A new dry photochemical etching process using HBr gas to selectively remove InGaAs layer over an InAlAs layer has been developed. A 172 nm Excimer lamp was used as the photon source for this experiment. The intensity of the 172 nm light on the surface of the samples was 3 mW/cm2. Etch rates of 62 Å/min and 0.61 Å/min are obtained for InGaAs and InAlAs, respectively, at a chamber pressure of 120 mTorr, sample temperature of 80 °C and HBr flow rate of 10 sccm. These rates translate into a selectivity of over 100 for this material system. Scanning electron microscopy pictures and visual inspection reveal that this process is almost damage free, compared to reactive ion etching. This is due to lack of ion bombardment in the photochemical process. X‐ray photoelectron spectroscopy analysis suggests that the etch selectivity mechanism is due to the formation of nonvolatile  Al2O3on the surface of the InAlAs layer. This etching system should be very useful for fabrication of electrical as well as optical devices based on InGaAs/InAlAs heterojunction.