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Structural defects in GaN crystals grown by HVPE on needle-shaped GaN seeds obtained under high N2 pressure
ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 1407–1410
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Structural defects in GaN crystals grown by HVPE on needle-shaped GaN seeds obtained under high N2 pressure J. Smalc-Koziorowska a,b,, G. Kamler b, B. Łucznik b, I. Grzegory b a b
Warsaw University of Technology, Department of Material Science, Wo!oska 141, 02-792 Warsaw, Poland Institute of High Pressure Physics, Polish Academy of Science, Sokolowska 29/37, 01-142 Warsaw, Poland
a r t i c l e in fo
abstract
Article history: Received 13 August 2008 Received in revised form 3 December 2008 Accepted 4 December 2008 Communicated by R. M. Biefeld Available online 16 December 2008
A bulk crystal of GaN grown by double hydride vapor phase epitaxy (HVPE) on a needle-shaped GaN single-crystalline seed, obtained by the high nitrogen pressure solution (HNPS) method, was studied by transmission electron microscopy (TEM). Structural defects at characteristic places, such as the seed–HVPE GaN interface, the first HVPE/second HVPE interface, and at morphological imperfections of the seed were characterized. The TEM studies have shown that despite high structural quality of the pressure-grown seed, structural defects are present in the HVPE-grown GaN crystal. A possible reason for strain and structural defects generation could be small lattice mismatch between highly oxygendoped seed crystal and the HVPE GaN layer as well as between particular sectors of the HVPE GaN layer growing in different crystallographic directions. It was also observed that morphological imperfections of the seed surface may lead to the introduction of the high number of defects extending into the overgrown layer. & 2008 Elsevier B.V. All rights reserved.
PACS: 68.37.Lp 81.05.Ea 81.15.Kk Keywords: A1. Transmission electron microscopy A3. Hydride vapor phase epitaxy B1.Gallium nitride
1. Introduction Recently, the field of bulk crystallization of GaN has developed very rapidly because of a great need for GaN single-crystalline substrates for epitaxy of AIIIN heterostructures. There are numerous methods considered as promising for the production of large (at least 2 in), relatively non-expensive and high-quality GaN wafers. These methods are hydride vapor phase epitaxy (HVPE) [1], ammonothermal solution growth [2], high nitrogen pressure solution (HNPS) [3] and Na-based flux growth [4]. At present, HVPE is the most advanced GaN crystallization method in terms of obtaining large-diameter single-crystalline free-standing substrates. There are two approaches to the fabrication of such GaN wafers: epitaxial growth on foreign substrates and bulk crystallization. Epitaxial growth on foreign substrates includes deposition of a few hundreds of micrometersthick GaN layer on GaAs or sapphire and separation of the substrate by chemical etching (GaAs), or by methods based on engineering of the strain in the GaN–sapphire system. Usually the initial substrates are patterned in a special way in order to Corresponding author at: Warsaw University of Technology, Department of Material Science, Wo"oska 141, 02-792 Warsaw, Poland. Tel.: +48 22 8880068; fax: +48 22 6324218. E-mail address: [email protected] (J. Smalc-Koziorowska).
0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.12.018
enforce lateral overgrowth mechanisms, allowing significant reduction of dislocation density in the final GaN wafer. The highest quality GaN substrates in terms of dislocation density and performance of lasers are those obtained by the advanced dislocation elimination by the epitaxial growth with inversepyramidal pits (A-DEEP) procedure developed in Sumitomo Electric Industries (Japan) [1]. The substrates contain 400-mmwide stripes of very low dislocation density GaN (o105 cm 2) separated by dislocated areas of GaN with inversed polarity. Good-quality (dislocation density 106–107 cm 2) free-standing GaN substrates based on epitaxial growth on sapphire are offered and/or reported by some companies and academic institutions [1,5]. An alternative approach, i.e. bulk growth with subsequent slicing and polishing of the resulting crystal, is technically more difficult because of parasitic deposition during long HVPE growth runs. Nevertheless there are reports on the successful growth of 2 in GaN boules with thicknesses up to 10 mm [5]. The ‘‘bulk approach’’ has an additional advantage that the crystal can be sliced in various directions. Thus, substrates of required orientations including non-polar and semi-polar ones can be obtained. In this work, the crystal grown by double hydride vapor phase epitaxy on a small needle-shaped GaN single-crystalline seed obtained by the high nitrogen pressure solution [6] method is characterized.
