Treatment of GaN with SiH4 and NH3 increases the size of surface pits associated with threading dislocations, allowing them to be easily imaged by atomic force microscopy. Here, we assess the effect of a similar treatment on AlxGa1-xN surfaces for x <= 0.4. For relaxed AlxGa1-xN epilayers, an increase in the observed size and density of threading dislocation pits is observed. However, if the AlxGa1-xN is under tensile strain, the treatment results in the appearance of nanometre-scale surface hillocks. These hillocks may prevent observation of the dislocation pits. The hillocks are found to consist of crystalline AlxGa1-xN, and hence are suggested to be formed by strain driven etching or transformation of the surface by SiH4 and NH3. (C) 2007 Elsevier B.V. All rights reserved.