- Email: [email protected]
Effect of AlN buffer thickness on GaN epilayer grown on Si(1 1 1)
Materials Science in Semiconductor Processing 14 (2011) 97–100
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Effect of AlN buffer thickness on GaN epilayer grown on Si(1 1 1) Meng Wei a,n, Xiaoliang Wang a,b, Xu Pan a, Hongling Xiao a,b, CuiMei Wang a,b, Qifeng Hou a, Zhanguo Wang b a b
Materials Science Center, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China
a r t i c l e i n f o
abstract
Available online 3 March 2011
We studied the influence of high temperature AlN buffer thickness on the property of GaN film on Si (1 1 1) substrate. Samples were grown by metal organic chemical vapor deposition. Optical microscopy, atomic force microscopy and X-ray diffraction were employed to characterize the samples. The results demonstrated that thickness of high temperature AlN buffer prominently influenced the morphology and the crystal quality of GaN epilayer. The optimized thickness of the AlN buffer is found to be about 150 nm. Under the optimized thickness, the largest crack-free range of GaN film is 10 mm 10 mm and the full width at half maximum of GaN (0 0 0 2) rocking curve peak is 621.7 arcsec. Using high temperature AlN/AlGaN multibuffer combined with AlN/GaN superlattices interlayer we have obtained 2 mm crack-free GaN epilayer on 2 in Si (1 1 1) substrates. & 2011 Elsevier Ltd. All rights reserved.
Keywords: GaN MOCVD Si(1 1 1) AlN
1. Introduction GaN compound semiconductor material system has been extensively of great interest for the large band gap, high breakdown electric strength, high electron saturated drift velocity and good thermal stability. Furthermore, the particular spontaneous and piezoelectric polarization effects lead to high concentration of two-dimensional electron gas (2DEG) near the interface. Consequently, GaN material has a large potential application in the fields of high power and high frequency electronic devices, light emitting diodes (LEDs) and laser diodes [1–5]. Generally GaN is grown on sapphire, SiC and Si substrates. Sapphire substrates limit the improvement of the power density of the device due to its bad thermal conduction. Meanwhile, the high cost of SiC substrates hinders the application of GaN material grown on SiC. Compared with sapphire and
n Corresponding author. Tel.: + 86 10 82304140; fax: + 86 10 82304045. E-mail address: [email protected] (M. Wei).
1369-8001/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2011.01.006
SiC, Si is the best alternative for its low cost, good thermal conductivity and ability to be integrated with the mature Si-based processing techniques. However, the cracks and high defect density seriously degrade the performance of GaN based devices on Si substrates because of the large crystal lattice mismatch (17%) and the coefficients of thermal expansion (CTE) mismatch (56%) between GaN and silicon [6,7]. In order to resolve these problems the selection of suitable buffer layer is crucial, such as AlN, AlGaN, Al2O3, ZnO, etc. [8,9]. It has been reported that high temperature (HT) AlN buffer owns many advantages for GaN epitaxy on Si (1 1 1), including the prevention of melt-back etching reaction [10], introduction of compressive stress into GaN epilayer and avoidance of impurity atoms into growth cavity. Based on the previous researches [11,12], this paper focused on the optimization of the HT AlN thickness. Effects of the HT AlN buffer thickness on the crack density and crystal quality of GaN film on Si (1 1 1) substrate were investigated. The research is very useful for a better understanding of the role of AlN buffer in the epitaxy of GaN. Using HT AlN/AlGaN multibuffer and AlN/GaN
98
M. Wei et al. / Materials Science in Semiconductor Processing 14 (2011) 97–100
superlattices interlayer we have successfully grown 2 mm crack-free GaN on 2 in Si (1 1 1) substrates.
