Showing papers by "Warren B. Jackson published in 1993"

Hydrogen passivation of grain boundary defects in polycrystalline silicon thin films

Abstract: 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.

Light-induced creation of metastable paramagnetic defects in hydrogenated polycrystalline silicon.

Abstract: Light-induced defect creation is demonstrated in hydrogenated polycrystalline silicon (poly-Si:H). The newly created defects are metastable as in hydrogenated amorphous silicon (a-Si:H). However, unlike a-Si:H the magnitude of the light-induced degradation decreases with repeated illumination and anneal cycles and is restored upon reexposure to monatomic hydrogen. This unique response arises from the inherent structural inhomogeneity of the polycrystalline material and establishes that hydrogen directly contributes to the metastability in poly-Si:H.

Nonequilibrium occupancy of tail states and defects in a -Si:H: Implications for defect structure

Abstract: A detailed investigation of the electron and hole occupancy of tail states in undoped amorphous silicon (a-Si:H) as well as changes in the dangling-bond occupancy as a function of excitation intensity was carried out using light-induced electron-spin-resonance (LESR) measurements. For very thick films the band-tail electron and hole densities are not proportional. Over a wide range of excitation conditions the excess hole density is constant, suggesting the presence of charged defects with a density that is 5\char21{}10 times larger than the neutral defect density in annealed or as-grown a-Si:H. Light soaking increases mainly the neutral defect density. The dependence of the excess hole density on film thickness and absorption profiles indicates that this effect is a bulk property, which may be masked in thinner films by the comparatively high interface defect density. Model calculations of nonequilibrium occupation statistics confirm the experimental results. For a defect distribution that includes charged defects, the calculations suggest a very small positive LESR signature of the dangling bond, in spite of the high density of charged defects in the material, as a necessary consequence of the asymmetries observed between electron and hole capture rates and tail-state distributions. The calculations demonstrate that the lack of this signature does not imply a defect structure that contains predominantly neutral defects.

Deposited silicon nitride with low electron trapping rates

Abstract: In this article, nitrogen‐rich silicon nitride plasma deposited under conditions minimizing Si—H bonding is shown to possess extremely low bulk electron trapping rates which are as low or lower than plasma‐deposited oxides produced using He dilution. The bulk trap density, measured by avalanche injection decreases as the rf power is decreased. The total charge trapped within these silicon nitrides reaches a saturation value determined by high field detrapping in thick nitride films.

Role of clustering in hydrogen transport in silicon.

Abstract: The effects of H concentration and thermal history on H trapping in hydrogenated amorphous silicon have been investigated through analysis of deuterium diffusion profiles. Unexpected results indicate that the number of traps increases with H concentration while the trap depth increases upon annealing. H equilibrated within the film is associated with traps while rapidly introduced H is not. The results are consistent with a model of trapping and release from H clusters which nucleate and grow in response to added H. It is proposed that the effective diffusion coefficients for high concentrations of H in all forms of Si, amorphous, polycrystalline, and crystalline, are determined by these clusters

Effect of hydrogen exchange on diffusion and evolution in hydrogenated amorphous silicon

Abstract: Evidence is presented showing that the H emission rate during exposure to an external atomic D source has the same activation energy (06–08 eV) as diffusion under the same circumstances, a result similar to the layer diffusion case This result as well as H exchange with the sample during deuteration and the results indicating a narrow H emission distribution can be explained in terms of an exchange mechanism whereby a mobile D exchanges places with a bound H with no significant barrier We also show through analysis of H evolution curves that the H chemical potential is pinned as H is lost from the sample, a result consistent with the negative U model for H bonding and H clustering

Thermodynamic equilibrium kinetics of phosphorus and boron doped a-Si:H

Abstract: Thermodynamic equilibrium kinetics of phosphorus (P) and boron (B) doped hydrogenated amorphous silicon (a-Si:H) are explored using dark conductivity measurements. Small variations in temperature of the semiconductor cause variations in active dopant concentration, which affects the conductivity. The equilibration kinetics of phosphorus- and boron-doped a-Si:H are different and there is a strong dependence on the deposition temperature (T s ). The transformation of P into the electronically active phase (activation) follows a stretched exponential time dependence with a temperature-independent dispersion parameter, β, of 0.85, whereas the transformation into the electronically passive bonding configuration (passivation) is also stretched exponential but with β ⩽ 0.8. The kinetics of boron metastability are similarly a stretched exponential, but with equal β values for activation and passivation. The time constant, τ, to achieve equilibrium for both transformations is thermally activated with energies E act ≈/ 1.1 eV for P and B. An increase in T 8 leads to an increase of E act .

