  The depth dependence of crystalline structure within thin films is critical
for many technological applications, but has been impossible to measure
directly using common techniques. In this work, by monitoring diffraction peak
intensity and location and utilizing the highly angle-dependent waveguiding
effects of X-rays near grazing incidence we quantitatively measure the
thickness, roughness and orientation of stratified crystalline layers within
thin films of a high-performance semiconducting polymer. In particular, this
diffractive X-ray waveguiding reveals a self-organized 5-nm-thick crystalline
surface layer with crystalline orientation orthogonal to the underlying
65-nm-thick layer. While demonstrated for an organic semiconductor film, this
approach is applicable to any thin film material system where stratified
crystalline structure and orientation can influence important interfacial
processes such as charge injection and field-effect transport.
