- Email: [email protected]
AlxGa1−xN (0⩽x⩽1) nanocrystalline powder by pyrolysis route
ARTICLE IN PRESS
Journal of Crystal Growth 308 (2007) 198–203 www.elsevier.com/locate/jcrysgro
AlxGa1xN (0pxp1) nanocrystalline powder by pyrolysis route R. Garciaa,, S. Srinivasana, O.E. Contrerasb, A.C. Thomasc, F.A. Poncea a
Department of Physics, Arizona State University, Tempe, AZ 85287-1504, USA Centro de Ciencias de la Materia Condensada, Universidad Nacional Auto´noma de Me´xico, Apdo. Postal 2681, C.P. 22800, Ensenada, Baja California, Mexico c Rogers Corporation, Durel Division, Chandler, AZ 85224, USA
b
Received 29 June 2007; received in revised form 20 July 2007; accepted 24 July 2007 Communicated by K.W. Benz Available online 6 August 2007
Abstract A novel method to synthesize nanocrystalline AlxGa1xN (0pxp1) powders is presented in this work. AlGaN nanocrystallites with the wurtzite structure were produced by thermal decomposition of a gallium–aluminum complex compound at 1000 1C in a three-zone horizontal quartz tube reactor under high-purity ammonia atmosphere. The crystallites showed a hexagonal structure, high homogeneity, and a narrow particle-size distribution at around 50 nm. A continuous composition range from 0 to 1 mol fraction can be reached by this method, allowing high control on the gallium and aluminum composition by monitoring the stoichiometry of the reaction between the metal nitrates and carbohydrazide. Low-temperature photoluminescence and cathodoluminescence studies showed that some impurities, such as carbon and oxygen, are unintentionally present in the final product and affect the optical properties. Subsequent thermal treatments between 900 and 1100 1C under an ammonia atmosphere significantly improved the quality of these materials. r 2007 Elsevier B.V. All rights reserved. PACS: 81.07.Bc; 81.07.b; 81.07.Wx; 81.16.Be; 82.30.Lp; 81.05.Ea Keywords: A1. Solid solutions; B1. Nanomaterials; B1. Nitrides; B2. Phosphors; B2. Semiconducting III–V materials
1. Introduction Group III nitrides and their ternary alloys have gained much attention because of their optoelectronic applications, such as in light emitting diodes, laser diodes, solidstate lighting, and solar cells [1–4]. On the other hand, ultraviolet LEDs and solar-blind photodetectors represent the next frontier in solid-state emitters and hold promise for many important applications in biology, medicine, dentistry, solid-state lighting, displays, dense data storage, and semiconductor manufacturing [5]. AlGaN alloys are important for UV emitters and detectors, and also for high electron mobility transistors. In the last two decades, much research has been focused on Corresponding author. Tel.: +1 480 727 6260; fax: +1 480 965 7954.
E-mail address: [email protected] (R. Garcia). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.07.048
thin films of these compounds. More recently, research on III–N powders has gained momentum for possible applications in electroluminescent devices as freestanding, micrometer-size materials, and for quantum-confinement effects in nanometer-sized particles [6]. In this paper, a novel method to synthesize nanocrystalline wurtzite-type AlxGa1xN (0pxp1) powders at relative low pressures (1 Torr) is reported. The thermal decomposition of a Ga–Al complex compound was conducted in a three-zone horizontal quartz-tube reactor at 1000 1C under low-pressure ammonia flow yielding nanostructured crystallites. Subsequent annealing treatments, at temperatures between 900 and 1100 1C in ammonia atmosphere (760 Torr), significantly improved the optoelectronic quality of the material. It is expected that this AlGaN powder can be applied as phosphor in electroluminescent lamps and other optoelectronic devices.
