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Mapping Thickness Dependent Thermal Conductivity of GaN

[+] Author and Article Information
Elbara Ziade

Department of Mechanical Engineering, Boston University, Boston, MA USA
eziade@bu.edu

Jia Yang

Department of Mechanical Engineering, Boston University, Boston, MA USA
yangjia@bu.edu

Gordie Brummer

Department of Electrical and Computer Engineering, Boston University, Boston, MA USA
gbrummer@bu.edu

Denis Nothern

Division of Material Science and Engineering, Boston University, Boston, MA USA
dnothern@bu.edu

Theodore Moustaks

Department of Electrical and Computer Engineering, Boston University, Boston, MA USA;Division of Material Science and Engineering, Boston University, Boston, MA USA
tdm@bu.edu

Aaron Schmidt

Department of Mechanical Engineering, Boston University, Boston, MA USA;Division of Material Science and Engineering, Boston University, Boston, MA USA
schmidt@bu.edu

1Corresponding author.

J. Heat Transfer 138(2), 020906 (Jan 18, 2016) Paper No: HT-15-1708; doi: 10.1115/1.4032234 History: Received November 06, 2015; Revised December 02, 2015

Abstract

Frequency domain thermoreflectance (FDTR) is used to create quantitative maps of thermal conductivity and thickness for a thinning gallium nitride (GaN) film on silicon carbide (SiC). GaN was grown by molecular beam epitaxy on a 4H-SiC substrate with a gradient in the film thickness found near the edge of the chip. The sample was then coated with a 5 nm nickel adhesion layer and a 85 nm gold transducer layer for the FDTR measurement. A piezo stage raster scans the sample to create phase images at different frequencies. For each pixel, a periodically modulated continuous-wave laser (the red pump beam) is focused to a Gaussian spot, less than 2 um in diameter, to locally heat the sample, while a second beam (the green probe beam) monitors the surface temperature through a proportional change in the reflectivity of gold. The pump beam is modulated simultaneously at six frequencies and the thermal conductivity and thickness of the GaN film are extracted by minimizing the error between the measured probe phase lag at each frequency and an analytical solution to the heat diffusion equation in a multilayer stack of materials. A scanning electron microscope image verifies the thinning GaN. We mark the imaged area with a red box. A schematic of the GaN sample in our measurement system is shown in the top right corner, along with the two fitting properties highlighted with a red box. We show the six phase images and the two obtained property maps: thickness and thermal conductivity of the GaN. Our results indicate a thickness dependent thermal conductivity of GaN, which has implications of thermal management in GaN-based high electron mobility transistors.

Copyright © 2016 by ASME
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