Grain Boundaries In Polycrystalline Silicon Research Articles

Examination of the optimization of thin film transistor passivation with hydrogen electron cyclotron resonance plasmas

Hydrogen plasmas generated by electron cyclotron resonance currently provide the most efficient plasma exposure technique available for passivating the grain boundaries of polycrystalline silicon. In this report, we show that careful optimization may be required when using this passivation approach on polycrystalline silicon gated, polycrystalline silicon thin film transistors fabricated using low temperature oxides. Optimization is found to be necessary for thin film transistors (TFTs) with polycrystalline Si gates in order to prevent the onset of high leakage currents which can develop when exposures are too long. The effects of exposure time, substrate temperature, microwave power level, pressure, and plasma dilution with an inert gas are examined to determine the conditions for optimal improvement in electrical performance. A model is also presented to explain this need for optimization of the electron cyclotron resonance hydrogen plasma passivation of poly-Si TFTs.

Grain boundary barrier heights and recombination velocities in polysilicon under optical illumination

A new model of recombination of carriers at grain boundaries in polycrystalline silicon under optical illumination is presented by considering the monoenergetic density of grain boundary states. Calculations have been performed on the grain boundary barrier heights ( V g), and on the interface and effective recombination velocities of minority carriers as a function of illumination level, grain size ( d) and bulk diffusion length of the minority carriers ( L b). These computations show that if the grain size lies in the range W g < d < L b (where W g is the depletion width) and the illumination level is high, the dependence of V g on grain size and illumination level will be much higher than that in the small and large grain size ranges. It is also found that, in the small grain size range, the dependence of interface and effective recombination velocities on the grain size is quite different, especially when illumination level is low. The dependence of the bending of the minority-carrier quasi-Fermi level in the grain boundary space-charge and quasi-neutral regions on the grain size and on the illumination level is studied. Calculations show that the validity of the quasi-equilibrium assumption decreases as grain size decreases and V g increases. This model predicts that the existing experimental studies cannot be used to calculate the effective recombination velocity of the minority carriers for polysilicon with small grain sizes. A number of experimental results have been fitted well on the basis of the theory.

Inactivation of implanted impurities during buried Si 3N 4 layer formation

The electrical activity of boron and phosphorus atoms in silicon layers previously implanted with nitrogen ions was investigated. The behavior of implanted impurities (boron and phosphorus) depends on the dose of implanted nitrogen, previous thermal treatment, dose and energy of boron and phosphorus ions. The decrease of electrical activity is connected with accumulation of impurity atoms on sinks as silicon nitride inclusions, grain boundaries in polycrystalline silicon. It is proposed that impurity affected distribution profiles are influenced elastic force fields appearing as a result of creation of silicon nitride inclusions. The conclusion was drawn that the distribution profile formation is connected with the mobility of impurity atoms during irradiation.

Theory of grain boundary recombination and carrier transport in polycrystalline silicon under optical illumination

The physics controlling the recombination of minority carriers at grain boundaries in polycrystalline silicon under optical illumination is described theoretically, and a model for the grain boundary space-charge potential barrier height is presented. The model is based on the assumption of a Gaussian energy distribution of grain boundary interface states. Attention is also focused on the electrical conduction in this material under illumination. The dependence of space-charge potential barrier height and the effective recombination velocity on the illumination level, the grain size, and the bulk diffusion length of minority carriers (L/sub b/) is investigated. Computations show that if the illumination level is high, the sensitivity of effective recombination velocity to grain size (d) in the intermediate grain size range (i.e. d approximately=L/sub b/) is much higher than that in the small and large grain size ranges. It is found that the resistivity of polysilicon decreases on increasing illumination level. The dependence of polysilicon resistivity on grain size under optical illumination is found to be much higher than that under dark conditions.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effect of hydrogen passivation on electrical transport in polycrystalline silicon

Hydrogen passivation of grain boundaries in polycrystalline silicon is shown to improve the efficiencies of devices fabricated from these materials. We have carried out hydrogen plasma passivation of polycrystalline silicon in RF discharge and have studied the variation in room temperature resistivity of the polysilicon wafer after H plasma treatment. In order to study intergrain changes in the polysilicon wafer after optimum H plasma treatment, we have carried out J( V, T) measurement in the temperature range 130 to 330 K. It is shown that after optimum hydrogenation, the temperature range over which thermionic emission occurs extrapolates to a much lower temperature value which is due to reduction of the grain boundary potential barrier after H passivation.

Infrared microcharacterization of grain boundaries in polycrystalline silicon

Fourier transform infrared analysis of boron-doped polycrystalline silicon, in the 4000-600 cm −1 wavenumber range, is performed by scanning inter- and intra-grain regions of the samples with a 20 μm size spot. The results provide the first direct optical evidence of free carrier excess at grain boundaries (GB), attributed to impurity segregation. Moreover, oxygen absorption band behaviour shows that infrared active species of oxygen segregate at the surface of the GB only, in agreement with results from SIMS and EBIC measurements on the same samples.

