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Growth of crack-free AlGaN film on thin AlN interlayer by MOCVD
ARTICLE IN PRESS
Journal of Crystal Growth 268 (2004) 35–40
Growth of crack-free AlGaN film on thin AlN interlayer by MOCVD R.Q. Jin*, J.P. Liu, J.C. Zhang, H. Yang State Key Laboratory of Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China Received 19 March 2004; accepted 28 April 2004
Communicated by M. Schieber
Abstract We have studied the effect of low-temperature-deposited (LT) and high-temperature-deposited (HT) AlN interlayer with various thickness on AlGaN film grown on GaN using c-plane sapphire as substrate. All the Al0.25Ga0.75N films thicker than 1 mm with LT-AlN interlayer or with HT-AlN interlayer were free of cracks, however, their surfaces were different: the Al0.25Ga0.75N films with LT-AlN interlayer showed smooth surface, while those with HT-AlN interlayer exhibit rough surface morphology. The results of X-ray double crystal diffraction and Rutherford backscattering showed that all of the AlGaN films were under compressive strain in the parallel direction. The compressive strain resulted from the effect of interlayer-induced stress relieving and the thermal mismatch for the samples with LT-AlN interlayer, and it was due to the thermal mismatch between AlGaN and the underlying layers for those with HT-AlN interlayer. r 2004 Elsevier B.V. All rights reserved. Keywords: A1. Crystal morphology; A1. X-ray diffraction; A3. Metalorganic chemical vapor deposition; B1. Aluminium gallium nitride
1. Introduction Group III nitrides have been promising materials for their direct and wide band gap energy between 3.4 and 6.2 eV.Especially AlGaN alloys are indispensable for the applications in blue-green *Corresponding author. Tel.: +86-1082304128; fax: +861082305033. E-mail address: [email protected] (R.Q. Jin).
light emitting diodes, laser diodes and visible-blind photodetectors [1–4]. Moreover, AlGaN was always grown on GaN, which bring a hamper to its applications because it is extremely difficult to grow thick and crack-free AlGaN on GaN due to the tensile strain resulting from the lattice mismatch between AlGaN and GaN layers. Since the realization of the technique low-temperature (LT)- deposited buffer layer directly on a sapphire substrate, the quality of GaN epilayer has
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.04.109
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R.Q. Jin et al. / Journal of Crystal Growth 268 (2004) 35–40
improved greatly. At present hith-crystalline-quality GaN layer with a smooth surface could be obtained easily, despite the large lattice mismatch of about 17% between GaN and sapphire. Additionally, Amano et al. [5] first introduced LT-AlN interlayer between the high-temperaturegrown GaN layers to further improve the structural quality of GaN layer. The LT-interlayer is thought to exert a similar effect to that of the LTbuffer layer, acting as a kind of nucleation layer. So the insertion of an LT-interlayer is very promising for the improvement of quality of the AlGaN/GaN heterostructure. Amano et al. [6] reported on the growth of crack-free thick AlGaN film by the insertion of a low-temperature-grown thin AlN interlayer between GaN and AlGaN layer. Han et al. [7] also reported on the control of cracking of AlGaN using a low-temperature AlGaN interlayer. Meanwhile, high-temperature AlN interlayer is also used as an interlayer for the growth of AlGaN epilayer on GaN. Lee et al. [8] obtained crack-free AlGaN film by using hightemperature AlN interlayer. However, there has been no report on a contrast to the effect of an LTAlN interlayer or a HT-AlN interlayer on the quality of AlGaN film. In this study, we investigate the effect of crack suppression of an LT-AlN interlayer or an HT-AlN interlayer on the AlGaN film.
2. Experiment In this section, we described the growth of thick AlGaN with various thickness of LT-AlN or HTAlN interlayer. The Al0.25Ga0.75N layers with thickness of 1 mm were grown on the 2 mm-thick GaN layer through LT-AlN or HT-AlN interlayer with various thickness by low-pressure metalorganic chemical vapor deposition, as shown in Fig. 1(a). Ammonia, trimethylgallium (TMGa), and trimethylaluminum (TMAl) were used as N source, Ga source, and Al source gases, respectively. H2 was used as the carrier gas. Firstly GaN was grown on sapphire (0 0 0 1) substrates using a low-temperature (600 C) GaN nucleation layer followed by growth of 2 mm GaN at 1040 C. Then the LT- or HT-AlN interlayer with
1.1µm-Al0.25Ga0.75N 1.1µm-Al0.25Ga0.75N
AlN interlayer 2µm-GaN
2µm-GaN
LT-GaN buffer
LT-GaN buffer
(0001) Sapphire Substrate
(0001) Sapphire Substrate
(a)
(b)
Fig. 1. Schematic layer structures: (a) with LT-AlN or HT-AlN interlayer between the GaN underlying layer and AlGaN top layer, (b) without any AlN interlayer.
