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Si film prepared by RF magnetron sputtering
ARTICLE IN PRESS
Journal of Crystal Growth 293 (2006) 258–262 www.elsevier.com/locate/jcrysgro
Effects of ZnO interlayers on thick GaN/Si film prepared by RF magnetron sputtering C.G. Zhang, L.F. Bian, W.D. Chen, C.C. Hsu State Key Laboratory for Surface Physics, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China Received 7 September 2005; received in revised form 19 April 2006; accepted 16 May 2006 Communicated by R.M. Biefeld
Abstract We have successfully prepared a high-quality 2 mm-thick GaN film with three inserted 30 nm-thick ZnO interlayers on Si (1 1 1) substrate without cracks by magnetron sputtering. The effects of the thickness and number of ZnO interlayers on the crystal quality of the GaN films were studied. It was found that the GaN crystal quality initially improved with the increase of the thickness of ZnO interlayers, but deteriorated quickly when the thickness exceeded 30 nm. Multiple ZnO interlayers were used as an effective means to further improve the crystal quality of the GaN film. By increasing the number of interlayers up to three, the cracks can be constrained to a certain extent, and the crystal quality of the GaN film greatly improved. r 2006 Elsevier B.V. All rights reserved. PACS: 78.55.Cr; 68.55.Jk; 81.15.Cd Keywords: A3. Radio-frequency magnetron sputtering; B1. Gallium nitride; B1. ZnO interlayers
1. Introduction Over the past decade, GaN has triggered great research interest [1] due to its many applications in optoelectronic and microelectronic devices. The use of GaN allows for large concentrations of RE dopant with no significant reduction of light emission at room temperature [2]. In particular, the ability of RE ions to emit red, green, and blue light [3] makes RE-doped GaN well-suited for applications in visible full-color display technology [4–7]. So far, the RE-doping of GaN has been performed only by ion implantation and molecular beam epitaxy (MBE). For applications in thin film display devices, the study of a low cost, large area deposition method is needed for the preparation of GaN film doped with RE ions. For depositing large area films at low cost, radio-frequency magnetron sputtering appears promising [8–12]. GaN on silicon (Si) substrate will pave the way for the integration of devices with mature Si IC technology. However, the Corresponding author. Tel.: +86 10 82304243; fax: +86 10 82304253.
E-mail address: [email protected] (C.G. Zhang). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.05.064
differences in lattice constant (20%) and thermal expansion coefficient (56%) between GaN and Si are even larger than those between GaN and sapphire. Cracks caused by the large difference in thermal expansion always occur when the thickness of the film exceeds 1 mm. In order to inhibit such cracks, interlayers must be introduced into the GaN film. The effectiveness of interlayers in inhibiting cracks has been attributed to the fact that the compressive stress introduced by the interlayers can compensate for part of the tensile stress in the GaN films. Kim and Kim [13] reported the preparation of high-quality GaN/Si film using a ZnO buffer layer applied by sputtering, but when the thickness of the film exceeded 1 mm, cracks would usually occur, thus seriously degrading the crystal quality. Cong et al. [14] reported the growth of high-quality GaN/Si films by metalorganic chemical vapor deposition (MOCVD) using AlN buffer layers and interlayers, but the films were limited in size and were still rather expensive. In this study, we have investigated the effects of the thickness and number of ZnO interlayers on the stress relaxation and the crystal quality of GaN films grown on Si (1 1 1) substrate by sputtering. To our knowledge, the growth of thick GaN
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(d) Fig. 1. Schematic diagram of samples with different numbers of ZnO interlayers inserted.
