Polycrystalline Silicon Films Research Articles

Stochastic Effects on the Dynamics of the Resonant Structure of a Lorentz Force MEMS Magnetometer

Resonance features of slender mechanical parts of Lorentz force MEMS magnetometers are affected by the (weakly) coupled thermo-electro-magneto-mechanical multi-physics governing their dynamics. We recently showed that reduced-order models for such parts can be written in the form of the Duffing equation, whose nonlinear term stems from the mechanical constraint on the vibrations and is affected by the driving voltage. As some device performance indices vary proportionally to the amplitude of oscillations at resonance, an optimization of the operational conditions may lead to extremely slender, imperfection-sensitive movable structures. In this work, we investigate the effects of imperfections on the mechanical response of a single-axis magnetometer. At the microscopic length-scale, imperfections are given in terms of uncertainties in the values of the over-etch depth and of the Young’s modulus of the vibrating polycrystalline silicon film. Their effects on the nonlinear structural dynamics are investigated through a Monte Carlo analysis, to show how the output of real devices can be scattered around the reference response trend.

Scalable Approach for Amorphous Thin Silicon Films Near‐IR Laser‐Induced Crystallization Using Nickel Absorption Layer

Technology for thin polycrystalline silicon films preparation is crucial for development of novel semiconductor devices, including flexible electronics. A method for thin polycrystalline silicon film preparation utilizing deposited nickel absorption layer which allows use of inexpensive 1064 nm pulsed YAG:Nd laser for laser annealing of magnetron‐sputtered amorphous silicon is reported here. Film morphology changes are visualized with scanning electron microscopy, its chemical composition and Ni fate upon laser irradiation are studied using X‐ray energy dispersion analysis and secondary ion mass spectrometry. Silicon crystalline structure changes and its homogeneity are characterized using Raman spectroscopy utilizing mapped spectral measurements. Range of laser radiation energy fluence (J cm−2) is established which allowed for preparation of fully polycrystalline silicon film with crystalline silicon Raman peak position and width comparable to that of single‐crystalline material. Nickel film is found to ablate substantially upon irradiation, leaving remains that can be easily removed by chemical etching.

Hydrogenation of Phosphorus-Doped Polycrystalline Silicon Films for Passivating Contact Solar Cells.

We characterize and discuss the impact of hydrogenation on the performance of phosphorus-doped polycrystalline silicon (poly-Si) films for passivating contact solar cells. Combining various characterization techniques including transmission electron microscopy, energy-dispersive X-ray spectroscopy, low-temperature photoluminescence spectroscopy, quasi-steady-state photoconductance, and Fourier-transform infrared spectroscopy, we demonstrate that the hydrogen content inside the doped poly-Si layers can be manipulated to improve the quality of the passivating contact structures. After the hydrogenation process of poly-Si layers fabricated under different conditions, the effective lifetime and the implied open circuit voltage are improved for all investigated samples (up to 4.75 ms and 728 mV on 1 Ω cm n-type Si substrates). Notably, samples with very low initial passivation qualities show a dramatic improvement from 350 μs to 2.7 ms and from 668 to 722 mV.

Trade-Off in Fire-Retardant Solar Cell Materials and Environmental Issues

In recent years, several types of solar cells, such as polycrystalline silicon, compound semiconductor and organic thin films, have been and grown and developed as one of the promising renewable energy devices for low cost and safety compared with nuclear power generation. They were composed from many components and their materials generally. As for Dye Sensitized Solar Cell (DSSC), which may be high efficiency and easy to assemble but not be expensive so much, not only inorganic titanium oxide and organic or metal complex dyes but also organic solvents as electrolyte. Furthermore, a condensing lens made of plastic are also used to improve power generating efficiency.