ARTICLE IN PRESS R. Garcia et al. / Journal of Crystal Growth 308 (2007) 198–203
199
xGa(NO3)3 6H2O xAl(NO3)3 9H2O Pyrolysis reaction (~10 min) (1000 °C / NH3= 300 sccm / P~ 1Torr) HPLC grade Toluene (~20 ml)
Fume Hood
Venting exhaust gases (~30 min) (N2, CO, H2O)
Dissolution (~111°C, ~10 min)
Stirring
Carbohydrazide CH6N4O
Cooling AlxGa1-xN powder (N2 flow, ~30 min)
Grinding and storage AlxGa1-xN powder
Viscous compound
Annealing AlxGa1-xN powder in NH3 atmosphere (~10h)
1h heating in a horizontal reactor (N2 flow, 150 sccm / P~1Torr)
Venting vapors (~200 °C, 30 min) Fig. 1. Flow diagram to synthesize AlxGa1xN (0pxp1) powders.
2. Experimental procedure The procedure for the synthesis of the AlxGa1xN nanocrystalline powders is illustrated in Fig. 1. High-purity (99.9995%) Al(NO3)3, Ga(NO3)3, and reagent-grade (98%) carbohydrazide (CH6N4O) were used as raw materials. The required mass of each reactant was calculated from the desired mass of the products, according to the following chemical equations: AlðNO3 Þ3ðsolÞ þ GaðNO3 Þ3ðsolÞ þ 3CH6 N4 OðsolÞ toluene
! ðCH6 N4 OÞ3 Al GaðNO3 Þ3 ðsÞ
111 C
ð1Þ
ðCH6 N4 OÞ3 Al GaðNO3 Þ3 ðsÞ N2
! AlGaNðsÞ þ 9H2 OðgÞ þ 3COðgÞ þ 7N2 ðgÞ
1000 C
ð2Þ
The number of grams of AlxGa1xN was selected typically between 1 and 2. The target mass values for each reactant were calculated to the nearest tenth of milligram. The reactants were placed into the weighing dish in an analytical balance until the weight was 1 mg of the target value. Once the desired amount of each reactant had been weighed, the nitrates were placed into a 100 ml Teflon beaker and around 20 ml of (99.8%) HPLC-grade toluene
was added. The mixture was magnetically stirred and heated in a hot plate at 111 1C. After approximately 10 min, the nitrates were fully in solution and the CH6N4O was added. After approximately 15 min of heating and stirring, the toluene evaporated completely and a viscous complex compound –(CH6N4O)3AlGa(NO3)3– was formed. When the compound looked dry (approximately 1 min later), the Teflon beaker was removed from the hot plate and allowed to cool down to room temperature to be used as reactant in the next step. A desired amount (1 g) of the compound previously synthesized was placed in a high-alumina boat inside of the three-zone horizontal quartz-tube reactor. During the whole process a 1 Torr pressure and a current (150 sccm) of ultra-high-purity NH3 was kept inside the tube. The zone 1 of the reactor was heated to 200 1C meanwhile the other zones were heated to 1000 1C. The boat with the complex compound was placed in the zone 1 of the furnace for 30 min, and the compound became foamy. Then the boat was positioned in the zone 2 at 1000 1C, for 10 min, and the formation of white vapors indicated that the pyrolysis reaction had happened. The boat with the AlGaN powder was placed in the entrance of the tube and allowed to cool down to room temperature. Finally the powders were taken out of the reactor, grounded in a mortar with pestle and stored in a glass vial until the following procedures. Annealing treatments on the as-synthesized
ARTICLE IN PRESS R. Garcia et al. / Journal of Crystal Growth 308 (2007) 198–203
200
AlGaN powders were carried out in the horizontal quartztube reactor under ultra-high-purity NH3 for 10 h, at temperatures between 900 and 1100 1C.
1000
57
58
59
60
61 62 63 2 Theta (Deg)
64
65
66
67
148 Vegard's Law (b=0) AlxGa1-xN alloy
146 144 142 140 138 136
Valloy = xVAlN + (1-x)VGaN + b(x)(1-x)
0.0
0.2
0.4 0.6. Al mole fraction (x)
0.8
1.0
Fig. 3. (a) X-ray diffraction spectra of the AlxGa1xN (0pxp1) powders, planes 1 1 2¯ 1 and 1 0 1¯ 3. (b) Relationship between the unit cell volume and the aluminum mole fraction in the AlxGa1xN alloy.
