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Separation of thick HVPE-GaN films from GaN templates using nanoporous GaN layers
Accepted Manuscript Separation of thick HVPE-GaN films from GaN templates using nanoporous GaN layers
Zengyin Dong, Ruixia Yang, Song Zhang, Zaien Wang, Jianli Chen, Xun Li PII:
S0749-6036(17)31616-6
DOI:
10.1016/j.spmi.2017.08.039
Reference:
YSPMI 5214
To appear in:
Superlattices and Microstructures
Received Date:
04 July 2017
Revised Date:
23 August 2017
Accepted Date:
23 August 2017
Please cite this article as: Zengyin Dong, Ruixia Yang, Song Zhang, Zaien Wang, Jianli Chen, Xun Li, Separation of thick HVPE-GaN films from GaN templates using nanoporous GaN layers, Superlattices and Microstructures (2017), doi: 10.1016/j.spmi.2017.08.039
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Separation of thick HVPE-GaN films from GaN templates using nanoporous GaN layers Zengyin Dong1,2, Ruixia Yang1,*, Song Zhang2, Zaien Wang2, Jianli Chen2, Xun Li1 1School
of Information Engineering, Hebei University of Technology, Tianjin 300401, China 2The 46th Research Institute, China Electronics Technology Group Corporation, Tianjin 300220, China *corresponding author e-mail: [email protected] Abstract: In this work, we have succeeded in growing an approximate 2-inch selfseparated thick GaN wafer by hydride vapor phase epitaxy with an introduction of a sacrificial layer of nanoporous GaN. Such nanoporous GaN layer is invented by using the HVPE growth of thin GaN layer on the spin-coating silica nanosphere layer followed by a hydrofluoric acid etching to the silica nanosphere layer. It has been found that the nanoporous GaN layer, enabling a reduction of stickiness between thick GaN films and the substrate, plays a significant role in the self-separation of thick GaN films during the cooling process. However, the thickness of the nanoporous GaN layer is another key issue to achieve good quality self-separated GaN thick films. In our study, we suggest that the nanoporous GaN layer with a thickness of approximately 150-240 nm can best serve as the sacrificial layer in self-separation process. Raman spectroscopy also indicates the self-separated thick GaN films by using the proposed approach are virtually strain-free. Key words: hydride vapor phase epitaxy; nanoporous GaN; silica colloid; spincoating; self-separation; GaN crystal 1. Introduction Gallium nitride (GaN) and related nitride compounds have attracted much attention due to their excellent performance in optoelectronic and high-power electronic devices in last few decades [1,2]. Nowadays, GaN-based devices are widely fabricated on foreign substrates, such as sapphire, silicon and SiC [3-5]. However, the poor lattice match and large difference in thermal expansion coefficients for these substrates often lead to a severe bowing of wafers, large densities of threading dislocations and great residual strains, which greatly affect the optical and electrical properties of devices [6]. One of the alternative to solve this problem is homoepitaxial growth by using free-standing bulk GaN substrate [7]. Hydride vapor phase epitaxy (HVPE) has been considered as one of the promising candidates to commercially fabricate free-standing GaN wafers among other proposed methods due to its high growth rate at a relatively low cost [8]. Generally speaking, the sapphire substrate is extremely difficult to remove, because of its toughness and resistance to chemical etching. Recently, several studies have been reported to separate HVPE-GaN from sapphire substrates, such as the laser lift-off 0
ACCEPTED MANUSCRIPT [9,10], the void-assisted separation (VAS) [11,12] and the natural stress-induced separation [13,14]. Compared with the laser lift-off and the VAS methods, the natural stress-induced separation method is relatively simple and less expensive. It has been reported that with an insertion of a porous interlayer at the GaN/sapphire interface, free-standing GaN wafers have been successfully peeled off from the substrates [1518]. In this report, a novel approach has been proposed to fabricate a nanoporous GaN layer in order to help with the process of the separation of the thick GaN film from the substrate. Our studies indicate that the nanoporous GaN layer is able to weaken the adhesive between the thick GaN film and the GaN template, which eases the occurrence of self-separation during the cooling process. By insertion of a nanoporous GaN layer with an approximately 150-240 nm, we have obtained an intact 2-inch free-standing GaN wafer with a dislocation density less than 107cm-2. The selfseparated GaN wafer also exhibits no residual compressive strain confirmed by Raman spectroscopy. 