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Gallium nitride thin layers via a liquid precursor route
Journal of Crystal Growth 208 (2000) 153}159
Gallium nitride thin layers via a liquid precursor route Manfred Puchinger!,*, Thomas Wagner!, Dieter Rodewald",1, Joachim Bill", Fritz Aldinger", Frederick F. Lange# !Max-Planck-Institut fu( r Metallforschung, Seestrasse 92, D-70174 Stuttgart, Germany "Max-Planck-Institut fu( r Metallforschung und Institut fu( r Nichtmetallische Anorganische Materialien, Pulvermetallurgisches Laboratorium, Universita( t Stuttgart, Heisenbergstrasse 5, D-70569 Stuttgart, Germany #Materials Department, University of California, Santa Barbara, CA 93106-5050, USA Received 27 April 1999; accepted 8 July 1999 Communicated by C.R. Abernathy
Abstract A chemical solution deposition method was used to grow thin epitaxial GaN "lms on C- and R-plane sapphire substrates. The "lms were grown by spin-coating a gallium carbodiimide based polymeric precursor onto sapphire and pyrolyzing in NH at 9003C. During heat treatment a-GaN formed on the R-plane of the sapphire with the following 3 epitaxial orientation relationship: a-Al O (0 1 11 2)Ea-GaN(1 1 21 0); a-Al O [2 11 11 0]Ea-GaN[1 11 0 0]. A multiple coat2 3 2 3 ing process resulted in "lms fully covering the R-plane substrates. Films deposited on C-plane sapphire were not continuous and single islands were present on the substrate surface. The morphology and microstructure of the "lms were characterised by SEM, XRD, and conventional and analytical electron microscopy. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 81.15.!z; 68.55.Jk Keywords: Thin "lms; Precursor pyrolysis; Chemical solution deposition; Gallium nitride (GaN); Sapphire (Al O ); Transmission 2 3 electron microscopy (TEM)
1. Introduction In recent time, gallium nitride has received increasing attention due to the development of highly e$cient blue light emitting diodes (LEDs) and laser diodes. Hexagonal GaN posesses a direct band gap
* Corresponding author. Tel.: #49-711-2095-241; fax: #49711-2095-120. E-mail address: [email protected] (M. Puchinger) 1 Present address: Elastogran GmbH, LemfoK rde, Germany.
of about 3.4 eV [1] which makes it well suited for short-wavelength optical devices [2,3]. Standard methods for the production of GaN thin "lms are molecular beam epitaxy (MBE) and chemical vapour deposition (CVD) [4,5]. MBE methods generally use metallic gallium and molecular nitrogen (plasma-enhanced or excited by electron cyclotron resonance) as material sources, whereas most CVD methods apply volatile metalorganic precursors like trimethylgallium or triethylgallium and ammonia (MOCVD). In these growth processes, the "lm can be deposited atomby-atom. a-Al O (sapphire) is the most common 2 3
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substrate material; bu!er layers of AlN or GaN were found to improve the crystal quality of the deposited GaN "lms [2,6]. A di!erent approach for the fabrication of thin "lms is the chemical solution deposition method where a precursor solution is deposited by spin- or dip-coating onto a single-crystal substrate [7,8]. After evaporation of the solvent, the precursor "lm is heat-treated (pyrolysed) in an appropriate atmosphere, until transformation of the polymeric precursor material into a poly- or single-crystal thin "lm takes place. The main advantage of this process is its simplicity compared to conventional CVD and MBE processes. However, relative to vapour phase epitaxy, little is known about the correlation between process parameters and materials properties for this method [7]. Recently, we developed a precursor for the preparation of hexagonal (wurtzite, a-) GaN thin "lms from a precursor solution by pyrolysis under ammonia [9]. In the present paper we present the precursor preparation, the deposition and the microstructure of thin GaN "lms which were produced by spin-coating on C-plane (0 0 0 1)- and R-plane (0 1 11 2) a-Al O . To our knowledge, this 2 3 is the "rst time that epitaxial GaN thin "lms have been grown successfully by the chemical solution deposition method. These "lms may have the potential to serve as bu!er layers or seeds in conventional GaN thin "lm growth techniques.
