Single-energy, MeV implant isolation of multilayer III-V device structures

A single-energy, implant isolation scheme for thick (≥1.5 μm) III-V semiconductor device structures such as heterojunction bipolar transistors (HBTs) is described. A 5-MeV O⁺ implant at doses around 10 ¹⁵ cm^-2 produces an almost uniform damage profile over ∼2 μm, sufficient to isolate structures containing highly doped (p=7×10¹⁹ cm^-3) individual layers. The heavily damaged region associated with the end of the O⁺ ions range is placed in the underlying semi-insulating substrate. Resistivities above 10⁸ Ω/D'Alembertian sign are obtained in GaAs/AlGaAs HBTs with such an implant, following annealing at ∼550°C. High-quality, 2×5 μm² HBTs with gains of 25 for base doping of 7×10 ¹⁹ cm^-3 have been fabricated using this isolation scheme. A considerable simplification is achieved over the use of conventional keV implants, where up to ten separate ion energies are required to isolate an HBT structure.