Carbon diffusion in undoped, n-type, and p-type GaAs

The effects of background doping, surface encapsulation, and As₄ overpressure on carbon diffusion have been studied by annealing samples with 1000 Å p-type carbon doping spikes grown within 1 μm layers of undoped (n^-), Se-doped (n⁺), and Mg-doped (p⁺) GaAs. The layers were grown by low-pressure metalorganic chemical vapor deposition using CCl₄ as the carbon doping source. Two different As₄ overpressure conditions were investigated: (1) the equilibrium p_As4 over GaAs (no excess As), and (2) p_As4 ∼2.5 atm. For each As ₄ overpressure condition, both capless and Si₃N ₄-capped samples of the n^--, n⁺-, and p ⁺-GaAs crystals were annealed simultaneously (825°C, 24 h). Secondary-ion mass spectroscopy was used to measure the atomic carbon depth profiles. The carbon diffusion coefficient is always low, but depends on the background doping, being highest in Mg-doped (p⁺) GaAs and lowest in Se-doped (n⁺) GaAs. The influence of surface encapsulation (Si ₃N₄) and p_As4 on carbon diffusion is minimal.