Effects of In Situ SiN Layer on the Leakage Current of AlGaN/GaN HEMTs

For conventional GaN based high electron mobility transistors (HEMTs) fabrication process, Au based ohmic contacts are usually adopted to form low resistance ohmic contact. However, Au based ohmic contact usually needs high temperature annealing process and it induces high density of surface states which causes high leakage current of devices [1]. In this work, in-situ SiN layer is employed to prevent degradation of the GaN surface from high temperature annealing during fabrication of GaN HEMT. Since in-situ SiN was grown at high temperature environment, it is heat resistant enough to protect GaN surface from high temperature stress. Three samples were fabricated and compared to analyze the effect of in-situ SiN layer.<br/><br/>The heterostructures was grown on a Si substrate consisting of 50-nm-thick SiN cap layer, 20-nm-thick Al_0.25Ga_0.75N barrier, GaN channel and buffer layer. The fabrication process starts with mesa-isolation with inductively-coupled-plasma reactive-ion-etching (ICP-RIE) using BCl₃/Cl₂ gas. The in-situ grown SiN on the region of ohmic contact was etched with ICP-RIE using SF₆gas. The multilayer Ti/Al/Ni/Au (30/180/40/150 nm) metallization was deposited using e-beam evaporator. Rapid thermal annealing (RTA) process was followed to form the ohmic contacts. The samples were annealed at the temperatures of 900°C for 70s in nitrogen ambient. For a sample A, any surface treatments or etching processes were not applied. For the sample B, TMAH treatment was applied on the surface of sample for 1min at the temperature of 100°C to remove high temperature stress related surface states [2]. For the sample C, the in-Situ SiN layer was etched with low power ICP-RIE using SF₆gas. The Ti/Au (20/300 nm) pad metallization was deposited using e-beam evaporator. Finally, Ni/Au (20/300 nm) was deposited to form the gate electrodes using e-beam evaporator. The fabricated GaN HEMTs have an 8.5-µm source-drain spacing, 3-µm gate length and 2×50µm gate width. The electrical characteristics of the GaN HEMTs were measured using HP4155A semiconductor parameter analyzer.<br/><br/>The leakage currents of fabricated samples were compared to analyze the effects of in-situ SiN. The gate leakage current densities of sample A, B and C were 73 µA/mm, 142 nA/mm, 5 nA/mm respectively at the gate bias of -10 V. The sample C exhibited the lowest gate leakage current, followed by the sample B and sample A. It was found that the TMAH treatment reduces the leakage current by removing surface states on SiN. The removal of SiN damaged due to the thermal stress further reduced the leakage current. The low leakage current of sample C implies that the in-situ SiN successfully protected the GaN surface from high temperature stress suppressing the formation of surface states on the GaN surface.<br/><br/>References<br/>[1] T. Hashizume and H. Hasegawa, "Effects of nitrogen deficiency on electronic properties of AlGaN surfaces subjected to thermal and plasma processes", Applied Surface Science, vol. 234, no. 1-4, pp. 387-394, 2004.<br/>[2] Y. Yoon et al., "TMAH-based wet surface pre-treatment for reduction of leakage current in AlGaN/GaN MIS-HEMTs", Solid-State Electronics, vol. 124, pp. 54-57, 2016.