μc-Si Films Research Articles

Controllability of structural and electrical properties of silicon films grown in atmospheric-pressure very high-frequency plasma

Amorphous silicon (a-Si) and microcrystalline silicon (µc-Si) films were deposited in atmospheric-pressure (AP) He/H2/SiH4 plasma excited by a 150 MHz very high-frequency (VHF) power at a temperature of 220 °C. The variations of thickness and film crystallinity in the gas flow direction were studied using two electrodes (length = 16 and 5 mm). The electrical properties of the deposited Si layers were evaluated by fabricating bottom-gate thin film transistors (TFTs). The results showed that the chemical reactions both in gas phase and on the growing film surface were significantly enhanced in AP-VHF plasma, promoting phase transition from amorphous to microcrystalline on a time scale of the order of less than 0.1 ms. The performance of bottom-gate TFTs showed that a-Si layers formed in the upstream portion of the plasma zone had good enough electrical property (field-effect mobility of 1–1.5 cm2 · V−1 · s−1) despite very high deposition rates (several tens of nm · s−1). Meanwhile, µc-Si layers deposited in the downstream portion of the plasma zone did not acquire their original superiority over a-Si films with reference to electrical properties, although the use of the shorter electrode greatly improved the layer quality. The precise control of gas residence time is primarily important for the deposition of µc-Si films with desired qualities through sufficiently passivating grain boundaries with a-Si tissues.

Microwave plasma assisted process for cleaning and deposition in future semiconductor technology

The epitaxial growth of silicon layers is an important step in the fabrication of semiconductor devices. For conventional silicon epitaxy, high temperatures, up to 900 °C are necessary. However, in future, semiconductor technology epitaxy processes at lower temperatures are required to increase the integration density. The goal of this study was to investigate microwave plasma assisted processes for the selective removing of thin silicon oxide, the cleaning of silicon surfaces and the depositing of high quality silicon films. The main focus was to apply these processes for low temperature epitaxy. All processes, such as oxide removal, cleaning and deposition, were done in one chamber and with microwave plasma assistance. In order to remove silicon dioxide, the etching behavior of hydrogen, fluorine, and hydrogen/fluorine plasma was studied. It was shown, that with hydrogen/fluorine plasma, the best selectivity of oxide to silicon was reached. The deposition process of silicon was studied by growing μc-Si films. The process was characterized and optimized by spectral ellipsometry. After a successful characterization of all process steps, silicon epitaxy layers have been grown with in-situ removal of native oxide and in-situ surface cleaning. The temperature for all process steps was reduced below 450 °C.

Carrier Selective Contacts Design for Efficient Photocatalytic Water Reduction

2-photon tandem device structures that consist of a wide band gap and narrow band gap photoabsorber can maximize photoelectrolysis efficiency since their I-V characteristics match well with that needed for photoelectrolysis [1]. In the case of the narrow band gap (Eg ) material, recent theoretical work has shown that Si has a suitable Eg for the bottom cell of tandem water splitting device [2]. Since current performance of Si PEC cells lag far behind the theoretical limit [3], the need for a different approach increases. Up to now, tremendous efforts have been made to improve the performance of Si-based photocathodes [4-6]. One approach the solar cells industry uses to increase performance is to use metal-oxide-semiconductor (MOS) carrier-selective contacts due to their efficient carrier transport and low recombination [5,6]. However, very little work has been done studying photocatalytic hydrogen evolution reaction (HER) with the MOS structure. Moreover, there has been no detailed report on energy band level analysis to understand how the carriers can be selectively injected.In this work we demonstrate μc-Si/SiO2/p-Si structure (see Figure 1) to both front and back contacts which provide interface passivation and selective injection of minority and majority carrier to solid/liquid interface and back contact, respectively. A transparent TiO2 is placed above the highly doped μc-Si thin film as a conducting protection layer, and then Pt nanoparticles were drop-casted on the samples for photocatalytic HER in 1M HClO4. Using thin film degenerately doped Si as a metallic layer has been employed in recent state-of-art MOS based PV cells [7], but combination with TiO2 as a conducting protection layer for PEC hydrogen production has not been shown so far. Moreover, the details working principles based on the band energy analysis are outlined to understand how the carriers can be injected selectively and transferred to solid/liquid interface in PEC system.1. B. Seger, I.E. Castelli, P.C.K. Vesborg, K.W. Jacobsen, O. Hansen, I. Chorkendorff, Energy Environ. Sci. 7 (2014) 2397–2413. 2. A.B. Laursen, S. Kegnæs, S. Dahl, I. Chorkendorff, Energy Environ. Sci. 5 (2012) 5577. 3. W. Shockley, H.J. Queisser, J. Appl. Phys. 32 (1961) 510–519. 4. D. Bae, T. Pedersen, B. Seger, M. Malizia, A. Kuznetsov, O. Hansen, I. Chorkendorff, P.C.K. Vesborg, Energy Environ. Sci. 8 (2015) 650–660. 5. D. V Esposito, I. Levin, T.P. Moffat, A.A. Talin, Nat. Mater. 12 (2013) 562–8. 6. A.G. Scheuermann, J.P. Lawrence, K.W. Kemp, T. Ito, A. Walsh, C.E.D. Chidsey, P.K. Hurley, P.C. McIntyre, Nat. Mater. (2015) 1–8. 7. F. Feldmann, M. Simon, M. Bivour, C. Reichel, M. Hermle, S.W. Glunz, Sol. Energy Mater. Sol. Cells. 131 (2014) 100–104. Figure 1

