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High quality crack-free GaN film grown on si (1 1 1) substrate without AlN interlayer
Journal of Crystal Growth 407 (2014) 58–62
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
High quality crack-free GaN film grown on si (1 1 1) substrate without AlN interlayer Dan-Wei Li, Jia-Sheng Diao, Xiang-Jing Zhuo, Jun Zhang, Xing-Fu Wang, Chao Liu, Bi-jun Zhao, Kai Li, Lei Yu, Yuan-Wen Zhang, Miao He, Shu-Ti Li n Laboratory of Nano-photonic Functional Materials and Devices, Institute of Opto-electronic Materials and Technology, South China Normal University, Guangzhou 510631, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 11 March 2014 Received in revised form 16 August 2014 Accepted 25 August 2014 Communicated by: R. M. Biefeld Available online 16 September 2014
High quality crack-free GaN film has been grown on 2 in. n-type Si (1 1 1) substrate without AlN interlayers by metalorganic chemical vapor deposition (MOCVD). By using a two-step-pressure growth technique for the AlN buffer layer, we have obtained crack-free 1.8 μm thick continuous GaN layer. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) results indicate that the twostep-pressure growth method can improve the crystalline quality of the GaN film by prolonging the three-dimensional (3D) growth process of the GaN layer, Raman spectrum analysis shows that tensile stress in the overlaying GaN layer can be reduced obviously to 0.62 GPa, which helps to reduce the possibility of cracking. & 2014 Elsevier B.V. All rights reserved.
Keywords: A1. Stresses A1. Substrates A3. Metalorganic chemical vapor deposition B1. Gallium compounds B1. Nitrides B2. Semiconducting III–V materials
1. Introduction Silicon is a promising substrate for the epitaxial growth of GaN related materials due to its high thermal conductivity, large available scale and low cost. However, the large mismatch in the lattice constant and the thermal expansion coefficient between GaN and Si substrate leads to a high density of defects and cracks. [1] A number of techniques have been reported for defect reduction and crack elimination. SiNx interlayer inserted at various positions has been proved to be effective for dislocation filtering. [2,3] AlN-based interlayer is also widely used to provide compressive stress for strain balancing utilizing AlN and has a smaller inplane lattice constant of 2%. [4] On one hand, AlN interlayers, [5] AlGaN interlayer [6] and AlN/GaN superlattice interlayer [7] between AlN nucleation layer and GaN have been reported to be beneficial for crack-free GaN film. The HT-/LT-/HT-AlN sandwich structure have also been used to grow GaN. [8] However, the AlNbased interlayer will lead to a degradation of electrical performance due to interlayer roughness appearance when the AlN has been tried to dope. In this work continuous high quality crack-free GaN film has been grown on Si (1 1 1) substrate without AlN-
n
Corresponding author. Tel.: þ 86 13119562953. E-mail address: [email protected] (S.-T. Li).
http://dx.doi.org/10.1016/j.jcrysgro.2014.08.025 0022-0248/& 2014 Elsevier B.V. All rights reserved.
based interlayer. By using the two-step-pressure AlN growth method, the 3D growth process of GaN can be prolonged, which leads to improved of crystalline quality of GaN and the reduction of stress, and a crack-free GaN layer can be achieved.
2. Experiments The GaN epitaxial layers were grown on 2 in. Si (1 1 1) substrates in a (MOCVD) system with the Thomas Swan closely coupled shower-head reactor. In order to obtain a clean surface, the Si (1 1 1) substrate was chemically cleaned by a typical RCA process and duel HF solution before being loaded into the reactor. [9] Trimethylgallium (TMGa), trimethylaluminum (TMAl) and ammonia (NH3) were used as Ga, Al and N sources, respectively. Hydrogen (H2) was used as the carrier gas. Before growth, the silicon substrate was heated to 1050 1C under hydrogen (H2) ambient for 5 min. Afterwards, a 80 nm thick AlN buffer layer was deposited at 820, followed by 1.8 μm thick GaN growth at 1010 1C. Firstly, three samples marked as samples A1, B1 and C1 were grown, which consisted of two-step AlN layers grown on silicon substrates. The first 40 nm AlN layer of all the three samples was grown under 100 mbar. The difference among the three samples lied in the second AlN layer which was grown under 100, 400, 650 mbar, respectively. Another three samples marked as
D.-W. Li et al. / Journal of Crystal Growth 407 (2014) 58–62
A2, B2 and C2 involved 500 nm thick GaN layer grown on AlN layer with the same growth parameters as A1, B1, C1, respectively. The last three samples labeled as A3, B3, C3 were composed of 1.8 μm GaN layer grown on AlN layer with identical growth parameters of A1, B1, C1, respectively. The surface morphology was examined by scanning electron microscopy (SEM), atomic force microscopy (AFM) and Optical Microscopy (OM). The dislocation density was measured by AFM and double crystal X-ray diffraction (DCXRD). Furthermore, the strain-statuses of the samples were analyzed by Raman scattering.