Precipitation of Copper and Cobalt at Grain Boundaries in Silicon

AbstractThe precipitation of copper and (radioactive) cobalt at low energy grain boundaries in polycrystalline silicon and bicrystals is investigated. The metals are diffused in from a surface source between 800 - 1000 °C and the precipitation after cooling down is studied by TEM (for Cu) and Mößbauer spectroscopy (for Co). The precipitates are metal suicides. For copper it is shown that they appear in form of colonies containing hundreds of precipitates with a particle size between 5-60 nm. In the grain boundary they nucleate preferentially at dislocations and steps. The distribution and size of the precipitates depend on the cooling rate after the diffusion. In the vicinity of the grain boundary the volume is depleted from the impurities.

Systematic study of the process parameters affecting hydrogen plasma passivation of polycrystalline silicon and polycrystalline silicon solar cells

We report detailed studies of hydrogen passivation of polycrystalline silicon wafers by RF plasma and the effect of various process parameters on the extent of passivation as seen through electrical measurements. Similar studies made on polysilicon solar cells are also reported. The need for such studies is emphasized via some interesting results which show an excessive dependence of effective passivation on the process conditions such as processing time, substrate temperature, gas pressure and RF power which seems to have been overlooked by earlier workers. Our results reveal that there is an optimum for almost all the above parameters which yield the most effective passivation of the grain boundaries in polycrystalline silicon. Physically, therefore, it implies that optimum hydrogen incorporation in a particular bonding configuration is a necessary condition to achieve best passivation. Results of samples passivated for several hours indicate the presence of yet another process along with the normal hydrogen diffusion. Possibilities of bond breaking or hydrogen incorporation in different bonding configurations cannot be completely ruled out. The dependence of effective passivation on these process conditions is also revealed by the systematic studies of variation in average carrier concentration, charge carrier mobility and resistivity with substrate temperature, RF power and gas pressure. Polycrystalline silicon solar cells passivated with the optimum process conditions show significant improvement in the efficiency and fill factor. Presence of hydrogen was confirmed by quadrupole mass spectrometry studies by heating the passivated samples.

Neutralization and bonding mechanisms of shallow acceptors at grain boundaries in polycrystalline silicon

Interactions between shallow acceptors (B, Al, Ga, and In) and hydrogen in polycrystalline Si are investigated. The bonding mechanisms involved in the acceptor neutralization process at grain boundaries are examined using microanalytical techniques. Differences in the incorporation of molecular and atomic hydrogen, and corresponding variations in electrical passivation at grain boundaries, are observed. Low-temperature Auger difference spectroscopy confirms Si–H bonding to dominate B, Ga, and In-doped cases, with no direct acceptor–hydrogen bonding. Al-rich grain boundaries show H-complex and hydroxyl bonding. The data confirm chemical bond strength trends with B&amp;lt;Ga&amp;lt;In. Volume-indexed Auger electron spectroscopy is utilized to compare bonding and H distributions in B- and Al-rich grain boundary regions.

Grain boundary segregation of oxygen and carbon in polycrystalline silicon

The electrical activity of grain boundaries (GB’s) in polycrystalline silicon is strongly affected by heat treatments, which are known to induce the segregation of oxygen. The large, quantitative variations of the electrical activity of GB’s observed experimentally, which are a function not only of the heat treatments but also of the carbon and oxygen content of the specific sample examined, could not be explained however, without assuming that carbon and oxygen play a synergistic role. We demonstrate in this work, by using secondary ion mass spectrometry, that oxygen and carbon segregate simultaneously, albeit spatially resolved, at grain boundaries. This result appears to be of major importance when interpreting electron or light beam induced current profiles at grain boundaries in polycrystalline silicon.

Effects of crystallization on trap state densities at grain boundaries in polycrystalline silicon

The relationship between crystallization processes in the formation of polycrystalline-silicon (poly-Si) films and trap state densities at grain boundaries is described. Three different crystallization techniques were used to obtain poly-Si films: 1) LPCVD, 2) solid-phase crystallization, and 3) laser recrystallization. Trap state densities in laser-recrystallized poly-Si are 9.2-9.6 × 1011cm-2, regardless of grain size. These values are half of those in LPCVD and solid-phase crystallized poly-Si. It is indicated quantitatively that laser-induced melting and the subsequent solidification process exert a significant influence on the electrical activity of silicon grain boundaries.

Tem Studies of Silicon-Silicon Oxide Interface Roughness

AbstractInterfaces between two kind of substrate, a bulk silicon wafer and a laser-recrystallized Silicon-On-Insulator (SOI), and its thermally grown oxide have been studied. High resolution transmission electron microscopy (HRTEM) of cross sectional specimen shows that the roughness at the interface is atomically flat and nearly uniform for the bulk single crystal silicon and silicon oxide, while being nonuniform and rough as much as 20 nm height for the recrystallized silicon and silicon oxide interface. Consideration of interface between recrystallized silicon and silicon oxide, and the oxide surface above, the observed roughness at the interface is due to original grain boundaries of polycrystalline silicon which was used as the material for the laser recrystallized silicon formation. It is also discussed HRTEM of the interface between polycrystalline silicon and silicon oxide.

