Nov_Dec_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 2 0 1 1 which have been used in energy-saving LEDs since the 2000s. The material has interesting optic and electronic prop- erties, making it attractive for many applications, including power-switch- ing devices in electric vehicles. But it can develop defects and impurities during its fabrication, affecting perfor- mance. Currently available methods for testing these crystals are expensive or too invasive. ODPL spectroscopy is a non-inva- sive technique that can test the crystals, but only at room temperature. The abil- ity to change the crystal’s temperature is important to properly test its prop- erties. The Tohoku team found a way to set up an ODPL instrument so that the crystal can be cooled. The process involves placing a gallium nitride crys- tal on an aluminum plate connected to a cooling device. This is placed under an “integrating sphere,” which collects light coming from many directions. For testing, a sample is placed outside the integrating sphere and onto an aluminum plate connected to a cooling device. Courtesy of Tohoku University. External light is shone through the sphere onto the crystal, exciting it. The crystal emits light back into the sphere in order to return to its initial unexcited state. The two lights, from the external source and the crystal, are integrated within the sphere and measured by a detec- tor. The result reveals the crystal’s internal quantum efficiency, which is reduced if it contains defects and impurities and can be mea- sured even at very low temperatures. The team’s modifica- tion—placing the crystal outside the sphere and con- necting it to something that cools it—means the tem- perature change happens only within the crystal and not within the sphere. The researchers say further work will use the method for testing other materials, such as perovskites, for use in highly efficient solar cells and boron nitride as an atomically thin 2D mate- rial. tohoku.ac.jp/en.