Impact of AlN buffer thickness on electrical and thermal characteristics of AlGaN/GaN/AlN HEMTs

We investigate the influence of AlN buffer thickness on the structural, electrical, and thermal properties of AlGaN/GaN high-electron mobility transistors (HEMTs) grown on semi-insulating SiC substrates by metal-organic chemical vapor deposition. X-ray diffraction and atomic force microscopy reveal that while thin AlN layers (120 nm) exhibit compressive strain and smooth step-flow surfaces, thicker single-layer buffers (550 nm) develop tensile strain and increased surface roughness. Multi-layer buffer structures up to 2 μm alleviate strain and maintain surface integrity. Low-temperature Hall measurements confirm that electron mobility decreases with increasing interface roughness, with the highest mobility observed in the structure with a thin AlN buffer. Transient thermoreflectance measurements show that thermal conductivity (ThC) of the AlN buffer increases with the thickness, reaching 188 W/m.K at 300 K for the 2 μm buffer layer, which is approximately 60% of the bulk AlN ThC value. These results highlight the importance of optimizing AlN buffer design to balance strain relaxation, thermal management, and carrier transport for high-performance GaN-based HEMTs.

💡 Research Summary

This work investigates how the thickness of an AlN buffer layer influences the structural, electrical, and thermal performance of AlGaN/GaN high‑electron‑mobility transistors (HEMTs) grown on semi‑insulating 4H‑SiC substrates by metal‑organic chemical vapor deposition (MOCVD). Four buffer configurations were examined: a thin single‑layer (120 nm), a thicker single‑layer (550 nm), and two multi‑layer designs (1 µm and 2 µm). X‑ray diffraction (ω‑2θ) showed that increasing the AlN thickness improves crystal quality, as evidenced by narrower and more intense AlN (0002) peaks. However, the 550 nm single‑layer exhibited two overlapping AlN peaks, indicating coexistence of a partially relaxed upper region and a lower region still under compressive stress due to lattice and thermal‑expansion mismatch with SiC. Multi‑layer buffers mitigated this stress, yielding a more uniform in‑plane strain state and preserving the near‑zero strain in the 150 nm GaN channel.

Atomic force microscopy revealed that the 120 nm buffer produced a smooth step‑flow surface (RMS ≈ 0.35 nm), whereas the 550 nm single‑layer showed increased roughness (RMS ≈ 0.63 nm). The multi‑layer structures restored surface smoothness (RMS ≈ 0.40 nm) despite their greater thickness, confirming that strain‑relief engineering can maintain interface quality.

Low‑temperature Hall measurements (77 K–300 K) demonstrated that the thin 120 nm buffer delivers the highest 2DEG mobility (≈ 1670 cm² V⁻¹ s⁻¹) and a sheet carrier density of 1.27 × 10¹³ cm⁻², reflecting minimal interface scattering. The 550 nm single‑layer suffers a mobility drop to ≈ 1357 cm² V⁻¹ s⁻¹, attributed to higher dislocation density and rougher interfaces. The 1 µm and 2 µm multi‑layer buffers recover mobility to ≈ 1500 cm² V⁻¹ s⁻¹ and ≈ 1326 cm² V⁻¹ s⁻¹, respectively, while maintaining low sheet resistance, indicating that the thicker buffers do not compromise carrier transport when strain is properly managed.

Thermal conductivity of the AlN buffer was measured by pump‑probe transient thermoreflectance (TTR). Thin buffers (120 nm) exhibit low thermal conductivity (~70 W m⁻¹ K⁻¹) due to dominant phonon‑boundary scattering. As thickness increases, boundary scattering diminishes and three‑phonon Umklapp processes become dominant, raising κ to 190 W m⁻¹ K⁻¹ for the 2 µm buffer—about 60 % of bulk AlN’s value (321 W m⁻¹ K⁻¹). The temperature dependence of κ aligns well with a modified Callaway model that incorporates boundary, defect, and Umklapp scattering, confirming the physical interpretation.

Overall, the study shows a trade‑off: ultra‑thin AlN buffers provide superior electron mobility and smooth interfaces but suffer from poor thermal conduction, limiting their suitability for high‑power operation. Thick single‑layer buffers relieve strain inadequately and degrade electrical performance. Multi‑layer AlN buffers of 1–2 µm thickness achieve strain relaxation, preserve interface quality, and substantially improve thermal conductivity, thereby offering a balanced solution for high‑performance GaN HEMTs. The authors conclude that optimal AlN buffer design—favoring multi‑layer structures with controlled thickness—is essential to simultaneously meet the electrical and thermal demands of next‑generation power and RF devices.