Figure 1: Fabrication and electronic transport in nanogranular tunnelling resistor sensors.
(a) Example of nanogranular tunnelling resistor (NTR) deposited on a 1 μm wide, 300 nm thick free standing SiN cantilever. The scanning electron microscopy (SEM) image is false coloured to distinguish the NTR sensor (orange) and the metal contacts (yellow). Scale bar, 1 μm. (b) Illustration of the NTR electron-beam-induced deposition process. Precursor gas molecules adsorb and diffuse on the surface, where they are dissociated by the scanned electron beam and form platinum clusters embedded in a carbonaceous matrix. (c) Schematic depiction of the inelastic co-tunnelling process in the NTR. Electrons tunnel through several grains at the same time via virtual energy levels. The co-tunnelling radius therefore is larger than the inter grain distance (blue halos). When the sensor is stretched, the inter grain distance increases and the co-tunnelling radius decreases that results in an increased resistance. (d) Phase diagram of the electronic transport regimes in granular metals. This phase diagram was theoretically predicted based on a recent theoretical investigations24 and was largely verified experimentally by very recent experiments on Pt(C)-based granular metal samples with finely tuned tunnel coupling g (ref. 25). The conductivity temperature dependence is given for each regime, where Δ′–Δ′′′ are temperature constants that depend on the NTR material properties and conduction regime22. At room temperature and the prevailing coupling strength, the NTR sensors operate within the inelastic co-tunnelling regime.