2. Experimental procedure 2.1. Precursor preparation The precursor solution was prepared in an argon atmosphere. 3.7 g (21 mmol) gallium chloride (GaCl ) were stirred with 120 ml (530 mmol) 3 of bis(trimethylsilyl)carbodiimide (Me Si)NCN3 (SiMe ) (Me"methyl) for three days at room tem3 perature, providing a homogeneous solution of the gallium chloride. No other solvents were added. The resulting solution was re#uxed for 9 h at &1703C. During the reaction, the trimethylsilyl groups are substituted by Ga atoms which can be bonded to three carbodiimide groups altogether.
Me SiCl is formed and some polymer percipitates. 3 After "ltering, a transparent, yellow solution was obtained, consisting of a gallium- and carbodiimide-containing polymer dissolved in bis(trimethylsilyl)carbodiimide. This served as the precursor solution. In a seperate experiment, bis(trimethylsilyl)carbodiimide was removed from the precursor solution by distillation. A weakly yellowish polymer powder was obtained after drying the residue in vacuum at 2103C. A chemical analysis revealed a composition of Ga(NCN) (SiMe ) Cl . 1.05 3 0.20 1.05 This indicates an incomplete reaction between chlorine atoms and trimethylsilyl groups during the precursor preparation. 2.2. Film growth Polished C- and R-plane a-Al O substrates 2 3 (CrysTec, Berlin, Germany) were mounted by tape onto a spincoater. Two to three drops of precursor solution were put on the substrate surface in argon atmosphere. Subsequently, the disc was rotated at high speed (see Table 1) for about 30 s. The samples were then put into a sealed schlenk tube and placed into a tube furnace. After a quick #ush with ammonia gas (NH ), a constant #ow of ammonia inside 3 the schlenk tube was adjusted (roughly 1}5 cm3/s, 105 Pa) avoiding a direct gas #ow over the sample surface. The furnace was heated at 3.5 K/min starting from room temperature (RT) to 1003C and at 5 K/min from 100 to 9003C. Following a dwell time of 6 min, the furnace was cooled at 4.3 K/min to RT. To grow thicker "lms, the above process was performed three times without exposure to air. 2.3. Characterisation The structure and chemical composition of the "lms were characterized by X-ray di!raction Table 1 GaN samples prepared by repeated spincoating a liquid precursor solution on a sapphire substrate Sample
Sapphire orientation
Spinning speeds (rpm)
1 2
R-plane C-plane
2000/2000/2000 6000/4000/4000
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155
Fig. 1. XRD (h}2h) scans of GaN thin "lms grown on sapphire prepared by precursor pyrolysis.
(XRD) (Siemens D 500), scanning electron microscopy (SEM) (Zeiss DSM 982 Gemini), and conventional and analytical electron microscopy (Jeol JEM 200 CX; Jeol JEM 2000 FX, VG HB501). Cross-sectional TEM specimen preparation followed a procedure described by Strecker et al. [10].
3. Experimental results 3.1. Films on R-plane a-Al2 O3 XRD h}2h-scans were performed to determine the structure of the thin "lms after pyrolysis (Fig. 1a). The XRD pattern of the R-plane specimen shows a strong re#ection at 57.93 which can be attributed to a-GaN(1 1 21 0). No other di!raction peaks from the "lm were detected. An SEM image of a multiple coated R-plane substrate is shown in Fig. 2, demonstrating that the surface morphology is extremely inhomogeneous. Strongly facetted regions (diameter of each facet 50}300 nm) are observed. Apart from these areas, structures with much smaller diameter (about 20 nm) can be seen. Between the facetted regions
Fig. 2. SEM picture of the surface of R-plane sapphire coated with GaN precursor. The surface is facetted.
and the areas with "ner features, both gradual and abrupt changes of the facet size are observable. Fig. 3 shows a bright "eld TEM image of the GaN "lm and the a-Al O substrate. The incident 2 3 electron beam was along the [2 11 11 0] a-Al O 2 3 direction. An array of regularly formed a-GaN facets covers the substrate. The facets have a pyramidal shape with a mean height of &50 nm
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Fig. 4. Cross-sectional TEM of R-plane sapphire coated with GaN precursor; zone axis a-Al O [0 11 1 1], respectively, a2 3 GaN [0 0 0 1]. Half hexagons are visible.