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2. Experimental procedure 2.1. Sample The GaN crystal used as a seed for HVPE bulk growth was obtained by the HNPS method [3], which results in crystals of very high structural quality (in terms of X-ray diffraction and defectselective etching (DSE) results), but of a size limited to approximately 1 cm. The needle-shaped crystals are grown at relatively
Fig. 1. (a) Photograph of high-pressure-grown GaN seed crystal and (b) schematic illustration of seeded growth by HVPE.
high supersaturations therefore they are often morphologically unstable (i.e. hollow inside). However, they have well-developed non-polar {1 0 1¯ 0} faces that can be used for seeding of HVPE crystallization. The needle-shaped HPNS-grown crystals are strongly n-type with free electron concentration of about 1020 cm 3 so their lattice constants are larger than in undoped material resulting in a lattice mismatch of about 0.01%. Fig. 1a shows the pressure-grown seed crystal and Fig. 1b shows schematically the HVPE crystallization on such a seed. The HVPE system used for growth of the sample [6] analyzed in this study was a horizontal home-built quartz reactor with a rotating quartz susceptor. GaCl was supplied vertically, just over the surface of the susceptor. The growth temperature was 1050 1C and the temperature of GaCl synthesis was 870 1C, with an HCl flow of 24 mls/min diluted with 500 mls/min of N2. A NH3 flow of 1200 and 3000 mls/min of N2 as a carrier gas were applied for two subsequent runs of 10 h. The rate of growth in /1 0 1¯ 0S directions was about 100 mm/h, similar to that in the (0 0 0 1) direction with the same growth conditions. The resulting crystal is shown in Fig. 2a. It’s hexagonal (0 0 0 1) cross-section (Fig. 2b) reveals the interface between the first and second HVPE runs (HVPE1/HVPE2 interface). It was shown by micro-Raman measurements [7] that the free electron concentration for the crystal grown by HVPE on the needleshaped seed is about 5 1018 cm 3 and one order of magnitude higher just at the HVPE1/HVPE2 interface. Crystallites visible on the side walls of the regular prismatic crystal (Fig. 2a) here called ‘‘parasitic grains’’—PGs originate from inside of the crystal as suggested by Fig. 2c where the non-polar (11 2¯ 0) cross-section is shown.
2.2. Sample preparation and TEM measurements
Fig. 2. GaN crystal grown on a needle-shaped GaN seed in two subsequent 10 h HVPE runs. (a) General view, dashed lines indicating the planes along which the cross-section platelets were cut; (b) hexagonal (0 0 0 1) cross-section of the crystal; c) non-polar (11 2¯ 0) cross-section of the crystal, the dark part is the seed, the bright part is the newly grown HVPE crystal. Boxed areas x, y and z were investigated by TEM. x is at the HPNS/HVPE1 interface between the HNPS GaN crystal and the first HVPE GaN layer (growth surface (1 0 2¯ 0)), y is at the HVPE1/ HVPE2/PG interface between the first HVPE GaN layer, the second HVPE GaN layer and a parasitic grain, z is an area of growth in a direction different from [1 0 1¯ 0], i.e. ‘‘semi-polar growth’’.
The non-polar (1 0 1¯ 0) cross-section shown in Fig. 2c was used for the preparation of transmission electron microscopy (TEM) specimens. Characteristic areas of the sample were chosen (Fig. 3) to study the behavior of the system at the HPNS/HVPE1 interface between the HPNS seed crystal and the HVPE GaN grown in the first run (areas x—area of non-polar growth and z—area of semipolar growth in Fig. 2c). Also the area at the HVPE1/HVPE2/PG interface between the first HVPE growth, the second HVPE growth and the parasitic grain (area y in Fig. 2c) was analyzed. Cross-section specimens oriented in [11¯ 0 0] and [11 2¯ 0] wurtzite zone axes were prepared by mechanical polishing followed by dimpling and Ar ion milling until perforation occurred. Observations were performed by conventional and high-resolution transmission electron microscopy (HRTEM) using a JEOL JEM3010 microscope operating at 300 kV equipped with a Gatan CCD camera. HRTEM images were filtered using Digital Micrograph software.
Fig. 3. Cross-section images of the interface between the HP needle-shaped GaN and the first HVPE GaN layer. (a) Image taken with gGaN=0002 near the [11 2¯ 0] zone axis and (b) HRTEM image viewed along the [11 2¯ 0] GaN zone axis, the HPNS/HVPE1 interface is indicated with dashed line.
