Determination of the indium content and layer thicknesses in InGaN/GaN quantum wells by x-ray scattering

We have determined the indium content and the layer thicknesses in an InGaN epilayer and InGaN/GaN quantum well structures by high-resolution x-ray diffraction (XRD) using the (002) reflection. The thickness of the total repeat (an InGaN well plus a GaN barrier) in the superlattice is easily determined from the spacing between the satellite peaks in an omega/2theta scan. Measurement of the individual thickness of InGaN and GaN layers and the indium content is less straightforward, since for multilayer structures the peak positions are influenced by both the indium content and the thickness ratio of the GaN to the InGaN layer. Thus, several different models may give reasonable fits to data collected over a limited range (about 1 deg. omega/2theta either side of the (002)) showing only lower-order (-3 to +3) satellite peaks. Whenever possible, we have collected data over a wide range (about 4 deg. omega/2theta) and determined the thickness ratio by examination of the relative intensities of weak higher-order satellite peaks (-7 to +5). An alternative but less sensitive method is to use relative intensities from x-ray reflectivity measurements to give the thickness ratio. Once the thickness of both the InGaN and GaN layers has been established, the InGaN composition can be determined from the peak positions. If the quality of the samples is poor, because of inhomogeneities or wide diffuse interfaces, there are insufficient data to characterize the structures. There is good agreement between the composition of the epilayer as determined by XRD and secondary ion mass spectroscopy and good agreement between x-ray and electron microscopy results for the quantum well structures. We find no variation from Vegard's rule for In contents less than 0.20. This article shows that structural parameters of high-quality InGaN/GaN superlattices with 10 and 5 repeats can be determined reliably by x-ray techniques: The InGaN and GaN thicknesses to ±1 Aa and the In content to ±0.01