Optimization of Ohmic Contacts and Surface Passivation for ‘Buffer-Free’ GaN HEMT Technologies

Licentiate thesis, 2020

Gallium nitride high electron mobility transistors (GaN HEMTs) draw attention from high frequency and high power industries due to unique properties including high electron mobility and saturation velocity combined with high breakdown voltage. This makes GaN HEMTs suitable for power devices with high switching speed and high frequency applications with high power density requirements. However, the device performance is still partly limited by problems associated with the formation of low resistivity ohmic contact, trapping effects, and the confinement of the two-dimensional electron gas (2DEG).

    In this work, reproducible deeply recessed Ta-based ohmic contacts with a low contact resistance of 0.2 - 0.3 Ωmm, a low annealing temperature of 550 - 600 °C, and a large process window were optimized. Low annealing temperature reduces the risk of 2DEG degradation and promotes better morphology of the ohmic contacts. Deeply recessed ohmic contacts beyond the barrier layers make the process less sensitive to the etching depth since the ohmic contacts are formed on the sidewall of the recess. The concept of deeply recessed low resistivity ohmic contacts is also successfully demonstrated on different epi-structures with different barrier designs.

    Passivation with silicon nitride (SiN) is an effective method to suppress electron trapping effects. Low Pressure Chemical Vapor Deposition (LPCVD) of SiN has shown to result in high quality dielectrics with excellent passivation effect. However, the surface traps are not fully removed after passivation due to dangling-bonds and native oxide layer at the interface of passivation and epi-structure. Therefore, a plasma-free in-situ NH3 pretreatment method before the deposition of the SiN passivation was studied. The samples with the pretreatment present a 38% lower surface-related current collapse and a 50% lower dynamic on-resistance than the samples without the pretreatment. The improved dynamic performance and lower dispersion directly yield a 30% higher output power of (3.4 vs. 2.6 W/mm) and a better power added efficiency (44% vs. 39%) at 3 GHz. Furthermore, it was found that a longer pretreatment duration improves the uniformity of device performance.

    Traditionally, decreasing leakage currents in the buffer and improving electron confinement to the 2DEG are achieved by intentional acceptor-like dopants (iron and carbon) in the GaN buffer and back-barrier layer made by a ternary III-nitride material. However, electron trapping effects and thermal resistivity increase due to the dopants and the ternary material, respectively. In this thesis, a novel approach, where a unique epitaxial scheme permits a thickness reduction of the unintentional-doped (UID) GaN layer down to 250 nm, as compared to a normal thickness of 2 μm. In this way, the AlN nucleation layer effectively act as a back-barrier. The approached, named QuanFINE is investigated and benchmarked to a conventional epi-structure with a thick Fe-doped-GaN buffer. A 2DEG mobility of 2000 cm^2/V-s and the 2DEG concentration of 1.1∙10^13 cm^-2 on QuanFINE indicate that the 2DEG properties are not sacrificed with a thin UID-GaN layer. Thanks to the thin UID-GaN layer of QuanFINE, trapping effects are reduced. Comparable output power of 4.1 W/mm and a PAE of 40% at 3 GHz of both QuanFINE and conventional Fe-doped thick GaN buffer sample are measured.