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Dislocation structure of GaN bulk crystals grown on SiC substrates by HVPE
Materials Science and Engineering B61 – 62 (1999) 325 – 329
Dislocation structure of GaN bulk crystals grown on SiC substrates by HVPE I. Nikitina a,*, G. Mosina a, Yu. Melnik b,c, A. Nikolaev a, K. Vassilevski a a
Ioffe Institute, 26 Politechnicheskaya str., St. Petersburg 194021, Russia b Crystal Growth Research Center, St. Petersburg 193036, Russia c TDI, Inc., Gaithersburg, MD 20877, USA
Abstract The dislocation structure of GaN wafers was studied by transmission electron microscopy. These wafers were fabricated using hydride vapor phase epitaxy of thick GaN layers on 6H – SiC substrates and subsequent removal of the substrate by reactive ion etching. Three major types of dislocations were observed in these GaN crystals: (1) threading dislocations (TD), which are mainly parallel to the [0001] direction and extend from the former interface to the crystal surface. They were found to have edge, screw and mixed type; (2) dislocations lying on basal (0001) planes and located near TD through all the thickness of the crystal; (3) dislocations lying preferentially on prismatic planes in the top region of the crystals. Based on experimental results, the influence of initial nucleation stage of heteroepitaxial growth and post-growth cooling on the dislocation distribution in bulk GaN crystals is discussed. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Silicon carbide; Gallium nitride; Transmission electron microscopy; Dislocations
1. Introduction III–V-nitrides are very promising semiconductors for light emitting and microwave devices. Recent developments in thin film epitaxial and device fabrication technology have led to the production at the commercial scale of blue light emitting diodes and to the demonstration of room temperature violet laser diodes (LD) based on InGaN GaN AlGaN heterostructures [1,2]. Fabrication of field effect transistors with improved microwave performance has been also reported [3]. Nitride epitaxial films for these devices were grown on foreign substrates, mainly on sapphire and silicon carbide. The properties of these films are unique and often incomprehensible in the frame of conventional semiconductor physics and technology. In particular, the nitride epitaxial layers with excellent optoelectronic performance have dislocation densities of 1010 cm − 2 [4]. Furthermore, epitaxy on SiC, which has lattice parame-
* Corresponding author. Fax: +7-812-2476425. E-mail address: [email protected] (I. Nikitina)
ters and thermal-expansion coefficient much better fitted to GaN than sapphire, does not lead to improvement in device characteristics. On the other hand, it was recently reported that the use of bulk GaN crystals as a substrate for the fabrication of nitride based LD epitaxial structure give an essential improvement in optoelectrical characteristics of laser diodes [5]. This fact makes more and more interesting to investigate the microstructure of GaN epitaxial films and bulk crystals. The structural characterization of GaN layers grown on sapphire under various growth conditions has been reported by a number of authors [4,6–8]. The most detailed analysis of dislocation structure of GaN layers with a thickness of 7 mm grown on sapphire by MOCVD was conducted by Ning et al. [7]. To the best of our knowledge, a similar study for GaN layers grown on SiC–6H has not been reported up today. There is also no circumstantial report on dislocation distribution in bulk GaN crystals. This paper presents results of microstructure investigation of thick (100 mm) GaN free standing platelets grown by hydride vapor phase epitaxy (HVPE).
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2. Samples preparation and measurement techniques Thick GaN films were grown on 30 mm diameter SiC–6H (0001) substrates using hydride vapor phase epitaxy in a horizontal open flow reactor at T 1050°C without any buffer layer [9]. GaN wafers were then fabricated by selective removal of the initial silicon carbide substrate using reactive ion etching (RIE) [10]. The thickness of the GaN platelets was ca. 100 mm. Hereafter we call the side of the GaN platelet adjacent to the former GaN SiC interface as ‘former interface’, and the opposite side as ‘GaN crystal surface’. To estimate the GaN crystal quality, X-ray diffraction (XRD) measurements were performed using double-crystal spectrometer on (0002) and (1124) CuKa reflections from both sides of bulk crystal. Cross-sectional TEM images were obtained from some portions of the GaN crystal situated at different distances from the former interface.