2. Experiments A 4-5 μm thick GaN template was grown by metal organic chemical vapor deposition (MOCVD) on a two-inch-diameter c-plane sapphire substrate. The GaN template was uniformly spin-coated with a silica nanosphere layer at a spinning speed of 4000rpm before it was loaded into a conventional vertical HVPE reactor. The HVPE growth was performed at atmospheric pressure using NH3, HCl and metallic Ga as sources for the GaN growth. Nitrogen was used as the carrier gas. The GaCl was synthesized in the gallium boat by the reaction between HCl and Ga at 900 ℃, and was then transported into the growth zone, where the substrate was heated up to 1020 ℃. In order to make GaN extend out from the voids on the silica nanosphere surface, a relatively high NH3 flow rate (~4000 sccm) was first introduced with a HCl flow rate of 2 sccm for 10 minutes. It has been reported that high NH3 flow rate could supply sufficient active nitrogen atoms to trap gallium atoms near the surface, which results in a fast vertical growth mode [19]. After that, the growth continued for another 30s at a 300-sccm NH3 and 32-sccm HCl flow rate. When the HVPE growth ended, the sample was unloaded from the reactor and was immersed into the 15 % HF solution for 30 minutes to remove the silica nanosphere. Consequently, one nanoporous GaN layer with a thickness of approximately 150-240 nm was formed on the GaN template surface. The GaN template with the nanoporous GaN layer was reloaded into the HVPE reactor followed by a 6-hour re-growth of thick GaN film, during which the flow rates of the HCl and the NH3 were kept at 48 sccm and 1100 sccm, respectively. A 500-μm thick GaN film was successfully separated from the substrate via the nanoporous GaN sacrificial layer during the cooling process. For the purpose of investigating the effect of the silica nanosphere layer thickness on quality of the nanoporous GaN layer and the subsequently grown GaN layer. Four samples (named A, B, C, and D) were prepared with different thickness of silica nanosphere layers. After HF chemical etching, another subsequent 65-μm thick GaN films were grown on each of the nanoporous GaN layers by HVPE. The 1
ACCEPTED MANUSCRIPT thickness of the silica nanosphere layer was controlled by the spin-coating times and the solid content of the silica colloid. Detailed information is summarized in Table 1. Table 1 Spin-coating conditions of silica colloid and the thickness of the obtained nanoporous GaN layers Sample
Solid content (%)
Spin-coating times
A
30
2
B C D
30 20 15
1 1 1
nanoporous GaN thickness (nm)
655 238 156 50
The thickness and the pore distribution of the nanoporous GaN layers were measured by PHILIPS XL30 scanning electron microscopy (SEM). The surface morphologies of the thick GaN films were observed by Nikon SL150 differential interference contrast microscope (DICM). The GaN symmetrical (002) reflection rocking curves (XRC) were obtained by using a Bruker D8 Discovery system with Cu Kα line at 0.15418 nm as a source. The radius of lattice curvature was estimated from the shift of the peak top angle of the 002 ω-scan XRC profiles with the incident X-ray directions parallel to the m-direction of the GaN. Threading dislocation density was evaluated according to etch-pit density. Raman spectrum of the freestanding GaN wafer was obtained with the LabRam HR800 system at the room temperature by using a 514 nm solid laser as the excitation source. 3. Results and discussion Figure 1 displays the cross-sectional SEM images of four nanoporous GaN layers after HF chemical etching to the initial silica colloid spin-coating layers. The thickness of each layer was measured to be 655 nm, 238 nm, 156 nm, and 50 nm respectively. The nanoporous GaN layers exhibit a number of pores with a diameter approximately 60 nm, close to that of the silica nanosphere used in spin-coating. It is suggested that during the early steps of HVPE growth, GaN tends to penetrate through the voids among silica nanospheres, forming one layer of GaN tangled with silica nanospheres. After HF chemical etching, the silica nanopheres have been removed, leaving the GaN layer with a nanoporous feature. From this point of view, the thickness of the nanoporous GaN layer could be manipulated by silica colloid spincoating procedure to some extent. However, for sample D, the nanoporous GaN layer is as thin as 50 nm, and the porous structure is not obvious compared with other three samples (see figure 1d). We conclude that the silica nanosphere layer is too thin and is completely covered by the thin GaN layer grown on it. As a result, the silica nanospheres have been protected from being etched away by the HF solution leading to a porous-free structure. One 65-μm thick GaN film has been grown on each of the remaining nanoporous GaN layer for samples A-C and the surface morphologies are revealed by DICM, as shown in figure 2 a-c respectively. In figure 2a, a rough and undulated surface of the 2
