The low-frequency 1/f noise – fluctuation process with the spectral density S~1/fγ, where γ≈1 – is a ubiquitous phenomenon, found everywhere from fluctuations of sea level in Bermuda and the intensity of a recording of Bach's Brandenburg Concerto, to human heart rates and electrical currents in semiconductor devices.
The low-frequency fluctuations in electrical current attract particular attention. The low-frequency electronic 1/f noise was first discovered in vacuum tubes, in 1925, and later observed in a wide variety of electronic materials and devices. The importance of this noise for electronic and communication devices motivated numerous studies of its physical mechanisms and methods for its control. However, after almost a century of investigations, the origin of 1/f noise in most material systems still remains a mystery. A question of particular importance for electronics is whether 1/f noise is generated on the surface of electrical conductors or inside their volumes.
A team of researchers from the University of California – Riverside (UCR), Rensselaer Polytechnic Institute (RPI) and Ioffe Physical-Technical Institute of The Russian Academy of Sciences was able to shed light on theorigin of the 1/f noise and its mechanism. The researchers used a set of multi-layered graphene samples with the thickness continuously varied from around 15 atomic planes to a single layer of graphene.
The team of researchers included Dr. Alexander A. Balandin
, Professor of Electrical Engineering and Founding Chair of Materials Science and Engineering at UCR, Dr. Guanxiong Liu, Research Associate in Professor Balandin’s Nano-Device Laboratory (NDL)
, Dr. Michael S. Shur, Patricia W. and C. Sheldon Roberts Professor of Solid State Electronics and Director of the NSF I/UCRC Center "Connection One" at RPI and Dr. Sergey Rumyantsev, Research Professor at RPI and Ioffe Institute. The results of research have been accepted for publication in the Applied Physics Letters
and will appear in print in March under the title “1/f Noise in Graphene Multilayers: Surface vs. Volume.” The pre-print is available at arXiv
Initially the UCR – RPI – Ioffe team planned to investigate how 1/f noise level scales with the thickness of the graphene multilayer film. Then the researchers realized that the results could provide an insight into the origin of the 1/f noise itself and answer the almost century-old “surface vs. volume” question.
“Unlike the thickness of metal or semiconductor films, the thickness of graphene multilayers can be continuously and uniformly varied all the way down to a single atomic layer of graphene – the actual “surface,” – explained Balandin. “Thus, multilayer graphene films allowed us to directly probe the origin of the 1/f noise.”
The UCR – RPI – Ioffe team found that 1/f noise becomes dominated by the volume noise when the thickness of the films exceeds ~7 atomic layers (corresponding to ~2.5 nm). The 1/f noise is a pure surface phenomenon below this thickness. Although the results were achieved for a particular material system – graphene and graphene multilayers – their implications go beyond graphene.
“We understand better the origin and sources of 1/f noise in graphene. However, the investigation of 1/f noise in this exotic system is still in its infancy. We still have to come up with the exact mechanism of 1/f noise and understand contributions of the mobility fluctuations and career number fluctuations in the overall noise level. There are results from many research groups with various explanations. There is still no conventionally accepted theory,” – stated Balandin.
The research at UCR was supported, in part, by the Semiconductor Research Corporation (SRC) and Defense Advanced Research Project Agency (DARPA) through Center for Function Accelerated nanoMaterial Engineering (FAME) and by the National Science Foundation (NSF). The work at RPI was supported by the US NSF under the auspices of I/UCRC “CONNECTION ONE” at RPI and by the NSF EAGER program.