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Growth and structural properties of thick GaN layers obtained by sublimation sandwich method
ARTICLE IN PRESS
Journal of Crystal Growth 303 (2007) 395–399 www.elsevier.com/locate/jcrysgro
Growth and structural properties of thick GaN layers obtained by sublimation sandwich method Michal Kaminskia,, Slawomir Podsiadloa, Krzysztof Wozniakb, Lukasz Dobrzyckib, Rafal Jakielac,d, Adam Barczc,d, Marek Psodae, Jaroslaw Mizerae a
Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland b Chemistry Department, Warsaw University, Pasteura 1, 02-093 Warsaw, Poland c Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland d Institute of Electron Technology, Al. Lotnikow 32/46, 02-668 Warsaw, Poland e Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, Poland Received 3 October 2006; received in revised form 22 November 2006; accepted 28 November 2006 Communicated by K.W. Benz Available online 23 January 2007
Abstract The synthesis and characterization of GaN thick layers are reported in this paper. The layers were prepared by sublimation sandwich method (SSM). Powder of GaN was used as the source of gallium and ammonia was used as the source of nitrogen. Sapphire with 3 mm GaN thin film grown by MOCVD was used as the substrate. The GaN layers having a current maximum size of 200 mm thickness and 10 mm 10 mm area were obtained. It is also shown that the crystals of best crystalline quality are obtained with a growth rate of 20 mm/ h. Characterization of the layers was performed using rocking curve, maps of reflection, structure refinement and SIMS. Quality of the material is good. r 2007 Elsevier B.V. All rights reserved. PACS: 71.55.Eq; 61.10.Nz Keywords: A1. Characterization; A3. Buffer layer; A3. Sublimation method; B1. GaN
1. Introduction Gallium nitride is a promising material for blue light emitting devices such as diodes and lasers [1,2]. Doped GaN is also a prospective spintronic material [3–5]. Growing of gallium nitride single crystals is difficult and needs to employ high pressure and temperature for obtaining large crystals [6]. Other methods allow to obtain good quality crystals but their sizes preclude any practical applications [7,8]. Nowadays, the methods of synthesis of layered single crystals of doped GaN are gaining the utmost importance. The most often used technique is hydride vapor phase epitaxy (HVPE), which allows to grow thick GaN layers [9]. After removal of the substrate, e.g. by Corresponding author. Tel.: +48 22 6607921; fax: +48 22 6282741.
E-mail address: [email protected] (M. Kaminski). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.11.338
laser lift-off (LLO) technique, these layers can be used as quasi-substrates for homoepitaxial growth [9,10]. However, the HVPE has some disadvantages—the use of the hydrogen chloride may introduce additional impurity in the form of chlorine. While in sublimation sandwich method (SSM) only high-purity ammonia is used as the reacting gas, this method has been applied to grow AlN [11,12], SiC [13,14] and appears to be promising for the growth of thick GaN layers [15,16]. 2. Experimental procedure 2.1. Synthesis Experiments of growing GaN layers were carried out in a horizontal quartz reactor. A graphite cylinder used as heating element was heated by RF induction. Sapphire
ARTICLE IN PRESS 396
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(0 0 0 1) with 3 mm GaN thin film grown by MOCVD was used as the substrate. To ensure gallium transfer, temperature gradient was employed between the gallium source and the substrate (DT ¼ 70 1C). GaN layers were prepared as follows. Powder of GaN prepared in our laboratory [7] was used as the source of gallium and ammonia was used as the source of nitrogen. GaN powder was placed in a quartz boat on the top of a graphite. The substrate was positioned in a distance of 5 mm from the GaN powder (above it). Ammonia was introduced into the gap between the Ga source and the substrate for growth. The reaction process was performed within the temperature range of 1140–1220 1C with a heating time of 5–120 min. The ammonia flow rate was 0.2–0.8 l/min. 2.2. X-ray measurements Data collection for a layer of GaN was performed on a KM4CCD k-axis diffractometer with graphite-monochromated MoKa radiation. The crystal was positioned at 62 mm from the CCD camera. Two thousand and four hundred frames were measured at 11 intervals with a counting time of 3 or 6 s, depending on detector position. The data were corrected for Lorentz and polarization effects. Analytical correction for absorption was applied [17]. Data reduction and analysis were carried out with the Oxford Diffraction programs [18]. The structure was solved by direct methods [19] and refined using SHELXL [20]. The refinement was based on squares of structure factors F2 for all reflections except those with very negative F2. Weighted R factors wR and all goodness-of-fit S values are based on F2. Conventional R factors are based on F with F set to zero for negative F2. The F 20 42sðF 20 Þ criterion was used only for calculating R factors and is not relevant to the choice of reflections for the refinement. The R factors based on F2 are about twice as large as those based on F. Scattering factors were taken from Tables 6.1.1.4 and 4.2.4.2 in Ref. [21]. Measurements of rocking curves and map of reflections were done using an X-ray diffractometer D8 Discover Series 2 by Bruker AXS. The rocking curves and maps of reflection from the (0 0 2) planes were determined. X-ray diameter was 0.1 mm 5 mm and the studied area had the diameter 5 mm. There were 72 scans for every layer (step 51). There were no differences between these 72 rocking curves, so only one for buffer layer and one for GaN by SSM were chosen to show in this study.