ACCEPTED MANUSCRIPT GaN film is exhibited with lots of pits (the black spots in the figure 2a) of different sizes. On the other hand, however, the morphologies of the GaN films grown on samples B (figure 2b) and C (figure 2c) have been remarkably improved with a relatively flat and smooth surface; even a mirror-like surface has been realized for sample C. The number of pits has also been notably reduced compared with that of sample A. These observations suggest that the thickness of the silica nanosphere layer have a significant influence on the coalescence and morphologies for the following thick HVPE-GaN film growth. Although excessively thick nanosphere layers are detrimental for the growth of the HVPE-GaN film, a proper control of the thickness for the nanoporous GaN layer will be under investigation in future. The crystalline quality of the 65-μm thick GaN film is studied by XRD, which can be determined by the full widths at half maximum (FWHM). The narrower FWHM represents the better crystalline quality of the GaN film [20]. Figure 3a shows the (002) XRCs of the thick GaN films grown on samples A-C. The FWHM of the XRCs for the thick GaN film grown on sample A is measured to be 531.3 arcsec, which is much larger than that of samples B (231.9 arcsec) and C (203.3 arcsec ). These results are consistent with the observations of the morphologies of GaN thick films under nanoporous GaN layers with different thickness, indicating that the thickness of the nanoporous GaN layer within a range from 150 to 240 nm could be usable for the further self-separation of the thick GaN film from the substrate. Figure 3b demonstrates the cross-sectional SEM image of interface between the nanoporous GaN layer and the 65-μm thick GaN film grown on sample B. It is evident that the pores in the nanoporous GaN layer are not completely stuffed by the subsequent GaN epitaxial layer. It could be explained that the source materials cannot be continuously transported into the region where the pores are distributed after the fast coalescence of the HVPE-GaN. It is the existence of these pores at the interface that dramatically weakens the adhesive between the GaN film and the substrate. The strain at the interface could be released through the regions where the pores are accumulated and thick GaN films could be self-separated from the substrate during the cooling process. A single piece of approximate 2-inch freestanding GaN wafer with a thickness of 500 μm was obtained by using a 240-nm thick nanoporous GaN layer, as shown in figure 4a. There are many concave pits on the rough surface of the backside for the separated HVPE-GaN observed in figure 4b. These concave pits are indicative of the locations of the accumulation of nano-pores and the occurrence of the self-separation. Moreover, the dislocation density has been estimated to be around 7.8×106 cm-2 by the counting of pit density from the molten KOH etching in figure 4c. The residual stress in the freestanding GaN wafer is assessed by Raman spectrum. In the range of 500-800 cm-1, the spectrum in figure 5 mainly includes the high-frequency E2 (E2 (high)) phonon peak at 567.4cm-1 and the Al (LO) phonon peak at 734.3cm-1, which is consistent with Raman selection rules for wurtzite GaN. The E2 (high) phonon peak position is sensitive to the stress state of GaN. The E2 (high) phonon peak of the stress-free GaN crystal is well located in 567.1±1cm-1 [21]. When there is the compressive stress in the GaN crystal, the E2 (high) phonon peak has a 3
ACCEPTED MANUSCRIPT blue shift. On the contrary, a red shift occurs when the GaN crystal exhibits a tensile stress. Compared with the stress-free GaN layer, the experimental result shows a blue shift of only 0.3 cm-1, which indicates that almost no residual compressive stress is left in the freestanding GaN wafer. The radius of lattice curvature of the freestanding GaN wafer is also calculated by the equation of R=Δx/Δω, where Δx is the distance between the neighboring measurement points (generally Δx=5 mm) and Δω is the difference between the neighboring XRC peak top angles [22]. Figure 6 illustrates the mapping of (002) XRC, which gives a 7.4-m radius of lattice curvature for the freestanding GaN wafer. 4. Conclusions In summary, we proposed a unique way to fabricate a nanoporous GaN layer at the interface between thick GaN films and the GaN templates, which is effective to realization of the self-separation during the cooling process. A single piece of 2-inch free-standing GaN wafer is obtained by this method. The thickness of the nanoporous GaN layer greatly affects the crystalline quality and morphologies of the subsequently thick GaN film grown by HVPE, which could be controlled by solid content of silica colloid and spin-coating times. We recommend a thickness within a range from 150 to 240 nm thick nanoporous GaN layer be sufficient to obtain not only good crystalline quality of GaN thick films but also ease the self-separation during cooling process. The free-standing GaN wafer by this method is free of any residual stress confirmed by Raman spectroscopy. Acknowledge This work is financially supported by the National Natural Science Foundation of China (Grant No. 61774054) and the Natural Science Foundation of Tianjin (Grant No. 15JCQNJC03700).
