LF noise research

Graphene has played a major role in providing an answer to an almost century-old problem of pink and flicker noise.

The research could lead to a further downsizing in electronic devices now that the mystery of this noise is better understood. The noise is a signal or process with power spectral density inversely proportional to the frequency. It was first discovered in thermionic valves in 1925 and has since been found in many areas from fluctuations of the intensity of music recordings to human heart rates and electrical currents in materials and devices.

The importance of this noise for electronics was behind several studies of its origins and methods for its control.  For example, the signal’s phase noise in a radar or communications device, such as a smartphone, is determined, to a large extent, by the LF noise level in the transistors used inside the radar or phone.

A question of particular importance for electronics is whether LF noise is generated on the surface of electrical conductors or inside them.

A team of researchers from the UC Riverside, Rensselaer Polytechnic Institute and Ioffe Physical-Technical Institute of the Russian Academy of Sciences, under the leadership of Alexander Balandin, has shed light on the noise’s origin using a set of multilayered graphene samples with their thickness continually varied from around 15 atomic planes to a single layer. Graphene, of course, is a single-atom-thick carbon crystal with properties that include superior electrical and heat conductivity, mechanical strength and optical absorption.

“The key to this interesting result was that unlike in 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 the material - the ultimate ‘surface’ of the film,”  Balandin said.

He added: “Thus we were able to accomplish with multilayer graphene films something that researchers could not do with metal films in the last century. We probed the origin of LF noise directly.”

He said that previous studies could not test metal films to the thicknesses below about 8 nm. The thickness of graphene is 0.35 nm and can be increased gradually, one atomic plane at a time.

“Apart from the fundamental science, the results are important for continuing the downscaling of conventional electronic devices,” Balandin said.

“Current technology is already at the level when many devices become essentially the surface. In this sense, the findings go beyond the graphene field.”

He also noted that the study was essential for the proposed applications of graphene in analog circuits, communications and sensors. This is because all these applications require acceptably low levels of LF noise which contribute to the phase noise of communications systems and limit sensor sensitivity and selectivity.

Balandin is the founding chairman of the materials science and engineering program at UC Riverside where the research was supported, in part, by the Semiconductor Research Corporation and Defence Advanced Research Project Agency through the Centre for Function Accelerated nanoMaterial Engineering and by the National Science Foundation.

The results of the investigation have been published in the Journal of Applied Physics Letters.