3. Results The values of full width at half maximum (FWHM) of XRD rocking curves (RC), measured with v- scan geometry on symmetric (0002) and asymmetric (1124) reflections from both sides of the studied GaN wafer are presented in Table 1. As it is seen from these data, (1) GaN bulk crystal has a high crystal quality; (2) the FWHMs of RC(v) measured from the former interface are higher than those measured from the surface; (3) the broadening of RC(v) measured on the asymmetric reflection are comparable with that on the symmetric one. Taking into account the X-ray penetration depth t0002 =8.5 mm [11], it may be concluded that edge, screw and mixed dislocations, are equally responsible for rocking curves broadening at least inside the regions 8.5 mm remote from both surfaces of bulk GaN crystal [7].
3.1. Dislocation structure of GaN platelets near the former interface Cross-section bright-field TEM images taken from a bulk GaN platelet in the region, close to the former interface, are shown in Fig. 1. Two regions having Table 1 FWHM of XRD rocking curves, measured with v- scan geometry Side of GaN crystal
FWHM (arcsec) (0002) CuKa
FWHM (arcsec) (1124) CuKa
Surface Former interface
165 200
170 240
Fig. 1. Cross-sectional bright-field TEM images taken near the [1120] zone axis, for an area in the vicinity of the former interface of GaN platelet:(a) with g =1120 and (b) with g =0002.
different microstructure with sharp boundary between them are clearly seen in these TEM images. First is a highly defected initial layer, located in the bottom parts of both images. This layer is typical for GaN heteroepitaxial growth on SiC [12] and sapphire [13] substrates. Its presence in the GaN free standing platelet confirms the selectivity of the RIE process employed to remove the SiC substrate. The density of defects in this initial layer is much higher than the one in the top part of the region shown in the same figure. Consecutive stacking faults on (0001) plane are observed in TEM bright-field cross-section images obtained from the initial layer at
I. Nikitina et al. / Materials Science and Engineering B61–62 (1999) 325–329
higher magnification. This is shown in Fig. 2. The average distance between the stacking faults is about 10 nm. Dislocation boundaries, which lie along [0001] direction were also detected here. Similar structure of initial layer in 15 mm GaN crystal grown on sapphire by HVPE was reported by Romano et al. [6]. TEM cross-sectional image taken from the region near the former interface clearly showed the layer, having a thickness about 50 nm. It consists from some small grains, growing preferentially in the [0001] direction (Fig. 3). Reflections (0002) GaN from the grains were detected in a microdiffraction pattern, obtained from this region (shown in the inset of the figure). These grains were identified as nucleation islands arising at the initial stage of the GaN growth on the SiC substrate. This fact is in an agreement with the conclusions drawn by Sitar et al. [14], that the growth of GaN on SiC has a three-dimensional mode. The formation of stacking faults was assumed to occur as a result of the mutual accommodation of GaN and SiC lattices inside
Fig. 2. Cross-sectional bright-field TEM images taken from the region near initial layer of a GaN platelet: (a) with g = 0002 and (b) with g= 1010
327
Fig. 3. Cross-sectional bright-field TEM image taken from an area located ca. 100 nm near the former interface of GaN platelet. Inset shows microdiffraction pattern from this region.
