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(a) Collinear four probe resistance measurements of bulk doped silicon substrates and highly doped, shallow Si:P dopant layers. V–I characteristics are shown for a p-type substrate before (b) and after (c) phosphine dosing and encapsulation with 4 nm of epitaxial silicon.
Probe spacing dependence of the four-terminal resistance of three different silicon substrates, before and after phosphorus -doping. The substrates are (a) 300 μm thick p-type, (b) 300 μm thick n-type, and (c) 2 μm thick p-type silicon-on-insulator. For all samples, the encapsulation thickness after -doping is (4.0 ± 0.5) nm.
The significance of two-terminal resistance in bulk sensitivity. (a) The predicted two-terminal source-to-drain resistances as a function of probe radius for a variety of substrate doping levels at a fixed probe spacing of 300 μm (from Eq.(4)). For the radius probes used here, the 2D -layer (green trace) is more conductive than the bulk substrate. (b) Experimental four-terminal resistances (grey circles) of -layers on different n-type substrates with varying doping densities confirm the predictions of (a). 2D conduction is still observed on a much lower doped () substrate, but bulk conduction dominates on a much heavier doped () substrate.
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