Today’s production processes are far from ideal. They often lead to colour deviations. In addition, the manufacturing costs are very high due to the materials contained, such as sapphire, and the time-consuming manufacturing process. In his dissertation on “Photoluminescence at high temperatures from epitaxially growing nitride semiconductor layers for in-situ material characterization”, Dr.-Ing. Christoph Prall explored ways of increasing production efficiency and thus reducing costs in HRW laboratories.
During the production process, which takes several hours, it has not yet been possible to determine whether the desired colour tone of an LED has actually been achieved or whether deviations have occurred. Even fluctuations in the per thousand range at a production temperature of around 1000 degrees cause colour deviations that can be perceived with the naked eye. Only when the production process was completed did the result become visible and could only be checked on the finished product. However, a correction was then no longer possible. In his dissertation, Prall has now developed a method that uses photoluminescence to directly control and readjust the desired color tone during the running process (in situ) for the first time.
“Conventional surveillance techniques in LED production have so far used the principle of thermal imaging cameras to check the temperature during the process. However, the infrared light passes through the transparent materials of an LED, which makes direct temperature measurement difficult and inevitably leads to distortions,” explains the researcher. “We reapplied an old concept by illuminating the LED crystal layers for a fraction of a second during the birth of an LED with UV laser flashes. This was a lengthy process, but ultimately very successful,” says Christoph Prall.
According to Professor Dirk Rüter from the HRW, there is already interest from industry and other renowned research groups in the promising and innovative results of the dissertation; a patent application has already been filed.
Here you find the dissertation text (sorry, only in German): https://duepublico.uni-duisburg-essen.de/servlets/DerivateServlet/Derivate-45607/Diss_Prall.pdf