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Anisotropic defect structure of GaN film grown by MOCVD
PERGAMON
Solid State Communications 114 (2000) 237–240 www.elsevier.com/locate/ssc
Anisotropic defect structure of GaN film grown by MOCVD D.P. Feng a,*, Y. Zhao a, C.C. Sorrell a, G.Y. Zhang b a
School of Materials Science and Engineering, University of New South Wales, Kensington, Sydney NSW 2052, Australia Department of Physics, Mesoscopic Physics Laboratory, Peking University, Beijing 100871, People’s Republic of China
b
Received 11 August 1999; accepted 15 November 1999 by Z.Z. Gan; received in final form by the Publisher 26 January 2000
Abstract The defect structure and morphology of h-GaN film grown by means of MOCVD on (0001) sapphire substrate has been studied by focused ion beam milling (FIB), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). Two different types of grain boundary defects having an anisotropic distribution are observed. For the first type, the grain boundaries surround the pyramid-shaped grains. These defects originate from the mismatch of the crystal lattices between the GaN film and the sapphire substrate. The grain boundary defects penetrate the GaN epilayer and the average grain size increases with increasing layer thickness up to about 500 nm from the interface to the top surface of the GaN film. For the second type, the grain boundaries surround the voids forming produced by impurities in the film. Both types of grain boundaries have a distribution with a hexagonal symmetry that may be the origin of the anisotropic transport properties observed previously in the GaN films. q 2000 Published by Elsevier Science Ltd. All rights reserved. Keywords: A. Semiconductors; C. Grain boundaries; C. Impurities in semiconductors; C. Scanning and transmission electron microscopy
1. Introduction The microstructure of gallium nitride film grown by metal organic chemical-vapour deposit (MOCVD) has been extensively studied due to the potential applications of GaN in optoelectronic devices working in the visible and ultraviolet light range [1,2]. Recently, GaN-based green and blue light emitting diodes (LED’s) have been realised [3]. But, it is still difficult to obtain high quality GaN films, mainly because the lattice mismatch of ,16% between h-GaN and Al2O3 sapphire substrate leads to unavoidably high density of misfits. Even if a buffer layer of the GaN or AlN has been applied, the as-grown GaN epilayer still contains a high density of defects (mainly threading dislocations, stacking faults and grain boundaries) [4,5]. These defects affect both electronic and optical properties of the GaN film. For example, broken bonds at defect sites may enhance free carrier recombination [6]. In this letter, we report our focused ion beam milling (FIB), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) results of grain boundary defects in the plain-view and cross-sectional h-GaN film, * Corresponding author. E-mail address: [email protected] (D.P. Feng).
respectively. Two different types of grain boundary defects are observed. Both types of grain boundaries have a distribution with a hexagonal symmetry that may be the origin of the anisotropic transport properties observed previously in the GaN films [7].
2. Experimental Hexagonal h-GaN films were grown on the (0001) sapphire basal plane by using a horizontal reactor for low pressure metal organic chemical-vapour deposit (LPMOCVD). High purity hydrogen (H2) was used as the carrier gas, and trimethylgallium (TMGa) and ammonia (NH3) were used as the Ga and N sources, respectively. During the growth process, a two-step growth technique was employed, that is, a GaN buffer layer was pre-deposited on the (0001) surface of the sapphire at a low temperature of 5508C. The GaN epilayer was subsequently deposited on this buffer layer at a higher temperature of 10508C. Details of the GaN film growth conditions were found elsewhere [8]. The cross-sectional preparation of sample was undertaken by MICRON focused ion beam miller (FIB). The morphology of the sample was performed by the secondary electron image of FIB, HITACHI S4500 field emission scanning electron
0038-1098/00/$ - see front matter q 2000 Published by Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(00)00036-3
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D.P. Feng et al. / Solid State Communications 114 (2000) 237–240
Fig. 1. (a) Morphology of the tilted 45 degree surface of a polycrystalline GaN grown on sapphire substrate, showing pyramidshaped growth hillocks. (b) The (0001) plain-view of a polycrystalline GaN grown on a sapphire substrate showing grain boundary.
microscope (FE-SEM) and HITACHI-9000 high-resolution transmission electron microscope (HR-TEM).