every grain. It is necessary to emphasise that the abrupt boundary between the initial layer and the top part of the GaN crystal is usually observed in thin GaN layers grown directly on foreign substrates [6,8,15]. Probably, such an abrupt boundary is defined by the so-called ‘secondary critical size’ [7] of grains, when the ‘residual elastic strain energy’ is almost totally released. As can be seen from Fig. 1, the region above the initial layer has three kind of defects: (1) randomly distributed single dislocations, forming at the interface; (2) dislocation boundaries along the c-axis, traversing the crystal from the former interface to the surface. Both types are called as threading dislocations (TD); and (3) dislocations located in the vicinity of the first types of dislocations and parallel to the former interface. A lot of single threading dislocations of the first kind is perpendicular to the former interface, some of them are inclined. These dislocations have generally an edge character. TD inclined to former interface can interact together. This interaction provides a decrease in the full dislocation density in the direction of the crystal growth. A g× b analysis [16] shows that the dislocation boundaries (second kind of dislocations in Fig. 1) consist of dislocations having as well edge type with b= 1/31120 as screw type with b= 0001 and mixed type. The distance between the dislocation boundaries located inside the region of 1–3 mm from the former interface is in the range from 0.5 to 1 mm. A high deformation contrast caused by stress around TD was usually detected in the vicinity of them. When the diffraction vector g was changed from one direction to the opposite, it was found that the crystal is divided by dislocation boundaries into vertical columns, which are slightly tilted one relative to each others. Based on these results, it was assumed that TD were initiated at the interface during the initial stage of the growth. When two or more neighboring grains come into contact, dislocation boundaries oriented along the c-axis,
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are formed to compensate a misorientation of one grain relatively to others in the basal plane. At last, third kind of dislocation in this region, which are parallel to the former interface, have Burgers vector b=1/31120, as evidenced from the g × b analysis. The distance between them ranged from 130 nm near dislocation boundaries to 500 nm near single TD. Interestingly, an oscillation contrast on some dislocations images was observed. This means, that the lines of these dislocations are curved on the (0001) planes and that dislocations parallel to the former interface may have a mixed type. Thus, the character of the dislocation structure near the former interface of bulk GaN crystal grown by HVPE on SiC substrates and released from it by RIE is similar to the one, observed in the same region of the GaN layers grown on sapphire substrate (see for examples [6–8]). The only distinction is the absence of dislocations half-loops, formed by two screw segments along the c-axis and one edge dislocation, which were observed in GaN grown on sapphire [7]. We were not able to observe this last type of defect in our specimens.
3.2. Dislocation structure of GaN platelets in regions remote from the former interface At a distance of about 30 mm from the former interface, the density of TD was found to be essentially lower (at least, by a factor of 10) than that in the interface region (Fig. 4). They were not detected at all
Fig. 4. Cross-sectional bright-field TEM image taken near the [1120] zone axis from a region located ca. 30 mm away from the former interface.
in the TEM specimen prepared from this region. We observed only two kinds of dislocations. They are (1) well resolved rows of parallel dislocations lying on prismatic planes and (2) dislocations lying parallel to the interface, as in the region close to the former interface. It was found that the distance between these last dislocations ( 1 mm) is essentially increased comparably to that in the region near the former interface. At last, in the area at 10 mm from the surface of GaN crystal the distance between dislocations, lying on basal planes, was found to increase up to 1.5 mm. Hence, the distinctive peculiarity of dislocation structure in studied GaN crystals is an existence of dislocations, lying parallel to the former interface. They were found in all regions of the crystal. The distance between these basal dislocations increases from former interface to the crystal surface. As it was noted above, TD are the sources of these basal dislocations. 4. Discussion From our experimental results we assume that stacking faults in the initial layer and all dislocations found in GaN platelets, excepting those which lie on basal planes, are formed during the growth process at high temperature. There are only small residual lattice mismatch stresses and local stress field around TD at the growth temperature. Thermal stress arises from the difference in thermal expansion coefficients (TEC) during post-growth cooling. Although this thermal stress can be more or less compensated by the residual lattice mismatch stress in the GaN SiC heterostructure [17], the resulting stress remain rather high and is mainly defined by the difference in TEC. Hence, the local stress field around TD has to be increased due to the addition of the thermal stress during the cooling process. The relaxation may come through two possible ways: (a) by generation of dislocations lying on (0001) planes and emanating from TD; and (b) by cracks formation when the limit of mechanical rigidity is exceeded near TD having the most powerful stress field. Then it may be assumed that there would be two possible ways in order to minimize cracks formation. First one is to decrease considerably the relaxation degree of lattice mismatch stresses and the difference in TEC to compensate thermal stress by residual lattice mismatch stress. For the GaN/SiC system this can be obtained by use of an AlGaN buffer layer, as discussed earlier [17]. In this case, the most total compensation of thermal stress by the lattice mismatch stress is possible. The second way is to decrease local stress field around TD, i.e. to prevent the formation of dislocation boundaries. This can be obtained by decreasing the misorientation of the GaN grains on the substrate surface at the nucleation stage of the growth, i.e. by increasing the number of nucleus.
