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Study of surface blistering in GaN by hydrogen implantation at elevated temperatures
Thin Solid Films 590 (2015) 64–70
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Study of surface blistering in GaN by hydrogen implantation at elevated temperatures B.S. Li ⁎, Z.G. Wang, H.P. Zhang Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China
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Article history: Received 2 December 2014 Received in revised form 27 June 2015 Accepted 19 July 2015 Available online 21 July 2015 Keywords: Implantation Cross-sectional transmission electron microscopy Dislocation loops Blisters Exfoliation
a b s t r a c t We have investigated mechanisms of ion-cut in H+ 2 -implanted GaN by analyzing microstructural features of H+ 2 -implanted GaN at room temperature, 573 K and 723 K. Using optical microscopy and transmission electron microscopy, it was found that the in-plane compressive stress induced by the H-implantation was necessary for H-platelet nucleation and growth. The control of implantation temperature is crucial for creating sufficient in-plane compressive stress to induce surface blistering. © 2015 Elsevier B.V. All rights reserved.
1. Introduction GaN is a great important wide bandgap semiconductor, which can be used in electronic and optoelectronic devices. Because of the high price of free-standing GaN wafers, the growth of epitaxial layers for device fabrication is mostly performed on sapphire, SiC and Si. The “Smart Cut” technology was firstly mentioned by Bruel [1]. The procedure of this technology comprises H-implanted Si, then bonding them to a substrate stiffener. Finally, a splitting treatment is used to achieve layer transfer upon annealing at about 673 K. The “Smart Cut” technology can provide inexpensive templates with high structural quality. The free-standing wafer of GaN can be utilized to transfer multiple layers on to other foreign substrates such as Si, SiC and sapphire via this method, which in turn serve as template layers for the growth of epitaxial device layers. The ion-cut process has been extensively studied on various semiconductor materials such as Si, SiC, Ge, GaAs and InP. Implantation dose, implantation temperature, annealing temperature and time play very important roles in the splitting process. All these parameters strongly depend on the type of semiconductor materials used for splitting. In contrast to the cases of Si, SiC, Ge, GaAs and InP, only a few studies of H-implantation-induced GaN layer splitting were reported [2–8]. Tong et al. [9] reported that blister formation and layer splitting have the same activation energy, indicating that formation of optically detectable surface blisters is regarded as onset of microcrack formation. ⁎ Corresponding author at: Laboratory of Advanced Nuclear Materials, Institute of Modern Physics, CAS, Lanzhou 730000, China. E-mail address: [email protected] (B.S. Li).
http://dx.doi.org/10.1016/j.tsf.2015.07.039 0040-6090/© 2015 Elsevier B.V. All rights reserved.
In fact, the physical mechanisms of layer splitting can be conveniently investigated by studying the occurrence of surface blisters/exfoliations in H-implantation and annealing without bonded wafers [6,7,10,11]. Kucheyev et al. [5] reported the H-implantation-induced exfoliation in GaN. The exfoliation is observed in a narrow window of the implantation dose above 3 × 1017 cm−2. Mountanabbir et al. [2,3] reported microstructural evolution of H-implanted GaN with annealing temperature. The wafers were implanted by H ions at 50 keV to a dose of 2.6 × 1017 cm−2 at RT. A high density of bubbles with 1–2 nm in diameter was found in the damaged layer. After annealing above 450 °C, platelets parallel to the surface were formed. Increasing annealing temperature to 600 °C, structural transitions from platelets to microcracks occurred. Tong et al. [4] reported that the blistering of GaN would only occur when the implantation was performed in a temperature window of 275–450 °C. Blistering of GaN also occurred after H-implantation to doses above 5 × 1017 cm−2. Hayashi et al. [12] regarded that blistering also depends on the crystalline quality of the GaN wafer. Some studies have been shown that hydrogen forms complexes with Ga vacancies induced by H-implantation [3]. The agglomeration of vacancies and hydrogen leads to the formation of nanoscopic bubbles in GaN filled with molecular hydrogen, which serve as precursor to the formation of microcracks in the damaged layer and ultimately surface blisters in GaN upon annealing [7]. However, Moutanabbir et al. [2] regarded that stress-induced Ga–N bond breaking would be the origin of the formation of platelets observed in the H-implanted GaN, rather than the coalescence of nanoscopic bubbles. For a better understanding of the microscopic mechanisms of ion-induced GaN thin layer splitting, the role of implantation-induced defects should be investigated in detail.
