TECHNOLOGY LEDs
partial pressure. Thus the oxygen stoichiometry, which is primordial for the properties of oxide-based materials, can be precisely tuned over a much larger range without compromising material quality.
LED structures
The pertinence of ZnO for GaN-based LEDs lies in the optoelectronic grade thin-fi lm pseudo-substrates that we have developed as templates for GaN regrowth.
Over the past few years, the Georgia Tech / CNRS lab has developed a novel MOCVD approach for capping our ZnO epilayers with GaN that is free from back-etching. Once the ZnO layer is encapsulated, a GaN LED structure can be grown under standard MOCVD conditions without damaging the oxide layer.
Characterisation of GaN fi lms that are deposited on ZnO reveals a very high material quality. After only 100 nm of growth, these layers are optically active, have a root-mean-square surface roughness of 1 nm, and have much less crystallographic dispersion than GaN fi lms of comparable thickness grown directly on sapphire.
This suggests that a switch from sapphire to ZnO/sapphire for LED production would allow far thinner layers of GaN to be used, which would be advantageous for light out-coupling and external quantum effi ciency.
That is not the end of the story, however. We have gone on to demonstrate that ZnO thin-fi lm templates can subsequently be used as sacrifi cial release layers for the GaN. This process, an alternative to laser lift-off, is based on selective chemical dissolution of the ZnO underlayer. It takes just a few hours and can be performed with a variety of dilute acids or alkalis – because GaN is highly resistant to chemical etching in acids and alkalis other than HF, while ZnO is not (see Figure 2, which is a full 2-inch wafer chemical lift-off progression). The lifted GaN layers show no trace of zinc in high- resolution electron microscopy energy dispersive X-ray microanalysis near the interface.
In parallel work on another materials system carried out at Heriot Watt University in the UK, Kevin Prior’s group have developed a process for the full
Figure 4. The costs for adoption of a native GaN substrate for vertical GaN LED manufacturing can be slashed by turning to cycles of chemical lift-off and reclaim.
transfer of ZnSe from GaAs to alternative substrates by means of direct wafer bonding after chemical lift-off with a sacrifi cial MgS underlayer. Mirroring this bonding process, we were able to demonstrate similar transfer of a GaN LED structure from sapphire to an alternative substrate (see Figure 3).
Thanks to funding in 2013 from the European Commission FP7 and the Scottish Universities Physics Alliance, we have been able to unite the partnerships with the groups of Prior and Ougazzaden and go on to demonstrate that a similar process is capable of delivering chemical lift-off of GaN from ZnO-coated bulk GaN substrates.
The advantage of this counter-intuitive approach is the higher quality of the GaN grown on top of the ZnO-coated GaN substrate. Indeed, X-ray diffraction and electron microscopy studies showed that the resulting GaN epilayers had signifi cantly larger grains, less strain and lower defect density than can be obtained by heteroepitaxy on ZnO/sapphire.
Moreover, the GaN fi lm is relaxed, thanks to the underlying GaN substrate straining the ZnO template such that it is lattice- matched to GaN.
X-ray diffraction reveals that a 100 nm- thick overlayer of GaN on the ZnO/GaN template replicates the omega rocking curve linewidth of the GaN substrate (~0.05°), which represents an order of magnitude reduction in crystallographic
dispersion compared with GaN layers of similar thickness grown on ZnO/sapphire. In addition to superior epitaxy, the expensive single crystal GaN substrate is not consumed in this process and multiple GaN substrate reclaim and reuse cycles are possible without signifi cant degradation of GaN layer quality (see Figure 4).
Moreover, chemical cleaning suffi ces and there is, therefore, no wafer loss through repolishing. Consequently, large quantities of GaN substrates are not required for chip production, which means that substrate costs per growth run are compressed to industrially viable levels. This should enable a breakthrough in LED chip manufacturing,in terms of output power per wafer and cost per lumen. There is no reason why this technology cannot even be applied to the rarer, even more expensive, non-polar GaN substrates that are being put forward as a way to combat “droop”, the decline in LED effi ciency at higher current densities.
This chemical lift-off technique is clearly very promising, and unlocking its true potential could play a major role in the future of solid-state lighting.
The author thanks Carlos Lee of EPIC, the FP7 Nexpresso Programme, the Scottish Universities Physics Alliance and the French Agence National de la Recherche for their support in this work.
© 2014 Angel Business Communications.
June 2014
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