Low cross-plane thermal conductivity of sub-1 µm polycrystalline silicon thin films for thermoelectric applications

Silicon thin films with sub-1 µm thickness are of technological significance in many electronic and energy conversion devices. While a number of previous works have investigated the in-plane thermal conductivity of silicon films, less reported is their cross-plane thermal conductivity, particularly at sub-1 µm thickness. Here, we report on the cross-plane thermal conductivity of sub-1 µm, undoped, polycrystalline silicon films (75 and 246 nm) with columnar grain structure at room temperature. Using an optical pump-probe thermoreflectance metrology and a semi-classical phonon transport model, we find the cross-plane thermal conductivity to be 4.5 ± 0.8 and 12.5 ± 2.3 W m−1 K−1 for the 75 and 246 nm films, respectively. To the best of our knowledge, the cross-plane thermal conductivities we report here are the lowest ever reported for undoped polycrystalline silicon films with columnar grain structure, which can be ascribed to the impact of increased phonon scattering by a higher density of defects as compared to previous data. Given that a significant reduction in phonon thermal conductivity is required to enhance the thermoelectric energy conversion efficiency, our findings may be promising particularly for thermoelectric energy conversion applications.

Sub-Bandgap Luminescence from Doped Polycrystalline and Amorphous Silicon Films and Its Application to Understanding Passivating-Contact Solar Cells

We report luminescence phenomena from doped polycrystalline silicon (poly-Si) films and their applications to study carrier transport properties in passivating-contact solar cells. Low-temperature ...

Annealing effect on grain boundary width of polycrystalline silicon for photovoltaic application

Nowadays, LPCVD (Low Pressure Chemical Vapor Deposition) and highly doped polycrystalline silicon films have numerous applications in microelectronic component manufacturing technologies, integrated circuits and solar cells. The complexity of the circuits, and the increasing degree of integration of the components, constantly require improvement and mastery of the properties of this type of material. As part of this work, we are interested in the study of the effect of doping and annealing Effect on grain boundary width of polycrystalline silicon thin films deposited by LPCVD. The results obtained showed, on the one hand, that arsenic doped layers are more resistive than boron doped ones, and on the other hand, and that the diffusion of dopants is generally much greater in grain boundaries than in grains; and considering the importance of the average grain size in the polycrystalline silicon material, which is an important parameter, on which the physical and electrical properties of this material depend.

Comparison of linear transformations for deriving kinetic constants during silicon etching in Cl2 environment

The chemical etching of n-type polycrystalline silicon films in Cl2 environment at three different temperatures is investigated. The experimental dependences of silicon etching rate on pressure of Cl2 molecules are described using the Michaelis–Menten equation. The experimental data are presented using Lineweaver–Burk, Hanes-Woolf, and Eadie–Hofstee plots. The linear transformations are used to derive reaction rate constants, desorption rate constants, and Michaelis constants. True values of the kinetic constants are determined using Michaelis–Menten saturation curves. The linear transformations are compared using mean absolute percentage errors (MAPEs) of the kinetic constants. It is found that the Hanes–Woolf plot provides the most accurate values of the kinetic constants.

23% efficient p-type crystalline silicon solar cells with hole-selective passivating contacts based on physical vapor deposition of doped silicon films

Of all the materials available to create carrier-selective passivating contacts for silicon solar cells, those based on thin films of doped silicon have permitted to achieve the highest levels of performance. The commonly used chemical vapour deposition methods use pyrophoric or toxic gases like silane, phosphine and diborane. In this letter, we propose a safer and simpler approach based on physical vapour deposition (PVD) of both the silicon and the dopant. An in-situ doped polycrystalline silicon film is formed, upon annealing, onto an ultrathin SiOx interlayer, thus providing selective conduction and surface passivation simultaneously. These properties are demonstrated here for the case of hole-selective passivating contacts, which present recombination current densities lower than 20 fA/cm2 and contact resistivities below 50 mΩ cm2. To further demonstrate the PVD approach, these contacts have been implemented in complete p-type silicon solar cells, together with a front phosphorus diffusion, achieving an open-circuit voltage of 701 mV and a conversion efficiency of 23.0%. These results show that PVD by sputtering is an attractive and reliable technology for fabricating high performance silicon solar cells.

