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Characterization of ALN buffer layers on (0 0 0 1)-sapphire substrates
Journal of Crystal Growth 189/190 (1998) 282—286
Characterization of ALN buffer layers on (0 0 0 1)-sapphire substrates Yves-Matthieu Le Vaillant!,*, Rene´ Bisaro", Jean Olivier", Olivier Durand", Jean-Yves Duboz", Sandra Ruffenach-Clur!, Olivier Briot!, Bernard Gil!, Roger-Louis Aulombard! ! GES, Universite& de Montpellier II, CC 074, Place Euge% ne Bataillon, 34095 Montpellier Cedex 5, France " Thomson-CSF LCR, Domaine de Corbeville, 91404 Orsay Cedex, France
Abstract It is now established that low-temperature grown buffer layers are needed to improve the structural and electronic properties of GaN layers grown on sapphire. We studied the recrystallization of AlN buffer layers grown by low-pressure MOVPE as a function of annealing time. The Warren—Averbach method was applied to the analysis of broadening and line shape of the (0 0 0 2) and (0 0 0 4) X-ray diffraction peaks. This method yielded a separation of the grain size distribution from microstrain effects. The evolution of the relative frequency distribution of the grain size with annealing is correlated with atomic force microscopy experiments. The distribution of the reflecting-planes orientation was determined by X-ray rocking-curve experiments. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Gallium nitride and related materials have direct band gaps ranging from 1.9 to 6.2 eV, and have been recently used in the fabrication of blue and green light-emitting diodes (LEDs) and laser diodes (LDs) [1]. The performance of GaN-based devices depends critically on the initially grown layer which conducts to the generation of misfit dislocations and polarity-related defects in the main layer. The C-face (0 0 0 1) sapphire has been successfully
* Corresponding author. Tel.: #33 4 67143924; fax: #33 4 67143760; e-mail: le—[email protected].
used as a substrate to grow GaN films by metalorganic chemical vapor deposition (MOCVD) method. The lattice parameter mismatch (&14%) and the difference in the thermal expansion coefficients (&23%) between sapphire and GaN require the predeposition of a buffer layer. Because of a lower interface energy and a higher density of nucleation sites, GaN heteroepitaxy on the AlN buffer layer is easier than a direct GaN heteroepitaxy on sapphire substrate. Structures using AlN [2,3] or GaN buffer layers [4—6] have already been investigated. These studies especially dealt with the buffer layer treatment process and its influence on the characteristics of the GaN epitaxial layer [2—6]. The methods of investigation used were often
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 2 5 9 - 0
Y.-M. Le Vaillant et al. / Journal of Crystal Growth 189/190 (1998) 282–286
283
indirect because of the difficulty of observation mainly due to the small thickness of the intermediate layer. Nowadays, little is known about the structural properties that the buffer layer should have for improving the quality of further GaN epitaxy. The aim of this paper is to study the effects of thermal annealing on AlN buffer layers. In addition to surface imaging by atomic force microscopy (AFM) and usual structure characterization by Xray diffraction (XRD), we applied the Warren— Averbach analysis of X-ray line shape [7] to extract the grain size and the intra-grain strain distributions.
a set of columns of unit cells, n represents the number of cells in each crystal column. The length ¸ of a column is: ¸"na . The Fourier coefficients 3 A (l), obtained experimentally, give access to the n size and the microstrain of the crystallites. X-ray rocking-curve experiments have been performed using a (u—fixed 2h ) geometry, a secondary (0 0 0 2) graphite curved monochromator and a thin back slit. The resulting divergence in the incidence plane is of about 0.03°. Hence, the curve shape of an u-scan only depends on the angular distribution of the c-axis in the grains.
