Warren B. Jackson

Hydrogen diffusion in amorphous silicon

Abstract Hydrogen diffusion in doped and compensated a-Si:H has been measured by secondary-ion mass spectrometry profiling in the temperature range 155-300°C. Doping reduces the activation energy and enhances the diffusion coefficient by up to three orders of magnitude at 200°C, and a correlation between the diffusion coefficient and the dangling-bond density is found. An analysis of three different diffusion models indicates that the breaking of weak Si – Si bonds by hydrogen may be an important process. The relation between the diffusion results and the thermal equilibration of the electronic structure is discussed. The hydrogen diffusion coefficient in a-Si:H has been measured over the temperature range 155-300°C, with particular emphasis on the effects of doping and compensation. In all cases D H is thermally activated with an energy 1[sdot]2-1[sdot]5 eV. The diffusion coefficient decreases slowly with time which is attributed to the disorder-induced variation in site energies. We find that D H is gre...

Passivation kinetics of two types of defects in polysilicon TFT by plasma hydrogenation

The effects and kinetics of hydrogen passivation on polycrystalline-silicon thin-film transistors (poly-TFTs) are investigated. Based on the response of device parameters with the progress of hydrogenation, two types of defects can be distinguished from the difference in passivation rate. The threshold voltage and subthreshold slope, which are strongly influenced by the density of dangling bond midgap states, have a faster response to hydrogenation. The off-state leakage current and field-effect mobility, related to stain-bond tail states, respond more slowly to hydrogenation, with an onset period of approximately 4 to 12 h depending on the grain size. Since the larger-grain-size samples showed a longer onset period, the contribution of intragranular defects to the strain-bond tail states appears to be significant.<<ETX>>

Density of gap states of silicon grain boundaries determined by optical absorption

The results of optical absorption measurements on fine‐grain polycrystalline‐silicon thin films indicate that the singly occupied dangling silicon bond lies 0.65±0.15 eV below the conduction‐band minimum in the grain boundary. The grain boundary band gap is ∼1.0 eV and there is evidence for exponential tailing of the band edges. The optical absorption was determined by photothermal deflection spectroscopy. The dangling silicon bond density has been measured on polycrystalline‐silicon thin films as a function of hydrogen passivation of the grain boundaries and on silicon‐on‐saphhire films. The optical absorption exhibits a defect shoulder which varies as the dangling bond density.

Mechanism of device degradation in n- and p-channel polysilicon TFTs by electrical stressing

The effects of electrical stress on hydrogenated n- and p-channel polysilicon thin-film transistors are discussed. The on-state caused the most significant degradation, whereas off-state and accumulation conditions resulted in negligible degradation. The on-state stress degraded the threshold voltage, trap state density, and subthreshold sharpness of both n- and p-channel devices toward perhydrogenated values, and the rates of degradation increased with stressing biases. The field-effect mobility and leakage current, however, were not degraded by stressing. The mechanism of device degradation may be attributed to the metastable creation of midgap states within the polysilicon channel, as opposed to gate dielectric charge trapping or interface state generation.<<ETX>>

Optical absorption spectra of surface or interface states in hydrogenated amorphous silicon

The optical absorption of doped and undoped hydrogenated amorphous silicon (a‐Si:H) films ranging from 5 nm to 10 μm was measured using photothermal deflection spectroscopy. The absorption spectra show that there is a high defect layer associated with the surface or interface of the film. From comparison of defect absorption and dangling bond spin densities, it is found that a‐Si:H films which have ∼1015 bulk defects/cm3 exhibit surface or interface layers with ∼1012 dangling bonds/cm2.

Kinetics of the Staebler–Wronski effect in hydrogenated amorphous silicon

We have investigated the time and intensity dependence of the creation process for light‐induced metastable defects (Staebler–Wronski effect) in hydrogenated amorphous silicon (a‐Si:H). The observed changes in electron spin resonance spin density (dangling bonds) and photoconductivity are consistent with a model which explains the Staebler–Wronski effect as a self‐limiting process intrinsic to a‐Si:H. A possible microscopic mechanism based on the nonradiative recombination of band tail carriers is discussed.