(Invited) Analysis of a Molten Region on Amorphous Silicon Film By High-Speed Camera and Contactless Temperature Measurement during Atmospheric Pressure Thermal Plasma Jet Annealing

In the production process of thin film transistors (TFTs), many researchers have discussed about crystallization of an amorphous silicon (a-Si) film by rapid thermal annealing for providing the polycrystalline silicon (poly-Si) film at a low temperature. The a-Si film was deposited on the glass substrate at a low temperature and annealed by rapid thermal annealing. It was important to reveal the mechanism of a silicon melt-solidification and grain growth for providing a poly-Si film having high crystallinity. So, they have proposed various measurement methods for finding the important information which is a lattice temperature, time-resolved electrical-conductance and optical-reflectance, and so on [1,2]. We have proposed our own analysis which is a high-speed camera (HSC) observation in real time in microsecond scale and a contactless temperature measurement of a substrate surface during an atmospheric pressure thermal plasma jet (TPJ) annealing [3,4]. We first disclosed leading wave crystallization (LWC) on the a-Si film during TPJ irradiation because of this HSC observation and revealed the two-dimensional temperature distribution for the surface of a glass or quartz substrate. However, we can’t obtain their information at the same time. Here, we demonstrate a temperature distribution of the molten region on a-Si film when we simultaneously use the contactless temperature measurement and the HSC observation in a microsecond scale. Amorphous silicon film with the thickness of 100 nm was formed on a 100-µm-thick flexible glass substrates by plasma-enhanced chemical vapor deposition at 250 °C and was dehydrated in N2 ambient at 450 °C. We carried out the temperature measurement and real-time observation of the molten silicon region by the contactless temperature measurement and the HSC observation when the glass substrate and the prepared film on the substrate was rapidly annealed by TPJ irradiation. In the result of the real-time reflectance by the contactless temperature measurement, we obtained a time resolved temperature on the surface of a glass substrate and a TPJ thermal profile by our own simulation analysis. Also, we got the three-dimensional temperature distribution from the obtained thermal information. We could get a cross-sectional image from the three-dimensional temperature distribution by cutting this distribution across the plane of the 1700 K parallel to a horizontal direction. We obtained isothermal lines from the cross-sectional image. We presumed that these isothermal lines correspond to the molten region on the a-Si film in the HSC image. So, we tried to superimpose the isothermal lines on the HSC image. The black area in the HSC image is the molten silicon region and this area is in the width of ~0.43 mm and length of ~1.13 mm. TPJ irradiation at 1 m/s was scanned from right to left of this image. The crystallization area is located at the rear of TPJ irradiation. The high speed lateral crystallization (HSLC) which is dendrite crystal growth appears in this area. We carried out the overlap of the HSC image and the isothermal lines with the edge of the molten region and the isothermal line of 1700 K and recognized that these images most closely matched as shown in the figure. We found that there is a liquid area even though the temperature is lower than the melting point. Also, it was revealed that this area is supercooling area and supercooling degree is ~100 K. Also, it was obvious that the crystal growth direction didn’t exactly align parallel to the direction of a temperature gradient. In this paper, we clarified that the temperature distribution in the molten region of the a-Si film can be visualized by the proposed method. This is a quite powerful tool to understand the crystalline growth in microsecond time regime. Reference: [1] G.J. Galvin, et al., Phys. Rev. B 27 (1983) 1079. [2] B.C. Larson, et al., Appl. Phys. Lett. 42 (1983) 282. [3] S. Hayashi, et al., Appl. Phys. Lett. 101 (2012) 172111. [4] T. Okada, et al., Jpn. J. Appl. Phys., 45 (2006) 4355. Figure 1

Ellipsometry-based conductivity extraction in case of phosphorus doped polysilicon