Fig. 3. Cross-sectional TEM and SAD of R-plane sapphire coated with GaN precursor; zone axis a-Al O [2 11 11 0], respec2 3 tively, a-GaN[1 11 0 0]. A pyramidal structure of epitaxial aGaN covers the substrate.
and a mean base of 100}150 nm, in good agreement with the SEM results. Apparently, the notch between two neighbouring pyramids does not reach the substrate. As revealed by SAD (selected area di!raction), all islands are epitaxial with the following orientation relationship (OR I): a-Al O (0 1 11 2)Ea-GaN(1 1 21 0) 2 3 and a-Al O [2 11 11 0]Ea-GaN[1 11 0 0]. 2 3 The same orientation relationship is found when GaN "lms are grown on sapphire R-plane
substrates by MBE and MOCVD [11}13]. Fig. 4 shows a bright-"eld TEM micrograph of the same interface along the [0 11 1 1] a-Al O direction. No 2 3 pyramidal shapes are observed in this direction, instead symmetrical structures resembling half hexagons are seen. The corresponding SAD pattern reveals the same epitaxial orientation relationship, OR I. In order to determine the chemical composition of the "lm, EELS (electron energy loss spectroscopy) measurements were performed on a thinner "lm (prepared by a single spincoating step) in a STEM (scanning transmission electron microscope). The electron beam was placed at the middle of a 50 nm wide pyramid, like those shown in Fig. 3. The typical area investigated was 3]4 nm. Results of the chemical analysis can be seen in Table 2. The atomic concentrations of Ga and N were almost equal and only oxygen impurities could be detected. The measured atomic concentration of oxygen was about 6%. 3.2. Films on C-plane a-Al2 O3 The XRD pattern of the sample prepared on C-plane sapphire (see Fig. 1b) shows two maxima that can be attributed to the "lm. The re#ection at 34.63 "ts a-GaN(0 0 0 2), while the peak at 36.93 belongs to a-GaN(1 0 11 1), the most intense peak in powder di!raction [14].
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Table 2 Chemical composition of a "lm prepared by a single coating step of an R-plane sapphire substrate, measured by EELS in the STEM Element Ga N O Other, each
Fraction (at%) 46.0 48.0 5.6 (1
Deviation (at%) $6.2 $6.5 $1.0 $1.0
Fig. 6. Cross-sectional TEM of C-plane sapphire coated with GaN precursor.
Fig. 5. SEM picture of the surface of C-plane sapphire coated with GaN precursor. A bimodal grain size distribution is found on the substrate surface.
The surface morphology of the "lms was studied by SEM. Fig. 5 shows an SEM image of the sample. The "lm consists of grains with a bimodal size distribution. The larger grains have a mean diameter of &150 nm and a drastically lower density per area than the smaller grains (&10 nm in diameter). The smaller grains cover most of the surface. Fig. 6 shows a cross-sectional bright "eld TEM micrograph of the multiple-coated C-plane substrate. Consistent with the SEM data, a high density of &10 nm grains was observed on the a-Al O sur2 3 face, along with a few larger a-GaN grains, which had an average size of &150 nm. Selected area di!raction revealed that the larger grains had no special orientation relationship with the substrate. A planview TEM micrograph of this "lm can be seen in Fig. 7. Consistent with images of the crosssectional sample, it could be shown that the small grains were randomly oriented (compare SAD
Fig. 7. Planview TEM and SAD of C-plane sapphire coated with GaN precursor near zone axis a-Al O [0 0 0 1]. The 2 3 a-GaN grains are randomly oriented, resulting in a SAD ring pattern.
