Session: 11-05-01: Thermophysical Properties: Characterization and Modeling Across Scales

Paper Number: 150588

150588 - Experimental Investigation of Thermal Conductivity Reduction in Alxga1-Xn: Effect of Composition, Small Hotspot, and Boundary Scattering 

Ultra-wide bandgap materials like Al_xGa_1-xN possess a large electronic bandgap (>4 eV). Their ability to sustain high voltage, switching frequency, and operating temperature makes them suitable for next-generation high energy-density electronics. Al_xGa_1-xN is stable from GaN to AlN and provides the opportunity to tune these properties by controlling composition. It also has high electron mobility, high saturation velocity, and relatively easy n-type doping with Si [1]. 

As the electronic design and manufacturing processes for Al_xGa_1-xN-based devices mature, there is an urgent need to develop device-level thermal simulations and characterization tools for electro-thermal co-design. Characteristic length scales in these devices, such as the hotspot size and the layer thicknesses, are expected to be comparable to the mean free paths of the phonons, the primary heat carriers in semiconductor materials. Increased phonon scattering will reduce thermal conductivity and alongside inefficient cooling of small hotspots, will lead to more severe heat accumulation than what is predicted by conventional modeling approaches [2].

We investigate the role of compositional disorder and small-size effects on nanoscale thermal transport in Al_xGa_1-xN. We measure the effective thermal conductivity of Al_xGa_1-xN thin films grown on AlN using frequency domain thermoreflectance (FDTR) [3]. In our FDTR setup, an intensity-modulated continuous-wave pump laser (488 nm wavelength) is focused on the sample coated with ~80 nm thick gold film. The pump is absorbed and creates a periodic heat flux on the gold surface that acts as a transducer. Since the reflectivity of gold changes with temperature, when a constant intensity probe beam (532 nm wavelength) is focused and reflected off the gold surface, it gets modulated according to the temperature variation on the surface. The phase lag between the pump and the reflected probe is then measured and the effective thermal conductivity is fitted using a heat conduction model of the sample stack [4] by matching with a simulated phase lag.

The effects of compositional disorder and boundary scattering are studied by measuring samples with different compositions (x between 0.2 and 0.9) and thicknesses (~100 nm to 1500 nm). To investigate the effect of hotspot size and learn the distribution of phonon MFPs, we vary the size of the heater in the FDTR measurement. A simultaneous co-fitting procedure allows for considering multiple independent measurements for a fixed composition of Al_xGa_1-xN to determine common fitting parameters, such as the interface conductances between the gold transducer and Al_xGa_1-xN and between Al_xGa_1-xN and AlN. Monte Carlo simulation-based uncertainty analysis establishes confidence intervals for the measured thermal conductivities. 

The effective thermal conductivity shows a decreasing trend with both a reduction in heater size and film thickness. The measured thermal conductivities are compared against the predictions from first-principles based calculations and solutions of phonon Boltzmann transport equation.

 

[1] “Ultra-Wide Bandgap Semiconductors Workshop III.” Accessed: May 10, 2023. [Online]. Available: https://apps.dtic.mil/sti/citations/AD1138076

[2] B. Chatterjee et al., “Interdependence of Electronic and Thermal Transport in AlxGa1–xN Channel HEMTs,” IEEE Electron Device Lett., vol. 41, no. 3, pp. 461–464, Mar. 2020, doi: 10.1109/LED.2020.2969515.

[3] K. T. Regner, D. P. Sellan, Z. Su, C. H. Amon, A. J. H. McGaughey, and J. A. Malen, “Broadband phonon mean free path contributions to thermal conductivity measured using frequency domain thermoreflectance,” Nat. Commun., vol. 4, no. 1, p. 1640, Mar. 2013, doi: 10.1038/ncomms2630.

[4] D. G. Cahill, “Analysis of heat flow in layered structures for time-domain thermoreflectance,” Rev. Sci. Instrum., vol. 75, no. 12, pp. 5119–5122, Dec. 2004, doi: 10.1063/1.1819431.

Presenting Author: Abhishek Pathak Carnegie Mellon University

Presenting Author Biography: I have been a Postdoc at Carnegie Mellon University since Jan 2023. I study thermal conductivity in AlGaN alloy semiconductors from first-principles and molecular dynamics simulations and optical measurement techniques such as frequency domain thermoreflectance. Before joining CMU, I was a Ph.D. student at University at Buffalo where I worked on developing a Monte Carlo solver for the Phonon Boltzmann transport equation.

Authors:

Abhishek Pathak Carnegie Mellon University
Kyuhwe Kang Pennsylvania State University
Yiwen Song Pennsylvania State University
Timothy Mirabito Pennsylvania State University
Andrew Allerman Sandia National Laboratories
Joan Redwing Pennsylvania State University
Sukwon Choi Pennsylvania State University
Alan Mcgaughey Carnegie Mellon University
Jonathan Malen Carnegie Mellon University