TECHNOLOGY LEDs
This problem is not insurmountable, however, and it has been overcome, by our ZnO team at Nanovation of Châteaufort, France, working in partnership with the GaN group of Abdallah Ougazzaden at the joint Georgia Tech and CNRS lab in France.
Nanovation is a ZnO dedicated epiwafer foundry, founded in 2001 by four research scientists who identifi ed the commercial potential of ZnO for emerging opto-semiconductor markets: extending from transparent conductors and electronics through to LEDs, lasers and photovoltaics.
Figure 2. Photographs taken through the backside of a 2-inch GaN/ZnO/sapphire wafer (immersed in ~ 1M HCl) showing a time sequence of the chemical dissolution of ZnO & the progressive detachment of the substrate.
To this end, we have pioneered the adoption of pulsed laser deposition as a production tool in the semiconductor industry, based on its unique capacity for forming state-of-the-art ZnO layers with a huge range of properties on almost any substrate.
can be formed (see Figure 1), which have current fl ow through the substrate. Such devices have a much more homogeneous current distribution than a conventional lateral LED, enabling them to be driven up to 25 times higher. VLEDs also avoid complex, costly lithographic steps required to make the top contacts, and have a footprint at least 30 percent smaller than a conventional LED, which means that manufacturers of these devices benefi t from a signifi cantly higher chip yield per wafer.
What’s more, a fl ipped device geometry can increase overall light extraction, thanks to laser-induced roughening of the GaN, which reduces the proportion of light trapped by total internal refl ection. And on top of this, the substrate can be reclaimed and reused after polishing away the surface residue left by the laser ablation. However, despite all of these advantages associated with laser lift-off, the performance of the transposed LED is still held back by the quality of the original expitaxy on the mismatched sapphire.
It might seem that the ultimate way forward is to grow such VLED structures on GaN substrates (because this would lead to very high crystal quality) and then perform laser lift-off in order to reclaim/ reuse the expensive GaN substrate. However, it is not possible to remove this native platform with laser lift-off, because GaN substrates are not transparent to the requisite short-wavelength lasers.
Another substrate option that has attracted a signifi cant amount of attention is ZnO, which not only has the same crystal structure as GaN, but also very similar lattice parameters and comparable thermal expansion coeffi cients. However, ZnO is much more chemically reactive than GaN.
Due to this, attempts by numerous groups to adopt bulk ZnO substrates have met with problems of back-etching of ZnO by the process gases used in the MOCVD process.
This remarkable propensity for tuneability results from the exceptionally high energy of the adatoms in the laser ablation plume – they typically have energies of 10 eV to 100 eV, compared to 1 eV to 10 eV for sputtering and 0.1 eV to 1 eV for more common semiconductor deposition methods, such as MBE and CVD.
Thanks to these higher energies, deposition can occur at lower growth temperatures while using a much- extended working range for oxygen
Figure 3. Cross-sectional transmission electron microscope images showing the epitaxial growth of GaN on ZnO/sapphire by MOCVD without back-etching of the ZnO and the excellent GaN/glass interface after chemical lift-off and direct fusion bonding. The lifted GaN layers show no trace of zinc in high-resolution electron microscopy energy dispersive X-ray microanalysis near the interface. [credit: G. Patriarche, LPN, Marcoussis, France].
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www.compoundsemiconductor.net June 2014
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