Atomic Force Microscopy (AFM)

The surface topography was analysed by Atomic Force Microscopy (AFM), that was performed in the non-contact (tapping) mode on a Vista Burleigh Instruments scanning probe microscope. The surface topography of each layer of the SC series is shown in Fig. 3.13. The samples deposited with SC = 5% to 7.5% have a quite rough surface, whereas the 8% sample surface is quite different. A cauliflower-like structure of the surface is particularly well visible for the 7% and 7.5% samples. The Root Mean Square (RMS) surface roughness corresponding to the AFM scan of Fig. 3.13 is given in Fig. 3.14: it is approximately constant around 25 nm for the samples deposited at SC between 5% and 6.5%, then increases for the 7% and 7.5% samples to reach more than 40 nm, and finally drops to almost zero for the 8% sample. In Fig. 3.14 is also represented the size of the conglomerates (D) emerging at the surface, evaluated from the Fourier transform power spectrum of the AFM scans: D is approximately constant around 500 - 600 nm for the samples 5% to 6.5%, then reaches a maximum at ~800 nm for the 7% sample, and finally rapidly decreases for the 7.5% and 8% samples, down to 150 nm.

Unlike what was observed previously in the case of XRD, Lamb or m0t0, the 7.5% sample

is very different from the 8% sample from the point of view of the surface nature. It should therefore still be considered as a sample in the mc-Si:H / a-Si:H transition region and not as a completely amorphous sample.

Fig. 3.13: AFM scan of the top of the layers of the SC series. The lateral size of the surface feature, or “conglomerate” lateral size, measured from the Fourier transform power spectrum of these AFM scans, is represented in Fig. 3.14.

0 20 40 60 80 100 0 200 400 600 800 1000 5 5.5 6 6.5 7 7.5 8 RMS conglomerate size RMS roughness [nm] Conglomerate size [nm] SC = SiH 4 /(SiH4+H2) [%]

Fig. 3.14: RMS surface roughness measured by AFM and conglomerate lateral size measured from the Fourier transform power spectrum of the AFM scan (see Fig. 3.13) of the SC series of layers.

3.4.3.4. Micro-Raman spectroscopy

Bifacial depth-dependent micro-Raman technique (see § 2.5), i.e. measurements performed with l = 514 nm and l = 633 nm excitation light from the top side and from the bottom side (through the glass), has been applied to the SC series of layers deposited on glass. Then, the Raman crystallinity factor (fc) has been evaluated according to § 2.6.2 from each of

the four Raman spectra obtained for each layer (i.e. for each value of SC).

Fig. 3.15 shows that the different fc-values vary smoothly with SC. The value fc ≈ 0

obtained for bottom illumination at 514 nm indicates that the beginning of growth is, for all values of SC of this series of layers, amorphous over a thickness which is at least equal to the corresponding Raman Collection Depth (RCD, see § 2.5.1), value (≈ 50 nm). The three other curves show a continuous decrease of fc with increasing value of SC. The two values of fc

measured at 633 nm are representative of the bottom (less crystalline) and top (more crystalline) parts of the layer; thereby, the crystallinity within the layers increases as

growth proceeds, in agreement with TEM observations (see Fig. 3.9). The fc-value for top

illumination at 514 nm is very close to that at 633 nm, indicating that the crystallinity does not change very much in the last hundreds of nanometres of growth of these 2 µm thick layers.

We can therefore say that there is a saturation of the crystallinity with the sample thickness, and that the saturated value depends on the deposition conditions (i.e. SC here).

0 0.2 0.4 0.6 0.8 1 5 5.5 6 6.5 7 7.5 8 top, 514nm top, 633nm bottom, 633nm bottom, 514nm Ram an cristallinity factor

f

Fig. 3.15: Raman crystallinity factor (fc) (see § 2.6.2) for the SC series of i-layer

(thickness ≈ 2.2 ± 0.2 µm) deposited on glass. fc is evaluated from the spectra measured by

the bifacial depth-dependant micro-Raman technique (see § 2.5), i.e. measurements were performed with l=514 nm (filled symbols) and l=633 nm (open symbols) excitation light, from the top side (diamonds) and from the bottom side through the glass (circles).

In the section 3.4 hereafter, we will use the Raman crystallinity factor fc for top

illumination at 514 nm (fctop, 514) as the monitoring parameter instead of the SC value used for

i-layer deposition. fctop, 514 is quite high for the samples deposited at SC between 5% and 6%,

and then decreases rapidly with increasing SC. Note that, from a point of view of the Raman crystallinity factor, only the 8% sample is completely amorphous, as already observed from the AFM measurements.

Similar Raman crystallinity factor evaluation has been performed with 514 nm excitation light on some other intrinsic silicon layers deposited at silane concentration value of 2%, 4%, 6% and 8%. Note that the thicknesses of these samples range from 1 to 1.5 µm and are therefore thinner than those of the SC series samples of Fig. 3.15 (ranging from 2 to 2.4 µm). Fig. 3.16 shows that the sample deposited at SC = 2% is as crystalline in the first tens (or hundreds) of nanometres of growth as in the last upper part. On the other hand, the 8% sample is completely amorphous. In between, the 4% and 6% samples make the transition. Note that the fctop, 514 for the 6% sample of Fig. 3.16 is lower than fctop, 514 for the 6% sample of the SC

series (Fig. 3.15). This variation can be explained by the difference in thickness of the samples. Indeed the thicker sample of the SC series (Fig. 3.15) will reach higher crystallinity values in its upper part, as the crystallinity increases as growth proceeds.

0 0.2 0.4 0.6 0.8 1 2 4 6 8 top, 514nm bottom, 514nm Ram an cristallinity factor

f

Fig. 3.16: Raman crystallinity factor (fc) for i-layers deposited at SC = 2%, 4%, 6% and 8%

on glass (thickness ≈ 1.2 ± 0.3 µm).