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

Abstract

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.

Figures (5)

• Figure 1: (a) Schematic cross-sectional structure of the AlGaN/GaN/AlN HEMT on a 4H-SiC substrate. The structure includes an AlGaN barrier, a 150 nm GaN channel layer, and an AlN buffer layer with various thicknesses (120, 550, 1000 and 2000 nm). (b) Energy-band diagram indicating the formation of a 2DEG at the AlGaN/GaN interface and a 2DHG at the GaN/AlN interface.
• Figure 2: (a) XRD $\omega$-2$\theta$ scans for the HEMTs samples with varying AlN buffer layer thicknesses. (b) In-plane strain ($\varepsilon_{xx}$) of AlN and GaN layers as a function of AlN buffer thickness. (c) The threading dislocation density in the AlN buffer and GaN channel layers.
• Figure 3: Atomic force microscopy (AFM) surface images of GaN layers grown on AlN buffer layers with different thicknesses: (a) 120 nm, (b) 550 nm, (c) 1000 nm, and (d) 2000 nm. All scans are performed over a $5 \times 5~\mu\mathrm{m}^2$ area.
• Figure 4: Temperature-dependent Hall measurements of GaN layers grown on AlN buffer layers with different thicknesses (120 nm, 550 nm, and 2000 nm): (a) Electron mobility, (b) Sheet carrier density, and (c) Sheet resistance. The measurements were performed from 77 K to 300 K. The sample with a 2000 nm AlN buffer shows the highest mobility and lowest sheet resistance at low temperatures.
• Figure 5: Thermal conductivity of the AlN buffer layers with thickness of 120 nm, 550 nm, 1 $\mu$m and 2 $\mu$m, measured in the temperature range of 100 - 400 K. The solid lines present results from calculations based on the modified Callaway model.