Comparative study between electrical, optical and structural properties of annealed heavily carbon doped GaAs

Heavily C-doped GaAs epilayers have been grown by atmospheric pressure metalorganic vapor phase epitaxy with hole concentration ranging from 2×10¹⁹ to 1.6×10²⁰ cm⁻³. In order to study the stability of C acceptors over this range, the films have been annealed at 810 °C for 10 min under two mixture gas AsH₃+H₂ or only N₂. Annealing of the layers resulted in a decrease in carrier concentration, carrier mobility and lattice mismatch with undoped GaAs. The lattice matching conditions of C-doped GaAs layers were systematically investigated by using X-ray high-resolution diffraction space mapping. The comparison between electrical and structural before and after annealing of layers properties indicates that the simultaneous decrease of carrier concentrations, Hall mobility and mismatch is probably related to an increase of compensation. Basing on a theoretical calculation of mobility as a function of hole concentration and Vegard's law, we estimate that the compensation comes from the formation of (C-C)⁺_[100] interstitial couples. This fact does not exclude definitively the possibility of the formation of other species such as H-C_As especially for hole concentration lower than 5×10¹⁹ cm⁻³. An annealing under AsH₃+H₂ ameliorates the crystalline properties contrarily to an annealing under N₂. The optical properties have been investigated using Raman spectroscopy. Two main Raman features are observed before and after annealing of the layers: the longitudinal-optic (LO) phonon mode and the coupled plasmon-LO phonon (LOPC). As for as grown layers, the intensity ratio I_LOPC/I_LO between the intensity of LOPC peak and the LO peak increases by increasing the hole concentration. This ratio is about 1 after an annealing of layers under AsH₃+H₂. An unusual change of I_LOPC/I_LO ratio is observed in samples annealed under N₂. Indeed, for high doping (∼10²⁰ cm⁻³) the ratio I_LOPC/I_LO<1 and for relatively low doping (∼2×10¹⁹ cm⁻³) the ratio I_LOPC/I_LO>1. This behaviour is probably related to the high sensibility of Raman measurement not only to the hole concentration change but also to the surface quality.