pattern in Fig. 7). Their di!raction patterns "t hexagonal GaN as well as hexagonal AlN since the lattice parameters di!er only slightly. A "rst EELS analysis, carried out in cross-sectional STEM, indicated that these grains consisted of Ga, Al, N and O. A more detailed analysis of the chemistry of the "lm is in preparation. 4. Discussion 4.1. Growth on R-plane a-Al2 O3 The precursor system developed by Rodewald et al. [9] is well suited for growing gallium nitride
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thin "lms on sapphire substrates. GaN grew on R-plane a-Al O with a well-de"ned epitaxial ori2 3 entation relationship. The formation of this orientation is favoured most likely by the low lattice mismatch f"1.3% between GaN and a-Al O 2 3 along the [0 1 11 1] a-Al O direction [15] and 2 3 presumably by a low interface energy. Although the sequential crystallisation process was not investigated, the epitaxial GaN nuclei formed in all probability at the interface of the sapphire substrate and the amorphous precursor layer as reported for many other systems [7]. Certainly, further work is needed to clarify details of particular nucleation and growth processes during pyrolysis [16,17]. The facetting of the "lm is caused most likely by the anisotropy of the surface energy of a-GaN. However, it cannot be excluded that other factors dominated this behaviour: (i) di!erences in growth rates of di!erent a-GaN facets, (ii) coherency strain in the "lm or islands in the very initial growth state, or (iii) materials transport phenomena in the gas phase during annealing in an ammonia atmosphere. All these factors may lead to surface morphologies unattainable by conventional thin "lm techniques. It is noteworthy that GaN "lms with a similar facetted surface morphology have been produced by metal-organic vapour phase epitaxy [18] and plasma-assisted MBE [19]. However, the size distributions of the facets were much narrower than the one produced by precursor pyrolysis. 4.2. Growth on C-plane a-Al2 O3 The growth behaviour of GaN on C-plane aAl O was very di!erent to that on R-plane. On 2 3 C-plane, a thin polycrystalline "lm formed with the production of only a few larger a-GaN grains. Although the XRD data indicated an orientation relation, a-Al O (0 0 0 1)Ea-GaN(0 0 0 1) (OR II), 2 3 the fraction of grains with this orientation was too small to be quanti"ed in the TEM. The di!erences of morphology are most likely due to a change in wetting and crystallisation behaviour of the liquid precursor on the C-plane sapphire substrate. Experiments dedicated to this questions will be carried out in the future. However, this bimodal island-like structure is apparently spe-
ci"c for the precursor route, since similar structures have not been reported for MBE- or CVD-produced GaN layers on C-plane sapphire yet. Surfaces are generally described to have a small roughness compared to the layer thickness; some authors have reported about a columnar structure in their "lms [12,20,21].
5. Conclusions Thin epitaxial "lms of hexagonal GaN could be grown on R-plane sapphire substrates by a chemical solution deposition method. The "lms consisted of small, highly oriented grains with a single epitaxial orientation. These results encourage further exploration of this method, especially in regard to nucleation, growth mechanism and the defect structure.
Acknowledgements We thank Dr. Ph. Kohler-Redlich and Dr. W. Sigle for the EELS measurements in the STEM. This research project was supported by the Volkswagen-Stiftung (Project No. I/72 527 and I/72 530). For F.F. Lange, this work was supported by the MRL Program of the National Science Foundation under Award No. DMR-96-32716 and a NSF Travel Grant, INT-9726295. The Max Planck Research Award Grant is also gratefully acknowledged.
References [1] D.E. Lacklison, J.W. Orton, I. Harrison, T.S. Cheng, L.C. Jenkins, C.T. Foxon, S.E. Hooper, J. Appl. Phys. 78 (3) (1995) 1838. [2] S. Nakamura, G. Fasol, The Blue Laser Diode: GaN based Light Emitters and Lasers, Springer, Berlin, 1997. [3] S.J. Pearton, C. Kuo, MRS Bull. 22 (2) (1997) 17. [4] M.E. Lin, B.N. Sverdlov, H. Morkoc, J. Appl. Phys. 74 (8) (1993) 5038. [5] C.-R. Lee, S.-J. Son, I.-H. Lee, J.-Y. Leem, S.K. Noh, J. Crystal Growth 182 (1997) 11. [6] S. Nakamura, Jpn. J. Appl. Phys. 30 (1991) L1705. [7] F.F. Lange, Science 273 (1996) 903.