reference. The 29Si isotope was measured to avoid interference with 14N2 signal. It allowed decreasing detection limit of silicon in GaN to 1017 at/cm3. Implanted GaN was used as a calibration standard. Measurements performed with O+ 2 primary beam, with the beam current kept at 800 nA, were used to characterize Ga and N ions. 3. Results and discussion The dimensions of the obtained layers were ca. 10 mm 10 mm growth rate varied from 5 to 100 mm/h. The best crystalline quality is obtained with a growth rate of 20 mm/h. However, we obtained a 200 mm thick layer in 120 min reaction but the morphology and the quality of that material were very poor. The growth rate depends on many factors: temperature, ammonia flow rate and distance between the Ga source and substrate. Their influence on the thickness of the layers will be discussed elsewhere. The photograph of the 10 mm GaN layer is shown in Fig. 1. The surface of the layer is almost smooth, but some places may be covered with growth figures that are circular or hexagonal like. In order to confirm the details of the 3D GaN structure, we applied X-ray diffraction methods. Selected crystallographic data and the refinement procedure of our layers of GaN obtained by the SSM are presented in Tables A1–A3 in Appendix A. The structure of GaN layer is illustrated in Fig. 2. Pure GaN obtained in our experiments crystallizes in the polar P6(3)mc (No. 186) hexagonal space group forming a layered structure of the Wurtzite 2H type. The unit cell parameters obtained are equal to a ¼ 3.1972(12) A˚ and c ¼ 5.207(3) A˚, which is in an excellent agreement with previous studies [22–33]. In such a crystal structure, cations and anions form separate honeycombed layers perpendicularly to the six-fold axis. The conformation of each sixmember ring in a given layer is close to a chair-type like conformation of cyclohexane. Because planes containing
2.3. SIMS Measurements were done using a CAMECA IMS6F microanalyser. H, C, O and Si measurements were performed with Cs+ primary beam, with the beam current kept at 170 nA. The secondary ions were collected from a region of 30 mm in diameter. H, C, O and Si concentrations were derived from the intensity of H, 12C, 16O and 29 Si species, and the matrix signal 69Ga was taken as a
1 mm
Fig. 1. Photograph of a GaN layer on the substrate.
ARTICLE IN PRESS M. Kaminski et al. / Journal of Crystal Growth 303 (2007) 395–399
cations and anions are slightly shifted relatively to each other, there are two different distances between them. Different layers of atoms in the polar Z-direction are shown in Fig. 2.
