3.5 Influence of emitter properties on contact formation
3.5.2 Boron surface concentration
As discussed in the introduction of this section, for pure silver pastes a high phospho-rous surface concentration resulting in high density of phosphophospho-rous precipitates at the silicon surface is necessary to facilitate the growth of silver crystals on the silicon surface and thereby obtain a reasonable contact resistance.
To investigate the influence of the boron surface concentration on the specific con-tact resistance and especially the growth of metal spikes into the silicon surface for aluminium-containing pastes, three emitters featuring similar emitter depthde but dif-ferent dopant surface concentrations were developed. The emitter profiles measured on alkaline textured wafers are depicted in figure 3.26. Additionally, to examine if an emitter is required for the growth of contact spots, wafers without emitter were pro-cessed. The relevant properties of the different emitters are summarized in table 3.4.
The depth of the emitters is evaluated in the same way as in the previous subsection. dE
is comparable for all emitters and lies around 0.45μm. With the variation of the boron surface concentration the total course of the dopant density also differs. Therefore, the sheet resistances of the three emitters vary significantly. In the experiment the differ-ent emitters were diffused on alkaline textured n-type silicon wafers. In the following,
3.5 Influence of emitter properties on contact formation
0.0 0.1 0.2 0.3 0.4 0.5
1E16 1E17 1E18 1E19 1E20
N B(cm-3 )
x (μm)
S1 S2 S3
Figure 3.26:Emitter profiles of BBr3-based boron emitters featuring different boron surface concentrations. The red line marks the doping density above which current transport via field emission dominates for the metal-semiconductor contact.
Table 3.4:Relevant emitter properties of the emitters S1-S3 featuring different boron surface concentrations and the sample without emitter (noEm).
emitter designation
boron surface concentration NB,s (1019cm−3)
emitter depth
de (μm) sheet resistance Rsh (Ω/)
S1 1.4±0.0 0.42±0.02 364±12
S2 1.8±0.0 0.48±0.01 250±5
S3 8.6±0.1 0.46±0.02 61±1
noEm no emitter
-75 nm PECVD-SiNx:H were deposited on all wafers and contacts were screen-printed using two different commercially available silver-aluminium pastes. As no significant impact of doping density on wafer peak temperature was found in the experiment pre-sented in the last subsection, contacts were fired in the standard firing process.
For the wafers with emitters TLM measurements were carried out. The measured spe-cific contact resistances are shown in figure 3.27. As expected, the spespe-cific contact resistance is remarkably lower on emitters with a high boron surface concentration/
low sheet resistance.
In figure 3.28 SEM images of contacts etched back in hydrofluoric acid are shown. To compare the density of contact spots on the different emitters, the contact areas are shown in low SEM magnification for the samples without emitter and with emitters S1 and S3. The contact spots are coloured green for better visibility. On all sam-ples, also for the wafer without emitter, contact spots can be observed on the surface.
1 10 100
8.6x1019 1.8x1019
1.6x1019 ρc(mΩcm2 )
Ag/Al-Ax Ag/Al-B
NB,s(cm-3)
Figure 3.27: Specific contact resistance in dependence of boron surface concentration.
Considering several micrographs, on the wafer without emitter and with emitter S1 (low surface concentration) no significant difference in the density of contact spots can be observed. For emitter S3, featuring a high boron surface concentration, however, slightly more and larger spikes are visible. Again, no difference in the shape of the contact spots is found on the different samples. An important result of the SEM anal-ysis is that even on wafers without emitter silver-aluminium contacts spots grow into the silicon surface below screen-printed contacts. In contrast to aluminium-free pastes on phosphorous emitters, no high dopant surface concentration is necessary for crystal growth. The used boron emitters feature no defects of inactive dopant at the silicon surface. This is supposed to be the reason, why aluminium-free pastes are not, but aluminium-containing pastes are able to contact boron emitters. However, a slightly increased number of contact spots could be found on the emitter with a high boron sur-face concentration. Possibly, the increased indiffusion of boron into the wafer induces a higher density of crystal defects, e.g. dislocations. Therefore, silicon could be easier dissolved from the surface, and a slightly larger number of crystals could form.
However, as for emitters featuring different depths, the main influence of the boron sur-face concentration on the contact formation is not found in microstructural differences in the contact, but in the varying current transport over the direct metal-semiconductor contact. Considering the emitters used in this investigation, only the profile of emit-ter S3 featuring the highest boron surface concentration exceeds partially the value of 1·1020cm−3. Therefore, for the other emitters current transported via field emission is negligible, which can explain the high contact resistances on these emitters compared to emitter S3. These considerations show that probably instead of the boron surface concentration the maximal boron concentration in the emitter profile is the relevant factor to explain the measured specific contact resistances.
3.5 Influence of emitter properties on contact formation
no emitter 20 μm
(a) 20 μm S1
(b)
20 μm S3
(c)
Figure 3.28: Low magnification SEM micrographs of contacts etched back in hydrofluoric acid. The contact spots are marked in green. (a) wafer without emitter, (b) emitter S1 (low NB,s), (c) emitter S3 (high NB,s).
In the following subsection the influence of the current transport over the metal-semiconduc-tor contact on the specific contact resistance of a single contact spot will be analyzed.