different thickness was grown at 540 C (for samples A, B, C and D) or 1040 C (for samples E, F, G and H), respectively. The 1.1 mm-thickness AlGaN growth was carried out at a temperature of 1040 C on AlN interlayer. All the Al-containing layers were grown at a low pressure of 100 mbar to minimize any parasitic reactions except the LT-AlN interlayer was grown at a pressure of 500 mbar. All the growth conditions for GaN buffer layers, GaN epilayers and AlGaN epilayers were nominally kept under the same parameters, but those of AlN interlayer were different among the samples. The flow rate of TMAl, NH3 and H2 were 30 mmol/min, 2.5 l/min and 2.5 l/ min for the LT-AlN interlayers, and 30 mmol/min, 1 and 4 l/min for the HT-AlN interlayers, respectively. The AlGaN epilayer without any AlN interlayer was grown directly on the underlying GaN for comparison, as shown in Fig. 1(b). The Al composition and full-width at halfmaximum (FWHM) of the AlGaN epilayers was estimated by X-ray double-crystal diffraction (XRD) with CuKa1 radiation. The surface morphology of Al0.25Ga0.75N epilayers was examined by scanning electron microscopy (SEM). In order to estimate the amount of plastic strain of the AlGaN epilayer, the random Rutherford backscattering (RBS) spectra was performed at a collimated 2.6 eV a particle beam.
3. Results and discussions The thickness of LT-AlN interlayer were 8, 16, 24 and 40 nm corresponding to the growth time 50,
ARTICLE IN PRESS R.Q. Jin et al. / Journal of Crystal Growth 268 (2004) 35–40
100, 150 and 250 s for samples A, B, C and D, respectively, and those of the HT-AlN interlayer were 9, 18.2, 26 and 40 nm relevant to the growth time 70, 140, 200 and 340 s for samples E, F, G and H, respectively. Sample I without any AlN interlayer was also grown for comparison. Fig. 2 showed the SEM surface morphology of Al0.25Ga0.75N epilayers labeled by sample A and I. As seen, sample I, where no interlayer was used, was full of a high-density crack network. These cracks are caused by excessive biaxial tensile strain induced by lattice mismatch during epitaxial growth. Unlike zinc-blended semiconductors, the (0 0 0 1)-oriented wurtzite III-nitrides lack of available low-energy slip systems so that they are unable to nucleate and glide dislocations to relieve the in-plane biaxial tension. The cracking is an effective way to release the tensile stress, which initiats the nucleation of misfit dislocations around the crack tips and their subsequent gliding when the AlGaN layer is over the critical thickness. The sample A with the thinnest thickness LT-AlN interlayer showed a crack-free and smooth surface, and the samples B, C and D all had the similar surface which has not been shown here, indicating that inserting a LT-AlN interlayer was an effective way to eliminate the crack. Blasing et al. [9] proposed that lower growth temperature would lead to a decoherent growth between the GaN underlayer and AlGaN overlayer, so AlGaN layer was compressively stressed due to the effect of the interlayer resulting in the crack-free AlGaN growth. In sample A, the AlGaN epilayer had 0.33% tensile stress in the perpendicular direction
37
calculated from the RBS and X-ray diffraction data. So it was under compressive strain in the parallel direction, consistent with the results of Blasing. The perpendicular elastic strain of the AlGaN epilayer e> was calculated from the following formula: e> ¼ ðcepi cb Þ=cb ; where, cepi was the lattice constant of the AlGaN epilayer, calculated from the spectra of the symmetric X-ray y–2y scan; the lattice constant cb was for the fully relaxed epilayer. The Al composition obtained by the simulation of the random spectrum of RBS, was used to calculate the value of cb from the lattice parameters of GaN and AlN using Vegard’s law. For sample A, the values of cepi and cb were 0.5144 and 0.5127 nm, respectively. So we could get that e> equal to 0.33%, indicating that the AlGaN epilayer was under tensile strain in the perpendicular direction. Inserting an LT-AlN interlayer would relieve the stress due to the lattice mismatch between the AlGaN top layer and the GaN underlying layer. Then it led to the growth of crack-free AlGaN epilayer due to the fact that the AlGaN epilayers were under compressive stress in the parallel direction. In conclusion, inserting an LT-AlN interlayer would destroy the coherent growth of the underlying GaN layer and the top AlGaN layer. So the top AlGaN epilayer was under compressively stressed at growth temperature due to the smaller lattice constant of the AlN. Moreover, the AlGaN layer subsequently underwent compressive strain again when cooled down to the room temperature. This was induced by the
Fig. 2. The SEM surface morphology of Al0.25Ga0.75N epilayers: (a) the sample A with 8 nm LT-AlN interlayer between the AlGaN toplayer and the GaN underlying layer, (b) the sample I without LT-AlN interlayer.
