Studying Heat Flow In InGaAs


Purdue University researchers have visualised temperature changes produced by ultra-small heat sources, gold strips formed on top of InGaAs

A team from Purdue University in the US has applied the same 'hydrodynamic transport model' used to study flow in fluids to explain heat transport in a compound semiconductor, with potential application in the design of high-speed transistors and lasers.

They believe that their findings could have important implications for 'thermal crosstalk' in which multiple heat sources next to each other impact the overall temperature of the system, hindering performance.

The researchers used a technique called full-field thermoreflectance thermal imaging to directly visualise temperature changes produced by ultra-small heat sources, gold strips formed on top of the semiconductor InGaAs.

(The images above depict, a) the device structure and experimental setup, b) an optical imageof the fabricated device and (c) an experimental thermal image.)

The research concerns the crucial role of phonons, quantum-mechanical objects, or quasiparticles that describe how vibrations travel through a material's crystal structure. The phonons are said to be 'heat carriers' in solid materials.

“This is the first time such hydrodynamic effects are indirectly observed for heat propagation in a solid,” said Ali Shakouri, Purdue University's Mary Jo and Robert L. Kirk Director of the Birck Nanotechnology centre and a professor of electrical and computer engineering.

“While structures called vortices are common in fluid flows such as water or air, this is the first time we've seen that they can be present inside solids for phonon flow in the typical semiconductor InGaAs, which is used in high-speed transistors and lasers.”

The findings are detailed in a research paper 'Full-field thermal imaging of quasiballistic crosstalk reduction in nanoscale devices' appearing on Jan. 17 in Nature Communications.

“The observed thermal crosstalk reduction has important implications in the design of nanoscale electronic and optoelectronic devices,” said Purdue postdoctoral research associate Amirkoushyar Ziabari, the paper's lead author.

“As the size of electronic and optoelectronic devices are getting smaller, there are more and more devices being packed into a smaller area, so the thermal crosstalk between these devices becomes important. Knowing the accurate thermal behaviour in the neighbourhood and a few microns from heat sources would help design better state-of-the-art devices in terms of performance, speed, thermal reliability, and so on.”

The researchers found that the reduced thermal crosstalk is caused by vortices generated near the edge of the heat sources.

“This is similar to the vortices that are observed at the edge of an obstacle placed inside of a current of air or water, such as behind an airplane wing,” Shakouri said.

The governing law of heat conduction, known as the Fourier Law or the heat-diffusion equation, does not accurately predict thermal transport for devices on the nanoscale. Because the Fourier diffusion equation doesn't explain the heat transport at those scales, this transport regime is called non-diffusive.

“As the size of electronic and optoelectronic devices are getting smaller, it is important to consider this non-diffusive behaviour for design and optimisation of such small devices,” Ziabari said. “These new measurements show that at nanoscales, heat propagation has interesting ‘fluid-like' behaviour.”

Conventional methods do not account for vortices of heat transport found at the nanoscale.

“Vorticity only becomes important when the characteristic source dimension is comparable to the hydrodynamic length scale of about 150 nanometers,” he said.

The Fourier theory substantially overestimates the experimentally observed temperature a short distance away from the heater lines.

“The surprising effect was that the temperature decays much faster than what Fourier theory predicted,” Shakouri said. “Within a distance of 1 or 2 microns of a small heat source - a line about 100 nanometers wide - temperature could be one-third to one-fourth what Fourier theory predicts.”

The thermoreflectance thermal imaging approach allows researchers to create maps of temperature rise at far higher resolution than otherwise possible using light in the visible range.

The work was performed by researchers at the Birck Nanotechnology centre in Purdue's Discovery Park, Purdue's School of Electrical and Computer Engineering, Universitat Autónoma de Barcelona, Commissariat à l'Énergie Atomique in Grenoble, France, and the Department of Mechanical and Materials Engineering at the University of Cincinnati. All co-authors are listed in the abstract.

The research was partially funded by the European Commission and the Spanish Ministry of Economy and Competitiveness.

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