- Email: [email protected]
High-pressure high-temperature synthesis of GaN with melamine as the nitrogen source
Diamond & Related Materials 15 (2006) 1242 – 1245 www.elsevier.com/locate/diamond
High-pressure high-temperature synthesis of GaN with melamine as the nitrogen source J. Zhang, Q. Cui *, Y. Xie, H. Jiao, X. Li, Y. Wang, H. Ma, L. Shen, G. Zou National Laboratory of Superhard Materials, Jilin University, Changchun, 130012, PR China Received 6 April 2005; received in revised form 26 July 2005; accepted 28 September 2005 Available online 23 November 2005
Abstract Micron-sized grains of gallium nitride (GaN) crystallizing in the Wurtzite phase were synthesized through a chemical reaction between gallium (Ga) metal and melamine (C3N6H6). The reaction occurred at the temperature range from 1073 to 1473 K and the pressure range from 3.5 to 5.5 GPa. X-ray diffraction (XRD) and transmission electron microscopy (TEM) investigations showed that the final black products mainly contained the clusters of tiny GaN crystals. Prism-like well-shaped single crystals were found in the TEM micrographs. A vapor – liquid – solid growth process was proposed to explain the growth mechanism of GaN in which the pyrolysis of melamine was responsible for the provision of reactive nitrogen. D 2005 Elsevier B.V. All rights reserved. Keywords: Gallium nitride; High-pressure high-temperature crystal growth
1. Introduction There has been a boom in GaN-based semiconductor technology in the past decades, which is regarded as one of the most important progress in material science and technology. Thin layers of GaN have been grown on foreign substrates by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) methods [1 – 5]. Blue lightemitting diodes (LED) based on GaN are now commercially available [6]. Due to its high saturated electron drift velocity, high thermal conductivity and thermal stability at high temperatures as well as a wide direct band gap (3.46 eV at 300 K), GaN is considered one of the most promising materials for blue to ultraviolet optoelectronics and high-power, highfrequency devices. A major hindrance in the development of GaN technology is the lack of a suitable substrate for MBE or MOCVD epitaxy in that the small lattice constants and thermal expansion coefficient of GaN bring about large lattice and/or thermal mismatches with all conventional substrates. However, a suitable substrate may be simply made of GaN itself. Thus, synthesis of substrate quality bulk crystals as well as fine powders of GaN has attracted considerable attention. * Corresponding author. Fax: +86 431 5168883. E-mail address: [email protected] (Q. Cui). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.09.044
Growth of GaN fine powders falls mainly into the category of heating metallic gallium or gallium oxide in an atmosphere containing ammonia. Examples abound in the literature such as GaN powders prepared from gallium and ammonia at temperatures of 1273 –1473 K [7], micron-sized particles of GaN synthesized in a Na flux at a temperature of 973– 1073 K [8,9] and by using Li metal as the nitrogen fixant at a temperature as low as 573 K [10]. In the latter two cases, the additional alkali metals form alloys with gallium and mediate the nucleation of GaN, thus lowering the reaction temperature. Two methods are generally employed to grow large-scale bulk crystals of GaN. One is the so-called sublimation method in which a chemical vapor transfer of gallium and nitrogen elements is involved. Up to 3-mm-long GaN single crystals have been grown by the sublimation of cold pressed GaN pellets in an ammonia flow [11]. On the other hand, Porowski and co-workers have developed a high nitrogen pressure high-temperature technique and single crystals with surface areas up to tens of mm2 have been successfully synthesized under nitrogen pressures up to 2 GPa and temperatures up to 2000 K [12 –14]. Both of these methods require a long run of tens of hours. In this letter, we report the synthesis of fine GaN powders using a high-pressure high-temperature technique. At the present study, metal gallium and melamine (C3N6H6) have been chosen as the starting materials.
