optically induced defects. The temperature dependence of the thermodynamic equilibrium defect density is calculated, based on the weak-bond\char21{}dangling-bond conversion model. Four specific defect reactions are analyzed, two of which involve the motion of bonded hydrogen. The defect density is sensitive to the details of the model because of entropy effects. The experimental data agree well with the analysis, but do not conclusively distinguish between the different possible defect reactions because of uncertainties in the parameters of the model. The different annealing rates of thermal and optical defects are accounted for by relating the distributions of hydrogen-bonding energies, the defect-formation energies, and the valence-band-tail states. It is proposed that the time dependence of the relaxation is related to the shape of the valence-band-tail distribution.

K. Winer

Papers

Defect equilibria in undoped a -Si:H