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Because of the efficient dynamic annealing of the implantation-induced defects in GaN crystalline, the nucleation and growth of extended defects in H-implanted GaN at elevated temperature are faster than in the case of RT implantation. Therefore, the microstructural evolution at elevated temperature implantation may be useful to study the mechanism of ion-cut in GaN. Up to now, the microstructural evolution of Himplanted GaN at elevated temperatures has been less investigated [13, 14]. In this paper, optical microscopy was employed to observe the evolution of surface morphology upon annealing, and transmission electron microscopy was employed to systematically study the microstructural evolution with the implantation temperature. 2. Experimental The material studied is an n-type GaN epilayer with 30 μm in thickness grown on the c-plane of a sapphire substrate by the metal-organic chemical vapor deposition (MOCVD) method. Samples with size 10 × 5 mm2 were used for the ion implantation. The implantation with 134 keV H+ 2 ions was performed at 320 kV High-voltage Platform in the Institute of Modern Physics, Chinese Acad2 emy of Sciences. Ion fluence was 1.5 × 1017 H+ 2 /cm . The experimental conditions are expected to produce irradiation effects similar to that of a 67 keV H+ ions to a fluence of 3 × 1017H+/cm2. The distributions of the damage level in displacements per atom (dpa) and deposition concentration were estimated by SRIM96 (the displacement energies of Ga = 20.5 eV and N = 10.8 eV, and density 6.1 g cm− 3) [15]. The mean projected range Rp is approximately 440 nm with a straggling of ΔRp is approximately 140 nm. The peak concentration is approximately 18% and the maximum damage is approximately 1.5 dpa. The current density was approximately 0.8 μA/cm2 and the implantation temperatures were room temperature (RT), 573 K and 723 K. Postimplantation, wafers were cut into several pieces and then isochronally annealed in flow of N2 at temperatures of 573 K, 723 K and 873 K for 15 min, respectively. The evolution of the surface morphology properties of the implanted as well as un-implanted samples was analyzed by Olympus optical microscopy. The magnification was 1000 times, which is able to observe micron-scale bubbles or exfoliations on the sample surface. Before observations, the samples were cleaned in ethanol by ultrasonic for 5 min. Post-implantation, the microstructural evolution as a function of the implantation temperature was investigated with Tecnai G20 transmission electron microscopy (TEM) operated at 200 kV and equipped with a double tilt goniometer stage. TEM samples were prepared by a conventional method; this is, mechanical thinning up to approximately 40 μm in thickness and then ion milling with Ar ions. Initially, the energy of the Ar ions was kept at 5 kV and the incident angle was maintained at 5° from the surface until the occurrence of a hole in the center of the sample. Then, the energy of the Ar ions was reduced to 2 kV at an incident angle of 3°, applied for 1 h, in order to minimize radiation damage induced by the Ar ions. The bubbles were imaged in under-focused and over-focused conditions to highlight the bubble edges with Fresnel contrast. The Burgers vector of dislocation loops was determined by standard g·b techniques. Weak-beam darkfield imaging was also employed to improve the contrast quality. 3. Results and discussion A series of surface morphology images of the annealed and asimplanted samples obtained by optical microscopy. In the asimplantation, whatever the implantation temperature performed, the sample surfaces have not any change as compared to un-implanted samples, as shown in Fig. 1(a), (e) and (h). After 573 K annealing for the RT implanted sample, many bright contrasts appear as circular in shape on the surface. The observed contrasts are attributed to blisters, which have the lateral size ranging from 1 to 4 μm, as shown in Fig. 1(b). The observed blisters increased up to 15 μm and some blisters
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burst up to form exfoliation when the RT-implanted sample was annealed at 723 K, as shown in Fig. 1(c). The growth of blisters reaches saturation with increasing annealing temperature up to 873 K. For the 573 K implanted sample upon 723 K annealing, few of blisters with sizes approached 20 μm and most of blisters with lateral sizes ranging from 1 to 5 μm. After annealing at 873 K, some blisters grown up to approximately 16 μm and then burst up to form exfoliation. For the sample implanted at 723 K upon 873 K annealing, there is no change compared to that of the as-implanted sample, as shown in Fig. 1(i). To investigate the underlying mechanisms of this phenomenon mentioned above, the studies of the formation of H-induced bubbles and extended defects in the as-implanted sample are critical. Fig. 2 presents bright-field cross-sectional transmission electron microscopy (XTEM) images of the damaged layer for the samples implanted with H+ 2 ions at RT, 573 K and 723 K. The images were taken along the GaN [01 1 0] zone axis. No amorphous layer was detected anywhere in the damaged layer due to the efficient dynamic annealing of GaN crystalline. High densities of black contrasts which are correlated to implantation-induced dislocation loops, stacking faults and strains were observed in Fig. 2. For the sample implanted at RT, the image exhibits a well-defined damage band at depths ranging from 230 to 580 nm below the sample surface, as shown in Fig. 2(a). For the sample implanted at 573 K, Fig. 2(b) shows a well-defined damage band at depths ranging from 350 to 710 nm below the sample surface. For the sample implanted at 723 K, Fig. 2(c) shows a damage band from the sample surface to a depth of 710 nm. The width of the damage layer increased with increasing implantation temperature. It is attributed to implantation-induced lattice defects migration toward the sample surface and/or toward the end-of-projected range at elevated temperatures. To increase in the contrast of bubbles, the sample was tilted 12° from the [0110] zone axis to reduce the intensity of the diffraction contrast. Fig. 3 presents the distribution of the bubbles in the damaged layer. The distribution of the bubbles observed in the damaged layer is related to the implantation temperature. For the sample implanted at RT, bubbles are less distributed and the majority of the bubbles with diameters smaller than 3 nm are located near the projected range, as shown in Fig. 3(a). The width of the bubble layers is approximately 140 nm. The distribution of the bubbles increased with increasing implantation temperature. For the sample implanted at 573 K [see Fig. 3(b)], bubbles are homogeneously distributed in the damaged layer. Some bubbles have diameters larger than 3 nm. The width of the bubble layers is approximately 230 nm. For the sample implanted at 723 K [see Fig. 3(c)], bubbles are homogeneously distributed in the damaged layer. Some bubbles have diameters larger than 5 nm. The width of the bubble layers is approximately 250 nm. To investigate the H-implantation-induced defects in the damaged layer, two-beam conditions were performed. Fig. 4 shows weak-beam dark-field XTEM images taken from the sample implanted at RT, 573 K and 723 K. The dislocation loops are visible in the weak-beam dark-field images as white spots. Images taken under g = 0002 condition indicate the presence of a high density of point defects and dislocation loops. Images taken under g = 2110 condition indicate the presence of some planar defects which are parallel to the sample surface. Under the different diffraction factors, the size and density of the H-implantation-induced dislocation loops are different. The measurement error of dislocation loops is about 0.5 nm due to the resolution of TEM. For the sample implanted at RT, under g = 0002 condition [see Fig. 4(a)], the distribution of dislocation loops with sizes ranges from 1 to 20 nm and a number density is about 1.09 × 1023/m3. Under g = 2110 condition [see Fig. 4(b)], the distribution of dislocation loops with sizes ranges from 1 to 13 nm and a number density is about 1.05 × 1023/m3. For the sample implanted at 573 K, under g = 0002 condition [see Fig. 4(c)], the distribution of dislocation loops with sizes ranges from 1 to 19 nm and a number density is about 9.7 × 1022/m3. Under g = 2110 condition [see Fig. 4(d)], the distribution of dislocation loops with sizes ranges from 1 to 16 nm and a number density is about 5.5 × 1022/m3. For the sample implanted at
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17 + 2 Fig. 1. Optical microscopy plane view images of the sample surfaces of GaN implanted 134 keV H+ 2 ions to a fluence of 1.5 × 10 H2 /cm at: (a–d) RT; (e–g) 523 K; (h, i) 723 K, (a, e, g) asimplantation, and then annealed at: (b) 523 K; (c, f) 723 K; (d, g, i) 873 K. The scale at left corner of images represents 10 μm.