films using interlayers by sputtering has not been reported to date. 2. Experimental procedure A conventional radio frequency magnetron sputtering system was used in this experiment. Both Ga2O3 (5N) and ZnO (5N) ceramic targets with a diameter and thickness of 75 and 5 mm were loaded into the sputtering chamber with a 5 104 Pa base pressure. Si (1 1 1) substrates were chemically cleaned by standard procedures. Then the substrates were loaded into the chamber and kept at 200 1C during sputtering process. The distance between target and substrate was 10.5 cm. Both targets were pre-sputtered in pure argon for 10 min. Then, a 100 nm-thick ZnO film as a buffer layer was deposited on Si (1 1 1) substrates at an RF power of 100 W and an argon pressure of 1.0 Pa for 30 min. Subsequently, Ga2O3 films were grown on a ZnO buffer layer at an RF power of 150 W and a pressure of 1.0 Pa. The Ga2O3 film was then annealed under flowing ammonia (NH3, 5N) at a rate of 100 l/min at 1000 1C. The annealing process was maintained for 10 min at atmospheric pressure of ammonia in order to allow the NH3 to react completely with the Ga2O3. GaN particles coalesced as crystal nuclei and grew gradually to form large GaN grains. After the GaN growth, ZnO interlayers were grown on it under the same conditions as mentioned above. The GaN film was again grown on ZnO interlayers. The above steps were repeated until the GaN film reached a thickness of 2 mm. In this study, two sets of samples were grown. The first set was grown to study the effect of the thickness of the ZnO interlayers. The sample with no buffer layer was
labeled A. Other samples with buffer layers were labeled A0(without interlayers), A10(10 nm), A30(30 nm), A50 (50 nm) in order of increasing interlayer thickness. Figs. 1(a) and (b) schematically show the structure of samples A0, A10, A30, A50. The second set was grown to study the effect of the number of interlayers. The structure of this set of samples is shown in Figs. 1(c) and (d). The samples were labeled B1 (one layer), B2 (two layers), B3 (three layers) and B4 (four layers) in order of the number of interlayers. The GaN films were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), Xray diffraction (XRD), and photoluminescence (PL) measurements. SEM images were taken with a Hitachi S450 SEM operated at 10–15 kV. The XRD measurements were performed using a Rigaku X-ray generator (SLX-1A) with a Cu-Ka1 rotating radiation source (l ¼ 0.15 405 nm) and a Ge (0 0 4) monochromator. The PL excitation source was a He-Cd laser (325 nm), and the PL signal at 365 nm was analyzed with a JY-HRD1 double grating monochromator equipped with an RCA-C31034 photo-multiplier tube in a PAR-1140A photon counting mode. A Nanoscope III digital Instrument was used for AFM characterization with a scan size of 5 mm 5 mm. TencorInstrument (Alpha-step200) was used to measure the film thickness. 3. Results and discussion 3.1. Effect of the thickness of ZnO interlayers on the properties of the GaN film Fig. 2 shows SEM images of the first series of samples. For sample A with no buffer layer, voids and large amounts of cracks were found on the surface (Fig. 2(a)).
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For sample A0 with a buffer layer and with no ZnO interlayer, the voids disappeared, but a high density of cracks of up to 102 cm2 can be found (Fig. 2(b)). The interlayer of sample A10 was 10 nm thick, and only a few cracks per millimeter were found (Fig. 2(c)). The crystal quality of sample A30, with a 30 nm interlayer, was the best among all samples. No apparent cracks could be observed on its surface. However, sample A50, with a 50 nm interlayer, exhibited a very rough surface compared with A30. It seems that an optimal thickness of the interlayer is beneficial to the crystallization of GaN film. To investigate the crystal quality of the GaN film, the Xray rocking curve (XRC) of GaN (0 0 0 2) diffraction, PL, and AFM were measured, as shown in Fig. 3. The fullwidth at half-maximum (FWHM) of the XRC of sample A0 with no ZnO interlayer was larger than those of the samples with ZnO interlayers. Sample A30, with a 30 nmthick interlayer, shows the best crystal quality among the samples with interlayers, because the FWHM of its XRC was the smallest, its integrated PL intensity was the strongest, smallest grain size and lowest RMS roughness.
FWHM of XRC(arcmin) Grain Size (nm)
Fig. 2. SEM surface images of the GaN films with ZnO interlayers of different thickness: (a) without buffer layer, (b) 0 nm (without the ZnO interlayer), (c) 10 nm, (d) 30 nm, and (e) 50 nm.
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Fig. 3. Dependence of the grain size, RMS roughness, the XRC FWHM of the GaN (0 0 0 2) diffraction, and the integrated PL intensity at 365 nm of the GaN films on the ZnO interlayer thickness.