1011
0002
Intensity (Counts)
1121 1013 1012
GaN 10 at% Al 30 at% Al 50 at% Al 70 at% Al 90 at% Al AlN
30
1121 1013
Unit cell volume (A3)
Powder X-ray diffraction spectra of seven samples of annealed AlxGa1xN are shown in Figs. 2 and 3a. The main peaks were indexed according the PDF cards 76-0703 and 79-2497. X-ray diffraction patterns show that AlxGa1xN powders produced by this new technique have a hexagonal structure, of the wurtzite type. There are no other crystalline phases present such as oxides or other nitrides with different stoichiometry. However, some impurities are expected due to the nature of the precursors as was shown afterwards by cathodoluminescence (CL). In Figs. 2 and 3a it is observed that the crystallinity of the pure GaN powder is better than the crystallinity of the AlxGa1xN powders and also how the lattice parameters change as the aluminum concentration (x) in the alloy is increased. The volume of the unit cell of each alloy goes from 136 A˚3 for x ¼ 1 to 147 A˚3 for x ¼ 0. These values were calculated using the positions of planes f1 1 2¯ 1g and f1 0 1¯ 3g (see Fig. 3a) measured by the diffraction of a monochromatic Cu Ka radiation of 0.154 nm wavelength. The relationship between the volumes of the unit cell of each alloy versus its aluminum concentration (x) is shown in Fig. 3b, the cell volumes of the ternary alloys follow a linear behavior with respect to the Al concentration. However, this linear relationship does not match Vegard’s law for ideal solutions using a bowing parameter (b) equal to zero. Scanning electron microscopy images and energy-dispersive X-ray spectra (EDS) of the AlxGa1xN (x ¼ 1, 0, 0.5) powders synthesized in this work is shown in Fig. 4.
Intensity (Counts)
3. Results and discussion
Al0.1GaN Al0.3GaN Al0.5GaN Al0.7GaN Al0.9GaN GaN AlN
35
40
45 50 2 Theta (Deg)
55
60
65
Fig. 2. X-ray diffraction spectra of the AlxGa1xN (0pxp1) powders.
The annealed GaN powder is composed by sub-micro polyhedra with very well defined hexagonal facets and narrow particle-size distribution between 200 and 800 nm as is shown in Fig. 4b. The EDS spectrum (Fig. 4e) shows only the peaks related to nitrogen (transition Ka at 0.392 keV) and gallium (transitions: L1a at 1.096 keV and L1b at 1.122 keV). The fact that there are no other transitions indicates the absence of impurities such as oxygen and carbon within the detection limits of the instrument in this material. The EDS spectra of the AlxGa1xN powders for x ¼ 1 and 0.5 (see Fig. 4d and f, respectively) show the peaks related to nitrogen (transition Ka at 0.392 keV), aluminum (transition: Ka at 1.560 keV), gallium (transitions: L1a at 1.096 keV and L1b at 1.122 keV), and some traces of oxygen (0.540 keV), which means that even after annealing at high temperatures under ammonia, these AlGaN powders retain some oxygen as contaminant. These traces of oxygen affect the optical properties of this
ARTICLE IN PRESS R. Garcia et al. / Journal of Crystal Growth 308 (2007) 198–203
Fig. 4. SEM images and energy-dispersive X-ray spectra of AlxGa1xN powders: (a) and (d) x ¼ 1, (b) and (e) x ¼ 0, (c) and (f) x ¼ 0.5.
201
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R. Garcia et al. / Journal of Crystal Growth 308 (2007) 198–203
Fig. 5. High-resolution TEM image and electron-diffraction pattern of Al0.5Ga0.5N crystallites.