2. Experimental procedure
3. Experimental results
AlN thin films of about 50 nm thick, have been grown on (0 0 0 1)-sapphire substrates, using the low-pressure metalorganic chemical vapor deposition (LP-MOCVD) technique. The precursors were trimethyl-aluminum (TMA) and ammonia (NH ). High-purity hydrogen has been employed 3 as the carrier gas. All substrates have been first etched (H PO : 3H SO , for 10 min at 80°C) and 3 4 2 4 then submitted to a pre-growth nitridation in an ammonia flow (10 min at 1050°C). AlN buffer layers have been grown at 800°C and 76 Torr. A series of buffer layers have been post-annealed at a temperature of 1070°C during time periods ranging from 30 s to 15 min. The surface morphology has been studied by atomic force microscopy (AFM) in a non-contact mode in order to avoid charge problems. For X-ray diffraction experiments, data have been recorded using a high-performance diffraction set-up in standard Bragg—Brentano (h—2h), asymmetrical (u—2h) and (u—fixed 2h) geometry. Based on an analysis of the Bragg-peak shape, the Warren—Averbach method is able to separate the effects of strain and particle size in the broadening of a diffraction peak. It has been shown [7] that a diffraction peak profile can be represented by a Fourier series. The distribution of the X-ray reflecting power per unit length can be expressed as: P JN&`=A (l)cos 2pnh , 2h ~= n 3 where N is the total number of unit cells in the crystal, h "2a sin h/j, and a is the norm of the 3 3 3 base vector of the unit cell, normal to the reflecting planes. If the crystallites are supposed to be built of
As a result of a low-temperature deposition process followed by a thermal annealing, the AlN buffer layers, as shown in the AFM images (Fig. 1), are made of small grains of various sizes, the distribution of which depends on annealing time. The profile of the surface shows a increased roughness, at 2 min annealing. Then, for an annealing time between 2 and 5 min, the surface becomes smoothed: the coalescence of the grains occurs. The cube of the mean column length has been found to be a linear function of the annealing time (Fig. 2). Obtained from the Warren—Averbach method, the frequency distribution function of the crystal column length is presented in Fig. 3 for different annealing times. Two characteristic column length values can be pointed out. The as-grown sample appears to be almost exclusively composed of small crystallized columns (&6 nm). With annealing time, an increasing proportion of larger columns is observed (&30 nm). After annealing for 2 min, the column size distribution is centered around both these particular values. No intermediate column size is observed. For a longer annealing time, the column size distribution is smoothed and an increasing proportion of column lengths reach the thickness of the layer (&50 nm). The distribution queues spreading above this value are relative to the last harmonics of the Fourier analysis and denote the high sensitivity of the fit. Nevertheless, their relative proportion never exceeds 1%. For measuring the microstrain, the variation of the lattice inter-planar spacing is compared to the nominal value for AlN.
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Y.-M. Le Vaillant et al. / Journal of Crystal Growth 189/190 (1998) 282–286
Fig. 1. Atomic force microscopy images and cross-sections of aluminum nitride surface for samples: (a) as-grown, annealed at 1070°C for 2 min (b) and 15 min (c). Roughness: r.m.s (0.891, 2.555, 1.258 nm); peak-to-valley (3.974, 8.307, 4.876 nm) for samples A, B and C, respectively.
Fig. 2. Evolution of the cubic column-length with annealing time. The slope of the linear extrapolation is 1915 nm3/min.
Fig. 3. Frequency distribution function of column lengths in the crystal for samples: as-grown (a), annealed at 1070°C for 0.5 min (b), 2 min (c), 5 min (d) and 15 min (e).
This distortion parameter gives a statistical information about the local strain in the crystallites through the relative variation of the column length (Fig. 4). In a crystallite, the local strain depends on
the volume-to-surface ratio and is inhomogeneously relaxed through the bulk. The smaller the column length, the higher the corresponding microstrain. During the first 2 min, the distortion increases
Y.-M. Le Vaillant et al. / Journal of Crystal Growth 189/190 (1998) 282–286
285
results. Small and large column lengths have their own angular distribution of the c-axis. The resulting distribution is the sum of these two components. The sharp orientation of the grains after 5 min annealing comes from the contribution of large crystallized domains obtained by coalescence. The shift of the peak denotes an averaged disorientation that could be due to a strain relaxation.
4. Discussion
Fig. 4. Root mean square strain versus column length.
Fig. 5. (0 0 0 2)-AlN rocking-curves for samples: (a) as-grown, annealed at 1070°C during (b) 0.5 min, (c) 2 min, (d) 5 min and (e) 15 min.
whatever the column length. Then, after a sudden strain relaxation between 2 and 5 min annealing, the strain increases again. The rocking curves (Fig. 5) denote a close correlation with the above
During annealing, after a primary recrystallization by atomic migration through a matrix, a secondary recrystallization occurs as a result of the growth of bigger grains to the detriment of smaller ones. The linear evolution of the cubic column length mean with annealing time (Fig. 2) is an evidence of the stage of secondary recrystallization or Ostwald ripening [8]. The shape of the distribution function of column sizes (Fig. 3), where two contributions have been shown, is the result of such a phenomenon. The driving force of Ostwald ripening is the lowering of the interfacial energy by decreasing the total wrap area of the crystallites. But this new distribution, by an increase of the area where epitaxy occurs, increases progressively the stress in the bulk. The morphological change observed is the result of a competition between these two mechanisms. As a consequence, a pseudo twodimensional GaN growth will be favored for the 2 min annealing buffer layer: firstly because it has the highest density of low-energy nucleation sites for chemical adsorption of GaN molecules, and secondly because the spreading of GaN molecules is favored on the smallest grains of the buffer layer where the highest microstrain (Fig. 4), hence the highest free surface energy is observed. Beyond 2 min annealing, the proportion of large grain sizes is sufficient to produce their coalescence. As a consequence the strain energy stored in the bulk increases again with the volume-to-surface ratio.
