Free hole absorption in doped AlxGa1-xAs films, grown by molecular-beam epitaxy on semi-insulating GaAs substrates, was investigated. Free carrier absorption for three different hole concentrations with the same Al fraction and for two different Al fractions with the same doping concentration was studied. Experimental absorption coefficients were obtained from the data using a model that includes multiple reflections in the substrate wafer. In the 100–400 μm range, (3,5,8)×1018 cm-3 Be-doped Al0.01Ga0.99As films have absorption coefficients of ∼(3,3.5,5)×103 cm-1, respectively, where the magnitude of the absorption is found to be almost independent of the wavelength. This allows replacing doped GaAs emitters in heterojunction interfacial work function internal photoemission far-infrared (HEIWIP) detectors with p-AlxGa1-xAs layers with x≪0.017 facilitating the extension of the threshold wavelength of HEIWIP detectors beyond the 92 μm limit due to the practical Al fraction growth limit of 0.005 in molecular-beam epitaxy.