Sumitomo snatches the green laser crown
Although this device’s performance characteristics are not good enough for commercialization, if they were improved this laser could then provide the green source laser TV and tiny, color projectors. These displays currently use cumbersome, expensive green lasers that combine an infra-red source with crystals that convert 1064 nm emission to 532 nm.
Sumitomo’s engineers grew their laser structures on the {202?1} plane of free-standing GaN substrates that were fabricated in-house.
The researchers say that the selection of this plane is the key to their success – it enables the fabrication of indium-rich InGaN quantum wells with a high degree of compositional uniformity.
A series of gain-guided lasers with 10 ?m stripes were fabricated by conventional deposition and lift-off techniques. The 531 nm variant was driven in pulsed mode with a 0.5 percent duty cycle, and had a threshold current density of 15.4 kA cm-2.
Devices emitting at 520 nm had a lower threshold current density of 8.2 kA cm-2, a threshold voltage of 17.7 V, and produced a maximum output power of 28 mW.
More recently, Sumitomo’s engineers have improved the 520 nm laser performance by adding a lattice-matched InAlGaN cladding layer, and switching to a ridge-stripe geometry. These refinements created a device that delivers 2.5 mW in continuous-wave mode, and has a threshold voltage and current density of 9.4 V and 7.9 kA cm-2, respectively.
Processing
Printing LED displays
Researchers at the University of Illinois at Urbana Champaign (UIUC) have led the development of a series of novel processes that can produce small and medium-sized displays featuring an array of LED pixels.
Conventional processes are unable to make LED displays of this size, because they cannot produce chips that are small enough. And even if this issue were overcome, this approach would not be attractive due to the large quantities of wiring needed to interconnect pixels.
Desktop monitors, home theatre systems and instrument gauges could all be produced with the US team’s technology. Wearable health monitors offer another opportunity, which would require the mounting of LED chips on a flexible platform.
The researchers built a range of small, red displays by first growing sacrificial 1.5 ?m-thick AlAs layers and GaInP-based LED structures on GaAs substrates. Square islands containing the LED epistructure and having a length of just 50 ?m were defined on the wafer by a combination of photolithography and inductively-coupled plasma, reactive-ion etching.
Immersion of the processed wafer in hydrofluoric acid etched away the vast majority of the AlAs layer. An automated printing tool with elastomeric stamp then pulled arrays of these tiny LED chips from the substrate, thanks to a sufficiently strong Van der Waals interaction, and printed them onto another surface. Fabrication of red displays was completed by adding electrical interconnects with planar processing methods.
Progressing to color displays will require the addition of blue and green LEDs. The team has already had success in this area, according to corresponding author John Rogers from UIUC, and a paper is now being written on that topic.
If this technology is to be transferred to the manufacturing arena it will have to demonstrate acceptable yields. “The yield of printed, working devices is very close to 100%,” says Rogers, “ and the yield for interconnected arrays interfaced to computer control systems is in the 85% range.” Most of these failures are associated with external connections to ribbon cables.
Similar etching and printing methods are also being employed by a partner in this research, Semprius, which is making small GaAs solar cells coupled to concentrating optics. “They have built working modules, and are on the verge of sending out research demos at the 1-10 kW level for customer evaluation,” says Rogers.
Lighting systems could also be produced with the processes developed by Rogers and his co-workers. “Options to licenses to the necessary IP have been obtained from Semprius by a separate startup, CoolEdge, that will pursue lighting exclusively, starting with systems for general illumination.”
Epitaxy
HVPE ups GaN resistivity