Sample A50, with a 50 nm-thick interlayer, showed a lower crystal quality than sample A30, which confirms that there exists an optimal thickness value for optimizing the overall
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Fig. 4. AFM surface images of the GaN films with different numbers of ZnO interlayers: (a) B1 sample (one interlayer), (b) B2 sample (two interlayers), (c) B3 sample (three interlayers), and (d) B4 sample (four interlayers).
quality of the final GaN layer. In our experiment, the optimal thickness was 30 nm. The experimental results above show that the ZnO interlayer thickness greatly influences the growth of the upper part of the GaN layer. Lu et al. [15] investigated the depth distribution of the strain-related tetragonal distortion in the GaN film with low-temperature AlN interlayers on Si (1 1 1) by Rutherford backscattering and channeling. It was found that there existed a tilt difference of about 0.011 between the two layers of GaN separated by the AlN interlayer, suggesting that these two GaN layers were partially decoupled by the AlN interlayer and confirming that the relaxed interlayer can bring about the compressive strain compensation [15]. In this experiment, it can be assumed that ZnO contributes to tensile strain compensation because its lattice constant is larger than that of GaN. Furthermore, the thicker the ZnO interlayer was, the more the tensile strain was compensated. This may explain the reason why the crystal quality of sample A0 was worse than that of the samples with ZnO interlayers. In addition, ZnO interlayers have poor quality after sputtering without annealing. If the ZnO interlayer was too thick, as in that of sample A50, the quality of the ZnO interlayer was insufficient after annealing, resulting in poorer quality of the upper GaN film. Based on the analysis above, we suggest that the ZnO interlayer should be carefully designed in order to compromise the stress relaxation and the crystal quality of the GaN film. Therefore, 30 nm was the optimal thickness here, which could meet two requirements at most.
3.2. Effect of the number of ZnO interlayers on the properties of the GaN film Fig. 4 shows surface AFM views of the second series of samples, exploring the effect of the ZnO interlayer on the crystal quality of the GaN film. One, two, three, and four ZnO interlayers with a thickness of 30 nm labeled B1, B2, B3, and B4, respectively, were sandwiched into a 2 mmthick GaN film. The surface of three samples was smooth and without cracks. The crystal quality improved with increased number of interlayers for three or fewer interlayers. When interlayer number reached four or more, the crystal quality deteriorated quickly. Therefore, the crystal quality of sample B3 was the best among all the samples, as shown in Fig. 4. In order to characterize the crystal change of the GaN film samples with different numbers of interlayers, the FWHM of GaN (0 0 0 2) diffraction of XRC and integrated PL intensity were measured, as shown in Fig. 5. With the increase of the number of ZnO interlayers, the FWHM of the XRC decreased and then increased, the PL intensity increased and then decreased, and both the grain size and RMS value as observed by AFM decreased and then increased. These results further confirm that three interlayers are the optimal number. Furthermore, we used cross-sectional SEM images (not shown) to study the mechanism of the multi-interlayer effect on the quality of the GaN crystal. We found that cracks can be prevented effectively by multiple interlayers. By increasing the number of the interlayers, the cracks can be constrained
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Fig. 5. Dependence of the grain size, RMS roughness, the XRC FWHM of the GaN (0 0 0 2) diffraction and the integrated PL intensity at 365 nm of the GaN films on the ZnO interlayer number.
to a certain extent. But it should be pointed out that some new cracks can be produced simultaneously. And when the number of interlayers was larger than a critical value, the number of new cracks produced by the interlayers exceeded the number of blocked cracks. Therefore, the number of interlayers should be optimized. The results show that inserting multiple ZnO interlayers into the GaN film not only effectively relaxed the stress, but also improved the crystal quality. Cong et al. [14] also observed the same phenomenon as they used cross-sectional TEM to study GaN film sandwiched with multiple AlN interlayers grown by MOCVD. 4. Conclusion The effects of the thickness and number of ZnO interlayers on the crystal quality of GaN films grown on Si (1 1 1) substrate were studied. It was found that voids and cracks can be eliminated by inserting ZnO interlayers, and a 30 nm ZnO interlayer is an appropriate thickness for better crystal quality of 2 mm GaN film. In addition, three