material, particularly the luminescence efficiency. Therefore, other purification methods are needed to get better quality in this kind of powders. The annealed AlN and Al0.5Ga0.5N powders shown in Fig. 4a and c respectively, are composed by flake shaped nanocrystallites with a narrow particle-size distribution between 30 and 100 nm as can also be seen in the highresolution transmission electron microscopy (TEM) image in Fig. 5. TEM observations on these powders indicate that the crystallites are mainly flake-shaped. A well-defined wurtzite structure of a single Al0.5Ga0.5N crystallite is identified by high-resolution TEM imaging as is shown in the lower left inset of Fig. 5. However, the wurtzite nature of the powder can be verified from typical electron diffraction as shown in the upper left inset of Fig. 5. Optical studies on GaN powders showed a linear relationship between the annealing temperature and the intensity of the band-to-band emission at around 357 nm (3.47 eV) [7]. For AlxGa1xN (0oxp1) powders, the expected alloy emission in the UV region between 200 nm (6.2 eV) and 350 nm (3.5 eV) is not observed. Instead the powders exhibit high-intensity luminescence in the visible region, which is believed to be related to oxygen contamination. Low-temperature (4 K) CL measurements were performed on the AlxGa1xN powders using a 5 keV electron beam as the excitation source. Fig. 6a shows CL spectra of seven different samples across the entire composition range 0pxp1 of AlxGa1xN alloy. All samples show a dominant donor–acceptor pair emission related to oxygen contamination [8]. The GaN powder shows the common
donor–acceptor pair transition at 380 nm (3.26 eV), with a phonon replica at 390 nm (3.18 eV). The near-band-edge emission appears as a small peak at 360 nm (3.44 eV). With the introduction of Al, the CL-emission intensity drops significantly, indicating a strong decline in the crystalline quality. This is consistent with the fact that GaN exhibits well-defined morphology, while AlGaN powders do not. Interestingly, the donor–acceptor pair intensity first decreases continuously with increasing Al content up to x ¼ 0.50, and then increases as shown in Fig. 6b. The peak position of the donor–acceptor pair shows a non-monotonic variation with Al content possibly due to impurities such as oxygen and carbon as was shown by the elemental analysis. The broad peak at 380 nm is believed to be the donor–acceptor pair band, which has been attributed to recombination between residual donors and oxygen complexes.
4. Conclusions A novel method to synthesize a continuous range of AlxGa1xN (0pxp1) alloys has been developed. The method produces wurtzite-type nanocrytallites with high homogeneity and relatively narrow particle-size distribution between 30 and 100 nm. Control of the crystallite size has been achieved by thermal annealing. Luminescent materials have been produced that can be applied as phosphor for electroluminescent lamps and other optoelectronics devices. However, the method needs improvement in order to reduce the impurities and native defects, such as residual oxygen, carbon, and vacancies.
ARTICLE IN PRESS R. Garcia et al. / Journal of Crystal Growth 308 (2007) 198–203
This research has been supported by Durel Division, Rogers Corporation.
380 nm (3.26 eV)
CL Intensity (counts)
432 nm (2.87eV)
300
350
400
GaN 10% AlGaN 30% AlGaN 50% AlGaN 70% AlGaN 90% AlGaN AlN
450 500 550 Wavelength (nm)
600
References [1] [2] [3] [4] [5]
650
Peak position (eV)
3.4
3.2
3.0
2.8
2.6 0
203
20 40 60 80 Aluminum concentration (atomic %)
100
Fig. 6. (a) 10 K temperature cathodoluminescence spectra of AlxGa1xN (0pxp1) powders. (b) Relationship between donor–acceptor pair peak position and the aluminum mole fraction in the AlxGa1xN alloy.
Acknowledgments The authors gratefully acknowledge the use of facilities within the Center for Solid State Science at Arizona State University.
F.A. Ponce, D.P. Bour, Nature (London) 386 (1997) 351. S. Nakamura, Science 281 (1998) 956. S. Srite, H. Morkoc, J. Vac. Sci. Technol. B 10 (1992) 1237. A.J. Stekle, J.M. Zavada, MRS Bull. 24 (1999) 33. M.S. Shur, A. Zukauskas, in: M.S. Shur, A. Zukauskas (Eds.), UV Solid-State Light Emitters and Detectors, vol. 144, NATO Science Series II, The Netherlands, 2004, p. 1. [6] T. Honda, H. Kimura, Y. Amahori, H. Kawanishi, J. Lumin. 102–103 (2003) 173. [7] R. Garcia, G.A. Hirata, A.C. Thomas, F.A. Ponce, Opt. Mater. 29 (2006) 19. [8] M.A. Reschikova, H. Morkoc, J. Appl. Phys. 97 (2005) 61301.
