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Supporting Information for:
In situ self-release of thick GaN wafer from sapphire
substrate via graded strain field engineering
Na Lin1, Jiejun Wu2, Hongmei Xu1, Nanliu Liu2, Tongchang Zheng1, Wei Lin1, Chuan Liu3 and
Duanjun Cai1,4, a
1
Fujian Key Laboratory of Semiconductor Materials and Applications and Department of Physics, Xiamen
University, Xiamen 361005, China
2 Research Center for Wide-gap Semiconductors, State Key Laboratory for Artificial Microstructures and
Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, P. R. China
3
State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen 361005,China
4 Institute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan
University, 1, Roosevelt Road, Section 4, Taipei 10617, Taiwan
Although GaN can crystallize in zinc-blende(ZB) cubic structure it normally stabilizes in
wurtzite (WZ) phase. Moreover, most studies focus on the metal-terminated c plane (0001) of
the structure because of its stability and wide applications with growth on (0001) sapphire
substrate. Meanwhile non-polar planes including both a plane (10-10) and m plane (11-20) were
also concerned in wurtzite structure crystal due to their virtue of absence of polarization. Before
the calculation of surface models, the bulk structures of c, a and m-plane cases were constructed,
as shown in Fig. S1, and the total energies of these primitive cells were obtained for comparison.
Fig. S1. The structure of WZ-GaN primitive cell on (a) c, (b) a, (c) m planes.
Figure S2 shows the band structures of the surface models for various polar and non-polar planes. One
can see that the surface states of m(2)-plane [Fig. S2d] largely occupy the energy range where the Fermi level
resides. This means that this type of surface would be more metallic, active and unstable. This may be
attributed to the fact that two dangling bonds exist on the m(2) surface, which provides more free electrons.
While, the c plane surface shows less overlap of its surface states with EF level, indicating a surface much
difficult to release. The partial density of states (PDOS) calculation shows the results of the density of surface
states on different surfaces, as displayed in Fig. S3. It can be found that the surface states mostly stem from
the s and p-states electrons of Ga atoms on the surface, which are the dangling bonds broken from the sp3
hybridization.
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Fig. S2. Band structure of surface models for (a) c-plane,(b) a-plane, (c) m(1)-plane, and (d) m(2)-plane. The
blue and red bands are assigned to the surface states contributed from the surface dangling bond.
Fig. S3. PDOS of Ga atoms in bulk (a), and on surfaces of c-plane (b), a-plane (c), m(1)-plane (d), and
m(2)-plane (e), respectively. The solid vertical lines define the region of energy band gap and the dash line
indicates the position where Fermi level locates.
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The setup of vertical type HVPE system for the growth of GaN thick epilayers is shown in
Fig. S4. All GaN samples were deposited on 2-μm thick MOCVD-GaN templates at a
low-pressure of 200 Torr with metallic gallium (Ga), hydrogen chloride (HCl) and ammonia
(NH3) as sources. The GaCl was obtained in a Ga source by reacting HCl with Ga at 920℃. The
growth temperature was about 1060 ℃. A hydrogen and nitrogen gas mixture was used as a
carrier. For the thick GaN with well strain control by using pre-treatment of sapphire and
introduction of LT GaN buffer, the perfect self-release could be achieved, as shown in Fig. S5.
The thickness of remaining GaN layer on sapphire substrate depends on the total thickness of
as-grown GaN epilayers.
Fig. S4. Schematic diagram of the vertical type HVPE system.
Fig. S5. Sideview photo of self-separation of GaN wafer from the sapphire substrate
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The surface smoothness of GaN epilayer grown by HVPE was determined by AFM
scanning (Fig. S6). The surface roughness of the as-grown sample is only about 0.21 nm (RMS),
which is comparable to epitaxial sample by MOCVD. The PL measurement at room temperature
was carried out for the released GaN thick wafer, as shown in Fig. S7. The band-edge emission
peak is at 3.397 eV. This indicates that the released GaN wafer has the residual strain fully
relaxed because of the removal of sapphire substrate.
Fig. S6. AFM image of the surface of GaN thick layer grown by HVPE. The surface roughness is determined
to be about 0.21 nm (RMS).
Fig. S7.
PL spectrum of released GaN wafer. The band-edge emission is at 3.397 eV, which indicates the
full relaxation of misfit strain.
In order to measure the strain distribution in the GaN epilayer in different thickness (depth),
a series of samples have been prepared, as shown in Fig. S8. The Raman spectra were recorded
for these samples. The characteristic peak at E2 mode is used to determine the stress type and
magnitude. The confocal micro Raman spectroscopy is employed to survey the phonon signal
from the sample surface and also the signal from different depth beneath the surface by moving
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the position of the focal plane. The frequency of the E2 peak for bulk GaN (stress free) is 568
cm-1.[P. Perlin, C. Jauberthiecarillon, J. P. Itie, A. San Miguel, I. Grzegory, and A. Polian,
Physical Review B 45, 83 (1992).] To check the lateral variation of strain within the wafer, three
points (in center, edge and between) were measured. It is found that the Raman peaks from
different points are unshifted, suggesting the uniformity of the strain distribution on the wafer
surface.
Fig. S8.
Pictures of GaN samples grown on sapphire substrates in different thicknesses.
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