3. Results and discussion The morphology of the GaN sample which shows anisotropic electronic transport behaviour was taken by FE-SEM and presented in Fig. 1(a) and (b), showing a polycrystalline and roughness GaN surface of pyramid-shaped growth hillocks. This is in agreement with the results reported by other authors [6]. The main interesting feature is the existence of the grain boundaries that have a distribution with a hexagonal symmetry. This anisotropic distribution may be the consequence of the intrinsic crystalline features of hGaN materials. For further revealing the microstructure characterisation of the GaN film, the FIB cross-sectional morphology
Fig. 2. (a) [10–10] FIB cross-sectional image of a GaN film grown on (1000) sapphire showing the grains defects. (b) [11–20] FIB cross-sectional image of a GaN film.
(secondary electron image) was taken in [10–10] and [11–20] directions of a well-grown GaN film and shown in Fig. 2(a) and (b), respectively. It is clearly shown that high density of misfits existed near the GaN and Al2O3 interface. The regions close to the interface have amorphous-like structures with the grain boundaries. The grain structures in less than about a half microns scale is apparently seen. The grain size gradually becomes bigger and reaches almost a constant value from the GaN/Al2O3 interface to the GaN film in about 500 nm thickness. This phenomenon is consistent with the result of Qian et al [9]. The top surface of the GaN epilayer was very uniform as shown in the crosssectional view in Fig. 2(a) and (b). It is visible also from the images that most of the grain boundaries surround the pyramid-shaped grains, and arise from the interface between the GaN buffer layer and the sapphire substrate, and extend from the bottom up to the top of the GaN epilayer. The most interesting fact is that the density and distribution of grain boundaries change with the direction
D.P. Feng et al. / Solid State Communications 114 (2000) 237–240
239
Fig. 4. Schemes illustrating the microstructure of the GaN growth mechanism.
Fig. 3. FE-SEM images in some regions of the GaN film grown by MOCVD on sapphire substrate: (a) void with an impurity; and (b) hexagonal shape holes with some impurities in the GaN film.
examined, as shown in Fig. 2a and b. In average, the grain boundary defects in the [11–20] direction have a lower density and a relatively homogeneous distribution, whereas in the [10–10] direction, the density of the grain boundary defects is higher. This further reveals that the defect structure is anisotropic even inside the well-grown epitaxial GaN films. This is in a good agreement with our results of highresolution images (HR-TEM) which reveals that the grain boundaries in the immediate vicinity of the GaN/Al2O3 interface are anisotropic. Similar microstructures were found in a number of samples with different growth conditions, grown on the (0001) sapphire substrate. This means that the microstructures shown in Figs. 1 and 2 might be common to the h-GaN film under our growth condition. Besides, in several of our samples different types of the grain boundaries were observed. As displayed in Fig. 3(a), pyramid-shaped holes are distributed in the h-GaN film. The edges of the holes form a kind of grain boundaries. Detailed investigation shows that the holes originate from the existence of impurities, which prevent the growth of the GaN film around
them in Fig. 4(b). In this case, the grain boundaries surround the voids forming from the impurities in the GaN film. Therefore, a common feature of the both types of the grain boundaries is the hexagonal symmetry of their distribution. As observed by us [7], a kind of anisotropic transport phenomenon exists in GaN films, which can be explained with a model in which the distribution of the defects is assumed to be of hexagonal symmetry. The abovementioned hexagonal symmetry in the distribution of the grain boundaries may be regarded as the evidence that we expect, and thus supports our explanation for the origin of the anisotropic transport properties observed in the GaN films. It may be considered that the grain-like structures were initially formed at the first stage of GaN buffer layer growth at its low deposition temperature (5508C). These grains are extended to the GaN epilayer even if in the high quality growth epilayer. It has been reported [8] that the grain defects in the GaN films are dislocations resulting from the misfit strain introduced by the lattice mismatch between the epilayer and the substrate, and low angle grain boundaries. Here, we use a simple growth model to explain the grain-like structure and growth mechanism of the GaN film, in which the GaN buffer in the two step growth process is initially grown by the grains on the basal plane of the sapphire substrate in roughness and polycrystalline form. As the GaN film is constantly growing, in average, the sizes of the grains become bigger and the grains gradually coalesce and the GaN film becomes more uniform, even though the GaN epilayer grows. Fig. 4 illustrates the scheme of our model on the GaN growth mechanism, which shows that the grain boundaries exist inside the GaN films and, due to the hexagonal symmetry of the crystal structure of h-GaN, having an anisotropic distribution.