The high quality of the crystalline structure was also confirmed by rocking curve (RC) and maps of reflections. Both, MOCVD GaN template (buffer) and GaN grown by SSM were investigated. RC and maps of reflection for buffer layer and layer grown by SSM are shown in Fig. 3a–d, respectively. In the first case, full-width at half-maximum (FWHM) value is 0.1291 for the (0 0 2) reflection (Fig. 3a) and the maximum on the map of reflection (Fig. 3b) is observed for the 2y ¼ 34.5701. The X-ray rocking curve of GaN layer grown by SSM (Fig. 3c) has an FWHM of 0.1591 for (0 0 2) with the maximum of the signal (Fig. 3d) for 2y ¼ 34.5751. By comparing these two values one can say that there are no big differences between the crystalline structure of the buffer GaN and GaN grown by the SSM. There was only one peak, which means that only a single phase was detected. Also, the lack of broadening and separating of rocking curve peak, as well as appearance only one maximum on the map of reflections testify that there are
Fig. 2. Crystal lattice of GaN.
a
397
b
2 / °→ →
Counts →
34.8
FWHM=0.129°
34.6
34.3
0 17.0
17.1
17.2
17.3 / °→ →
17.4
17.5
17.6
17.0
17.1
17.2
17.3
17.4
17.5
Omega Scale / °→ →
c
d
2 / °→ →
Counts →
36.0
FWHM = 0.159°
34.5
34.0
0 17.0
17.1
17.2
17.3 / °→ →
17.4
17.5
17.6
17.0
17.1
17.2
17.3
17.4
17.5
Omega Scale / °→ →
Fig. 3. (a) Rocking curve for buffer GaN, (b) 2D map of reflection for buffer GaN, (c) rocking curve for GaN grown by SSM and (d) 2D map of reflection for GaN grown by SSM.
ARTICLE IN PRESS M. Kaminski et al. / Journal of Crystal Growth 303 (2007) 395–399
398
no twinnings and significant mosaicity of the material. But some essential differences can be seen in the 2D shape of the equivalent reflections. A bit elongated shape of the reflection in the Y-direction confirms that the c parameter in the SSM layer is slightly different from the c parameter in the substrate GaN. This parameter is 5.186 A˚ in the buffer GaN and 5.207 A˚ in GaN layer grown by the SSM. An increase in the c parameter is probably a consequence of the point defects or impurities, for example oxygen. Secondary ion mass spectrometry (SIMS) measurements were done in order to measure quantity of impurities such as H, C, O and Si in GaN layers. Concentration profiles of particular species are shown in Fig. 4. High concentrations of impurities at the beginning and end of the profiles come from surface and GaN/Al2O3 interface contamination, respectively. The concentration of impurities drops rapidly away from the surface down to about 1017 cm3 for Si, 1018 cm3 for H and 1018–1019 cm3 for O and C.
X-ray and SIMS investigations. The crystals of best quality were obtained with growth rate of 20 mm/h. Further improvement of the system could take effect in obtaining thicker layers, depending on the size of the substrate used, the size of the reactor (e.g. amount of the gallium source, free path between GaN powder and the substrate) and time. However, as mentioned by Baranov et al. [15], vertical configuration would be more suitable for longlasting processes. Acknowledgment This work was supported by the State Committee for Scientific Research (Grant no. 3T09B 056 28). Appendix A Selected crystallographic data and the results of the refinement procedure (see Tables A1–A3).
4. Conclusions In this paper, we have reported on the synthesis of good quality GaN layers. Presented method is simple, cheap, fast and potentially effective. It does not need high pressure nor hazardous conditions. The use of simple substances is one of the advantages. We have obtained layers of GaN by SSM with good crystalline structure, which was proved by
H C O Si
Concentration [at/cm3]
Empirical formula Formula weight Temperature Wavelength Crystal system Space group Unit cell dimensions
GaN 83.73 100(2) K 0.71073 A˚
Volume Z Calculated density Absorption coefficient F(0 0 0) Crystal size Theta range for data collection Index ranges
1022
1021
Table Al Crystal data and structure refinement for GaN obtained by SSM
Reflections collected/unique Completeness to theta ¼ 36.301 Maximum and minimum transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I42s(I)] R indices (all data) Absolute structure parameter Extinction coefficient w Largest diffraction peak and hole
20
10
1019
Hexagonal P63mc a ¼ 3.1972(12) A˚ c ¼ 5.207(3) A˚ 46.10(4) A˚3 2 6.033 mg/m3 28.723 mm1 76 0.18 0.11 0.02 mm3 7.36–44.761 6 ( h ( 6, 6 ( k ( 6, 10 ( l ( 10 2048/173 [R(int) ¼ 0.0182] 94.9% 0.58 and 0.005 Full-matrix least squares on F2 173/1/8 1.234 R1 ¼ 0.0088, wR2 ¼ 0.0194 R1 ¼ 0.0119, wR2 ¼ 0.0196 0.03(3) 0.113(9) 0.774 and 0.463 e A˚3
1018
1017
Table A2 Atomic coordinates (104) and anisotropic displacement parameters (A2 103) for GaN obtained by SSM 0
1
2
3
4
5
Atom
x
y
z
Ueq
U11
U22
U33
U23
U13
U12
Ga(1) N(1)
6667 6667
3333 3333
5912(2) 9688(2)
3(1) 4(1)
2(1) 3(1)
2(1) 3(1)
3(1) 4(1)
0 0
0 0
0 0
Depth [μm]
Fig. 4. H, C, O and Si concentration profiles obtained by SIMS in a 5 mm GaN layer.