723 K, under g = 0002 condition [see Fig. 4(e)], the distribution of dislocation loops with sizes ranges from 1 to 35 nm and a number density is about 3.5 × 1022/m3. Under g = 2110 condition [see Fig. 4(f)], the distribution of dislocation loops with sizes ranges from 1 to 13 nm and a
number density is about 1.9 × 1022/m3. The Burgers vectors were analyzed by the invisibility criterion g∙b = 0. According to the previous report of O+-implanted GaN [16], we assume these dislocation loops with b = 1/2[0001] or b = 1/3 b 1010 N. In the present study, for the RT
17 + Fig. 2. XTEM bright field micrograph of the damage band in GaN implanted 134 keV H+ H2 /cm2 at: (a) RT; (b) 523 K; and (c) 723 K. The microstructures with 2 ions to a fluence of 1.5 × 10
the electron beam close to [0110] zone axis. Adapted from Ref. [13].
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17 + Fig. 3. XTEM bright field micrograph of the bubble layers in GaN implanted 134 keV H+ H2 /cm2 at: (a) RT; (b) 523 K; and (c) 723 K. Underfocus view of the 2 ions to a fluence of 1.5 × 10
bubble layers in Fig. 2 with the electron beam incident at an angle of 12° from the [0110] zone axis. Adapted from Ref. [13].
implanted sample, the dislocation loops with b = 1/2[0001] almost has the same number density as the dislocation loops with b = 1/3b1010N. For the 573 K and 723 K implanted samples, the ratios of the number density of the dislocation loops with b = 1/2[0001] to b = 1/3 b 1010N are 1.76 and 1.84, respectively. The results of optical microscopy and XTEM have clearly shown that the implantation temperature can affect the ion-cut in GaN. The ion-cut is more efficient for the sample implanted at 573 K than that of the sample implanted at RT, whereas the ion-cut did not occur for the sample implanted at 723 K. These observations suggest that high temperature implantation should be very careful in order to fulfill the ion-cut in GaN. This behavior differs completely from the phenomenon in H-implanted SiC where ion-cut easily occurred at elevated temperature implantation [17,18]. The phenomenon of ion-cut in H-implanted Si has been widely studied [19–21]. The chemical environment of H-implanted Si, H shares its electron with Si neighbors to form the atomic configuration of Si–H– Si, weakening the original Si–Si bond. The atomic configuration of Si– H–H–Si further weakens the Si–Si bond. The formation of the H–H planes (platelets) reduces the energy needed to fracture the Si {100} plane. The platelets provide a favorable site for the formation and growth of H2 gas bubbles because of a low surface energy of the platelets. Upon annealing, free atomic hydrogen evaporating from VH and Si–H defects can diffuse into the platelets and then agglomerate, forming pressurized H2 gas platelets. The high pressure in the H2 gas platelets provides the force needed to generate crack opening displacement and results in the propagation of microcracks. The coalescence of the microcracks leads to splitting, and the implanted layer can be transferred to the handle substrate via using wafer bonding before the microcrack formation [19]. Therefore, the formation and growth of platelets that are related to the concentration of H2 are critical for ion-cut. For the case of H-implanted Si at RT, the threshold dose of implanted H is about 1.5 × 1016 cm−2 for ion-cut [21], whereas ion-cut did not occur when H+ ion implanted GaN to a dose of 1 × 1017 cm−2. In the H-implanted GaN sample, no platelets that are usually formed in H-implanted Si or SiC were observed in the damaged layer [22,23]. Previous studies show that the surface layer splitting is due to the nucleation and growth of the H-decorated platelets. Therefore, no platelets formed in the as-implanted sample can explain why the threshold dose for splitting in H-implanted GaN is almost one order of magnitude larger than that of H-implanted Si or SiC. Barbot et al. [24] reported the formation of gas-filled platelets along the caxis in the GaN crystalline implanted with He ions at 750 °C to a fluence of 1 × 1017 cm− 2. The platelets are only observed in the end of the projected range where the vacancy concentration is rather low as compared to the He concentration. In this region, numerous helium agglomerate to form a strong out-of-plane strain that provides natural space for
bubble nucleation. Due to the relative low concentration of vacancies, bubbles appear as platelet-shape. However, we did not find the platelets formed in the projected range of the H-implanted GaN. This phenomenon may be attributed to the efficient dynamic annealing and many atomic H decorated vacancies and therefore, the absence of sufficient H atoms agglomeration to form H2 and no enough spaces for H2 accumulation. In addition, there is another possible reason that H2 in GaN requires a high formation energy of about 2.4 eV in vacuum [23]. Moutanabbir et al. [2] investigated mechanisms of H-implantationinduced GaN thin layer splitting and regarded that stress-induced Ga–N bond breaking could be the origin of the formation of platelets rather than the coalescence of bubbles. A similar phenomenon termed implantation-induced stresses has been well recognized in H-implanted Si used for Si ion-cut. Hochbauer et al. [20] regarded that H-implantation-induced damage introduces out-of-plane tensile strain, corresponding to an in-plane compressive stress. The in-plane compressive stress assists in nucleating Si–H defects that evolve into H platelets. The in-plane compressive stress is related to the defect clustering behavior observed in H-implanted Si. The results of XTEM have shown that there are two kinds of dislocation loops that are 1/2[0001] and 1/3b10 1 0 N. Besides, the number density of 1/2[0001] dislocation loops have similar the number density of 1/3b1010 N in the RT implanted sample. With increasing implantation temperature, the formation of 1/3b10 10 N dislocation loops is hindered. The results of the present study demonstrate that the efficient dynamic annealing in GaN is related to the observed dislocation loops. The growth or shrinkage of dislocation loops through the absorption of point defects generated during H-implantation. It may also be implied that the point defects in the GaN have rather high diffusivities. The dynamic annealing behavior would become more evident with increasing implantation temperature. The results show that the number density of dislocation loops decreased with increasing implantation temperature. Moreover, the width of the damaged band increased evidently when the sample was implanted at 723 K. In the deeper side of the damaged band, some dislocation loops of habit planes (0110) with b = 1/3b1010 N, were observed as shown in Fig. 5(a). A high-resolution TEM image taken from this region is shown in Fig. 5(b). In order to facilitate visualization of possible lattice defects in the periphery of the dislocation loops, the image was Fourier filtered image with the (0002) spot, as shown Fig. 5(c). A high concentration of planar defects and dislocation loops can be clearly observed. What's more, most of the observed planar defects are of interstitial-type defects, as indicated by circles in Fig. (c). The (0001) plane is the most densely packed plane due to wurtzitetype structured GaN. H-implantation-induced vacancies and interstitials would preferentially nucleate two-dimensional clusters on the basal plane, resulting in the formation of basal stacking faults with a prismatic dislocation loops with a Burger vector 1/2[0001] as previous
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17 + Fig. 4. XTEM (3 g) weak-beam dark field micrographs of GaN implanted 134 keV H+ H2 /cm2 at: RT using (a) g = 0002 and (b) g = 2110; 573 K using 2 ions to a fluence of 1.5 × 10
(c) g = 0002 and (d) g = 2110; 723 K using (e) g = 0002 and (f) g = 2110.
