Titanium dioxide (TiO2) cap layers were deposited onto C-doped InGaAs/AlGaAS QW laser structures by electron-beam evaporation in order to investigate their effect on atomic interdiffusion. In comparison to the as-grown sample, a negligible shift of the photoluminescence peak was observed after annealing at 900°C, indicating that the atomic interdiffusion was greatly suppressed by TiO2 capping layers. For the uncapped sample, the high temperature annealing step significantly improves the threshold current density in laser diode devices but leaves the internal efficiency unchanged. We attribute these effects to the activation of the carbon p-type dopant, as demonstrated by electrochemcial C-V capacitance-voltage and X-ray measurements. SIMS analysis shows that the carbon atomic profile does not significantly change after annealing. In contrast, a similar Zn doped laser structure shows an almost flat Zn profile after annealing at 925°C, due to considerable indiffusion from the highly doped p++ GaAs top contact layer in to the rest of the structure.