Preparation and optical properties of GaN nanocrystalline powders L. Jia, E. Q. Xie*, X. J. Pan, Z. X. Zhang and Y. Z. Zhang GaN nanocrystalline powders were synthesised by decomposition of gallium nitrate, followed by nitrogenising with ammonia under different temperature. X-ray diffraction (XRD) and the transmission electron microscopy (TEM) indicated that the crystallinity of the powder is improved and the average size of the GaN nanocrystallites increases from 4?8 to 23?9 nm as the temperature increases from 850 to 1050uC. The Raman spectra displayed four broadened peaks corresponding to A1 (LO), A1 (TO), E1 (TO) and E2 (high) modes of wu¨rtzite GaN respectively. Two additional modes at 252 and 421 cm21 attributed to boundary phonons activated by the finite size effects and octahedral Ga–N6 bonds were observed respectively. A strong blue photoluminescence (y353 nm) was detected for room temperature measurement, indicating that the GaN nanocrystalline powders have few defects and high quality. Keywords: Gallium nitride, Nanocrystalline, Raman spectrum, Photoluminescence
Introduction Wide band gap semiconductor gallium nitride (GaN, Eg53?4 eV) and related materials have attracted more and more attention for their potential electronic and optoelectronic applications, such as high brightness blue ultraviolet light emitting diodes, short wavelength laser diodes,1,2 high performance ultraviolet detector and high temperature or high power electronic devices.3 In recent years, GaN nanocrystalline powders in particular have drawn great interest because of their new exotic optical, magnetic, electronic and catalytic properties, which largely depend on the grain size and native defects concentration.4 If the particle size is small enough, the quantum confinement effect would lead to modified states which are different from the bulk material.5 GaN in nanostructure has been synthesised by many techniques such as metal organic chemical vapour deposition in situ synthesis of amorphous GaN nanoparticles in a polymer, pyrolysis at high temperatures, direct current arc plasma, ion assisted deposited and detonation of gallium azides.6–12 However, finding a relatively simple method which is capable of producing nanoparticles of high quality is a big challenge to for electronic device mass production. Of all these method, decomposition of gallium nitrate followed by nitrogenisation into their nitrides under a flow of ammonia (NH3) gas is a cheap and simple route with high production rates. The knowledge of structural and optical properties of GaN nanocrystalline powders, which is largely influenced by the sample preparation, is very limited to the authors’ understanding. Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Department of Physics, Lanzhou University, Lanzhou 730000, China *Corresponding author, email
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ß 2009 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 4 November 2008; accepted 3 December 2008 DOI 10.1179/174328409X405698
In the present paper, nanocrystalline GaN powders were obtained by decomposition of gallium nitrate followed by nitrogenising with ammonia under different temperature. The structural properties, surface morphology and optical properties were characterised by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectra and photoluminescence (PL) emission spectra. The structures and optical properties influenced by different heating temperatures were investigated.
Experimental High purity (99?999%) Ga(NO3)3.xH2O (x57–9) in Al2O3 crucible was placed in a horizontal quartz furnace. At first, the reactor was heated to set temperature (850, 950 and 1050uC). Then ultrahigh purity (99?999%) NH3 was flowing inside the furnace at a flux of 30 sccm. After that, the temperature was kept constant for 1 h. Finally, the furnace was self-cooled to room temperature under the same NH3 ambience. The structure characteristics of the GaN nanocrystalline powders were determined by XRD using a Philips X’Pert diffractometer with Cu Ka1 radiation (l5 ˚ ). The TEM images were obtained with a 1?54056 A JEM-1230 transmission electron microscope. The Raman and PL spectra were collected on Jobin-Yvon LabRAM HR80 microarea spectrometer at room temperature. Raman spectra were excited by 532 nm line of Torus 50 mW diode pumped solid state laser and PL measurement was performed by a 35 mW He–Cd laser (KIMMON) using 325 nm as excitation source.