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D.P. Feng et al. / Solid State Communications 114 (2000) 237–240
4. Conclusions We have extensively observed, by means of the FE-SEM, FIB and TEM, that the microstructure characteristic of a h-GaN film grown by MOCVD on a sapphire substrate is mainly the existence of grain boundaries near the GaN/ Al2O3 interface. Two different types of grain boundary defects are observed: the first surrounds pyramid-shaped grains; the second occurs around voids. Both types of grain boundaries have a distribution with a hexagonal symmetry that may be the origin of the anisotropic transport properties in the GaN films. References [1] T. Kawabata, T. Matsuda, S. Koike, J. Appl. Phys. 56 (1984) 2367.
[2] H. Amano, T. Tanaka, Y. Kunii, K. Kato, S.T. Kim, I. Akasaki, Appl. Phys. Lett. 64 (1994) 1377. [3] T. Nakamura, T. Mukai, M. Senoh, Jpn. J. Appl. Phys. 30 (1991) L1998. [4] B. Heying, X.H. Wu, S. Keller, Y. Li, D. Kapolnek, B.P. Keller, S.P. DenBaars, J.S. Speck, Appl. Phys. Lett. 68 (1996) 643. [5] F.A. Ponce, J.J.S. Major, W.E. Plano, D.F. Welch, Appl. Phys. Lett. 65 (1994) 2302. [6] Y. Xin, P.D. Brown, C.J. Humphreys, T.S. Cheng, C.T. Foxon, Appl. Phys. Lett. 70 (1997) 1308. [7] D.P. Feng, Y. Zhao, G.Y. Zhang, Phys. Status Solidi (a) 176 (1999) 1003. [8] L.S. Cheng, Y. Zhang, D.P. Yu, Z. Zhang, Appl. Phys. Lett. 70 (1997) 1408. [9] W. Qian, M. Skowronski, M. DeGraef, K. Doverspike, L.B. Rowland, D.K. Gaskill, Appl. Phys.Lett. 66 (1995) 1252.
Solid State Communications 114 (2000) 237–240 www.elsevier.com/locate/ssc
Anisotropic defect structure of GaN film grown by MOCVD D.P. Feng a,*, Y. Zhao a, C.C. Sorrell a, G.Y. Zhang b a
School of Materials Science and Engineering, University of New South Wales, Kensington, Sydney NSW 2052, Australia Department of Physics, Mesoscopic Physics Laboratory, Peking University, Beijing 100871, People’s Republic of China
b
Received 11 August 1999; accepted 15 November 1999 by Z.Z. Gan; received in final form by the Publisher 26 January 2000
Abstract The defect structure and morphology of h-GaN film grown by means of MOCVD on (0001) sapphire substrate has been studied by focused ion beam milling (FIB), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). Two different types of grain boundary defects having an anisotropic distribution are observed. For the first type, the grain boundaries surround the pyramid-shaped grains. These defects originate from the mismatch of the crystal lattices between the GaN film and the sapphire substrate. The grain boundary defects penetrate the GaN epilayer and the average grain size increases with increasing layer thickness up to about 500 nm from the interface to the top surface of the GaN film. For the second type, the grain boundaries surround the voids forming produced by impurities in the film. Both types of grain boundaries have a distribution with a hexagonal symmetry that may be the origin of the anisotropic transport properties observed previously in the GaN films. q 2000 Published by Elsevier Science Ltd. All rights reserved. Keywords: A. Semiconductors; C. Grain boundaries; C. Impurities in semiconductors; C. Scanning and transmission electron microscopy
1. Introduction The microstructure of gallium nitride film grown by metal organic chemical-vapour deposit (MOCVD) has been extensively studied due to the potential applications of GaN in optoelectronic devices working in the visible and ultraviolet light range [1,2]. Recently, GaN-based green and blue light emitting diodes (LED’s) have been realised [3]. But, it is still difficult to obtain high quality GaN films, mainly because the lattice mismatch of ,16% between h-GaN and Al2O3 sapphire substrate leads to unavoidably high density of misfits. Even if a buffer layer of the GaN or AlN has been applied, the as-grown GaN epilayer still contains a high density of defects (mainly threading dislocations, stacking faults and grain boundaries) [4,5]. These defects affect both electronic and optical properties of the GaN film. For example, broken bonds at defect sites may enhance free carrier recombination [6]. In this letter, we report our focused ion beam milling (FIB), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) results of grain boundary defects in the plain-view and cross-sectional h-GaN film, * Corresponding author. E-mail address: [email protected] (D.P. Feng).
respectively. Two different types of grain boundary defects are observed. Both types of grain boundaries have a distribution with a hexagonal symmetry that may be the origin of the anisotropic transport properties observed previously in the GaN films [7].