M. Puchinger et al. / Journal of Crystal Growth 208 (2000) 153}159 [8] J. Bill, F. Aldinger, Z. Metallk. 87 (1996) 827. [9] D. Rodewald, J. Bill, U. Beck, M. Puchinger, T. Wagner, A. Greiner, F. Aldinger, personal communication. [10] A. Strecker, U. Salzberger, J. Mayer, Prakt. Metallogr. 30 (10) (1993) 482. [11] W.A. Melton, J.I. Pankove, J. Crystal Growth 178 (1997) 168. [12] T. Lei, K.F. Ludwig Jr., T.D. Moustakas, J. Appl. Phys. 74 (7) (1993) 4430. [13] T. Metzger, H. Angerer, O. Ambacher, M. Stutzmann, E. Born, Phys. Stat. Sol. (b) 193 (1996) 391. [14] JCPDS-ICDD "le 2-1078 (hexagonal GaN).
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[15] T.D. Moustakas, R.J. Molnar, T. Lei, G. Menon, C.R. Eddy, Mat. Res. Soc. Symp. Proc. 242 (1992) 427. [16] C.V. Thompson, J. Floro, H.I. Smith, J. Appl. Phys. 67 (9) (1990) 4099. [17] K.T. Miller, F.F. Lange, D.B. Marshall, J. Mater. Res. 5 (1) (1990) 151. [18] T. Sasaki, S. Zembutsu, J. Appl. Phys. 61 (7) (1987) 2533. [19] C.R. Eddy, T.D. Moustakas, J. Scanlon, J. Appl. Phys. 73 (1) (1993) 448. [20] F.A. Ponce, MRS Bull. 22 (2) (1997) 51. [21] X.J. Ning, F.R. Chien, P. Pirouz, J.W. Yang, M.A. Khan, J. Mater. Res. 11 (3) (1996) 580.
Gallium nitride thin layers via a liquid precursor route Manfred Puchinger!,*, Thomas Wagner!, Dieter Rodewald",1, Joachim Bill", Fritz Aldinger", Frederick F. Lange# !Max-Planck-Institut fu( r Metallforschung, Seestrasse 92, D-70174 Stuttgart, Germany "Max-Planck-Institut fu( r Metallforschung und Institut fu( r Nichtmetallische Anorganische Materialien, Pulvermetallurgisches Laboratorium, Universita( t Stuttgart, Heisenbergstrasse 5, D-70569 Stuttgart, Germany #Materials Department, University of California, Santa Barbara, CA 93106-5050, USA Received 27 April 1999; accepted 8 July 1999 Communicated by C.R. Abernathy
Abstract A chemical solution deposition method was used to grow thin epitaxial GaN "lms on C- and R-plane sapphire substrates. The "lms were grown by spin-coating a gallium carbodiimide based polymeric precursor onto sapphire and pyrolyzing in NH at 9003C. During heat treatment a-GaN formed on the R-plane of the sapphire with the following 3 epitaxial orientation relationship: a-Al O (0 1 11 2)Ea-GaN(1 1 21 0); a-Al O [2 11 11 0]Ea-GaN[1 11 0 0]. A multiple coat2 3 2 3 ing process resulted in "lms fully covering the R-plane substrates. Films deposited on C-plane sapphire were not continuous and single islands were present on the substrate surface. The morphology and microstructure of the "lms were characterised by SEM, XRD, and conventional and analytical electron microscopy. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 81.15.!z; 68.55.Jk Keywords: Thin "lms; Precursor pyrolysis; Chemical solution deposition; Gallium nitride (GaN); Sapphire (Al O ); Transmission 2 3 electron microscopy (TEM)
1. Introduction In recent time, gallium nitride has received increasing attention due to the development of highly e$cient blue light emitting diodes (LEDs) and laser diodes. Hexagonal GaN posesses a direct band gap
* Corresponding author. Tel.: #49-711-2095-241; fax: #49711-2095-120. E-mail address: [email protected] (M. Puchinger) 1 Present address: Elastogran GmbH, LemfoK rde, Germany.