In this work, we investigated a new method for thin films electrical conductivity extraction based on spectroscopic ellipsometry. This has been enabled through the correlation between the films conductivity and their ellipsometric properties. Indeed, it has been demonstrated that numerous ellipsometric fitting-based approaches can provide, in an indirect way, the electrical characteristics of thin films. The study was focused on electrical conductivity, but doping level or carriers’ mobility can also be extrapolated from ellipsometric measurements. Among various possibilities leading to electrical properties extraction, we can cite the extremal values of Ψ and Δ ellipsometric angles, their associated wavelengths, the mean square error and the maximal and minimal reflectivities ratio. Otherwise, the correlation between extrinsic conductivity and ellipsometric parameters evolution has been confirmed in case of low doping levels with particular behavior after annealing. This contactless method has been successfully applied to polycrystalline silicon films deposited on oxidized, p-type monocrystalline substrates, by low pressure chemical vapor deposition technique, and lightly or heavily phosphorus doped by diffusion. The feasibility of the method has been proven in this case, but also in other cases like implanted polysilicon layers or silicon-on-insulator (not included here).

Mechanical Characterization of Polysilicon MEMS: A Hybrid TMCMC/POD-Kriging Approach.

Microscale uncertainties related to the geometry and morphology of polycrystalline silicon films, constituting the movable structures of micro electro-mechanical systems (MEMS), were investigated through a joint numerical/experimental approach. An on-chip testing device was designed and fabricated to deform a compliant polysilicon beam. In previous studies, we showed that the scattering in the input–output characteristics of the device can be properly described only if statistical features related to the morphology of the columnar polysilicon film and to the etching process adopted to release the movable structure are taken into account. In this work, a high fidelity finite element model of the device was used to feed a transitional Markov chain Monte Carlo (TMCMC) algorithm for the estimation of the unknown parameters governing the aforementioned statistical features. To reduce the computational cost of the stochastic analysis, a synergy of proper orthogonal decomposition (POD) and kriging interpolation was adopted. Results are reported for a batch of nominally identical tested devices, in terms of measurement error-affected probability distributions of the overall Young’s modulus of the polysilicon film and of the overetch depth.

Influence of phosphorus doping on the chemical etching rate of polycrystalline silicon films

The chemical etching of intrinsic and n-type poly-Si with phosphorus concentration 5×1018cm−3 in Cl2 environment is investigated. It is found that phosphorus doping increases formation and desorption of SiCl2 molecules. The activation energy of Si+Cl2→SiCl2 reaction for intrinsic poly-Si is equal to 1.73±0.24eV, and it decreases to 1.51±0.16eV when n-type poly-Si films are used. Meanwhile, the desorption activation energy of SiCl2 molecules for intrinsic poly-Si is equal to 1.86±0.16eV, and it decreases to 1.54±0.21eV when n-type poly-Si films are used. The etching-rate limiting process is determined using the calculated mean times of elementary processes.

Formation of Poly-Si Films on Glass Substrates by Using Microwave Plasma Heating and Fabrication of TFT’s on the Films

We have developed an apparatus for producing high-density hydrogen plasma. The atomic hydrogen density was 3.0 × 1021 m?3 at a pressure of 30 Pa, a microwave power of 1000 W, and a hydrogen gas flow rate of 5 sccm. We confirmed that the temperatures of tungsten films increased to above 1000?C within 5 s when they were exposed to hydrogen plasma formed using the apparatus. We applied this phenomenon to the selective heat treatment of tungsten films deposited on amorphous silicon films on glass substrates and formed polycrystalline silicon films. To utilize this method, we can perform the crystalline process only on device regions. TFTs were fabricated on the polycrystalline silicon films and the electron mobilities of 60 cm2/Vs were obtained.

Liquid‐Tin‐Assisted Molten Salt Electrodeposition of Photoresponsive n‐Type Silicon Films

AbstractProduction of silicon film directly by electrodeposition from molten salt would have utility in the manufacturing of photovoltaic and optoelectronic devices owing to the simplicity of the process and the attendant low capital and operating costs. Here, dense and uniform polycrystalline silicon films (thickness up to 60 µm) are electrodeposited on graphite sheet substrates at 650 °C from molten KCl–KF‐1 mol% K2SiF6 salt containing 0.020–0.035 wt% tin. The growth of such high‐quality tin‐doped silicon films is attributable to the mediation effect of tin in the molten salt electrolyte. A four‐step mechanism is proposed for the generation of the films: nucleation, island formation, island aggregation, and film formation. The electrodeposited tin‐doped silicon film exhibits n‐type semiconductor behavior. In liquid junction photoelectrochemical measurement, this material generates a photocurrent about 38–44% that of a commercial n‐type Si wafer.