3. Results and discussion Fig. 1(a)–(c) shows the top view SEM images of the surface morphology of AlN buffer layer on samples A1, B1, and C1. It can be seen that the diameter of AlN grains in sample A1 is about 20 nm. As the second-step growth pressure is increased in sample B1 AlN grains become smaller and align more closely to one another. The AlN grains become further smaller when the second-step growth pressure is increased to 650 mbar. It is well known that the large mismatch in the lattice constant between AlN and Si leads to3D growth of AlN on silicon. Thus isolated AlN islands are observed on the other hand, the mean free paths of atoms are increased when the growth pressure is reduced. Thus, under the growth pressure of 100 mbar (samples A1), the diameter of AlN grains is the largest because 3D-growth mode of AlN buffer layer is most apparent, since the atoms move to the most appropriate place under low pressure. AlN grains become smaller and align more closely with increasing pressure during the second-step growth because the mean free paths of atoms are decreased under high growth pressure. When the second-step pressure growth of AlN is increased to 650 mbar, 3D growth is further weakened reinforced (samples C1). The SEM images of the surface morphology of 500 nm thick GaN are presented in Fig. 1(d)–(f). It can be seen that the surface morphology roughness of the samples is increased with the second growth pressure of AlN is increased, especially in sample C2. In addition, an AFM measurement is carried out in a 50 μm 50 μm
59
area to study the surface morphology of GaN layer as shown in Fig. 2. The Root Mean Square (RMS) values for samples A2, B2 and C2 are 59.98 nm, 106.52 nm and 219.08 nm, respectively, which also exhibits the same trend that the surface roughness of the sample increases with higher growth pressure during the second AlN layer growth, which is consistent with the SEM results. Besides, from the SEM and AFM results, we observe that the surface grain size of the AlN grains obviously increases with the growth pressure of the second AlN buffer layer. Three samples with 1.8 μm thick GaN layer have been grown on the AlN layers with identical growth parameters as samples A1, B1, C1, respectively. Sample A3 and sample B3 have a surface like mirror, which indicates that a two-dimension (2D) growth is formed. However, the surface of sample C3 is quite rough, indicating failure in transformation of the GaN growth mode from 3D mode to 2D mode, i.e. a smooth surface GaN can hardly be achieved when the second-step growth pressure of AlN buffer layer is too high. Fig. 3 shows Optical Microscopy (OM) images of the surface morphology of samples A3, B3 and C3. It can be seen that sample A3 encounters a serious crack problem. For sample B3, the crack is obviously decreased, and a crack free surface could be observed. However, the surface of sample C3 is too rough to achieve a surface like mirror, which agrees with the results of AFM. The AFM images of the three 1.8 μm thick GaN samples are shown in Fig. 4. The 2D step-flow growth is dominant on the surface of GaN layer, as evidenced by the well-defined steps and terraces. The surface of sample C3 is too rough to see dislocation point. The threading dislocations (TD) densities of samples A3 and B3 obtained by AFM is about 1.6 109 cm 2 and 5.2 108 cm 2, respectively, The RMS values carry out by AFM for samples A3, B3 and C3 are 0.261 nm, 0.238 nm and 14.207 nm, respectively, which indicated that surface morphology of the GaN layer is improved when the second-step growth pressure of AlN is 400 mbar. Fig. 5 shows the rocking curves of DCXRD for (0 0 2) and (1 0 2) planes of the three 1.8 μm thick GaN samples. The values of the full-width at half-maximum (FWHM) are about 486 arcsec, 439 arcsec and 898 arcsec for samples A3, B3 and C3. The FWHM values of (1 0 2) planes are about 811 arcsec, 577 arcsec and 1203 arcsec for samples A3, B3 and C3, which mean that GaN layer
Fig. 1. SEM images of the surface morphology of AlN: sample A1 (a), sample B1 (b) and sample C1 (c), the surface morphology of the 500 nm high temperature GaN: sample A2 (d), sample B2 (e) and sample C2 (f).