Journal of Crystal Growth 308 (2007) 198–203 www.elsevier.com/locate/jcrysgro
AlxGa1xN (0pxp1) nanocrystalline powder by pyrolysis route R. Garciaa,, S. Srinivasana, O.E. Contrerasb, A.C. Thomasc, F.A. Poncea a
Department of Physics, Arizona State University, Tempe, AZ 85287-1504, USA Centro de Ciencias de la Materia Condensada, Universidad Nacional Auto´noma de Me´xico, Apdo. Postal 2681, C.P. 22800, Ensenada, Baja California, Mexico c Rogers Corporation, Durel Division, Chandler, AZ 85224, USA
b
Received 29 June 2007; received in revised form 20 July 2007; accepted 24 July 2007 Communicated by K.W. Benz Available online 6 August 2007
Abstract A novel method to synthesize nanocrystalline AlxGa1xN (0pxp1) powders is presented in this work. AlGaN nanocrystallites with the wurtzite structure were produced by thermal decomposition of a gallium–aluminum complex compound at 1000 1C in a three-zone horizontal quartz tube reactor under high-purity ammonia atmosphere. The crystallites showed a hexagonal structure, high homogeneity, and a narrow particle-size distribution at around 50 nm. A continuous composition range from 0 to 1 mol fraction can be reached by this method, allowing high control on the gallium and aluminum composition by monitoring the stoichiometry of the reaction between the metal nitrates and carbohydrazide. Low-temperature photoluminescence and cathodoluminescence studies showed that some impurities, such as carbon and oxygen, are unintentionally present in the final product and affect the optical properties. Subsequent thermal treatments between 900 and 1100 1C under an ammonia atmosphere significantly improved the quality of these materials. r 2007 Elsevier B.V. All rights reserved. PACS: 81.07.Bc; 81.07.b; 81.07.Wx; 81.16.Be; 82.30.Lp; 81.05.Ea Keywords: A1. Solid solutions; B1. Nanomaterials; B1. Nitrides; B2. Phosphors; B2. Semiconducting III–V materials
1. Introduction Group III nitrides and their ternary alloys have gained much attention because of their optoelectronic applications, such as in light emitting diodes, laser diodes, solidstate lighting, and solar cells [1–4]. On the other hand, ultraviolet LEDs and solar-blind photodetectors represent the next frontier in solid-state emitters and hold promise for many important applications in biology, medicine, dentistry, solid-state lighting, displays, dense data storage, and semiconductor manufacturing [5]. AlGaN alloys are important for UV emitters and detectors, and also for high electron mobility transistors. In the last two decades, much research has been focused on Corresponding author. Tel.: +1 480 727 6260; fax: +1 480 965 7954.
E-mail address: [email protected] (R. Garcia). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.07.048
thin films of these compounds. More recently, research on III–N powders has gained momentum for possible applications in electroluminescent devices as freestanding, micrometer-size materials, and for quantum-confinement effects in nanometer-sized particles [6]. In this paper, a novel method to synthesize nanocrystalline wurtzite-type AlxGa1xN (0pxp1) powders at relative low pressures (1 Torr) is reported. The thermal decomposition of a Ga–Al complex compound was conducted in a three-zone horizontal quartz-tube reactor at 1000 1C under low-pressure ammonia flow yielding nanostructured crystallites. Subsequent annealing treatments, at temperatures between 900 and 1100 1C in ammonia atmosphere (760 Torr), significantly improved the optoelectronic quality of the material. It is expected that this AlGaN powder can be applied as phosphor in electroluminescent lamps and other optoelectronic devices.
