Deposition of Thin-Film Silicon for Photovoltaics: Use of VHF-GD and OES

Deposition rates over 10 Å/s can be obtained for device-grade microcrystalline silicon with the VHF-GD (Very High Frequency Glow Discharge) method, as applied to hydrogendiluted silane plasmas. The morphological phase transition from amorphous to microcrystalline silicon can be controlled by varying e.g. the applied VHF-power or the dilution level. The glow of the plasma associated with this morphological phase transition is monitored by optical emission spectroscopy (OES). Thereby, we find that the decisive criteria for μc-Si:H growth is the OES-ratio between the Hα line (atomic hydrogen line at 656 nm) and the SiH* line (at 414 nm); in our case, as soon as this ratio becomes lower than 1.7 one obtains microcrystalline growth. Introduction: a-Si:H and μc-Si:H for solar cells The search for new device concepts, for suitable photosensitive materials and for economically attractive, low-temperature processes has become a priority in the field of photovoltaic solar cells. However, of all the possible materials, only silicon holds at present the promise of providing solar cells that can be used on a large scale. In fact, silicon is the only non-toxic and widely available photovoltaic material, that has already proved its effectiveness in commercial solar cells. Silicon solar cells based, however, on the use of wafers clearly necessitate the investment of too large quantities of raw material (ultra pure silicon), as well as of tremendous amounts of production energy. They therefore do not constitute a viable solution for the future large-scale application of photovoltaics. One has therefore to concentrate efforts on thin-film silicon. Here, amorphous silicon thin films (a-Si:H) have been the first option. A new option, as pioneered by our institute, is the use of plasma-deposited microcrystalline silicon thin films (μc-Si:H). The viability of using hydrogenated microcrystalline silicon (μc-Si:H) deposited within a silane-hydrogen plasma as an active photovoltaic material has been demonstrated by fabricating a 8.5 % efficient single-junction thin-film silicon solar cell [1]. This result became possible mainly thanks to three factors: First, an efficient passivation of grain boundaries and other defects by atomic hydrogen due to the plasma. Secondly, the use of a "soft" plasma deposition technique, such as the VHF-GD that avoids high energy bombardment, as discussed elsewhere [2]. Thirdly, the use of p-i-n and n-i-p diodes with the intrinsic (i) layer as photovoltaically active layer; thereby care is taken to keep the Fermi-level of this (i) layer at midgap, by careful exclusion (or alternatively compensation) of impurities with a dopant character, such as oxygen [3]. μc-Si:H has the same gap as crystalline silicon (1.1 eV) but shows an enhanced apparent optical absorption. Recent studies suggest that this effect is due to scattering on the surface and in the bulk [4]. It is therefore the ideal partner for a-Si:H with its gap of 1.7 eV in a double-junction tandem cell. We have called such a μc-Si:H/a-Si:H tandem cell the micromorph cell and have reached sofar a stable cell efficiency of 12 % [5]. For the fabrication of a micromorph tandem cell we need an a-Si:H absorber for the top cell and a μc-Si:H layer for the bottom cell. Hydrogen-diluted silane plasmas lead both to the deposition of high quality μc-Si:H as well as to the deposition of more stable a-Si:H. Several process parameters can influence the morphological transition from a-Si:H to μc-Si:H, as the applied dilution, the VHF-power, the temperature, the excitation frequency and the process pressure. In order to get more insight in hydrogen-diluted silane plasmas we used OES as a process monitor. OES is a quite simple and efficient in-situ characterisation technique which does not disturb the plasma. OES for hydrogen-diluted silane plasmas OES consists in our case of decomposing the light emitted from the plasma with a monochromator and spectrally detecting the intensity with a photodiode at the exit slit (see Fig. 1). Comparisons to other systems can be only done on a relative basis, since no absolute calibration of the spectra is done. Basically, we performed OES for two selected series deposited under VHF-GD conditions in the range 110-130 MHz where the morphological transition (a-Si:H/μc-Si:H) has been controlled by varying one parameter alone. In the first series, the morphology has been controlled by the applied dilution of silane alone; the dilutions (SiH4/(SiH4+H2)) were set to 1.25 / 2.5 / 5 and 7.5 %. All deposited layers are μc-Si:H except the one deposited at 7.5 % which showed to be amorphous [6, 7]. Here the VHF-power was set to 6 W. In the second series we fixed the dilution at 7.5 % and systematically increased the applied VHF-power from 20 to 70 W. All layers are μc-Si:H except the one deposited at 20 W is amorphous [8, 9]. Fig. 1 shows a typical OES as obtained with our set-up, for a deposition condition leading to μc-Si:H growth (2.5 % dilution, 6 W VHF-power). The spectral lines of main interest which will be considered hereafter are situated at 414 and 656 nm. The former line is associated to the SiH* radical and the latter to atomic hydrogen Hα. These lines of our interest are marked in Fig. 1 and we will further analyse how they evolve by changing the process parameters.