30 nm thick ZnO interlayers sandwiched into the 2 mm thick GaN film is the optimal condition to further improve the overall crystal quality of the film. In our study, a highquality 2 mm-thick GaN film without cracks was prepared successfully by radio frequency (RF) magnetron sputtering and three 30 nm-thick ZnO interlayers were sandwiched into the GaN film. Acknowledgements The authors are grateful to Mr. H. W. Diao for kindly helping with experiments. This work was supported by the National Natural Science Foundation of China (NSFC) (Grant no. 60576004). References [1] J.M. Zavada, U. Hommerich, A.J. Stachl, III-nitride semiconductors, optical properties, Taylor & Francis Books, London, 2002 (Chapter 9). [2] W.D. Chen, J.J. Liang, C.C. Hsu, Mater. Res. Soc. Symp. Proc. 562 (1999) 107. [3] H. Morkoc, Nitride Semiconductors and Devices, Springer, Berlin, New York, 1999. [4] A.J. Steckl, J. Heikenfeld, D.S. Lee, M. Garter, Mater. Sci. Eng. B 81 (2001) 97–101. [5] U. Hommerich, E.E. Nyein, D.S. Lee, J. Heikenfeld, A.J. Steckl, J.M. Zavada, Mater. Sci. Eng. B 105 (2003) 91–96. [6] D.S. Lee, A.J. Steckl, Appl. Phys. Lett. 79 (2001) 1962. [7] D.S. Lee, A.J. Steckl, Appl. Phys. Lett. 81 (2002) 2331. [8] J.H. Kim, N. Shepherd, M. Davidson, P. Holloway, Appl. Phys. Lett. 83 (2003) 641. [9] J.H. Kim, N. Shepherd, M. Davidson, P. Holloway, Appl. Phys. Lett. 83 (2003) 4279. [10] J.H. Kim, P. Holloway, Appl. Phys. Lett. 85 (2004) 1689. [11] S. Shirakata, R. Sasaki, T. Kataoka, Appl. Phys. Lett. 85 (2004) 2247. [12] C.G. Zhang, W.D. Chen, L.F. Bian, S.F. Song, C.C. Hsu, Appl. Surf. Sci. 252 (2006) 2153. [13] H.W. Kim, N.H. Kim, Appl. Surf. Sci. 236 (2004) 192–197. [14] G.W. Cong, Y. Lu, W.Q. Peng, X.L. Liu, X.H. Wang, Z.G. Wang, J. Crystal Growth 276 (2005) 381–388. [15] Y. Lu, G.W. Cong, X.L. Liu, D.C. Lu, Z.G. Wang, Appl. Phys. Lett. 85 (2004) 5562.
Journal of Crystal Growth 293 (2006) 258–262 www.elsevier.com/locate/jcrysgro
Effects of ZnO interlayers on thick GaN/Si film prepared by RF magnetron sputtering C.G. Zhang, L.F. Bian, W.D. Chen, C.C. Hsu State Key Laboratory for Surface Physics, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China Received 7 September 2005; received in revised form 19 April 2006; accepted 16 May 2006 Communicated by R.M. Biefeld
Abstract We have successfully prepared a high-quality 2 mm-thick GaN film with three inserted 30 nm-thick ZnO interlayers on Si (1 1 1) substrate without cracks by magnetron sputtering. The effects of the thickness and number of ZnO interlayers on the crystal quality of the GaN films were studied. It was found that the GaN crystal quality initially improved with the increase of the thickness of ZnO interlayers, but deteriorated quickly when the thickness exceeded 30 nm. Multiple ZnO interlayers were used as an effective means to further improve the crystal quality of the GaN film. By increasing the number of interlayers up to three, the cracks can be constrained to a certain extent, and the crystal quality of the GaN film greatly improved. r 2006 Elsevier B.V. All rights reserved. PACS: 78.55.Cr; 68.55.Jk; 81.15.Cd Keywords: A3. Radio-frequency magnetron sputtering; B1. Gallium nitride; B1. ZnO interlayers
1. Introduction Over the past decade, GaN has triggered great research interest [1] due to its many applications in optoelectronic and microelectronic devices. The use of GaN allows for large concentrations of RE dopant with no significant reduction of light emission at room temperature [2]. In particular, the ability of RE ions to emit red, green, and blue light [3] makes RE-doped GaN well-suited for applications in visible full-color display technology [4–7]. So far, the RE-doping of GaN has been performed only by ion implantation and molecular beam epitaxy (MBE). For applications in thin film display devices, the study of a low cost, large area deposition method is needed for the preparation of GaN film doped with RE ions. For depositing large area films at low cost, radio-frequency magnetron sputtering appears promising [8–12]. GaN on silicon (Si) substrate will pave the way for the integration of devices with mature Si IC technology. However, the Corresponding author. Tel.: +86 10 82304243; fax: +86 10 82304253.