ARTICLE IN PRESS J. Smalc-Koziorowska et al. / Journal of Crystal Growth 311 (2009) 1407–1410
3. Results To study the structural quality of the HPNS/HVPE1 interface, the cross-section specimens oriented in [11¯ 0 0] and [11 2¯ 0] GaN zone axes were prepared. The image taken with gGaN=0002 near the [11 2¯ 0] zone axis (Fig. 3a) shows the interface as a dark line. The high-resolution TEM image (Fig. 3b) shows the HPNS/HVPE1 interface, where matching between (0 0 0 2) planes in HPNSgrown GaN needle and (0 0 0 2) planes in first HVPE run GaN is visible. Matching lattice fringes at this interface indicate that very few misfit dislocations are present. However, a large number of dislocation loops lying in the (0 0 0 1) basal plane were observed in the HVPE region at a distance of about 1 mm from the interface line with estimated density in the range 5 108 cm 2. Additionally, planar defects such as basal stacking faults (BSFs) of I1 type (Fig. 4c) and prismatic stacking faults (PSFs) exist in different areas of the HVPE-grown crystal. We can distinguish isolated PSFs close to the HPNS/HVPE1 interface, and in the area of semi-polar growth (area z of Fig. 2c). Also observed are PSFs terminating with BSFs, located around 1 mm and further from the HPNS/HVPE1 interface line. In order to investigate the defect distribution at the HVPE1/HVPE2/PG interface between the first and second HVPE growths and the parasitic grain, a cross-section sample oriented along [11 2¯ 0] was prepared from the area labeled as y in Fig. 2c. Since there is no mismatch between the first and second HVPE layers, the HVPE1/HVPE2 interface between these layers was not visible by TEM. However, as already mentioned, the HVPE1/HVPE2 interface is visible in optical microscopy (Figs. 2b,c and 6a) since the electron concentration at interfaces between HVPE layers is different than inside HVPE layers. The defects observed in the area close to the HVPE1/HVPE2/PG interface are mainly BSFs of I1 type and a small number of dislocation loops lying in the (0 0 0 1) basal plane. Most BSFs propagate through this interface and have a
Fig. 4. Images of defects observed in the investigated crystal found in areas x, y and z indicated on Fig. 2c. Bright field images taken with gGaN=11¯00 near the [11 2¯ 0] zone axis show the distribution of BSFs: (a) in the area close to the parasitic grain—area y; (b) in the area 100 mm from HVPE1/HVPE2/PG interface—area y; (c) HRTEM image viewed along [11 2¯0] of an I1 type BSF; (d) bright field image taken with gGaN=11 2¯ 0 near the [0 0 0 1] zone axis of the PSF created near the HPNS/HVPE1 interface—area z; (e) dark field image taken with gGaN=112¯0 near the [11¯ 0 0] zone axis showing the distribution of dislocation loops in the area z.
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density around 2.8 105 cm 1 in the area close to the parasitic grain (Fig. 4a). The measured density of BSFs is reduced in the second HVPE layer (Fig. 4b) at a distance of around 100 mm from the parasitic grain, at a value of 8 104 cm 1. The region of the parasitic grain is highly defected and the measured dislocation density is in the range of 109 cm 2. Selected area diffraction patterns (not shown here) taken from the interface between the HVPE layers and the parasitic grain show that the grain is misoriented about 301 with respect to the orientation of the HVPE layers. The area of semi-polar growth (area z in Fig. 2c) was investigated by cross-section specimens oriented along [11¯ 0 0] and [0 0 0 1] GaN directions. Close to the HP GaN needle, isolated PSFs (Fig. 4d) and a high number of dislocation loops lying in the (0 0 0 1) plane (Fig. 4e) with a density of 4 109 cm 2 were observed.
4. Discussion Both specification and distribution of structural defects deduced from the experimental results described in the previous section are schematically illustrated in Fig. 5. The TEM analysis has shown that despite high structural quality of the seed crystal, a number of lattice defects are present in the HVPE-grown part of the sample. There are both basal and prismatic stacking faults as well as dislocation loops at concentrations depending on the area of the crystal. One can distinguish a few possible reasons for generation and relaxation of strain leading to the formation of structural defects in the investigated system. As was mentioned earlier, the needle-like seeds grown under pressure are strongly n-type crystals with free electron concentrations approaching 1020 cm 3 due to high oxygen content incorporated during the high-pressure growth. On the other hand, the electron concentration in GaN obtained by the HVPE method depends strongly on the orientation of the crystallization front during the HVPE growth. It was shown that for growth in the Ga-polar [0 0 0 1] direction, typical electron concentration is about 1017 cm 3 or lower, whereas for the growth in the opposite N-polar [0 0 0 1¯] direction it can be more than two orders of magnitude higher, and usually exceeds 1019 cm 3 [7]. This very high electron concentration was also observed for growth on semi-polar (inclined) {1 0 1¯ 1} faces of GaN. As already mentioned in Section 2.1, for crystallization on the non-polar {1 0 1¯ 0} faces dominating the sample studied in this work, the free electron concentrations in the range 3–5 1018 cm 3 were measured [8] by the micro-Raman scattering technique. Additionally, for a multiple HVPE growth a sharp increase in electron concentration at regions corresponding to the beginning of the subsequent HVPE run has been found. This strong anisotropy in physical properties of the HVPEgrown GaN crystals is due to a different efficiency of oxygen incorporation on particular crystallographic faces of growing crystals.
Fig. 5. Schematic illustration of defects distribution in the crystal.
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Fig. 6. Images of (0 0 0 1) cross-section of four times HVPE-grown GaN crystal on HP GaN seed. (a) Photograph of the cut platelet, HP seed crystal (area of dark contrast) and interfaces between subsequent HVPE growths are indicated; (b) optical microscopy image of the same platelet as in image (a) after etching in molten bases. Feature 1 shows the seed crystal, features 2 and 3 are areas with etch pits spreading from morphological imperfections of the seed, features 4 and 5 show low defect density parts of the HVPE bulk GaN and features 6 and 7 are parasitic, strongly defected grains; (c) scanning electron microscopy image of the boxed area on the image (b).