Table 2 Crack-free ranges of samples. ID
A
B
C
D
E
AlN thickness (nm) Crack-free range (mm2)
50 45
100 77
150 10 10
200 56
250 33
2. Experimental In this work, the epitaxy of GaN on 2 in Si (1 1 1) substrates was performed in a horizontal metal organic chemical vapor deposition (MOCVD) reactor. Prior to growth, Si (1 1 1) substrates were degreased in boiling sulphuric acid, boiling HCl:H2O2:H2O (1:1:5) solution, boiling NH4OH:H2O2:H2O (1:1:5) solution and diluted HF solution in turn, which provided an oxide-free hydrogen-terminated Si (1 1 1) surface. After the Si substrates were loaded in reactor, an in-situ thermal clean at 1100 1C in hydrogen atmosphere was employed. Trimethylgallium (TMG), trimetheylaluminum (TMA) and ammonia (NH3) were used as Ga, Al and N precursors, respectively, while N2 and H2 as carrier gases. Five samples were grown containing different HT AlN buffers (50, 100, 150, 200 and 250 nm) and 1.2 mm GaN film, labeled by samples A, B, C, D and E, respectively. Schematic diagram of the sample growth structure is shown in Fig. 1, and the detailed descriptions of the samples are given in Table 1. Before AlN growth, the Si (1 1 1) substrates were exposed to TMA flow for 15 s without any ammonia in order to deposit an ultra-Al layer preventing the formation of a silicon nitride phase on the Si surfaces. GaN and HT AlN were grown at 1060 and 1050 1C, respectively. The pressure was kept constant at 100 Torr for all deposition steps. The V/III ratio of the HT AlN growth was about 114. The crack-free range of the GaN film was evaluated by optical microscopy (OM). The surface morphologies were studied by OM and atomic force microscopy (AFM). X-ray diffraction (XRD) was performed to measure the full width at half maximum (FWHM) of GaN (0 0 0 2) rocking curve peaks to characterize the crystal quality. 3. Results and discussion The crack density of the GaN film was investigated by OM. The crack-free ranges of the five samples were 4 mm 5 mm, 7 mm 7 mm, 10 mm 10 mm, 5 mm 6 mm and 3 mm 3 mm, as shown in Table 2. Compared
Fig. 1. Schematic diagram of sample structure. Table 1 Detailed messages of five samples. ID
Sample description
A B C D E
Si(1 1 1) Si(1 1 1) Si(1 1 1) Si(1 1 1) Si(1 1 1)
/50 nm HT AlN/1.2 mm GaN /100 nm HT AlN/1.2 mm GaN /150 nm HT AlN/1.2 mm GaN /200 nm HT AlN/1.2 mm GaN /250 nm HT AlN/1.2 mm GaN
with the OM images, we found that the crack-free range and the surface morphology of the samples were evidently dependent on the thickness of HT AlN buffer layer. It was suggested that the AlN buffer could introduce a compression stress to the GaN layer, due to its smaller lattice constant than that of GaN. The compression stress could compensate for the tensile stress in GaN generated from large CTE mismatch between Si (1 1 1) substrate and GaN film in the process of cooling down, thus resulted in the reduction of crack density. Feltin et al. [13] speculated that there was a transition thickness of HT AlN buffer, when AlN thickness exceeded the critical thickness the residual tensile stress in GaN would increase, leading to an increase in crack density. Raghavan et al. [14] indicated that crystalline grain mergence was one reason of the tensile stress generation. It seemed that AlN buffer was optimized under the thickness of 150 nm, which yielded the largest crack-free range of 10 mm 10 mm and the most specular surface of the GaN film (Fig. 2(c)). On the surface of sample A many circular defects were apparent to the naked eyes, some of which inclined to distribute in the junctures of cracks, as shown in Fig. 2(a). The surfaces of samples B, C, D and E were specular to naked eyes. However, OM images showed some pits on the surface of sample B, while none in samples C, D and E, as shown in Fig. 2. Energy diffraction spectrum (EDS) analysis indicated that the components of Si element in the circular defects of sample A and in the pits of sample B were as high as 65% and 32%, respectively. It proved that the circular defects and pits were caused by the reaction of Ga and Si atoms outdiffusing from substrate, coincident with Bougrioua et al. [15]. Dadgar et al. [16] reported it as melt-back etching reaction, which was severely destructive to GaN epilayer. Sanchez et al. [17] interpreted the circular defects and pits origination by step theory. It was confirmed by comparing the OM images that HT AlN buffer could prevent Ga–Si reaction to form circular defects, and with the increase in AlN buffer thickness the hindrance effect was strengthened. It was suggested to be the reason for the observation of circular defects in sample A and pits in sample B while none was shown in samples C, D and E. The AlN buffer thicknesses of samples A and B were too less to prevent Si atoms diffusing out in the HT growth process. The surface morphologies of samples were characterized by AFM with 2 mm 2 mm scan scope. As shown in Fig. 3, the root mean square roughness (RMS) of samples decreased sharply from 4.34 to 1.82 nm (the AFM image of sample C was shown in Fig. 4) with the HT AlN buffer thickness increasing from 50 to 150 nm, while decreased slightly from 1.82 to 1.57 nm as the thickness increased from 150 to 250 nm. The results revealed that increase in HT AlN thickness from 50 to 150 nm could significantly improve the surface morphology of GaN film, because AlN
M. Wei et al. / Materials Science in Semiconductor Processing 14 (2011) 97–100
99
Fig. 2. OM image of sample A (a), sample B (b), sample C (c), sample D (d) and sample E (e).
