What causes the low energy efficiency of GaN-based lasers?
The energy efficiency is the fraction of the electrical input energy that is emitted as laser light. It is usually given as power conversion efficiency (PCE) and it is surprisingly low for GaN-based lasers. OSRAM just announced a record number of PCE=43% at SPIE Photonics West. This is certainly a remarkable achievement, considering the struggle to break the mysterious 40% limit. However, 43% is far below the record PCE of 84% reported for GaN LEDs. The inherently low hole conductivity and large series resistance on the p-doped side of GaN lasers are usually blamed for the efficiency deficit. However, the series resistance is known to shrink with rising temperature, which can be attributed to the increasing density of free holes in p-doped layers. Thus, one would expect that the PCE improves at elevated temperatures. But the OSRAM paper reported that the measured PCE drops with higher ambient temperature despite the shrinking series resistance. Ergo, there seems to be an even stronger loss mechanism involved.
Immediately after my return from Photonics West, I therefore went back to my previous GaN laser efficiency studies and simulated the laser performance at 80ºC ambient temperature, as measured by OSRAM. Compared to the results at room-temperature, electron leakage and free-carrier absorption are not much larger. However, carrier losses inside the quantum wells (QWs) now contribute more strongly to the power loss (see picture). The main reason behind this phenomenon is the rising QW carrier density which leads to enhanced Auger recombination. In other words, the common assumption is obviously incorrect, that Auger recombination has little influence on the high-power performance of GaN-lasers.
By the way, the record PCE of 43% was measured for blue light emission. Green InGaN/GaN lasers exhibit a much lower efficiency, which points to defect-related recombination as additional major loss mechanism.