It is well established [9] that a high number of free electrons in GaN leads to a slight expansion of the crystal lattice. The mismatch between an undoped GaN crystal (therefore semiinsulating) and a crystal containing 5 1019 cm 3 of free electrons is of the order of 0.01% for both a and c lattice constants of the wurtzite structure. So for the investigated system, in the case of bulk growth one can expect strain generation at the interface between the seed crystal and the HVPE-grown GaN, as well as in the areas where the crystal sectors growing in different directions, thus having different electron concentrations, meet. The latter situation can take place at the apex of the considered needleshaped seed where the growth proceeds in various directions and where a significant number of structural defects have been found. Another possible reason for the deterioration of the crystal quality is related with morphological imperfections of the seed. If a side face of the seed is not flat, but contains surfaces of orientations different from {1 0 1¯ 0}, the start of growth can be again accompanied by the formation of domains with large relative variations in free electron concentration, which causes strain in the crystal. This situation is illustrated in Fig. 6 where a polar (0 0 0 1) slice of the crystal similar to the one investigated by TEM is shown. The (0 0 0 1) Ga-polar surface of this slice was subjected to a defect-selective etching procedure [10] in molten KOH–NaOH eutectics to reveal structural defects. The crystal was overgrown with four separate HVPE growth cycles. The DSE result indicates that the seed crystal is almost defect-free (no etch pits), as seen in area 1 in Fig. 6b. The etch pits spread mostly from places of strong deviation from the (1 0 1¯ 0) orientation of the side faces of the seed, as seen in areas 2 and 4 in Fig. 6b and c. The strain induced at the seed is so high that its relaxation requires the formation of parasitic, strongly defected grains, indicated in areas 6 and 7 of Fig. 6b. The highest quality material containing BSFs at density lower than 105 cm 1 and no dislocations, has been found in the second HVPE layer in the area for which the HVPE growth started from flat (1 0 1¯ 0) face of the seed. It seems therefore that for crystallization of bulk GaN by the HVPE method seeded with GaN single crystals elongated in /0 0 0 1S directions, the crucial is to avoid abrupt changes of free electron concentration in the growth system. It is difficult for small seeds in general, if an increase of the crystal size in three dimensions corresponding to the growth in different directions is expected. For prismatic seeds like the one considered in this study, the growth in the non-polar /1 0 1¯ 0S directions leading to an increase of diameter of the needle is most suitable for obtaining good-quality crystal with uniform physical properties.
5. Summary The TEM studies of bulk GaN crystal grown by the HVPE method on the needle-shaped GaN single-crystalline seed have shown that despite high structural quality of the pressure-grown seed, structural defects are present in the HVPE-grown GaN crystal. A possible reason for strain and structural defects generation could be small lattice mismatch between highly oxygen-doped seed crystal and the HVPE GaN layer as well as between particular sectors of the HVPE GaN layer growing in different crystallographic directions. It seems, however, that goodquality GaN bulk crystal containing BSFs at density lower than 105 cm 1 and without dislocations can be obtained by the HVPE growth on the needle-like prismatic seed for the growth in the non-polar /1 0 1¯ 0S directions leading to an increase of lateral size of the needle. The growth leading to an elongation of the needle (in the polar and semi-polar directions) should be suppressed to avoid the strains caused by the different concentrations of free electrons in neighboring sectors. Obviously, the multiple growth should be replaced by a single, long duration run to avoid inhomogeneities related to the beginning of the HVPE crystallization process.
Acknowledgement We would like to acknowledge Dr Suman-Lata Sahonta for fruitful discussion of the results obtained in this work. References [1] K. Motoki, et al., J. Crystal Growth 237–239 (2002) 912. [2] A. Yoshikawa, E. Oshima, T. Fukuda, H. Tsuji, K. Oshima, J. Crystal Growth 260 (2004) 67. [3] I. Grzegory, M. Boc´kowski, S. Porowski, Optical and optoelectronic materials, in: P. Capper (Ed.), Bulk Crystal Growth of Electronic, Wiley New York, 2005, pp. 173–207, Chapter 6. [4] H. Yamane, M. Shimada, T. Sekiguchi, F.J. Di Salvo, J. Crystal Growth 186 (1998) 8. [5] D. Hanser, L. Liu, E.A. Preble, D. Thomas, M. Williams, Mat. Res. Soc. Symp. Proc. 798 (2004) Y2.1.1. [6] B. Łucznik, B. Pastuszka, I. Grzegory, M. Boc´kowski, G. Kamler, E. LitwinStaszewska, S. Porowski, J. Crystal Growth 281 (2005) 38. [7] J.L. Weyher, R. Lewandowska, L. Macht, B. Łucznik, I. Grzegory, Mater. Sci. Semicond. Process. 9 (2006) 175. [8] R. Lewandowska, unpublished results. [9] M. Krys´ko, M. Sarzyn´ski, J. Domaga"a, I. Grzegory, B. Łucznik, G. Kamler, S. Porowski, M. Leszczyn´ski, J. Alloys Compds. 401 (2005) 261. [10] J.L. Weyher, P.D. Brown, J.-L. Rouviere, T. Wosinski, A.R.A. Zauner, I. Grzegory, J. Crystal Growth 210 (1–3) (2000) 151.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Structural defects in GaN crystals grown by HVPE on needle-shaped GaN seeds obtained under high N2 pressure J. Smalc-Koziorowska a,b,, G. Kamler b, B. Łucznik b, I. Grzegory b a b