Effects of silver nanoparticles size and shape on light scattering

The scattering from metal nanoparticles near their localized surface plasmon resonance is a promising way to increase the light absorption in solar cells. In this article, we investigate the light scattering of silver (Ag) nanoparticles on the microcrystalline silicon (μc-Si) substrate with various sizes and shape by finite element method. The results show that large spherical particles lead to enhanced scattering efficiency, whereas reduced coupling efficiency. The scattering cross section and coupling efficiency are very sensitive to the shape of the particles. Compared with spherical particles, hemispherical and cylindrical particles show much lower and broader scattering cross section, containing two surface plasmon resonance modes, and much higher coupling efficiency at longer wavelengths. However, with the increase in the longitudinal size (or height) of the particles, the coupling efficiency decreases, especially at shorter wavelengths, due to backscattering by the longitudinal resonance mode. Finally, we simulate the absorption enhancement in μc-Si thin film solar cells by Ag nanoparticle arrays with various shape. These results will be useful for enhancing performance of the μc-Si thin film solar cells by optimizing the light-trapping design.

Enhanced passivation at amorphous/crystalline silicon interface and suppressed Schottky barrier by deposition of microcrystalline silicon emitter layer in silicon heterojunction solar cells

In silicon heterojunction (SHJ) solar cells, the embedded p-layer (emitter) between transparent conductive oxide (TCO) and crystalline silicon is depleted form both sides. Hence, high level of doping is required to preserve flat band. In this report, the role of the emitter conductivity on the electrical properties of SHJ solar cells is investigated. We offer depositing microcrystalline silicon (μc-Si) films to satisfy the required emitter conductivity and to suppress the Schottky barrier when employing low work function TCO material. Finally, by depositing μc-Si layers, the passivation of solar cell precursors improves and current increases, due to their low absorption coefficient.

μc-Si thin film transistors with very thin active layer

N-type microcrystalline silicon (μc-Si) top-gate Thin Film Transistors (TFTs) are fabricated at a maximum temperature of 180°C using different thicknesses of undoped μc-Si active layers. The effect of the thickness on the TFT performance is experimentally studied and then modeled using Silvaco software tools. The experimental high improvement of the subthreshold swing and the limitation of the rear channel effect, when using very thin active layer, are shown to be due to the increase of the lateral electrical field between the source or drain and the active layer. This increase of the lateral field is shown to be much more important for defected active layer as the microcrystalline silicon one compared to single crystalline silicon active layer. The importance of the use of very thin active layer for amorphous or ploy–micro–nano-crystalline silicon based TFTs is then demonstrated.

Effect of reducing impurity concentration of microcrystalline silicon thin films for solar cells using radio frequency hollow electrode enhanced glow plasma

Microcrystalline silicon (μc-Si) thin films with low impurity (oxygen) concentrations were prepared by radio frequency hollow electrode enhanced glow plasma (RF-HEEPT). By using metal gasket seals with the RF-HEEPT system, the oxygen concentration in the thin films was reduced to 3.0 × 1017 cm−3, which is approximately two orders of magnitude less than that of films fabricated in a system using rubber gasket seals. Under the reduced oxygen conditions, we were able to deposit μc-Si thin films at a maximum deposition rate of 10.0 nm/s. We also investigated the spectral response of μc-Si thin-film solar cells. The spectral response of the solar cells fabricated with a low oxygen concentration was improved, especially in the near-infrared region. However, with increasing deposition rate of the thin films, the peaks of the spectral response shifted toward shorter wavelengths and the magnitudes decreased, especially in the near-infrared region, indicating that grain boundaries or defects in the thin films increased at higher deposition rate.

