Raman studies of heavily carbon doped GaAs
Optical measurements, mainly Raman spectroscopy, are used to study GaAs heavily doped with carbon. Hole concentration in these samples ranges from 2.3 x 10 to 1x10 cm^20^-3. Three main Raman features are investigated: the longitudinal-optic (LO) phonon mode, the substitutional carbon-at-arsenic local vibrational mode (CAS LVM), and the coupled plasmon-LO phonon mode (LOPC). CAS LVM intensity is directly proportional to carrier concentration. This implies that CAS LVM intensity is a good carrier density indicator, even though its practical use is limited by its weakness. Only one phonon-like coupled mode is observed due to the large plasmon damping and high effective carrier masses. The coupled mode is seen to systematically red shift as p increases even though the peak width of the mode stays constant. This behavior is described by a model which includes the effects of high hole concentrations on the dielectric function and an additional shift in the LO phonon we attribute to carbon size effect. Interestingly, the local mode intensity shows good correlation with that of LOPC mode as a function of p. Based on these results, the intensity of LOPC to that of the LO phonon is determined to be a good indicator of the carrier concentration. ILOPC/ILO decreases upon annealing, implying p reduction. Simultaneously, two peaks around 1375 and 1600 cm"' appear in all the annealed samples. The two peaks are assigned to carbon precipitates. From the observation, it is believed that carbon precipitates into a nanocrystalline graphite phase upon annealing. The crystal size of the carbon precipitates was estimated from the peak intensity ratio. Emission due to a conduction band to acceptor level transition (e.A) was observed from photoluminescence (PL) spectra. The band redshifts as carrier concentration increases due to band gap shrinkage. PL intensity of the (e.A) transition in annealed samples is drastically decreased. Carrier concentration reduction and the formation of nonradiative recombination centers are suggested as the cause of the behavior, and carbon precipitates observed in Raman spectra are suggested as the possible nonradiative recombination centers.