ARTICLE IN PRESS R. Garcia et al. / Journal of Crystal Growth 308 (2007) 198–203
199
xGa(NO3)3 6H2O xAl(NO3)3 9H2O Pyrolysis reaction (~10 min) (1000 °C / NH3= 300 sccm / P~ 1Torr) HPLC grade Toluene (~20 ml)
Fume Hood
Venting exhaust gases (~30 min) (N2, CO, H2O)
Dissolution (~111°C, ~10 min)
Stirring
Carbohydrazide CH6N4O
Cooling AlxGa1-xN powder (N2 flow, ~30 min)
Grinding and storage AlxGa1-xN powder
Viscous compound
Annealing AlxGa1-xN powder in NH3 atmosphere (~10h)
1h heating in a horizontal reactor (N2 flow, 150 sccm / P~1Torr)
Venting vapors (~200 °C, 30 min) Fig. 1. Flow diagram to synthesize AlxGa1xN (0pxp1) powders.
2. Experimental procedure The procedure for the synthesis of the AlxGa1xN nanocrystalline powders is illustrated in Fig. 1. High-purity (99.9995%) Al(NO3)3, Ga(NO3)3, and reagent-grade (98%) carbohydrazide (CH6N4O) were used as raw materials. The required mass of each reactant was calculated from the desired mass of the products, according to the following chemical equations: AlðNO3 Þ3ðsolÞ þ GaðNO3 Þ3ðsolÞ þ 3CH6 N4 OðsolÞ toluene
! ðCH6 N4 OÞ3 Al GaðNO3 Þ3 ðsÞ
111 C
ð1Þ
ðCH6 N4 OÞ3 Al GaðNO3 Þ3 ðsÞ N2
! AlGaNðsÞ þ 9H2 OðgÞ þ 3COðgÞ þ 7N2 ðgÞ
1000 C
ð2Þ
The number of grams of AlxGa1xN was selected typically between 1 and 2. The target mass values for each reactant were calculated to the nearest tenth of milligram. The reactants were placed into the weighing dish in an analytical balance until the weight was 1 mg of the target value. Once the desired amount of each reactant had been weighed, the nitrates were placed into a 100 ml Teflon beaker and around 20 ml of (99.8%) HPLC-grade toluene
was added. The mixture was magnetically stirred and heated in a hot plate at 111 1C. After approximately 10 min, the nitrates were fully in solution and the CH6N4O was added. After approximately 15 min of heating and stirring, the toluene evaporated completely and a viscous complex compound –(CH6N4O)3AlGa(NO3)3– was formed. When the compound looked dry (approximately 1 min later), the Teflon beaker was removed from the hot plate and allowed to cool down to room temperature to be used as reactant in the next step. A desired amount (1 g) of the compound previously synthesized was placed in a high-alumina boat inside of the three-zone horizontal quartz-tube reactor. During the whole process a 1 Torr pressure and a current (150 sccm) of ultra-high-purity NH3 was kept inside the tube. The zone 1 of the reactor was heated to 200 1C meanwhile the other zones were heated to 1000 1C. The boat with the complex compound was placed in the zone 1 of the furnace for 30 min, and the compound became foamy. Then the boat was positioned in the zone 2 at 1000 1C, for 10 min, and the formation of white vapors indicated that the pyrolysis reaction had happened. The boat with the AlGaN powder was placed in the entrance of the tube and allowed to cool down to room temperature. Finally the powders were taken out of the reactor, grounded in a mortar with pestle and stored in a glass vial until the following procedures. Annealing treatments on the as-synthesized
ARTICLE IN PRESS R. Garcia et al. / Journal of Crystal Growth 308 (2007) 198–203
200
AlGaN powders were carried out in the horizontal quartztube reactor under ultra-high-purity NH3 for 10 h, at temperatures between 900 and 1100 1C.
1000
57
58
59
60
61 62 63 2 Theta (Deg)
64
65
66
67
148 Vegard's Law (b=0) AlxGa1-xN alloy
146 144 142 140 138 136
Valloy = xVAlN + (1-x)VGaN + b(x)(1-x)
0.0
0.2
0.4 0.6. Al mole fraction (x)
0.8
1.0
Fig. 3. (a) X-ray diffraction spectra of the AlxGa1xN (0pxp1) powders, planes 1 1 2¯ 1 and 1 0 1¯ 3. (b) Relationship between the unit cell volume and the aluminum mole fraction in the AlxGa1xN alloy.