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FWHM of XRD from (0002) diffraction for AlGaN film (sec)
thermal mismatch between the AlGaN epilayer and the underlying layers. The value of compressive strain for this kind of sample was the sum of those introduced by the LT-interlayer and by the thermal mismatch. The crystalline quality of all the AlGaN epilayers was characterized by X-ray diffraction pattern. From Fig. 3, we could find that the values FWHM for the (0 0 0 2) XRD o-scan peaks increased with increasing the thickness of LT-AlN interlayer, moreover, the RMS surface roughness showed the similar rule. In other words, thick LT-AlN interlayer would deteriorate both the quality of the AlGaN epilayer and the surface roughness. Fig. 4 showed the SEM images of the top Al0.25Ga0.75N surface of samples E–G. As seen,
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Fig. 3. Rocking curve FWHM of (0 0 0 2) plane for AlGaN with different thickness AlN interlayer (a), and RMS surface roughness of AlGaN epilayer with different thickness of LTAlN interlayer (b).
these three samples with the HT-AlN interlayers’ thickness in the range of 9–40 nm had a crack-free but rough surface, which were different from the results by Lee et al. [8]; he found that the lower thickness of the HT-AlN interlayer led to cracking because it could not relieve stress completely and the higher one would result in broad XRD line width. However, in our study, 9 nm HT-AlN interlayer was enough to relieve the stress so as to avoid the crack formation. Lee proposed the mechanism of strain relaxation of the HT-AlN interlayer: the forming of the initial crack and subsequent dislocation would release the tensile strain when the thickness of HT-AlN interlayer exceed the critical values, and then the crack-free AlGaN layers were grown lateral over the cracks, from cathodoluminescence (CL) spectra. In our study, AlGaN sample with the thinnest HT-AlN interlayer at the thickness of 9 nm was crack free, but it maybe cracked due to stress relieving incompletely inserting an HT-AlN interlayer thinner than 9 nm. In addition, the surface roughness and the FWHM values for the (0 0 0 2) XRD oscan peaks of the AlGaN epilayer increased when the thickness of the HT-AlN interlayer increased. At the nearly same thickness of AlN interlayer, the FWHM of (0 0 0 2) diffraction for the samples with LT-AlN interlayer was much higher than the values for the samples with HT-AlN interlayer, as shown in Fig. 3(a). This showed that inserting HT-AlN interlayer was more favored to improve the crystalline quality than inserting LT-AlN interlayer, because HT-AlN interlayers were pseudomorphic with the underlying GaN. Sample G with the largest value of the thickness of the HT-AlN interlayer showed a more high-density mesa than sample F as shown in Fig. 4, symboling the Npolarity growth for the AlGaN epilayer resulted from the inappropriate growth parameters of the HT-AlN interlayer such as the relatively low growth temperature (GaN layer was usually grown at this growth temperature in our study) and the high flow rate of the TMAl. Both these factors reduced the diffusion length of the Al atoms. We also got the values of the tensile stress in the perpendicular direction in sample E and F, which were 0.14% and 0.16%, respectively, less than that of the sample A with an LT-AlN interlayer. Thus,
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by the thermal mismatch between AlGaN and sapphire substrate. AlGaN epilayer was coherently grown on the underlying HT layers, so it was under tensile stress during the growth temperature. Subsequently, the tensile stress would turn into the compressive stress due to the thermal mismatch between AlGaN and the underlying layers when the AlGaN epilayer was cooling down to the room temperature. In conclusion, AlGaN epilayer was under compressive strain at room temperature, which was the balance of the tensile strain induced by the lattice mismatch during the growth and the compressive strain induced by the thermal mismatch between the AlGaN epilayer and the underlying layer.
