Many advanced III - V devices require highly strained heteroepitaxial layers less than 25 nm in thickness, with tight specifications on both the layer thickness and composition. In many cases the layers required are close to the critical thickness.

The growth conditions for these thin layers are often extrapolated from established conditions for thicker layers. This method can result in layers which have the incorrect thickness and composition because of the transients which occur as growth commences. To minimise this problem it is desirable to establish growth conditions for layers which are as close to device requirements as possible. X-ray diffraction is capable of measuring layer thicknesses and compositions non-destructively. The minimum measurable layer thickness is usually within a small factor (typically 0.5 to 5 times) of device requirements.

A single x-ray rocking curve is required to determine the thickness and composition of an unrelaxed (strained) layer. At least two rocking curves are required when relaxation is present. This paper discusses the appropriate choice of measurement conditions for a given sample.