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

摘要

Heavily C-doped GaAs epilayers have been grown by atmospheric pressure metalorganic vapor phase epitaxy with hole concentration ranging from 2 X 10~(19) to 1.6 X 10~(20) cm~(-3). In order to study the stability of C acceptors over this range, the films have been annealed at 810℃ for 10 min under two mixture gas AsH_3 + H_2 or only N_2. 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 X 10~(19) cm~3. An annealing under AsH_3 + H_2 ameliorates the crystalline properties contrarily to an annealing under N_2. 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 AsH_3 + H_2. An unusual change of I_(LOPC)/I_(LO) ratio is observed in samples annealed under N_2. Indeed, for high doping (～ 10~(20) cm~(-3)) the ratio I_(LOPC)/I_(LO) < 1 and for relatively low doping (～2 X 10~(19) 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.