(Al,In)N layers and (Al,In)N/GaN heterostructures grown by plasma-assisted molecular beam epitaxy on 6H-SiC(0001)

We study the properties of (Al,In)N layers and (Al,In)N/GaN heterostructures grown on 6H-SiC(0001) by plasma-assisted molecular beam epitaxy. The (Al,In)N films are deposited on a GaN buffer layer. A growth temperature of 500 degrees C and above results in low In contents which give rise to cracks due to the large tensile strain experienced from the underlying GaN buffer layer. In addition, these layers exhibit strong phase separation leading to inhomogeneous In composition and rough surfaces. In contrast, samples with homogeneous and well-controlled In-contents between 10%-30% are reproducibly obtained in the temperature range of 250-350 degrees C. Surprisingly, nominally lattice-matched layers with an In content of 17%-18% also exhibit cracks. Symmetric omega-2 theta x-ray diffraction scans and reciprocal space maps reveal the presence of a strain gradient in these layers despite the apparently lattice-matched conditions. Transmission electron microscopy indicates that these cracks are the result of tensile stresses induced by crystallite coalescence and grain-boundary formation. This mechanism can be counteracted by augmenting the adatom mobility through increasing the growth temperature and the N flux. However, phase separation sets an upper limit on the growth temperature and a moderate increase to 350-400 degrees C is sufficient to obtain crack-free and homogeneous (Al,In)N layers. The results of our growth experiments lead to a phase diagram which shows the optimum growth window for (Al,In)N layers. By choosing the growth conditions within this window, we are able to obtain crack-free Al0.82In0.18N/GaN multilayers with abrupt interfaces.