Boundary Trap States Research Articles

On-Current Modeling of Polycrystalline Silicon Thin-Film Transistors

We propose an on-current (above threshold voltage) model of polycrystalline silicon thin-film transistors (poly-Si TFTs). The model includes the study of the effect of trap state density, poly-Si inversion layer thickness and temperature on the TFT characteristics. Effective carrier mobility and I-V characteristics are described by considering the mechanism of capture and release of carriers at grain boundary trap states and the thermionic emission theory. It is found that at low as well as at high doping concentrations, the effective carrier mobility (µeff) increases with increasing temperature whereas a dip is observed at intermediate doping concentration. At very high and very low doping concentration the effect of temperature on the mobility is found to be almost negligible. Calculations reveal that effective carrier mobility and drain current increase as the gate bias increases and are larger for a lower trap state density. The calculated value of activation energy decreases as the gate bias increases and is larger for a larger poly-Si inversion layer thickness. A comparison between the present predictions and the experimental results shows reasonably good agreement.

Evaluation of grain boundary trap states in polycrystalline–silicon thin-film transistors by mobility and capacitance measurements

A method for evaluating localized grain boundary (GB) trap states in polycrystalline–silicon thin-film transistors (poly-Si TFTs) is proposed. Field effects in poly-Si TFTs are analyzed by decomposition of the two-dimensional Poisson’s equation. This analysis gives the theoretical basis for evaluating the localized GB trap state density. The trap states are evaluated with the GB barrier potential given by the activation energy of the effective mobility and with the surface potential calculated from the capacitance–voltage characteristics in poly-Si TFTs. This evaluating method was applied to several different types of poly-Si TFTs fabricated with excimer laser annealing crystallization processes. This method has succeeded experimentally in eliminating the error effects related to oxide–semiconductor interface trap states and in clarifying that the GB barrier potential has a peak in its gate bias dependence. This method has also elucidated that the GB trap state density has almost the same value independent of the grain size when the hydrogenation efficiency in poly-Si is in the same level between TFTs. The good agreement of the simulated and the measured drain currents confirm the validity of this method.

Two-Dimensional Numerical Simulation of Solid-Phase-Crystallized Polysilicon Thin-Film Transistor Characteristics

A new two-dimensional (2-D) numerical simulation model for the solid-phase-crystallized polysilicon (poly-Si) thin-film transistors (TFTs) is presented in this paper. The proposed model is mainly based on the defects located at the poly-Si/gate oxide and poly-Si/buffer oxide interfaces and at the intergrain boundaries. The simulation results have shown to be consistent with the experimental TFT data. It has been demonstrated that the interface/grain boundary trap states have different effects on the conduction mechanisms of the poly-Si TFTs. The characteristics of the poly-Si TFTs are discussed in detail.

Characteristics of low temperature poly-Si TFT device and circuits fabricated with disilane

Various fabrication techniques have been employed in order to optimize the performance of poly-Si TFTs device and circuits. We have prepared poly-Si active film by LPCVD and PECVD using monosilane and disilane recrystallized by the low temperature process of ELA and the high temperature process of SPC. The poly-Si TFTs with ELA active films exhibit higher field effect mobility and lower threshold voltages than those with SPC active film due to low intragranular trap state density. The poly-Si TFTs with active film deposited using Si2H6 exhibit higher mobility and sub-threshold slope than those with active film deposited using SiH4 due to larger poly-Si grain size and lower grain boundary trap state density. The oscillation frequency of a 23-stage CMOS inverter chain and the current driving capability of a CMOS transmission gate indicate that the low temperature processed circuits with active film deposited using Si2H6 and annealed by ELA method exhibit remarkable performances.

In Situ Electron Holography of Grain Boundary Schottky Barrier Dynamics in 0.5 Wt% NB Doped SrTiO3 36.8° Symmetric Tilt Bicrystals

Abstract Electrical phenomena such as positive temperature coefficient of resistance (PTCR), grain boundary layer capacitance, and varistors are controlled by Schottky barriers to current transport formed in grain boundary regions. These grain boundary potential barriers are a result of charged defect accumulation, either through equilibrium segregation or kinetic diffusion processes. Recent work on Nb and Mn doped BaTiO3 highlights the importance of substitutional defects which activate grain boundary trap states and give rise to the PTCR effect. The dynamic interaction of charge carriers with such trap states also produces the rapid electrical breakdown characteristic of varistors. This study explores the issues of defect species, space charge, and electrical transport in donor doped SrTi03 by electron holography and electron energy loss spectroscopy. The specific defect species and mechanisms of current transport through donor doped SrTiO3 remain unresolved because of the difficulty in connecting defect distributions (structural information) and resultant Schottky barrier effects (chemical information).

Optical characterization of AlN films: measurement of stress

AlN films were deposited on quartz and silicon substrates by d.c. magnetron sputtering of an Al target in argon + nitrogen plasma (20–100 vol.% nitrogen). Transmission electron microscopy (TEM) and X-ray diffraction (XRD) studies indicated the films to be polycrystalline for higher nitrogen (>30 vol.%) and amorphous for lower nitrogen in the sputtering gas. Direct optical transition with high band gap ( E g ≥ 5 eV) was observed for the polycrystalline films while for amorphous films indirect transition could be identified. The absorption band tail at photon energies less than the band gap energy ( E g) in the polycrystalline films could be analysed by considering the contribution of stress (1.4–3.0 GPa) and strain ((3–7) × 10 −3) due to band gap fluctuation in the grain boundary region. The effects of grain boundary trap state density ( Q t ≈ (2.0–3.8) × 10 12 cm −2) on the optical properties of the polycrystalline films were ascertained. The microstructural information, obtained from the scanning electron microscopy, TEM and XRD studies, were correlated with the above measurements.