I. Nikitina et al. / Materials Science and Engineering B61–62 (1999) 325–329
5. Conclusion A study of the dislocation distribution in GaN wafers grown on SiC-6H by HVPE and free of substrates has been carried out by TEM. It was found that the distinctive peculiarity of the dislocation structure in these bulk GaN crystals is an existence of dislocations lying on basal (0001) planes in all regions across the crystal thickness. Based on our investigations, we conclude that (1) stacking faults in the initial layer, single threading dislocations and dislocation boundaries, are formed at the earlier stage of the growth process at the growth temperature; (2) dislocations lying on basal planes and located near TD are likely formed during post-growth cooling as a result of relaxation of the local stress field near TD and thermal stress.
Acknowledgements This work was supported by EC INTAS Program under Grant c 96-2131. Two of the authors (I.N. and Yu.M.) are grateful for the support from the PICS program of CNRS France.
References [1] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, Berlin,
.
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1997. [2] J. Edmond, G. Bulman, H.S. Kong, M. Leonard, et al., Mater. Sci. Forum 264-268 (1998) 1421. [3] M.S. Shur, M.A. Khan, MRS Bull. 22 (2) (1997) 44. [4] F.A. Ponce, MRS Bull. 22 (2) (1997) 51. [5] S. Nakamura, M. Senoh, S. Nagahama, et al., Appl. Phys. Lett. 72 (1998) 2014. [6] L.T. Romano, B.S. Krusor, R.J. Molnar, Appl. Phys. Lett. 71 (1997) 2283. [7] X.J. Ning, F.R. Chien, P. Pirouz, J.W. Yang, M. Asif Khan, J. Mat. Res. 11 (1996) 580. [8] R.J. Molnar, R. Aggarwal, Z.L. Liau, E.R. Brown, I. Melngailis, W. Gotz, L.T. Romano, N.M. Johnson, MRS Symp. Proc. 395 (1996) 189. [9] Yu. Melnik, A. Nikolaev, I. Nikitina, K. Vassilevski, V. Dmitriev, Mat. Res. Soc. Symp. Proc. 482 (1998) 269. [10] V.E. Sizov, K.V. Vassilevski, NATO ASI Series 3. High Technology, Wide Bandgap Electronic Materials, vol. 1, Kluwer Academic Publishers, Dordrecht, 1995, p. 427. [11] A.A. Rusakov, Rentgenografiya metallov, Atomizdat, Moskva, 1977. [12] Yu. V. Melnik, I.P. Nikitina, A.S. Zubrilov, A.A. Sitnikova, V.A. Dmitriev, Inst. Phys. Conf. Ser. 142 (1996) 863. [13] P. Vennegues, B. Beaumont, P. Gibart, Mater. Sci. Eng. B43 (1997) 274. [14] Z. Sitar, L.L. Smith, R.F. Davis, J. Cryst. Growth 141 (1994) 11. [15] Yu. V. Melnik, I.P. Nikitina, A.S. Zubrilov, A.A. Sitnikova, Y.G. Musikhin, V.A. Dmitriev, Inst. Phys. Conf. 142 (1996) 863. [16] P.B. Hirsch, A. Howie, R.B. Nicholson, D.W. Pashley, M.J. Whelan, Electron Microscopy of Thin Crystals, Butterworth, London, 1967. [17] I.P. Nikitina, M.P. Sheglov, Yu. V. Melnik, K.G. Irvine, V.A. Dmitriev, Diam. Rel. Mater. 6 (1997) 1524.