ARTICLE IN PRESS M. Kaminski et al. / Journal of Crystal Growth 303 (2007) 395–399 Table A3 Bond lengths and angles between atoms in GaN obtained by SSM Ga(1)–N(1)#1 Ga(1)–N(1) N(1)#1–Ga(1)–N(1)#2 N(1)#1–Ga(1)–N(1)
1.9527(8) (A) 1.9664(19) (A) 109.90(5) (deg) 109.04(5) (deg)
Symmetry transformations used to generate equivalent atoms: #1, x+1, y, z1/2; #2, x+2, y+1, z1/2.
References [1] S. Nakamura, J. Crystal Growth 170 (1997) 11. [2] F. Tinjod, et al., J. Crystal Growth 285 (2005) 450. [3] S. Podsiad"o, T. Szyszko, W. Gebicki, J. Gosk, R. Bacewicz, L. Dobrzycki, K. Wozniak, M. Zajac, A. Twardowski, Chem. Mater. 15 (2003) 4533. [4] T. Szyszko, M. Kamin´ski, S. Podsiad"o, K. Woz´niak, Ł. Dobrzycki, W. G˛ebicki, J. Crystal Growth 276 (2005) 419. [5] H. Ohno, Science 291 (2001) 840. [6] I. Grzegory, M. Bockowski, B. Lucznik, S. Krukowski, Z. Romanowski, M. Wroblewski, S. Porowski, J. Crystal Growth 246 (2002) 177. [7] G. Kamler, J. Zachara, S. Podsiadlo, L. Adamowicz, W. Gebicki, J. Crystal Growth 212 (2000) 39. [8] Y. Song, W. Wang, W. Yuan, X. Wu, X. Chen, J. Crystal Growth 247 (2003) 275. [9] M. Hermann, D. Gogova, D. Siche, M. Schmidbauer, B. Monemar, M. Stutzmann, M. Eickhoff, J. Crystal Growth 293 (2006) 462. [10] E.A. Stach, M. Kelsch, E.C. Nelson, W.S. Wong, T. Sands, N.W. Cheung, Appl. Phys. Lett. 77 (2000) 1819. [11] Y. Shi, Z.Y. Xie, L. Liu, B. Liu, J.H. Edgar, M. Kuball, J. Crystal Growth 233 (2001) 177. [12] A.S. Segal, S.Yu. Karpov, Yu.N. Makarov, E.N. Mokhov, A.D. Roenkov, M.G. Ramm, Yu.A. Vodakov, J. Crystal Growth 211 (2000) 68.