4. Summary
Fig. 4. The SEM surface morphology of Al0.25Ga0.75N epilayers with different thickness of HT-AlN interlayer (a) the sample E with 9 nm HT-AlN interlayer, (b) the sample F with 18.2 nm HT-AlN interlayer, (c) the sample G with 26 nm HTAlN interlayer.
we could get the conclusion that the samples were under compressive stress in the parallel direction as expected from the compressive strain induced
We found that inserting not only an LT-AlN interlayer but also an HT-AlN interlayer between AlGaN and GaN would be an effective ways to relieve stress so as to avoid the crack formation. Both the thickness of LT-AlN interlayer and HTAlN interlayer were critical for material quality of AlGaN epilayer. X-ray analysis along with AFM data showed good structural quality and smooth surface for the AlGaN sample with 8 nm LT-AlN interlayer. The AlGaN sample with 9 nm HT-AlN interlayer also displayed excellent structural quality but rough surface due to the unsuitable growth parameters for the HT-AlN interlayer. All the AlGaN samples with a LT-AlN or HT-AlN interlayer were under compressive stress in the parallel to the growth direction, but due to the effect of the interlayer-induced stress relieving during growth and the thermal mismatch for the samples with LT-AlN interlayer, and due to the thermal mismatch between AlGaN and sapphire substrate during cooling from the growth temperature to the room temperature for those with HT-AlN interlayer.
References [1] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, Berlin 1997, p. 277.
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[2] A.V. Sakharov, W.V. Lundin, A. Usikov, U.I. Ushakov, MRS Internet J. Nitride Semicond. Res. 3 (1998) 28. [3] J. Han, M.H. Crakford, R.J. Shul, J.J. Figiel, M. Banas, L. Zhang, Y.K. Song, H. Zhou, A.V. Nuramikko, Appl. Phys. Lett. 73 (1998) 1688. [4] Y. Koide, H. Itoh, M.R.H. Khan, K. Hiramatsu, N. Sawaki, I. Akasaki, J. Appl. Phys. 61 (1987) 4540. [5] 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.
[6] H. Amano, M. Iwaya, N. Hayashi, T. Kashima, S. Nitta, C. Wetzel, I. Akasaki, Phys. Stat. Sol. b 216 (1999) 683. [7] J. Han, K.E. Waldrip, S.R. Lee, J.J. Figiel, S.J. Hearne, G.A. Petersen, S.M. Myers, Appl. Phys. Lett. 78 (2001) 67. [8] I.H. Lee, T.G. Kim, Y. Park, J. Crystal Growth 234 (2002) 305. [9] J. Blasing, A. Reiher, A. Dadgar, A. Diez, A. Krost, Appl. Phys. Lett. 81 (2002) 2722.
Journal of Crystal Growth 268 (2004) 35–40
Growth of crack-free AlGaN film on thin AlN interlayer by MOCVD R.Q. Jin*, J.P. Liu, J.C. Zhang, H. Yang State Key Laboratory of Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China Received 19 March 2004; accepted 28 April 2004
Communicated by M. Schieber
Abstract We have studied the effect of low-temperature-deposited (LT) and high-temperature-deposited (HT) AlN interlayer with various thickness on AlGaN film grown on GaN using c-plane sapphire as substrate. All the Al0.25Ga0.75N films thicker than 1 mm with LT-AlN interlayer or with HT-AlN interlayer were free of cracks, however, their surfaces were different: the Al0.25Ga0.75N films with LT-AlN interlayer showed smooth surface, while those with HT-AlN interlayer exhibit rough surface morphology. The results of X-ray double crystal diffraction and Rutherford backscattering showed that all of the AlGaN films were under compressive strain in the parallel direction. The compressive strain resulted from the effect of interlayer-induced stress relieving and the thermal mismatch for the samples with LT-AlN interlayer, and it was due to the thermal mismatch between AlGaN and the underlying layers for those with HT-AlN interlayer. r 2004 Elsevier B.V. All rights reserved. Keywords: A1. Crystal morphology; A1. X-ray diffraction; A3. Metalorganic chemical vapor deposition; B1. Aluminium gallium nitride
1. Introduction Group III nitrides have been promising materials for their direct and wide band gap energy between 3.4 and 6.2 eV.Especially AlGaN alloys are indispensable for the applications in blue-green *Corresponding author. Tel.: +86-1082304128; fax: +861082305033. E-mail address: [email protected] (R.Q. Jin).