Effects of PECVD temperature and RF power on surface structure and refractive index of amorphous and polycrystalline silicon films

The amorphous and polycrystalline silicon films were obtained by plasma enhanced chemical vapor deposition. The effect of power and temperature over a wide range on the surface morphology and structure, deposition rate films and refractive index have been investigated by atomic force microscopy, profilometry and ellipsometry. The deposition rate increased from 19.1 to 58.2 nm/min with RF power from 10 to 40 W. The refractive index of the films obtained at a temperature of 550 to 700 °C was in the range from 3.5 to 5.5.

Stress Distribution Profile Imaging With Spectral Fabry-Perot Interferometry in Thin Layer Substrates for Surface Micromachining

We have used spectral two-layer interferometry (STLI) imaging for estimation of the stress distribution profiles (SDPs) in thin film substrates, enabling fast and reliable all-optical methodology for the evaluation of pre-stress topography profiles in silicon wafers deposited with thin films. Specifically, in polycrystalline silicon (PS) and silicon nitride (SN) thin films, we demonstrate a nondestructive, systematic, and robust capability for consistent stress distribution profile (SDP) evaluation relying on STLI. In particular, for PS and SN devices, the SDP estimation is consistent and is compared with complementary characterization of the films.

Improved quality of flash-lamp-crystallized polycrystalline silicon films by using low defect density Cat-CVD a-Si films

Abstract We investigate the influence of the quality of precursor amorphous silicon (a-Si) films on the quality of flash-lamp-crystallized (FLC) polycrystalline Si (poly-Si) films by tuning the conditions of a-Si deposition by catalytic chemical vapor deposition. Electron spin resonance measurement reveals that the defect density of FLC poly-Si films is affected by the defect density of a-Si films, and FLC poly-Si films with lower defect density can be formed by using precursor a-Si films with lower defect density. The same tendency is also confirmed through μ-PCD measurement. Improvement in the characteristics of thin-film crystalline Si (c-Si) solar cells can be expected by using high-quality FLC poly-Si films formed from a-Si films with low defect density.

Crystallization of amorphous silicon films by photon-involved rapid thermal annealing

Polycrystalline silicon films with a preferential (2 2 0) orientation have been fabricated by annealing amorphous silicon films using a photon-involved rapid thermal annealing process. In contrast, conventional thermal annealing of the same amorphous silicon films results in crystallization at a preferential (1 1 1) orientation. This difference reveals direct photons interactions upon the silicon atoms, which results in preferential crystallization at the atom plane with the smallest atom densities on their projection plane, instead of the ones with the lowest energies at conventional thermal annealing.

Evolution of surface morphology during the growth of amorphous and polycrystalline silicon films

The evolution of the surface morphology of LPCVD poly-Si films (deposition temperature 620°C), a-Si films (deposition temperature 550°C) and poly-Si films, obtained by the crystallization of a-Si is investigated in the thickness range 40–500 nm. It is found that upon an increase in the thickness from 40 to 500 nm, the surface roughness (parameters S q , S z , S v ) is increased for poly-Si, while in the case of a-Si and poly-Si obtained by crystallization a-Si, on the contrary, decreases. The correlation length (S al ) increases for all three types of silicon films. Poly-Si films, obtained by the crystallization of a-Si, as compared to LPCVD poly-Si films have a significantly lower surface roughness, respectively, S q two times less at a thickness of 40 nm and sixteen times less at 500 nm. In contrast to thick films, thin a-Si films (at thicknesses of less than 40 nm) have a granular structure, which is especially pronounced at an average thickness of about 20 nm and there is a maximum on the dependence of the roughness S q on the thickness.