We have measured the reflection spectra of ultra-thin InxGa1-xN/GaN layers on sapphire substrates with thick GaN buffer layer at temperatures of 300, 200 and 90K. We could not observe any signal associated with these layers. Comparing the measured spectra with the calculated ones for well-mixed InxGa1-xN layers, we concluded that our samples had no mixed state. We also concluded that the separated In and N atoms did not form InN layers. The reason for the fact that we did not observe a response from these layers is the inhomogeneous broadening of the InxGa1-xN reflection line. We have calculated the absorption spectra of small parallelepiped-like InN particles in the GaN matrix. It has shown that the absorption line width for such small parallelepiped-like particles is bigger than that for the bulk InN material. We concluded that our samples contained separated In and demonstrated formation of InN clusters.
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