5. Conclusions In this paper, we have described quantitatively several effects of the annealing time on the structure
286
Y.-M. Le Vaillant et al. / Journal of Crystal Growth 189/190 (1998) 282–286
and the morphology of AlN buffer layers: the surface imaging by AFM, the size and corresponding microstrain relative to the crystallites by the Warren—Averbach statistical X-ray method and the texture of the material by the X-ray rocking-curve experiments. We have pointed out the Ostwald ripening as the secondary nucleation stage and a complete change of the morphology when the annealing lasts between 2 and 5 min. Our results underline the influence of the strain in the recrystallization process. Such measurements give a physical meaning to the empirical choice of annealing time previously determined by the photoluminescence or Hall effect quality of the GaN overlayer.
Acknowledgements This work was partly supported by the French DRET/DGA.
References [1] GaN and Related Materials for Device Application, Material Research Society Bull., 22 (2) (1997). [2] H. Amano, N. Sawaki, I. Akasaki, Y. Toyoda, Appl. Phys. Lett. 48 (1986) 353. [3] T. Sasaki, T. Matsuoka, J. Appl. Phys. 77 (1995) 192. [4] S. Nakamura, Jpn. J. Appl. Phys. 30 (1991) L1705. [5] S.D. Hersee, J. Ramer, K. Zheng, C. Kranenberg, K. Malloy, M. Banas, M. Goorsky, J. Electron. Mater. 24 (11) (1995) 1519. [6] C.F. Lin, G.C. Chi, M.S. Feng, J.D. Guo, J.S. Tsang, J. Minghuang Hong, Appl. Phys. Lett. 68 (26) (1996) 3758. [7] B.E. Warren, Progress in Metal Physics, Pergamon Press, London, vol. 8, 1959, pp. 147, and references therein. [8] J. Zarzycki, Les Verres et l’E¨tat Vitreux, Ch. VI, Masson, 1982.
Characterization of ALN buffer layers on (0 0 0 1)-sapphire substrates Yves-Matthieu Le Vaillant!,*, Rene´ Bisaro", Jean Olivier", Olivier Durand", Jean-Yves Duboz", Sandra Ruffenach-Clur!, Olivier Briot!, Bernard Gil!, Roger-Louis Aulombard! ! GES, Universite& de Montpellier II, CC 074, Place Euge% ne Bataillon, 34095 Montpellier Cedex 5, France " Thomson-CSF LCR, Domaine de Corbeville, 91404 Orsay Cedex, France
Abstract It is now established that low-temperature grown buffer layers are needed to improve the structural and electronic properties of GaN layers grown on sapphire. We studied the recrystallization of AlN buffer layers grown by low-pressure MOVPE as a function of annealing time. The Warren—Averbach method was applied to the analysis of broadening and line shape of the (0 0 0 2) and (0 0 0 4) X-ray diffraction peaks. This method yielded a separation of the grain size distribution from microstrain effects. The evolution of the relative frequency distribution of the grain size with annealing is correlated with atomic force microscopy experiments. The distribution of the reflecting-planes orientation was determined by X-ray rocking-curve experiments. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Gallium nitride and related materials have direct band gaps ranging from 1.9 to 6.2 eV, and have been recently used in the fabrication of blue and green light-emitting diodes (LEDs) and laser diodes (LDs) [1]. The performance of GaN-based devices depends critically on the initially grown layer which conducts to the generation of misfit dislocations and polarity-related defects in the main layer. The C-face (0 0 0 1) sapphire has been successfully
* Corresponding author. Tel.: #33 4 67143924; fax: #33 4 67143760; e-mail: le—[email protected].
