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EpiTT for UV-C LEDs

280nm reflectance senses AlGaN surface morphology

Above: growth of AlN/AlGaN (60 percent Al) on a sapphire/AlN template in Aixtron CCS 6x2 reactor: black "“ 280nm reflectance; blue "“ 405 nm reflectance; green "“ high-resolution wafer bow; red "“ true temperature.

AlGaN buffer layers with high aluminum content are necessary for optimal UV-C LED performance. But their band-edge lies below 300nm, so the established 405nm in- situ reflectance is insensitive to the surface morphology of such AlGaN layers.

To monitor precisely both AlGaN growth rate and surface morphology during UV-CLED epitaxy, Laytec now offers an additional 280nm reflectance channel in its EpiTT in-situ system metrology tool that employs a UV-C LED as a light source.

The graph above shows the results measured in-situ during the growth of an AlGaN layer: The Fabry-Perot oscillations of the final AlGaN layer are damping out because the band edge of the material shifts toward longer wavelength at the growth temperature.

The small reflectance reduction at 12000s indicates a small roughening of the AlGaN surface. The green line delivers the high-resolution wafer bow data. This study is supported by Advanced UV for Life funding (grant number 03ZZ0105C, BMBF). 

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