Dislocation structure of GaN bulk crystals grown on SiC substrates by HVPE I. Nikitina a,*, G. Mosina a, Yu. Melnik b,c, A. Nikolaev a, K. Vassilevski a a
Ioffe Institute, 26 Politechnicheskaya str., St. Petersburg 194021, Russia b Crystal Growth Research Center, St. Petersburg 193036, Russia c TDI, Inc., Gaithersburg, MD 20877, USA
Abstract The dislocation structure of GaN wafers was studied by transmission electron microscopy. These wafers were fabricated using hydride vapor phase epitaxy of thick GaN layers on 6H – SiC substrates and subsequent removal of the substrate by reactive ion etching. Three major types of dislocations were observed in these GaN crystals: (1) threading dislocations (TD), which are mainly parallel to the [0001] direction and extend from the former interface to the crystal surface. They were found to have edge, screw and mixed type; (2) dislocations lying on basal (0001) planes and located near TD through all the thickness of the crystal; (3) dislocations lying preferentially on prismatic planes in the top region of the crystals. Based on experimental results, the influence of initial nucleation stage of heteroepitaxial growth and post-growth cooling on the dislocation distribution in bulk GaN crystals is discussed. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Silicon carbide; Gallium nitride; Transmission electron microscopy; Dislocations
1. Introduction III–V-nitrides are very promising semiconductors for light emitting and microwave devices. Recent developments in thin film epitaxial and device fabrication technology have led to the production at the commercial scale of blue light emitting diodes and to the demonstration of room temperature violet laser diodes (LD) based on InGaN GaN AlGaN heterostructures [1,2]. Fabrication of field effect transistors with improved microwave performance has been also reported [3]. Nitride epitaxial films for these devices were grown on foreign substrates, mainly on sapphire and silicon carbide. The properties of these films are unique and often incomprehensible in the frame of conventional semiconductor physics and technology. In particular, the nitride epitaxial layers with excellent optoelectronic performance have dislocation densities of 1010 cm − 2 [4]. Furthermore, epitaxy on SiC, which has lattice parame-
* Corresponding author. Fax: +7-812-2476425. E-mail address: [email protected] (I. Nikitina)
ters and thermal-expansion coefficient much better fitted to GaN than sapphire, does not lead to improvement in device characteristics. On the other hand, it was recently reported that the use of bulk GaN crystals as a substrate for the fabrication of nitride based LD epitaxial structure give an essential improvement in optoelectrical characteristics of laser diodes [5]. This fact makes more and more interesting to investigate the microstructure of GaN epitaxial films and bulk crystals. The structural characterization of GaN layers grown on sapphire under various growth conditions has been reported by a number of authors [4,6–8]. The most detailed analysis of dislocation structure of GaN layers with a thickness of 7 mm grown on sapphire by MOCVD was conducted by Ning et al. [7]. To the best of our knowledge, a similar study for GaN layers grown on SiC–6H has not been reported up today. There is also no circumstantial report on dislocation distribution in bulk GaN crystals. This paper presents results of microstructure investigation of thick (100 mm) GaN free standing platelets grown by hydride vapor phase epitaxy (HVPE).
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I. Nikitina et al. / Materials Science and Engineering B61–62 (1999) 325–329
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2. Samples preparation and measurement techniques Thick GaN films were grown on 30 mm diameter SiC–6H (0001) substrates using hydride vapor phase epitaxy in a horizontal open flow reactor at T 1050°C without any buffer layer [9]. GaN wafers were then fabricated by selective removal of the initial silicon carbide substrate using reactive ion etching (RIE) [10]. The thickness of the GaN platelets was ca. 100 mm. Hereafter we call the side of the GaN platelet adjacent to the former GaN SiC interface as ‘former interface’, and the opposite side as ‘GaN crystal surface’. To estimate the GaN crystal quality, X-ray diffraction (XRD) measurements were performed using double-crystal spectrometer on (0002) and (1124) CuKa reflections from both sides of bulk crystal. Cross-sectional TEM images were obtained from some portions of the GaN crystal situated at different distances from the former interface.
3. Results The values of full width at half maximum (FWHM) of XRD rocking curves (RC), measured with v- scan geometry on symmetric (0002) and asymmetric (1124) reflections from both sides of the studied GaN wafer are presented in Table 1. As it is seen from these data, (1) GaN bulk crystal has a high crystal quality; (2) the FWHMs of RC(v) measured from the former interface are higher than those measured from the surface; (3) the broadening of RC(v) measured on the asymmetric reflection are comparable with that on the symmetric one. Taking into account the X-ray penetration depth t0002 =8.5 mm [11], it may be concluded that edge, screw and mixed dislocations, are equally responsible for rocking curves broadening at least inside the regions 8.5 mm remote from both surfaces of bulk GaN crystal [7].