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References [1] W. B. Lanford, T. Tanaka, Y. Otoki, I. Adesida, Recessed-gate enhancementmode GaN HEMT with high threshold voltage, Electronics Letters 41 (2005) 449-450. [2] Wu Junqiao, When group-III nitrides go infrared: New properties and perspectives, Journal of Applied Physics 106 (2009) 5. [3] Shunfeng Li, Andreas Waag, GaN based nanorods for solid state lighting, Journal of Applied Physics 111 (2012) 071101. [4] Pengelly RS, Wood SM, Milligan JW, Sheppard ST, Pribble WL, A review of GaN on SiC high electron-mobility power transistors and MMICs, IEEE Transactions on Microwave Theory and Techniques 60 (2012) 1764-1783. [5] Uemoto Y, Hikita M, Ueno H, Matsuo H, Ishida H, Yanagihara M, Ueda T, Tanaka T, Ueda D, Gate injection transistor (GIT)-A normally-off AlGaN/GaN power transistor using conductivity modulation, IEEE Transactions on Electron Devices (54) 2007 3393-3399. [6] Wu Jie-Jun, Wang Kun, Yu Tong-Jun, Zhang Guo-Yi, GaN substrate and GaN homo-epitaxy for LEDs: Progress and challenges, Chin. Phys. B 24 (2015) 068106. [7] Tanya Paskova, Keith R. Evans, GaN substrates—Progress, status, and prospects, IEEE Journal of Selected Topics in Quantum Electronics 15 (2009) 1041-1052. [8] Hajime Fujikura, Takehiro Yoshida, Masatomo Shibata, Yohei Otoki, Recent progress of high-quality GaN substrates by HVPE method, Proc. of SPIE 10104 (2017) 1010403. [9] Hae-Yong Lee, Young-Jun Choi, Jin-Hun Kim, Hyun-Soo Jang, Hae-Kon Oh, Jun Young Kim, Jung Young Jung, Jonghee Hwang, The control of mechanical bow for GaN substrate grown by HVPE with relatively longer radius of lattice curvature, Phys. Status Solidi C 11 (2014) 477-482. [10]X J Su, K Xu, Y Xu, G Q Ren, J C Zhang, J F Wang, H Yang, Shock-induced brittle cracking in HVPE-GaN processed by laser lift-off techniques, J. Phys. D: Appl. Phys. 46 (2013) 205103. [11]Yuichi Oshima, Takeshi Eri, Masatomo Shibata, Haruo Sunakawa, Kenji Kobayashi, Toshinari Ichihashi, Akira Usui, Preparation of Freestanding GaN Wafers by Hydride Vapor Phase Epitaxy with Void-Assisted Separation, Jpn. J. Appl. Phys. 42 (2003) L1-L3. [12]Takehiro Yoshida, Yuichi Oshima, Takeshi Eri, Ken Ikeda, Shunsuke Yamamoto, Kazutoshi Watanabe, Masatomo Shibata, Tomoyoshi Mishima,
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ACCEPTED MANUSCRIPT Fabrication of 3-in GaN substrates by hydride vapor phase epitaxy using voidassisted separation method, Journal of Crystal Growth 310 (2008) 5-7. [13]K. Yamane, M. Ueno, H. Furuya, N. Okada, K. Tadatomo, Successful natural stress-induced separation of hydride vapor phase epitaxy-grown GaN layers on sapphire substrates, Journal of Crystal Growth 358 (2012) 1-4. [14]Xingbin Li, Jiejun Wu, Nanliu Liu, Tong Han, Xiangning Kang, Tongjun Yu, Guoyi Zhang, Self-separation of two-inch-diameter freestanding GaN by hydride vapor phase epitaxy and heat treatment of sapphire, Materials Letters 132 (2014) 94-97. [15]Hyun-Jae Lee, S. W. Lee, H. Goto, Sang-Hyun Lee, Hyo-Jong Lee, J. S. Ha, Takenari Goto, M. W. Cho, T. Yao, Self-separated freestanding GaN using a NH4Cl interlayer, Appl. Phys. Lett. 91 (2007), 192108. [16]Hyun-Jae Lee, Jun-Seok Ha, Hyo-Jong Lee, S. W. Lee, K. Fujii, S. K. Hong, J. H. Chang, M. W. Cho, Takenari Goto, T. Yao, Fabrication of free-standing GaN substrate using evaporable buffer layer (EBL), Phys. Status Solidi C 6 (2009) S313-S316. [17]Sinae Kim, Hyunjae Lee, Siyoung Kim, Sungkuk Choi, Jieun Koo, Jiho Chang, Fabrication of free-standing GaN by using thermal decomposition of GaN, Journal of Crystal Growth 398 (2014) 13-17. [18]V. Nikolaev, A. Golovatenko, M. Mynbaeva, I. Nikitina, N. Seredova, A. Pechnikov, V. Bougrov, M. Odnobludov, Effect of nano-column properties on self-separation of thick GaN layers grown by HVPE, Phys. Status Solidi C 11 (2014) 502–504. [19]X. Zhang, P. D. Dapkus, D. H. Rich, Lateral epitaxy overgrowth of GaN with NH3 flow rate modulation, Appl. Phys. Lett. 77 (2000) 1496-1498. [20]R. Chierchia, T. Bottcher, H. Heinke, S. Einfeldt, S. Figge, D. Hommel, Microstructure of heteroepitaxial GaN revealed by x-ray diffraction, J. Appl. Phys. 93 (2003) 8918-8925 [21]Yuanbin Dai, Yongliang Shao, Yongzhong Wu, Xiaopeng Hao, Peng Zhang, Xingzhong Cao, Lei Zhang, Yuan Tian, Haodong Zhang, Influence of V/III ratio on stress control in GaN grown on different templates by hydride vapour phase epitaxy, RSC Adv. 4 (2014) 21504-21509. [22]Humberto M. Foronda, Alexey E. Romanov, Erin C. Young, Christian A. Robertson, Glenn E. Beltz, James S. Speck, Curvature and bow of bulk GaN substrates, J. Appl. Phys. 120 (2016) 035104.
