Plasma processing of microcrystalline silicon films : filling in the gaps

Hydrogenated microcrystalline silicon (µc-Si:H) is a mixed-phase material consisting of crystalline silicon grains, hydrogenated amorphous silicon (a-Si:H) tissue, and voids. Microcrystalline silicon is extensively used as absorber layer in thin-film tandem solar cells, combining the advantages of a low (indirect) band gap (1.1 eV), which results in an enhanced absorption of red and (near) infrared light, with an improved stability under light exposure (reduced Staebler–Wronski effect). However, due to the indirect nature of the band gap, relatively thick (1–2 µm) µc-Si:H films are necessary to achieve an efficient absorption of red and (near) infrared light, even when light trapping concepts are applied. Therefore, from a cost-perspective point of view, high growth rates (>1 nm/s) are required, preferably in combination with large-area (roll-to-roll) processing. The most common, and so far most successful, deposition technique is the capacitively-coupled plasma (CCP) in parallel plate configuration using radio or very high excitation frequencies (RF or VHF, respectively) and highly hydrogen diluted hydrogen and silane gas mixtures. Approaches to increase the growth rate include an increase of the plasma power, moving from a low-pressure to a high-pressure depletion (LPD to HPD, respectively) regime, and/or by increasing the excitation frequency from 13.56 MHz to 27–300 MHz. The combination of HPD-VHF has resulted in high deposition rates (2-3 nm/s) while maintaining high solar cell efficiencies (7-8%). In this work, the use of an ultra-fast (2-20 nm/s) deposition technique, i.e. the expanding thermal plasma, has been explored for the deposition of µc-Si:H films. Characteristic for ETP-grown µc-Si:H films is the lack of a sufficient amount of a-Si:H tissue, which is necessary to passivate the grain boundaries and fill the intergranular space, resulting in a network of (inter-connected) cracks and voids. As a consequence, the µc-Si:H films are prone to post-deposition oxidation, resulting in low solar cells efficiencies (<2%). The post-deposition oxidation has been monitored by means of Fourier transform infrared (FTIR) spectroscopy over a period of 8 months. This study revealed a two-timescale oxidation: on short timescales (<3 months) the crystalline silicon grain boundaries oxidize, on longer timescales the oxidation involves also the a-Si:H tissue. This indicates that in order to prevent post-deposition oxidation, it is not sufficient to fill the intergranular space, but that the a-Si:H tissue needs to be of sufficient quality, i.e. dense and not susceptible for post-deposition oxidation. One process that could be responsible for the insufficient amount of a-Si:H tissue, is hydrogen-induced etching of a-Si:H tissue. Atomic hydrogen is, under µc-Si:H growth conditions, abundant in the plasma, and is known to preferentially etch a-Si:H over crystalline silicon (c-Si). In addition, the interaction of atomic hydrogen with the (growing) film can result in the formation of an hydrogen-rich sub-surface layer, caused by the insertion of atomic hydrogen into strained Si-Si bonds, which possibly explains the porous quality of the a-Si:H tissue. Monitoring the etch rate of a-Si:H films during Ar/H2 plasma exposure by real time spectroscopic ellipsometry showed that the hydrogen-induced etch rate was at least one order of magnitude lower than typical deposition rates. In addition, FTIR spectroscopy revealed that insertion of atomic H in the sub-surface layer (top ~30 nm) during Ar/H2 plasma exposure did not result in an increased porosity. These results suggest that the interaction of atomic hydrogen with the growing film is not responsible for the insufficient amount of (dense) a-Si:H tissue. The fact that the interaction of atomic hydrogen is not responsible for the poor material properties of ETP-grown µc-Si:H, the question "what mechanism is then responsible?" arises. To address this question the plasma chemistry and the resulting growth mechanism of ETP is compared to CCP, which so far is the only technique with which solar-grade µc-Si:H is obtained. One difference between the two techniques is the absence of an ion bombardment effect in ETP. In CCP the HPD and the use of VHF are employed to suppress a (potentially uncontrolled) ion bombardment effect, hypothesized to be responsible for an amorphization of the crystalline growth and defect incorporation. However, there is always some form of ion bombardment present. The extent to which ions contribute to the growth depends on the ion flux, the ion energy, and the chemical nature of the ion. Under HPD-VHF conditions SinHm+ is identified as the dominant ion in H2/SiH4 plasmas, but no direct ion energy and ion flux measurements under HPD conditions have been reported so far. Therefore, the ion energy and flux in a CCP reactor have been studied. For this purpose, a capacitively-coupled plasma reactor in parallel plate configuration has been designed and built, in close collaboration with the Institute of Photovoltaics at Forschungszentrum Jülich (Germany). This reactor has been especially designed for the implementation of plasma and (in situ) film diagnostics. Under solar-grade µc-Si:H deposition conditions the contribution of ions to the film growth has been studied by means of a capacitive probe. The ion to Si deposition flux ratio was found to be large, ~0.30. However, since the ion energy is rather low

