Conduction In Polycrystalline Silicon Research Articles

Studies on the polycrystalline silicon/SiO2 stack as front surface field for IBC solar cells by two-dimensional simulations**Project supported by the National Natural Science Foundation of China (Grant Nos. 11104319, 11274346, 51202285, 61234005, 51172268, 51602340, 61274059, and 51402347), the Solar Energy Action Plan of Chinese Academy of Sciences (Grant Nos. Y1YT064001, Y1YF034001, and Y2YF014001), the Graduate and College Student’s Innovative Project

Interdigitated back contact (IBC) solar cells can achieve a very high efficiency due to its less optical losses. But IBC solar cells demand for high quality passivation of the front surface. In this paper, a polycrystalline silicon/SiO2 stack structure as front surface field to passivate the front surface of IBC solar cells is proposed. The passivation quality of this structure is investigated by two dimensional simulations. Polycrystalline silicon layer and SiO2 layer are optimized to get the best passivation quality of the IBC solar cell. Simulation results indicate that the doping level of polycrystalline silicon should be high enough to allow a very thin polycrystalline silicon layer to ensure an effective passivation and small optical losses at the same time. The thickness of SiO2 should be neither too thin nor too thick, and the optimal thickness is 1.2 nm. Furthermore, the lateral transport properties of electrons are investigated, and the simulation results indicate that a high doping level and conductivity of polycrystalline silicon can improve the lateral transportation of electrons and then the cell performance.

Study of the transverse conductivity of polycrystalline silicon films fabricated by magnetron sputtering

The transverse conductivity of undoped polycrystalline silicon (PCS) films 0.2–0.6 µm thick, deposited by magnetron sputtering on heavily doped single-crystal substrates is studied. Molybdenum films 0.4 µm thick were used as top electrodes. The dependences of the film resistivity on the electric field strength are obtained, which are described by the space-charge limited conduction model for the quasi-uniform distribution of traps in the PCS band gap.

Hydrogen-induced metastable changes in the electrical conductivity of polycrystalline silicon.

Measurements of the dark electrical conductivity ${\mathrm{\ensuremath{\sigma}}}_{\mathit{D}}$ performed on hydrogenated polycrystalline silicon (poly-Si:H) reveal a cooling-rate dependent metastable increase of ${\mathrm{\ensuremath{\sigma}}}_{\mathit{D}}$ below 268 K. This nonequilibrium state relaxes slowly and the time to reach equilibrium is thermally activated with ${\mathit{E}}_{\mathrm{\ensuremath{\tau}}}$\ensuremath{\simeq}0.74 eV. Since thermal quenching does not affect unhydrogenated specimens, the observed metastable changes are clearly due to the formation and dissociation of an electrically active hydrogen complex. We propose that this complex consists of an isolated H atom at the bond-center site of a prestrained Si-Si bond at a grain boundary.

Exploratory observations of post-breakdown conduction in polycrystalline-silicon and metal-gated thin-oxide metal-oxide-semiconductor capacitors

The post-breakdown conduction of thin-oxide metal-oxide-semiconductor structures with different gate electrodes and substrates is studied. Due to the extreme localization of the breakdown, many breakdown events can be produced in one capacitor during a constant voltage stress. In some cases, these events have been found to be reversible and this suggests that the breakdown is a reversible phenomenon (i.e., that the breakdown is a reversible switching between two conduction states of different conductivities). This reversibility is further supported by the observation of bistable conduction in the post-breakdown I-V characteristic when the breakdown current is externally limited. The experimental results are interpreted assuming that the breakdown is a three-stage process (degradation-breakdown-thermal effects), and a simple phenomenological model is presented. The role of the gate electrode (chromium, aluminum, or polycrystalline-silicon) and that of the substrate doping are analyzed within this framework. The presented results show that the analysis of the post-breakdown properties is a powerful technique to investigate the physics of the breakdown.