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FIGURE CAPTIONS Figure 1. Cross-sectional SEM images of nanoporous GaN layer with different thickness; Figure 2. Surface morphologies of the subsequent grown 65-μm thick GaN on nanoporous GaN layers for sample A-C; Figure 3. (a). X-ray rocking curves for (002) reflection for the thick GaN films of sample A-C, (b). Cross-sectional SEM image of a 65-μm GaN film for sample B; Figure 4. (a). Nearly 2-inch free-standing GaN wafer with a thickness of 500 μm, grid: 5 mm; (b). SEM images of the backside of the self-separated GaN wafer; (c). SEM images of the GaN surface etched in molten KOH at 260 ℃ for 10 mins; Figure 5. Raman spectroscopy of the self-separated GaN wafer; Figure 6. Dependence of peak top angle of XRC on measurement point. The solid squares represent the shift in XRC peak top angle of the freestanding GaN wafer. The inverse of the slope of a line represents the radius of lattice curvature;
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Figure 1.
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Figure 2.
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Figure 3.
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Figure 4.
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Intensity
E2 (high)
A1 (LO)
500
550
600
650
700
750
800
-1
Raman Shift (cm )
Figure 5.
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Peak top angle of (002) XRC (degree)
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17.85
XRC peak top angle of GaN the linear fit of XRC peak top angle
17.80 17.75 17.70 17.65 17.60 17.55
-15
-10
-5
0
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Position (mm)
Figure 6.
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ACCEPTED MANUSCRIPT Highlights
The porous structured GaN layer enables a reduced stickiness between the thick HVPE-GaN film and the GaN template grown on the sapphire substrate.
A method for preparing nanoporous GaN layer is proposed, which combines the processes of the spin-coating of silica colloid, the HVPE-GaN growth and the hydrofluoric acid etching.
The 150-240 nm thick nanoporous GaN layer can serve as the sacrificial layer to enable the separation between the thick HVPE-GaN film and the GaN template.
Zengyin Dong, Ruixia Yang, Song Zhang, Zaien Wang, Jianli Chen, Xun Li PII:
S0749-6036(17)31616-6
DOI:
10.1016/j.spmi.2017.08.039
Reference:
YSPMI 5214
To appear in:
Superlattices and Microstructures
Received Date:
04 July 2017
Revised Date:
23 August 2017
Accepted Date:
23 August 2017
Please cite this article as: Zengyin Dong, Ruixia Yang, Song Zhang, Zaien Wang, Jianli Chen, Xun Li, Separation of thick HVPE-GaN films from GaN templates using nanoporous GaN layers, Superlattices and Microstructures (2017), doi: 10.1016/j.spmi.2017.08.039
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Separation of thick HVPE-GaN films from GaN templates using nanoporous GaN layers Zengyin Dong1,2, Ruixia Yang1,*, Song Zhang2, Zaien Wang2, Jianli Chen2, Xun Li1 1School
of Information Engineering, Hebei University of Technology, Tianjin 300401, China 2The 46th Research Institute, China Electronics Technology Group Corporation, Tianjin 300220, China *corresponding author e-mail: [email protected] Abstract: In this work, we have succeeded in growing an approximate 2-inch selfseparated thick GaN wafer by hydride vapor phase epitaxy with an introduction of a sacrificial layer of nanoporous GaN. Such nanoporous GaN layer is invented by using the HVPE growth of thin GaN layer on the spin-coating silica nanosphere layer followed by a hydrofluoric acid etching to the silica nanosphere layer. It has been found that the nanoporous GaN layer, enabling a reduction of stickiness between thick GaN films and the substrate, plays a significant role in the self-separation of thick GaN films during the cooling process. However, the thickness of the nanoporous GaN layer is another key issue to achieve good quality self-separated GaN thick films. In our study, we suggest that the nanoporous GaN layer with a thickness of approximately 150-240 nm can best serve as the sacrificial layer in self-separation process. Raman spectroscopy also indicates the self-separated thick GaN films by using the proposed approach are virtually strain-free. Key words: hydride vapor phase epitaxy; nanoporous GaN; silica colloid; spincoating; self-separation; GaN crystal 1. Introduction Gallium nitride (GaN) and related nitride compounds have attracted much attention due to their excellent performance in optoelectronic and high-power electronic devices in last few decades [1,2]. Nowadays, GaN-based devices are widely fabricated on foreign substrates, such as sapphire, silicon and SiC [3-5]. However, the poor lattice match and large difference in thermal expansion coefficients for these substrates often lead to a severe bowing of wafers, large densities of threading dislocations and great residual strains, which greatly affect the optical and electrical properties of devices [6]. One of the alternative to solve this problem is homoepitaxial growth by using free-standing bulk GaN substrate [7]. Hydride vapor phase epitaxy (HVPE) has been considered as one of the promising candidates to commercially fabricate free-standing GaN wafers among other proposed methods due to its high growth rate at a relatively low cost [8]. Generally speaking, the sapphire substrate is extremely difficult to remove, because of its toughness and resistance to chemical etching. Recently, several studies have been reported to separate HVPE-GaN from sapphire substrates, such as the laser lift-off 0