Warsaw University of Technology, Department of Material Science, Wo!oska 141, 02-792 Warsaw, Poland Institute of High Pressure Physics, Polish Academy of Science, Sokolowska 29/37, 01-142 Warsaw, Poland
a r t i c l e in fo
abstract
Article history: Received 13 August 2008 Received in revised form 3 December 2008 Accepted 4 December 2008 Communicated by R. M. Biefeld Available online 16 December 2008
A bulk crystal of GaN grown by double hydride vapor phase epitaxy (HVPE) on a needle-shaped GaN single-crystalline seed, obtained by the high nitrogen pressure solution (HNPS) method, was studied by transmission electron microscopy (TEM). Structural defects at characteristic places, such as the seed–HVPE GaN interface, the first HVPE/second HVPE interface, and at morphological imperfections of the seed were characterized. The TEM studies have shown that despite high structural quality of the pressure-grown seed, structural defects are present in the HVPE-grown GaN crystal. A possible reason for strain and structural defects generation could be small lattice mismatch between highly oxygendoped seed crystal and the HVPE GaN layer as well as between particular sectors of the HVPE GaN layer growing in different crystallographic directions. It was also observed that morphological imperfections of the seed surface may lead to the introduction of the high number of defects extending into the overgrown layer. & 2008 Elsevier B.V. All rights reserved.
PACS: 68.37.Lp 81.05.Ea 81.15.Kk Keywords: A1. Transmission electron microscopy A3. Hydride vapor phase epitaxy B1.Gallium nitride
1. Introduction Recently, the field of bulk crystallization of GaN has developed very rapidly because of a great need for GaN single-crystalline substrates for epitaxy of AIIIN heterostructures. There are numerous methods considered as promising for the production of large (at least 2 in), relatively non-expensive and high-quality GaN wafers. These methods are hydride vapor phase epitaxy (HVPE) [1], ammonothermal solution growth [2], high nitrogen pressure solution (HNPS) [3] and Na-based flux growth [4]. At present, HVPE is the most advanced GaN crystallization method in terms of obtaining large-diameter single-crystalline free-standing substrates. There are two approaches to the fabrication of such GaN wafers: epitaxial growth on foreign substrates and bulk crystallization. Epitaxial growth on foreign substrates includes deposition of a few hundreds of micrometersthick GaN layer on GaAs or sapphire and separation of the substrate by chemical etching (GaAs), or by methods based on engineering of the strain in the GaN–sapphire system. Usually the initial substrates are patterned in a special way in order to Corresponding author at: Warsaw University of Technology, Department of Material Science, Wo"oska 141, 02-792 Warsaw, Poland. Tel.: +48 22 8880068; fax: +48 22 6324218. E-mail address: [email protected] (J. Smalc-Koziorowska).
0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.12.018
enforce lateral overgrowth mechanisms, allowing significant reduction of dislocation density in the final GaN wafer. The highest quality GaN substrates in terms of dislocation density and performance of lasers are those obtained by the advanced dislocation elimination by the epitaxial growth with inversepyramidal pits (A-DEEP) procedure developed in Sumitomo Electric Industries (Japan) [1]. The substrates contain 400-mmwide stripes of very low dislocation density GaN (o105 cm 2) separated by dislocated areas of GaN with inversed polarity. Good-quality (dislocation density 106–107 cm 2) free-standing GaN substrates based on epitaxial growth on sapphire are offered and/or reported by some companies and academic institutions [1,5]. An alternative approach, i.e. bulk growth with subsequent slicing and polishing of the resulting crystal, is technically more difficult because of parasitic deposition during long HVPE growth runs. Nevertheless there are reports on the successful growth of 2 in GaN boules with thicknesses up to 10 mm [5]. The ‘‘bulk approach’’ has an additional advantage that the crystal can be sliced in various directions. Thus, substrates of required orientations including non-polar and semi-polar ones can be obtained. In this work, the crystal grown by double hydride vapor phase epitaxy on a small needle-shaped GaN single-crystalline seed obtained by the high nitrogen pressure solution [6] method is characterized.