Plasma processing of microcrystalline silicon films : filling in the gaps

Hydrogenated microcrystalline silicon (µc-Si:H) is a mixed-phase material consisting of crystalline silicon grains, hydrogenated amorphous silicon (a-Si:H) tissue, and voids. Microcrystalline silicon is extensively used as absorber layer in thin-film tandem solar cells, combining the advantages of a low (indirect) band gap (1.1 eV), which results in an enhanced absorption of red and (near) infrared light, with an improved stability under light exposure (reduced Staebler–Wronski effect). However, due to the indirect nature of the band gap, relatively thick (1–2 µm) µc-Si:H films are necessary to achieve an efficient absorption of red and (near) infrared light, even when light trapping concepts are applied. Therefore, from a cost-perspective point of view, high growth rates (>1 nm/s) are required, preferably in combination with large-area (roll-to-roll) processing. The most common, and so far most successful, deposition technique is the capacitively-coupled plasma (CCP) in parallel plate configuration using radio or very high excitation frequencies (RF or VHF, respectively) and highly hydrogen diluted hydrogen and silane gas mixtures. Approaches to increase the growth rate include an increase of the plasma power, moving from a low-pressure to a high-pressure depletion (LPD to HPD, respectively) regime, and/or by increasing the excitation frequency from 13.56 MHz to 27–300 MHz. The combination of HPD-VHF has resulted in high deposition rates (2-3 nm/s) while maintaining high solar cell efficiencies (7-8%). In this work, the use of an ultra-fast (2-20 nm/s) deposition technique, i.e. the expanding thermal plasma, has been explored for the deposition of µc-Si:H films. Characteristic for ETP-grown µc-Si:H films is the lack of a sufficient amount of a-Si:H tissue, which is necessary to passivate the grain boundaries and fill the intergranular space, resulting in a network of (inter-connected) cracks and voids. As a consequence, the µc-Si:H films are prone to post-deposition oxidation, resulting in low solar cells efficiencies (<2%). The post-deposition oxidation has been monitored by means of Fourier transform infrared (FTIR) spectroscopy over a period of 8 months. This study revealed a two-timescale oxidation: on short timescales (<3 months) the crystalline silicon grain boundaries oxidize, on longer timescales the oxidation involves also the a-Si:H tissue. This indicates that in order to prevent post-deposition oxidation, it is not sufficient to fill the intergranular space, but that the a-Si:H tissue needs to be of sufficient quality, i.e. dense and not susceptible for post-deposition oxidation. One process that could be responsible for the insufficient amount of a-Si:H tissue, is hydrogen-induced etching of a-Si:H tissue. Atomic hydrogen is, under µc-Si:H growth conditions, abundant in the plasma, and is known to preferentially etch a-Si:H over crystalline silicon (c-Si). In addition, the interaction of atomic hydrogen with the (growing) film can result in the formation of an hydrogen-rich sub-surface layer, caused by the insertion of atomic hydrogen into strained Si-Si bonds, which possibly explains the porous quality of the a-Si:H tissue. Monitoring the etch rate of a-Si:H films during Ar/H2 plasma exposure by real time spectroscopic ellipsometry showed that the hydrogen-induced etch rate was at least one order of magnitude lower than typical deposition rates. In addition, FTIR spectroscopy revealed that insertion of atomic H in the sub-surface layer (top ~30 nm) during Ar/H2 plasma exposure did not result in an increased porosity. These results suggest that the interaction of atomic hydrogen with the growing film is not responsible for the insufficient amount of (dense) a-Si:H tissue. The fact that the interaction of atomic hydrogen is not responsible for the poor material properties of ETP-grown µc-Si:H, the question "what mechanism is then responsible?" arises. To address this question the plasma chemistry and the resulting growth mechanism of ETP is compared to CCP, which so far is the only technique with which solar-grade µc-Si:H is obtained. One difference between the two techniques is the absence of an ion bombardment effect in ETP. In CCP the HPD and the use of VHF are employed to suppress a (potentially uncontrolled) ion bombardment effect, hypothesized to be responsible for an amorphization of the crystalline growth and defect incorporation. However, there is always some form of ion bombardment present. The extent to which ions contribute to the growth depends on the ion flux, the ion energy, and the chemical nature of the ion. Under HPD-VHF conditions SinHm+ is identified as the dominant ion in H2/SiH4 plasmas, but no direct ion energy and ion flux measurements under HPD conditions have been reported so far. Therefore, the ion energy and flux in a CCP reactor have been studied. For this purpose, a capacitively-coupled plasma reactor in parallel plate configuration has been designed and built, in close collaboration with the Institute of Photovoltaics at Forschungszentrum Jülich (Germany). This reactor has been especially designed for the implementation of plasma and (in situ) film diagnostics. Under solar-grade µc-Si:H deposition conditions the contribution of ions to the film growth has been studied by means of a capacitive probe. The ion to Si deposition flux ratio was found to be large, ~0.30. However, since the ion energy is rather low