ACCEPTED MANUSCRIPT [9,10], the void-assisted separation (VAS) [11,12] and the natural stress-induced separation [13,14]. Compared with the laser lift-off and the VAS methods, the natural stress-induced separation method is relatively simple and less expensive. It has been reported that with an insertion of a porous interlayer at the GaN/sapphire interface, free-standing GaN wafers have been successfully peeled off from the substrates [1518]. In this report, a novel approach has been proposed to fabricate a nanoporous GaN layer in order to help with the process of the separation of the thick GaN film from the substrate. Our studies indicate that the nanoporous GaN layer is able to weaken the adhesive between the thick GaN film and the GaN template, which eases the occurrence of self-separation during the cooling process. By insertion of a nanoporous GaN layer with an approximately 150-240 nm, we have obtained an intact 2-inch free-standing GaN wafer with a dislocation density less than 107cm-2. The selfseparated GaN wafer also exhibits no residual compressive strain confirmed by Raman spectroscopy. 2. Experiments A 4-5 μm thick GaN template was grown by metal organic chemical vapor deposition (MOCVD) on a two-inch-diameter c-plane sapphire substrate. The GaN template was uniformly spin-coated with a silica nanosphere layer at a spinning speed of 4000rpm before it was loaded into a conventional vertical HVPE reactor. The HVPE growth was performed at atmospheric pressure using NH3, HCl and metallic Ga as sources for the GaN growth. Nitrogen was used as the carrier gas. The GaCl was synthesized in the gallium boat by the reaction between HCl and Ga at 900 ℃, and was then transported into the growth zone, where the substrate was heated up to 1020 ℃. In order to make GaN extend out from the voids on the silica nanosphere surface, a relatively high NH3 flow rate (~4000 sccm) was first introduced with a HCl flow rate of 2 sccm for 10 minutes. It has been reported that high NH3 flow rate could supply sufficient active nitrogen atoms to trap gallium atoms near the surface, which results in a fast vertical growth mode [19]. After that, the growth continued for another 30s at a 300-sccm NH3 and 32-sccm HCl flow rate. When the HVPE growth ended, the sample was unloaded from the reactor and was immersed into the 15 % HF solution for 30 minutes to remove the silica nanosphere. Consequently, one nanoporous GaN layer with a thickness of approximately 150-240 nm was formed on the GaN template surface. The GaN template with the nanoporous GaN layer was reloaded into the HVPE reactor followed by a 6-hour re-growth of thick GaN film, during which the flow rates of the HCl and the NH3 were kept at 48 sccm and 1100 sccm, respectively. A 500-μm thick GaN film was successfully separated from the substrate via the nanoporous GaN sacrificial layer during the cooling process. For the purpose of investigating the effect of the silica nanosphere layer thickness on quality of the nanoporous GaN layer and the subsequently grown GaN layer. Four samples (named A, B, C, and D) were prepared with different thickness of silica nanosphere layers. After HF chemical etching, another subsequent 65-μm thick GaN films were grown on each of the nanoporous GaN layers by HVPE. The 1
ACCEPTED MANUSCRIPT thickness of the silica nanosphere layer was controlled by the spin-coating times and the solid content of the silica colloid. Detailed information is summarized in Table 1. Table 1 Spin-coating conditions of silica colloid and the thickness of the obtained nanoporous GaN layers Sample
Solid content (%)
Spin-coating times
A
30
2
B C D
30 20 15
1 1 1
nanoporous GaN thickness (nm)
655 238 156 50
The thickness and the pore distribution of the nanoporous GaN layers were measured by PHILIPS XL30 scanning electron microscopy (SEM). The surface morphologies of the thick GaN films were observed by Nikon SL150 differential interference contrast microscope (DICM). The GaN symmetrical (002) reflection rocking curves (XRC) were obtained by using a Bruker D8 Discovery system with Cu Kα line at 0.15418 nm as a source. The radius of lattice curvature was estimated from the shift of the peak top angle of the 002 ω-scan XRC profiles with the incident X-ray directions parallel to the m-direction of the GaN. Threading dislocation density was evaluated according to etch-pit density. Raman spectrum of the freestanding GaN wafer was obtained with the LabRam HR800 system at the room temperature by using a 514 nm solid laser as the excitation source. 3. Results and discussion Figure 1 displays the cross-sectional SEM images of four nanoporous GaN layers after HF chemical etching to the initial silica colloid spin-coating layers. The thickness of each layer was measured to be 655 nm, 238 nm, 156 nm, and 50 nm respectively. The nanoporous GaN layers exhibit a number of pores with a diameter approximately 60 nm, close to that of the silica nanosphere used in spin-coating. It is suggested that during the early steps of HVPE growth, GaN tends to penetrate through the voids among silica nanospheres, forming one layer of GaN tangled with silica nanospheres. After HF chemical etching, the silica nanopheres have been removed, leaving the GaN layer with a nanoporous feature. From this point of view, the thickness of the nanoporous GaN layer could be manipulated by silica colloid spincoating procedure to some extent. However, for sample D, the nanoporous GaN layer is as thin as 50 nm, and the porous structure is not obvious compared with other three samples (see figure 1d). We conclude that the silica nanosphere layer is too thin and is completely covered by the thin GaN layer grown on it. As a result, the silica nanospheres have been protected from being etched away by the HF solution leading to a porous-free structure. One 65-μm thick GaN film has been grown on each of the remaining nanoporous GaN layer for samples A-C and the surface morphologies are revealed by DICM, as shown in figure 2 a-c respectively. In figure 2a, a rough and undulated surface of the 2