J. Zhang et al. / Diamond & Related Materials 15 (2006) 1242 – 1245
1243
2. Experiments The reactants we used were gallium metal with the purity higher than 99.999% and fine powders of melamine with the purity no less than 99%. A schematic diagram of the sample chamber is shown in Fig. 1. At first the melamine powders were shaped into a solid column with 10 mm in diameter and 8 mm in height by press at room temperature. Then a hole of 6 mm in diameter was drilled through the axis of the column. After that, the gallium metal was melted and poured into the hole under a nitrogen atmosphere by using an alcohol burner. Finally, melamine pellets were used to enclose the two end surfaces of the column. The synthesis of gallium nitride was carried out using a six-anvil high-pressure apparatus with a sample chamber of 23 mm on the edges at the temperature range from 1073 to 1473 K and the pressure range from 3.5 to 5.5 GPa. The pressure-transmitting medium was pyrophyllite. Heat was transferred into the system by a graphite heater. A 30-minute experimental run was adopted for all of the samples during which the experimental pressure and temperature were kept almost invariable. Structural analysis of the products was carried out by powder X-ray diffractometry (XRD) on a D8 DISCOVER GADDS diffractometer with Cu Ka radiation. The XRD patterns were indexed by using the reflex module combined in the Materials Studio program (Accelrys Inc.). Morphologies of the clusters and the grains of GaN were examined by transmission electron microscopy (TEM) on a H-8100 electron microscope. 3. Results and discussions XRD patterns of the GaN samples synthesized at several typical conditions are shown in Fig. 2. The diffractions of GaN, Ga2O3 and amorphous carbon are marked with N, O and a-C, respectively. It can be seen that at 3.5 GPa, the shapes of the peaks of GaN grow sharper with increasing temperature (Fig. 2(a), (b) and (c)). At higher pressure and temperature (Fig. 2(d)), the diffractions of Ga2O3 (for reference, see PDF # 85-
Fig. 2. XRD patterns of the GaN samples synthesized at several typical conditions: (a) 3.5 GPa, 1173 K; (b) 3.5 GPa, 1273 K; (c) 3.5 GPa, 1373 K and (d) 5.5 GPa, 1473 K.
0988) emerge besides those of GaN. The two bands around 26.5- and 43.5- are the characteristic diffractions of amorphous carbon. They persist at low pressures (Fig. 2(a), (b) and (c)) and disappear at higher pressures (Fig. 2(d)). In order to get specific information from the XRD patterns, we have indexed the XRD pattern of the sample synthesized at 3.5 GPa and 1373 K by using the reflex module combined in the Materials Studio program. Ten peaks are distinguished and used for the indexing. The peaks can be indexed unambiguously as the diffractions of GaN crystals with Wurtzite structure. The lattice ˚ and c = 5.192 T 0.004 A ˚ . The parameters are a = 3.193 T 0.002 A indexing results are listed in Table 1. The Miller indices corresponding to the peaks are also shown in Fig. 2 in brackets. From Fig. 2, it can be concluded that crystalline GaN has been grown by the high-pressure high-temperature technique we have designed and that 3.5 GPa and 1373 K may be the Table 1 The indexing results of the XRD pattern of the GaN sample synthesized at 3.5 GPa and 1373 K HKL DOBS DCAL DIF.D 100 002 101 102 110 103 200 112 201 004
Fig. 1. A schematic diagram of the sample chamber: 1 — steel cap, 2 — pyrophyllite, 3 — dolomite tube, 4 — graphite heater, 5 — quartz tube, 6 — gallium, 7 — melamine, 8 — quartz pellet, 9 — multilayer of graphite and alloys.
2.757 2.589 2.433 1.890 1.597 1.467 1.383 1.360 1.338 1.299
2.766 2.596 2.441 1.893 1.597 1.467 1.383 1.360 1.336 1.298
0.009 0.007 0.008 0.003 0.000 0.000 0.000 0.000 0.002 0.001
2TH.OBS 2TH.CAL DIF.2TH. INT. (a. u.) 32.45 34.62 36.92 48.11 57.67 63.34 67.69 68.98 70.32 72.74
32.34 34.52 36.79 48.03 57.69 63.34 67.70 69.00 70.40 72.81
0.11 0.10 0.13 0.08 0.02 0.00 0.01 0.02 0.08 0.07
422 352 999 145 192 195 40 162 96 28
XRD—x-ray diffraction; HKL—the miller indices; DOBS—the observed dspacings; DCAL—the calculated d-spacings; DIF.D—the differences between the observed and the calculated d-spacings; 2TH.OBS—the observed diffraction angle (2 theta); 2TH.CAL—the calculated diffraction angle (2 theta); DIF.2TH.—the differences between the observed and the calculated diffraction angle (2 theta); INT. (a. u.)—the intensity (arbitrary unit).
