US researchers suggest alternative design for blue-emitting laser diodes
Photoelectrochemical-etched current aperture creates a more powerful laser
For the manufacturers of nitride laser diodes, ridge etching offers a fair trade-off between performance and cost. But the selection of the etch depth involves compromise: if it is shallow, that is, it leaves a p-side waveguide thickness of 100 nm or more, a high lateral current leakage and optical mode instability in the waveguide will impair performance; but if it is deep, the benefits of a decrease in threshold current density, strong index guiding and a high slope efficiency must be weighed against the challenges of realising high etching quality and good thermal management.
This dilemma for makers of laser chips could soon disappear, however, thanks to recent work by researchers at the University of California, Santa Barbara (UCSB). They have revealed promising results for an alternative design of blue-emitting laser, featuring a current aperture formed by photoelectrochemical etching.
Current aperture, edge-emitting blue laser diodes made using photoelectrochemical etching (left) differ from conventional designs (right).
Ludovico Megalini, spokesman for the UCSB team, claims that one important aspect of this work is that it shows that selective lateral etching, in a controllable way, is possible in a complex nitride-based structure, such as a laser diode.
"We selectively etched an InGaN-based active region from GaN-based bottom and top epilayers, but this could be applied also to etch GaN layers from AlGaN-based bottom and top epilayers," explains Megalini.
According to him, the other big breakthrough of this work is that it shows how to make nitride-based optoelectronic devices "“ and eventually electronic devices "“ that deliver superior performance to those made with traditional dry etching techniques. "Reactive-ion etching and inductively-coupled-plasma etching are known to cause sub-surface damage to the epitaxial structure."
Megalini believes that another advantage of photoelectrochemical etching, which is an established technique for etching silicon and zinc blende III-Vs, is that it is not just a technique for the lab "“ it can be used for high-volume manufacturing. "Being a wet-etched based technique, photoelectrochemical etching does not require expensive dry etching tools, and in general it is low cost, easy to do, and doesn't involve hazardous gases, like chlorine."
The researchers at UCSB fabricated their novel laser by applying a photoelectrochemical etch to an epistructure with an InGaN/GaN active region formed on the (20-2-1) plane of GaN. To create the current aperture, they added an opaque metal mask. This blocked light needed to generate carriers during the photoelectrochemical etching step involving potassium hydroxide.
After etching, Megalini and co-workers formed 1800 µm stripe-length lasers with an 8 µm wide ridge width and an active region width of 2.5 µm. Comparisons were then made between this chip and a shallow-etched laser with an identical strip-length. This control had a p-GaN ridge width and active region width of 2.5 µm and an etch depth of 420 nm (the remnant p-side thickness is 220 nm).
Switching to this novel design wrought many improvements in performance: It cut series resistance from 6 Ω to 4.7 Ω, trimmed threshold current density from 8.1 kA cm-2 to 4.4 kA cm-2, reduced threshold voltage from 7.6 V to 6.1 V, and increased slope efficiency from 0.07 W/A to 0.13 W/A.
Photoelectrochemical etching of the aperture to a narrower width resulted in further improvement. A device with a 1.5 µm-wide active region produced 15 mW, the highest single-mode CW output power for this type of laser.
One of the weaknesses of the photoelectrochemically etched lasers is their injection efficiency, which is just 65 percent. Much higher values should be possible, according to Megalini, who believes that nitride lasers produced by other groups may have injection efficiencies of 90 percent. To improve their lasers, the UCSB team will try to refine the design of the chip and reduce optical scattering from the rough edge of the device.
"I am working on both the material aspect "“ that is, I am trying to improve the general epitaxial structure of my laser diodes "“ and also on the fabrication side, trying to improve the photoelectrochemical etching technique," explains Megalini.
