We investigate the impact of the carbon tetrabromine (CBr4) and arsine (AsH3) concentrations, the V/III ratio at the gas inlet (R-inlet) and the temperature (T-g) on the growth rate, the solid composition, the incorporated carbon (C) and free hole concentration in GaAsSb:C grown by metal-organic vapor-phase epitaxy (MOVPE). The introduction of CBr4 deeply perturbs the growth when compared to when no C-doping is used. The growth of GaAsSb:C using CBr4 behaves as if an effective V/III ratio (R-eff) were defined as the R-inlet leading to the same growth rate in the absence of CBr4 under the same group V inlet flux. This consideration allows (and/or accounts) for the growth of high-quality layers with R-inlet << 1, and explains alloy composition and growth rate variations as well as non-monotonic variations of the free hole concentration with increasing CBr4 flow. We show how R-eff similar to 1 can be experimentally determined to optimize the growth conditions in MOVPE reactors. An increase of the CBr4 flux enhances the solid arsenic (As) concentration and reduces the growth rate. The As concentration dependence on the CBr4 flux becomes stronger at higher R-inlet. Although the incorporated C density monotonically increases with increasing CBr4 flow, the hole concentration increases and then drops. This drop is attributed to the amphoteric character of C and/or to the formation of C-C bonds. The peak hole density is enhanced from 2.5 x 10(19) to over 1.6 x 10(20) cm(-3) by reducing R-inlet from 0.72 to 0.25. Furthermore, the hole density decreases from 1.6 x 10(20) to 5.5 x 10(19) cm(-3) if the AsH3 concentration increases from 0.31 to 0.51. Thus, alloys with As-rich composition obtained by increasing the AsH3 composition in the gas phase cannot achieve hole concentrations as high as 1 x 10(20) cm(-3) at T-g=550 degrees C. However, we demonstrate that GaAs0.7Sb0.3 with doping levels of 1 x 10(20) cm(-3) could only be achieved by decreasing T-g to 510 degrees C. (C) 2009 Elsevier B.V. All rights reserved.