used as a substrate to grow GaN films by metalorganic chemical vapor deposition (MOCVD) method. The lattice parameter mismatch (&14%) and the difference in the thermal expansion coefficients (&23%) between sapphire and GaN require the predeposition of a buffer layer. Because of a lower interface energy and a higher density of nucleation sites, GaN heteroepitaxy on the AlN buffer layer is easier than a direct GaN heteroepitaxy on sapphire substrate. Structures using AlN [2,3] or GaN buffer layers [4—6] have already been investigated. These studies especially dealt with the buffer layer treatment process and its influence on the characteristics of the GaN epitaxial layer [2—6]. The methods of investigation used were often
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 2 5 9 - 0
Y.-M. Le Vaillant et al. / Journal of Crystal Growth 189/190 (1998) 282–286
283
indirect because of the difficulty of observation mainly due to the small thickness of the intermediate layer. Nowadays, little is known about the structural properties that the buffer layer should have for improving the quality of further GaN epitaxy. The aim of this paper is to study the effects of thermal annealing on AlN buffer layers. In addition to surface imaging by atomic force microscopy (AFM) and usual structure characterization by Xray diffraction (XRD), we applied the Warren— Averbach analysis of X-ray line shape [7] to extract the grain size and the intra-grain strain distributions.
a set of columns of unit cells, n represents the number of cells in each crystal column. The length ¸ of a column is: ¸"na . The Fourier coefficients 3 A (l), obtained experimentally, give access to the n size and the microstrain of the crystallites. X-ray rocking-curve experiments have been performed using a (u—fixed 2h ) geometry, a secondary (0 0 0 2) graphite curved monochromator and a thin back slit. The resulting divergence in the incidence plane is of about 0.03°. Hence, the curve shape of an u-scan only depends on the angular distribution of the c-axis in the grains.
2. Experimental procedure
3. Experimental results
AlN thin films of about 50 nm thick, have been grown on (0 0 0 1)-sapphire substrates, using the low-pressure metalorganic chemical vapor deposition (LP-MOCVD) technique. The precursors were trimethyl-aluminum (TMA) and ammonia (NH ). High-purity hydrogen has been employed 3 as the carrier gas. All substrates have been first etched (H PO : 3H SO , for 10 min at 80°C) and 3 4 2 4 then submitted to a pre-growth nitridation in an ammonia flow (10 min at 1050°C). AlN buffer layers have been grown at 800°C and 76 Torr. A series of buffer layers have been post-annealed at a temperature of 1070°C during time periods ranging from 30 s to 15 min. The surface morphology has been studied by atomic force microscopy (AFM) in a non-contact mode in order to avoid charge problems. For X-ray diffraction experiments, data have been recorded using a high-performance diffraction set-up in standard Bragg—Brentano (h—2h), asymmetrical (u—2h) and (u—fixed 2h) geometry. Based on an analysis of the Bragg-peak shape, the Warren—Averbach method is able to separate the effects of strain and particle size in the broadening of a diffraction peak. It has been shown [7] that a diffraction peak profile can be represented by a Fourier series. The distribution of the X-ray reflecting power per unit length can be expressed as: P JN&`=A (l)cos 2pnh , 2h ~= n 3 where N is the total number of unit cells in the crystal, h "2a sin h/j, and a is the norm of the 3 3 3 base vector of the unit cell, normal to the reflecting planes. If the crystallites are supposed to be built of
As a result of a low-temperature deposition process followed by a thermal annealing, the AlN buffer layers, as shown in the AFM images (Fig. 1), are made of small grains of various sizes, the distribution of which depends on annealing time. The profile of the surface shows a increased roughness, at 2 min annealing. Then, for an annealing time between 2 and 5 min, the surface becomes smoothed: the coalescence of the grains occurs. The cube of the mean column length has been found to be a linear function of the annealing time (Fig. 2). Obtained from the Warren—Averbach method, the frequency distribution function of the crystal column length is presented in Fig. 3 for different annealing times. Two characteristic column length values can be pointed out. The as-grown sample appears to be almost exclusively composed of small crystallized columns (&6 nm). With annealing time, an increasing proportion of larger columns is observed (&30 nm). After annealing for 2 min, the column size distribution is centered around both these particular values. No intermediate column size is observed. For a longer annealing time, the column size distribution is smoothed and an increasing proportion of column lengths reach the thickness of the layer (&50 nm). The distribution queues spreading above this value are relative to the last harmonics of the Fourier analysis and denote the high sensitivity of the fit. Nevertheless, their relative proportion never exceeds 1%. For measuring the microstrain, the variation of the lattice inter-planar spacing is compared to the nominal value for AlN.
284
Y.-M. Le Vaillant et al. / Journal of Crystal Growth 189/190 (1998) 282–286
Fig. 1. Atomic force microscopy images and cross-sections of aluminum nitride surface for samples: (a) as-grown, annealed at 1070°C for 2 min (b) and 15 min (c). Roughness: r.m.s (0.891, 2.555, 1.258 nm); peak-to-valley (3.974, 8.307, 4.876 nm) for samples A, B and C, respectively.
