InGaN epitaxial layers grown by metalorganic chemical vapor deposition were investigated in order to understand the occurrence of composition modulations in the GaN–InN system. The In contents of the samples were determined to be x=0.21 and 0.31. Transmission electron microscopy was performed on [0001], [101̄0], and [112̄0] zone-axis specimens. Plan-view images display a domain structure, representing regions in which the directions of the modulations differ. Intersections between domains occur in 〈101̄0〉, and 〈112̄0〉, and other directions. Satellite spots appear in selected-area diffraction patterns. These observations can be explained by diffraction effects resulting from periodic composition modulations. An equation was derived relating the spacing between the satellites and the reflections to the wavelength of the modulations in the wurtzite structure. The sample with x=0.21 had a wavelength of λ=3.1±1.3 nm and the one with x=0.31 had λ=3.2±1.3 nm. Since Young’s modulus is isotropic in the (0001) plane, no particular direction is favored for the modulations based on strain energy considerations. This result is consistent with the observation of the variously oriented domains and satellites.
We report the determination of the energy-band offsets between GaN and AlN using the linewidth (full width at half maximum) of an extremely sharp excitonic luminescence transition in Alx Ga1-x N alloy with x=0.18 at 10 K. Our sample was grown on C -plane sapphire substrate by metal-organic chemical-vapor deposition at 1050 °C. The observed value of the excitonic linewidth of 17 meV is the smallest ever reported in literature. On subtracting a typical value of the excitonic linewidth in high-quality GaN, namely, 4.0 meV, we obtain a value of 13.0 meV, which we attribute to compositional disorder. This value is considerably smaller than that calculated using a delocalized exciton model [S. M. Lee and K. K. Bajaj, J. Appl. Phys. 73, 1788 (1993)]. The excitons are known to be strongly localized by defects and/or the potential fluctuations in this alloy system. We have simulated this localization assuming that the hole, being much more massive than the electron, is completely immobile, i.e., the hole mass is treated as infinite. Assuming that the excitonic line broadening is caused entirely by the potential fluctuations experienced by the conduction electron, the value of the conduction-band offset between GaN and AlN is determined to be about 57% of the total-band-gap discontinuity. Using our model we have calculated the variation of the excitonic linewidth as a function of Al composition in our samples with higher Al content larger than 18% and have compared it with the experimental data. We also compare our value of the conduction-band offset with those recently proposed by several other groups using different techniques. © 2006 American Institute of Physics
Epitaxial layers of InGaN were deposited by metalorganic chemical vapor deposition on a GaN layer/sapphire substrate in order to ascertain compositional variation in the GaN-InN system. Samples were examined with In contents from x = 0.09 to 0.31. Plan-view images obtained by transmission electron microscopy reveal a domain structure within which the composition is modulated. Satellites appear around the fundamental reflections in the diffraction pattern. The spacing between the satellite and the reflection can be related to the wavelength of the modulations. Equations are derived for modulations in the ½10 1 10 and ½11 2 20 directions. Modulations were measured in the ½10 1 10 direction and found to be about l = 3.2 nm for all samples. Strain energy considerations explain the observation of modulations along different directions.Introduction GaN and its alloys have attracted considerable attention for optoelectronic devices. Emission over the entire visible spectrum can be obtained by suitable alloying. InGaN has provided the active layer in blue laser diodes [1]. It was predicted [2] to undergo spinodal decomposition, which researchers [3] have found indeed to be the case. Non-circular spots in the diffraction patterns have been described as resulting from a multicomposition structure [4,5]. Although this is true, the means by which the spots obtain this shape have not yet been elucidated. Analysis of the spots can provide important information about the microstructure.
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