2. Experimental Hexagonal h-GaN films were grown on the (0001) sapphire basal plane by using a horizontal reactor for low pressure metal organic chemical-vapour deposit (LPMOCVD). High purity hydrogen (H2) was used as the carrier gas, and trimethylgallium (TMGa) and ammonia (NH3) were used as the Ga and N sources, respectively. During the growth process, a two-step growth technique was employed, that is, a GaN buffer layer was pre-deposited on the (0001) surface of the sapphire at a low temperature of 5508C. The GaN epilayer was subsequently deposited on this buffer layer at a higher temperature of 10508C. Details of the GaN film growth conditions were found elsewhere [8]. The cross-sectional preparation of sample was undertaken by MICRON focused ion beam miller (FIB). The morphology of the sample was performed by the secondary electron image of FIB, HITACHI S4500 field emission scanning electron
0038-1098/00/$ - see front matter q 2000 Published by Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(00)00036-3
238
D.P. Feng et al. / Solid State Communications 114 (2000) 237–240
Fig. 1. (a) Morphology of the tilted 45 degree surface of a polycrystalline GaN grown on sapphire substrate, showing pyramidshaped growth hillocks. (b) The (0001) plain-view of a polycrystalline GaN grown on a sapphire substrate showing grain boundary.
microscope (FE-SEM) and HITACHI-9000 high-resolution transmission electron microscope (HR-TEM).
3. Results and discussion The morphology of the GaN sample which shows anisotropic electronic transport behaviour was taken by FE-SEM and presented in Fig. 1(a) and (b), showing a polycrystalline and roughness GaN surface of pyramid-shaped growth hillocks. This is in agreement with the results reported by other authors [6]. The main interesting feature is the existence of the grain boundaries that have a distribution with a hexagonal symmetry. This anisotropic distribution may be the consequence of the intrinsic crystalline features of hGaN materials. For further revealing the microstructure characterisation of the GaN film, the FIB cross-sectional morphology
Fig. 2. (a) [10–10] FIB cross-sectional image of a GaN film grown on (1000) sapphire showing the grains defects. (b) [11–20] FIB cross-sectional image of a GaN film.
(secondary electron image) was taken in [10–10] and [11–20] directions of a well-grown GaN film and shown in Fig. 2(a) and (b), respectively. It is clearly shown that high density of misfits existed near the GaN and Al2O3 interface. The regions close to the interface have amorphous-like structures with the grain boundaries. The grain structures in less than about a half microns scale is apparently seen. The grain size gradually becomes bigger and reaches almost a constant value from the GaN/Al2O3 interface to the GaN film in about 500 nm thickness. This phenomenon is consistent with the result of Qian et al [9]. The top surface of the GaN epilayer was very uniform as shown in the crosssectional view in Fig. 2(a) and (b). It is visible also from the images that most of the grain boundaries surround the pyramid-shaped grains, and arise from the interface between the GaN buffer layer and the sapphire substrate, and extend from the bottom up to the top of the GaN epilayer. The most interesting fact is that the density and distribution of grain boundaries change with the direction
D.P. Feng et al. / Solid State Communications 114 (2000) 237–240
239
Fig. 4. Schemes illustrating the microstructure of the GaN growth mechanism.