Fig. 3. 2 mm 2 mm scan scope RMS of samples vs. thicknesses of HT AlN buffer. Fig. 4. 2 mm 2 mm scan scope AFM image of sample C.
transformed from three-dimension (3D) mode to twodimension (2D) mode in this stage; thus the crystal quality and surface morphology of AlN buffer improved with the increase in thickness. Nevertheless, AlN changed into 2D growth mode absolutely when the thickness was further increased from 150 to 250 nm, as a result the surface morphology improvement of AlN; therefore GaN was limited. The X-ray o-scan for samples was carried out with symmetric GaN (0 0 0 2) reflections. As shown in Fig. 4(a), GaN (0 0 0 2) XRD FWHMs decreased sharply with the HT AlN buffer thicknesses increasing from 50 to 150 nm while increased slightly when the thicknesses further increased from 150 to 250 nm. GaN (0 0 0 2) XRD FWHM
of sample C was 621.7 arcsec, as shown in Fig. 4(b). The results suggested that the crystal quality of GaN film was evidently affected by the thickness of HT AlN buffer, and an optimized thickness lied around 150 nm. It is suggested that HT AlN buffer layer could act as nucleation layer for GaN film. With the increase in thickness, AlN islands were gradually mergenced. When the thickness of HT AlN increased to 150 nm, AlN islands accomplished mergence and changed into 2D growth mode, followed by the merged AlN covering Si substrates completely. The mergence of AlN islands prevented the Si atoms from outdiffusing and reacting with Ga atoms, resulted in the improvement of GaN crystal quality [18], which confirmed the OM observation discussed above. When the
100
M. Wei et al. / Materials Science in Semiconductor Processing 14 (2011) 97–100
Fig. 5. (a) GaN(0 0 0 2) XRD FWHMs vs. thicknesses of HT AlN buffer. (b) GaN(0 0 0 2) XRD FWHM of sample C.
AlN thickness exceeded 150 nm, the nucleation effect was degraded by the smooth surface of the AlN buffer, leading to the GaN film crystal quality deterioration. The results suggested that an optimized thickness of HT AlN was required to obtain perfect crystal quality (Fig. 5).
(nos. 2006CB604905 and 2010CB327503) and the Knowledge Innovation Program of the Chinese Academy of Sciences (nos. ISCAS2008T01, ISCAS2009L01 and ISCAS2009L02).
