micro-Raman 3D thermometry with diffraction-limit spatial resolution for GaN-based light-emitting diodes*

Confocal micro-Raman microscopy performed in the transparent spectral region of a semiconductor can in principle be used for operando 3D thermometry with optical diffraction-limit spatial resolution. However, when applied to high power GaN-based light-emitting diodes (LEDs), the applicability is hindered by the often strong secondary electroluminescence (EL) in the visible spectral region that overwhelms the Raman signal. We develop a “split-time-window” scheme that can mimic the continuous wave (CW) operation but without the interference of the secondary emission, which allows us to carry out noninvasive 3D temperature profiling, thus, a comprehensive thermal analyses of the whole device, at any operation current. The technique is applied to an InGaN/GaN LED to extract its 3D temperature distribution when operated at 350 mA with µm scale resolution when using 532 nm laser. We show that although a conventional technique can yield reliable average temperature difference between the heat sink and the LED junction (a few degrees), the spatial fluctuations are much larger than the average difference. Furthermore, we show that using anti-Stokes to Stokes Raman intensity ratio as metric can yield more reliable and accurate results than using Raman frequency shift.