Fig. 3. FE-SEM images in some regions of the GaN film grown by MOCVD on sapphire substrate: (a) void with an impurity; and (b) hexagonal shape holes with some impurities in the GaN film.
examined, as shown in Fig. 2a and b. In average, the grain boundary defects in the [11–20] direction have a lower density and a relatively homogeneous distribution, whereas in the [10–10] direction, the density of the grain boundary defects is higher. This further reveals that the defect structure is anisotropic even inside the well-grown epitaxial GaN films. This is in a good agreement with our results of highresolution images (HR-TEM) which reveals that the grain boundaries in the immediate vicinity of the GaN/Al2O3 interface are anisotropic. Similar microstructures were found in a number of samples with different growth conditions, grown on the (0001) sapphire substrate. This means that the microstructures shown in Figs. 1 and 2 might be common to the h-GaN film under our growth condition. Besides, in several of our samples different types of the grain boundaries were observed. As displayed in Fig. 3(a), pyramid-shaped holes are distributed in the h-GaN film. The edges of the holes form a kind of grain boundaries. Detailed investigation shows that the holes originate from the existence of impurities, which prevent the growth of the GaN film around
them in Fig. 4(b). In this case, the grain boundaries surround the voids forming from the impurities in the GaN film. Therefore, a common feature of the both types of the grain boundaries is the hexagonal symmetry of their distribution. As observed by us [7], a kind of anisotropic transport phenomenon exists in GaN films, which can be explained with a model in which the distribution of the defects is assumed to be of hexagonal symmetry. The abovementioned hexagonal symmetry in the distribution of the grain boundaries may be regarded as the evidence that we expect, and thus supports our explanation for the origin of the anisotropic transport properties observed in the GaN films. It may be considered that the grain-like structures were initially formed at the first stage of GaN buffer layer growth at its low deposition temperature (5508C). These grains are extended to the GaN epilayer even if in the high quality growth epilayer. It has been reported [8] that the grain defects in the GaN films are dislocations resulting from the misfit strain introduced by the lattice mismatch between the epilayer and the substrate, and low angle grain boundaries. Here, we use a simple growth model to explain the grain-like structure and growth mechanism of the GaN film, in which the GaN buffer in the two step growth process is initially grown by the grains on the basal plane of the sapphire substrate in roughness and polycrystalline form. As the GaN film is constantly growing, in average, the sizes of the grains become bigger and the grains gradually coalesce and the GaN film becomes more uniform, even though the GaN epilayer grows. Fig. 4 illustrates the scheme of our model on the GaN growth mechanism, which shows that the grain boundaries exist inside the GaN films and, due to the hexagonal symmetry of the crystal structure of h-GaN, having an anisotropic distribution.
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D.P. Feng et al. / Solid State Communications 114 (2000) 237–240
4. Conclusions We have extensively observed, by means of the FE-SEM, FIB and TEM, that the microstructure characteristic of a h-GaN film grown by MOCVD on a sapphire substrate is mainly the existence of grain boundaries near the GaN/ Al2O3 interface. Two different types of grain boundary defects are observed: the first surrounds pyramid-shaped grains; the second occurs around voids. Both types of grain boundaries have a distribution with a hexagonal symmetry that may be the origin of the anisotropic transport properties in the GaN films. References [1] T. Kawabata, T. Matsuda, S. Koike, J. Appl. Phys. 56 (1984) 2367.
[2] H. Amano, T. Tanaka, Y. Kunii, K. Kato, S.T. Kim, I. Akasaki, Appl. Phys. Lett. 64 (1994) 1377. [3] T. Nakamura, T. Mukai, M. Senoh, Jpn. J. Appl. Phys. 30 (1991) L1998. [4] B. Heying, X.H. Wu, S. Keller, Y. Li, D. Kapolnek, B.P. Keller, S.P. DenBaars, J.S. Speck, Appl. Phys. Lett. 68 (1996) 643. [5] F.A. Ponce, J.J.S. Major, W.E. Plano, D.F. Welch, Appl. Phys. Lett. 65 (1994) 2302. [6] Y. Xin, P.D. Brown, C.J. Humphreys, T.S. Cheng, C.T. Foxon, Appl. Phys. Lett. 70 (1997) 1308. [7] D.P. Feng, Y. Zhao, G.Y. Zhang, Phys. Status Solidi (a) 176 (1999) 1003. [8] L.S. Cheng, Y. Zhang, D.P. Yu, Z. Zhang, Appl. Phys. Lett. 70 (1997) 1408. [9] W. Qian, M. Skowronski, M. DeGraef, K. Doverspike, L.B. Rowland, D.K. Gaskill, Appl. Phys.Lett. 66 (1995) 1252.