light emitting diodes, laser diodes and visible-blind photodetectors [1–4]. Moreover, AlGaN was always grown on GaN, which bring a hamper to its applications because it is extremely difficult to grow thick and crack-free AlGaN on GaN due to the tensile strain resulting from the lattice mismatch between AlGaN and GaN layers. Since the realization of the technique low-temperature (LT)- deposited buffer layer directly on a sapphire substrate, the quality of GaN epilayer has
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.04.109
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R.Q. Jin et al. / Journal of Crystal Growth 268 (2004) 35–40
improved greatly. At present hith-crystalline-quality GaN layer with a smooth surface could be obtained easily, despite the large lattice mismatch of about 17% between GaN and sapphire. Additionally, Amano et al. [5] first introduced LT-AlN interlayer between the high-temperaturegrown GaN layers to further improve the structural quality of GaN layer. The LT-interlayer is thought to exert a similar effect to that of the LTbuffer layer, acting as a kind of nucleation layer. So the insertion of an LT-interlayer is very promising for the improvement of quality of the AlGaN/GaN heterostructure. Amano et al. [6] reported on the growth of crack-free thick AlGaN film by the insertion of a low-temperature-grown thin AlN interlayer between GaN and AlGaN layer. Han et al. [7] also reported on the control of cracking of AlGaN using a low-temperature AlGaN interlayer. Meanwhile, high-temperature AlN interlayer is also used as an interlayer for the growth of AlGaN epilayer on GaN. Lee et al. [8] obtained crack-free AlGaN film by using hightemperature AlN interlayer. However, there has been no report on a contrast to the effect of an LTAlN interlayer or a HT-AlN interlayer on the quality of AlGaN film. In this study, we investigate the effect of crack suppression of an LT-AlN interlayer or an HT-AlN interlayer on the AlGaN film.
2. Experiment In this section, we described the growth of thick AlGaN with various thickness of LT-AlN or HTAlN interlayer. The Al0.25Ga0.75N layers with thickness of 1 mm were grown on the 2 mm-thick GaN layer through LT-AlN or HT-AlN interlayer with various thickness by low-pressure metalorganic chemical vapor deposition, as shown in Fig. 1(a). Ammonia, trimethylgallium (TMGa), and trimethylaluminum (TMAl) were used as N source, Ga source, and Al source gases, respectively. H2 was used as the carrier gas. Firstly GaN was grown on sapphire (0 0 0 1) substrates using a low-temperature (600 C) GaN nucleation layer followed by growth of 2 mm GaN at 1040 C. Then the LT- or HT-AlN interlayer with
1.1µm-Al0.25Ga0.75N 1.1µm-Al0.25Ga0.75N
AlN interlayer 2µm-GaN
2µm-GaN
LT-GaN buffer
LT-GaN buffer
(0001) Sapphire Substrate
(0001) Sapphire Substrate
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(b)
Fig. 1. Schematic layer structures: (a) with LT-AlN or HT-AlN interlayer between the GaN underlying layer and AlGaN top layer, (b) without any AlN interlayer.
different thickness was grown at 540 C (for samples A, B, C and D) or 1040 C (for samples E, F, G and H), respectively. The 1.1 mm-thickness AlGaN growth was carried out at a temperature of 1040 C on AlN interlayer. All the Al-containing layers were grown at a low pressure of 100 mbar to minimize any parasitic reactions except the LT-AlN interlayer was grown at a pressure of 500 mbar. All the growth conditions for GaN buffer layers, GaN epilayers and AlGaN epilayers were nominally kept under the same parameters, but those of AlN interlayer were different among the samples. The flow rate of TMAl, NH3 and H2 were 30 mmol/min, 2.5 l/min and 2.5 l/ min for the LT-AlN interlayers, and 30 mmol/min, 1 and 4 l/min for the HT-AlN interlayers, respectively. The AlGaN epilayer without any AlN interlayer was grown directly on the underlying GaN for comparison, as shown in Fig. 1(b). The Al composition and full-width at halfmaximum (FWHM) of the AlGaN epilayers was estimated by X-ray double-crystal diffraction (XRD) with CuKa1 radiation. The surface morphology of Al0.25Ga0.75N epilayers was examined by scanning electron microscopy (SEM). In order to estimate the amount of plastic strain of the AlGaN epilayer, the random Rutherford backscattering (RBS) spectra was performed at a collimated 2.6 eV a particle beam.