1244
J. Zhang et al. / Diamond & Related Materials 15 (2006) 1242 – 1245
optimum condition. The diffractions marked with U may originate form the pyrolysis products of melamine. They are not of interest here and will be discussed elsewhere. A typical TEM graph of the GaN sample synthesized at 3.5 GPa and 1373 K is shown in Fig. 3. Together with it is the transmission electron beam diffraction pattern of the area marked with F. It can be seen in Fig. 3 that the sample contains mainly grains with 0.2 to 1.5 Am in size. The grains have irregular shapes, yet the electron beam diffraction pattern of that selected area shows clear diffraction spots indicating that small single crystals may be found. The diffraction spots possess hexagonal symmetry. While the carbon contained in the sample has an amorphous nature, the diffraction pattern is supposed to be originated from a GaN single crystal that has underwent unstable growing process due to edge nucleation. Several well-shaped prisms of GaN are seen in another TEM graph as shown in Fig. 4. Fig. 4(b) is the magnified copy of the lower right region of Fig. 4(a). It can be seen from Fig. 4 that regular columnar shaped single crystals with about 0.6 Am in length and 0.1 Am in diameter may grow under conditions of 1373 K and 3.5 GPa. We assume that vapor– liquid – solid growth may be the most likely mechanism for the formation of GaN in our case. At a proper temperature and pressure condition, the pyrolysis of melamine takes place following such formula C3 N6 H6 6 3CðamorphousÞ þ 3N2 þ 2NH3 The amorphous carbon may be deemed as the result of the irregular stacking of tiny graphite-like carbon layers. The simultaneous nucleation of large numbers of sites is a reasonable result of the pyrolysis of melamine under high pressure and the growth into large and perfect graphite crystals is prevented. The nitrogen and ammonia vapors are driven to diffuse towards gallium by the pressure gradient. Reactive nitrogen atoms are produced by the dissociation of those vapors at the surface of liquid state gallium. N2 6 2N NH3 6 N þ 3H NðfreeÞ 6 Nðdissolved in GaÞ
Fig. 4. TEM of a well-shaped GaN single crystal.
Thus, fast surface nucleation of GaN occurs due to the high supersaturation generated by high pressure [13] and a surface layer of GaN may be formed. Since the gallium melt is in a temperature gradient, the GaN surface layer will dissolve in gallium and be transported by convection and diffusion to the central parts of gallium where the solution becomes supersaturated since the temperature is a little lower. Thus, the crystallization of GaN takes place. The reactions mentioned above will be obeyed exactly at 3.5 GPa and 1173 to 1373 K and thus gallium can be fully nitrified. At higher pressures, the pyrolysis of melamine will produce some CN compounds and complexes and the yield of reactive nitrogen is reduced. Thus, gallium is only partially nitrified and the remaining gallium will be oxidated upon exposure to air (Fig. 2). Since the surface layer is the natural source of growth centers, high supersaturation will lead to the simultaneous crystallization of a large amount of GaN crystals, which inhibits the further growth of the nitride. Very high supersaturation will also induce the edge nucleation of the GaN grains. Such grains will undergo unstable growth process and take irregular shapes (Fig. 3). 4. Conclusions In conclusion, we have carried out experiments on metal gallium and melamine at the temperature range from 1073 to 1473 K and the pressure range from 3.5 to 5.5 GPa. to synthesize GaN. Irregularly shaped GaN grains with the size of 0.1 to 2 Am have been grown at about 1373 K and 3.5 GPa within 30 min. Regular columnar-shaped GaN single crystals with about 0.6 Am in length and 0.1 Am in diameter have also been fabricated. A vapor – liquid –solid (VLS) process has been proposed to explain the growth mechanism and the experimental phenomena. Acknowledgements
Fig. 3. TEM and electron diffraction pattern of the GaN sample synthesized at 3.5 GPa and 1373 K.
