ARTICLES https://doi.org/10.1038/s41928-021-00595-9 Integration of boron arsenide cooling substrates into gallium nitride devices Joon Sang Kang1,3, Man Li1,3, Huan Wu1,3, Huuduy Nguyen1,3, Toshihiro Aoki2 and Yongjie Hu 1 ✉ Thermal management is critical in modern electronic systems. Efforts to improve heat dissipation have led to the exploration of novel semiconductor materials with high thermal conductivity, including boron arsenide (BAs) and boron phosphide (BP). However, the integration of such materials into devices and the measurement of their interface energy transport remain unex- plored. Here, we show that BAs and BP cooling substrates can be heterogeneously integrated with metals, a wide-bandgap semiconductor (gallium nitride, GaN) and high-electron-mobility transistor devices. GaN-on-BAs structures exhibit a high ther- mal boundary conductance of 250 MW m−2 K−1, and comparison of device-level hot-spot temperatures with length-dependent scaling (from 100 μm to 100 nm) shows that the power cooling performance of BAs exceeds that of reported diamond devices. Furthermore, operating AlGaN/GaN high-electron-mobility transistors with BAs cooling substrates exhibit substantially lower hot-spot temperatures than diamond and silicon carbide at the same transistor power density, illustrating their potential for use in the thermal management of radiofrequency electronics. We attribute the high thermal management performance of BAs and BP to their unique phonon band structures and interface matching. n most electronic systems, large amounts of waste heat are dis- been reported in boron phosphide (BP, ref. 7) and 1,300 W m−1 K−1 in sipated from hot spots to heat sinks across a series of device lay- boron arsenide (BAs, refs. 8,11). BAs has a thermal conductivity over ers and interfaces that have a thermal resistance. A large thermal three times that of industrial HTC standards such as copper and sil- I −1 −1 resistance—and hence rising hot-spot temperatures—can degrade icon carbide (SiC)—both around 400 W m K —and twice that of device operation, so thermal management is an important techno- cubic boron nitride. Furthermore, the mechanical and thermophys- logical challenge in the semiconductor industry1–3. Recent research ical properties of BAs have been measured to be highly compatible on improving heat dissipation has focused on replacing common with power semiconductors11, as desired for device integration. The substrates (such as silicon carbide, silicon and sapphire) with heterogeneous integration and characterization of BAs and BP with high-thermal-conductivity (HTC) materials to reduce the overall other material layers is critical to achieving their future implemen- thermal resistance4–11. A key challenge for high-performance ther- tation in devices for thermal management applications, but these mal management is to achieve the combination of HTC with a low remain unexplored. thermal boundary resistance (TBR—the resistance of an interface to In this Article, we report the interface characterization and inte- thermal flow) near electronics junctions3. gration of BAs and BP with other metal and semiconductor materi- Diamond is currently the leading research prototype HTC mate- als. We examine the heat dissipation performance and mechanisms at rial for high-performance power electronics cooling. It has been these interfaces via material characterizations, spectroscopy measure- studied with wide-bandgap semiconductors and shown reduced ments and atomistic phonon transport theory simulations. Because hot-spot temperatures in gallium nitride (GaN)–diamond devices of their unique phonon band structures, BAs and BP show a combi- compared with traditional radiofrequency (RF) systems. However, nation of high HTC and low TBR. Here, we develop GaN-on-BAs GaN–diamond interfaces have poor TBR, which compromises structures using metamorphic heteroepitaxy and measure a thermal the application potential of diamond for thermal management3. boundary conductance of 250 MW m−2 K−1. In the following, we Conventional HTC materials have also been limited by their ther- highlight the potential of BAs as an alternative cooling substrate to mal properties and other intrinsic issues. For example, diamond and diamond and other state-of-the-art HTC materials by using experi- cubic boron nitride are challenging for applications because of their mental data from GaN–BAs structures with a variable-width heat high-temperature and high-pressure synthesis requirements, slow source to determine and investigate hot-spot temperatures of GaN growth rate, high cost, degraded quality and difficulty integrating transistors as a function of length scaling (from 100 μm to 100 nm) in with semiconductors. Graphite is highly anisotropic and mechani- both diffusive and ballistic transport regimes. We also develop device cally soft due to having weak cross-plane van der Waals bonding. integration and provide experimental measurements of operational Nanomaterials such as graphene and nanotubes can be good con- AlGaN/GaN high-electron-mobility transistors (HEMTs), verifying