E-mail address: [email protected] (C.G. Zhang). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.05.064
differences in lattice constant (20%) and thermal expansion coefficient (56%) between GaN and Si are even larger than those between GaN and sapphire. Cracks caused by the large difference in thermal expansion always occur when the thickness of the film exceeds 1 mm. In order to inhibit such cracks, interlayers must be introduced into the GaN film. The effectiveness of interlayers in inhibiting cracks has been attributed to the fact that the compressive stress introduced by the interlayers can compensate for part of the tensile stress in the GaN films. Kim and Kim [13] reported the preparation of high-quality GaN/Si film using a ZnO buffer layer applied by sputtering, but when the thickness of the film exceeded 1 mm, cracks would usually occur, thus seriously degrading the crystal quality. Cong et al. [14] reported the growth of high-quality GaN/Si films by metalorganic chemical vapor deposition (MOCVD) using AlN buffer layers and interlayers, but the films were limited in size and were still rather expensive. In this study, we have investigated the effects of the thickness and number of ZnO interlayers on the stress relaxation and the crystal quality of GaN films grown on Si (1 1 1) substrate by sputtering. To our knowledge, the growth of thick GaN
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(d) Fig. 1. Schematic diagram of samples with different numbers of ZnO interlayers inserted.
films using interlayers by sputtering has not been reported to date. 2. Experimental procedure A conventional radio frequency magnetron sputtering system was used in this experiment. Both Ga2O3 (5N) and ZnO (5N) ceramic targets with a diameter and thickness of 75 and 5 mm were loaded into the sputtering chamber with a 5 104 Pa base pressure. Si (1 1 1) substrates were chemically cleaned by standard procedures. Then the substrates were loaded into the chamber and kept at 200 1C during sputtering process. The distance between target and substrate was 10.5 cm. Both targets were pre-sputtered in pure argon for 10 min. Then, a 100 nm-thick ZnO film as a buffer layer was deposited on Si (1 1 1) substrates at an RF power of 100 W and an argon pressure of 1.0 Pa for 30 min. Subsequently, Ga2O3 films were grown on a ZnO buffer layer at an RF power of 150 W and a pressure of 1.0 Pa. The Ga2O3 film was then annealed under flowing ammonia (NH3, 5N) at a rate of 100 l/min at 1000 1C. The annealing process was maintained for 10 min at atmospheric pressure of ammonia in order to allow the NH3 to react completely with the Ga2O3. GaN particles coalesced as crystal nuclei and grew gradually to form large GaN grains. After the GaN growth, ZnO interlayers were grown on it under the same conditions as mentioned above. The GaN film was again grown on ZnO interlayers. The above steps were repeated until the GaN film reached a thickness of 2 mm. In this study, two sets of samples were grown. The first set was grown to study the effect of the thickness of the ZnO interlayers. The sample with no buffer layer was
labeled A. Other samples with buffer layers were labeled A0(without interlayers), A10(10 nm), A30(30 nm), A50 (50 nm) in order of increasing interlayer thickness. Figs. 1(a) and (b) schematically show the structure of samples A0, A10, A30, A50. The second set was grown to study the effect of the number of interlayers. The structure of this set of samples is shown in Figs. 1(c) and (d). The samples were labeled B1 (one layer), B2 (two layers), B3 (three layers) and B4 (four layers) in order of the number of interlayers. The GaN films were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), Xray diffraction (XRD), and photoluminescence (PL) measurements. SEM images were taken with a Hitachi S450 SEM operated at 10–15 kV. The XRD measurements were performed using a Rigaku X-ray generator (SLX-1A) with a Cu-Ka1 rotating radiation source (l ¼ 0.15 405 nm) and a Ge (0 0 4) monochromator. The PL excitation source was a He-Cd laser (325 nm), and the PL signal at 365 nm was analyzed with a JY-HRD1 double grating monochromator equipped with an RCA-C31034 photo-multiplier tube in a PAR-1140A photon counting mode. A Nanoscope III digital Instrument was used for AFM characterization with a scan size of 5 mm 5 mm. TencorInstrument (Alpha-step200) was used to measure the film thickness. 3. Results and discussion 3.1. Effect of the thickness of ZnO interlayers on the properties of the GaN film Fig. 2 shows SEM images of the first series of samples. For sample A with no buffer layer, voids and large amounts of cracks were found on the surface (Fig. 2(a)).
