In this paper, we investigated C doped Al0.33Ga0.67As thin film epitaxially grown on a Zn-doped GaAs substrate using spectrally resolved modulated photothermal infrared radiometry (SR-PTR). We assumed that thermal conductivity and diffusivity of the thin layer are known and estimate values of the infrared absorption coefficient of the thin layer and the thermal boundary resistance of the interface between the thin layer and the substrate. We found out that the thermal boundary resistance is two orders of magnitude greater than thermal boundary resistance of studied recently undoped AlGaAs/GaAs sample. We attribute this effect to formation of quasi 2-dimensional hole gas due to modulation doping. Finally, the infrared absorption coefficient of the thin layer decreases with increasing wavelength due to inter-valence band absorption. We found out that with increasing infrared absorption coefficient of the thin film, the sensitivity of the method for estimation of the infrared absorption coefficient increases, while the estimation error decreases.