Roy B, Tsui A, Oni O and Vlahovic B
In a contactless photo conductance measurement system the radio-frequency (RF) probe transmission (ΔV/ V0) should be proportional to the product of laser-induced carrier concentration and carrier mobility (???Σμ) through a sensitivity factor (A). We use 532 nm laser (pump)-millimeter wave (mmw-probe) system whose concentrations (???) are calculated by considering single-surface reflection of the laser beam and mobility (Σμ) derived from a model. In order to ascertain A we use five c-Si (100) samples having resistivity in the range 15-130 Ω-cm. For relating (ΔV/V0) with ???Σμ to find A, we take their ratio and quantify A once using a quadratic-fit functional form of the ratio of sample resistivity to air resistivity (ρ/ρ0), and another time using product of free-space impedance and sample thickness (ρ/Z0t). A is ascertained for (ΔV/V0)-laser fluence linear region while fluence is in range 0-1.7 μJ/cm2 and probe frequency is fixed at 140 GHz. Value of A is further fine-tuned by multiplying with 0.85 (to linearize the ratio with the non-dimensional function) and finally obtain sensitivity A=0.291. Standard error in mmw photo conductance (obtained using calculated A) between the two approaches diminish with laser attenuation roughly at a rate ± 0.53 × 10-5 S, per decimal neutral density filter size.
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