Phase separation and atomic ordering in indium gallium nitride epitaxial layers

Phase separation and atomic ordering were investigated in InGaN layers grown by metalorganic chemical vapor deposition on (0001) sapphire substrates. Transmission electron microscopy (TEM) of InGaN layers during their early stages of growth reveal 2-D quantum rings that form spontaneously. In thick layers at InN contents of 3%, planview TEM images show a random distribution of atomic species and selected area diffraction (SAD) patterns do not exhibit satellite spots continuous to Bragg reflections. InN contents of 12% result in a speckled microstructure and satellites are present in SAD patterns. No satellites are observed along the [0001] direction, implying that phase separation is two-dimensional in nature and may occur on the surface while the layer is growing. These results are indicative of composition modulations lying in the (0001) growth plane. Samples containing InN fractions of between 22 and 34% exhibit microstructures having stronger contrast variations and SAD patterns with satellites further spaced from fundamental reflections. In cross-sectional TEM images, contrast striations oriented along [0001] are present except near the InGaN/GaN interface. The spacing of these striations is comparable to the composition modulation wavelengths calculated from SADPs and decreases with increasing InN content. Similarly, plan view TEM images taken from very thin specimens exhibit a domain structure with well aligned stripes within the domains. Increasing the growth rate from 400nm/h to 900nm/h results in a reduction in the intensity of satellite spots, indicating that the amplitude of composition modulations is reduced. The absence of contrast near the InGaN/GaN interface suggest reduced In incorporation, resulting in the absence of phase separation. Reduced In incorporation is confirmed by high angle angular dark field (HAADF) imaging and energy dispersive x-ray spectroscopy (EDS). X-ray diffraction and photoluminescence data are consistent with the occurrence of phase separation. The stability of phase separated microstructures were assessed at different temperatures. Annealing above the growth temperature eliminates satellite spots in diffraction patterns and results in a less periodic microstructure. No changes were observed by annealing at temperatures lower than the growth temperature. Diffraction spots at (0001) as well as diffuse intensity along <-1,1/2,1/2,1> are observed in (10-10) SAD patterns, implying the existence of atomic ordering.