This work is supported by the National Foundations for Natural Science of China, No. 50072005, and the International Cooperation Plan of the Ministry of Science and Technology in China, No. 2001CB711201.
J. Zhang et al. / Diamond & Related Materials 15 (2006) 1242 – 1245
References [1] H. Amano, M. Kito, K. Hiramatsu, I. Akasaki, Jpn. J. Appl. Phys. 28 (1989) 2112. [2] S. Nakamura, T. Mukai, M. Senoh, N. Iwasa, Jpn. J. Appl. Phys. 31 (1992) 139. [3] S. Gennari, G. Attolini, C. Pelosi, P. Lottici, F. Ricco, M. Labardi, M. Allegrini, C. Frediani, J. Cryst. Growth 166 (1996) 309. [4] R. Moon, J. Cryst. Growth 170 (1997) 1. [5] T. Tokuda, A. Wakahara, S. Noda, A. Sasaki, J. Cryst. Growth 173 (1997) 237. [6] Nichia Chemical Industries Ltd., Ogawa Building, Shiba 5-Chrome, Minotaku, Tokyo 108, Japan.
1245
[7] G. Kamler, J. Zachara, S. Podsiadlo, L. Adamowicz, W. Gebicki, J. Cryst. Growth 212 (2000) 39. [8] S. Barry, S. Ruoff, A. Ruoff, Chem. Mater. 10 (1998) 2571. [9] H. Yamane, M. Shimada, T. Sekiguchi, F. DiSalvo, J. Cryst. Growth 186 (1998) 8. [10] M. Yano, M. Okamoto, Y. Yap, M. Yoshimura, Y. Mori, T. Sasaki, Diamond Relat. Mater. 9 (2000) 512. [11] C. Balkas, Z. Sitar, L. Bergman, I. Shmagin, J. Muth, R. Kolbas, R. Nemanich, R. Davis, J. Cryst. Growth 208 (2000) 100. [12] S. Porowski, J. Cryst. Growth 166 (1996) 583. [13] S. Porowski, I. Grzegory, J. Cryst. Growth 178 (1997) 174. [14] M. Bockowski, Physica B 265 (1999) 1.
High-pressure high-temperature synthesis of GaN with melamine as the nitrogen source J. Zhang, Q. Cui *, Y. Xie, H. Jiao, X. Li, Y. Wang, H. Ma, L. Shen, G. Zou National Laboratory of Superhard Materials, Jilin University, Changchun, 130012, PR China Received 6 April 2005; received in revised form 26 July 2005; accepted 28 September 2005 Available online 23 November 2005
Abstract Micron-sized grains of gallium nitride (GaN) crystallizing in the Wurtzite phase were synthesized through a chemical reaction between gallium (Ga) metal and melamine (C3N6H6). The reaction occurred at the temperature range from 1073 to 1473 K and the pressure range from 3.5 to 5.5 GPa. X-ray diffraction (XRD) and transmission electron microscopy (TEM) investigations showed that the final black products mainly contained the clusters of tiny GaN crystals. Prism-like well-shaped single crystals were found in the TEM micrographs. A vapor – liquid – solid growth process was proposed to explain the growth mechanism of GaN in which the pyrolysis of melamine was responsible for the provision of reactive nitrogen. D 2005 Elsevier B.V. All rights reserved. Keywords: Gallium nitride; High-pressure high-temperature crystal growth
1. Introduction There has been a boom in GaN-based semiconductor technology in the past decades, which is regarded as one of the most important progress in material science and technology. Thin layers of GaN have been grown on foreign substrates by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) methods [1 – 5]. Blue lightemitting diodes (LED) based on GaN are now commercially available [6]. Due to its high saturated electron drift velocity, high thermal conductivity and thermal stability at high temperatures as well as a wide direct band gap (3.46 eV at 300 K), GaN is considered one of the most promising materials for blue to ultraviolet optoelectronics and high-power, highfrequency devices. A major hindrance in the development of GaN technology is the lack of a suitable substrate for MBE or MOCVD epitaxy in that the small lattice constants and thermal expansion coefficient of GaN bring about large lattice and/or thermal mismatches with all conventional substrates. However, a suitable substrate may be simply made of GaN itself. Thus, synthesis of substrate quality bulk crystals as well as fine powders of GaN has attracted considerable attention. * Corresponding author. Fax: +86 431 5168883. E-mail address: [email protected] (Q. Cui). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.09.044