Electrical properties of polycrystalline silicon under optical illumination

A new theoretical model for electrical conduction in polycrystalline silicon under optical illumination is introduced to investigate the effect of grain size, illumination level, temperature, and grain-boundary scattering effect on the carrier mobility and resistivity of polycrystalline silicon by considering the variation of grain-boundary space-charge potential barrier height (Vg) with these parameters. This theory is based on the assumption that the grain-boundary scattering effect on carrier transport is represented by a rectangular potential barrier with a constant width of 20 Å and height φ. It is found that this barrier cannot be completely eliminated by lowering the temperature, while the barrier Vg can be removed in some cases, especially when the illumination level is very high and the grain size is large. Our theory predicts that the carrier mobility is approximately proportional to the temperature in the temperature range 100–125 K and approximately proportional to T−2/5 between 125 and 200 K. Computations show that the dependence of resistivity and carrier mobility of illuminated polysilicon on grain size is large at small grain sizes and at low temperatures and illumination levels.

Effect of Grain Boundary Segregation on the Transbarrier Conductivity of Polycrystalline Silicon

AbstractThe conductivity of single grain boundaries of electronic grade, Bridgman grown polycrystalline silicon samples was measured using the dc polarization technique with the aim of detecting any influence on the carrier transport regime resulting from the segregation of oxygen and carbon, which, by themselves, should behave as electrically inactive impurities. To this scope as grown samples and samples heat treated at 1123 and 1223 K, differing in their initial oxygen and carbon content, were used and the conductivity measured in the 298-100 K range.The results indicate that for both the carbon rich and for the oxygen rich samples the conductivity across the grain boundaries is of the thermally activated type and that it could be discussed in terms of carriers which are thermally emitted over the barrier and cross the barrier in the limit of the short mean free path.Samples which do not present a net excess of oxygen or carbon, apparently behave as single crystal specimens, instead.The deconvolution of the I-V curves is then used for obtaining the densitl of the interface states responsible of the set-up of the grain boundaries barrier, whose shape supports a completely new hypothesis about the configuration of the impurity cloud at the grain boundaries.

Thermionic-emission diffusion model of current conduction in polycrystalline silicon

A comprehensive model of current conduction in polycrystalline silicon based on thermionic-emission diffusion theory is developed. On the basis of this model, a more general expression for the effective majority-carrier mobility is derived which reduces to thermionic-emission (TE) theory-based expressions under certain simplifying assumptions and also helps in understanding the physical significance of the scaling factor used by earlier workers to explain their experimental results.

Thermionic emission diffusion model of current conduction in polycrystalline silicon and temperature dependence of mobility

In this paper a comprehensive model of current conduction in polycrystalline silicon (polysilicon) based on the thermionic-emission-diffusion (TED) theory is developed, and on the basis of this model an expression for the effective majority carrier mobility μeff is derived. This expression is quite general in nature and some thermionic emission (TE) theory based expressions for μeff can be obtained from it straightaway under certain simplifying assumptions. In addition, it helps in understanding the physical significance of the scaling factor used by earlier workers to explain their experimental results. Also, the experimental data on Hall mobility, which we obtained under an ohmic conduction regime in the 300–440 K temperature range for dark and illuminated conditions in lightly doped n-type polysilicon samples of different grain sizes, are presented and are interpreted on the basis of the TED model. Under strong illumination, the Hall mobility μHL was observed to vary with temperature T according to the relation μHL=aT−b, where a depended on grain size and was found to be smaller for the smaller grain size. The dark mobility data fitted well into the TED-based expression for μeff considering the interface states associated with grain boundaries to be localized at Ev+0.63 eV in the band gap. The analysis reveals that, generally, the scaling factor is needed if the effect of diffusion is neglected in comparison with the thermionic emission while in essence it is appreciable to be considered. However, in the TED model, as diffusion contribution in controlling the current transport across the grain-boundary potential barrier is well taken care of, the scaling factor is not required.

Electrical conduction model for polycrystalline silicon under illumination

Considering the grain-boundary scattering effect, a new theoretical model for the electrical conduction in polycrystalline silicon under illumination is presented. A current-voltage relationship is developed which includes the thermionic field-emission and the thermionic-emission across the grain-boundary scattering barrier. The effect of temperature on the resistivity of polycrystalline silicon has also been investigated theoretically. It is found that the actual grain-boundary barrier resistivity increases with decreasing temperature. Excellent agreement has been achieved between available results and theoretical predictions.