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Fig. 5. XTEM images showing dislocation loops in the deeper region of the damage band formed by H-implantation in GaN at 723 K, (a) bright field, (b) dark field, high resolution TEM image taken from [1120] direction showing the dislocation loop at the deeper region of the damage band as well as the corresponding Fourier filtered image (c). Some planar defects are bowled up as indicated by circles due to interstitial condensation.
reported [16]. The basal plane faults in GaN have three different types. The first type is formed by inserting an extra Ga–N bilayer. The second type is formed by removal of a basal layer of Ga–N followed by a slip of 1/3b1010 N to reduce the energy. The third type is formed from a shear of 1/3b1010N in a perfect crystal. In these three types of stacking faults in GaN, the first type has the highest stacking fault energy. With increasing implantation temperature, 1/3b1010 N dislocation loops become unstable, making it favorable for the formation of 1/2[0001] dislocation loops. The results of the surface morphology suggest that ion-cut is more efficient for the sample implanted at 573 K. This result demonstrates that 1/2[0001] dislocation loops have positive while 1/3b1010N dislocation loops have negative effects on ion-cut in (0001) GaN films. The direction of in-plane stress observed in the damaged layer is related to the damage clustering behavior. It has been reported that stressinduced Ga–N bond breaking could be the origin of the formation of platelets [2]. This interpretation could hold for the present results that the direction of platelets formed in the damaged layer depends on the direction of stress. When the direction of stress is parallel to the basal plane of GaN, platelets parallel to the basal plane of GaN could be formed. Therefore, It may imply that the direction of in-plane stress produced by 1/2[0001] is parallel to the basal plane of GaN. Because of
effective dynamic annealing, the decrease in 1/2[0001] dislocation loops lead to the in-plane stress that could not break enough numbers of Ga–N bonds. As a result, surface blisters and exfoliation were not observed for the sample implanted at 723 K upon subsequent annealing. However, the formation of platelets is still poorly understood for Himplanted GaN and more work is necessary.
4. Conclusions The effects of implantation temperature on the ion-cut process in H+ 2 -implanted GaN have been investigated. Nano-bubbles with 1–2 nm diameters and dislocation loops with b = 1/2[0001] or b = 1/3b1010N were formed by H-implantation in the damage band. With increasing implantation temperature, the ratio of the 1/3b1010N dislocation loops to 1/2[0001] dislocation loops decreased. The splitting efficiency is related to types of dislocation loops created by H-implantation. It is found that the in-plane compressive stress created by H-implantation-induced 1/2[0001] dislocation loops is necessary for platelet nucleation, growth, and finally surface blistering. A moderate implantation temperature is required for the ion-cut process of GaN.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 11005130 and 11475229) and West Light Foundation of the Chinese Academy of Sciences. The author sincerely acknowledge the members in the 320 kV High-voltage Platform in Institute of Modern Physics for their help in the ion implantation experiment. References [1] M. Bruel, Application of hydrogen ion beams to silicon on insulator material technology, Nucl. Instrum. Methods Phys. Res., Sect. B 108 (1996) 313. [2] O. Moutanabbir, R. Scholz, S. Senz, U. Gosele, M. Chicoine, F. Schiettekatte, F. Subraut, R.K. Rehberg, Microstructural evolution in H ion induced splitting of freestanding GaN, Appl. Phys. Lett. 93 (2008) 031916. [3] O. Mountanabbir, Y.J. Chabal, M. Chicoine, S. Christiansen, R.K. Rehberg, F. Schiettekatte, R. Scholz, O. Seitz, S. Senz, F. Subkraut, U. Gosele, Mechanisms of ion-induced GaN thin layer splitting, Nucl. Instrum. Methods Phys. Res., Sect. B 267 (2009) 1264. [4] Q.-T. Tong, L.J. Huang, U. Gosele, Transfer of semiconductor and oxide films by wafer bonding and layer cutting, J. Electron. Mater. 29 (2000) 928. [5] S.O. Kucheyev, J.S. Williams, C. Jagadish, J. Zou, G. Li, Blistering of H-implanted GaN, J. Appl. Phys. 91 (2002) 3928. [6] H.J. Woo, H.W. Choi, W. Hong, J.H. Park, C.H. Eum, Blistering kinetics of GaN by hydrogen implanted at high temperature, Surf. Coat. Technol. 203 (2009) 2375. [7] R. Singh, U. Dadwal, R. Scholz, O. Mountanabbir, S. Christiansen, U. Gosele, Study of implantation-induced blistering/exfoliation in wide bandgap semiconductors for layer transfer applications, Phys. Status Solidi C 7 (2009) 44. [8] R. Singh, I. Radu, U. Gosele, S.H. Christiansen, Investigation of hydrogen implantation induced blistering in GaN, Phys. Status Solidi C 3 (2006) 1754. [9] Q.-T. Tong, K. Gutjahr, S. Hopfe, U. Gosele, T.-H. Lee, Layer splitting process in hydrogen-implanted Si, Ge, SiC, and diamond substrates, Appl. Phys. Lett. 70 (1997) 1390. [10] B.S. Li, C.H. Zhang, H.H. Zhang, Y. Zhang, Y.T. Yang, L.H. Zhou, L.Q. Zhang, Thermodynamic model of helium and hydrogen co-implanted silicon surface layer splitting, Instrum. Methods Phys. Res., Sect. B 268 (2010) 555.