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[13] E.N. Mokhov, M.G. Ramm, A.D. Roenkov, Y.A. Vodakov, Mater. Sci. Eng. B B46 (1997) 317. [14] A.S. Segal, A.N. Vorob’ev, S.Yu. Karpov, E.N. Mokhov, M.G. Ramm, M.S. Ramm, A.D. Roenkov, Yu.A. Vodakov, Yu.N. Makarov, J. Crystal Growth 208 (2000) 431. [15] P.G. Baranov, et al., J. Nitride Semicond. Res. 3 (1998) 50. [16] Yu.A. Vodakov, E.N. Mokhov, A.D. Roenkov, M.E. Boiko, P.G. Baranov, J. Crystal Growth 183 (1998) 10. [17] CrysAlis RED, Oxford Diffraction Ltd., Version 1.171.28cycle2 beta (release 25-10-2005 CrysAlis171 .NET) (compiled Oct 25 2005, 08:50:05) Analytical numeric absorption correction using a multifaceted crystal model based on expressions derived by R.C. Clark and J.S. Reid. [18] CrysAlis CCD, Oxford Diffraction Ltd., Version 1.171.28cycle2 beta; CrysAlis RED, Oxford Diffraction Ltd., Version 1.171.28cycle2 beta. [19] G.M. Sheldrick, Acta Crystallogr. A 46 (1990) 467. [20] G.M. Sheldrick, SHELXL93. Program for the Refinement of Crystal Structures, University of Go¨ttingen, Germany. [21] A.J.C. Wilson (Ed.), International Tables for Crystallography, vol. C, Kluwer, Dordrecht, 1992. [22] R. Juza, H. Hahn, Zeitschrift fuer Anorganische und Allgemeine Chemie 239 (1938) 282. [23] H. Schulz, K.H. Thiemann, Solid State Commun. 23 (1997) 815. [24] K. Miwa, A. Fukumoto, Phys. Rev. Ser. 3 B 48 (1993) 7897. [25] Y.N. Xu, W.Y. Ching, Phys. Rev. Serie 3, B: Condens. Matter 48 (1993) 4335. [26] Y.N. Xu, W.Y. Ching, Phys. Rev. Ser. 3 B 10 (1974) 676. [27] A.F. Wright, J.S. Nelson, Phys. Rev. Ser. 3 B 51 (1995) 7866. [28] N.E. Christensen, I. Gorczyca, Phys. Rev. Ser. 3 B 50 (1994) 4397. [29] A.F. Wright, J.S. Nelson, Phys. Rev. Ser. 3 B 50 (1994) 2159. [30] W. Paszkowicz, S. Podsiadlo, R. Minikayev, J. Alloys Compds. 382 (2004) 100. [31] C. Yeh, et al., Phys. Rev. Ser. 3 B 46 (16) (1992) 10086. [32] X.-L. Chen, et al., Chin. Phys. Lett. 16 (2) (1999) 107. [33] X.-L. Chen, et al., Mod. Phys. Lett. B 13 (9) (1999) 285.
Journal of Crystal Growth 303 (2007) 395–399 www.elsevier.com/locate/jcrysgro
Growth and structural properties of thick GaN layers obtained by sublimation sandwich method Michal Kaminskia,, Slawomir Podsiadloa, Krzysztof Wozniakb, Lukasz Dobrzyckib, Rafal Jakielac,d, Adam Barczc,d, Marek Psodae, Jaroslaw Mizerae a
Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland b Chemistry Department, Warsaw University, Pasteura 1, 02-093 Warsaw, Poland c Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland d Institute of Electron Technology, Al. Lotnikow 32/46, 02-668 Warsaw, Poland e Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, Poland Received 3 October 2006; received in revised form 22 November 2006; accepted 28 November 2006 Communicated by K.W. Benz Available online 23 January 2007
Abstract The synthesis and characterization of GaN thick layers are reported in this paper. The layers were prepared by sublimation sandwich method (SSM). Powder of GaN was used as the source of gallium and ammonia was used as the source of nitrogen. Sapphire with 3 mm GaN thin film grown by MOCVD was used as the substrate. The GaN layers having a current maximum size of 200 mm thickness and 10 mm 10 mm area were obtained. It is also shown that the crystals of best crystalline quality are obtained with a growth rate of 20 mm/ h. Characterization of the layers was performed using rocking curve, maps of reflection, structure refinement and SIMS. Quality of the material is good. r 2007 Elsevier B.V. All rights reserved. PACS: 71.55.Eq; 61.10.Nz Keywords: A1. Characterization; A3. Buffer layer; A3. Sublimation method; B1. GaN
1. Introduction Gallium nitride is a promising material for blue light emitting devices such as diodes and lasers [1,2]. Doped GaN is also a prospective spintronic material [3–5]. Growing of gallium nitride single crystals is difficult and needs to employ high pressure and temperature for obtaining large crystals [6]. Other methods allow to obtain good quality crystals but their sizes preclude any practical applications [7,8]. Nowadays, the methods of synthesis of layered single crystals of doped GaN are gaining the utmost importance. The most often used technique is hydride vapor phase epitaxy (HVPE), which allows to grow thick GaN layers [9]. After removal of the substrate, e.g. by Corresponding author. Tel.: +48 22 6607921; fax: +48 22 6282741.
