Systematic Study of Insulator Deposition Effect (Si3N4, 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 Al2O3) have been deposited on Al0.3Ga0.7N/GaN heterostructures. A significant increase in two-dimensional electron gas (2DEG) density (Ns) was observed for all the insulators with the order of Ns(Al2O3) > Ns(AlN) ~ Ns(SiO2) > Ns(Si3N4) > N0 (N0: Ns without insulators). This resulted in a decrease in sheet resistance (R) with the smallest order of R(Al2O3) < R(AlN) < R(Si3N4) < R0 ~ R(SiO2) (R0: R without insulators). This order is the same as that of Ns except for SiO2, where the 2DEG mobility largely degraded due to the diffusion of Si atoms into nitride layers. The increase in Ns was theoretically analyzed in terms of the change in the potential profile, and the following parameters were extracted: (i) the surface potential barrier (B), and (ii) the interface charge (NInt) between an insulator and AlGaN. B (eV) was estimated to be 1.7 (Si3N4), 2.2 (AlN), 2.7 (Al2O3), and 3.6 (SiO2), exhibiting a positive correlation between B and the bandgap of the insulator. NInt (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.

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