3. Results and discussions The thickness of LT-AlN interlayer were 8, 16, 24 and 40 nm corresponding to the growth time 50,
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100, 150 and 250 s for samples A, B, C and D, respectively, and those of the HT-AlN interlayer were 9, 18.2, 26 and 40 nm relevant to the growth time 70, 140, 200 and 340 s for samples E, F, G and H, respectively. Sample I without any AlN interlayer was also grown for comparison. Fig. 2 showed the SEM surface morphology of Al0.25Ga0.75N epilayers labeled by sample A and I. As seen, sample I, where no interlayer was used, was full of a high-density crack network. These cracks are caused by excessive biaxial tensile strain induced by lattice mismatch during epitaxial growth. Unlike zinc-blended semiconductors, the (0 0 0 1)-oriented wurtzite III-nitrides lack of available low-energy slip systems so that they are unable to nucleate and glide dislocations to relieve the in-plane biaxial tension. The cracking is an effective way to release the tensile stress, which initiats the nucleation of misfit dislocations around the crack tips and their subsequent gliding when the AlGaN layer is over the critical thickness. The sample A with the thinnest thickness LT-AlN interlayer showed a crack-free and smooth surface, and the samples B, C and D all had the similar surface which has not been shown here, indicating that inserting a LT-AlN interlayer was an effective way to eliminate the crack. Blasing et al. [9] proposed that lower growth temperature would lead to a decoherent growth between the GaN underlayer and AlGaN overlayer, so AlGaN layer was compressively stressed due to the effect of the interlayer resulting in the crack-free AlGaN growth. In sample A, the AlGaN epilayer had 0.33% tensile stress in the perpendicular direction
37
calculated from the RBS and X-ray diffraction data. So it was under compressive strain in the parallel direction, consistent with the results of Blasing. The perpendicular elastic strain of the AlGaN epilayer e> was calculated from the following formula: e> ¼ ðcepi cb Þ=cb ; where, cepi was the lattice constant of the AlGaN epilayer, calculated from the spectra of the symmetric X-ray y–2y scan; the lattice constant cb was for the fully relaxed epilayer. The Al composition obtained by the simulation of the random spectrum of RBS, was used to calculate the value of cb from the lattice parameters of GaN and AlN using Vegard’s law. For sample A, the values of cepi and cb were 0.5144 and 0.5127 nm, respectively. So we could get that e> equal to 0.33%, indicating that the AlGaN epilayer was under tensile strain in the perpendicular direction. Inserting an LT-AlN interlayer would relieve the stress due to the lattice mismatch between the AlGaN top layer and the GaN underlying layer. Then it led to the growth of crack-free AlGaN epilayer due to the fact that the AlGaN epilayers were under compressive stress in the parallel direction. In conclusion, inserting an LT-AlN interlayer would destroy the coherent growth of the underlying GaN layer and the top AlGaN layer. So the top AlGaN epilayer was under compressively stressed at growth temperature due to the smaller lattice constant of the AlN. Moreover, the AlGaN layer subsequently underwent compressive strain again when cooled down to the room temperature. This was induced by the
Fig. 2. The SEM surface morphology of Al0.25Ga0.75N epilayers: (a) the sample A with 8 nm LT-AlN interlayer between the AlGaN toplayer and the GaN underlying layer, (b) the sample I without LT-AlN interlayer.
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FWHM of XRD from (0002) diffraction for AlGaN film (sec)
thermal mismatch between the AlGaN epilayer and the underlying layers. The value of compressive strain for this kind of sample was the sum of those introduced by the LT-interlayer and by the thermal mismatch. The crystalline quality of all the AlGaN epilayers was characterized by X-ray diffraction pattern. From Fig. 3, we could find that the values FWHM for the (0 0 0 2) XRD o-scan peaks increased with increasing the thickness of LT-AlN interlayer, moreover, the RMS surface roughness showed the similar rule. In other words, thick LT-AlN interlayer would deteriorate both the quality of the AlGaN epilayer and the surface roughness. Fig. 4 showed the SEM images of the top Al0.25Ga0.75N surface of samples E–G. As seen,
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The thicknessof LT-AlN interlayer (nm)
Fig. 3. Rocking curve FWHM of (0 0 0 2) plane for AlGaN with different thickness AlN interlayer (a), and RMS surface roughness of AlGaN epilayer with different thickness of LTAlN interlayer (b).