E-mail address: [email protected] (M. Kaminski). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.11.338
laser lift-off (LLO) technique, these layers can be used as quasi-substrates for homoepitaxial growth [9,10]. However, the HVPE has some disadvantages—the use of the hydrogen chloride may introduce additional impurity in the form of chlorine. While in sublimation sandwich method (SSM) only high-purity ammonia is used as the reacting gas, this method has been applied to grow AlN [11,12], SiC [13,14] and appears to be promising for the growth of thick GaN layers [15,16]. 2. Experimental procedure 2.1. Synthesis Experiments of growing GaN layers were carried out in a horizontal quartz reactor. A graphite cylinder used as heating element was heated by RF induction. Sapphire
ARTICLE IN PRESS 396
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(0 0 0 1) with 3 mm GaN thin film grown by MOCVD was used as the substrate. To ensure gallium transfer, temperature gradient was employed between the gallium source and the substrate (DT ¼ 70 1C). GaN layers were prepared as follows. Powder of GaN prepared in our laboratory [7] was used as the source of gallium and ammonia was used as the source of nitrogen. GaN powder was placed in a quartz boat on the top of a graphite. The substrate was positioned in a distance of 5 mm from the GaN powder (above it). Ammonia was introduced into the gap between the Ga source and the substrate for growth. The reaction process was performed within the temperature range of 1140–1220 1C with a heating time of 5–120 min. The ammonia flow rate was 0.2–0.8 l/min. 2.2. X-ray measurements Data collection for a layer of GaN was performed on a KM4CCD k-axis diffractometer with graphite-monochromated MoKa radiation. The crystal was positioned at 62 mm from the CCD camera. Two thousand and four hundred frames were measured at 11 intervals with a counting time of 3 or 6 s, depending on detector position. The data were corrected for Lorentz and polarization effects. Analytical correction for absorption was applied [17]. Data reduction and analysis were carried out with the Oxford Diffraction programs [18]. The structure was solved by direct methods [19] and refined using SHELXL [20]. The refinement was based on squares of structure factors F2 for all reflections except those with very negative F2. Weighted R factors wR and all goodness-of-fit S values are based on F2. Conventional R factors are based on F with F set to zero for negative F2. The F 20 42sðF 20 Þ criterion was used only for calculating R factors and is not relevant to the choice of reflections for the refinement. The R factors based on F2 are about twice as large as those based on F. Scattering factors were taken from Tables 6.1.1.4 and 4.2.4.2 in Ref. [21]. Measurements of rocking curves and map of reflections were done using an X-ray diffractometer D8 Discover Series 2 by Bruker AXS. The rocking curves and maps of reflection from the (0 0 2) planes were determined. X-ray diameter was 0.1 mm 5 mm and the studied area had the diameter 5 mm. There were 72 scans for every layer (step 51). There were no differences between these 72 rocking curves, so only one for buffer layer and one for GaN by SSM were chosen to show in this study.
