Systematic study of insulator deposition effect (Si₃N ₄, SiO₂, AlN, and Al₂O₃) on electrical properties in AlGaN/GaN heterostructures

To systematically examine the effect of insulator deposition on the electrical properties in AlGaN/GaN heterostructures, the Si- and Al-based insulators (Si₃N₄, SiO₂, AlN, and Al ₂O₃) have been deposited on Al_0.3Ga _0.7N/GaN heterostructures. A significant increase in two-dimensional electron gas (2DEG) density (N_s) was observed for all the insulators with the order of N_s(Al₂O₃) > N _s(AlN) ∼ N_s(SiO₂) > N _s(Si₃N₄) > N₀ (N₀: N_s without insulators). This resulted in a decrease in sheet resistance (R) with the smallest order of R(Al₂O₃) < R(AlN) < R(Si₃N₄) < R₀ ∼ R(SiO ₂) (R₀: R without insulators). This order is the same as that of N_s except for SiO₂, where the 2DEG mobility largely degraded due to the diffusion of Si atoms into nitride layers. The increase in N_s was theoretically analyzed in terms of the change in the potential profile, and the following parameters were extracted: (i) the surface potential barrier (φ_B). and (ii) the interface charge (N_Int) between an insulator and AlGaN. φ_B (eV) was estimated to be 1.7 (Si₃N₄), 2.2 (AlN), 2.7 (Al ₂O₃), and 3.6 (SiO₂), exhibiting a positive correlation between φ_B and the bandgap of the insulator. N _Int (10¹³ cm^-2) was estimated to be ∼0 (Si₃N₄), 0.1 (SiO₂), 0.3 (AlN), and 0.5 (Al₂O₃); thus, the interface was found to be positively charged for AlN and Al₂O₃, whereas it was found to be almost neutral for Si₃N₄ and SiO₂. Thus, the insulator deposition effect has been shown to be significant and to vary among insulators. The analysis shown here offers a guideline for understanding and designing the electrical properties in AlGaN/GaN heterostructures, where insulators are deposited as surface passivation and/or gate insulators.