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D.-W. Li et al. / Journal of Crystal Growth 407 (2014) 58–62
Fig. 2. The 500 nm high temperature GaN from AFM 50 μm 50 μm scan of sample A2 (a), sample B2 (b), and sample C2 (c), (d)–(e): three dimensional images for sample A2 (d), sample B2 (e), and sample C2 (f).
Fig. 3. Optical Microscopy image of sample A3, sample B3 and sample C3.
had better crystalline quality when the second-step growth pressure of AlN is 400 mbar. The crystalline quality of GaN layer got worse when the second-step growth pressure of AlN buffer layer is further increased to 650 mbar. In order to clarify the stress behaviors among the three samples, micro-Raman scattering spectroscopy at room temperature is performed, and the results are shown in Fig. 6. Both GaN E2 (high) (Eh2 ), and A1 (LO) phonon modes can be clearly observed. It is known that the GaN Eh2 phonon peak is related to the residual stress inside the layer. With respect to the stress-free GaN Eh2 peak at 567.5 cm 1, [10] the relationship between biaxial stress and Raman shift can be shown by formula [11]: σ xx ¼
Δω cm GPa 4:3
ð1Þ
where σxx is the biaxial stress and Δω is the Raman shift. The exact values of shift corresponding to the stress are listed in Table 1. From the data of Table 1, it becomes clear that the magnitude of tensile stress decrease when the second-step growth pressure of AlN is increased. The tensile stress is decreased from 0.89 GPa to 0.41 GPa when the second-step growth pressure of AlN is
increased from 100 mbar to 650 mbar. On the other hand, compared with the surface morphology of the original 500 nm GaN (Figs. 1 and 2), the roughness of sample C2 is increased, compared to the other two samples, which meant that the 3D-growth process of GaN layer in sample C2 is prolonged. It indicated that the 3D-growth of GaN contributes to reduced tensile stress. The residual stress in the upper GaN layer can be effectively reduced by prolonging the 3D-growth time, which contributes to the reduction of the crack. It is a well-known phenomenon that the highly-mismatched GaN/AlN system resulted in the formation of isolated GaN islands in the early stage due to the Stranski–Krastanov process. [12] AlN particle could be regarded as nuclei for the subsequent GaN growth. The size and arrangement of AlN particle will influence the growth of GaN layer apparently. Compared to the SEM images of the surface morphology of the 500 nm thick GaN (Fig. 1(d)–(f), we believe the grain size of GaN can be increased and the 3D-growth process of GaN can be prolonged with increasing of the second-step growth pressure of AlN buffer layer, which influence the GaN crystal quality and stress accordingly. When the second-step growth pressure of AlN was increased from 100 mbar to 400 mbar, the grain size of GaN was reasonably
D.-W. Li et al. / Journal of Crystal Growth 407 (2014) 58–62
61
Fig. 4. Surface images from AFM 3 μm 3 μm scan of sample A3, sample B3 and sample C3.
Fig. 5. X-ray rocking curves of sample A3, sample B3 and sample C3 measured from GaN the (a) (0 0 2) and (b) (1 0 2) reflection of GaN layers
4. Conclusions In summary, we have studied high quality crack-free GaN film growth on Si (1 1 1) substrate without AlN-based interlayer, and found that the two-step-pressure growth of AlN buffer layer could significantly affect the crystalline quality and stress of the following GaN layer. A crack-free 1.8 μm thick GaN film can be obtained by optimizing the growth condition of the two-step AlN layer.
Acknowledgments
Fig. 6. Raman spectrum for sample A3, sample B3 and sample C3, the inset shows the dependence of the GaN E2 (TO) growth for sample A3, sample B3 and sample C3.
