Reflectance together with pyrometry and deflection (bow) measurements are nowadays standard monitoring techniques during the epitaxy of nitride semiconductors. But even with those, the composition of a typical InGaN quantum well (QW) cannot be measured during growth. Also the defects are not seen since their densities usually are not high enough to influence the reflection. The only exceptions are defects that change the topography by causing roughness or facets.
Thickness and composition of QWs as well as defect densities can be found out afterwards. However, changes to the surface during growth (like single or double metal layers) cannot be found out later. For instance adding indium strong enhances the metal coverage on the surface. Using very sensitive optical monitoring with ellipsometry in the UV region (above 4.5 eV photon energy) allows following such surface changes. Altering the V/III ratio during growth of AlN showed similar indications for a surface transition. In both cases the topography of the layers was different depending on growth conditions.
Growing on patterned substrates/templates is another field where the analysis of reflectance signals is not straight forward. But in this case there is a signal that can be analyzed. From beating pattern coalescence as well as growth rate can be extracted. This will be demonstrated by the growth monitoring of semipolar GaN (11-22) grown on patterned silicon (113) or m-plane Sapphire templates. Still the growth monitoring is limited to the growth rate, and access to defect densities is not possible.
As an outlook, some further monitoring techniques will be discussed, which could improve resolution even in commercial epitaxy reactors.

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2. The influence of surfactant and the nucleation process to InN growth by metal-organic vapor phase epitaxy
3. Quantitative Dopant Profiling in the SEM including surface states.
4. Does electron microscopy produce In clustering in InGaN?
5. MOVPE growth of InN on Sapphire