3.1. Dislocation structure of GaN platelets near the former interface Cross-section bright-field TEM images taken from a bulk GaN platelet in the region, close to the former interface, are shown in Fig. 1. Two regions having Table 1 FWHM of XRD rocking curves, measured with v- scan geometry Side of GaN crystal
FWHM (arcsec) (0002) CuKa
FWHM (arcsec) (1124) CuKa
Surface Former interface
165 200
170 240
Fig. 1. Cross-sectional bright-field TEM images taken near the [1120] zone axis, for an area in the vicinity of the former interface of GaN platelet:(a) with g =1120 and (b) with g =0002.
different microstructure with sharp boundary between them are clearly seen in these TEM images. First is a highly defected initial layer, located in the bottom parts of both images. This layer is typical for GaN heteroepitaxial growth on SiC [12] and sapphire [13] substrates. Its presence in the GaN free standing platelet confirms the selectivity of the RIE process employed to remove the SiC substrate. The density of defects in this initial layer is much higher than the one in the top part of the region shown in the same figure. Consecutive stacking faults on (0001) plane are observed in TEM bright-field cross-section images obtained from the initial layer at
I. Nikitina et al. / Materials Science and Engineering B61–62 (1999) 325–329
higher magnification. This is shown in Fig. 2. The average distance between the stacking faults is about 10 nm. Dislocation boundaries, which lie along [0001] direction were also detected here. Similar structure of initial layer in 15 mm GaN crystal grown on sapphire by HVPE was reported by Romano et al. [6]. TEM cross-sectional image taken from the region near the former interface clearly showed the layer, having a thickness about 50 nm. It consists from some small grains, growing preferentially in the [0001] direction (Fig. 3). Reflections (0002) GaN from the grains were detected in a microdiffraction pattern, obtained from this region (shown in the inset of the figure). These grains were identified as nucleation islands arising at the initial stage of the GaN growth on the SiC substrate. This fact is in an agreement with the conclusions drawn by Sitar et al. [14], that the growth of GaN on SiC has a three-dimensional mode. The formation of stacking faults was assumed to occur as a result of the mutual accommodation of GaN and SiC lattices inside
Fig. 2. Cross-sectional bright-field TEM images taken from the region near initial layer of a GaN platelet: (a) with g = 0002 and (b) with g= 1010
327
Fig. 3. Cross-sectional bright-field TEM image taken from an area located ca. 100 nm near the former interface of GaN platelet. Inset shows microdiffraction pattern from this region.
every grain. It is necessary to emphasise that the abrupt boundary between the initial layer and the top part of the GaN crystal is usually observed in thin GaN layers grown directly on foreign substrates [6,8,15]. Probably, such an abrupt boundary is defined by the so-called ‘secondary critical size’ [7] of grains, when the ‘residual elastic strain energy’ is almost totally released. As can be seen from Fig. 1, the region above the initial layer has three kind of defects: (1) randomly distributed single dislocations, forming at the interface; (2) dislocation boundaries along the c-axis, traversing the crystal from the former interface to the surface. Both types are called as threading dislocations (TD); and (3) dislocations located in the vicinity of the first types of dislocations and parallel to the former interface. A lot of single threading dislocations of the first kind is perpendicular to the former interface, some of them are inclined. These dislocations have generally an edge character. TD inclined to former interface can interact together. This interaction provides a decrease in the full dislocation density in the direction of the crystal growth. A g× b analysis [16] shows that the dislocation boundaries (second kind of dislocations in Fig. 1) consist of dislocations having as well edge type with b= 1/31120 as screw type with b= 0001 and mixed type. The distance between the dislocation boundaries located inside the region of 1–3 mm from the former interface is in the range from 0.5 to 1 mm. A high deformation contrast caused by stress around TD was usually detected in the vicinity of them. When the diffraction vector g was changed from one direction to the opposite, it was found that the crystal is divided by dislocation boundaries into vertical columns, which are slightly tilted one relative to each others. Based on these results, it was assumed that TD were initiated at the interface during the initial stage of the growth. When two or more neighboring grains come into contact, dislocation boundaries oriented along the c-axis,
328
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are formed to compensate a misorientation of one grain relatively to others in the basal plane. At last, third kind of dislocation in this region, which are parallel to the former interface, have Burgers vector b=1/31120, as evidenced from the g × b analysis. The distance between them ranged from 130 nm near dislocation boundaries to 500 nm near single TD. Interestingly, an oscillation contrast on some dislocations images was observed. This means, that the lines of these dislocations are curved on the (0001) planes and that dislocations parallel to the former interface may have a mixed type. Thus, the character of the dislocation structure near the former interface of bulk GaN crystal grown by HVPE on SiC substrates and released from it by RIE is similar to the one, observed in the same region of the GaN layers grown on sapphire substrate (see for examples [6–8]). The only distinction is the absence of dislocations half-loops, formed by two screw segments along the c-axis and one edge dislocation, which were observed in GaN grown on sapphire [7]. We were not able to observe this last type of defect in our specimens.