Table 1 Raman GaN E2 (TO) peak and the stress corresponding to shift. Samples E2 (TO) peak (cm Stress (GPa)
1
)
A3
B3
C3
563.68 0.89
564.81 0.62
565.75 0.41
increased and the 3D-growth of GaN is prolonged, which improved the crystal quality and the residual stress in the upper GaN layer is reduced. [13,14] However, when the second-step growth pressure of AlN layer increases to 650 mbar, it is difficult to obtain the flat surface of the GaN film because the 3D-growth process of GaN is too sharp to lead to 2D-growth GaN film.
This work was supported by the National Natural Science Foundation of China (Grant 51172079), the Science and Technology Program of Guangdong province, China (Grants 2010B090400456, 2009B011100003, and 2010A081002002), the Science and Technology Program of Guangzhou City, China (Grants 2010U1-D00191 and 11A52091257), the National High Technology Research and Development Programs of China (Grant 2013AA03A101), and Program for Changjiang Scholars and Innovative Research Team in University, Project no. IRT13064.
References [1] S. Pal, C. Jacob, Bull. Mater. Sci. 27 (2004) 501–504. [2] S. Tanaka, M. Takeuchi, Y. Aoyog, Jap. J. Appl. Phys. 39 (2000) L831–L 834. [3] A. Dadgar, M. Poschenrieder, A. Reiher, J. Bläsing, J. Christen, A. Krtschil, T. Finger, T. Hempel, A. Diez, A. Krost, Appl. Phys. Lett. 82 (2003) 28–30. [4] H. Amano, M. Iwaya, T. Kashima, M. Katsuragawa, I. Akasaki, J. Han, S. Hearne, J.A. Floro, E. Chason, J. Figiel, Jpn. J. Appl. Phys. 37 (1998) L1540–L1542. [5] A. Dadgar, J. Bläsing, A. Diez, A. Alam, M. Heuken, A. Krost, Jpn. J. Appl. Phys. 39 (2000) L1183–L1185. [6] S. Fritze, P. Drechsel, P. Stauss, P. Rode, T. Markurt, T. Schulz, M. Albrecht, J. Bläsing, A. Dadgar, A. Krost, J. Appl. Phys. 111 (2012) 124505–124511.
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[7] E. Feltin, B. Beaumont, P. Vennéguès, M Vaille, P. Gibart, T. Riemann, J. Christen, A. Dobos, B. Pécz, J. Appl. Phys. 93 (2003) 182–184. [8] K.L. Lin, E.Y. Chang, Y.L. Hsiao, W.C. Huang, T. Li, D. Tweet, J.S. Ma, S.T. Hsu, C.T. Lee, Appl. Phys. Lett. 91 (2007) 222111–222113. [9] M. Grundmann, A. Krost, D. Bimberg, Appl. Phys. Lett. 58 (1991) 284–286. [10] H. Harima, J. Phys. 14 (2002) R967–R969. [11] T. Kozawa, T. Kachi, H. Kano, H. Nagase, N. Koide, K. Manabe, J. Appl. Phys. 77 (1995) 4389–4392.
[12] B. Daudin, F. Widman, G. Feuillet, Y. Samson, M. Arlery, J.L. Rouviere, Phys. Rev. B 56 (1997) R7069–R7072. [13] S. Figge, T. Böttcher, S. Einfeldt, D. Hommel, J. Cryst. Growth 221 (2000) 262–266. [14] S. Kim, J. Oh, J. Kang, D. Kim, J. Won, J.W. Kim, H.K. Cho, J. Cryst. Growth 262 (2004) 7–13.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
High quality crack-free GaN film grown on si (1 1 1) substrate without AlN interlayer Dan-Wei Li, Jia-Sheng Diao, Xiang-Jing Zhuo, Jun Zhang, Xing-Fu Wang, Chao Liu, Bi-jun Zhao, Kai Li, Lei Yu, Yuan-Wen Zhang, Miao He, Shu-Ti Li n Laboratory of Nano-photonic Functional Materials and Devices, Institute of Opto-electronic Materials and Technology, South China Normal University, Guangzhou 510631, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 11 March 2014 Received in revised form 16 August 2014 Accepted 25 August 2014 Communicated by: R. M. Biefeld Available online 16 September 2014
High quality crack-free GaN film has been grown on 2 in. n-type Si (1 1 1) substrate without AlN interlayers by metalorganic chemical vapor deposition (MOCVD). By using a two-step-pressure growth technique for the AlN buffer layer, we have obtained crack-free 1.8 μm thick continuous GaN layer. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) results indicate that the twostep-pressure growth method can improve the crystalline quality of the GaN film by prolonging the three-dimensional (3D) growth process of the GaN layer, Raman spectrum analysis shows that tensile stress in the overlaying GaN layer can be reduced obviously to 0.62 GPa, which helps to reduce the possibility of cracking. & 2014 Elsevier B.V. All rights reserved.
