The Universities of Strathclyde (lead author Dr Zheng Gong right) and Peking, together with Shanghai-based Epilight Technology Co, Ltd focused on gallium nitride-based high-bright blue and green EPI wafers and LED chips, have made what is believed to be the first demonstration of yellow–green (560nm) and amber (600nm) III-nitride micro-LED arrays, based on new epitaxial structures.

Anticipating applications for micro pixelated InGaN LED arrays coming from scientific instrumentation, optical communications and micro-displays, the researchers need LED emissions can be tuned to the absorption lines of biological active molecules, allowing optogenetic experiments to be controlled in both time and space, as with optoelectronic tweezer (OET) control.

Rather than using expensive nitride substrates, the epitaxial material was grown on standard (0001) sapphire using metal-organic chemical vapour deposition (MOCVD). The freestanding nitride substrates are used for producing devices with longer wavelengths (green, yellow, amber) reducing defects and polarisation field effects which are critical to performance.

Different buffer and active layers of InGaN/GaN wells and barriers were used in the two device types emitting light of wavelengths 560nm and 600nm. Using a low-indium-content InGaN/GaN electron reservoir layer is believed to increase carrier capture in the QWs above, improving radiative efficiency.
The micro-LEDs can sustain a high current density, up to 4.5 kA cm−2, before thermal rollover. A significant blueshift of the emission wavelength with increasing injection current is observed.

The blueshift saturates at 45nm (50nm) for the yellow–green (amber) LED array, and numerical simulations have been used to gain insight into the responsible mechanisms in this microstructured format of device. In the relatively low-current-density regime (<3.5 kA cm−2) the blueshift is attributable to both the screening of the piezoelectric field by the injected carriers and the band-filling effect. 

MacEtch MOCVD optimised for GaAs
 

Metal-assisted chemical etch has two steps. A thin layer of gold is patterned on top of a semiconductor wafer with soft lithography.  The gold catalyses a chemical reaction that etches the semiconductor from top down, creating 3D structures for optoelectronic applications (Left Graphic by Xiuling Li)

Illinois researchers have developed an efficient, lower-cost method of manufacturing compound semiconductors such as gallium arsenide for many electronic device applications. i

[Below: the research team, from the left: Professor Xiuling Li, student Ik Su Chun, postdoctoral researchers Sungjin Jo and Jongseung Yoon, and professor John Rogers. Photo by Liz Ahlberg.]

MacEtch is essentially a wet etching method yet produces anisotropic high aspect ratio semiconductor micro and nano structures without incurring lattice damage. MacEtch produced semiconductor nano structures can be integrated into solar cells with better absorption and collection efficiency, for thermoelectric devices with low thermal conductivity when the sidewalls are rough, and for batteries with greater energy density.

"It is a big deal to be able to etch GaAs this way,"  says Li. "Realisation of high-aspect-ratio III-V nanostructure arrays by wet etching can potentially transform the fabrication of semiconductor lasers where surface grating is currently fabricated by dry etching, which is expensive and causes surface damage."

To create metal film patterns on GaAs surfaces, Li's team used a patterning technique pioneered by John Rogers,  Lee J. Flory founder chair and  Professor of materials science and engineering at UI.
The research teams joined forces to optimise the soft lithography, for chemical compatibility while protecting the GaAs surface. Soft lithography is applied to the whole semiconductor wafer, as opposed to small segments, creating patterns over large areas—without expensive optical equipment.

"The combination of soft lithography and MacEtch make the perfect combination to produce large-area, high-aspect-ratio III-V nanostructures in a low-cost fashion," said Li.

The researchers hope to further optimise conditions for GaAs etching and establish parameters for MacEtch of other III-V semiconductors. Then hope to demonstrate device fabrication, including distributed Bragg reflector lasers and photonic crystals.