Hydrogen passivation of grain boundary defects in polycrystalline silicon thin films

The dependence of defect passivation in undoped polycrystalline silicon on hydrogenation conditions (i.e., time and temperature) was examined. At long hydrogenation times the spin density NS saturates. The saturation value of NS depends strongly on the hydrogenation temperature. The lowest residual spin density was obtained at 350 °C. Model calculations of the time and temperature dependence of the defect passivation suggest that the amount of hydrogen necessary for defect passivation exceeds the density of grain boundary defects by a factor that is significantly larger than unity and which depends on the hydrogenation temperature.

The correlation energy of the dangling silicon bond in a Si:H

Abstract Photothermal deflection spectroscopy measurements of the optical absorption for undoped and phosphorus doped a  Si:H films are presented. Comparision of the energy shift of the dangling bond absorption shoulder shows that the correlation energy is between 0.25 and 0.45 eV. The singly and doubly occupied dangling silicon bonds are approximately ∼1.25 eV and ∼0.9 eV from the conduction band, respectively.

Comment on ‘‘Reduction of hot electron degradation in metal oxide semiconductor transistors by deuterium processing’’ [Appl. Phys. Lett. 68, 2526 (1996)]

Lyding et al. have recently reported significant improvements in the lifetime of metal oxide semiconductor ~MOS! transistors due to incorporation of deuterium ~D!, rather than hydrogen ~H!, at the Si/SiO2 interface. This remarkable achievement indicates that the Si–D bond is more resistant to hot-electron excitation than the Si–H bond. Lyding et al. pointed out that the phenomenon is probably analogous to the observed reduction in desorption of deuterium versus hydrogen from hydrogenated Si~100!:H surfaces using the scanning tunneling microscope ~STM!. In this comment, we propose a specific pathway for the dissociation of Si–H and Si–D bonds, providing a natural explanation for the difference in dissociation rates. Shen et al. have proposed that the low-voltage STMinduced desorption of Si–H bonds from Si~100! proceeds via a multiple-vibrational excitation by tunneling electrons. Electrons excite Si–H vibrational transitions with a rate proportional to the tunneling current. The extent to which vibrational energy can be stored in the bond depends on the lifetime, i.e., on the rate at which energy is lost by coupling to phonons. Because the lifetime of H on Si is long, efficient vibrational excitation is expected. In the quantitative analysis of Ref. 3, it was assumed that the vibrational energy is deposited in the stretch mode of the Si–H bond, which has a frequency around 2100 cm. The same assumption is usually implicitly made in discussions of dissociation of Si–H bonds. Our main purpose here is to point out that both the vibrational lifetime and carrier-enhanced dissociation mechanisms are most likely controlled by the Si–H bending modes. The vibrational frequency of the bending mode for Si–H is around 650 cm, and the estimated frequency for Si–D is around 460 cm. This value is close to the frequency of bulk TO phonon states at the X point ~463 cm21). We therefore expect the coupling of the Si–D bending mode to the Si bulk phonons to result in an efficient channel for deexcitation. While it is quite possible to reach a highly excited vibrational state in the case of Si–H, this will be more difficult for Si–D. Deuterium should therefore be much more resistant to STM-induced desorption and hot-electron induced dissociation, due to the relaxation of energy through the bending mode.

Electronic structure of silicon nitride and amorphous silicon/silicon nitride band offsets by electron spectroscopy

The film thickness, chemical state, and polarization screening for a‐SiN1.4 :H films deposited by glow discharge over hydrogenated amorphous silicon (a‐Si:H) were determined by x‐ray photoelectron spectroscopy (XPS) and Auger spectroscopy. The nitride films were observed to be single phase and the escape depth for 1400‐eV electrons in the a‐SiN1.4 :H film was determined to be 30 A. The band offsets for the a‐Si:H/a‐SiN1.4 :H interface were determined by XPS and Bremsstrahlung isochromat spectroscopy (BIS) to be 1.2 eV for the valence band and 2.2 eV for the conduction band, while the band gap for a‐SiN1.4 :H was found to be 5.3 eV in accordance with the optical gap. By combining optical absorption measurements with the valence‐band density of states and conduction‐band density of states determined by electron spectroscopy, a semiquantitative estimate of the band tailing within the nitride gap was obtained. Correlation of the defect absorption with the electron spin resonance measurements suggest that the ...