Keywords: A1. Stresses A1. Substrates A3. Metalorganic chemical vapor deposition B1. Gallium compounds B1. Nitrides B2. Semiconducting III–V materials
1. Introduction Silicon is a promising substrate for the epitaxial growth of GaN related materials due to its high thermal conductivity, large available scale and low cost. However, the large mismatch in the lattice constant and the thermal expansion coefficient between GaN and Si substrate leads to a high density of defects and cracks. [1] A number of techniques have been reported for defect reduction and crack elimination. SiNx interlayer inserted at various positions has been proved to be effective for dislocation filtering. [2,3] AlN-based interlayer is also widely used to provide compressive stress for strain balancing utilizing AlN and has a smaller inplane lattice constant of 2%. [4] On one hand, AlN interlayers, [5] AlGaN interlayer [6] and AlN/GaN superlattice interlayer [7] between AlN nucleation layer and GaN have been reported to be beneficial for crack-free GaN film. The HT-/LT-/HT-AlN sandwich structure have also been used to grow GaN. [8] However, the AlNbased interlayer will lead to a degradation of electrical performance due to interlayer roughness appearance when the AlN has been tried to dope. In this work continuous high quality crack-free GaN film has been grown on Si (1 1 1) substrate without AlN-
n
Corresponding author. Tel.: þ 86 13119562953. E-mail address: [email protected] (S.-T. Li).
http://dx.doi.org/10.1016/j.jcrysgro.2014.08.025 0022-0248/& 2014 Elsevier B.V. All rights reserved.
based interlayer. By using the two-step-pressure AlN growth method, the 3D growth process of GaN can be prolonged, which leads to improved of crystalline quality of GaN and the reduction of stress, and a crack-free GaN layer can be achieved.
2. Experiments The GaN epitaxial layers were grown on 2 in. Si (1 1 1) substrates in a (MOCVD) system with the Thomas Swan closely coupled shower-head reactor. In order to obtain a clean surface, the Si (1 1 1) substrate was chemically cleaned by a typical RCA process and duel HF solution before being loaded into the reactor. [9] Trimethylgallium (TMGa), trimethylaluminum (TMAl) and ammonia (NH3) were used as Ga, Al and N sources, respectively. Hydrogen (H2) was used as the carrier gas. Before growth, the silicon substrate was heated to 1050 1C under hydrogen (H2) ambient for 5 min. Afterwards, a 80 nm thick AlN buffer layer was deposited at 820, followed by 1.8 μm thick GaN growth at 1010 1C. Firstly, three samples marked as samples A1, B1 and C1 were grown, which consisted of two-step AlN layers grown on silicon substrates. The first 40 nm AlN layer of all the three samples was grown under 100 mbar. The difference among the three samples lied in the second AlN layer which was grown under 100, 400, 650 mbar, respectively. Another three samples marked as
D.-W. Li et al. / Journal of Crystal Growth 407 (2014) 58–62
A2, B2 and C2 involved 500 nm thick GaN layer grown on AlN layer with the same growth parameters as A1, B1, C1, respectively. The last three samples labeled as A3, B3, C3 were composed of 1.8 μm GaN layer grown on AlN layer with identical growth parameters of A1, B1, C1, respectively. The surface morphology was examined by scanning electron microscopy (SEM), atomic force microscopy (AFM) and Optical Microscopy (OM). The dislocation density was measured by AFM and double crystal X-ray diffraction (DCXRD). Furthermore, the strain-statuses of the samples were analyzed by Raman scattering.
