Steven C. Deane

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

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.

Current induced drift mechanism in amorphous SiNx:H thin film diodes

It is shown that the drift in the current–voltage characteristics of silicon‐rich amorphous silicon nitride metal–semiconductor–metal diodes can be explained by a mechanism whereby electron trapping centers are created via hole–electron recombination. A first order model which includes excitation of holes by hot electrons moving into the anode and recombination of electrons with holes trapped in the valence band tail is in good quantitative agreement with the measured dependencies between drift, device thickness, current density, time, and charge passed through the device.

Dangling-bond defect state creation in microcrystalline silicon thin-film transistors

We analyze the threshold voltage shift in microcrystalline Si thin-film transistors (TFTs), in terms of a recently developed thermalization energy concept for dangling-bond defect state creation in amorphous Si TFTs. The rate of the threshold voltage shift in microcrystalline Si TFTs is much lower than in amorphous Si TFTs, but the characteristic energy for the process, which we identify as the mean energy to break a Si–Si bond, is virtually the same. This suggests that the same basic Si–Si bond breaking process is responsible for the threshold voltage shift in both cases. The lower magnitude in microcrystalline Si TFTs is due to a much lower attempt frequency for the process. We interpret the attempt frequency in amorphous and microcrystalline silicon in terms of the localization length of the electron wave function and the effect of stabilizing H atoms being located only at grain boundaries.

Field‐effect conductance in amorphous silicon thin‐film transistors with a defect pool density of states

A new computer program to analyze field‐effect conductance measurements has been developed. In this program a defect pool model, where the equilibrium density of state is determined by the Fermi level, has been incorporated. Transistors with finite band bending, due to fixed charge in the insulator, will therefore have a density of states that is spatially inhomogeneous. The inhomogeneous density of states means that the subthreshold slope of a device is not always controlled by the density of states near the interface, but can become dominated by the bulk density of states, contrary to simpler models. Both electron and hole branches are modeled simultaneously and self‐consistently with no assumptions made about the flatband voltage. Indeed, it is demonstrated that there is no flatband voltage in a transistor with an inhomogeneous density of state; however, a true flatband voltage can be achieved by a process of thermal bias annealing. Finite thickness effects and defect correlation energies are taken int...

Kinetics of defect creation in amorphous silicon thin film transistors

We have developed a theoretical model to account for the kinetics of defect state creation in amorphous silicon thin film transistors, subjected to gate bias stress. The defect forming reaction is a transition with an exponential distribution of energy barriers. We show that a single-hop limit for these transitions can describe the defect creation kinetics well, provided the backward reaction and the charge states of the formed defects are properly taken into account. The model predicts a rate of defect creation given by (NBT)α(t/t0)(β−1), with the key result that α=3β. The time constant t0 is also found to depend on band-tail carrier density. Both results are in excellent agreement with experimental data. The t0 dependence means that comparing defect creation kinetics for different thin film transistors can only be done for the same value of band-tail carrier density. Normalization of bias stress data on different thin film transistors made at different band-tail densities is not possible.We have developed a theoretical model to account for the kinetics of defect state creation in amorphous silicon thin film transistors, subjected to gate bias stress. The defect forming reaction is a transition with an exponential distribution of energy barriers. We show that a single-hop limit for these transitions can describe the defect creation kinetics well, provided the backward reaction and the charge states of the formed defects are properly taken into account. The model predicts a rate of defect creation given by (NBT)α(t/t0)(β−1), with the key result that α=3β. The time constant t0 is also found to depend on band-tail carrier density. Both results are in excellent agreement with experimental data. The t0 dependence means that comparing defect creation kinetics for different thin film transistors can only be done for the same value of band-tail carrier density. Normalization of bias stress data on different thin film transistors made at different band-tail densities is not possible.

Stability of plasma deposited thin film transistors comparison of amorphous and microcrystalline silicon

Abstract We compared threshold voltage shifts in amorphous Si, microcrystalline Si and polycrystalline Si thin-film transistors (TFTs) in terms of a recently developed thermalization energy concept for a dangling-bond defect state creation in amorphous Si TFTs. The rate of the threshold voltage shift in microcrystalline Si TFTs was much lower than in amorphous Si TFTs, but the characteristic energy for the process, which we identified as the mean energy to break a Si–Si bond, was virtually the same. This suggests that the same basic Si–Si bond breaking process was responsible for the threshold voltage shift in both cases. The lower magnitude in microcrystalline Si TFTs was due to a much lower attempt frequency for the process. We interpreted the attempt frequency in amorphous and microcrystalline silicon in terms of the localization length of the electron wavefunction and the effect of stabilizing H atoms being located only at grain boundaries.

Below threshold conduction in a‐Si:H thin film transistors with and without a silicon nitride passivating layer

We report temperature measurements of inverted staggered amorphous silicon thin film transistor subthreshold conductance for devices with and without a top silicon nitride passivating layer. Subthreshold conductance activation energies clearly show the different conductance paths in the active layer of these devices. Transistors with no top nitride layer conduct in the bulk amorphous silicon, whereas the devices with a top nitride layer conduct at the interface between the amorphous silicon and the top nitride (a ‘‘back’’ channel). Gate bias stressing and light soaking experiments uphold the existence of the back channel. We also present two‐dimensional simulations that support our interpretation of the experimental data.

Effect of amorphous silicon material properties on the stability of thin film transistors: evidence for a local defect creation model

Abstract We present an improved description of the defect creation kinetics in amorphous silicon thin film transistors and analyze in detail the dependence on the key amorphous silicon material properties: Urbach energy, hydrogen content, hydrogen bonding and intrinsic stress. The results support a model for defect creation, involving Si–Si bond breaking, with only local hydrogen bonding rearrangements to stabilize the broken bond, as opposed to models involving long range hydrogen diffusion. On the other hand, defect annealing proceeds by breaking Si–H bonds and long-range diffusion of hydrogen.

Thermal bias annealing evidence for the defect pool in amorphous silicon thin‐film transistors

Thin‐film transistors were thermally annealed while a bias voltage was applied to the gate electrode. The transfer characteristics were then measured, and the density of states distributions derived by field‐effect analysis. The results indicate that the equilibrium distribution and number of defects in the transistor channel region depend on the position of the Fermi energy during annealing. Thus the density of states can be increased or decreased in parts of the band gap. A high Fermi energy during annealing results in few states high in the gap and more states low in the gap. The reverse is true for annealing while the Fermi energy is low. This is consistent with the defect pool model for silicon dangling bond states and suggests that most deep states are part of the defect pool.

Urbach energy dependence of the stability in amorphous silicon thin-film transistors

We investigate the relationship between the stability of amorphous silicon thin-film transistors (a-Si:H TFTs) and the bulk properties of a-Si:H films. Threshold voltage shifts in a-Si:H TFTs are characterized by the thermalization energy Eth for different times and temperatures and fitted by {1+exp[(Eth−Ea)/kT0]}−2. We find that kT0 exhibits a clear correlation to the Urbach energy, but the more significant parameter Ea seems to depend only on the deposition-induced microstructure and not on the Urbach energy, the hydrogen content, or the hydrogen diffusion coefficient.