ductors for individual materials, but, when integrated at practical the superior cooling performance of BAs and showing its substan- sizes, their thermal conductivity degrades due to ambient interac- tially lower hot-spot temperature (~60 K) than diamond (~110 K) tions and disorder scattering. and SiC (~140 K) at the same transistor power density. Recently, new compound semiconductors7–11 have been devel- oped experimentally based on ab initio theory12–20 and exhibit high Thermal management using BAs and BP cooling substrates thermal conductivity beyond that of most common heat conductors. TBR is limited by the scattering of energy carriers from both sides In particular, an isotropic thermal conductivity of 500 W m−1 K−1 has of the interface4,21,22 and has been shown to exist at all heterogeneous 1School of Engineering and Applied Science, University of California, Los Angeles, Los Angeles, CA, USA. 2Irvine Materials Research Institute, Irvine, CA, USA. 3These authors contributed equally: Joon Sang Kang, Man Li, Huan Wu, Huuduy Nguyen. ✉e-mail: [email protected] 416 NatURE ELECTRONICS | VOL 4 | JUNE 2021 | 416–423 | www.nature.com/natureelectronics NATURE ELECTRONICS ARTICLES a b BAs Diamond 1,000 Electronic chip BP ) Q –1 BN Temperature T Au Al GaN TBR = ∆ K Q –1 Ni Si Heat spreader 100 Pd (W m Pt Heat flux Ge Thermal conductivity GaAs Ti Distance 10 100 1,000 Debye temperature (K) Fig. 1 | Electronics thermal management using integrated HTC materials as a cooling substrate to improve heat dissipation. a, Schematic illustrating heat dissipation and thermal boundary resistance (TBR) at the interfaces in microchip packaging. TBR = ΔT/Q, where ΔT and Q are the temperature drop and heat flux across the interface, respectively. b, Room-temperature thermal conductivities and Debye temperatures of representative metals, semiconductors and HTC materials. interfaces, regardless of atomic perfection. Despite improvements palladium, platinum and titanium films were deposited on top in material quality, such as surface roughness and defects4,23, the of BAs and BP thin films by electron-beam evaporation to form ultimate limit of TBR for semiconductor device interfaces is usu- a clean metal–HTC interface, as verified by cross-sectional scan- ally dominated by the mismatch of atomistic vibrations (the modes ning electron microscopy (SEM; Fig. 2b). Our measurements of of which are defined as phonons21) across the interface (Fig. 1a). the temperature-dependent thermal boundary conductance (G, Under the classical Debye approximation, the vibrational properties the reciprocal value of TBR) are shown in Fig. 2d,e. In general, of materials are approximated by linear phonon dispersion, and the metal interfaces with BAs and BP are measured to have high ther- maximum temperature of the highest phonon frequency is defined mal conductance. The value of G varies for each metal but has a 24 as the Debye temperature (ΘD) . A simple metric to qualitatively similar trend for BAs and BP. For example, gold and titanium have evaluate the overlap between phonon spectra of two materials is the lowest and highest G values, respectively. The measured ther- 22 to compare their ΘD values (ref. ), where a smaller difference in mal boundary conductance values (at room temperature) between ΘD across the interface will be expected to result in a smaller TBR. BAs and gold, platinum, aluminium, palladium, nickel and titanium −2 −1 Figure 1b compares ΘD values for typical semiconductors, metals are 85, 133, 250, 290, 309 and 310 MW m K , respectively. These and HTC materials. values are slightly higher than those obtained with BP, as expected Most semiconductors (such as silicon, germanium, GaAs and from the difference in ΘD. From 300 K to 600 K, no substantial tem- GaN) and metals have ΘD values below 700 K, while prototype HTC perature dependence is observed for G, indicating that complete materials such as diamond and cubic BN have much higher values phonon excitation and a saturated phonon population are involved of ΘD (over 2,000 K) due to their large phonon group velocity. It is in the interface transport. The thermal boundary conductances therefore expected, and studies have confirmed, that there is a large of BAs and BP with metals are typically over 4× and 2.5× higher, TBR for diamond and BN interfaces after integration with typical respectively, than those of a metal–diamond interface30, verifying semiconductors, which substantially compromises their applica- their high heat dissipation efficiency. tion potential for thermal management, despite their high thermal conductivities. For example, the interface between diamond and Ab initio calculations and MD simulations of interface GaN has a mismatch in ΘD of over 1,500 K, resulting in a TBR of energy transport 2 −1 25–27 ~30 m K GW (refs. ). By comparison, BAs and BP have much To understand the experimental results, we performed atomistic lower ΘD values (for example, BAs has a ΘD of ~700 K), suggesting calculations to understand the phonon spectral contributions to they may show improved TBR upon integration.