reference. The 29Si isotope was measured to avoid interference with 14N2 signal. It allowed decreasing detection limit of silicon in GaN to 1017 at/cm3. Implanted GaN was used as a calibration standard. Measurements performed with O+ 2 primary beam, with the beam current kept at 800 nA, were used to characterize Ga and N ions. 3. Results and discussion The dimensions of the obtained layers were ca. 10 mm 10 mm growth rate varied from 5 to 100 mm/h. The best crystalline quality is obtained with a growth rate of 20 mm/h. However, we obtained a 200 mm thick layer in 120 min reaction but the morphology and the quality of that material were very poor. The growth rate depends on many factors: temperature, ammonia flow rate and distance between the Ga source and substrate. Their influence on the thickness of the layers will be discussed elsewhere. The photograph of the 10 mm GaN layer is shown in Fig. 1. The surface of the layer is almost smooth, but some places may be covered with growth figures that are circular or hexagonal like. In order to confirm the details of the 3D GaN structure, we applied X-ray diffraction methods. Selected crystallographic data and the refinement procedure of our layers of GaN obtained by the SSM are presented in Tables A1–A3 in Appendix A. The structure of GaN layer is illustrated in Fig. 2. Pure GaN obtained in our experiments crystallizes in the polar P6(3)mc (No. 186) hexagonal space group forming a layered structure of the Wurtzite 2H type. The unit cell parameters obtained are equal to a ¼ 3.1972(12) A˚ and c ¼ 5.207(3) A˚, which is in an excellent agreement with previous studies [22–33]. In such a crystal structure, cations and anions form separate honeycombed layers perpendicularly to the six-fold axis. The conformation of each sixmember ring in a given layer is close to a chair-type like conformation of cyclohexane. Because planes containing
2.3. SIMS Measurements were done using a CAMECA IMS6F microanalyser. H, C, O and Si measurements were performed with Cs+ primary beam, with the beam current kept at 170 nA. The secondary ions were collected from a region of 30 mm in diameter. H, C, O and Si concentrations were derived from the intensity of H, 12C, 16O and 29 Si species, and the matrix signal 69Ga was taken as a
1 mm
Fig. 1. Photograph of a GaN layer on the substrate.
ARTICLE IN PRESS M. Kaminski et al. / Journal of Crystal Growth 303 (2007) 395–399
cations and anions are slightly shifted relatively to each other, there are two different distances between them. Different layers of atoms in the polar Z-direction are shown in Fig. 2.
The high quality of the crystalline structure was also confirmed by rocking curve (RC) and maps of reflections. Both, MOCVD GaN template (buffer) and GaN grown by SSM were investigated. RC and maps of reflection for buffer layer and layer grown by SSM are shown in Fig. 3a–d, respectively. In the first case, full-width at half-maximum (FWHM) value is 0.1291 for the (0 0 2) reflection (Fig. 3a) and the maximum on the map of reflection (Fig. 3b) is observed for the 2y ¼ 34.5701. The X-ray rocking curve of GaN layer grown by SSM (Fig. 3c) has an FWHM of 0.1591 for (0 0 2) with the maximum of the signal (Fig. 3d) for 2y ¼ 34.5751. By comparing these two values one can say that there are no big differences between the crystalline structure of the buffer GaN and GaN grown by the SSM. There was only one peak, which means that only a single phase was detected. Also, the lack of broadening and separating of rocking curve peak, as well as appearance only one maximum on the map of reflections testify that there are
Fig. 2. Crystal lattice of GaN.
a
397
b
2 / °→ →
Counts →
34.8
FWHM=0.129°
34.6
34.3
0 17.0
17.1
17.2
17.3 / °→ →
17.4
17.5
17.6
17.0
17.1
17.2
17.3
17.4
17.5
Omega Scale / °→ →
c
d
2 / °→ →
Counts →
36.0
FWHM = 0.159°
34.5
34.0
0 17.0
17.1
17.2
17.3 / °→ →
17.4
17.5
17.6
17.0
17.1
17.2
17.3
17.4
17.5
Omega Scale / °→ →
Fig. 3. (a) Rocking curve for buffer GaN, (b) 2D map of reflection for buffer GaN, (c) rocking curve for GaN grown by SSM and (d) 2D map of reflection for GaN grown by SSM.
