Bulk-like Intrinsic Phonon Thermal Conductivity of Micrometer-Thick AlN Films

Yee Rui Koh; Zhe Cheng; Abdullah Mamun; Md Shafkat Bin Hoque; Zeyu Liu; Tingyu Bai; Kamal Hussain; Michael E Liao; Ruiyang Li; John T Gaskins; Ashutosh Giri; John Tomko; Jeffrey L Braun; Mikhail Gaevski; Eungkyu Lee; Luke Yates; Mark S Goorsky; Tengfei Luo; Asif Khan; Samuel Graham; Patrick E Hopkins

doi:10.1021/acsami.0c03978

Bulk-like Intrinsic Phonon Thermal Conductivity of Micrometer-Thick AlN Films

ACS Appl Mater Interfaces. 2020 Jul 1;12(26):29443-29450. doi: 10.1021/acsami.0c03978. Epub 2020 Jun 22.

Authors

Yee Rui Koh¹, Zhe Cheng², Abdullah Mamun³, Md Shafkat Bin Hoque¹, Zeyu Liu⁴, Tingyu Bai⁵, Kamal Hussain³, Michael E Liao⁵, Ruiyang Li⁴, John T Gaskins¹, Ashutosh Giri¹, John Tomko⁶, Jeffrey L Braun¹, Mikhail Gaevski³, Eungkyu Lee⁴, Luke Yates², Mark S Goorsky⁵, Tengfei Luo⁴, Asif Khan³, Samuel Graham^{2

7}, Patrick E Hopkins^{1

6

8}

Affiliations

¹ Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia 22904, United States.
² George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States.
³ Department of Electrical Engineering, University of South Carolina, Columbia, South Carolina 29208, United States.
⁴ Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States.
⁵ Department of Materials Science and Engineering, University of California at Los Angeles, Los Angeles, California 90095, United States.
⁶ Department of Materials Science and Engineering, University of Virginia, Charlottesville, Virginia 22904, United States.
⁷ School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States.
⁸ Department of Physics, University of Virginia, Charlottesville, Virginia 22904, United States.

PMID: 32491824
DOI: 10.1021/acsami.0c03978

Abstract

Aluminum nitride (AlN) has garnered much attention due to its intrinsically high thermal conductivity. However, engineering thin films of AlN with these high thermal conductivities can be challenging due to vacancies and defects that can form during the synthesis. In this work, we report on the cross-plane thermal conductivity of ultra-high-purity single-crystal AlN films with different thicknesses (∼3-22 μm) via time-domain thermoreflectance (TDTR) and steady-state thermoreflectance (SSTR) from 80 to 500 K. At room temperature, we report a thermal conductivity of ∼320 ± 42 W m^-1 K^-1, surpassing the values of prior measurements on AlN thin films and one of the highest cross-plane thermal conductivities of any material for films with equivalent thicknesses, surpassed only by diamond. By conducting first-principles calculations, we show that the thermal conductivity measurements on our thin films in the 250-500 K temperature range agree well with the predicted values for the bulk thermal conductivity of pure single-crystal AlN. Thus, our results demonstrate the viability of high-quality AlN films as promising candidates for the high-thermal-conductivity layers in high-power microelectronic devices. Our results also provide insight into the intrinsic thermal conductivity of thin films and the nature of phonon-boundary scattering in single-crystal epitaxially grown AlN thin films. The measured thermal conductivities in high-quality AlN thin films are found to be constant and similar to bulk AlN, regardless of the thermal penetration depth, film thickness, or laser spot size, even when these characteristic length scales are less than the mean free paths of a considerable portion of thermal phonons. Collectively, our data suggest that the intrinsic thermal conductivity of thin films with thicknesses less than the thermal phonon mean free paths is the same as bulk so long as the thermal conductivity of the film is sampled independent of the film/substrate interface.

Keywords: TDTR; aluminum nitride; phonon; thermal conductivity; thin film.