Chapter 3: Coplanar electrical transport in intrinsic mc-Si:H layers and its
3.4. Electrical transport versus microstructure in a silane concentration (SC)
3.4.4. Relation between electronic transport and microstructure
In the preceding sections (§ 3.4.2 and § 3.4.3), we have studied, in one series of layers deposited on glass at various silane concentrations, the coplanar electronic transport and the evolution of the microstructure, respectively. We will now compare these two aspects. With this goal in mind, we have plotted in Fig. 3.20 electrical and microstructural features as a function of the Raman crystallinity factor measured with 514 nm excitation light on the top of the samples (instead of SC as used in the preceding sections).
From Fig. 3.20 (bottom), one can see that m0t0 is lower in the mc-Si:H samples than in the a-Si:H ones, while remaining constant as long as mc-Si:H material is detected in XRD spectra (i.e. for fc between 0.2 and 0.8). On the other hand, the average nanocrystal size increases
roughly linearly with fc. This demonstrates, that at least for this particular series of samples,
0 100 5 10-7 1 10-6 1.5 10-6 5 10 15 20 25 0 0.2 0.4 0.6 0.8 1 m0t0 nanocrystal size m 0 t 0 [cm 2 /V] nanocrystal size [nm]
Raman crystallinity factor fc514nm, top
a-Si mc-Si 0 200 400 600 800 1000 1200 150 200 250 300 350 400 450 conglom. size Lamb
conglomerate size [nm]
L
amb
[nm]
Fig. 3.20: Top: Conglomerate size (D) evaluated from AFM surface topography (see Fig. 3.21 for comparison) and ambipolar diffusion length (Lamb), with corresponding linear fits (for the
mc-Si:H samples); Bottom: mobility ¥ lifetime product (m0t0) and nanocrystal size (d)
evaluated from the (220) XRD peaks, with guides to the eye, as a function of the Raman crystallinity for the SC series of silicon layers deposited on glass.
Fig. 3.21: TEM dark field micrograph of a mc-Si:H layer deposited with SC = 7%. The ambipolar diffusion length (Lamb) measured in coplanar geometry and the conglomerate size
(D) as measured from AFM are also indicated. Lamb
D
top
Fig. 3.20 (top) shows that both conglomerate size (D) and Lamb slightly decreases with fc
for the mc-Si:H samples. Moreover, one sees that the ambipolar diffusion length is equal to
about half of the conglomerate size (Lamb ≈ 0.5·D) in this series (see also Fig. 3.21). This
relation can be expected if the diffusion length within the conglomerate is very large, but the carriers recombine at the boundaries.
The decrease of the conglomerate size with increasing crystallinity can be explained on the basis of the nuclei density. A nucleus is the starting point of a conglomerate, i.e. the tip of the inverted mc-Si:H cone (see Fig. 3.21). One can assume that, as it was observed for the case of entire solar cells [Bailat 2003], the nuclei density decreases as SC increases and the opening angle of the mc-Si:H cones is independent of SC. As a result, for a low nuclei density (i.e. high SC-value, leading also to a low crystallinity), the (few) microcrystalline cones will become quite large before coalescing. On the other hand, if the nuclei density is high (i.e. low SC-value, leading to high crystallinity), the (numerous) cones will coalesce rapidly and the resulting conglomerate size emerging at the surface will be smaller in this case.
From these observations, we can deduce that transport is not affected by the average nanocrystal size, but that it rather behaves in accordance with the conglomerate size, as already suggested by Kocka et al. [Kocka 2002]. Moreover, Fig. 3.22 shows that the activation energy (Eact), the dark conductivity prefactor (s0) and the dark conductivity (sdark)
rapidly change for low values of fc (a-Si:H ‡ mc-Si:H transition) and then smoothly evolve
for the mc-Si:H samples. The values measured for s0 and for Eact on our mc-Si:H samples
(fc > 0.2) are actually lower than the threshold values given by Kocka et al. [Kocka 2002,
Mates 2003] under which transport is limited by the boundaries of the conglomerates (see Fig. 3.22). Indeed, for s0 ≥ 100 W-1cm-1 and Eact ≥ 0.5 eV, Kocka et al. showed that material is
only made out of nanocrystals, and transport properties are therefore not deteriorated by the formation of conglomerate boundaries.
These observations are in accordance with what has been seen in the section 3.3, where similar m0tR products have been observed in mc-Si:H as in a-Si:H. We can therefore
conclude that the conglomerate boundaries are made of amorphous material and that they limit the electronic transport in mc-Si:H.
10-12 10-10 10-8 10-6 10-4 10-2 100 102 104 106 0 0.5 1 1.5 0 0.2 0.4 0.6 0.8 1 s0 sdark Eact s dark -- s 0 [ W -1 cm -1 ] E act [eV]
Raman crystallinity factor fc514nm, top
a-Si
mc-Si
E
act= 0.5 eV
s0= 100 W-1cm-1
Fig. 3.22: Dark conductivity, conductivity prefactor and activation energy as a function of the Raman crystallinity factor measured at 514nm from the layer-side of the silane concentration series of layers deposited on glass. The dotted lines give the limit values (s0 ≥ 100 W-1cm-1
and Eact ≥ 0.5 eV) found by Kocka [Kocka 2002] over which the transport is not affected by
the aggregates (or conglomerates), as they are not present.
3.5. Conclusions
In this chapter, we have first seen that the coplanar electronic transport under illumination in mc-Si:H material is similar to that in a-Si:H material, as observed here for a large variety of a-Si:H and mc-Si:H samples. The similitude in the values of the mobility ¥ lifetime products for mc-Si:H and a-Si:H (mt a-Si ≈ mtmc-Si) is quite surprising, as transport properties are much better (i.e. the mt-products are higher) in crystalline silicon than in a-Si:H. These observations suggest that electronic transport in mc-Si:H layers is governed by the amorphous (or defective) phase present at the grain boundaries (nanocrystal boundaries or conglomerate boundaries) of mc-Si:H. Then, for one series of layers deposited on glass at various silane concentrations, we have pointed out that the coplanar electronic transport is not
affected by the average nanocrystal size. Moreover, the ambipolar diffusion length measured in these samples was found to be about half of the size of the conglomerates. It seems thus that it is the conglomerates (or more precisely the conglomerates boundaries) that are limiting the transport in mc-Si:H layers. We can therefore conclude that the conglomerate boundaries are made of amorphous material and that they limit the electronic transport in mc-Si:H layers.