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Study of surface blistering in GaN by hydrogen implantation at elevated temperatures B.S. Li ⁎, Z.G. Wang, H.P. Zhang Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China
a r t i c l e
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Article history: Received 2 December 2014 Received in revised form 27 June 2015 Accepted 19 July 2015 Available online 21 July 2015 Keywords: Implantation Cross-sectional transmission electron microscopy Dislocation loops Blisters Exfoliation
a b s t r a c t We have investigated mechanisms of ion-cut in H+ 2 -implanted GaN by analyzing microstructural features of H+ 2 -implanted GaN at room temperature, 573 K and 723 K. Using optical microscopy and transmission electron microscopy, it was found that the in-plane compressive stress induced by the H-implantation was necessary for H-platelet nucleation and growth. The control of implantation temperature is crucial for creating sufficient in-plane compressive stress to induce surface blistering. © 2015 Elsevier B.V. All rights reserved.
1. Introduction GaN is a great important wide bandgap semiconductor, which can be used in electronic and optoelectronic devices. Because of the high price of free-standing GaN wafers, the growth of epitaxial layers for device fabrication is mostly performed on sapphire, SiC and Si. The “Smart Cut” technology was firstly mentioned by Bruel [1]. The procedure of this technology comprises H-implanted Si, then bonding them to a substrate stiffener. Finally, a splitting treatment is used to achieve layer transfer upon annealing at about 673 K. The “Smart Cut” technology can provide inexpensive templates with high structural quality. The free-standing wafer of GaN can be utilized to transfer multiple layers on to other foreign substrates such as Si, SiC and sapphire via this method, which in turn serve as template layers for the growth of epitaxial device layers. The ion-cut process has been extensively studied on various semiconductor materials such as Si, SiC, Ge, GaAs and InP. Implantation dose, implantation temperature, annealing temperature and time play very important roles in the splitting process. All these parameters strongly depend on the type of semiconductor materials used for splitting. In contrast to the cases of Si, SiC, Ge, GaAs and InP, only a few studies of H-implantation-induced GaN layer splitting were reported [2–8]. Tong et al. [9] reported that blister formation and layer splitting have the same activation energy, indicating that formation of optically detectable surface blisters is regarded as onset of microcrack formation. ⁎ Corresponding author at: Laboratory of Advanced Nuclear Materials, Institute of Modern Physics, CAS, Lanzhou 730000, China. E-mail address: [email protected] (B.S. Li).
http://dx.doi.org/10.1016/j.tsf.2015.07.039 0040-6090/© 2015 Elsevier B.V. All rights reserved.
In fact, the physical mechanisms of layer splitting can be conveniently investigated by studying the occurrence of surface blisters/exfoliations in H-implantation and annealing without bonded wafers [6,7,10,11]. Kucheyev et al. [5] reported the H-implantation-induced exfoliation in GaN. The exfoliation is observed in a narrow window of the implantation dose above 3 × 1017 cm−2. Mountanabbir et al. [2,3] reported microstructural evolution of H-implanted GaN with annealing temperature. The wafers were implanted by H ions at 50 keV to a dose of 2.6 × 1017 cm−2 at RT. A high density of bubbles with 1–2 nm in diameter was found in the damaged layer. After annealing above 450 °C, platelets parallel to the surface were formed. Increasing annealing temperature to 600 °C, structural transitions from platelets to microcracks occurred. Tong et al. [4] reported that the blistering of GaN would only occur when the implantation was performed in a temperature window of 275–450 °C. Blistering of GaN also occurred after H-implantation to doses above 5 × 1017 cm−2. Hayashi et al. [12] regarded that blistering also depends on the crystalline quality of the GaN wafer. Some studies have been shown that hydrogen forms complexes with Ga vacancies induced by H-implantation [3]. The agglomeration of vacancies and hydrogen leads to the formation of nanoscopic bubbles in GaN filled with molecular hydrogen, which serve as precursor to the formation of microcracks in the damaged layer and ultimately surface blisters in GaN upon annealing [7]. However, Moutanabbir et al. [2] regarded that stress-induced Ga–N bond breaking would be the origin of the formation of platelets observed in the H-implanted GaN, rather than the coalescence of nanoscopic bubbles. For a better understanding of the microscopic mechanisms of ion-induced GaN thin layer splitting, the role of implantation-induced defects should be investigated in detail.
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Because of the efficient dynamic annealing of the implantation-induced defects in GaN crystalline, the nucleation and growth of extended defects in H-implanted GaN at elevated temperature are faster than in the case of RT implantation. Therefore, the microstructural evolution at elevated temperature implantation may be useful to study the mechanism of ion-cut in GaN. Up to now, the microstructural evolution of Himplanted GaN at elevated temperatures has been less investigated [13, 14]. In this paper, optical microscopy was employed to observe the evolution of surface morphology upon annealing, and transmission electron microscopy was employed to systematically study the microstructural evolution with the implantation temperature. 2. Experimental The material studied is an n-type GaN epilayer with 30 μm in thickness grown on the c-plane of a sapphire substrate by the metal-organic chemical vapor deposition (MOCVD) method. Samples with size 10 × 5 mm2 were used for the ion implantation. The implantation with 134 keV H+ 2 ions was performed at 320 kV High-voltage Platform in the Institute of Modern Physics, Chinese Acad2 emy of Sciences. Ion fluence was 1.5 × 1017 H+ 2 /cm . The experimental conditions are expected to produce irradiation effects similar to that of a 67 keV H+ ions to a fluence of 3 × 1017H+/cm2. The distributions of the damage level in displacements per atom (dpa) and deposition concentration were estimated by SRIM96 (the displacement energies of Ga = 20.5 eV and N = 10.8 eV, and density 6.1 g cm− 3) [15]. The mean projected range Rp is approximately 440 nm with a straggling of ΔRp is approximately 140 nm. The peak concentration is approximately 18% and the maximum damage is approximately 1.5 dpa. The current density was approximately 0.8 μA/cm2 and the implantation temperatures were room temperature (RT), 573 K and 723 K. Postimplantation, wafers were cut into several pieces and then isochronally annealed in flow of N2 at temperatures of 573 K, 723 K and 873 K for 15 min, respectively. The