Effective phase control of silicon films during high-rate deposition in atmospheric-pressure very high-frequency plasma: Impacts of gas residence time on the performance of bottom-gate thin film transistors

Hydrogenated amorphous silicon (a-Si) and microcrystalline silicon (μc-Si) films were grown in atmospheric-pressure (AP) He/H2/SiH4 plasma excited by a 150-MHz very high-frequency (VHF) power at a temperature of 220°C. The variations in thickness and crystallinity of the deposited Si films along the gas flow direction were studied as functions of gas residence time in the plasma, VHF power density and H2 flow rate. Furthermore, the electrical characteristics of bottom-gate thin film transistors (TFTs) were investigated to evaluate the film quality. The results revealed that the chemical reactions both in gas phase and on the growing film surface were significantly enhanced in AP-VHF plasma, promoting phase transition from amorphous to microcrystalline in a time of the order of 0.1ms. The performance of the TFTs showed that a-Si layers formed in the upstream portion of the plasma zone had reasonably good electrical property (field-effect mobility of approximately 2cm2/Vs) despite very high deposition rates around 20nm/s. While μc-Si layers deposited in the downstream portion were very defective, which might come from the insufficient passivation of grain boundaries with a-Si tissues due to a too long gas residence time in the plasma. The precise control of gas residence time by adjusting the length of plasma will be effective for the phase control of Si films with desired quality.

Influence of substrate on the growth of microcrystalline silicon thin films deposited by plasma enhanced chemical vapor deposition

Microcrystalline silicon (μc-Si) thin films are widely used for silicon thin film solar cells, especially in the high performance tandem solar cells which comprise an amorphous silicon junction at the top and a μc-Si junction at the bottom. One of the major factors affecting the photovoltaic properties of μc-Si thin film solar cells of thin films is the quality of the μc-Si thin films. In this work, we investigated the effect of substrates on the crystallization characteristics and growth behaviors of μc-Si thin films grown by the plasma enhanced chemical vapor deposition method (PECVD), and found that substrates have a strong effect on the crystallization characteristics of μc-Si thin films. In addition, the growth rate of μc-Si thin films was also highly influenced by the substrates. Three types of substrates, quartz glass, single crystalline silicon and thermally oxidized single crystalline silicon, were used for growing μc-Si thin films from SiH4/H2 with a flow rate ratio 2:98 at different temperatures. Crystallization characteristics of these μc-Si thin films were studied by Raman scattering and X-ray diffraction techniques.

Determination factor of direction growth in microcrystalline silicon thin film deposition

We have proposed the mechanism of the <110> directional growth of microcrystalline silicon (μc-Si) thin films deposited by PECVD (plasma enhanced chemical vapor deposition) from SiH4 and H2 gas mixture, where dimeric radicals act a key role to form bridge nuclei for the ledge formation on the (110) facet. In order to look further into details of the mechanism, we investigated other important factors that influence the growth of μc-Si in <110> direction in terms of their impact on crystallinity with varying deposition temperature. The enhancement of surface diffusion length of radicals is inferred from the enlargement of the crystalline grain size accompanied with the increase of the deposition temperature. The growth in <110> direction is also promoted as the deposition temperature increases. Therefore, it is suggested that the surface diffusion length of radicals is another key factor that governs the crystalline growth in <110> direction. The growth mechanism of μc-Si thin films in <110> direction is discussed in terms of the relation between the surface diffusion length of monomeric radicals depending on the substrate surface temperature and the average space of bridges depending on the density of dimeric radicals on the growing surface.

Optical emission spectroscopy as a process control tool during plasma enhanced chemical vapor deposition of microcrystalline silicon thin films

The decisive criterion associated with the species emission intensity ratio (Hα/SiH*) which characterizes the crystallinity of microcrystalline silicon (μc-Si) film was found to display an unstable behavior resulting from species concentration variation during μc-Si film growth with optical emission spectroscopy (OES) tool. In this study, a real-time process control system i.e. closed-loop system was developed. It aims to control the species intensity ratio with OES device in a very high frequency (VHF) plasma enhanced chemical vapor deposition reactor, via modulating the VHF power and silane dilution to improve μc-Si film growth for high efficiency a-Si/μc-Si tandem solar cell. The experiment results show that the closed-loop system stabilized the Hα/SiH* intensity ratio within a variation of 5% during the μc-Si film deposition process. Higher growth rate of μc-Si film with the same crystallinity was obtained in the closed loop system which consumed less power and SiH4 gas than in the open loop system, i.e. without process control.

