InGaN

The InGaN alloys have attracted much interest as they can be used to cover the emission spectral range from the ultraviolet to the near infrared by varying the InN fraction. The large difference in interatomic spacing between GaN and InN gives rise to compositional inhomogeneities and strains that strongly affect the optical properties of the InGaN epilayers. As the InN fraction is varied between the end member compounds, the InGaN band gap crosses the laser excitation energy and selective resonance excitation of inhomogeneous regions shows in the frequency and lineshape of the resonantly-enhanced A1(LO) mode. We have studied these effects by changing the excitation energy of the Raman experiments. Figure 1 shows the Raman spectra of InGaN epilayers covering the entire composition range. The frequency dependence of the E2 and A1 modes is found to be in good agreement with the one-mode behaviour predicted by the modified random isodesplacement model.

The proximity of the band gap energy to the excitation energy for certain InN fractions gives rise to strong out-going resonances for scattering by the polar A1(LO) mode. Figure 2 displays the Raman spectrum of the In0.27Ga0.73N layer obtained with the 2.71 eV line showing multiphonon scattering up to 3A1(LO).

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