evolution of the surface morphology properties of the implanted as well as un-implanted samples was analyzed by Olympus optical microscopy. The magnification was 1000 times, which is able to observe micron-scale bubbles or exfoliations on the sample surface. Before observations, the samples were cleaned in ethanol by ultrasonic for 5 min. Post-implantation, the microstructural evolution as a function of the implantation temperature was investigated with Tecnai G20 transmission electron microscopy (TEM) operated at 200 kV and equipped with a double tilt goniometer stage. TEM samples were prepared by a conventional method; this is, mechanical thinning up to approximately 40 μm in thickness and then ion milling with Ar ions. Initially, the energy of the Ar ions was kept at 5 kV and the incident angle was maintained at 5° from the surface until the occurrence of a hole in the center of the sample. Then, the energy of the Ar ions was reduced to 2 kV at an incident angle of 3°, applied for 1 h, in order to minimize radiation damage induced by the Ar ions. The bubbles were imaged in under-focused and over-focused conditions to highlight the bubble edges with Fresnel contrast. The Burgers vector of dislocation loops was determined by standard g·b techniques. Weak-beam darkfield imaging was also employed to improve the contrast quality. 3. Results and discussion A series of surface morphology images of the annealed and asimplanted samples obtained by optical microscopy. In the asimplantation, whatever the implantation temperature performed, the sample surfaces have not any change as compared to un-implanted samples, as shown in Fig. 1(a), (e) and (h). After 573 K annealing for the RT implanted sample, many bright contrasts appear as circular in shape on the surface. The observed contrasts are attributed to blisters, which have the lateral size ranging from 1 to 4 μm, as shown in Fig. 1(b). The observed blisters increased up to 15 μm and some blisters
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burst up to form exfoliation when the RT-implanted sample was annealed at 723 K, as shown in Fig. 1(c). The growth of blisters reaches saturation with increasing annealing temperature up to 873 K. For the 573 K implanted sample upon 723 K annealing, few of blisters with sizes approached 20 μm and most of blisters with lateral sizes ranging from 1 to 5 μm. After annealing at 873 K, some blisters grown up to approximately 16 μm and then burst up to form exfoliation. For the sample implanted at 723 K upon 873 K annealing, there is no change compared to that of the as-implanted sample, as shown in Fig. 1(i). To investigate the underlying mechanisms of this phenomenon mentioned above, the studies of the formation of H-induced bubbles and extended defects in the as-implanted sample are critical. Fig. 2 presents bright-field cross-sectional transmission electron microscopy (XTEM) images of the damaged layer for the samples implanted with H+ 2 ions at RT, 573 K and 723 K. The images were taken along the GaN [01 1 0] zone axis. No amorphous layer was detected anywhere in the damaged layer due to the efficient dynamic annealing of GaN crystalline. High densities of black contrasts which are correlated to implantation-induced dislocation loops, stacking faults and strains were observed in Fig. 2. For the sample implanted at RT, the image exhibits a well-defined damage band at depths ranging from 230 to 580 nm below the sample surface, as shown in Fig. 2(a). For the sample implanted at 573 K, Fig. 2(b) shows a well-defined damage band at depths ranging from 350 to 710 nm below the sample surface. For the sample implanted at 723 K, Fig. 2(c) shows a damage band from the sample surface to a depth of 710 nm. The width of the damage layer increased with increasing implantation temperature. It is attributed to implantation-induced lattice defects migration toward the sample surface and/or toward the end-of-projected range at elevated temperatures. To increase in the contrast of bubbles, the sample was tilted 12° from the [0110] zone axis to reduce the intensity of the diffraction contrast. Fig. 3 presents the distribution of the bubbles in the damaged layer. The distribution of the bubbles observed in the damaged layer is related to the implantation temperature. For the sample implanted at RT, bubbles are less distributed and the majority of the bubbles with diameters smaller than 3 nm are located near the projected range, as shown in Fig. 3(a). The width of the bubble layers is approximately 140 nm. The distribution of the bubbles increased with increasing implantation temperature. For the sample implanted at 573 K [see Fig. 3(b)], bubbles are homogeneously distributed in the damaged layer. Some bubbles have diameters larger than 3 nm. The width of the bubble layers is approximately 230 nm. For the sample implanted at 723 K [see Fig. 3(c)], bubbles are homogeneously distributed in the damaged layer. Some bubbles have diameters larger than 5 nm. The width of the bubble layers is approximately 250 nm. To investigate the H-implantation-induced defects in the damaged layer, two-beam conditions were performed. Fig. 4 shows weak-beam dark-field XTEM images taken from the sample implanted at RT, 573 K and 723 K. The dislocation loops are visible in the weak-beam dark-field images as white spots. Images taken under g = 0002 condition indicate the presence of a high density of point defects and dislocation loops. Images taken under g = 2110 condition indicate the presence of some planar defects which are parallel to the sample surface. Under the different diffraction factors, the size and density of the H-implantation-induced dislocation loops are different. The measurement error of dislocation loops is about 0.5 nm due to the resolution of TEM. For the sample implanted at RT, under g = 0002 condition [see Fig. 4(a)], the distribution of dislocation loops with sizes ranges from 1 to 20 nm and a number density is about 1.09 × 1023/m3. Under g = 2110 condition [see Fig. 4(b)], the distribution of dislocation loops with sizes ranges from 1 to 13 nm and a number density is about 1.05 × 1023/m3. For the sample implanted at 573 K, under g = 0002 condition [see Fig. 4(c)], the distribution of dislocation loops with sizes ranges from 1 to 19 nm and a number density is about 9.7 × 1022/m3. Under g = 2110 condition [see Fig. 4(d)], the distribution of dislocation loops with sizes ranges from 1 to 16 nm and a number density is about 5.5 × 1022/m3. For the sample implanted at
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17 + 2 Fig. 1. Optical microscopy plane view images of the sample surfaces of GaN implanted 134 keV H+ 2 ions to a fluence of 1.5 × 10 H2 /cm at: (a–d) RT; (e–g) 523 K; (h, i) 723 K, (a, e, g) asimplantation, and then annealed at: (b) 523 K; (c, f) 723 K; (d, g, i) 873 K. The scale at left corner of images represents 10 μm.
