What Is To Blame For The Low Energy Efficiency Of GaN-based Lasers?
Why is the power conversion efficiency of the leading GaN lasers just half of that of the best LEDs?
BY JOACHIM PIPREK FROM THE NUSOD INSTITUTE LLC
Back in 2014 Shuji Nakamura received a Nobel Prize for Physics for the invention of the efficient GaN-based blue LED, a device that has enabled energy-saving white light sources. At the time he predicted that this device could soon be usurped by the GaN-based laser. But this is yet to happen "“ and it's not going to any time soon.
Although such lasers are being implemented in the headlights of high-end cars, such as BMWs, they are failing to make much impact in the general solid-state lighting market. That's predominantly because the blue LED's power conversion efficiency can hit 84 percent, while that for the laser is, at best, just 43 percent (the power conversion efficiency is defined as the fraction of electrical input power emitted as light output power).
If lasers are to displace LEDs in solid-state lighting, this gap will have to close. And if this is going to happen, efforts must begin with a comprehensive understanding of the physical mechanisms that are behind the low laser efficiency. At the NUSOD Institute we have been trying to do just that: read on to discover our findings.
To uncover a deeper understanding of the efficiency deficit, we have used advanced laser simulations to reproduce and analyse measured laser characteristics. These efforts have focused on a study of Panasonic's 7.2 W laser that emits at 405 nm. It delivers a record-breaking output power, due in part to a novel double-heat-flow packaging technology that trims the thermal resistance to about 7 K W-1.
Another attribute of Panasonic's laser is its minimal internal optical loss (see Figure 1 for vertical profiles of refractive index and lasing mode intensity). Thanks to a small overlap between the lasing mode and the highly absorbing p-doped AlGaN cladding layer, the modal absorption coefficient falls to a record-low value of just 2.5 cm-1.
One insight from our numerical analysis is that most of the remaining absorption is caused by free carriers inside the waveguide layers, which are located between the quantum wells and electron blocker layer (this is shown in the dashed line in Figure 1). Triggering free-carrier absorption is electron leakage from the quantum wells, which rises with increasing current injection.
Self-heating of the laser diode is behind the escalation in electron leakage. Despite the device's low thermal resistance, the temperature of the quantum well increases by 120 degrees when the injection current hits 4 A. It is well-known that self-heating is detrimental, reducing gain in the quantum well, so more carriers are required to maintain lasing (see Figure 2 for plots of laser power and carrier density with and without self-heating).
It is often assumed that the carrier density in the quantum well is constant. However, this cannot be the case when the temperature in the well changes. If it rises, the density of carrier increases. In turn, there is an increase in carrier losses inside the quantum wells via Auger recombination, defect-related recombination, and spontaneous photon emission. Of these three, Auger recombination rises the fastest, because it is proportional to the third power of the carrier density.