The effect of post-hydrogenation on the equilibrium and metastable properties of hydrogenated amorphous silicon

Abstract: Hydrogenated amorphous silicon films were exposed to monatomic deuterium at 350°C up to 8 hours. The defect densities in the annealed state, after illumination and after deuteration were determined using CPM measurements following each exposure sequence. We find that an increase of the concentration of Si-H bonds by as much as 3 × 1021cm−3 changes neither the defect density, the weak Si-Si bond density nor the defect metastability. This suggests that the weak Si-Si bond density as well as the dangling bond density is determined by equilibration with strong Si-Si bonds through the interchange of H. The implications of these resultsfor H bonding are discussed.

Amplitude Dependence of Metastable Defect Formation

Abstract: The kinetics of small changes from thermal equilibrium are measured and simulated for thermally induced metastable changes. We find that only a distribution of rate constants is consistent with the data, and therefore we can eliminate certain types of microscopic models for metastability which depend on a single variable such as the carrier density.

Comparison of Current and Light Induced Defects in A-SI:H

Abstract: Metastable defect creation by illumination and by a forward current in p-i-n devices are compared using CPM and reverse current measurements of the defect density. The data show that the same defects are formed by the two mechanisms, but with different spatial profiles. Numerical modelling shows how the spatial profile influences the reverse bias current.Acknowledgements

The Effect of Post-Deuteration on Metastability in a-Si:H

Abstract: Hydrogenated amorphous silicon films were deuterated through a sequence of 1 h exposures to a remote deuterium plasma at 350°C. The concentration profiles of hydrogen and deuterium were determined by SIMS at various times during the exposure sequence. The defect density in state A, after deuteration and after illumination with white light were determined using CPM measurements following each 1h exposure sequence. We find that post-deuteration does not alter the defect density in state A, change the Urbach edge, nor significantly alter metastable defect formation. Intense light soaking increases the defect density by about 5×1016cm−3 independent of the total H + D concentration. These results suggest that D always enters the sample in pairs pinning the hydrogen chemical potential which supports the idea of a negative U system for hydrogen and deuterium. Despite an increase of Si-H bonds by as much as 3×1021cm−3, the annealed dangling bond density and the weak Si-Si bond density did not change. This suggests that the density of weak Si-Si bonds as well as the dangling bond density is determined by equilibration with strong Si-Si bonds through the interchange of H. The implications of these results for H bonding will be discussed.

Metastability of phosphorus and boron in hydrogenated amorphous silicon

Abstract: Thermodynamic equilibration kinetics of phosphorus- and boron-doped hydrogenated amorphous silicon (a-Si:H) are explored using dark conductivity measurements. the equilibration kinetics of phorphorus- and boron-doped a-Si:H are different and there is a strong dependence on the deposition temperature (Ts). Small variations in temperature of the semiconductor cause variations in active dopant concentration, which affects the conductivity. the transformation of phorphorus into the electronically active phase (activation) follows a stretched exponential time dependence with a temperature-independent dispersion parameter, β, of 0.85, whereas the transformation into the electronically passive bonding configuration (passivation) is also a stretched exponential but with β ⩽ 0.8. the kinetics of boron metastability are similarly a stretched exponential, but with equal β values for activation and passivation. the time constant, τ, to achieve equilibrium for both transformations is thermally activated with energies Eact ∽ 1.1 eV for phosphorus and boron. an increase in Ts leads to an increase of Eact. the data are discussed and interpreted based on the hydrogen migration model.