A model of electrical conduction in polycrystalline silicon

This paper presents a modified version of the conduction model for polycrystalline silicon which includes the thermionic field emission of carriers through the space-charge potential barrier, carrier tunneling through the grain-boundary rectangular potential barrier after being thermally emitted over the space-charge barriers, and the thermionic emission of carriers over these barriers. It is found that if the height of the space-charge potential barrier is much smaller than the height of the grain-boundary barrier, the conduction is mainly controlled by the second mechanism. As grain size decreases, the contribution to current by second mechanism increases. The model predicts that the grain-boundary width in phosphorus-doped polycrystalline silicon film is a strong function of dopant concentration at intermediate dopant concentrations, while the grain-boundary width in boron-doped polycrystalline silicon is independent of dopant concentration in the range of 1016to 5 × 1019cm-3. Considering the potential drop across the grain-boundary barriers, the computed variation of resistivity with dopant concentration for different grain sizes is found to agree with the available experimental data.

Theory of conduction in polysilicon: Drift-diffusion approach in crystalline-amorphous-crystalline semiconductor system—Part I: Small signal theory

A theory of conduction in polycrystalline silicon is presented. The present approach fundamentally differs from previous theories in its treatment of the grain boundary. This theory regards the grain boundary as amorphous semiconductor in equilibrium contact with crystalline grain. The model explains the electrical properties of polysilicon in terms of the electronic and structural parameters of the material and is in excellent agreement with the experimental data. The formulation is applicable for arbitrary grain size, temperature, doping concentration, and applied voltage. Specifically, the temperature dependence of resistivity is explained in terms of conduction channels inherent in the amorphous grain boundary. Also, this paper explicitly compares the previous emission theories with the present model in terms of voltage partition scheme and I - V predictions.

Conduction in polycrystalline silicon: Diffusion theory and extended state mobility model

We present a new model for conduction in polycrystalline silicon based on an extended state mobility in the disordered grain boundary and the diffusion theory of current. This analysis for the first time satisfactorily explains-without the use of scaling or artificial factors-experimental data on current density, mobility, resistivity, and the activation energy for carriers in polysilicon. An attractive feature of this theory is that it provides simple expressions for J, µ, and ρ which may even be derived from an equivalent circuit model. Also, these expressions reduce, within appropriate limits, to the corresponding terms for single crystal or amorphous material.

A model for conduction in polycrystalline silicon—Part I: Theory

A new phenomenological model for the electrical conduction in polycrystalline silicon is developed. The combined mechanisms of dopant segregation, carrier trapping, and carrier reflection at grain boundaries are proposed to explain the electrical conduction in polycrystalline silicon. The grain boundaries are assumed to behave as an intrinsic wide-band-gap semiconductor forming a heterojunction with the grains. Thermionic emission over the potential barriers created within the grains due to carrier trapping at the grain boundaries and then tunneling through the grain boundaries is proposed as the carrier transport mechanism. A generalized current-voltage relationship is developed which shows that the electrical properties of polycrystalline silicon depend on the properties of the grain boundaries.

A model for conduction in polycrystalline silicon—Part II: Comparison of theory and experiment

In the preceding paper [1], a new phenomenological model for the electrical conduction in polycrystalline silicon was developed. Electrical conduction in polycrystalline silicon was shown to be controlled by dopant segregation, carrier trapping, and carrier tunneling through the grain boundaries. In this paper, the theoretical model is compared to experiment. The electrical behavior of polycrystalline silicon is shown to be influenced by the properties of the grain boundaries. In arsenic and phosphorus-doped polycrystalline-silicon films the grain boundaries are best modeled by rectangular barriers with a height of 0.66 eV and an approximate width of 7 A. The width of the grain-boundary barriers and the density of carrier trapping states are found to be weak functions of the dopant species and sample processing. The resistivity is found to be a strong function of dopant concentration, dopant species, and processing history at low and intermediate dopant concentrations, and the model can be used to predict this behavior.

Growth and Characterization of Polycrystalline Silicon

Polycrystalline silicon is deposited by pyrolysis of silane in an rf heated epitaxial reactor. The grains exhibit a fibrous microstructure having an 〈110〉 preferred orientation in the growth direction. Growth is inhibited in the presence of excess arsine and accelerated in the presence of diborane. The results are explained in terms of catalysis and poisoning of surface adsorption sites responsible for reaction. A simple model for current conduction in polycrystalline silicon is described based on grain size, grain doping, and effective barrier height due to the grain boundary. This model satisfactorily explains the observed temperature dependence of the resistivity of undoped films and also the large values of resistivity which are observed for dopant concentrations .