(In)GaN p-i-n structures were grown by MOVPE on both GaN- and ZnO-coated c-sapphire substrates. XRD studies of
the as-grown layers revealed that a strongly c-axis oriented wurtzite crystal structure was obtained on both templates and
that there was a slight compressive strain in the ZnO underlayer which increased after GaN overgrowth. The InGaN
peak position gave an estimate of 13.6at% for the indium content in the active layer. SEM and AFM revealed that the
top surface morphologies were similar for both substrates, with an RMS roughness (5 μm x 5 μm) of about 10 nm.
Granularity appeared slightly coarser (40nm for the device grown on ZnO vs 30nm for the device grown on the GaN
template) however. CL revealed a weaker GaN near band edge UV emission peak and a stronger broad defect-related
visible emission band for the structure grown on the GaN template. Only a strong ZnO NBE UV emission was observed
for the sample grown on the ZnO template. Quarter-wafer chemical lift-off (CLO) of the InGaN-based p-i-n structures
from the sapphire substrate was achieved by temporary-bonding the GaN surface to rigid glass support with wax and
then selectively dissolving the ZnO in 0.1M HCl. XRD studies revealed that the epitaxial nature and strong preferential
c-axis orientation of the layers had been maintained after lift-off. This demonstration of CLO scale-up, without
compromising the crystallographic integrity of the (In)GaN p-i-n structure opens up the perspective of transferring GaN
based devices off of sapphire substrates industrially.
Free-standing (0002)-oriented GaN substrates (φ = 2”) were coated with 200 nm of ZnO and used as templates for the
growth of GaN thin films. SEM and AFM revealed that such GaN layers had a relatively homogenous surface
morphology with an RMS roughness (5 μm x 5 μm) of less than 4nm. XRD studies revealed strained ZnO growth on the GaN substrate and the reproduction of the substrate rocking curve for the GaN overlayers after only a hundred nm of
growth, thus indicating that the GaN films had superior crystallographic quality compared to those grown on sapphire or
ZnO/sapphire substrates. Quarter-wafer areas of GaN were removed from the GaN substrate (by selective chemical
etching away of the ZnO interlayer). The expensive GaN substrates were then reclaimed/reused (without the need for
polishing) for a second cycle of ZnO and GaN growth, which gave similar XRD, SEM, CL and AFM results to the first
cycle.
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