1011
0002
Intensity (Counts)
1121 1013 1012
GaN 10 at% Al 30 at% Al 50 at% Al 70 at% Al 90 at% Al AlN
30
1121 1013
Unit cell volume (A3)
Powder X-ray diffraction spectra of seven samples of annealed AlxGa1xN are shown in Figs. 2 and 3a. The main peaks were indexed according the PDF cards 76-0703 and 79-2497. X-ray diffraction patterns show that AlxGa1xN powders produced by this new technique have a hexagonal structure, of the wurtzite type. There are no other crystalline phases present such as oxides or other nitrides with different stoichiometry. However, some impurities are expected due to the nature of the precursors as was shown afterwards by cathodoluminescence (CL). In Figs. 2 and 3a it is observed that the crystallinity of the pure GaN powder is better than the crystallinity of the AlxGa1xN powders and also how the lattice parameters change as the aluminum concentration (x) in the alloy is increased. The volume of the unit cell of each alloy goes from 136 A˚3 for x ¼ 1 to 147 A˚3 for x ¼ 0. These values were calculated using the positions of planes f1 1 2¯ 1g and f1 0 1¯ 3g (see Fig. 3a) measured by the diffraction of a monochromatic Cu Ka radiation of 0.154 nm wavelength. The relationship between the volumes of the unit cell of each alloy versus its aluminum concentration (x) is shown in Fig. 3b, the cell volumes of the ternary alloys follow a linear behavior with respect to the Al concentration. However, this linear relationship does not match Vegard’s law for ideal solutions using a bowing parameter (b) equal to zero. Scanning electron microscopy images and energy-dispersive X-ray spectra (EDS) of the AlxGa1xN (x ¼ 1, 0, 0.5) powders synthesized in this work is shown in Fig. 4.
Intensity (Counts)
3. Results and discussion
Al0.1GaN Al0.3GaN Al0.5GaN Al0.7GaN Al0.9GaN GaN AlN
35
40
45 50 2 Theta (Deg)
55
60
65
Fig. 2. X-ray diffraction spectra of the AlxGa1xN (0pxp1) powders.
The annealed GaN powder is composed by sub-micro polyhedra with very well defined hexagonal facets and narrow particle-size distribution between 200 and 800 nm as is shown in Fig. 4b. The EDS spectrum (Fig. 4e) shows only the peaks related to nitrogen (transition Ka at 0.392 keV) and gallium (transitions: L1a at 1.096 keV and L1b at 1.122 keV). The fact that there are no other transitions indicates the absence of impurities such as oxygen and carbon within the detection limits of the instrument in this material. The EDS spectra of the AlxGa1xN powders for x ¼ 1 and 0.5 (see Fig. 4d and f, respectively) show the peaks related to nitrogen (transition Ka at 0.392 keV), aluminum (transition: Ka at 1.560 keV), gallium (transitions: L1a at 1.096 keV and L1b at 1.122 keV), and some traces of oxygen (0.540 keV), which means that even after annealing at high temperatures under ammonia, these AlGaN powders retain some oxygen as contaminant. These traces of oxygen affect the optical properties of this
ARTICLE IN PRESS R. Garcia et al. / Journal of Crystal Growth 308 (2007) 198–203
Fig. 4. SEM images and energy-dispersive X-ray spectra of AlxGa1xN powders: (a) and (d) x ¼ 1, (b) and (e) x ¼ 0, (c) and (f) x ¼ 0.5.