these three samples with the HT-AlN interlayers’ thickness in the range of 9–40 nm had a crack-free but rough surface, which were different from the results by Lee et al. [8]; he found that the lower thickness of the HT-AlN interlayer led to cracking because it could not relieve stress completely and the higher one would result in broad XRD line width. However, in our study, 9 nm HT-AlN interlayer was enough to relieve the stress so as to avoid the crack formation. Lee proposed the mechanism of strain relaxation of the HT-AlN interlayer: the forming of the initial crack and subsequent dislocation would release the tensile strain when the thickness of HT-AlN interlayer exceed the critical values, and then the crack-free AlGaN layers were grown lateral over the cracks, from cathodoluminescence (CL) spectra. In our study, AlGaN sample with the thinnest HT-AlN interlayer at the thickness of 9 nm was crack free, but it maybe cracked due to stress relieving incompletely inserting an HT-AlN interlayer thinner than 9 nm. In addition, the surface roughness and the FWHM values for the (0 0 0 2) XRD oscan peaks of the AlGaN epilayer increased when the thickness of the HT-AlN interlayer increased. At the nearly same thickness of AlN interlayer, the FWHM of (0 0 0 2) diffraction for the samples with LT-AlN interlayer was much higher than the values for the samples with HT-AlN interlayer, as shown in Fig. 3(a). This showed that inserting HT-AlN interlayer was more favored to improve the crystalline quality than inserting LT-AlN interlayer, because HT-AlN interlayers were pseudomorphic with the underlying GaN. Sample G with the largest value of the thickness of the HT-AlN interlayer showed a more high-density mesa than sample F as shown in Fig. 4, symboling the Npolarity growth for the AlGaN epilayer resulted from the inappropriate growth parameters of the HT-AlN interlayer such as the relatively low growth temperature (GaN layer was usually grown at this growth temperature in our study) and the high flow rate of the TMAl. Both these factors reduced the diffusion length of the Al atoms. We also got the values of the tensile stress in the perpendicular direction in sample E and F, which were 0.14% and 0.16%, respectively, less than that of the sample A with an LT-AlN interlayer. Thus,
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by the thermal mismatch between AlGaN and sapphire substrate. AlGaN epilayer was coherently grown on the underlying HT layers, so it was under tensile stress during the growth temperature. Subsequently, the tensile stress would turn into the compressive stress due to the thermal mismatch between AlGaN and the underlying layers when the AlGaN epilayer was cooling down to the room temperature. In conclusion, AlGaN epilayer was under compressive strain at room temperature, which was the balance of the tensile strain induced by the lattice mismatch during the growth and the compressive strain induced by the thermal mismatch between the AlGaN epilayer and the underlying layer.
4. Summary
Fig. 4. The SEM surface morphology of Al0.25Ga0.75N epilayers with different thickness of HT-AlN interlayer (a) the sample E with 9 nm HT-AlN interlayer, (b) the sample F with 18.2 nm HT-AlN interlayer, (c) the sample G with 26 nm HTAlN interlayer.
we could get the conclusion that the samples were under compressive stress in the parallel direction as expected from the compressive strain induced
We found that inserting not only an LT-AlN interlayer but also an HT-AlN interlayer between AlGaN and GaN would be an effective ways to relieve stress so as to avoid the crack formation. Both the thickness of LT-AlN interlayer and HTAlN interlayer were critical for material quality of AlGaN epilayer. X-ray analysis along with AFM data showed good structural quality and smooth surface for the AlGaN sample with 8 nm LT-AlN interlayer. The AlGaN sample with 9 nm HT-AlN interlayer also displayed excellent structural quality but rough surface due to the unsuitable growth parameters for the HT-AlN interlayer. All the AlGaN samples with a LT-AlN or HT-AlN interlayer were under compressive stress in the parallel to the growth direction, but due to the effect of the interlayer-induced stress relieving during growth and the thermal mismatch for the samples with LT-AlN interlayer, and due to the thermal mismatch between AlGaN and sapphire substrate during cooling from the growth temperature to the room temperature for those with HT-AlN interlayer.
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