4. Summary and conclusions
[1] Hansen M, Fini P, Zhao L, Abare AC, Coldren LA, Speck JS, et al. Appl Phys Lett 2000;76:529. [2] Wu XH, Fini P, Tarsa EJ, Heying B, Keller S, Mishra UK, et al. J Cryst Growth 1998;189:231. [3] Tanaka S, Iwai S, Aoyagi Y. J Cryst Growth 1997;170:329. [4] Lahreche H, Leroux M, Lau€gt M, Vaille M, Beaumont B, Gibart P. J Appl Phys 2000;87:577. [5] Egawa T, Nakada N, Ishikawa H, Umeno M. Electron Lett 2000;36: 1816. [6] Nikishin SA, Faleev NN, Antipov G, Francoeur S, Seryogin GA, Temkin H, et al. Appl Phys Lett 1999;75:2073. [7] Arslan E, Ozturk MK, Teke A, Ozcelik S, Ozbay E. J Phys D: Appl Phys 2008;41:155317. [8] Able A, Wegscheider W, Engl K. J Cryst Growth 2005;276:415. [9] Krost A, Dadgar A. Phys Status Solidi A 2002;194:361. [10] Akenori U, Kazutada I, Nakao A, et al. J Cryst Growth 2007;298:201. [11] Liu Z, Wang XL, Wang JX, et al. J Cryst Growth 2007;298:281. [12] Luo WJ, Wang XL, Guo LC, et al. Microelectron J 2008;39:1710. [13] Feltin E, Beaumont B, Laugt M, et al. Appl Phys Lett 2001;79:3230. [14] Raghavan S, Redwing JM, et al. J. Appl. Phys. 2005;98:023514. [15] Bougrioua Z, Moerman I, Nistor L, Van Daele B, Monroy E, Palacios T, et al. Phys Status Solidi (A) 2003;195:93. [16] Dadgar A, Strittmatter A, Blasing J, et al. Phys Status Solidi (C) 2003;0:1583. [17] Sanchez AM, Dimitrakopulos GP, Ruterana P, et al. Appl Phys Lett 2005;86:011917. [18] Zang KY, Chua SJ, Wang LS, et al. Phys Status Solidi (C) 2003;7: 2067.
This article reports the influence of HT AlN buffer thickness on the surface morphology and crystal quality of GaN film on Si (1 1 1) substrates. An obvious dependence of GaN crack density and crystal quality on the HT AlN thickness is observed. Furthermore, the optimized thickness is found to be around 150 nm. Under the optimized HT AlN thickness, the largest crack-free range of GaN film is 10 mm 10 mm, RMS of AFM 2 2 mm scan is 1.82 nm and GaN (0 0 0 2) XRD FWHM is 621.7 arcsec. This work is helpful for a better understanding of the effect of HT AlN buffer on GaN heteroepitaxy.
Acknowledgments This work has been supported by the Knowledge Innovation Engineering of Chinese Academy of Sciences (no. YYYJ-0701-02), the National Nature Sciences Foundation of China (nos. 60890193 and 60906006), the State Key Development Program for Basic Research of China
References
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Effect of AlN buffer thickness on GaN epilayer grown on Si(1 1 1) Meng Wei a,n, Xiaoliang Wang a,b, Xu Pan a, Hongling Xiao a,b, CuiMei Wang a,b, Qifeng Hou a, Zhanguo Wang b a b
Materials Science Center, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China
a r t i c l e i n f o
abstract
Available online 3 March 2011
We studied the influence of high temperature AlN buffer thickness on the property of GaN film on Si (1 1 1) substrate. Samples were grown by metal organic chemical vapor deposition. Optical microscopy, atomic force microscopy and X-ray diffraction were employed to characterize the samples. The results demonstrated that thickness of high temperature AlN buffer prominently influenced the morphology and the crystal quality of GaN epilayer. The optimized thickness of the AlN buffer is found to be about 150 nm. Under the optimized thickness, the largest crack-free range of GaN film is 10 mm 10 mm and the full width at half maximum of GaN (0 0 0 2) rocking curve peak is 621.7 arcsec. Using high temperature AlN/AlGaN multibuffer combined with AlN/GaN superlattices interlayer we have obtained 2 mm crack-free GaN epilayer on 2 in Si (1 1 1) substrates. & 2011 Elsevier Ltd. All rights reserved.
Keywords: GaN MOCVD Si(1 1 1) AlN
1. Introduction GaN compound semiconductor material system has been extensively of great interest for the large band gap, high breakdown electric strength, high electron saturated drift velocity and good thermal stability. Furthermore, the particular spontaneous and piezoelectric polarization effects lead to high concentration of two-dimensional electron gas (2DEG) near the interface. Consequently, GaN material has a large potential application in the fields of high power and high frequency electronic devices, light emitting diodes (LEDs) and laser diodes [1–5]. Generally GaN is grown on sapphire, SiC and Si substrates. Sapphire substrates limit the improvement of the power density of the device due to its bad thermal conduction. Meanwhile, the high cost of SiC substrates hinders the application of GaN material grown on SiC. Compared with sapphire and
n Corresponding author. Tel.: + 86 10 82304140; fax: + 86 10 82304045. E-mail address: [email protected] (M. Wei).
