research review
structures. They found that inserting an AlN layer between the buffer and LED improved surface morphology – when GaN is grown directly onto hexagonal BN, it forms a rough, irregular island-shaped surface morphology, and is polycrystalline. Thanks to the addition of AlN, it is possible to form GaN films with a step-like flat surface and a root-mean-square roughness over a 5 µm by 5 µm area of just 0.69 nm, according to atomic force microscopy measurements. Dark-field transmission electron microscopy reveals another benefit of the AlN layer: It acts as a dislocation filter, decreasing the density of threading dislocations in GaN. In this layer, the predominant type of defect is a mixed dislocation, which has a density of 8.6 x 109
cm-2 . Deposition of a 25 nm-thick Al0.28 Ga0.72 N layer demonstrated the device quality of GaN.
When removed from the substrate, characterization of the 2 cm square sample revealed a two-dimensional electron gas mobility of 1,100 cm2 carrier density of 1 x 1013 temperature.
V-1 s-1 cm-2
and a sheet at room
The engineers from NTT have also fabricated an LED with a ten period multiple-quantum well and compared its performance to a conventional device, grown on a typical low-temperature AlN buffer layer. They found that electroluminescence intensities of the transferred LED at currents ranging from 10 mA to 50 mA were comparable to, or higher than, those produced by the control. The reason: Reflection from the backside contact indium.
Spectral width of the electroluminescence produced by both types of LEDs is similar,
indicating that the active region maintains its quality during the transfer process.
A battery-powered LED prototype has also been fabricated by the team. A 2 mm square-release LED that is 3.4 µm-thick has been sandwiched between two commercially available laminate films featuring T-shaped Pd/Au electrodes. Application of indium tin oxide contact layers and a thermally activated sealing process creates a violet- blue emitting, flexible source.
Today’s targets for the team include an increase in the area of detachable devices, followed by improvements in the performance of conventional devices, such as LEDs and transistors, that can be transferred onto other materials.
Y. Kobayashi et al. Nature 484 223 (2012)
Slashing defects in GaN-on-sapphire films Serpentine mask paves the way to ultra-high-quality GaN epilayers
AN INTERNATIONAL collaboration has produced incredibly high-quality GaN on sapphire with a fabrication process involving a single epitaxial growth step.
“ We expect that the devices benefiting the most from this technology are the ones requiring very high performance,” says Tahong Xie from the University of California Los Angeles. “These devices include lasers, power transistors pushing the limit for power handling, photodetectors requiring extremely low dark current, and LEDs for solid-state lighting,”
Xie’s group, working with researchers at Peking University, has produced GaN films with a dislocation density of just 7 x 105
cm-2 by using a serpentine mask to block threading dislocations.
His initial idea for using this form of mask structure dates back to 2001 and draws on popular epitaxial lateral overgrowth approaches and dislocation necking. owever, it overcomes the deficiencies associated with these technologies.
If device manufacturers were to employ this serpentine mask technology, they could trim production costs, thanks to a simplified epitaxial process. What’s more, chipmakers would benefit from a four-fold reduction in the size of highly defective regions, such as coalescence fronts, compared to GaN
44
www.compoundsemiconductor.net June 2012
Fabrication of GaN films by the process pioneered by Xie and his co-workers begins with CVD deposition of a 100 nm- thick layer of SiNx
on (0001) sapphire. A
set of [1120]-orientated stripes are then formed with standard photolithographic processes, before 200 nm-thick films of SiO2 and SiNx
are deposited onto this mask. Another set of stripes in SiNx is then
formed, separated by window areas and offset from the lower openings (see Figure 1). Finally, etching with hydrofluoric acid exposes the sapphire in the lower window. To form high-quality nitride layers, engineers deposit a 25 nm-thick GaN
Figure 2: As more GaN is deposited, material quality improves,because this structure can block threading dislocations
nucleation layer on this serpentine mask at 530 °C, followed by epitaxial growth at 1040 °C (see Figure 2).
GaN islands formed at the edge of this mask that are probably riddled with gallium vacancies. In the centre of the mask material quality is far higher, thanks to complete coalescence of GaN to create a film with a mirror-like surface.
Figure 1: The serpentine mask is formed by a combination of SiNx
and SiO2 film growth,photolithography and etching.
material formed by conventional epitaxial layer overgrowth techniques.
Here the width of X-ray diffraction peaks is narrower than it is for GaN grown on sapphire, defect densities are lower by several orders of magnitude and photoluminescence intensity is 16 times higher. The US-China collaboration is now partnering with engineers at National Cheng Kung University, Taiwan, to grow lasers and LEDs on these low-defect-density substrates.
L. Li et al. Appl. Phys. Express 5 051001 (2012)
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