Abstract
Graphene tunnel junctions are a promising experimental platform for single molecule electronics and biosensing. Ultimately their noise properties will play a critical role in developing these applications. Here we report a study of electrical noise in graphene tunnel junctions fabricated through feedback-controlled electroburning. We observe random telegraph signals characterized by a Lorentzian noise spectrum at cryogenic temperatures (77 K) and a 1/ f noise spectrum at room temperature. To gain insight into the origin of these noise features, we introduce a theoretical model that couples a quantum mechanical tunnel barrier to one or more classical fluctuators. The fluctuators are identified as charge traps in the underlying dielectric, which through random fluctuations in their occupation introduce time-dependent modulations in the electrostatic environment that shift the potential barrier of the junction. Analysis of the experimental results and the tight-binding model indicate that the random trap occupation is governed by Poisson statistics. In the 35 devices measured at room temperature, we observe a 20-60% time-dependent variance of the current, which can be attributed to a relative potential barrier shift of between 6% and 10%. In 10 devices measured at 77 K, we observe a 10% time-dependent variance of the current, which can be attributed to a relative potential barrier shift of between 3% and 4%. Our measurements reveal a high sensitivity of the graphene tunnel junctions to their local electrostatic environment, with observable features of intertrap Coulomb interactions in the distribution of current switching amplitudes.
Highlights
Graphene tunnel junctions provide a two-dimensional platform for probing individual molecules
Recent experiments have demonstrated charge transport through single molecules that were firmly anchored between a pair of graphene electrodes via π−π stacking[1−3] or covalent bonding.[4−8] graphene tunnel junctions have been proposed as candidate systems for molecular sensing, in particular for sequencing DNA molecules as they translocate through the gap.[9]
The observed current fluctuations are evident from the bimodal Gaussian distribution of current values (Figure 1F) and can be measured for up to 6 h. A histogram of the room temperature current in graphene tunnel junctions (Figure 1D)
Summary
Graphene tunnel junctions provide a two-dimensional platform for probing individual molecules. Low-frequency 1/f noise or “flicker” noise is ubiquitous in nanoscale electronic systems, leading to prominent current fluctuations in semiconductor devices,[35−39] tunnel junctions,[40−43] and nanopores.[44−49] While the physical mechanisms that generate these fluctuations may vary and are often not known, it is generally accepted that 1/f noise is the result of a distribution of nonidentical random telegraph signals (RTSs).[11,35,36,39,50] These RTSs each have a Lorentzian noise power spectral density, the superposition of which results in a 1/f power spectral density. Fluctuations in tunneling current in graphene tunnel junctions and resulting noise spectra: (C) Nonspecific fluctuations in tunneling current at room temperature and (D) The corresponding log-normal distribution of current values. (E) RTS in I−t traces and (F) bimodal current distribution with two Gaussian peaks upon cooling the device to 77 K. (G) Current noise PSD measured in graphene tunnel junctions has 1/f form at room temperature and Lorentzian form at 77 K, with lower overall noise level
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