201
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Fig. 5. High-resolution TEM image and electron-diffraction pattern of Al0.5Ga0.5N crystallites.
material, particularly the luminescence efficiency. Therefore, other purification methods are needed to get better quality in this kind of powders. The annealed AlN and Al0.5Ga0.5N powders shown in Fig. 4a and c respectively, are composed by flake shaped nanocrystallites with a narrow particle-size distribution between 30 and 100 nm as can also be seen in the highresolution transmission electron microscopy (TEM) image in Fig. 5. TEM observations on these powders indicate that the crystallites are mainly flake-shaped. A well-defined wurtzite structure of a single Al0.5Ga0.5N crystallite is identified by high-resolution TEM imaging as is shown in the lower left inset of Fig. 5. However, the wurtzite nature of the powder can be verified from typical electron diffraction as shown in the upper left inset of Fig. 5. Optical studies on GaN powders showed a linear relationship between the annealing temperature and the intensity of the band-to-band emission at around 357 nm (3.47 eV) [7]. For AlxGa1xN (0oxp1) powders, the expected alloy emission in the UV region between 200 nm (6.2 eV) and 350 nm (3.5 eV) is not observed. Instead the powders exhibit high-intensity luminescence in the visible region, which is believed to be related to oxygen contamination. Low-temperature (4 K) CL measurements were performed on the AlxGa1xN powders using a 5 keV electron beam as the excitation source. Fig. 6a shows CL spectra of seven different samples across the entire composition range 0pxp1 of AlxGa1xN alloy. All samples show a dominant donor–acceptor pair emission related to oxygen contamination [8]. The GaN powder shows the common
donor–acceptor pair transition at 380 nm (3.26 eV), with a phonon replica at 390 nm (3.18 eV). The near-band-edge emission appears as a small peak at 360 nm (3.44 eV). With the introduction of Al, the CL-emission intensity drops significantly, indicating a strong decline in the crystalline quality. This is consistent with the fact that GaN exhibits well-defined morphology, while AlGaN powders do not. Interestingly, the donor–acceptor pair intensity first decreases continuously with increasing Al content up to x ¼ 0.50, and then increases as shown in Fig. 6b. The peak position of the donor–acceptor pair shows a non-monotonic variation with Al content possibly due to impurities such as oxygen and carbon as was shown by the elemental analysis. The broad peak at 380 nm is believed to be the donor–acceptor pair band, which has been attributed to recombination between residual donors and oxygen complexes.
4. Conclusions A novel method to synthesize a continuous range of AlxGa1xN (0pxp1) alloys has been developed. The method produces wurtzite-type nanocrytallites with high homogeneity and relatively narrow particle-size distribution between 30 and 100 nm. Control of the crystallite size has been achieved by thermal annealing. Luminescent materials have been produced that can be applied as phosphor for electroluminescent lamps and other optoelectronics devices. However, the method needs improvement in order to reduce the impurities and native defects, such as residual oxygen, carbon, and vacancies.
ARTICLE IN PRESS R. Garcia et al. / Journal of Crystal Growth 308 (2007) 198–203
This research has been supported by Durel Division, Rogers Corporation.
380 nm (3.26 eV)
CL Intensity (counts)
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GaN 10% AlGaN 30% AlGaN 50% AlGaN 70% AlGaN 90% AlGaN AlN
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References [1] [2] [3] [4] [5]
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Fig. 6. (a) 10 K temperature cathodoluminescence spectra of AlxGa1xN (0pxp1) powders. (b) Relationship between donor–acceptor pair peak position and the aluminum mole fraction in the AlxGa1xN alloy.
Acknowledgments The authors gratefully acknowledge the use of facilities within the Center for Solid State Science at Arizona State University.
F.A. Ponce, D.P. Bour, Nature (London) 386 (1997) 351. S. Nakamura, Science 281 (1998) 956. S. Srite, H. Morkoc, J. Vac. Sci. Technol. B 10 (1992) 1237. A.J. Stekle, J.M. Zavada, MRS Bull. 24 (1999) 33. M.S. Shur, A. Zukauskas, in: M.S. Shur, A. Zukauskas (Eds.), UV Solid-State Light Emitters and Detectors, vol. 144, NATO Science Series II, The Netherlands, 2004, p. 1. [6] T. Honda, H. Kimura, Y. Amahori, H. Kawanishi, J. Lumin. 102–103 (2003) 173. [7] R. Garcia, G.A. Hirata, A.C. Thomas, F.A. Ponce, Opt. Mater. 29 (2006) 19. [8] M.A. Reschikova, H. Morkoc, J. Appl. Phys. 97 (2005) 61301.