Conductivity fluctuations in proton-implanted ZnO microwires

The electric noise can be an important limitation for applications of conducting elements of size in the nanometer range. The intrinsic electrical noise of prospective materials for opto-spintronics applications like ZnO has not been characterized yet. In this study we have investigated the conductivity fluctuations in 10~nm thick current paths produced by proton implantation of ZnO microwires at room temperature. The voltage noise under a constant dc current bias in undoped as well as in Li-doped microwires is characterized by $1/f^a$ power spectra with $a \sim 1$. The noise intensity scales with the square of the bias current pointing out to bias-independent resistivity fluctuations as a source of the observed noise. The normalized power spectral density appears inversely proportional to the number of carriers in the probed sample volume, in agreement with the phenomenological Hooge law. For the proton-implanted ZnO microwire and at 1~Hz we obtain a normalized power spectral density as low as $\sim 10^{-11}~$Hz$^{-1}$.