Selective area regrowth of silane-doped GaN achieving record carrier density for ultra-low resistive Ohmic contacts for AlGaN/GaN HEMT

Abstract

Low-temperature (LT) selective area growth (SAG) of degenerately silicon (Si) doped GaN by metal-organic-chemical vapor deposition (MOCVD) technique yielded a record-high charge carrier concentration (ns) of 2–2.2 × 1020 cm−3 with a mobility of 116–119 cm2/V s, utilizing triethylgallium and silane (SiH4) as gallium and Si precursors, respectively, and trimethylindium as a surfactant. While SiH4-doped GaN typically yields lower carrier concentrations (≤1020 cm−3) than disilane (Si2H6), this work demonstrates that defect and dislocation suppression enables degenerately doped GaN with ns exceeding 1020 cm−3 using the widely adopted SiH4 precursor. The use of an indium surfactant enhanced both ns and mobility by suppressing defect formation, enhancing 2D-step flow deposition mode compared to 3D island deposition mode in highly Si-doped GaN, enabling ns exceeding 1020 cm−3. SAG of LT-n++-GaN contact layer using SiH4 dopant in an AlGaN/GaN high-electron mobility transistor (HEMT) yielded an ultra-low sheet resistance (RSh) of 22 Ω/□ and a surface roughness of 0.33 nm, resulting in a record low Ohmic contact resistance of 0.065 Ω mm, along with a metal/n++-GaN contact resistance of 0.023 Ω mm, two-dimensional electron-gas-to-n++-GaN contact resistance of 0.025 Ω mm, and an access resistance (Rn+) of 0.017 Ω mm. A scaled AlGaN/GaN HEMT showed ultra-low on resistance (RON) of 0.36 Ω mm and a superior RF performance with current gain cutoff frequency (fT) and maximum oscillation frequency (fmax) of 160 and 195 GHz, respectively. Advancements in LT selective area MOCVD growth of n++-GaN using SiH4 enable regrown contacts in ultra-scaled AlGaN/GaN HEMTs, supporting commercialization for mm-wave applications.