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For sample A0 with a buffer layer and with no ZnO interlayer, the voids disappeared, but a high density of cracks of up to 102 cm2 can be found (Fig. 2(b)). The interlayer of sample A10 was 10 nm thick, and only a few cracks per millimeter were found (Fig. 2(c)). The crystal quality of sample A30, with a 30 nm interlayer, was the best among all samples. No apparent cracks could be observed on its surface. However, sample A50, with a 50 nm interlayer, exhibited a very rough surface compared with A30. It seems that an optimal thickness of the interlayer is beneficial to the crystallization of GaN film. To investigate the crystal quality of the GaN film, the Xray rocking curve (XRC) of GaN (0 0 0 2) diffraction, PL, and AFM were measured, as shown in Fig. 3. The fullwidth at half-maximum (FWHM) of the XRC of sample A0 with no ZnO interlayer was larger than those of the samples with ZnO interlayers. Sample A30, with a 30 nmthick interlayer, shows the best crystal quality among the samples with interlayers, because the FWHM of its XRC was the smallest, its integrated PL intensity was the strongest, smallest grain size and lowest RMS roughness.
FWHM of XRC(arcmin) Grain Size (nm)
Fig. 2. SEM surface images of the GaN films with ZnO interlayers of different thickness: (a) without buffer layer, (b) 0 nm (without the ZnO interlayer), (c) 10 nm, (d) 30 nm, and (e) 50 nm.
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Fig. 3. Dependence of the grain size, RMS roughness, the XRC FWHM of the GaN (0 0 0 2) diffraction, and the integrated PL intensity at 365 nm of the GaN films on the ZnO interlayer thickness.
Sample A50, with a 50 nm-thick interlayer, showed a lower crystal quality than sample A30, which confirms that there exists an optimal thickness value for optimizing the overall
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Fig. 4. AFM surface images of the GaN films with different numbers of ZnO interlayers: (a) B1 sample (one interlayer), (b) B2 sample (two interlayers), (c) B3 sample (three interlayers), and (d) B4 sample (four interlayers).
quality of the final GaN layer. In our experiment, the optimal thickness was 30 nm. The experimental results above show that the ZnO interlayer thickness greatly influences the growth of the upper part of the GaN layer. Lu et al. [15] investigated the depth distribution of the strain-related tetragonal distortion in the GaN film with low-temperature AlN interlayers on Si (1 1 1) by Rutherford backscattering and channeling. It was found that there existed a tilt difference of about 0.011 between the two layers of GaN separated by the AlN interlayer, suggesting that these two GaN layers were partially decoupled by the AlN interlayer and confirming that the relaxed interlayer can bring about the compressive strain compensation [15]. In this experiment, it can be assumed that ZnO contributes to tensile strain compensation because its lattice constant is larger than that of GaN. Furthermore, the thicker the ZnO interlayer was, the more the tensile strain was compensated. This may explain the reason why the crystal quality of sample A0 was worse than that of the samples with ZnO interlayers. In addition, ZnO interlayers have poor quality after sputtering without annealing. If the ZnO interlayer was too thick, as in that of sample A50, the quality of the ZnO interlayer was insufficient after annealing, resulting in poorer quality of the upper GaN film. Based on the analysis above, we suggest that the ZnO interlayer should be carefully designed in order to compromise the stress relaxation and the crystal quality of the GaN film. Therefore, 30 nm was the optimal thickness here, which could meet two requirements at most.