Growth of GaN fine powders falls mainly into the category of heating metallic gallium or gallium oxide in an atmosphere containing ammonia. Examples abound in the literature such as GaN powders prepared from gallium and ammonia at temperatures of 1273 –1473 K [7], micron-sized particles of GaN synthesized in a Na flux at a temperature of 973– 1073 K [8,9] and by using Li metal as the nitrogen fixant at a temperature as low as 573 K [10]. In the latter two cases, the additional alkali metals form alloys with gallium and mediate the nucleation of GaN, thus lowering the reaction temperature. Two methods are generally employed to grow large-scale bulk crystals of GaN. One is the so-called sublimation method in which a chemical vapor transfer of gallium and nitrogen elements is involved. Up to 3-mm-long GaN single crystals have been grown by the sublimation of cold pressed GaN pellets in an ammonia flow [11]. On the other hand, Porowski and co-workers have developed a high nitrogen pressure high-temperature technique and single crystals with surface areas up to tens of mm2 have been successfully synthesized under nitrogen pressures up to 2 GPa and temperatures up to 2000 K [12 –14]. Both of these methods require a long run of tens of hours. In this letter, we report the synthesis of fine GaN powders using a high-pressure high-temperature technique. At the present study, metal gallium and melamine (C3N6H6) have been chosen as the starting materials.
J. Zhang et al. / Diamond & Related Materials 15 (2006) 1242 – 1245
1243
2. Experiments The reactants we used were gallium metal with the purity higher than 99.999% and fine powders of melamine with the purity no less than 99%. A schematic diagram of the sample chamber is shown in Fig. 1. At first the melamine powders were shaped into a solid column with 10 mm in diameter and 8 mm in height by press at room temperature. Then a hole of 6 mm in diameter was drilled through the axis of the column. After that, the gallium metal was melted and poured into the hole under a nitrogen atmosphere by using an alcohol burner. Finally, melamine pellets were used to enclose the two end surfaces of the column. The synthesis of gallium nitride was carried out using a six-anvil high-pressure apparatus with a sample chamber of 23 mm on the edges at the temperature range from 1073 to 1473 K and the pressure range from 3.5 to 5.5 GPa. The pressure-transmitting medium was pyrophyllite. Heat was transferred into the system by a graphite heater. A 30-minute experimental run was adopted for all of the samples during which the experimental pressure and temperature were kept almost invariable. Structural analysis of the products was carried out by powder X-ray diffractometry (XRD) on a D8 DISCOVER GADDS diffractometer with Cu Ka radiation. The XRD patterns were indexed by using the reflex module combined in the Materials Studio program (Accelrys Inc.). Morphologies of the clusters and the grains of GaN were examined by transmission electron microscopy (TEM) on a H-8100 electron microscope. 3. Results and discussions XRD patterns of the GaN samples synthesized at several typical conditions are shown in Fig. 2. The diffractions of GaN, Ga2O3 and amorphous carbon are marked with N, O and a-C, respectively. It can be seen that at 3.5 GPa, the shapes of the peaks of GaN grow sharper with increasing temperature (Fig. 2(a), (b) and (c)). At higher pressure and temperature (Fig. 2(d)), the diffractions of Ga2O3 (for reference, see PDF # 85-
Fig. 2. XRD patterns of the GaN samples synthesized at several typical conditions: (a) 3.5 GPa, 1173 K; (b) 3.5 GPa, 1273 K; (c) 3.5 GPa, 1373 K and (d) 5.5 GPa, 1473 K.