ACCEPTED MANUSCRIPT GaN film is exhibited with lots of pits (the black spots in the figure 2a) of different sizes. On the other hand, however, the morphologies of the GaN films grown on samples B (figure 2b) and C (figure 2c) have been remarkably improved with a relatively flat and smooth surface; even a mirror-like surface has been realized for sample C. The number of pits has also been notably reduced compared with that of sample A. These observations suggest that the thickness of the silica nanosphere layer have a significant influence on the coalescence and morphologies for the following thick HVPE-GaN film growth. Although excessively thick nanosphere layers are detrimental for the growth of the HVPE-GaN film, a proper control of the thickness for the nanoporous GaN layer will be under investigation in future. The crystalline quality of the 65-μm thick GaN film is studied by XRD, which can be determined by the full widths at half maximum (FWHM). The narrower FWHM represents the better crystalline quality of the GaN film [20]. Figure 3a shows the (002) XRCs of the thick GaN films grown on samples A-C. The FWHM of the XRCs for the thick GaN film grown on sample A is measured to be 531.3 arcsec, which is much larger than that of samples B (231.9 arcsec) and C (203.3 arcsec ). These results are consistent with the observations of the morphologies of GaN thick films under nanoporous GaN layers with different thickness, indicating that the thickness of the nanoporous GaN layer within a range from 150 to 240 nm could be usable for the further self-separation of the thick GaN film from the substrate. Figure 3b demonstrates the cross-sectional SEM image of interface between the nanoporous GaN layer and the 65-μm thick GaN film grown on sample B. It is evident that the pores in the nanoporous GaN layer are not completely stuffed by the subsequent GaN epitaxial layer. It could be explained that the source materials cannot be continuously transported into the region where the pores are distributed after the fast coalescence of the HVPE-GaN. It is the existence of these pores at the interface that dramatically weakens the adhesive between the GaN film and the substrate. The strain at the interface could be released through the regions where the pores are accumulated and thick GaN films could be self-separated from the substrate during the cooling process. A single piece of approximate 2-inch freestanding GaN wafer with a thickness of 500 μm was obtained by using a 240-nm thick nanoporous GaN layer, as shown in figure 4a. There are many concave pits on the rough surface of the backside for the separated HVPE-GaN observed in figure 4b. These concave pits are indicative of the locations of the accumulation of nano-pores and the occurrence of the self-separation. Moreover, the dislocation density has been estimated to be around 7.8×106 cm-2 by the counting of pit density from the molten KOH etching in figure 4c. The residual stress in the freestanding GaN wafer is assessed by Raman spectrum. In the range of 500-800 cm-1, the spectrum in figure 5 mainly includes the high-frequency E2 (E2 (high)) phonon peak at 567.4cm-1 and the Al (LO) phonon peak at 734.3cm-1, which is consistent with Raman selection rules for wurtzite GaN. The E2 (high) phonon peak position is sensitive to the stress state of GaN. The E2 (high) phonon peak of the stress-free GaN crystal is well located in 567.1±1cm-1 [21]. When there is the compressive stress in the GaN crystal, the E2 (high) phonon peak has a 3
ACCEPTED MANUSCRIPT blue shift. On the contrary, a red shift occurs when the GaN crystal exhibits a tensile stress. Compared with the stress-free GaN layer, the experimental result shows a blue shift of only 0.3 cm-1, which indicates that almost no residual compressive stress is left in the freestanding GaN wafer. The radius of lattice curvature of the freestanding GaN wafer is also calculated by the equation of R=Δx/Δω, where Δx is the distance between the neighboring measurement points (generally Δx=5 mm) and Δω is the difference between the neighboring XRC peak top angles [22]. Figure 6 illustrates the mapping of (002) XRC, which gives a 7.4-m radius of lattice curvature for the freestanding GaN wafer. 4. Conclusions In summary, we proposed a unique way to fabricate a nanoporous GaN layer at the interface between thick GaN films and the GaN templates, which is effective to realization of the self-separation during the cooling process. A single piece of 2-inch free-standing GaN wafer is obtained by this method. The thickness of the nanoporous GaN layer greatly affects the crystalline quality and morphologies of the subsequently thick GaN film grown by HVPE, which could be controlled by solid content of silica colloid and spin-coating times. We recommend a thickness within a range from 150 to 240 nm thick nanoporous GaN layer be sufficient to obtain not only good crystalline quality of GaN thick films but also ease the self-separation during cooling process. The free-standing GaN wafer by this method is free of any residual stress confirmed by Raman spectroscopy. Acknowledge This work is financially supported by the National Natural Science Foundation of China (Grant No. 61774054) and the Natural Science Foundation of Tianjin (Grant No. 15JCQNJC03700).