On the Origin of the OER Activity of Ultrathin Manganese Oxide Films

There is an urgent need for cheap, stable, and abundant catalyst materials for photoelectrochemical water splitting. Manganese oxide is an interesting candidate as an oxygen evolution reaction OER catalyst, but the minimum thickness above which MnOx thin films become OER active has not yet been established. In this work, ultrathin lt;10 nm manganese oxide films are grown on silicon by atomic layer deposition to study the origin of OER activity under alkaline conditions. We found that MnOx films thinner than 1.5 nm are not OER active. X ray photoelectron spectroscopy shows that this is due to electrostatic catalyst support interactions that prevent the electrochemical oxidation of the manganese ions close to the interface with the support, while in thicker films, MnIII and MnIV oxide layers appear as OER active catalysts after oxidation and electrochemical treatment. From our investigations, it can be concluded that one MnIII,IV O monolayer is sufficient to establish oxygen evolution under alkaline conditions. The results of this study provide important new design criteria for ultrathin manganese oxide oxygen evolution catalyst

Remote plasma deposition of microcrystalline silicon thin-films: Film structure and the role of atomic hydrogen

Microcrystalline silicon films grown in an expanding thermal plasma, i.e. in the absence of ion bombardment, are found to be porous and rich in nano-sized voids. By carrying out an extensive investigation on the material quality of films deposited in the amorphous-to-microcrystalline transition regime, on the microcrystalline silicon growth development, and on the influence of the substrate temperature, it is concluded that the inferior material quality is related to the lack of a sufficient amount of amorphous silicon tissue. As possible cause for the insufficient amount of amorphous silicon tissue, the interaction of atomic hydrogen with amorphous silicon films has been studied in order to highlight a possible competition between film growth and H-induced etching of amorphous silicon, and between film growth and H-induced surface/film modification. The etch rates obtained are too low to compete with film growth. Furthermore, atomic H cannot be considered responsible for the poor quality of amorphous tissue present in the microcrystalline silicon films, as the H up-take mainly takes place in divacancies. These results suggest that ion bombardment may be a necessary condition to provide good quality microcrystalline silicon films. (C) 2011 Elsevier B.V. All rights reserved

On the role of atomic hydrogen during microcrystalline silicon thin-film deposition

The expanding thermal plasma, which is a promising technique for microcrystalline silicon (μc-Si:H) thin-film deposition because of its high growth rates over large areas, witnesses, so far, specific challenges in the deposition of device-grade μc-Si:H material. The μc-Si:H films show post-deposition oxidation, caused by an insufficient amount of (dense) amorphous tissue between the grains, resulting in low solar cell efficiencies. Atomic hydrogen, which is crucial for the formation of μc-Si:H films, is hypothesized to be responsible for this lack of amorphous tissue because of its ability to etch amorphous silicon (a-Si:H) by insertion in Si-Si bonds. Therefore, we studied the interaction of atomic H with thin a-Si:H films. Results show that etching does not compete with film growth, as etch rates are one order of magnitude lower than deposition rates. Furthermore, atomic H cannot be held responsible for the poor quality of amorphous tissue present in ETP-grown μc-Si:H, as the H up-take takes mainly place in divacancies.</p

On the oxidation mechanism of microcrystalline silicon thin films studied by Fourier transform infrared spectroscopy

Insight into the oxidation mechanism of microcrystalline silicon thin films has been obtained by means of Fourier transform infrared spectroscopy. The films were deposited by using the expanding thermal plasma and their oxidation upon air exposure was followed in time. Transmission spectra were recorded directly after deposition and at regular intervals up to 8 months after deposition. The interpretation of the spectra is focused on the Si–Hx stretching (2000–2100 cm-1), Si–O–Si (1000–1200 cm-1), and OxSi–Hy modes (2130–2250 cm-1). A short time scale (<3 months) oxidation of the crystalline grain boundaries is observed, while at longer time scales, the oxidation of the amorphous tissue and the formation of O–H groups on the grain boundary surfaces play a role. The implications of this study on the quality of microcrystalline silicon exhibiting no post-deposition oxidation are discussed: it is not sufficient to merely passivate the surface of the crystalline grains and fill the gap between the grains with amorphous silicon. Instead, the quality of the amorphous silicon tissue should also be taken into account, since this oxidation can affect the passivating properties of the amorphous tissue on the surface of the crystalline silicon grains

Operando Attenuated Total Reflection Fourier-transform Infrared (ATR-FTIR) Spectroscopy for Water Splitting

Operando attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy is discussed in this paper for water splitting application. The first part of the paper focuses on the discussion of the opportunities and challenges of this method for the characterization of the solid-liquid interface in water splitting. The second part of the paper focuses on recent results and future perspectives: We present stable and robust operando ATR-FTIR measurements using low temperature processing of hematite and a set-up where the functional thin film is integrated on the ATR crystal. We find increased absorbance as a function of applied potential at wavenumbers of 1000 cm-1 to 900 cm-1 and relate this to changes in the surface species during water oxidation. We argue that this approach has the potential to be developed to a routine characterization method for the characterization of interfaces in water splitting. Such ATR-FTIR data is of crucial importance for the validation of models in microkinetic modeling. We show some recent results of microkinetic modeling of the hematite-electrolyte interface and explain how a combination of operando ATR-FTIR measurements and microkinetic modeling enables the identification of the reaction mechanism in water splitting. We discuss how this combined approach will enable designing of tailored catalysts and accelerating their development in the future