Anomalously high collection probability in thin film polycrystalline silicon solar cells from numerical modeling

The impact of grain size on the average collection probability of fine grained (<10 μm) polycrystalline silicon has been investigated using a numerical device simulator. This model predicts that the collection probability can improve dramatically once the depletion regions around adjacent grain boundaries overlap. Thus, contrary to most analytical models, the collection probability is not monotonic with grain size. The collection probability has a local maximum at a grain size determined by the grain boundary trap state density. The magnitude of the improvement is dependent upon the grain boundary recombination velocity.

Characteristics of polycrystalline films grown by ultrahigh vacuum chemical vapor deposition system

In situ boron-doped polycrystalline Si 1− x Ge x (poly-Si 1− x Ge x ) films deposited by ultrahigh vacuum chemical vapor deposition (UHV/CVD) system were characterized. Optimum fitted values of grain boundary trap state densities, 4.0 × 10 12 cm −2 and 4.9 × 10 12 cm −2 were obtained for poly-Si and poly-Si 0.79Ge 0.21, respectively. The extracted average carrier concentration in the grain agrees with secondary ion mass spectroscopy (SIMS) analysis. In turn, we found that these films are suitable Hall elements to sense magnetic field. Experimental results show that the sensitivity decreased with the increasing input current, which can be well explained using the thermionic emission theory. Finally, we use these films to fabricate thin film transistors.

Grain boundary trap distribution in polycrystalline silicon thin-film transistors

The grain boundary trap state density is evaluated in polysilicon thin-film transistors by a method based on the dependence of the grain boundary potential barrier height on the gate voltage. Assuming a Gaussian energy distribution of the grain boundary trap states, the distribution parameters are determined by fitting the grain boundary barrier height experimental data with the theory. In low-pressure chemical vapor deposited polysilicon films, the influence of deposition pressure on the grain boundary trap distribution is examined by using this method. A large number of traps exist at the grain boundaries near the midgap of the material deposited at lower pressure due probably to an increased impurity contamination.

A New Method to Estimate Grain Boundary Trap State Density in Poly-Si TFTs

A new method of estimating grain boundary trap state density in poly-Si thin film transistors (TFTs) is proposed by modifying an assumption used in Levinson's method. Our method assumes that only the carrier near the surface contributes to the total current, and the other carrier is neglected. Then the carrier density at the surface is used to express the potential barrier height induced at the grain boundary instead of the averaged carrier density as in Levinson's method. The validity of our assumption is investigated using our two-dimensional device simulator, and it is shown that our assumption on the carrier density is suitable to derive the true trap state density.

Spontaneous Polarization Screening Effect and Trap‐State Density at Grain Boundaries of Semiconducting Barium Titanate Ceramics

The grain‐boundary trap‐state density and the polarization screening effect were studied for a series of semiconducting PTCR barium titanate ceramic samples with different manganese (Mn) additives and different thermal treatments. The grain‐boundary resistance and capacitance data were measured by the ac complex impedance method. The grain‐boundary data obtained were analyzed using a simple double‐depletion‐layer model and an absolute‐zero‐temperature approach for the Fermi distribution function for the boundary trap states. The energy density distributions of the boundary trap states were found to be V‐shaped for the energy range studied, from 0.35 to 1.35 eV, as measured from the conduction band downward. The “neutral” Fermi level at the grain boundary is taken as 1.35 eV and the bulk Fermi level is taken as 0.15 eV from the conduction band. For the samples without the Mn additive, the PTCR effect is controlled by the trap densities located near 0.35 eV. The trap centers are believed to be barium and oxygen vacancies, or chemisorped oxygens. For the samples with Mn additives, the trap densities increase dramatically near 1.35 eV and play a dominant role in the PTCR effect. These trap centers are believed to be Mn4+ ions at the titanium sites. The charge‐compensation effect of spontaneous polarizations on the trap charges was found to be linearly proportional to the total amount of trap charges at that temperature.

Effective mobility of polycrystalline semiconductors

ABSTRACT Theoretical investigations are presented to explain the behaviour of effective mobility (μ*) as a function of doping density, valid for all grain sizes, in polycrystalline silicon, by considering the dynamics of capture and release of free carriers at the grain boundary trapping sites, the finite thicknesses of the grain boundaries and the contributions of the bulk and depletion regions towards the electrical conductivity. An empirical formula is used for the value of grain boundary trapping states density (N T) as a function of grain size. Computations show that (a) μ* has a minimum for a particular doping concentration for a given grain size D, and it shifts towards lower doping concentration with increasing D; and (b) the polycrystalline behaviour approaches the single-crystal behaviour for D ≥: 3000μm. The predicted results reasonably represent the experimentally observed variations of effective resistivity and effective mobility of poly-semiconductors as a function of doping density for all grain sizes.

Passivation of grain boundaries in silicon

Several papers have demonstrated that the introduction of hydrogen into polycrystalline silicon can remove grain boundary trapping states. Evidence for this comes from studies of both majority carrier transport over grain boundary potential barriers and minority carrier recombination at these defects. Data on hydrogen passivation will be reviewed and the relative utility of both these transport measurements will be discussed with regard to the optimization of this process. Recent studies utilizing a Kaufman ion source have shown that increasing the proton energies and the dose rate greatly facilitates passivation in silicon. Simple diffusion models will be presented which account for these observations. This new treatment method has reduced the time necessary to improve photovoltaic cell parameters to the domain of commercial feasibility; the prospects for further improvements will be assessed. Because this passivation technique is essentially a low energy implantation process, there are accompanying dama...