723 K, under g = 0002 condition [see Fig. 4(e)], the distribution of dislocation loops with sizes ranges from 1 to 35 nm and a number density is about 3.5 × 1022/m3. Under g = 2110 condition [see Fig. 4(f)], the distribution of dislocation loops with sizes ranges from 1 to 13 nm and a
number density is about 1.9 × 1022/m3. The Burgers vectors were analyzed by the invisibility criterion g∙b = 0. According to the previous report of O+-implanted GaN [16], we assume these dislocation loops with b = 1/2[0001] or b = 1/3 b 1010 N. In the present study, for the RT
17 + Fig. 2. XTEM bright field micrograph of the damage band in GaN implanted 134 keV H+ H2 /cm2 at: (a) RT; (b) 523 K; and (c) 723 K. The microstructures with 2 ions to a fluence of 1.5 × 10
the electron beam close to [0110] zone axis. Adapted from Ref. [13].
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17 + Fig. 3. XTEM bright field micrograph of the bubble layers in GaN implanted 134 keV H+ H2 /cm2 at: (a) RT; (b) 523 K; and (c) 723 K. Underfocus view of the 2 ions to a fluence of 1.5 × 10
bubble layers in Fig. 2 with the electron beam incident at an angle of 12° from the [0110] zone axis. Adapted from Ref. [13].
implanted sample, the dislocation loops with b = 1/2[0001] almost has the same number density as the dislocation loops with b = 1/3b1010N. For the 573 K and 723 K implanted samples, the ratios of the number density of the dislocation loops with b = 1/2[0001] to b = 1/3 b 1010N are 1.76 and 1.84, respectively. The results of optical microscopy and XTEM have clearly shown that the implantation temperature can affect the ion-cut in GaN. The ion-cut is more efficient for the sample implanted at 573 K than that of the sample implanted at RT, whereas the ion-cut did not occur for the sample implanted at 723 K. These observations suggest that high temperature implantation should be very careful in order to fulfill the ion-cut in GaN. This behavior differs completely from the phenomenon in H-implanted SiC where ion-cut easily occurred at elevated temperature implantation [17,18]. The phenomenon of ion-cut in H-implanted Si has been widely studied [19–21]. The chemical environment of H-implanted Si, H shares its electron with Si neighbors to form the atomic configuration of Si–H– Si, weakening the original Si–Si bond. The atomic configuration of Si– H–H–Si further weakens the Si–Si bond. The formation of the H–H planes (platelets) reduces the energy needed to fracture the Si {100} plane. The platelets provide a favorable site for the formation and growth of H2 gas bubbles because of a low surface energy of the platelets. Upon annealing, free atomic hydrogen evaporating from VH and Si–H defects can diffuse into the platelets and then agglomerate, forming pressurized H2 gas platelets. The high pressure in the H2 gas platelets provides the force needed to generate crack opening displacement and results in the propagation of microcracks. The coalescence of the microcracks leads to splitting, and the implanted layer can be transferred to the handle substrate via using wafer bonding before the microcrack formation [19]. Therefore, the formation and growth of platelets that are related to the concentration of H2 are critical for ion-cut. For the case of H-implanted Si at RT, the threshold dose of implanted H is about 1.5 × 1016 cm−2 for ion-cut [21], whereas ion-cut did not occur when H+ ion implanted GaN to a dose of 1 × 1017 cm−2. In the H-implanted GaN sample, no platelets that are usually formed in H-implanted Si or SiC were observed in the damaged layer [22,23]. Previous studies show that the surface layer splitting is due to the nucleation and growth of the H-decorated platelets. Therefore, no platelets formed in the as-implanted sample can explain why the threshold dose for splitting in H-implanted GaN is almost one order of magnitude larger than that of H-implanted Si or SiC. Barbot et al. [24] reported the formation of gas-filled platelets along the caxis in the GaN crystalline implanted with He ions at 750 °C to a fluence of 1 × 1017 cm− 2. The platelets are only observed in the end of the projected range where the vacancy concentration is rather low as compared to the He concentration. In this region, numerous helium agglomerate to form a strong out-of-plane strain that provides natural space for
bubble nucleation. Due to the relative low concentration of vacancies, bubbles appear as platelet-shape. However, we did not find the platelets formed in the projected range of the H-implanted GaN. This phenomenon may be attributed to the efficient dynamic annealing and many atomic H decorated vacancies and therefore, the absence of sufficient H atoms agglomeration to form H2 and no enough spaces for H2 accumulation. In addition, there is another possible reason that H2 in GaN requires a high formation energy of about 2.4 eV in vacuum [23]. Moutanabbir et al. [2] investigated mechanisms of H-implantationinduced GaN thin layer splitting and regarded that stress-induced Ga–N bond breaking could be the origin of the formation of platelets rather than the coalescence of bubbles. A similar phenomenon termed implantation-induced stresses has been well recognized in H-implanted Si used for Si ion-cut. Hochbauer et al. [20] regarded that H-implantation-induced damage introduces out-of-plane tensile strain, corresponding to an in-plane compressive stress. The in-plane compressive stress assists in nucleating Si–H defects that evolve into H platelets. The in-plane compressive stress is related to the defect clustering behavior observed in H-implanted Si. The results of XTEM have shown that there are two kinds of dislocation loops that are 1/2[0001] and 1/3b10 1 0 N. Besides, the number density of 1/2[0001] dislocation loops have similar the number density of 1/3b1010 N in the RT implanted sample. With increasing implantation temperature, the formation of 1/3b10 10 N dislocation loops is hindered. The results of the present study demonstrate that the efficient dynamic annealing in GaN is related to the observed dislocation loops. The growth or shrinkage of dislocation loops through the absorption of point defects generated during H-implantation. It may also be implied that the point defects in the GaN have rather high diffusivities. The dynamic annealing behavior would become more evident with increasing implantation temperature. The results show that the number density of dislocation loops decreased with increasing implantation temperature. Moreover, the width of the damaged band increased evidently when the sample was implanted at 723 K. In the deeper side of the damaged band, some dislocation loops of habit planes (0110) with b = 1/3b1010 N, were observed as shown in Fig. 5(a). A high-resolution TEM image taken from this region is shown in Fig. 5(b). In order to facilitate visualization of possible lattice defects in the periphery of the dislocation loops, the image was Fourier filtered image with the (0002) spot, as shown Fig. 5(c). A high concentration of planar defects and dislocation loops can be clearly observed. What's more, most of the observed planar defects are of interstitial-type defects, as indicated by circles in Fig. (c). The (0001) plane is the most densely packed plane due to wurtzitetype structured GaN. H-implantation-induced vacancies and interstitials would preferentially nucleate two-dimensional clusters on the basal plane, resulting in the formation of basal stacking faults with a prismatic dislocation loops with a Burger vector 1/2[0001] as previous
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17 + Fig. 4. XTEM (3 g) weak-beam dark field micrographs of GaN implanted 134 keV H+ H2 /cm2 at: RT using (a) g = 0002 and (b) g = 2110; 573 K using 2 ions to a fluence of 1.5 × 10
(c) g = 0002 and (d) g = 2110; 723 K using (e) g = 0002 and (f) g = 2110.