High rate deposition of microcrystalline silicon films using jet-type inductively coupled plasma chemical vapor deposition

Microcrystalline silicon (μc-Si) films were deposited at a high rate and low temperature using jet-type inductively coupled plasma chemical vapor deposition (jet-ICPCVD), and the deposition rate, microstructure and electrical properties of the deposited films were investigated. It was demonstrated that a high deposition rate of over 20 nm/s can be achieved while maintaining high crystallinity and low dark conductivity. The deposition rate is well controlled by regulating the generation rate and transport of growth precursors. High crystallinity of the films results principally from hydrogen-induced chemical annealing. Furthermore, the excellent electrical properties benefit from the low oxygen content and/or low deposition temperature.

An Advanced 405-nm Laser Diode Crystallization Method of a-Si Film for Fabricating Microcrystalline-Si TFTs

This report describes a crystallization method we developed for amorphous (a)-Si film by using 405-nm laser diodes (LDs). The proposed method has been used to fabricate bottom gate (BG) microcrystalline (µc)-Si TFTs for the first time. A µc-Si film with high crystallinity was produced and high-performance BG µc-Si TFTs with a field effect mobility of 3.6cm2/Vs and a current on/off ratio exceeding 108 were successfully demonstrated. To determine the advantages of a 405-nm wavelength, a heat flow simulation was performed with full consideration of light interference effects. Among commercially available solid-state lasers and LDs with wavelengths having relatively high optical absorption coefficients for a-Si, three (405, 445, and 532nm) were used in the simulation for comparison. Results demonstrated that wavelength is a crucial factor for the uniformity, efficiency, and process margin in a-Si crystallization for BG µc-Si TFTs. The 405-nm wavelength had the best simulation results. In addition, the maximum temperature profile on the gate electrode through the simulation well explained the actual crystallinity distributions of the µc-Si films.

Exploration in the Two-Photon Absorption Effect of Silicon Based Films

In this work, the crystallization of different silicon based thin films as the precursor of crystallization was investigated by femto-second laser with 800 nm wavelength. The linear absorption coefficient of a-Si films at that wavelength is quit lower than the other structures of Si-based thin film, which has no related with the incident light energy. However, we found that the crystallization of a-Si films was better than μc-Si films as the precursor. We use Z-scan techniques to prove that the two-photon absorption effect would be responsible for the crystallization. And unlike the linear absorption, the two-photon absorption effect is correlated with the incident light energy, as well as the micro-structure of the silicon based film. At the end of the paper, the crystallization by laser with wavelength longer than the absorption limit was discussed.

High Performance Micro-Crystalline Silicon TFT Using Indirect Thermal Crystallization Technique

High performance n-channel micro-crystalline silicon (μc-Si) TFT was fabricated by 5 mask process. Indirect thermal crystallization (ITC) method was applied to crystallize amorphous silicon (a-Si) thin films. ITC technique adopted infrared laser with 808 nm wavelength and heat transition layer of Molybdenum on a-Si. We analyzed the crystallinity of a-Si with the width of crystallization area which is marked by semiconductor laser beam on the surface of a-Si films. Crystallinity of μc-Si films was calculated by Raman spectra and it shows correlation of field effect mobility with crystallinity fraction. In this study, we presented field effect mobility of 6.2 cm2 /Vs and sub-threshold slope of 0.41 V/dec. And bias temperature stress characteristics confirmed the reasonable reliability of device employing ITC technique compare to that of a-Si TFT.

SILICON THIN FILMS PREPARED BY PECVD USING VHF POWER IN A CIRCULAR-PARALLEL-PLATE PLASMA REACTOR

The deposition rate of μc-Si films was investigated for four excitation frequencies 30, 40, 60, 70 and 80 MHz with other deposition parameters fixed. Deposition rate increases with the increasing of excitation frequency, while Raman crystallinity behaves more complicated. With the optimization of deposition parameters, p-i-n solar cells at an initial efficiency of 5.41% were fabricated. With the increasing of plasma excitation frequency, the non-uniformity of these thin films increases. To better understand the cause of the non-uniformity of these films, a numerical simulation was carried out. The numerical results generally followed the experimental data. It turned out that the standing waves and the evanescent wave guide modes on the electrode surface played an important role. In order to achieve highly uniform thin films, a triode-electrode was employed together with a pulsed power source. We found that with a proper choice of pulse frequency and DC voltage applied to the mesh, non-uniformity is less than 8% for films deposited on 10×10 cm 2 substrates. Simulations were also applied to analyze the results.