0988) emerge besides those of GaN. The two bands around 26.5- and 43.5- are the characteristic diffractions of amorphous carbon. They persist at low pressures (Fig. 2(a), (b) and (c)) and disappear at higher pressures (Fig. 2(d)). In order to get specific information from the XRD patterns, we have indexed the XRD pattern of the sample synthesized at 3.5 GPa and 1373 K by using the reflex module combined in the Materials Studio program. Ten peaks are distinguished and used for the indexing. The peaks can be indexed unambiguously as the diffractions of GaN crystals with Wurtzite structure. The lattice ˚ and c = 5.192 T 0.004 A ˚ . The parameters are a = 3.193 T 0.002 A indexing results are listed in Table 1. The Miller indices corresponding to the peaks are also shown in Fig. 2 in brackets. From Fig. 2, it can be concluded that crystalline GaN has been grown by the high-pressure high-temperature technique we have designed and that 3.5 GPa and 1373 K may be the Table 1 The indexing results of the XRD pattern of the GaN sample synthesized at 3.5 GPa and 1373 K HKL DOBS DCAL DIF.D 100 002 101 102 110 103 200 112 201 004
Fig. 1. A schematic diagram of the sample chamber: 1 — steel cap, 2 — pyrophyllite, 3 — dolomite tube, 4 — graphite heater, 5 — quartz tube, 6 — gallium, 7 — melamine, 8 — quartz pellet, 9 — multilayer of graphite and alloys.
2.757 2.589 2.433 1.890 1.597 1.467 1.383 1.360 1.338 1.299
2.766 2.596 2.441 1.893 1.597 1.467 1.383 1.360 1.336 1.298
0.009 0.007 0.008 0.003 0.000 0.000 0.000 0.000 0.002 0.001
2TH.OBS 2TH.CAL DIF.2TH. INT. (a. u.) 32.45 34.62 36.92 48.11 57.67 63.34 67.69 68.98 70.32 72.74
32.34 34.52 36.79 48.03 57.69 63.34 67.70 69.00 70.40 72.81
0.11 0.10 0.13 0.08 0.02 0.00 0.01 0.02 0.08 0.07
422 352 999 145 192 195 40 162 96 28
XRD—x-ray diffraction; HKL—the miller indices; DOBS—the observed dspacings; DCAL—the calculated d-spacings; DIF.D—the differences between the observed and the calculated d-spacings; 2TH.OBS—the observed diffraction angle (2 theta); 2TH.CAL—the calculated diffraction angle (2 theta); DIF.2TH.—the differences between the observed and the calculated diffraction angle (2 theta); INT. (a. u.)—the intensity (arbitrary unit).
1244
J. Zhang et al. / Diamond & Related Materials 15 (2006) 1242 – 1245
optimum condition. The diffractions marked with U may originate form the pyrolysis products of melamine. They are not of interest here and will be discussed elsewhere. A typical TEM graph of the GaN sample synthesized at 3.5 GPa and 1373 K is shown in Fig. 3. Together with it is the transmission electron beam diffraction pattern of the area marked with F. It can be seen in Fig. 3 that the sample contains mainly grains with 0.2 to 1.5 Am in size. The grains have irregular shapes, yet the electron beam diffraction pattern of that selected area shows clear diffraction spots indicating that small single crystals may be found. The diffraction spots possess hexagonal symmetry. While the carbon contained in the sample has an amorphous nature, the diffraction pattern is supposed to be originated from a GaN single crystal that has underwent unstable growing process due to edge nucleation. Several well-shaped prisms of GaN are seen in another TEM graph as shown in Fig. 4. Fig. 4(b) is the magnified copy of the lower right region of Fig. 4(a). It can be seen from Fig. 4 that regular columnar shaped single crystals with about 0.6 Am in length and 0.1 Am in diameter may grow under conditions of 1373 K and 3.5 GPa. We assume that vapor– liquid – solid growth may be the most likely mechanism for the formation of GaN in our case. At a proper temperature and pressure condition, the pyrolysis of melamine takes place following such formula C3 N6 H6 6 3CðamorphousÞ þ 3N2 þ 2NH3 The amorphous carbon may be deemed as the result of the irregular stacking of tiny graphite-like carbon layers. The simultaneous nucleation of large numbers of sites is a reasonable result of the pyrolysis of melamine under high pressure and the growth into large and perfect graphite crystals is prevented. The nitrogen and ammonia vapors are driven to diffuse towards gallium by the pressure gradient. Reactive nitrogen atoms are produced by the dissociation of those vapors at the surface of liquid state gallium. N2 6 2N NH3 6 N þ 3H NðfreeÞ 6 Nðdissolved in GaÞ
Fig. 4. TEM of a well-shaped GaN single crystal.