1369-8001/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2011.01.006
SiC, Si is the best alternative for its low cost, good thermal conductivity and ability to be integrated with the mature Si-based processing techniques. However, the cracks and high defect density seriously degrade the performance of GaN based devices on Si substrates because of the large crystal lattice mismatch (17%) and the coefficients of thermal expansion (CTE) mismatch (56%) between GaN and silicon [6,7]. In order to resolve these problems the selection of suitable buffer layer is crucial, such as AlN, AlGaN, Al2O3, ZnO, etc. [8,9]. It has been reported that high temperature (HT) AlN buffer owns many advantages for GaN epitaxy on Si (1 1 1), including the prevention of melt-back etching reaction [10], introduction of compressive stress into GaN epilayer and avoidance of impurity atoms into growth cavity. Based on the previous researches [11,12], this paper focused on the optimization of the HT AlN thickness. Effects of the HT AlN buffer thickness on the crack density and crystal quality of GaN film on Si (1 1 1) substrate were investigated. The research is very useful for a better understanding of the role of AlN buffer in the epitaxy of GaN. Using HT AlN/AlGaN multibuffer and AlN/GaN
98
M. Wei et al. / Materials Science in Semiconductor Processing 14 (2011) 97–100
superlattices interlayer we have successfully grown 2 mm crack-free GaN on 2 in Si (1 1 1) substrates.
Table 2 Crack-free ranges of samples. ID
A
B
C
D
E
AlN thickness (nm) Crack-free range (mm2)
50 45
100 77
150 10 10
200 56
250 33
2. Experimental In this work, the epitaxy of GaN on 2 in Si (1 1 1) substrates was performed in a horizontal metal organic chemical vapor deposition (MOCVD) reactor. Prior to growth, Si (1 1 1) substrates were degreased in boiling sulphuric acid, boiling HCl:H2O2:H2O (1:1:5) solution, boiling NH4OH:H2O2:H2O (1:1:5) solution and diluted HF solution in turn, which provided an oxide-free hydrogen-terminated Si (1 1 1) surface. After the Si substrates were loaded in reactor, an in-situ thermal clean at 1100 1C in hydrogen atmosphere was employed. Trimethylgallium (TMG), trimetheylaluminum (TMA) and ammonia (NH3) were used as Ga, Al and N precursors, respectively, while N2 and H2 as carrier gases. Five samples were grown containing different HT AlN buffers (50, 100, 150, 200 and 250 nm) and 1.2 mm GaN film, labeled by samples A, B, C, D and E, respectively. Schematic diagram of the sample growth structure is shown in Fig. 1, and the detailed descriptions of the samples are given in Table 1. Before AlN growth, the Si (1 1 1) substrates were exposed to TMA flow for 15 s without any ammonia in order to deposit an ultra-Al layer preventing the formation of a silicon nitride phase on the Si surfaces. GaN and HT AlN were grown at 1060 and 1050 1C, respectively. The pressure was kept constant at 100 Torr for all deposition steps. The V/III ratio of the HT AlN growth was about 114. The crack-free range of the GaN film was evaluated by optical microscopy (OM). The surface morphologies were studied by OM and atomic force microscopy (AFM). X-ray diffraction (XRD) was performed to measure the full width at half maximum (FWHM) of GaN (0 0 0 2) rocking curve peaks to characterize the crystal quality. 3. Results and discussion The crack density of the GaN film was investigated by OM. The crack-free ranges of the five samples were 4 mm 5 mm, 7 mm 7 mm, 10 mm 10 mm, 5 mm 6 mm and 3 mm 3 mm, as shown in Table 2. Compared
Fig. 1. Schematic diagram of sample structure. Table 1 Detailed messages of five samples. ID
Sample description
A B C D E
Si(1 1 1) Si(1 1 1) Si(1 1 1) Si(1 1 1) Si(1 1 1)
/50 nm HT AlN/1.2 mm GaN /100 nm HT AlN/1.2 mm GaN /150 nm HT AlN/1.2 mm GaN /200 nm HT AlN/1.2 mm GaN /250 nm HT AlN/1.2 mm GaN