3.2. Dislocation structure of GaN platelets in regions remote from the former interface At a distance of about 30 mm from the former interface, the density of TD was found to be essentially lower (at least, by a factor of 10) than that in the interface region (Fig. 4). They were not detected at all
Fig. 4. Cross-sectional bright-field TEM image taken near the [1120] zone axis from a region located ca. 30 mm away from the former interface.
in the TEM specimen prepared from this region. We observed only two kinds of dislocations. They are (1) well resolved rows of parallel dislocations lying on prismatic planes and (2) dislocations lying parallel to the interface, as in the region close to the former interface. It was found that the distance between these last dislocations ( 1 mm) is essentially increased comparably to that in the region near the former interface. At last, in the area at 10 mm from the surface of GaN crystal the distance between dislocations, lying on basal planes, was found to increase up to 1.5 mm. Hence, the distinctive peculiarity of dislocation structure in studied GaN crystals is an existence of dislocations, lying parallel to the former interface. They were found in all regions of the crystal. The distance between these basal dislocations increases from former interface to the crystal surface. As it was noted above, TD are the sources of these basal dislocations. 4. Discussion From our experimental results we assume that stacking faults in the initial layer and all dislocations found in GaN platelets, excepting those which lie on basal planes, are formed during the growth process at high temperature. There are only small residual lattice mismatch stresses and local stress field around TD at the growth temperature. Thermal stress arises from the difference in thermal expansion coefficients (TEC) during post-growth cooling. Although this thermal stress can be more or less compensated by the residual lattice mismatch stress in the GaN SiC heterostructure [17], the resulting stress remain rather high and is mainly defined by the difference in TEC. Hence, the local stress field around TD has to be increased due to the addition of the thermal stress during the cooling process. The relaxation may come through two possible ways: (a) by generation of dislocations lying on (0001) planes and emanating from TD; and (b) by cracks formation when the limit of mechanical rigidity is exceeded near TD having the most powerful stress field. Then it may be assumed that there would be two possible ways in order to minimize cracks formation. First one is to decrease considerably the relaxation degree of lattice mismatch stresses and the difference in TEC to compensate thermal stress by residual lattice mismatch stress. For the GaN/SiC system this can be obtained by use of an AlGaN buffer layer, as discussed earlier [17]. In this case, the most total compensation of thermal stress by the lattice mismatch stress is possible. The second way is to decrease local stress field around TD, i.e. to prevent the formation of dislocation boundaries. This can be obtained by decreasing the misorientation of the GaN grains on the substrate surface at the nucleation stage of the growth, i.e. by increasing the number of nucleus.
I. Nikitina et al. / Materials Science and Engineering B61–62 (1999) 325–329
5. Conclusion A study of the dislocation distribution in GaN wafers grown on SiC-6H by HVPE and free of substrates has been carried out by TEM. It was found that the distinctive peculiarity of the dislocation structure in these bulk GaN crystals is an existence of dislocations lying on basal (0001) planes in all regions across the crystal thickness. Based on our investigations, we conclude that (1) stacking faults in the initial layer, single threading dislocations and dislocation boundaries, are formed at the earlier stage of the growth process at the growth temperature; (2) dislocations lying on basal planes and located near TD are likely formed during post-growth cooling as a result of relaxation of the local stress field near TD and thermal stress.
Acknowledgements This work was supported by EC INTAS Program under Grant c 96-2131. Two of the authors (I.N. and Yu.M.) are grateful for the support from the PICS program of CNRS France.
References [1] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, Berlin,
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