In-situ monitoring tools for III-Nitride MOVPE are typically based on optics - because photons can penetrate the gas-phase easily and can sense the actual growth status even for complex or very fast wafer rotation configurations. The physical wafer-related parameters that can be detected today in-situ are: wafer temperature, growth rate, material composition, and - if present - wafer curvature and surface roughness. In addition, complete finger-printing of device growth (e.g., stop-band wavelength and cavity dip of laser structures) by means of spectroscopic in-situ sensors is increasingly used.
This talk will focus on the physics background of the optical in-situ methods presently in use for monitoring III-Nitride growth: emissivity corrected pyrometry, spectroscopic reflectance, and laser beam deflection. Based on this we will discuss how the optimum wavelength range of a sensor can be chosen with respect to the material grown in the actual III-Nitride process.
We will compare the different techniques regarding their applicability to process development, process calibration, process monitoring and control. Here the accuracy and reproducibility of the different methods will be discussed and the related sensitivity limits will be estimated. It will be demonstrated that the over-all performance of a specific in-situ sensor depends not only on the sensor's degree of sophistication but also on the application conditions at the very different types of epitaxial growth systems.
Finally, we will review very recent results on actually challenging in-situ applications like the effect of wafer curvature and wafer doping on the true wafer temperature, absolute wafer temperature calibration for process transfer between different epitaxial growth systems and the quantitative analysis of in-situ optical data measured on structured and masked substrates.

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