of about 3.4 eV [1] which makes it well suited for short-wavelength optical devices [2,3]. Standard methods for the production of GaN thin "lms are molecular beam epitaxy (MBE) and chemical vapour deposition (CVD) [4,5]. MBE methods generally use metallic gallium and molecular nitrogen (plasma-enhanced or excited by electron cyclotron resonance) as material sources, whereas most CVD methods apply volatile metalorganic precursors like trimethylgallium or triethylgallium and ammonia (MOCVD). In these growth processes, the "lm can be deposited atomby-atom. a-Al O (sapphire) is the most common 2 3
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substrate material; bu!er layers of AlN or GaN were found to improve the crystal quality of the deposited GaN "lms [2,6]. A di!erent approach for the fabrication of thin "lms is the chemical solution deposition method where a precursor solution is deposited by spin- or dip-coating onto a single-crystal substrate [7,8]. After evaporation of the solvent, the precursor "lm is heat-treated (pyrolysed) in an appropriate atmosphere, until transformation of the polymeric precursor material into a poly- or single-crystal thin "lm takes place. The main advantage of this process is its simplicity compared to conventional CVD and MBE processes. However, relative to vapour phase epitaxy, little is known about the correlation between process parameters and materials properties for this method [7]. Recently, we developed a precursor for the preparation of hexagonal (wurtzite, a-) GaN thin "lms from a precursor solution by pyrolysis under ammonia [9]. In the present paper we present the precursor preparation, the deposition and the microstructure of thin GaN "lms which were produced by spin-coating on C-plane (0 0 0 1)- and R-plane (0 1 11 2) a-Al O . To our knowledge, this 2 3 is the "rst time that epitaxial GaN thin "lms have been grown successfully by the chemical solution deposition method. These "lms may have the potential to serve as bu!er layers or seeds in conventional GaN thin "lm growth techniques.
2. Experimental procedure 2.1. Precursor preparation The precursor solution was prepared in an argon atmosphere. 3.7 g (21 mmol) gallium chloride (GaCl ) were stirred with 120 ml (530 mmol) 3 of bis(trimethylsilyl)carbodiimide (Me Si)NCN3 (SiMe ) (Me"methyl) for three days at room tem3 perature, providing a homogeneous solution of the gallium chloride. No other solvents were added. The resulting solution was re#uxed for 9 h at &1703C. During the reaction, the trimethylsilyl groups are substituted by Ga atoms which can be bonded to three carbodiimide groups altogether.
Me SiCl is formed and some polymer percipitates. 3 After "ltering, a transparent, yellow solution was obtained, consisting of a gallium- and carbodiimide-containing polymer dissolved in bis(trimethylsilyl)carbodiimide. This served as the precursor solution. In a seperate experiment, bis(trimethylsilyl)carbodiimide was removed from the precursor solution by distillation. A weakly yellowish polymer powder was obtained after drying the residue in vacuum at 2103C. A chemical analysis revealed a composition of Ga(NCN) (SiMe ) Cl . 1.05 3 0.20 1.05 This indicates an incomplete reaction between chlorine atoms and trimethylsilyl groups during the precursor preparation. 2.2. Film growth Polished C- and R-plane a-Al O substrates 2 3 (CrysTec, Berlin, Germany) were mounted by tape onto a spincoater. Two to three drops of precursor solution were put on the substrate surface in argon atmosphere. Subsequently, the disc was rotated at high speed (see Table 1) for about 30 s. The samples were then put into a sealed schlenk tube and placed into a tube furnace. After a quick #ush with ammonia gas (NH ), a constant #ow of ammonia inside 3 the schlenk tube was adjusted (roughly 1}5 cm3/s, 105 Pa) avoiding a direct gas #ow over the sample surface. The furnace was heated at 3.5 K/min starting from room temperature (RT) to 1003C and at 5 K/min from 100 to 9003C. Following a dwell time of 6 min, the furnace was cooled at 4.3 K/min to RT. To grow thicker "lms, the above process was performed three times without exposure to air. 2.3. Characterisation The structure and chemical composition of the "lms were characterized by X-ray di!raction Table 1 GaN samples prepared by repeated spincoating a liquid precursor solution on a sapphire substrate Sample
Sapphire orientation
Spinning speeds (rpm)
1 2
R-plane C-plane
2000/2000/2000 6000/4000/4000
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Fig. 1. XRD (h}2h) scans of GaN thin "lms grown on sapphire prepared by precursor pyrolysis.
(XRD) (Siemens D 500), scanning electron microscopy (SEM) (Zeiss DSM 982 Gemini), and conventional and analytical electron microscopy (Jeol JEM 200 CX; Jeol JEM 2000 FX, VG HB501). Cross-sectional TEM specimen preparation followed a procedure described by Strecker et al. [10].
