Study of the piezoresistivity of doped nanocrystalline silicon thin films

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 = 0º and 90º, respectively, are negative for n-type and positive for p-type films. For other values of θ (30º, 45º, 120º and 135º) 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 2 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 250ºC and working pressure of 0.250 Torr, with dark conductivity 1.6 Ω-1 cm-1. The strongest n-type response (GF L =-28.1, GF T =-5.60) was found in a film deposited by PECVD at 150ºC and working pressure of 3 Torr, with dark conductivity 9.7 Ω-1 cm-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.

A Novel SU8 Polymer Anchored Low Temperature HWCVD Nitride Polysilicon Piezoresitive Cantilever

“…The slope of the plot gives the gauge factor for piezoresistor. The value of gauge factor obtained from the plot is 22 which compares with the literature [22] and [23].…”

Piezoresistivity of thin film semiconductors with application to thin film silicon solar cells

“…3) of the gauge factor when applied voltage and applied stress are perpendicular to each other. This effect on p-type piezoresistors is expected from other experimental (see Table 2) and theoretical works [45] even though there are a few studies that report the opposite [41,46]. Generally speaking, the piezoresistive effect weakens whenever we measure a component of the conductivity tensor (like here in our coordinate system σ yy ) that is not aligned with the principal value of the strain tensor (here ε xx ).…”

“…Material type Fabrication Gl Gt Maximal strain [41] n-type μ-Si:H RF-PECVD À 25 À 6 0.04% [41] p-type μ-Si:H RF-PECVD 25 7 0.04% [42] p-type μ-Si:H CMOS-MEMS 35 À 10 0.01% [43] n-type μ-Si:H HW-CVD À 14 n.s. 1% [44] n-type μ-Si:H RF-PECVD À 39 À 7.6 0.01% [44] p-type μ-Si:H RF-PECVD 20.8 À 7.8 0.01% [39] n-type a-Si:H Glow discharge À 18 À 7 0.06% [39] intrinsic a-Si:H Glow discharge À 8 n.s.…”

Piezoresistive silicon thin film sensor array for biomedical applications

“…Previous work has shown that, as far as piezoresistance is concerned, nc-Si:H films are isotropic in planes perpendicular to the growth direction [9]. The combined characteristics of flexibility, piezoresistance and isotropy allow the design of novel types of strain or shape sensing devices, for example for biomedical applications.…”