Amorphous silicon films with high deposition rate prepared using argon and hydrogen diluted silane for stable solar cells

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Abstract

Hydrogenated amorphous silicon films with high deposition rate (4–5 Å/s) and reduced Staebler–Wronski effect are prepared using a mixture of silane (SiH4), hydrogen and argon. The films show an improvement in short and medium range order. The structural, transport and stability studies on the films are done using X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman scattering studies, electrical conductivity and diffusion length measurement. Presence of both atomic hydrogen and Ar* in the plasma causes breaking of weak Sisingle bondSi bonds and subsequent reconstruction of strong bonds resulting in improvement of short and medium range order. The improved structural order enhances the stability of these films against light soaking. High deposition rate is due to the lesser etching of growing surface compared to the case of only hydrogen diluted silane.

Introduction

Hydrogenated amorphous silicon (a-Si:H) has drawn considerable attention worldwide because of its potential use in fabricating large-area, low-cost devices such as thin film solar cells, thin film transistors, radiation detectors, liquid crystal displays, etc. [1]. Though the opto-electronic properties of a-Si:H films have improved considerably over the last two decades, complete eradication of light-induced degradation, which is better known as Staebler–Wronski effect [2], has not been achieved yet and still remains a major bottleneck in making stable photovoltaic and other devices. It has been observed that dilution of precursor gas silane (SiH4) by hydrogen or argon causes improvement in optoelectronic properties and stability of the films. However, dilution by hydrogen reduces the deposition rate considerably [3], whereas argon dilution causes the columnar growth [4], [5]. Very high hydrogen and argon dilution results in the poly-crystalline/microcrystalline films [6], [7], [8], [9]. The deposition rate can be increased with hydrogen dilution alone using very high pressure and very high power during deposition; however, these films are highly microcrystalline [10].

In this paper, we report structural, transport and stability studies on amorphous silicon films prepared near the onset of microcrystallinity using a mixture of silane, hydrogen and argon as precursor gas. The deposition rate of the films is in the range of 4–5 Å/s for moderate argon dilution and decreased to ∼2 Å/s for very high dilution. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies on the films suggest these to be amorphous in nature, whereas Raman scattering studies suggest the presence of very small nanocrystallites (3–5 nm) embedded in amorphous network of the films and improved short and medium range order. The dark (σd) and photoconductivity (σph) values favor amorphous nature with smaller defect density. These films have shown less degradation after long-term light soaking and the light-induced changes could be annealed at much lower temperature (<150 °C).

Section snippets

Experimental

Thin films of a-Si:H (about 160–535 nm) are deposited on Corning 7059 glass substrates using hydrogen diluted silane (5% silane in hydrogen) and argon in a load lock-based PECVD chamber. The base pressure of the chamber has been better than 10−7 Torr. Prior to deposition, the argon flow rate is fixed so that a constant pressure of 0.355 Torr is achieved in the chamber without any silane flow. Silane flow rate is then varied from 6.4 to 40 sccm to get different argon dilution ratio (R). The

Results

The XRD studies, done at grazing angle of incidence, reveal the amorphous nature of all the samples. SEM studies also show smooth conchoidal surface morphology. No columnar growth is observed in these films. The PL emission studies (excitation wavelengths 300–400 nm) give weak intensity broad peaks for all the films with center of the peak ranging from 550 to 670 nm. The thickness of the films is found to be around 460–540 nm, which gives the high deposition rate of about 4–5 Å/s for argon dilution

Discussion

We have observed a higher deposition rate for the films in comparison to that of the regular films deposited by only hydrogen dilution [9]. In case of heavily hydrogen diluted silane, main film forming precursor is SiH3, which is a very less reactive species having a long diffusion length. During the growth, SiH3 does not find a Si site on the surface well covered by hydrogen, which passivates all the bonds on the surface, and hence traverses a long distance before it finds an un-terminated

Conclusion

In this paper, we have reported the high deposition rate (∼4–5 Å/s) and improved transport properties and stability against exposure to light of a-Si:H films prepared by argon and hydrogen dilution of silane. Presence of hydrogen prevents the columnar growth, which is observed in the case of films prepared by only argon-diluted silane. In our case, both Ar* and atomic hydrogen help in the efficient dissociation of silane and in the reconstruction of strong Sisingle bondSi bonds thus improving the medium

Acknowledgment

The work reported in this paper is supported by a grant from CSIR, New Delhi, India.

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    Its growth conditions, in terms of the gas dilution, dopant concentration, and other factors that dictate the extent of disorder in the as-grown films play an important role in determining the mechanisms of electronic conduction. The focus of recent studies in amorphous silicon (a-Si) thin films has been on the effect of argon dilution during growth [1] for solar cell applications [2–7] since it was shown that argon dilution provided better stability and reduced degradation of thin films resulting from the Staebler-Wronski effect. [8] Further research explored nanocrystalline (nc) inclusions in a-Si resulting from high argon dilution [9–12] and the properties of mixed-phase amorphous/nanocrystalline (a+nc) hydrogenated amorphous silicon (a-Si:H). [13,14]

  • Review: Progress in solar cells from hydrogenated amorphous silicon

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    Most research laboratories and industry use only H2 and SiH4 as gas precursors for intrinsic a-Si:H deposition by PECVD. However, many deposition regimes have been explored adding the inert gases Ar or He [110–112] that can play different roles depending on the process conditions: they can densify layers or create defects through ion bombardment, help the nucleation (enhancing the surface mobility of adatoms), etch already deposited material, dilute the plasma similar to H2, provide access to deposition conditions that would not be accessible according to the Paschen curve of the residual gas mixture, or simply enhance the safety by diluting SiH4 which is auto-flammable in high concentrations. Another approach is the (partial) substitution of SiH4 with Si2H6 [113,114], with SiH2Cl2 [115,116], or with SiF4 that led to promising results in μc-Si:H solar cells [117,118].

  • Effects of H<inf>2</inf> and Ar flow rates on the deposition of hydrogenated silicon thin films by an inductive coupled plasma-chemical vapor deposition system

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    For instance, the crystallinity drops as the silane concentration increases (SiH4/(SiH4 + H2) = 0.71% → 1.04%) [5]; Considerably reduced deposition rate was observed as Ar dilution ratio increased (Ar/(Ar + SiH4) = 60 → 97) with a slightly nano-size crystal growth [6]; Both the deposition rate and crystallinity were increased by the increased Ar and decreased hydrogen flow rates (Ar/(Ar + H2) = 0 → 0.65, Ar = 0 → 13 sccm, H2 = 20 → 7 sccm) [7]; The deposition rate was enhanced but crystallinity obviously diminished as SiH4/H2 (more H2) increased from 3% to 7.5%. Meanwhile, when SiH4/H2 = 5% and Ar raised from 0 to 40 sccm the crystallinity enhanced but deposition rate increased at first and then lowered down [8]; The deposition rate went down as the Ar/SiH4 ratio increased (Ar/SiH4 = 100 → 630, Ar = 200 sccm, SiH4 varied) while the crystallinity enhanced and then decreased [9]; hydrogen dilution (H2/SiH4 = 0 → 50) usually caused higher crystallinity [10,11], and higher deposition rate (SiH4/(H2 + SiH4) = 0.02 → 0.2, SiH4 = 10 sccm) [12]. These studies, among many others reported recently, indicate that higher SiH4 flow rate can expedite the deposition of hydrogenated silicon films and hence the higher deposition rate, the lower crystallinity of the films.

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