Si-implanted ohmic contacts with modulated pulse activation annealing for down-scaled RF GaN HEMTs

Abstract

A modulated pulse annealing method combined with an Al2O3/SiNx annealing cap is utilized for activating implanted Si in n+ ohmic contacts for AlGaN/GaN HEMTs. The objective is to prevent GaN decomposition during annealing and to reduce the alloying temperature of ohmic contacts. This enables the alloying of SiNx-encapsulated contacts for obtaining smooth ohmic contact edges, which is important for downscaled Ka/K-band HEMTs with a short gate–source distance. Al2O3 functions as a diffusion barrier against nitrogen, and SiNx protects the epitaxial layer stack during cap etching and prevents GaN decomposition due to the loss of N. Furthermore, the Al2O3/SiNx cap acts as a retardation layer for Si implantation to achieve a maximum Si concentration in the GaN channel under the AlGaN barrier. Before activation annealing, H is outgassed from the amorphous Al2O3 at 500 °C, which is then crystallized at 900 °C for enhancing its strength. Then it follows the modulated pulsed activation annealing with five pulses peaking at 1250 °C, four pulses at 1300 °C, and one at 1325 °C. The Al2O3/SiNx cap is then removed with dry and wet etching to leave a clean surface, before the deposition of a Ti-based ohmic contact metal. At an implantation dose of 3.5 × 1015 cm−2, 27% activation is achieved without any GaN decomposition, providing a contact resistance of 0.32 Ω mm after contact alloying only at 650 °C for 5 min.