Reflection absorption infrared spectroscopy analysis of the evolution of ErSb on InSb

► We demonstrated an ex-situ technique based on infrared absorption to determine small amounts of ErSb on InSb surfaces. ► Infrared absorption associated with In-H stretching mode on an InSb surface decreased as ErSb surface coverage increased. ► In the early stage of ErSb deposition, indium atoms on an InSb surface are replaced by erbium resulting in increase in the In-H mode absorption. ► Using this simple ex-situ technique, we showed that it is possible to calibrate the amount of ErSb of few to tens of monolayers.