The impact of film thickness and substrate surface roughness on the thermal resistance of aluminum nitride nucleation layers

Abstract

Thickness dependent thermal conductivity measurements were made on aluminum nitride (AlN) thin films grown by two methods on the (0001) surfaces of silicon carbide (SiC) and sapphire substrates with differing surface roughness. We find that the AlN itself makes a small contribution to the overall thermal resistance. Instead, the thermal boundary resistance (TBR) of 5.1 6 2.8 m 2 K/GW between the AlN and substrate is equivalent to 240 nm of highly dislocated AlN or 1450 nm of single crystal AlN. An order-of-magnitude larger TBR was measured between AlN films and SiC substrates with increased surface roughness (1.2 nm vs. 0.2 nm RMS). Atomic resolution TEM images reveal near-interface planar defects in the AlN films grown on the rough SiC that we hypothesize are the source of increased TBR. Nitride semiconductors are essential for blue/green light emitting diodes (LEDs) and high electron mobility transistors (HEMTs). LEDs are now being pushed to high power for lighting applications causing considerable heat generation due to Joule heating and inefficiencies in light production. 1 HEMTs are essential to high-power and high-speed switching operations that generate heat as a byproduct. 2,3 While thermal packaging is essential to minimize operating temperatures, nearly half of the total thermal resistance comes from the nitride device itself. 2,3 Experiments on bulk nitrides show that thermal transport is phonon-dominated. [4][5][6][7] Internal thermal resistance of nitride devices is complicated by the presence of interfaces between films and defects within films that scatter phonons and suppress thermal conductivity below bulk values.Non-native silicon carbide (SiC) and sapphire substrates are used for the commercial growth of nitride films because gallium nitride (GaN) and aluminum nitride (AlN) substrates are not yet economically viable. Growth is initiated with an AlN nucleation layer because direct growth of GaN on these substrates is not favorable at high temperatures. Due to a mismatch in the lattice parameters (a) of AlN (a AlN ¼ 3.07 Å ) with SiC (a SiC ¼ 3.11 Å ) and sapphire (a Sapp ¼ 4.785 Å ), dislocations and surface defects form at the interface and impact the quality of subsequent growth. Though SiC and AlN have a small mismatch (1%), their high elastic moduli require stress relief through such defect formation. 8,9 Prior studies agree that the AlN nucleation layer is a dominant thermal resistance in both LED 10 and HEMT architectures. 2,3,11,12 The source of this thermal resistance, however, is as yet experimentally unresolved as the total thermal resistance R T consists of three components: (1) the AlN/substrate TBR (TBR sub ), (2) the AlN intrinsic resistance L AlN /k AlN (where L is film thickness and k is thermal conductivity), and (3) the GaN/AlN TBR. Our prior study suggests that TBR sub is largest for AlN films grown on mechanically polished (MP) SiC substrates. 5 Nonetheless, it is unclear whether this conclusion holds for SiC vs. sapphire substrates, different growth techniques, or varied su...

Phonon scattering in strained transition layers for GaN heteroepitaxy

“…Although the transmission coefficients do not directly account for defects on either side of the medium, the fact that the model requires considerably lower values than those predicted by the DMM may suggest the potential role played by near-interfacial defects around the AlN interfaces with the adjacent GaN and substrate. A recent transmission electron micrograph [58] (not of our samples but relevant) illustrated planar defects within a few nanometers of the AlN interface with the rough SiC, which further supports the potential importance of near-interfacial defects. As a consequence, a combination of near-interfacial defects and point defects within the AlN transition film may be responsible for the AlN resistance, rather than only defects in the AlN.…”

Differential 3ω method for measuring thermal conductivity of AlN and Si3N4 thin films

“…Prior studies have shown that surface roughness affects the interface and the thermal properties of thin films [9,17,18]. Su et al [17] showed that a reduced thermal conductivity of AlN thin films can result from high surface roughness of the substrate material. It has been shown that for a substrate with 1.2 nm surface roughness defects at the interface impact the thermal boundary resistance (TBR) significantly [17].…”

“…Su et al [17] showed that a reduced thermal conductivity of AlN thin films can result from high surface roughness of the substrate material. It has been shown that for a substrate with 1.2 nm surface roughness defects at the interface impact the thermal boundary resistance (TBR) significantly [17]. However, for substrate surface roughness less than 0.2 nm no planar Table 1 Experimental results of the thermal conductivity measurements of AlN and Si 3 N 4 thin films using the differential 3ω method.…”

Thermal conductivity and mechanical properties of AlN-based thin films

“…In case, the film thermal conductivity was independent of the film thickness, the contribution of the film to the total thermal resistance increases linearly with its thickness. Since the contributions from thermal boundary resistances are independent of the film thickness, they could, as previously suggested by Cho et al 19 and Su et al, 20 be evaluated by studying a series of films with different thicknesses.…”