ARTICLE IN PRESS 1408
J. Smalc-Koziorowska et al. / Journal of Crystal Growth 311 (2009) 1407–1410
2. Experimental procedure 2.1. Sample The GaN crystal used as a seed for HVPE bulk growth was obtained by the HNPS method [3], which results in crystals of very high structural quality (in terms of X-ray diffraction and defectselective etching (DSE) results), but of a size limited to approximately 1 cm. The needle-shaped crystals are grown at relatively
Fig. 1. (a) Photograph of high-pressure-grown GaN seed crystal and (b) schematic illustration of seeded growth by HVPE.
high supersaturations therefore they are often morphologically unstable (i.e. hollow inside). However, they have well-developed non-polar {1 0 1¯ 0} faces that can be used for seeding of HVPE crystallization. The needle-shaped HPNS-grown crystals are strongly n-type with free electron concentration of about 1020 cm 3 so their lattice constants are larger than in undoped material resulting in a lattice mismatch of about 0.01%. Fig. 1a shows the pressure-grown seed crystal and Fig. 1b shows schematically the HVPE crystallization on such a seed. The HVPE system used for growth of the sample [6] analyzed in this study was a horizontal home-built quartz reactor with a rotating quartz susceptor. GaCl was supplied vertically, just over the surface of the susceptor. The growth temperature was 1050 1C and the temperature of GaCl synthesis was 870 1C, with an HCl flow of 24 mls/min diluted with 500 mls/min of N2. A NH3 flow of 1200 and 3000 mls/min of N2 as a carrier gas were applied for two subsequent runs of 10 h. The rate of growth in /1 0 1¯ 0S directions was about 100 mm/h, similar to that in the (0 0 0 1) direction with the same growth conditions. The resulting crystal is shown in Fig. 2a. It’s hexagonal (0 0 0 1) cross-section (Fig. 2b) reveals the interface between the first and second HVPE runs (HVPE1/HVPE2 interface). It was shown by micro-Raman measurements [7] that the free electron concentration for the crystal grown by HVPE on the needleshaped seed is about 5 1018 cm 3 and one order of magnitude higher just at the HVPE1/HVPE2 interface. Crystallites visible on the side walls of the regular prismatic crystal (Fig. 2a) here called ‘‘parasitic grains’’—PGs originate from inside of the crystal as suggested by Fig. 2c where the non-polar (11 2¯ 0) cross-section is shown.
2.2. Sample preparation and TEM measurements
Fig. 2. GaN crystal grown on a needle-shaped GaN seed in two subsequent 10 h HVPE runs. (a) General view, dashed lines indicating the planes along which the cross-section platelets were cut; (b) hexagonal (0 0 0 1) cross-section of the crystal; c) non-polar (11 2¯ 0) cross-section of the crystal, the dark part is the seed, the bright part is the newly grown HVPE crystal. Boxed areas x, y and z were investigated by TEM. x is at the HPNS/HVPE1 interface between the HNPS GaN crystal and the first HVPE GaN layer (growth surface (1 0 2¯ 0)), y is at the HVPE1/ HVPE2/PG interface between the first HVPE GaN layer, the second HVPE GaN layer and a parasitic grain, z is an area of growth in a direction different from [1 0 1¯ 0], i.e. ‘‘semi-polar growth’’.
The non-polar (1 0 1¯ 0) cross-section shown in Fig. 2c was used for the preparation of transmission electron microscopy (TEM) specimens. Characteristic areas of the sample were chosen (Fig. 3) to study the behavior of the system at the HPNS/HVPE1 interface between the HPNS seed crystal and the HVPE GaN grown in the first run (areas x—area of non-polar growth and z—area of semipolar growth in Fig. 2c). Also the area at the HVPE1/HVPE2/PG interface between the first HVPE growth, the second HVPE growth and the parasitic grain (area y in Fig. 2c) was analyzed. Cross-section specimens oriented in [11¯ 0 0] and [11 2¯ 0] wurtzite zone axes were prepared by mechanical polishing followed by dimpling and Ar ion milling until perforation occurred. Observations were performed by conventional and high-resolution transmission electron microscopy (HRTEM) using a JEOL JEM3010 microscope operating at 300 kV equipped with a Gatan CCD camera. HRTEM images were filtered using Digital Micrograph software.
Fig. 3. Cross-section images of the interface between the HP needle-shaped GaN and the first HVPE GaN layer. (a) Image taken with gGaN=0002 near the [11 2¯ 0] zone axis and (b) HRTEM image viewed along the [11 2¯ 0] GaN zone axis, the HPNS/HVPE1 interface is indicated with dashed line.