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Fig. 5. XTEM images showing dislocation loops in the deeper region of the damage band formed by H-implantation in GaN at 723 K, (a) bright field, (b) dark field, high resolution TEM image taken from [1120] direction showing the dislocation loop at the deeper region of the damage band as well as the corresponding Fourier filtered image (c). Some planar defects are bowled up as indicated by circles due to interstitial condensation.
reported [16]. The basal plane faults in GaN have three different types. The first type is formed by inserting an extra Ga–N bilayer. The second type is formed by removal of a basal layer of Ga–N followed by a slip of 1/3b1010 N to reduce the energy. The third type is formed from a shear of 1/3b1010N in a perfect crystal. In these three types of stacking faults in GaN, the first type has the highest stacking fault energy. With increasing implantation temperature, 1/3b1010 N dislocation loops become unstable, making it favorable for the formation of 1/2[0001] dislocation loops. The results of the surface morphology suggest that ion-cut is more efficient for the sample implanted at 573 K. This result demonstrates that 1/2[0001] dislocation loops have positive while 1/3b1010N dislocation loops have negative effects on ion-cut in (0001) GaN films. The direction of in-plane stress observed in the damaged layer is related to the damage clustering behavior. It has been reported that stressinduced Ga–N bond breaking could be the origin of the formation of platelets [2]. This interpretation could hold for the present results that the direction of platelets formed in the damaged layer depends on the direction of stress. When the direction of stress is parallel to the basal plane of GaN, platelets parallel to the basal plane of GaN could be formed. Therefore, It may imply that the direction of in-plane stress produced by 1/2[0001] is parallel to the basal plane of GaN. Because of
effective dynamic annealing, the decrease in 1/2[0001] dislocation loops lead to the in-plane stress that could not break enough numbers of Ga–N bonds. As a result, surface blisters and exfoliation were not observed for the sample implanted at 723 K upon subsequent annealing. However, the formation of platelets is still poorly understood for Himplanted GaN and more work is necessary.
4. Conclusions The effects of implantation temperature on the ion-cut process in H+ 2 -implanted GaN have been investigated. Nano-bubbles with 1–2 nm diameters and dislocation loops with b = 1/2[0001] or b = 1/3b1010N were formed by H-implantation in the damage band. With increasing implantation temperature, the ratio of the 1/3b1010N dislocation loops to 1/2[0001] dislocation loops decreased. The splitting efficiency is related to types of dislocation loops created by H-implantation. It is found that the in-plane compressive stress created by H-implantation-induced 1/2[0001] dislocation loops is necessary for platelet nucleation, growth, and finally surface blistering. A moderate implantation temperature is required for the ion-cut process of GaN.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 11005130 and 11475229) and West Light Foundation of the Chinese Academy of Sciences. The author sincerely acknowledge the members in the 320 kV High-voltage Platform in Institute of Modern Physics for their help in the ion implantation experiment. References [1] M. Bruel, Application of hydrogen ion beams to silicon on insulator material technology, Nucl. Instrum. Methods Phys. Res., Sect. B 108 (1996) 313. [2] O. Moutanabbir, R. Scholz, S. Senz, U. Gosele, M. Chicoine, F. Schiettekatte, F. Subraut, R.K. Rehberg, Microstructural evolution in H ion induced splitting of freestanding GaN, Appl. Phys. Lett. 93 (2008) 031916. [3] O. Mountanabbir, Y.J. Chabal, M. Chicoine, S. Christiansen, R.K. Rehberg, F. Schiettekatte, R. Scholz, O. Seitz, S. Senz, F. Subkraut, U. Gosele, Mechanisms of ion-induced GaN thin layer splitting, Nucl. Instrum. Methods Phys. Res., Sect. B 267 (2009) 1264. [4] Q.-T. Tong, L.J. Huang, U. Gosele, Transfer of semiconductor and oxide films by wafer bonding and layer cutting, J. Electron. Mater. 29 (2000) 928. [5] S.O. Kucheyev, J.S. Williams, C. Jagadish, J. Zou, G. Li, Blistering of H-implanted GaN, J. Appl. Phys. 91 (2002) 3928. [6] H.J. Woo, H.W. Choi, W. Hong, J.H. Park, C.H. Eum, Blistering kinetics of GaN by hydrogen implanted at high temperature, Surf. Coat. Technol. 203 (2009) 2375. [7] R. Singh, U. Dadwal, R. Scholz, O. Mountanabbir, S. Christiansen, U. Gosele, Study of implantation-induced blistering/exfoliation in wide bandgap semiconductors for layer transfer applications, Phys. Status Solidi C 7 (2009) 44. [8] R. Singh, I. Radu, U. Gosele, S.H. Christiansen, Investigation of hydrogen implantation induced blistering in GaN, Phys. Status Solidi C 3 (2006) 1754. [9] Q.-T. Tong, K. Gutjahr, S. Hopfe, U. Gosele, T.-H. Lee, Layer splitting process in hydrogen-implanted Si, Ge, SiC, and diamond substrates, Appl. Phys. Lett. 70 (1997) 1390. [10] B.S. Li, C.H. Zhang, H.H. Zhang, Y. Zhang, Y.T. Yang, L.H. Zhou, L.Q. Zhang, Thermodynamic model of helium and hydrogen co-implanted silicon surface layer splitting, Instrum. Methods Phys. Res., Sect. B 268 (2010) 555.
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