3. Experimental results 3.1. Films on R-plane a-Al2 O3 XRD h}2h-scans were performed to determine the structure of the thin "lms after pyrolysis (Fig. 1a). The XRD pattern of the R-plane specimen shows a strong re#ection at 57.93 which can be attributed to a-GaN(1 1 21 0). No other di!raction peaks from the "lm were detected. An SEM image of a multiple coated R-plane substrate is shown in Fig. 2, demonstrating that the surface morphology is extremely inhomogeneous. Strongly facetted regions (diameter of each facet 50}300 nm) are observed. Apart from these areas, structures with much smaller diameter (about 20 nm) can be seen. Between the facetted regions
Fig. 2. SEM picture of the surface of R-plane sapphire coated with GaN precursor. The surface is facetted.
and the areas with "ner features, both gradual and abrupt changes of the facet size are observable. Fig. 3 shows a bright "eld TEM image of the GaN "lm and the a-Al O substrate. The incident 2 3 electron beam was along the [2 11 11 0] a-Al O 2 3 direction. An array of regularly formed a-GaN facets covers the substrate. The facets have a pyramidal shape with a mean height of &50 nm
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Fig. 4. Cross-sectional TEM of R-plane sapphire coated with GaN precursor; zone axis a-Al O [0 11 1 1], respectively, a2 3 GaN [0 0 0 1]. Half hexagons are visible.
Fig. 3. Cross-sectional TEM and SAD of R-plane sapphire coated with GaN precursor; zone axis a-Al O [2 11 11 0], respec2 3 tively, a-GaN[1 11 0 0]. A pyramidal structure of epitaxial aGaN covers the substrate.
and a mean base of 100}150 nm, in good agreement with the SEM results. Apparently, the notch between two neighbouring pyramids does not reach the substrate. As revealed by SAD (selected area di!raction), all islands are epitaxial with the following orientation relationship (OR I): a-Al O (0 1 11 2)Ea-GaN(1 1 21 0) 2 3 and a-Al O [2 11 11 0]Ea-GaN[1 11 0 0]. 2 3 The same orientation relationship is found when GaN "lms are grown on sapphire R-plane
substrates by MBE and MOCVD [11}13]. Fig. 4 shows a bright-"eld TEM micrograph of the same interface along the [0 11 1 1] a-Al O direction. No 2 3 pyramidal shapes are observed in this direction, instead symmetrical structures resembling half hexagons are seen. The corresponding SAD pattern reveals the same epitaxial orientation relationship, OR I. In order to determine the chemical composition of the "lm, EELS (electron energy loss spectroscopy) measurements were performed on a thinner "lm (prepared by a single spincoating step) in a STEM (scanning transmission electron microscope). The electron beam was placed at the middle of a 50 nm wide pyramid, like those shown in Fig. 3. The typical area investigated was 3]4 nm. Results of the chemical analysis can be seen in Table 2. The atomic concentrations of Ga and N were almost equal and only oxygen impurities could be detected. The measured atomic concentration of oxygen was about 6%. 3.2. Films on C-plane a-Al2 O3 The XRD pattern of the sample prepared on C-plane sapphire (see Fig. 1b) shows two maxima that can be attributed to the "lm. The re#ection at 34.63 "ts a-GaN(0 0 0 2), while the peak at 36.93 belongs to a-GaN(1 0 11 1), the most intense peak in powder di!raction [14].
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Table 2 Chemical composition of a "lm prepared by a single coating step of an R-plane sapphire substrate, measured by EELS in the STEM Element Ga N O Other, each
Fraction (at%) 46.0 48.0 5.6 (1
Deviation (at%) $6.2 $6.5 $1.0 $1.0
Fig. 6. Cross-sectional TEM of C-plane sapphire coated with GaN precursor.
Fig. 5. SEM picture of the surface of C-plane sapphire coated with GaN precursor. A bimodal grain size distribution is found on the substrate surface.