Recombination effects and impurity segregation at grain boundaries in polycrystalline silicon

Recombination effects and impurity segregation at grain boundaries in polycrystalline silicon

Hydrogen Passivation of Grain Boundaries in Polycrystalline Silicon Deposited by Molecular Beams

ABSTRACTGrain boundary properties of polysilicon deposited by molecular beams have been investigated by electron spin resonance and conductivity measurements. The variations of the conductivity activation energy with doping can be explained by a density of states model consisting mainly of two exponential bandtails, implying that dangling bonds play a minor role. A hydrogen plasma treatment at 500 °C reduces the spin density by a factor of three but also passivates weak Si-Si bonds thus leading to steeper bandtails. The possibility of hydrogen-related intra-grain gap states is also discussed.

Formation of silicides by rapid thermal annealing over polycrystalline silicon

We have investigated the formation of titanium silicide by rapid thermal annealing in nitrogen and argon ambients over polycrystalline silicon. A sheet resistance of about 3 Ω per square for a 300-Å Ti layer was achieved after 900 °C/10-s annealing treatment, which decreased to about 2 Ω per square after 1100 °C/10-s treatment. The silicides were found to be stable during rapid thermal annealing up to 1100 °C/10 s with no or negligible migration of titanium along the grain boundaries in polycrystalline silicon. An external layer (titanium rich, mixture of titanium oxide and nitride) was observed to form during rapid thermal annealing treatment in the nitrogen ambient, but the surface remained clean in the argon ambient.

Carrier recombination at grain boundaries in polycrystalline silicon under optical illumination

A new relation for the grain boundary potential barrier height under optical illumination is presented. Calculations have been performed of the grain boundary potential barrier heights and effective recombination velocities of minority carriers at the edge of space-charge regions as a function of illumination level. These computations show that the sensitivity of grain boundary potential barrier height to illumination level is determined by the bulk diffusion length and the ratio of hole capture cross-section to electron capture cross-section of the grain boundary recombination centres. The dependence of the barrier height at grain boundary on the grain size has also been investigated theoretically. The available experimental data are found to be in good agreement with the theoretical predictions.

Grain Boundaries in Polycrystalline Silicon

▪ Abstract The phase-field method has recently emerged as a powerful computational approach to modeling and predicting mesoscale morphological and microstructure evolution in materials. It describes a microstructure using a set of conserved and ...Read More

Grain Boundaries Obsevation of Polycrystalline Sillcon Using Nematic Liquid Crystals with Positive Dielectric Anisotropy

Abstract A new method to analyze electrically active grain boundaries of polycrystalline silicon by using a field effect of nematic liquid crystal with positive dielectric anisotropy is described. In addition, threshold voltage of a liquid crystal planar cell with interdigited comb metal electrdes is measured as a function of frequency and temperature. From the results, the fringing electric field effect on the liquid crystal molecular orientation is clarified. This method is advantageous in the wide frequency range (0.1–105 Hz), fast response time and low threshild voltage by comparison with the method using nematic liquid crystal with negative dielectric anisotropy.

Compositional microcharacterization of electrically active and chemically passivated silicon grain boundaries

The mechanisms controlling the electrical passivation of grain boundaries in polycrystalline silicon, using the incorporation of hydrogen in these regions, are investigated. For the first time, direct and correlated microelectrical and microcompositional measurements of the effects of hydrogen on Si grain boundaries are presented. The detection, spatial mapping and quantification of hydrogen within the polycrystalline Si has been accomplished by a specially developed secondary ion mass spectrometry (SIMS) method. In this, selected secondary ion species corresponding to elements of molecules of interest are measured and stored (indexed for intensity, type, and spatial origin) digitally for an incremental volume encompassing the region of interest (i.e., the grain boundary plane). These data are utilized to determine the diffusion coefficients (temperature dependencies, activation energies, and preexponential coefficient terms) of hydrogen in the grain boundaries and the grains as functions of the processing parameters. The effects of the hydrogen passivation process on the performance of solar cells are discussed.

Effect of arsenic segregation on the electrical properties of grain boundaries in polycrystalline silicon

Equilibrium arsenic segregation to the grain boundaries of polycrystalline silicon was measured directly by x-ray microanalysis in the temperature range 700–1000 °C. A direct link was observed between arsenic segregation and resistivity. Increasing arsenic segregation at the lower annealing temperatures is consistent with an observed increase in resistivity. Fitting the enhancement levels at various temperatures with the McLean segregation isotherm, a binding energy of 0.65 eV/atom and a boundary saturation limit of 12 at. % for arsenic was obtained. A model for the effect of the segregation of arsenic to silicon grain boundaries is proposed. Segregation to boundary defects that cause trapping states can remove these interfacial traps, and segregation to other boundary sites can create a degenerately-doped interfacial layer. The electrical consequences of this segregation are considered, and by comparison of the measured resistivity changes with temperatures with those predicted from these simple models it is proposed that the major contribution to the resistivity in heavily-doped polycrystalline silicon comes from scattering of carriers by the high density of positive charge at the grain boundaries.