Effect of hydrogen dilution on the silicon cluster volume fraction of a hydrogenated amorphous silicon film prepared using plasma-enhanced chemical vapor deposition
Introduction
Hydrogenated amorphous silicon (a-Si:H) thin film, commonly fabricated using plasma-enhanced chemical vapor deposition (PECVD), is a fascinating material for use in thin-film microelectronics such as thin-film transistors, panel displays, and thin film solar cells [[1], [2], [3]]. In particular, the material processability including the ease of tuning the energy band gap with the substrate temperature, doping with various dopant gases, and the capability of stacking layers without damage make the material favorable for application to hetero junction thin-film solar cells [[4], [5], [6]]. However, the device performance of the solar cell is severely limited by light-induced degradation, referred to as the Staebler–Wronski effect (SWE) [7]. It has been reported to be related to the increase in dangling bonds resulting from a breaking of weak Si–Si bonds under illumination, leading to a reduction in photo and dark conductivity [[8], [9], [10]]. Thus, many research efforts have been made to develop a method for suppressing the dangling bond in the a-Si:H film. The most commonly suggested technique is to deposit films under a hydrogen dilution condition. Indeed, many research groups have reported that dilution of hydrogen with silane gases during film deposition improves optical and electrical properties by passivating dangling bond defects on the film growth surface [[11], [12], [13]].
Since Guha et al. first demonstrated the improvement in film stability after light exposure in films deposited with hydrogen dilution, the technique has been widely adopted to obtain high-quality a-Si:H films [14]. It is believed that the film growth occurs by physisorption of SiH3 species onto the hydrogen-terminated a-Si:H surface, creating a Si-dangling bond via abstraction of hydrogen from the Si–H bond. The subsequent adsorption of SiH3 onto the previously formed Si-dangling bond contributes to film growth by forming a Si–Si bonding structure. Here, the excess hydrogen plays an important role by providing enhanced hydrogen coverage on the growing film surface, in which impinging SiH3 can diffuse to a greater distance to find more energetically favorable sites because of the fully hydrogen-terminated Si:H surface [15]. This means that a higher hydrogen dilution promotes the production of Si–Si bonds, resulting in a morphological transition of the amorphous silicon network to a more ordered structure. Indeed, high-quality a-Si:H film is observed immediately below the edge of the amorphous to microcrystalline transition [16]. Several research groups have suggested the role of hydrogen acting as an etchant [17,18]. It is well known that incorporated hydrogens not only passivate existing unterminated dangling bonds but also break disordered weak Si–Si bonds from the silicon network. However, some of the remaining weak Si–Si bonds near the Si–H bond are broken under illumination as a result of non-radiative electron-hole pair recombination in the weak Si–Si bond [[19], [20], [21]]. Furthermore, it has been reported that in addition to the direct effect of hydrogen on the growth film surface, hydrogen dilution affects the gas-phase reaction in plasma by reducing the electron temperature associated with the polymerization of highly reactive radical species such as SiH2 species [[22], [23], [24]].
Polymerization of reactive silane radicals is associated with the electron temperature of the bulk plasma because the highly reactive SiH2 radicals, known as a trigger species of polymerization, can be easily produced at a high electron temperature (Te > 9.47 eV) [25]. The produced SiH2 radicals subsequently react with SinH2n+2 species with a high probability to form polymerized species [[26], [27], [28]]. Notably, several research groups have reported that hydrogen dilution leads to a decreased electron temperature, resulting in a stable a-Si:H film with low Si cluster inclusion [29,30]. In this regard, Si cluster incorporation is believed to increase the Si–H2 bonding configurations in silicon networks, which is reported to be closely related to light-induced degradation [[31], [32], [33]]. However, the correlation of hydrogen dilution and silicon cluster volume fraction (Vf) in the resulting films has not yet been fully clarified. Thus, in this study, we assessed the effect of hydrogen dilution on Vf by varying the hydrogen dilution ratio (Rh) and comparing it to that of the pure silane discharge. The electron temperature is discussed by comparing Si*(1P0, 288.2 nm) to SiH*(A2Δ, 412.7 nm) under hydrogen dilution and pure silane discharge regimes. The plasma characteristics, obtained using optical emission spectroscopy (OES), and the deposition rates, measured using quartz crystal microbalances (QCMs), are described. Based on the experimental results, the characteristics of the Si cluster volume fraction in the hydrogen dilution plasma regime are discussed.
Section snippets
Experimental method
The experiments were conducted using a multi-hollow plasma discharge reactor, as shown in Fig. 1. Because of the radio-frequency (RF) glow discharge, hydrogen dilution plasma was generated in each hole of the multi-hole electrode installed in the central location of the reactor. The gases were injected through a ring-shaped tube line placed around the electrode and flowed upward. Owing to the transportation of particles in the downstream direction, the diffusion of the particles in the upstream
Results and discussion
In our previous research, the measurement of an Si cluster Vf in a film growth was expressed as the ratio of the deposition rate with clusters (DRw/clusters) to that without clusters (DRw/o clusters) [37]. In particular, the measurement of those without clusters was achieved through the cluster-eliminating filter in which incorporation of clusters in the film was suppressed because of their high sticking probability s = ~100%, while a large number of SiH3 radicals can pass through the filter
Conclusion
In summary, we observed variation in plasma emissive species and their contribution to film deposition to determine the effect of hydrogen dilution on Si cluster, Vf, by varying the hydrogen dilution ratio. As the hydrogen dilution ratio increased, the deposition rate with the cluster volume ratio decreased, accompanied by the silicon cluster volume fraction. The effective electron temperature deduced by comparing Si* to SiH* under the hydrogen dilution conditions was lower than that of the
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