research review
think that there is another factor, which is more important, [saturation of the re- combination rate,” says Shim.
According to him, the model proposed by Schubert’s group is predominantly based on a p-n homojunction, and fails to consider the active region where recombination occurs. In particular, it fails to explain the following three experimental observations: Photoluminescence induces efficiency droop, LEDs of different colours are subjected to different levels of droop, and variations in device architecture impact LED behaviour.
Regarding his first point, Shim cites independent work by Osram and Philips Lumileds showing that resonant photoluminescence (PL) induces an efficiency droop similar to
electroluminescence (EL). “This indicates that there is another factor in the efficiency droop, which is inherent in quantum wells,” says Shim.
Expanding on his second point, Shim adds that the ultraviolet LEDs incorporating p-type AlGaN are expected to show more carrier asymmetry than the p-GaN used in blue and green LEDs. “However, UV LEDs show
less efficiency droop than blue green LEDs.”
Schubert can counter many of the issues raised by Shim. He argues that measurements performed by his team show that resonant-PL droop occurs at much higher excitation densities than EL droop. “Therefore, the resonant-PL droop does not need to have the same physical origin as the EL droop. I would not rule out carrier leakage as an explanation for the PL droop.”
According to Schubert, two factors can explain why green LEDs are more prone to droop than their blue cousins: Stronger polarization fields that supress carrier capture; and inferior p-type material quality, including lower p-type concentration and mobility that ultimately stems from the need for lower growth temperatures.
Schubert’s team have just published a paper with an analytical model for LED droop (see Appl. Phys. Lett. 100 161106 (2012)). With this model, a widening of the active region reduces droop – in other words, differences in the design of the active region do impact device behaviour. Claims based indium clustering that appear in the
Mechanical transfer with boron nitride
Unlike etching or laser-lift off, mechanical approaches for separating GaN epitaxial structures from their substrates can be quick, simple and scalable to large areas.
A mechanical approach for separating GaN devices from their sapphire substrates promises to replace the sacrificial etching and laser lift-off techniques used in today’s fabs, thanks to the recent development of BN films by engineers at NTT Basic Research Laboratories, Taiwan.
This team has demonstrated the strength of its approach by first depositing hexagonal BN films on sapphire substrates, and then growing various structures onto these templates , including: AlGaN/GaN structures with high electron mobility, and epitaxial layers for making multiple-quantum well LEDs. High-quality chips from 5 mm square to 2 cm square have been extracted from these wafers by mechanical lift-off and transferred to other substrates.
Corresponding author Yasuyuki Kobayashi believes that the BN-based mechanical transfer process could be popular with both LED and HEMT manufacturers: “We have already demonstrated that a very thin
flexible GaN-based LED can be fabricated in a pair of laminate films, which may be of interest to LED manufacturers. Our technology also makes it feasible to transfer AlGaN/GaN HEMTs onto any materials having high thermal conductivity, which may be attractive for transistor manufacturers who have been suffering from heating problems.”
NTT has already filed several patent applications for its process, and it plans to license the technology to other chipmakers.
The Japanese lab started working on BN, a wide bandgap material that could be used for making devices operating in the deep ultraviolet spectral range, in 2005. Growth of single phase, hexagonal BN by MOCVD followed in 2007, and in 2008 this growth technology yielded epitaxial films of this wide bandgap semiconductor on sapphire.
“This success opened an avenue of one- step, damage-free release and transfer of a wide range of GaN-based devices,” claims Kobayashi. Epitaxial growth of BN by MOCVD is very challenging. “No lattice- matched substrate is available for BN, at least in affordable form,” says Kobayashi. He explained that specially designed substrate heating equipment is needed to achieve substrate temperatures of 1300 °C to 1500 °C, which are reported to be optimal for the growth of BN films.
To demonstrate the promise of their technology, the team deposited a BN layer on sapphire, before adding different
June 2012
www.compoundsemiconductor.net 43
Korean conjecture are also controversial – Colin Humphreys’ group at Cambridge University, which has investigated GaN-based structures using scanning electron microscopes and atom probe tomography, claims that there is no indium clustering in quantum wells.
Shim’s response to this body of work is that indium can be locally segregated around point or line defects. He cites reports by others of clustering from atom probe tomography and near-field scanning optical microscopy measurements (see Stat. Sol. RRL 3 100 (2009) and Appl. Phys Lett. 87 161104 (2005), respectively).
He and his co-workers will continue to study LED droop. In particular, they plan to carry out measurements of radiative and non-radiative recombination lifetimes, study the correlation between the degree of saturation in radiative recombination rate and efficiency droop, and investigate structures that suppress efficiency droop.
D.-S. Shin et al. Appl. Phys. Lett. 100 153506 (2012)
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