Carbon doping in an InAl(Ga)As material system grown by LP-MOVPE lids been studied. Reduction of oxygen in carbon-doped InAlAs has been demonstrated by investigating the incorporation mechanisms of carbon and oxygen, and the effect of removal of oxygen by bromine. Through use of these growth procedures, we have successfully made a C-doped InAlAs with low oxygen contamination for the first time. The electrical and optical characteristics of the C-doped InAlAs have been investigated. In the material with low oxygen incorporation, the p-type carrier density is almost the same as the carbon concentration. For optical properties, photo-luminescence measurements have revealed that reduction in oxygen is the key to achieving InAlAs layers with high optical quality. The results indicate that C-doped InAlAs crystals with low oxygen would be well worth using in laser structures. (C) 2003 Elsevier Science B.V. All rights reserved.