Effect of Ar in the source gas on the microstructure and optoelectronic properties of microcrystalline silicon films deposited by plasma-enhanced CVD

Hydrogenated microcrystalline silicon films have been prepared by plasma-enhanced chemical vapor deposition technique using silane diluted in H 2 or H 2 + Ar. The microstructures for silicon films have been evaluated by Raman scattering spectroscopy, X-ray diffraction and Fourier-transform infrared spectroscopy. Optical characterization has been done by UV–vis spectroscopy. It is found that the addition of Ar in diluent gases efficiently improves the deposition rate and crystallinity due to an enhanced dissociation of the source gas and the energy of deexcitation of Ar* released within the growth zone. Meanwhile, the enhanced crystallinity and the reducing of hydrogen ion bombardment with increasing Ar dilution lead to the polymerization and also a bad passivation of the hydrogen which cause the widening of the optical gap and increase of defect states in the μc-Si films. The absorption coefficient and dark conductivity are found to decrease basically with increasing Ar dilution corresponding to the widening optical gap and more defects. That the activation energy increases with increasing Ar dilution or decreasing hydrogen dilution is due to the fact that more defect states lead to a pulling down of the Fermi level.

Enhanced Nucleation of Microcrystalline Silicon Thin Films Deposited by Inductively Coupled Plasma Chemical Vapor Deposition with Low-Frequency Pulse Substrate Bias

Microcrystalline silicon (µc-Si) films were deposited by inductively coupled plasma chemical vapor deposition with a low-frequency and low-duty pulse substrate bias (PSB). The crystallinity of the films was significantly improved by the PSB. In the case of the low-frequency and low-duty PSB, the duty ratio affected the crystallinity more than the negative peak voltage. Cross-sectional transmission electron microscopy measurements revealed that the nucleation density at the µc-Si/glass interface was increased by the PSB. This technique will be useful in fabricating high-performance bottom-gate µc-Si thin-film transistors for large-area electronics.

Development of microcrystalline silicon thin films with high deposition rate (over 10nm/s) using VHF hollow electrode enhanced glow plasma

Abstract Microcrystalline silicon (μc-Si) thin films with high deposition rate have been fabricated by VHF hollow electrode enhanced glow plasma transportation (VHF–HEEPT). Plasma observation is performed in the discharge chamber and the deposition chamber, and plasma emission spectroscopy is performed in the deposition chamber. The results of the plasma emission spectroscopy indicate that VHF–HEEPT yields a plasma with lower electron temperature compared to RF–HEEPT and conventional capacitively-coupled VHF plasmas. The structural and electrical properties of μc-Si thin films are estimated as a function of the SiH 4 /H 2 ratio. The thin films made by VHF–HEEPT are microcrystalline and have a high deposition rate (12.5 nm/s), while thin films made by RF–HEEPT become amorphous for deposition rates over 10 nm/s.

Material properties of microcrystalline silicon for solar cell application

The paper reviews the material requirements of microcrystalline silicon (μc-Si) in terms of the device operation and configuration for thin film solar cells and thin film transistors (TFTs). We investigated the material properties of μc-Si films deposited by using 13.56 MHz plasma-enhanced chemical vapor deposition (PECVD) from a conventional H 2 dilution in SiH 4. Two types of intrinsic μc-Si films deposited at the high pressure narrow electrode gap and the low pressure wide electrode gap were studied for the solar cell absorption layers. The material properties were characterized using dark conductivity, Raman spectroscopy, and transmission electron microscope (TEM) measurements. The μc-Si quality and solar cell performance were mainly determined by microstructure characteristics. Solar cells adopting the optimized μc-Si film demonstrated high stability with no significant changes in solar cell performance after air exposure for six months and subsequent illumination for over 300 h. The results can be explained that low ion bombardment and high atomic hydrogen density under the PECVD condition of the high pressure narrow electrode gap produce high-quality μc-Si films for solar cell application.