Study of the piezoresistivity of doped nanocrystalline silicon thin films: Journal of Applied Physics: Vol 109, No 12

Abstract

The piezoresistive response of n- and p-type hydrogenated nanocrystalline

silicon

thin films,

deposited by hot-wire (HW) and plasma-enhanced chemical vapor deposition (PECVD) on thermally oxidized silicon wafers, has been studied using four-point bending tests. The piezoresistive gauge factor (GF) was measured on patterned thin-film micro-resistors rotated by an angle θ with respect to the principal strain axis. Both longitudinal (GF_L) and transverse (GF_T) GFs, corresponding to θ = 0° and 90°, respectively, are negative for n-type and positive for p-type films. For other values of θ (30°, 45°, 120°, and 135°) GFs have the same signal as GF_L and GF_T and their value is proportional to the normal strain associated with planes rotated by θ relative to the principal strain axis. It is concluded that the films are isotropic in the growth plane since the GF values follow a Mohr’s circle with the principal axes coinciding with those of the strain tensor. The strongest p-type pirezoresistive response (GF_L = 41.0, GF_T = 2.84) was found in a film deposited by PECVD at a substrate temperature of 250 °C and working pressure of 0.250 Torr, with dark conductivity 1.6 Ω⁻¹cm⁻¹. The strongest n-type response (GF_L =− 28.1, GF_T =− 5.60) was found in a film deposited by PECVD at 150 °C and working pressure of 3 Torr, with dark conductivity 9.7 Ω⁻¹cm⁻¹. A model for the piezoresistivity of nc-Si is proposed, based on a mean-field approximation for the conductivity of an ensemble of randomly oriented crystallites and neglecting grain boundary effects. The model is able to reproduce the measured GF_L values for both n- and p-type films. It fails, however, to explain the transversal GF_T data. Both experimental and theoretical data show that nanocrystalline silicon can have an isotropic piezoresistive effect of the order of 40% of the maximum response of crystalline silicon.

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