Thus, fast surface nucleation of GaN occurs due to the high supersaturation generated by high pressure [13] and a surface layer of GaN may be formed. Since the gallium melt is in a temperature gradient, the GaN surface layer will dissolve in gallium and be transported by convection and diffusion to the central parts of gallium where the solution becomes supersaturated since the temperature is a little lower. Thus, the crystallization of GaN takes place. The reactions mentioned above will be obeyed exactly at 3.5 GPa and 1173 to 1373 K and thus gallium can be fully nitrified. At higher pressures, the pyrolysis of melamine will produce some CN compounds and complexes and the yield of reactive nitrogen is reduced. Thus, gallium is only partially nitrified and the remaining gallium will be oxidated upon exposure to air (Fig. 2). Since the surface layer is the natural source of growth centers, high supersaturation will lead to the simultaneous crystallization of a large amount of GaN crystals, which inhibits the further growth of the nitride. Very high supersaturation will also induce the edge nucleation of the GaN grains. Such grains will undergo unstable growth process and take irregular shapes (Fig. 3). 4. Conclusions In conclusion, we have carried out experiments on metal gallium and melamine at the temperature range from 1073 to 1473 K and the pressure range from 3.5 to 5.5 GPa. to synthesize GaN. Irregularly shaped GaN grains with the size of 0.1 to 2 Am have been grown at about 1373 K and 3.5 GPa within 30 min. Regular columnar-shaped GaN single crystals with about 0.6 Am in length and 0.1 Am in diameter have also been fabricated. A vapor – liquid –solid (VLS) process has been proposed to explain the growth mechanism and the experimental phenomena. Acknowledgements
Fig. 3. TEM and electron diffraction pattern of the GaN sample synthesized at 3.5 GPa and 1373 K.
This work is supported by the National Foundations for Natural Science of China, No. 50072005, and the International Cooperation Plan of the Ministry of Science and Technology in China, No. 2001CB711201.
J. Zhang et al. / Diamond & Related Materials 15 (2006) 1242 – 1245
References [1] H. Amano, M. Kito, K. Hiramatsu, I. Akasaki, Jpn. J. Appl. Phys. 28 (1989) 2112. [2] S. Nakamura, T. Mukai, M. Senoh, N. Iwasa, Jpn. J. Appl. Phys. 31 (1992) 139. [3] S. Gennari, G. Attolini, C. Pelosi, P. Lottici, F. Ricco, M. Labardi, M. Allegrini, C. Frediani, J. Cryst. Growth 166 (1996) 309. [4] R. Moon, J. Cryst. Growth 170 (1997) 1. [5] T. Tokuda, A. Wakahara, S. Noda, A. Sasaki, J. Cryst. Growth 173 (1997) 237. [6] Nichia Chemical Industries Ltd., Ogawa Building, Shiba 5-Chrome, Minotaku, Tokyo 108, Japan.
1245
[7] G. Kamler, J. Zachara, S. Podsiadlo, L. Adamowicz, W. Gebicki, J. Cryst. Growth 212 (2000) 39. [8] S. Barry, S. Ruoff, A. Ruoff, Chem. Mater. 10 (1998) 2571. [9] H. Yamane, M. Shimada, T. Sekiguchi, F. DiSalvo, J. Cryst. Growth 186 (1998) 8. [10] M. Yano, M. Okamoto, Y. Yap, M. Yoshimura, Y. Mori, T. Sasaki, Diamond Relat. Mater. 9 (2000) 512. [11] C. Balkas, Z. Sitar, L. Bergman, I. Shmagin, J. Muth, R. Kolbas, R. Nemanich, R. Davis, J. Cryst. Growth 208 (2000) 100. [12] S. Porowski, J. Cryst. Growth 166 (1996) 583. [13] S. Porowski, I. Grzegory, J. Cryst. Growth 178 (1997) 174. [14] M. Bockowski, Physica B 265 (1999) 1.