with the OM images, we found that the crack-free range and the surface morphology of the samples were evidently dependent on the thickness of HT AlN buffer layer. It was suggested that the AlN buffer could introduce a compression stress to the GaN layer, due to its smaller lattice constant than that of GaN. The compression stress could compensate for the tensile stress in GaN generated from large CTE mismatch between Si (1 1 1) substrate and GaN film in the process of cooling down, thus resulted in the reduction of crack density. Feltin et al. [13] speculated that there was a transition thickness of HT AlN buffer, when AlN thickness exceeded the critical thickness the residual tensile stress in GaN would increase, leading to an increase in crack density. Raghavan et al. [14] indicated that crystalline grain mergence was one reason of the tensile stress generation. It seemed that AlN buffer was optimized under the thickness of 150 nm, which yielded the largest crack-free range of 10 mm 10 mm and the most specular surface of the GaN film (Fig. 2(c)). On the surface of sample A many circular defects were apparent to the naked eyes, some of which inclined to distribute in the junctures of cracks, as shown in Fig. 2(a). The surfaces of samples B, C, D and E were specular to naked eyes. However, OM images showed some pits on the surface of sample B, while none in samples C, D and E, as shown in Fig. 2. Energy diffraction spectrum (EDS) analysis indicated that the components of Si element in the circular defects of sample A and in the pits of sample B were as high as 65% and 32%, respectively. It proved that the circular defects and pits were caused by the reaction of Ga and Si atoms outdiffusing from substrate, coincident with Bougrioua et al. [15]. Dadgar et al. [16] reported it as melt-back etching reaction, which was severely destructive to GaN epilayer. Sanchez et al. [17] interpreted the circular defects and pits origination by step theory. It was confirmed by comparing the OM images that HT AlN buffer could prevent Ga–Si reaction to form circular defects, and with the increase in AlN buffer thickness the hindrance effect was strengthened. It was suggested to be the reason for the observation of circular defects in sample A and pits in sample B while none was shown in samples C, D and E. The AlN buffer thicknesses of samples A and B were too less to prevent Si atoms diffusing out in the HT growth process. The surface morphologies of samples were characterized by AFM with 2 mm 2 mm scan scope. As shown in Fig. 3, the root mean square roughness (RMS) of samples decreased sharply from 4.34 to 1.82 nm (the AFM image of sample C was shown in Fig. 4) with the HT AlN buffer thickness increasing from 50 to 150 nm, while decreased slightly from 1.82 to 1.57 nm as the thickness increased from 150 to 250 nm. The results revealed that increase in HT AlN thickness from 50 to 150 nm could significantly improve the surface morphology of GaN film, because AlN
M. Wei et al. / Materials Science in Semiconductor Processing 14 (2011) 97–100
99
Fig. 2. OM image of sample A (a), sample B (b), sample C (c), sample D (d) and sample E (e).
Fig. 3. 2 mm 2 mm scan scope RMS of samples vs. thicknesses of HT AlN buffer. Fig. 4. 2 mm 2 mm scan scope AFM image of sample C.
transformed from three-dimension (3D) mode to twodimension (2D) mode in this stage; thus the crystal quality and surface morphology of AlN buffer improved with the increase in thickness. Nevertheless, AlN changed into 2D growth mode absolutely when the thickness was further increased from 150 to 250 nm, as a result the surface morphology improvement of AlN; therefore GaN was limited. The X-ray o-scan for samples was carried out with symmetric GaN (0 0 0 2) reflections. As shown in Fig. 4(a), GaN (0 0 0 2) XRD FWHMs decreased sharply with the HT AlN buffer thicknesses increasing from 50 to 150 nm while increased slightly when the thicknesses further increased from 150 to 250 nm. GaN (0 0 0 2) XRD FWHM
of sample C was 621.7 arcsec, as shown in Fig. 4(b). The results suggested that the crystal quality of GaN film was evidently affected by the thickness of HT AlN buffer, and an optimized thickness lied around 150 nm. It is suggested that HT AlN buffer layer could act as nucleation layer for GaN film. With the increase in thickness, AlN islands were gradually mergenced. When the thickness of HT AlN increased to 150 nm, AlN islands accomplished mergence and changed into 2D growth mode, followed by the merged AlN covering Si substrates completely. The mergence of AlN islands prevented the Si atoms from outdiffusing and reacting with Ga atoms, resulted in the improvement of GaN crystal quality [18], which confirmed the OM observation discussed above. When the
100
M. Wei et al. / Materials Science in Semiconductor Processing 14 (2011) 97–100
Fig. 5. (a) GaN(0 0 0 2) XRD FWHMs vs. thicknesses of HT AlN buffer. (b) GaN(0 0 0 2) XRD FWHM of sample C.