3. Results and discussion Fig. 1(a)–(c) shows the top view SEM images of the surface morphology of AlN buffer layer on samples A1, B1, and C1. It can be seen that the diameter of AlN grains in sample A1 is about 20 nm. As the second-step growth pressure is increased in sample B1 AlN grains become smaller and align more closely to one another. The AlN grains become further smaller when the second-step growth pressure is increased to 650 mbar. It is well known that the large mismatch in the lattice constant between AlN and Si leads to3D growth of AlN on silicon. Thus isolated AlN islands are observed on the other hand, the mean free paths of atoms are increased when the growth pressure is reduced. Thus, under the growth pressure of 100 mbar (samples A1), the diameter of AlN grains is the largest because 3D-growth mode of AlN buffer layer is most apparent, since the atoms move to the most appropriate place under low pressure. AlN grains become smaller and align more closely with increasing pressure during the second-step growth because the mean free paths of atoms are decreased under high growth pressure. When the second-step pressure growth of AlN is increased to 650 mbar, 3D growth is further weakened reinforced (samples C1). The SEM images of the surface morphology of 500 nm thick GaN are presented in Fig. 1(d)–(f). It can be seen that the surface morphology roughness of the samples is increased with the second growth pressure of AlN is increased, especially in sample C2. In addition, an AFM measurement is carried out in a 50 μm 50 μm
59
area to study the surface morphology of GaN layer as shown in Fig. 2. The Root Mean Square (RMS) values for samples A2, B2 and C2 are 59.98 nm, 106.52 nm and 219.08 nm, respectively, which also exhibits the same trend that the surface roughness of the sample increases with higher growth pressure during the second AlN layer growth, which is consistent with the SEM results. Besides, from the SEM and AFM results, we observe that the surface grain size of the AlN grains obviously increases with the growth pressure of the second AlN buffer layer. Three samples with 1.8 μm thick GaN layer have been grown on the AlN layers with identical growth parameters as samples A1, B1, C1, respectively. Sample A3 and sample B3 have a surface like mirror, which indicates that a two-dimension (2D) growth is formed. However, the surface of sample C3 is quite rough, indicating failure in transformation of the GaN growth mode from 3D mode to 2D mode, i.e. a smooth surface GaN can hardly be achieved when the second-step growth pressure of AlN buffer layer is too high. Fig. 3 shows Optical Microscopy (OM) images of the surface morphology of samples A3, B3 and C3. It can be seen that sample A3 encounters a serious crack problem. For sample B3, the crack is obviously decreased, and a crack free surface could be observed. However, the surface of sample C3 is too rough to achieve a surface like mirror, which agrees with the results of AFM. The AFM images of the three 1.8 μm thick GaN samples are shown in Fig. 4. The 2D step-flow growth is dominant on the surface of GaN layer, as evidenced by the well-defined steps and terraces. The surface of sample C3 is too rough to see dislocation point. The threading dislocations (TD) densities of samples A3 and B3 obtained by AFM is about 1.6 109 cm 2 and 5.2 108 cm 2, respectively, The RMS values carry out by AFM for samples A3, B3 and C3 are 0.261 nm, 0.238 nm and 14.207 nm, respectively, which indicated that surface morphology of the GaN layer is improved when the second-step growth pressure of AlN is 400 mbar. Fig. 5 shows the rocking curves of DCXRD for (0 0 2) and (1 0 2) planes of the three 1.8 μm thick GaN samples. The values of the full-width at half-maximum (FWHM) are about 486 arcsec, 439 arcsec and 898 arcsec for samples A3, B3 and C3. The FWHM values of (1 0 2) planes are about 811 arcsec, 577 arcsec and 1203 arcsec for samples A3, B3 and C3, which mean that GaN layer
Fig. 1. SEM images of the surface morphology of AlN: sample A1 (a), sample B1 (b) and sample C1 (c), the surface morphology of the 500 nm high temperature GaN: sample A2 (d), sample B2 (e) and sample C2 (f).
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D.-W. Li et al. / Journal of Crystal Growth 407 (2014) 58–62
Fig. 2. The 500 nm high temperature GaN from AFM 50 μm 50 μm scan of sample A2 (a), sample B2 (b), and sample C2 (c), (d)–(e): three dimensional images for sample A2 (d), sample B2 (e), and sample C2 (f).