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3. Results To study the structural quality of the HPNS/HVPE1 interface, the cross-section specimens oriented in [11¯ 0 0] and [11 2¯ 0] GaN zone axes were prepared. The image taken with gGaN=0002 near the [11 2¯ 0] zone axis (Fig. 3a) shows the interface as a dark line. The high-resolution TEM image (Fig. 3b) shows the HPNS/HVPE1 interface, where matching between (0 0 0 2) planes in HPNSgrown GaN needle and (0 0 0 2) planes in first HVPE run GaN is visible. Matching lattice fringes at this interface indicate that very few misfit dislocations are present. However, a large number of dislocation loops lying in the (0 0 0 1) basal plane were observed in the HVPE region at a distance of about 1 mm from the interface line with estimated density in the range 5 108 cm 2. Additionally, planar defects such as basal stacking faults (BSFs) of I1 type (Fig. 4c) and prismatic stacking faults (PSFs) exist in different areas of the HVPE-grown crystal. We can distinguish isolated PSFs close to the HPNS/HVPE1 interface, and in the area of semi-polar growth (area z of Fig. 2c). Also observed are PSFs terminating with BSFs, located around 1 mm and further from the HPNS/HVPE1 interface line. In order to investigate the defect distribution at the HVPE1/HVPE2/PG interface between the first and second HVPE growths and the parasitic grain, a cross-section sample oriented along [11 2¯ 0] was prepared from the area labeled as y in Fig. 2c. Since there is no mismatch between the first and second HVPE layers, the HVPE1/HVPE2 interface between these layers was not visible by TEM. However, as already mentioned, the HVPE1/HVPE2 interface is visible in optical microscopy (Figs. 2b,c and 6a) since the electron concentration at interfaces between HVPE layers is different than inside HVPE layers. The defects observed in the area close to the HVPE1/HVPE2/PG interface are mainly BSFs of I1 type and a small number of dislocation loops lying in the (0 0 0 1) basal plane. Most BSFs propagate through this interface and have a
Fig. 4. Images of defects observed in the investigated crystal found in areas x, y and z indicated on Fig. 2c. Bright field images taken with gGaN=11¯00 near the [11 2¯ 0] zone axis show the distribution of BSFs: (a) in the area close to the parasitic grain—area y; (b) in the area 100 mm from HVPE1/HVPE2/PG interface—area y; (c) HRTEM image viewed along [11 2¯0] of an I1 type BSF; (d) bright field image taken with gGaN=11 2¯ 0 near the [0 0 0 1] zone axis of the PSF created near the HPNS/HVPE1 interface—area z; (e) dark field image taken with gGaN=112¯0 near the [11¯ 0 0] zone axis showing the distribution of dislocation loops in the area z.
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density around 2.8 105 cm 1 in the area close to the parasitic grain (Fig. 4a). The measured density of BSFs is reduced in the second HVPE layer (Fig. 4b) at a distance of around 100 mm from the parasitic grain, at a value of 8 104 cm 1. The region of the parasitic grain is highly defected and the measured dislocation density is in the range of 109 cm 2. Selected area diffraction patterns (not shown here) taken from the interface between the HVPE layers and the parasitic grain show that the grain is misoriented about 301 with respect to the orientation of the HVPE layers. The area of semi-polar growth (area z in Fig. 2c) was investigated by cross-section specimens oriented along [11¯ 0 0] and [0 0 0 1] GaN directions. Close to the HP GaN needle, isolated PSFs (Fig. 4d) and a high number of dislocation loops lying in the (0 0 0 1) plane (Fig. 4e) with a density of 4 109 cm 2 were observed.
4. Discussion Both specification and distribution of structural defects deduced from the experimental results described in the previous section are schematically illustrated in Fig. 5. The TEM analysis has shown that despite high structural quality of the seed crystal, a number of lattice defects are present in the HVPE-grown part of the sample. There are both basal and prismatic stacking faults as well as dislocation loops at concentrations depending on the area of the crystal. One can distinguish a few possible reasons for generation and relaxation of strain leading to the formation of structural defects in the investigated system. As was mentioned earlier, the needle-like seeds grown under pressure are strongly n-type crystals with free electron concentrations approaching 1020 cm 3 due to high oxygen content incorporated during the high-pressure growth. On the other hand, the electron concentration in GaN obtained by the HVPE method depends strongly on the orientation of the crystallization front during the HVPE growth. It was shown that for growth in the Ga-polar [0 0 0 1] direction, typical electron concentration is about 1017 cm 3 or lower, whereas for the growth in the opposite N-polar [0 0 0 1¯] direction it can be more than two orders of magnitude higher, and usually exceeds 1019 cm 3 [7]. This very high electron concentration was also observed for growth on semi-polar (inclined) {1 0 1¯ 1} faces of GaN. As already mentioned in Section 2.1, for crystallization on the non-polar {1 0 1¯ 0} faces dominating the sample studied in this work, the free electron concentrations in the range 3–5 1018 cm 3 were measured [8] by the micro-Raman scattering technique. Additionally, for a multiple HVPE growth a sharp increase in electron concentration at regions corresponding to the beginning of the subsequent HVPE run has been found. This strong anisotropy in physical properties of the HVPEgrown GaN crystals is due to a different efficiency of oxygen incorporation on particular crystallographic faces of growing crystals.
Fig. 5. Schematic illustration of defects distribution in the crystal.
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Fig. 6. Images of (0 0 0 1) cross-section of four times HVPE-grown GaN crystal on HP GaN seed. (a) Photograph of the cut platelet, HP seed crystal (area of dark contrast) and interfaces between subsequent HVPE growths are indicated; (b) optical microscopy image of the same platelet as in image (a) after etching in molten bases. Feature 1 shows the seed crystal, features 2 and 3 are areas with etch pits spreading from morphological imperfections of the seed, features 4 and 5 show low defect density parts of the HVPE bulk GaN and features 6 and 7 are parasitic, strongly defected grains; (c) scanning electron microscopy image of the boxed area on the image (b).