Fig. 2. Evolution of the cubic column-length with annealing time. The slope of the linear extrapolation is 1915 nm3/min.
Fig. 3. Frequency distribution function of column lengths in the crystal for samples: as-grown (a), annealed at 1070°C for 0.5 min (b), 2 min (c), 5 min (d) and 15 min (e).
This distortion parameter gives a statistical information about the local strain in the crystallites through the relative variation of the column length (Fig. 4). In a crystallite, the local strain depends on
the volume-to-surface ratio and is inhomogeneously relaxed through the bulk. The smaller the column length, the higher the corresponding microstrain. During the first 2 min, the distortion increases
Y.-M. Le Vaillant et al. / Journal of Crystal Growth 189/190 (1998) 282–286
285
results. Small and large column lengths have their own angular distribution of the c-axis. The resulting distribution is the sum of these two components. The sharp orientation of the grains after 5 min annealing comes from the contribution of large crystallized domains obtained by coalescence. The shift of the peak denotes an averaged disorientation that could be due to a strain relaxation.
4. Discussion
Fig. 4. Root mean square strain versus column length.
Fig. 5. (0 0 0 2)-AlN rocking-curves for samples: (a) as-grown, annealed at 1070°C during (b) 0.5 min, (c) 2 min, (d) 5 min and (e) 15 min.
whatever the column length. Then, after a sudden strain relaxation between 2 and 5 min annealing, the strain increases again. The rocking curves (Fig. 5) denote a close correlation with the above
During annealing, after a primary recrystallization by atomic migration through a matrix, a secondary recrystallization occurs as a result of the growth of bigger grains to the detriment of smaller ones. The linear evolution of the cubic column length mean with annealing time (Fig. 2) is an evidence of the stage of secondary recrystallization or Ostwald ripening [8]. The shape of the distribution function of column sizes (Fig. 3), where two contributions have been shown, is the result of such a phenomenon. The driving force of Ostwald ripening is the lowering of the interfacial energy by decreasing the total wrap area of the crystallites. But this new distribution, by an increase of the area where epitaxy occurs, increases progressively the stress in the bulk. The morphological change observed is the result of a competition between these two mechanisms. As a consequence, a pseudo twodimensional GaN growth will be favored for the 2 min annealing buffer layer: firstly because it has the highest density of low-energy nucleation sites for chemical adsorption of GaN molecules, and secondly because the spreading of GaN molecules is favored on the smallest grains of the buffer layer where the highest microstrain (Fig. 4), hence the highest free surface energy is observed. Beyond 2 min annealing, the proportion of large grain sizes is sufficient to produce their coalescence. As a consequence the strain energy stored in the bulk increases again with the volume-to-surface ratio.
5. Conclusions In this paper, we have described quantitatively several effects of the annealing time on the structure
286
Y.-M. Le Vaillant et al. / Journal of Crystal Growth 189/190 (1998) 282–286
and the morphology of AlN buffer layers: the surface imaging by AFM, the size and corresponding microstrain relative to the crystallites by the Warren—Averbach statistical X-ray method and the texture of the material by the X-ray rocking-curve experiments. We have pointed out the Ostwald ripening as the secondary nucleation stage and a complete change of the morphology when the annealing lasts between 2 and 5 min. Our results underline the influence of the strain in the recrystallization process. Such measurements give a physical meaning to the empirical choice of annealing time previously determined by the photoluminescence or Hall effect quality of the GaN overlayer.
Acknowledgements This work was partly supported by the French DRET/DGA.
References [1] GaN and Related Materials for Device Application, Material Research Society Bull., 22 (2) (1997). [2] H. Amano, N. Sawaki, I. Akasaki, Y. Toyoda, Appl. Phys. Lett. 48 (1986) 353. [3] T. Sasaki, T. Matsuoka, J. Appl. Phys. 77 (1995) 192. [4] S. Nakamura, Jpn. J. Appl. Phys. 30 (1991) L1705. [5] S.D. Hersee, J. Ramer, K. Zheng, C. Kranenberg, K. Malloy, M. Banas, M. Goorsky, J. Electron. Mater. 24 (11) (1995) 1519. [6] C.F. Lin, G.C. Chi, M.S. Feng, J.D. Guo, J.S. Tsang, J. Minghuang Hong, Appl. Phys. Lett. 68 (26) (1996) 3758. [7] B.E. Warren, Progress in Metal Physics, Pergamon Press, London, vol. 8, 1959, pp. 147, and references therein. [8] J. Zarzycki, Les Verres et l’E¨tat Vitreux, Ch. VI, Masson, 1982.