Fig. 3. Optical Microscopy image of sample A3, sample B3 and sample C3.
had better crystalline quality when the second-step growth pressure of AlN is 400 mbar. The crystalline quality of GaN layer got worse when the second-step growth pressure of AlN buffer layer is further increased to 650 mbar. In order to clarify the stress behaviors among the three samples, micro-Raman scattering spectroscopy at room temperature is performed, and the results are shown in Fig. 6. Both GaN E2 (high) (Eh2 ), and A1 (LO) phonon modes can be clearly observed. It is known that the GaN Eh2 phonon peak is related to the residual stress inside the layer. With respect to the stress-free GaN Eh2 peak at 567.5 cm 1, [10] the relationship between biaxial stress and Raman shift can be shown by formula [11]: σ xx ¼
Δω cm GPa 4:3
ð1Þ
where σxx is the biaxial stress and Δω is the Raman shift. The exact values of shift corresponding to the stress are listed in Table 1. From the data of Table 1, it becomes clear that the magnitude of tensile stress decrease when the second-step growth pressure of AlN is increased. The tensile stress is decreased from 0.89 GPa to 0.41 GPa when the second-step growth pressure of AlN is
increased from 100 mbar to 650 mbar. On the other hand, compared with the surface morphology of the original 500 nm GaN (Figs. 1 and 2), the roughness of sample C2 is increased, compared to the other two samples, which meant that the 3D-growth process of GaN layer in sample C2 is prolonged. It indicated that the 3D-growth of GaN contributes to reduced tensile stress. The residual stress in the upper GaN layer can be effectively reduced by prolonging the 3D-growth time, which contributes to the reduction of the crack. It is a well-known phenomenon that the highly-mismatched GaN/AlN system resulted in the formation of isolated GaN islands in the early stage due to the Stranski–Krastanov process. [12] AlN particle could be regarded as nuclei for the subsequent GaN growth. The size and arrangement of AlN particle will influence the growth of GaN layer apparently. Compared to the SEM images of the surface morphology of the 500 nm thick GaN (Fig. 1(d)–(f), we believe the grain size of GaN can be increased and the 3D-growth process of GaN can be prolonged with increasing of the second-step growth pressure of AlN buffer layer, which influence the GaN crystal quality and stress accordingly. When the second-step growth pressure of AlN was increased from 100 mbar to 400 mbar, the grain size of GaN was reasonably
D.-W. Li et al. / Journal of Crystal Growth 407 (2014) 58–62
61
Fig. 4. Surface images from AFM 3 μm 3 μm scan of sample A3, sample B3 and sample C3.
Fig. 5. X-ray rocking curves of sample A3, sample B3 and sample C3 measured from GaN the (a) (0 0 2) and (b) (1 0 2) reflection of GaN layers
4. Conclusions In summary, we have studied high quality crack-free GaN film growth on Si (1 1 1) substrate without AlN-based interlayer, and found that the two-step-pressure growth of AlN buffer layer could significantly affect the crystalline quality and stress of the following GaN layer. A crack-free 1.8 μm thick GaN film can be obtained by optimizing the growth condition of the two-step AlN layer.
Acknowledgments
Fig. 6. Raman spectrum for sample A3, sample B3 and sample C3, the inset shows the dependence of the GaN E2 (TO) growth for sample A3, sample B3 and sample C3.
Table 1 Raman GaN E2 (TO) peak and the stress corresponding to shift. Samples E2 (TO) peak (cm Stress (GPa)
1
)
A3
B3
C3
563.68 0.89
564.81 0.62
565.75 0.41
increased and the 3D-growth of GaN is prolonged, which improved the crystal quality and the residual stress in the upper GaN layer is reduced. [13,14] However, when the second-step growth pressure of AlN layer increases to 650 mbar, it is difficult to obtain the flat surface of the GaN film because the 3D-growth process of GaN is too sharp to lead to 2D-growth GaN film.
This work was supported by the National Natural Science Foundation of China (Grant 51172079), the Science and Technology Program of Guangdong province, China (Grants 2010B090400456, 2009B011100003, and 2010A081002002), the Science and Technology Program of Guangzhou City, China (Grants 2010U1-D00191 and 11A52091257), the National High Technology Research and Development Programs of China (Grant 2013AA03A101), and Program for Changjiang Scholars and Innovative Research Team in University, Project no. IRT13064.
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