It is well established [9] that a high number of free electrons in GaN leads to a slight expansion of the crystal lattice. The mismatch between an undoped GaN crystal (therefore semiinsulating) and a crystal containing 5 1019 cm 3 of free electrons is of the order of 0.01% for both a and c lattice constants of the wurtzite structure. So for the investigated system, in the case of bulk growth one can expect strain generation at the interface between the seed crystal and the HVPE-grown GaN, as well as in the areas where the crystal sectors growing in different directions, thus having different electron concentrations, meet. The latter situation can take place at the apex of the considered needleshaped seed where the growth proceeds in various directions and where a significant number of structural defects have been found. Another possible reason for the deterioration of the crystal quality is related with morphological imperfections of the seed. If a side face of the seed is not flat, but contains surfaces of orientations different from {1 0 1¯ 0}, the start of growth can be again accompanied by the formation of domains with large relative variations in free electron concentration, which causes strain in the crystal. This situation is illustrated in Fig. 6 where a polar (0 0 0 1) slice of the crystal similar to the one investigated by TEM is shown. The (0 0 0 1) Ga-polar surface of this slice was subjected to a defect-selective etching procedure [10] in molten KOH–NaOH eutectics to reveal structural defects. The crystal was overgrown with four separate HVPE growth cycles. The DSE result indicates that the seed crystal is almost defect-free (no etch pits), as seen in area 1 in Fig. 6b. The etch pits spread mostly from places of strong deviation from the (1 0 1¯ 0) orientation of the side faces of the seed, as seen in areas 2 and 4 in Fig. 6b and c. The strain induced at the seed is so high that its relaxation requires the formation of parasitic, strongly defected grains, indicated in areas 6 and 7 of Fig. 6b. The highest quality material containing BSFs at density lower than 105 cm 1 and no dislocations, has been found in the second HVPE layer in the area for which the HVPE growth started from flat (1 0 1¯ 0) face of the seed. It seems therefore that for crystallization of bulk GaN by the HVPE method seeded with GaN single crystals elongated in /0 0 0 1S directions, the crucial is to avoid abrupt changes of free electron concentration in the growth system. It is difficult for small seeds in general, if an increase of the crystal size in three dimensions corresponding to the growth in different directions is expected. For prismatic seeds like the one considered in this study, the growth in the non-polar /1 0 1¯ 0S directions leading to an increase of diameter of the needle is most suitable for obtaining good-quality crystal with uniform physical properties.
5. Summary The TEM studies of bulk GaN crystal grown by the HVPE method on the needle-shaped GaN single-crystalline seed have shown that despite high structural quality of the pressure-grown seed, structural defects are present in the HVPE-grown GaN crystal. A possible reason for strain and structural defects generation could be small lattice mismatch between highly oxygen-doped seed crystal and the HVPE GaN layer as well as between particular sectors of the HVPE GaN layer growing in different crystallographic directions. It seems, however, that goodquality GaN bulk crystal containing BSFs at density lower than 105 cm 1 and without dislocations can be obtained by the HVPE growth on the needle-like prismatic seed for the growth in the non-polar /1 0 1¯ 0S directions leading to an increase of lateral size of the needle. The growth leading to an elongation of the needle (in the polar and semi-polar directions) should be suppressed to avoid the strains caused by the different concentrations of free electrons in neighboring sectors. Obviously, the multiple growth should be replaced by a single, long duration run to avoid inhomogeneities related to the beginning of the HVPE crystallization process.
Acknowledgement We would like to acknowledge Dr Suman-Lata Sahonta for fruitful discussion of the results obtained in this work. References [1] K. Motoki, et al., J. Crystal Growth 237–239 (2002) 912. [2] A. Yoshikawa, E. Oshima, T. Fukuda, H. Tsuji, K. Oshima, J. Crystal Growth 260 (2004) 67. [3] I. Grzegory, M. Boc´kowski, S. Porowski, Optical and optoelectronic materials, in: P. Capper (Ed.), Bulk Crystal Growth of Electronic, Wiley New York, 2005, pp. 173–207, Chapter 6. [4] H. Yamane, M. Shimada, T. Sekiguchi, F.J. Di Salvo, J. Crystal Growth 186 (1998) 8. [5] D. Hanser, L. Liu, E.A. Preble, D. Thomas, M. Williams, Mat. Res. Soc. Symp. Proc. 798 (2004) Y2.1.1. [6] B. Łucznik, B. Pastuszka, I. Grzegory, M. Boc´kowski, G. Kamler, E. LitwinStaszewska, S. Porowski, J. Crystal Growth 281 (2005) 38. [7] J.L. Weyher, R. Lewandowska, L. Macht, B. Łucznik, I. Grzegory, Mater. Sci. Semicond. Process. 9 (2006) 175. [8] R. Lewandowska, unpublished results. [9] M. Krys´ko, M. Sarzyn´ski, J. Domaga"a, I. Grzegory, B. Łucznik, G. Kamler, S. Porowski, M. Leszczyn´ski, J. Alloys Compds. 401 (2005) 261. [10] J.L. Weyher, P.D. Brown, J.-L. Rouviere, T. Wosinski, A.R.A. Zauner, I. Grzegory, J. Crystal Growth 210 (1–3) (2000) 151.