3.2. Effect of the number of ZnO interlayers on the properties of the GaN film Fig. 4 shows surface AFM views of the second series of samples, exploring the effect of the ZnO interlayer on the crystal quality of the GaN film. One, two, three, and four ZnO interlayers with a thickness of 30 nm labeled B1, B2, B3, and B4, respectively, were sandwiched into a 2 mmthick GaN film. The surface of three samples was smooth and without cracks. The crystal quality improved with increased number of interlayers for three or fewer interlayers. When interlayer number reached four or more, the crystal quality deteriorated quickly. Therefore, the crystal quality of sample B3 was the best among all the samples, as shown in Fig. 4. In order to characterize the crystal change of the GaN film samples with different numbers of interlayers, the FWHM of GaN (0 0 0 2) diffraction of XRC and integrated PL intensity were measured, as shown in Fig. 5. With the increase of the number of ZnO interlayers, the FWHM of the XRC decreased and then increased, the PL intensity increased and then decreased, and both the grain size and RMS value as observed by AFM decreased and then increased. These results further confirm that three interlayers are the optimal number. Furthermore, we used cross-sectional SEM images (not shown) to study the mechanism of the multi-interlayer effect on the quality of the GaN crystal. We found that cracks can be prevented effectively by multiple interlayers. By increasing the number of the interlayers, the cracks can be constrained
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Fig. 5. Dependence of the grain size, RMS roughness, the XRC FWHM of the GaN (0 0 0 2) diffraction and the integrated PL intensity at 365 nm of the GaN films on the ZnO interlayer number.
to a certain extent. But it should be pointed out that some new cracks can be produced simultaneously. And when the number of interlayers was larger than a critical value, the number of new cracks produced by the interlayers exceeded the number of blocked cracks. Therefore, the number of interlayers should be optimized. The results show that inserting multiple ZnO interlayers into the GaN film not only effectively relaxed the stress, but also improved the crystal quality. Cong et al. [14] also observed the same phenomenon as they used cross-sectional TEM to study GaN film sandwiched with multiple AlN interlayers grown by MOCVD. 4. Conclusion The effects of the thickness and number of ZnO interlayers on the crystal quality of GaN films grown on Si (1 1 1) substrate were studied. It was found that voids and cracks can be eliminated by inserting ZnO interlayers, and a 30 nm ZnO interlayer is an appropriate thickness for better crystal quality of 2 mm GaN film. In addition, three
30 nm thick ZnO interlayers sandwiched into the 2 mm thick GaN film is the optimal condition to further improve the overall crystal quality of the film. In our study, a highquality 2 mm-thick GaN film without cracks was prepared successfully by radio frequency (RF) magnetron sputtering and three 30 nm-thick ZnO interlayers were sandwiched into the GaN film. Acknowledgements The authors are grateful to Mr. H. W. Diao for kindly helping with experiments. This work was supported by the National Natural Science Foundation of China (NSFC) (Grant no. 60576004). References [1] J.M. Zavada, U. Hommerich, A.J. Stachl, III-nitride semiconductors, optical properties, Taylor & Francis Books, London, 2002 (Chapter 9). [2] W.D. Chen, J.J. Liang, C.C. Hsu, Mater. Res. Soc. Symp. Proc. 562 (1999) 107. [3] H. Morkoc, Nitride Semiconductors and Devices, Springer, Berlin, New York, 1999. [4] A.J. Steckl, J. Heikenfeld, D.S. Lee, M. Garter, Mater. Sci. Eng. B 81 (2001) 97–101. [5] U. Hommerich, E.E. Nyein, D.S. Lee, J. Heikenfeld, A.J. Steckl, J.M. Zavada, Mater. Sci. Eng. B 105 (2003) 91–96. [6] D.S. Lee, A.J. Steckl, Appl. Phys. Lett. 79 (2001) 1962. [7] D.S. Lee, A.J. Steckl, Appl. Phys. Lett. 81 (2002) 2331. [8] J.H. Kim, N. Shepherd, M. Davidson, P. Holloway, Appl. Phys. Lett. 83 (2003) 641. [9] J.H. Kim, N. Shepherd, M. Davidson, P. Holloway, Appl. Phys. Lett. 83 (2003) 4279. [10] J.H. Kim, P. Holloway, Appl. Phys. Lett. 85 (2004) 1689. [11] S. Shirakata, R. Sasaki, T. Kataoka, Appl. Phys. Lett. 85 (2004) 2247. [12] C.G. Zhang, W.D. Chen, L.F. Bian, S.F. Song, C.C. Hsu, Appl. Surf. Sci. 252 (2006) 2153. [13] H.W. Kim, N.H. Kim, Appl. Surf. Sci. 236 (2004) 192–197. [14] G.W. Cong, Y. Lu, W.Q. Peng, X.L. Liu, X.H. Wang, Z.G. Wang, J. Crystal Growth 276 (2005) 381–388. [15] Y. Lu, G.W. Cong, X.L. Liu, D.C. Lu, Z.G. Wang, Appl. Phys. Lett. 85 (2004) 5562.