Laser Lift-Off Mechanisms of GaN Epi-Layer Grown on Pattern Sapphire Substrate

Wafer bonding and laser lift-off (LLO) processes were employed to fabricate pattern sapphire thin-GaN light-emitting diodes LEDs (PT-LED). During the LLO process, the required laser energy for PT-LED was much higher than that for flat thin-GaN LED (FTLED). The yield rate of PT-LED was low, and the leakage current was high. In this study, the laser lift-off mechanisms of PT-LEDs were investigated. © The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0101502jss] All rights reserved.

GaN-based light-emitting diodes (LEDs) are usually grown on sapphire substrate.Epitaxial growth techniques have significantly improved their luminance intensity.However, the poor electrical and thermal conductivities of sapphire substrate have a negative effect on the LED performance. 1,2Thin-GaN LED process can solve these problems, [3][4][5] in which GaN LED epi-layer was stripped off and transferred to conductive substrates by wafer bonding and laser lift-off (LLO) technologies. 1,2n addition, pattern sapphire substrate (PSS) has been employed to improve both internal quantum efficiency (IQE) and light-extraction efficiency (LEE).This is because most of the growth of GaN is initiated from c-planes.7][8] Also, these patterns can redirect photons back into the escape cone.As a result, LEE was improved. 5,6,9][12][13][14][15] However, compared with conventional flat thin-GaN LED (FT-LED), the critical laser energy of PT-LED was high, 14 the yield rate was very low, and the leakage current was high. 15In this study, the requirement of high laser energy and the root causes of low yield rate and high leakage current were investigated.

Experimental
PSS-LED was used to fabricate PT-LED by bonding and LLO technologies.The patterns on sapphire are shown in Fig. 1a.The width of period pattern was 2.3 μm, the space was 0.7 μm, and the height was 1.5 μm.The LED structure was grown by metal-organic chemical vapor deposition (MOCVD) 1,2 The LED wafer was then bonded to a p-type Si substrate with a metal layer. 3The LLO process used a KrF pulsed excimer laser (248 nm) as the laser source.The pulse length of KrF laser was 38 ns.For the purpose of comparison, LED grown on flat sapphire substrate was also used to fabricate thin-GaN LED, designated as "FLAT-thin-GaN LED (FT-LED)".

