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

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.

High In-Plane Thermal Conductivity of Aluminum Nitride Thin Films

“…As discussed extensively in recent literature, − TDTR and FDTR have failed to measure the thermal conductivities of Si at low temperatures due to obfuscations from thermal boundary resistances, non-equilibrium processes, and limiting heater length scales. Similar phenomena have also been reported for several other high thermal conductivity crystals and thick alloys. − We note that though TDTR fails for Si and several other crystals, the thermal conductivity of sapphire and AlN can be accurately measured by TDTR at low temperatures. ,, The nature of such material specific failure of TDTR is an active area of research , and beyond the scope of this work. However, due to the measurement time and length scales, SSTR measurements are less sensitive to the above-mentioned limitations.…”

“…The AlN thin films used in this work are grown on sapphire substrates using metal–organic chemical vapor deposition (MOCVD). , We study three AlN films with thicknesses of 3.05, 3.75, and 6 μm. The AlN films of this study and those of Koh et al were grown in the same batch. Scanning transmission electron microscopy (STEM) reveals the existence of a nucleation layer near the AlN/sapphire interface in all samples, as shown in Supporting Information Figure S1.…”

“…Scanning transmission electron microscopy (STEM) reveals the existence of a nucleation layer near the AlN/sapphire interface in all samples, as shown in Supporting Information Figure S1. The thickness of this nulceation layer is less than 22% of the total film thickness . The region above the nucleation layer is a single crystal AlN layer.…”

“…Compared to the other III–V compound semiconductors, bulk, high-purity aluminum nitride (AlN) crystals have advantageous electronic properties, requisite transparency and higher thermal conductivity, making them suitable for use in optoelectronics, light-emitting diodes and high-power RF devices. ,− However, the use of AlN as heat spreaders in high-power electronic devices comes with the challenge of achieving high thermal conductivities in thin film forms. Recently, Koh et al reported bulk-like cross-plane thermal conductivities in high-quality AlN films with thicknesses ranging from 3 to 22 μm. These promising results suggest that high-quality micrometer-thick films of AlN could act as heat spreaders in high-power devices depending on their in-plane transport properties.…”

Recent Advances in Fabricating Wurtzite AlN Film on (0001)-Plane Sapphire Substrate

“…They improved the crystalline quality of AlN/sapphire by introducing air-voids near the AlN/sapphire interface, which allowed for the growth of high-quality and crack-free AlN films with a thickness >15 µm. The physical thought is similar to that which we mentioned in the MSG growth technique section [114,115]. Meanwhile, the high growth temperature and high-purity precursors effectively suppressed the incorporation of impurities such as O and C. As a result, the TDD values were estimated to be ~(1-3) × 10 8 cm −2 by counting etching pit densities.…”