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ACCEPTED MANUSCRIPT Fabrication of 3-in GaN substrates by hydride vapor phase epitaxy using voidassisted separation method, Journal of Crystal Growth 310 (2008) 5-7. [13]K. Yamane, M. Ueno, H. Furuya, N. Okada, K. Tadatomo, Successful natural stress-induced separation of hydride vapor phase epitaxy-grown GaN layers on sapphire substrates, Journal of Crystal Growth 358 (2012) 1-4. [14]Xingbin Li, Jiejun Wu, Nanliu Liu, Tong Han, Xiangning Kang, Tongjun Yu, Guoyi Zhang, Self-separation of two-inch-diameter freestanding GaN by hydride vapor phase epitaxy and heat treatment of sapphire, Materials Letters 132 (2014) 94-97. [15]Hyun-Jae Lee, S. W. Lee, H. Goto, Sang-Hyun Lee, Hyo-Jong Lee, J. S. Ha, Takenari Goto, M. W. Cho, T. Yao, Self-separated freestanding GaN using a NH4Cl interlayer, Appl. Phys. Lett. 91 (2007), 192108. [16]Hyun-Jae Lee, Jun-Seok Ha, Hyo-Jong Lee, S. W. Lee, K. Fujii, S. K. Hong, J. H. Chang, M. W. Cho, Takenari Goto, T. Yao, Fabrication of free-standing GaN substrate using evaporable buffer layer (EBL), Phys. Status Solidi C 6 (2009) S313-S316. [17]Sinae Kim, Hyunjae Lee, Siyoung Kim, Sungkuk Choi, Jieun Koo, Jiho Chang, Fabrication of free-standing GaN by using thermal decomposition of GaN, Journal of Crystal Growth 398 (2014) 13-17. [18]V. Nikolaev, A. Golovatenko, M. Mynbaeva, I. Nikitina, N. Seredova, A. Pechnikov, V. Bougrov, M. Odnobludov, Effect of nano-column properties on self-separation of thick GaN layers grown by HVPE, Phys. Status Solidi C 11 (2014) 502–504. [19]X. Zhang, P. D. Dapkus, D. H. Rich, Lateral epitaxy overgrowth of GaN with NH3 flow rate modulation, Appl. Phys. Lett. 77 (2000) 1496-1498. [20]R. Chierchia, T. Bottcher, H. Heinke, S. Einfeldt, S. Figge, D. Hommel, Microstructure of heteroepitaxial GaN revealed by x-ray diffraction, J. Appl. Phys. 93 (2003) 8918-8925 [21]Yuanbin Dai, Yongliang Shao, Yongzhong Wu, Xiaopeng Hao, Peng Zhang, Xingzhong Cao, Lei Zhang, Yuan Tian, Haodong Zhang, Influence of V/III ratio on stress control in GaN grown on different templates by hydride vapour phase epitaxy, RSC Adv. 4 (2014) 21504-21509. [22]Humberto M. Foronda, Alexey E. Romanov, Erin C. Young, Christian A. Robertson, Glenn E. Beltz, James S. Speck, Curvature and bow of bulk GaN substrates, J. Appl. Phys. 120 (2016) 035104.
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FIGURE CAPTIONS Figure 1. Cross-sectional SEM images of nanoporous GaN layer with different thickness; Figure 2. Surface morphologies of the subsequent grown 65-μm thick GaN on nanoporous GaN layers for sample A-C; Figure 3. (a). X-ray rocking curves for (002) reflection for the thick GaN films of sample A-C, (b). Cross-sectional SEM image of a 65-μm GaN film for sample B; Figure 4. (a). Nearly 2-inch free-standing GaN wafer with a thickness of 500 μm, grid: 5 mm; (b). SEM images of the backside of the self-separated GaN wafer; (c). SEM images of the GaN surface etched in molten KOH at 260 ℃ for 10 mins; Figure 5. Raman spectroscopy of the self-separated GaN wafer; Figure 6. Dependence of peak top angle of XRC on measurement point. The solid squares represent the shift in XRC peak top angle of the freestanding GaN wafer. The inverse of the slope of a line represents the radius of lattice curvature;
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Figure 1.
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Figure 2.
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Figure 3.
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Figure 4.
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ACCEPTED MANUSCRIPT Highlights
The porous structured GaN layer enables a reduced stickiness between the thick HVPE-GaN film and the GaN template grown on the sapphire substrate.
A method for preparing nanoporous GaN layer is proposed, which combines the processes of the spin-coating of silica colloid, the HVPE-GaN growth and the hydrofluoric acid etching.
The 150-240 nm thick nanoporous GaN layer can serve as the sacrificial layer to enable the separation between the thick HVPE-GaN film and the GaN template.