The surface morphology of the "lms was studied by SEM. Fig. 5 shows an SEM image of the sample. The "lm consists of grains with a bimodal size distribution. The larger grains have a mean diameter of &150 nm and a drastically lower density per area than the smaller grains (&10 nm in diameter). The smaller grains cover most of the surface. Fig. 6 shows a cross-sectional bright "eld TEM micrograph of the multiple-coated C-plane substrate. Consistent with the SEM data, a high density of &10 nm grains was observed on the a-Al O sur2 3 face, along with a few larger a-GaN grains, which had an average size of &150 nm. Selected area di!raction revealed that the larger grains had no special orientation relationship with the substrate. A planview TEM micrograph of this "lm can be seen in Fig. 7. Consistent with images of the crosssectional sample, it could be shown that the small grains were randomly oriented (compare SAD
Fig. 7. Planview TEM and SAD of C-plane sapphire coated with GaN precursor near zone axis a-Al O [0 0 0 1]. The 2 3 a-GaN grains are randomly oriented, resulting in a SAD ring pattern.
pattern in Fig. 7). Their di!raction patterns "t hexagonal GaN as well as hexagonal AlN since the lattice parameters di!er only slightly. A "rst EELS analysis, carried out in cross-sectional STEM, indicated that these grains consisted of Ga, Al, N and O. A more detailed analysis of the chemistry of the "lm is in preparation. 4. Discussion 4.1. Growth on R-plane a-Al2 O3 The precursor system developed by Rodewald et al. [9] is well suited for growing gallium nitride
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thin "lms on sapphire substrates. GaN grew on R-plane a-Al O with a well-de"ned epitaxial ori2 3 entation relationship. The formation of this orientation is favoured most likely by the low lattice mismatch f"1.3% between GaN and a-Al O 2 3 along the [0 1 11 1] a-Al O direction [15] and 2 3 presumably by a low interface energy. Although the sequential crystallisation process was not investigated, the epitaxial GaN nuclei formed in all probability at the interface of the sapphire substrate and the amorphous precursor layer as reported for many other systems [7]. Certainly, further work is needed to clarify details of particular nucleation and growth processes during pyrolysis [16,17]. The facetting of the "lm is caused most likely by the anisotropy of the surface energy of a-GaN. However, it cannot be excluded that other factors dominated this behaviour: (i) di!erences in growth rates of di!erent a-GaN facets, (ii) coherency strain in the "lm or islands in the very initial growth state, or (iii) materials transport phenomena in the gas phase during annealing in an ammonia atmosphere. All these factors may lead to surface morphologies unattainable by conventional thin "lm techniques. It is noteworthy that GaN "lms with a similar facetted surface morphology have been produced by metal-organic vapour phase epitaxy [18] and plasma-assisted MBE [19]. However, the size distributions of the facets were much narrower than the one produced by precursor pyrolysis. 4.2. Growth on C-plane a-Al2 O3 The growth behaviour of GaN on C-plane aAl O was very di!erent to that on R-plane. On 2 3 C-plane, a thin polycrystalline "lm formed with the production of only a few larger a-GaN grains. Although the XRD data indicated an orientation relation, a-Al O (0 0 0 1)Ea-GaN(0 0 0 1) (OR II), 2 3 the fraction of grains with this orientation was too small to be quanti"ed in the TEM. The di!erences of morphology are most likely due to a change in wetting and crystallisation behaviour of the liquid precursor on the C-plane sapphire substrate. Experiments dedicated to this questions will be carried out in the future. However, this bimodal island-like structure is apparently spe-
ci"c for the precursor route, since similar structures have not been reported for MBE- or CVD-produced GaN layers on C-plane sapphire yet. Surfaces are generally described to have a small roughness compared to the layer thickness; some authors have reported about a columnar structure in their "lms [12,20,21].
5. Conclusions Thin epitaxial "lms of hexagonal GaN could be grown on R-plane sapphire substrates by a chemical solution deposition method. The "lms consisted of small, highly oriented grains with a single epitaxial orientation. These results encourage further exploration of this method, especially in regard to nucleation, growth mechanism and the defect structure.
Acknowledgements We thank Dr. Ph. Kohler-Redlich and Dr. W. Sigle for the EELS measurements in the STEM. This research project was supported by the Volkswagen-Stiftung (Project No. I/72 527 and I/72 530). For F.F. Lange, this work was supported by the MRL Program of the National Science Foundation under Award No. DMR-96-32716 and a NSF Travel Grant, INT-9726295. The Max Planck Research Award Grant is also gratefully acknowledged.
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