ARTICLE IN PRESS M. Kaminski et al. / Journal of Crystal Growth 303 (2007) 395–399
398
no twinnings and significant mosaicity of the material. But some essential differences can be seen in the 2D shape of the equivalent reflections. A bit elongated shape of the reflection in the Y-direction confirms that the c parameter in the SSM layer is slightly different from the c parameter in the substrate GaN. This parameter is 5.186 A˚ in the buffer GaN and 5.207 A˚ in GaN layer grown by the SSM. An increase in the c parameter is probably a consequence of the point defects or impurities, for example oxygen. Secondary ion mass spectrometry (SIMS) measurements were done in order to measure quantity of impurities such as H, C, O and Si in GaN layers. Concentration profiles of particular species are shown in Fig. 4. High concentrations of impurities at the beginning and end of the profiles come from surface and GaN/Al2O3 interface contamination, respectively. The concentration of impurities drops rapidly away from the surface down to about 1017 cm3 for Si, 1018 cm3 for H and 1018–1019 cm3 for O and C.
X-ray and SIMS investigations. The crystals of best quality were obtained with growth rate of 20 mm/h. Further improvement of the system could take effect in obtaining thicker layers, depending on the size of the substrate used, the size of the reactor (e.g. amount of the gallium source, free path between GaN powder and the substrate) and time. However, as mentioned by Baranov et al. [15], vertical configuration would be more suitable for longlasting processes. Acknowledgment This work was supported by the State Committee for Scientific Research (Grant no. 3T09B 056 28). Appendix A Selected crystallographic data and the results of the refinement procedure (see Tables A1–A3).
4. Conclusions In this paper, we have reported on the synthesis of good quality GaN layers. Presented method is simple, cheap, fast and potentially effective. It does not need high pressure nor hazardous conditions. The use of simple substances is one of the advantages. We have obtained layers of GaN by SSM with good crystalline structure, which was proved by
H C O Si
Concentration [at/cm3]
Empirical formula Formula weight Temperature Wavelength Crystal system Space group Unit cell dimensions
GaN 83.73 100(2) K 0.71073 A˚
Volume Z Calculated density Absorption coefficient F(0 0 0) Crystal size Theta range for data collection Index ranges
1022
1021
Table Al Crystal data and structure refinement for GaN obtained by SSM
Reflections collected/unique Completeness to theta ¼ 36.301 Maximum and minimum transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I42s(I)] R indices (all data) Absolute structure parameter Extinction coefficient w Largest diffraction peak and hole
20
10
1019
Hexagonal P63mc a ¼ 3.1972(12) A˚ c ¼ 5.207(3) A˚ 46.10(4) A˚3 2 6.033 mg/m3 28.723 mm1 76 0.18 0.11 0.02 mm3 7.36–44.761 6 ( h ( 6, 6 ( k ( 6, 10 ( l ( 10 2048/173 [R(int) ¼ 0.0182] 94.9% 0.58 and 0.005 Full-matrix least squares on F2 173/1/8 1.234 R1 ¼ 0.0088, wR2 ¼ 0.0194 R1 ¼ 0.0119, wR2 ¼ 0.0196 0.03(3) 0.113(9) 0.774 and 0.463 e A˚3
1018
1017
Table A2 Atomic coordinates (104) and anisotropic displacement parameters (A2 103) for GaN obtained by SSM 0
1
2
3
4
5
Atom
x
y
z
Ueq
U11
U22
U33
U23
U13
U12
Ga(1) N(1)
6667 6667
3333 3333
5912(2) 9688(2)
3(1) 4(1)
2(1) 3(1)
2(1) 3(1)
3(1) 4(1)
0 0
0 0
0 0
Depth [μm]
Fig. 4. H, C, O and Si concentration profiles obtained by SIMS in a 5 mm GaN layer.
ARTICLE IN PRESS M. Kaminski et al. / Journal of Crystal Growth 303 (2007) 395–399 Table A3 Bond lengths and angles between atoms in GaN obtained by SSM Ga(1)–N(1)#1 Ga(1)–N(1) N(1)#1–Ga(1)–N(1)#2 N(1)#1–Ga(1)–N(1)
1.9527(8) (A) 1.9664(19) (A) 109.90(5) (deg) 109.04(5) (deg)
Symmetry transformations used to generate equivalent atoms: #1, x+1, y, z1/2; #2, x+2, y+1, z1/2.
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