Materials Science and Engineering B91 – 92 (2002) 425 – 432 X-ray determination of the composition of partially strained group-III nitride layers using the Extended Bond Method N. Herres *, L. Kirste, H. Obloh, K. Ko ¨ hler, J. Wagner, P. Koidl Fraunhofer -Institut fu ¨r Angewandte Festko ¨rperphysik (FhG-IAF), Tullastr. 72, D-79108 Freiburg, Germany Abstract An extension of Bond’s method for determining precision lattice parameters serves to determine the lattice parameters of (strained) unit cells of a film from the peak positions of the film alone. We call this measurement and evaluation procedure the ‘Extended Bond Method’ (EBM). The procedure avoids recurrence to possibly unreliable or unavailable lattice parameters of the substrate; it is successful, irrespective of whether a layer is pseudomorphically strained, partially relaxed, or completely relaxed, as long as the strain can be described as a uniaxial distortion parallel to the growth direction. Using a mathematical/graphical evaluation procedure, the chemical composition of a layer is obtained, once its strained lattice parameters have been determined. The technique is of particular value for the determination of the strain and the composition of group-III nitride layers. © 2002 Elsevier Science B.V. All rights reserved. Keywords: X-ray diffraction; Extended Bond Method; Strain relaxation; InGaN www.elsevier.com/locate/mseb 1. Introduction High-resolution X-ray diffraction (HRXRD) is an established tool for the structural characterization of crystalline substrates and thin films. Lattice parameters, superlattice periods, layer thicknesses and the structural parameters deduced from these can be determined with high reliability and non-destructively. Materials devel- opment in the micro-electronics industry rely to a large extent on the data provided by HRXRD techniques. Heteroepitaxial layer systems for which materials problems have been solved, rarely pose any problems to the industrial diffractionist. In daily life, it is when technologists encounter problems during growth or processing, that the X-ray diffraction (XRD) labora- tory is challenged. In these cases, the X-ray investiga- tion very often is hampered by unwanted and unintentional effects like sample curvature, tilt of the layer relative to the substrate lattice, subgrain tilts, partial relaxation, etc. Here, we present an X-ray dif- fractometric technique based on the Bond method [1] which helps to overcome many, if not most, of these problems by virtue of the goniometer design and the measurement procedures. (Al, Ga, In)-nitrides are important materials for pho- tonic and electronic applications. In many cases it is the structural perfection that will determine if group-III nitride films are suited for the application. In an earlier paper [2] we showed how to deal with problems related to phase purity, texture and mosaicity. Here, we present a rapid and accurate technique to measure the strained metric (i.e. the lattice parameters of the unit cell) and deduce the chemical composition of partially relaxed hexagonal Al x Ga 1 -x N and Ga 1 -x In x N layers grown on sapphire substrates. 2. Motivation Hexagonal nitride films grown on sapphire tend to be strained uniaxially along the [0001] growth direction through stresses induced by thermal expansion differ- ences between the film and the substrate on cooling down from deposition temperatures of around 1000 °C (Fig. 1). As the structures of the layer and the substrate material differ distinctly with respect to interatomic distances and symmetry within the growth plane, no * Corresponding author. Present address: NTB, Werdenbergstr. 4, CH-9471 Buchs SG, Switzerland. E-mail address: herres@gmxpro.de (N. Herres). 0921-5107/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0921-5107(01)01036-4