Quantitative nanoscale temperature mapping across the multi-quantum well of a light-emitting diode in operation using vacuum null-point scanning thermal microscopy to evaluate local energy conversion efficiency

Electrical energy that is not converted into light in light emitting diodes (LEDs) is locally dissipated as heat in the active layers. Therefore, by measuring the temperature distribution with nanoscale resolution across the multi-quantum well (MQW) of an LED in operation, the effect of nanostructures inside the LED on the local energy conversion efficiency can be observed. In this study, we first demonstrated that vacuum null-point scanning thermal microscopy (VNP SThM) could be used to quantitatively map the two-dimensional temperature distribution across the MQW of an LED in operation with a sufficient signal-to-noise ratio. Subsequently, by increasing the injection current in four steps, we quantitatively mapped the temperature distribution across the MQW at each step and observed the shift in the temperature peak across the active layers due to the increase in injection current. The measurements of the temperature distribution around the MQW indicate that as the injection current increased, the overall temperature around the MQW increased significantly, and the temperature peak position shifted. These results show that the main cause of the dissipation of electrical energy into thermal energy inside an LED changes as the injection current increases, and the nanostructures inside an LED affect the dissipation of electrical energy into thermal energy. The high thermal sensitivity, nanoscale resolution, and convenience of VNP SThM may enable the direct observation of the effect of the nanostructures inside various types of nanophotonic devices on local energy conversion even under intense localized radiation.