A PhD student at the University of Duisburg-Essen has developed a new method for monitoring LED production processes. Instead of after production, the emission spectra can already be controlled and readjusted during the production process.
The method developed is based on photoluminescence (PL). The principle is already used extensively by LED manufacturers to test the emission spectrum at the very end (ex-situ) of the manufacturing process. As part of his doctoral thesis, Dr.-Ing. Christoph Prall developed an in-situ procedure for this purpose. Since the emission spectrum is determined at production temperatures of a few hundred degrees, a model for the transfer to room temperature is additionally necessary, which Dr. Prall also created.
Manufacturers can predict the resulting emission spectrum of an LED with an accuracy in the nanometer range as soon as the first semiconductor layers have grown. If deviations have occurred here, the emission wavelength can be corrected for all other layers to be grown according to Dr. Prall. »Since only the top layers of an LED structure significantly contribute to the emission, real-time post-control could be realized,« Dr. Prall points out the possible applications on page 4 of his dissertation.
For PL measurements on GaN at high temperatures, continuously emitting argon-ion lasers and helium-cadmium lasers were previously used. The newly developed approach plans the use of a pulsed neodymium YAG laser. In pulsed operation, significantly higher excitation energies can be applied on the GaN for a short time without any damaging effect. This would generate sufficiently high measurement signals to carry out the PL method also on standard industrial plants with typically very narrow optical paths, according to Prall.
Temperature is a critical parameter during semiconductor growth of gallium nitride (GaN) and indium gallium nitride (InGaN), which is commonly required for LEDs. It is around 1000 °C and fluctuations in the thousandth range already lead to deviations in the emission spectrum that the human eye can detect.
Due to the structure of the industrial production facilities and the materials used, temperature measurements are not yet possible directly on the semiconductor layer, but only on the wafer sample carrier. Effects such as substrate curvature can cause temperature deviations between substrate and semiconductor layer.
The research took place as part of an external doctorate at the University of Duisburg-Essen. Dr. Prall was one of 14 cooperative doctoral students at the Ruhr West University of Applied Sciences. The high-tech company Laytec from Berlin and the epitaxy plant manufacturer Aixtron were involved in the project as industrial partners.