Stress and Crystal Quality Analysis of GaN Layers on Cost-Effective Substrates for Vertical High-Power Applications

For vertical transistor technology in the high-power regime, Gallium Nitride (GaN) layers grown on foreign substrates provide a cost-effective alternative to native GaN substrates but face challenges due to structural and thermomechanical mismatch. This study compares the structural properties of GaN grown on silicon (Si) and sapphire $\left(\text{Al}_{2} \mathrm{O}_{3}\right)$ substrates with those on a native GaN substrate. The analysis focuses on micrometer-thick $\mathrm{n}^{+}-\text{GaN}$ (drain) and n-GaN (drift) layers, which are critical for vertical transistor functionality. The high crystal quality of these layers is essential, as GaN's large piezoelectric constants make it highly sensitive to strain. Defects alter the electrical potential and field, increasing threshold voltage and leakage currents while reducing charge carrier mobility and thermal conductivity. Raman spectroscopy with sub-micrometer spatial resolution reveals substratedependent stress distributions: slight tensile stress (about 100 MPa) in GaN on the native substrate, compressive stress $(\sim-790 \text{MPa})$ on $\text{Al}_{2} \mathrm{O}_{3}$, and minimal residual stress ($\sim 10 \text{MPa}$) on Si. The reduced stress on the Si substrate is attributed to an advanced buffer structure incorporating superlattice layers. Crystal quality, assessed through the full width at half maximum (FWHM) of the Raman $E_{2}$ (high) phonon mode, shows that $\mathrm{n}^{+}-\mathrm{GaN}$ on $\mathrm{Al}_{2} \mathrm{O}_{3}$ achieves a FWHM of $2.4 ~\text{cm}^{-1}$, comparable to $2.3 ~\text{cm}^{-1}$ on the native substrate, while Si has broader peaks of about $4.0 ~\text{cm}^{-1}$. This work demonstrates the viability of GaN on Si and $\text{Al}_{2} \mathrm{O}_{3}$ as cost-effective platforms for vertical power transistors, supporting the development of scalable, high-performance GaN-based devices on foreign substrates at Si costs.