AlN thickness exceeded 150 nm, the nucleation effect was degraded by the smooth surface of the AlN buffer, leading to the GaN film crystal quality deterioration. The results suggested that an optimized thickness of HT AlN was required to obtain perfect crystal quality (Fig. 5).
(nos. 2006CB604905 and 2010CB327503) and the Knowledge Innovation Program of the Chinese Academy of Sciences (nos. ISCAS2008T01, ISCAS2009L01 and ISCAS2009L02).
4. Summary and conclusions
[1] Hansen M, Fini P, Zhao L, Abare AC, Coldren LA, Speck JS, et al. Appl Phys Lett 2000;76:529. [2] Wu XH, Fini P, Tarsa EJ, Heying B, Keller S, Mishra UK, et al. J Cryst Growth 1998;189:231. [3] Tanaka S, Iwai S, Aoyagi Y. J Cryst Growth 1997;170:329. [4] Lahreche H, Leroux M, Lau€gt M, Vaille M, Beaumont B, Gibart P. J Appl Phys 2000;87:577. [5] Egawa T, Nakada N, Ishikawa H, Umeno M. Electron Lett 2000;36: 1816. [6] Nikishin SA, Faleev NN, Antipov G, Francoeur S, Seryogin GA, Temkin H, et al. Appl Phys Lett 1999;75:2073. [7] Arslan E, Ozturk MK, Teke A, Ozcelik S, Ozbay E. J Phys D: Appl Phys 2008;41:155317. [8] Able A, Wegscheider W, Engl K. J Cryst Growth 2005;276:415. [9] Krost A, Dadgar A. Phys Status Solidi A 2002;194:361. [10] Akenori U, Kazutada I, Nakao A, et al. J Cryst Growth 2007;298:201. [11] Liu Z, Wang XL, Wang JX, et al. J Cryst Growth 2007;298:281. [12] Luo WJ, Wang XL, Guo LC, et al. Microelectron J 2008;39:1710. [13] Feltin E, Beaumont B, Laugt M, et al. Appl Phys Lett 2001;79:3230. [14] Raghavan S, Redwing JM, et al. J. Appl. Phys. 2005;98:023514. [15] Bougrioua Z, Moerman I, Nistor L, Van Daele B, Monroy E, Palacios T, et al. Phys Status Solidi (A) 2003;195:93. [16] Dadgar A, Strittmatter A, Blasing J, et al. Phys Status Solidi (C) 2003;0:1583. [17] Sanchez AM, Dimitrakopulos GP, Ruterana P, et al. Appl Phys Lett 2005;86:011917. [18] Zang KY, Chua SJ, Wang LS, et al. Phys Status Solidi (C) 2003;7: 2067.
This article reports the influence of HT AlN buffer thickness on the surface morphology and crystal quality of GaN film on Si (1 1 1) substrates. An obvious dependence of GaN crack density and crystal quality on the HT AlN thickness is observed. Furthermore, the optimized thickness is found to be around 150 nm. Under the optimized HT AlN thickness, the largest crack-free range of GaN film is 10 mm 10 mm, RMS of AFM 2 2 mm scan is 1.82 nm and GaN (0 0 0 2) XRD FWHM is 621.7 arcsec. This work is helpful for a better understanding of the effect of HT AlN buffer on GaN heteroepitaxy.
Acknowledgments This work has been supported by the Knowledge Innovation Engineering of Chinese Academy of Sciences (no. YYYJ-0701-02), the National Nature Sciences Foundation of China (nos. 60890193 and 60906006), the State Key Development Program for Basic Research of China
References