Results and Discussion
After LLO, for FT-LED, no visible alteration of the GaN layer was observed when the incident laser power was below ∼700 mJ/cm 2 .However, with increased laser power, the absorbed photon energy caused the destruction of the GaN layer.As for PT-LED, the critical laser power was 920 mJ/cm 2 , which was much higher than that of FT-LED.
The surface morphologies of PSSs after LLO are shown in Fig. 1b.The height of the pattern was reduced from 1.5 to 1.40 μm.At the same time, residues were found on the bottom c-plane and the sidewalls of patterns.It had been thought that these residues were metallic Ga. 1,2This is because the absorbed laser energy induces a rapid thermal decomposition of GaN, and yielding metallic Ga and N 2 gas.The Ga residues were subsequently removed by a wet chemical etch using HCl: H 2 O (1:1) solution.In this case, most of the particles on the bottom c-plane were removed, as shown in Fig. 1c.However, significant residues remained on the sidewalls, none of which were Ga.In addition, the height of pattern did not change.
Energy dispersive X-ray spectroscopy (EDS) analysis showed few Ga element presents in the sidewall residues, suggesting that these residues might be GaN.Samples were then etched in H 3 PO  Cross-sectional transmission electron microscopy (TEM) was used to investigate this top of pattern before H 3 PO 4 etching.As shown in Fig. 2, there was a light region at the top after LLO.Surprisingly, EDS and selected area diffraction patterns (SAD) analysis revealed that this light region was not GaN but rather amorphous/polycrystalline aluminum oxide (α-Al 2 O 3 /poly-Al 2 O 3 ) with the composition similar  Figure 3 shows the LLO-surface morphology of GaN.It has been reported that this LLO-GaN contained many defects, such as lattice dilation and deformation, dislocations, and stacking faults. 1,2,16Residues were found on the bottom of the GaN concave pattern.To examine the nature of residue, platinum was deposited on top of the concave pattern for image contrast in the TEM sample preparation.Figure 4a shows the cross-sectional TEM image of the residue on the concave   pattern.EDS and SAD analysis (Fig. 4d) revealed that the residue was α-Al 2 O 3 /poly-Al 2 O 3 .SAD analysis also revealed that the GaN concave pattern had a Wurtzite structure (Fig. 4b and 4c).As shown in Fig. 4c, several dimmer spots with irregular distribution were found near the GaN/Al 2 O 3 interface.Those spots could be poly-GaN grains with a disorder zone axis, which was caused by the LLO process.
Figure 5 is a schematic illustration of the α-Al 2 O 3 residues at the GaN/sapphire interface after LLO process.This is because the top of the sapphire (c-Al 2 O 3 ) pattern melted during the laser process.Then, during the rapid cooling process, it turned into α-Al 2 O 3 /poly-Al 2 O 3 and stuck to the GaN and sapphire surfaces.
The melting temperature of c-Al 2 O 3 is extremely high, 2030 • C. We believe the temperature increase at the top of the PSS pattern is due to the shape of the pattern and partial transmission and reflection amplitudes when light moves from a low (n sapphire =1.7) to high refractive index medium (n GaN =2.5). 17he shape and sidewalls of the PSS patterns were measured by sideview projection image and cross-sectional image.It was illustrated in Fig. 6, the sapphire/GaN interface can be simplified as four zones: 1 bottom c, 1 triangle, and 3 trapezoid zones (a, b, and c).Their base angles and laser incident angles are listed in Table I.
The incident laser was unpolarized (containing a mix of s-and p-polarizations).The reflectance (reflected fraction, R) and transmittance (transmitted fraction, T) for s-polarized light and p-polarized light are and T where n 1 (=1.7) and n 2 (=2.5) are the refractive indexes of sapphire and GaN, θ i and θ t are the incident and transmit angles.The relationship between these two angles is given by Snell's law.The related transmit and reflected energy densities on the sidewalls in where 920 mJ/cm 2 is the critical energy used in PT-LED.The related energies are listed in Table I.Note that, on the sidewalls of the patterns, the transmit energies of the p-polarized light (E Tp ) are higher than those of the s-polarized light (E Ts ).In other words, the p-polarized light would be effective for the sidewall GaN separation.
For the purpose of comparison, the related E T and E R of critical energy 700 mJ/cm 2 on FT-LED are also listed in Table I.Both critical transmit energies (E Tp * and E Ts * ) of FT-LED are 674 mJ/cm 2 (E T * ).Table I also shows that transmit energies on sidewalls of 3 trapezoid zones (a, b, and c) were much less than E T * (674 mJ/cm 2 ).In other words, the transmit energy was too low to lift off GaN on trapezoid zones. 1,2This was confirmed by the observation of GaN residues left on the sidewalls of trapezoid as shown in Fig. 1c.
For the triangle zone, the transmit energies (especially for the ppolarized light) on the triangle zone were almost the same as E T * (674 mJ/cm 2 ).In addition, as shown in Fig. 3, most of the reflected energies from trapezoid zones were redirected to triangle zone.The collected energy on the triangle zone must be higher than E T * .This was confirmed by the observation that the top of the sapphire (c-Al 2 O 3 ) pattern was melted as shown in Fig. 2a and Fig. 4.
Table I also shows that the transmit energy densities on the bottom c zone were much higher than E T * , meaning that the temperature on the bottom c-plane was very high.This was confirmed by TEM analysis, which shows that there was about 150 nm α-Al 2 O 3 on the top of c-plane.
In summary, in PT-LED, when incident laser power was less than the critical energy density, GaN layers cannot be lifted off because the energy density was not high enough to separate the sidewalls of trapezoid zones.In other words, transmit energy densities on sidewalls of 3 trapezoid zones were much less than E T * (674 mJ/cm 2 ).The adhesion force between GaN and sapphire on sidewalls of trapezoid zones were still strong.When incident laser beam power reached 920 mJ/cm 2 , a GaN thin film was lifted off, even though the transmit energies on the sidewalls of the trapezoid zones were still low.However, at the same time, on the bottom c and triangle zones, the energy densities were much higher than E T * .This high energy induced a local heating of the layer above the critical sublimation temperature of Ga, yielding Ga and N 2 gases on these two zones. 2As a result, these gases penetrated/separated the GaN/sapphire interfaces of the trapezoid zones.The sidewalls of the trapezoid zones were not separated by the laser lift-off (decomposition of GaN) but rather by the penetration of the Ga and N 2 gases.In other words, they were separated by "mechanical stress".This was confirmed by the fact that manyGaN residues remained on the sidewalls (Fig. 1c and Fig. 5).This mechanical stress can cause the deformation (formation of the dislocations and stacking faults) and an increase in leakage current. 1,2

Conclusions
The laser lift-off mechanism of PT-LED was investigated and it was found that the critical laser lift-off power of PT-LED was 920 mJ/cm 2 , which was much higher than that of FT-LED (700 mJ/cm 2 ).When incident laser power was below critical power, the laser energy density was not high enough to separate the sidewalls of the trapezoid zones of patterns.When incident power reached 920 mJ/cm 2 , a GaN thin film was lifted from the sapphire substrate and the triangle zone of PSS pattern was melted.This is because, on the bottom c and triangle zones, high energy induced a local heating of the layer above the critical sublimation temperature of Ga, and yielding Ga and N 2 gases.These gases penetrated the sidewalls of the trapezoid zones.The melted triangle zone was due to the shape of the pattern and partial transmission and reflection amplitudes when light moves from a low to a high refractive index medium.
4 solution at 150 • C to remove GaN residues.As shown in Fig 1d, residues on the sidewalls were removed.Interestingly, at the same time, the pattern height decreased from 1.40 to 1.16 μm, meaning the top of PSS patterns was also etched by H 3 PO 4 .

Figure 5 .
Figure 5. Schematic illustration of α-Al 2 O 3 residues at the GaN/sapphire interface after LLO process.

Figure 6 .
Figure 6.Schematic illustration of the shape and zones of PSS pattern.) unless CC License in place (see abstract).ecsdl.org/site/terms_useaddress.Redistribution subject to ECS terms of use (see 54.191.40.80 Downloaded on 2017-08-27 to IP