Results and discussion Figure 1 shows the XRD patterns for the GaN nanocrystalline powders synthesised at temperatures of
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1 XRD spectra of synthesised GaN nanocrystalline powders prepared at different temperatures: curve (a) 850uC; curve (b) 950uC; curve (c) 1050uC
850, 950 and 1050uC respectively. All the XRD patterns could indicate the formation of hexagonal gallium nitride (h-GaN) structure.4 Three prominent peaks could be observed at 2h532?38, 34?58 and 36?82u, corresponding to (100), (002) and (101) of h-GaN phase respectively. The minor peaks at 2h548?12, 57?74, 63?43, 67?77, 69?07, 70?48, 72?88, 78?33 and 82?08u could also be observed, which are derived from the plane (102), (110), (103), (200), (112), (201), (004), (202) and (104) in h-GaN phase respectively. The spectra of the sample prepared at 850uC display several packets, while the patterns of the other samples which synthesised at higher temperatures show sharp lines. This indicates that the crystallinity of the powder is improved as the temperature increases. It is noted that the diffraction peaks are broadened compared with that of GaN bulk, indicating that the GaN grain sizes obtained are very small. Using Scherrer formula D(nm)~ Kl=½(b21 {b20 )1=2 cosh, the average sizes of the particles are calculated as 4?8, 20?4 and 23?9 nm, corresponding to 850, 950 and 1050uC respectively. The particle size increases with increasing temperature, because under higher temperature, the NH3 is more reactive and accelerates the growing process of GaN crystallites.5 No diffraction peaks of Ga2O3 or other phases are observed. The TEM images of GaN nanocrystalline powders prepared at different temperatures are shown in Fig. 2. The GaN particle size is y5 nm when the temperature is 850uC, as shown in Fig. 2a; the particle size increases to y20 nm while the temperature increases to 950uC, as shown in Fig. 2b; and the particle size increases to more than 30 nm, while the temperature reaches 1050uC, as
Preparation and optical properties of GaN nanocrystalline powders
3 Raman spectra of GaN nanocrystalline powders prepared at different temperatures: curve (a) 850uC; curve (b) 950uC; curve (c) 1050uC; inset is Lorentzian fitting of curve (c) in range of 500–640 cm21, in which dash lines display fitting lines and solid line displays experiment curve
shown in Fig. 2c. It is visible that the sizes of individual nanocrystals are comparable with those determined from XRD, but a little larger, which is probably because of the particle aggregation. In addition, the size of the particles is quite uniform. Figure 3 displays the Raman spectra of GaN nanocrystalline powders synthesised at temperatures of 850, 950 and 1050uC respectively. In the authors’ samples, broadened peaks at 535, 554 and 721 cm21 could be observed and these main features can all be assigned to the known first order GaN modes. Peak at 721 cm21 is derived from A1 (LO) mode, which also indicates the low carrier density of the particles. However, the frequency of the A1 (LO) is smaller compared to well established experimental values, which could be explained by strain or zone folding effect.13 The strong peak at y554 cm21 derived from E2 (high) mode reflects the characteristic scattering of wu¨rtzite phase of GaN, and it is accompanied by a shoulder on the low energy side which is identified as A1 (TO). In order to resolve the shoulders of E2 (high) mode, the peak was decomposed into Lorentzian peaks.13 The results of the decomposition of curve (c) are displayed in the inset of Fig. 3, where two peaks constituting the shoulder at frequency positions of 515 and 542 cm21 can be seen, corresponding to A1 (TO) and E1 (TO) modes respectively. Compared with GaN single crystals, the positions of the first order Raman modes discussed
a 850uC; b 950uC; c 1050uC 2 TEM images of GaN nanocrystalline powders obtained at different temperatures
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indicate that the crystallinity of the powder is improved as the temperature increases. Raman scattering studies of the GaN nanocrystalline powders reveal four first order phonon modes, with frequencies close to those of the bulk material but significantly broadened, which is indicative of the phonon confinement effects brought out by the nanoscale dimensions of the system. Furthermore, the appearance of two additional modes at 252 and 421 cm21 are attributed to zone boundary phonons activated by the finite size effects and octahedrally bonded Ga–N6 structures. A strong blue photoluminescence is observed for room temperature measurement, indicating that the GaN nanocrystalline powders have few defects and high quality.
Acknowledgement 4 Room temperature PL emission spectra of GaN nanocrystalline powders obtained at different temperatures: curve (a) 850uC; curve (b) 950uC; curve (c) 1050uC; inset shows peak details in range of 352–355 nm
This work was supported by Program for New Century Excellent Talents in University of China (grant no. NCET-04-0975).
above slightly shift to lower frequency and significantly broadened, which would be ascribed to phonon confinement brought by nanoscale dimensions of the systems.14 Additionally, as the selection rule is released, the nanoscale also introduced new features which are 4 space group in first order Raman not allowed by the C6v scattering at the zone centre: the two peaks near 251 and 415 cm21. The former is possibly attributed to zone boundary phonon activated by surface disorders and finite size effects.14–16 The latter are generally assigned to a second order Raman scattering mode by overtones of transverse acoustic phonons, a two phonon subtractive mode or local vibration of vacancy related defects.17–20 In the authors’ work, no overtone activity is shown and the behaviour seems not similar to the vacancy related mode (such as the VN related band at 655 cm21).9 The authors consider that the mode at 415 cm21 quite probably originates from N rich octahedrally bonded Ga–N6 structures, as reported by Ning et al.15 The GaN nanocrystalline powders produced in the present work show strong band gap photoluminescence emission. Figure 4 shows PL emission spectra (325 nm excitation) at room temperature. All the samples display a very intensive and sharp (FWHM,1 nm) peak at y353 nm (3?51 eV), which is 0?12 eV larger than that in bulk wu¨rtzite GaN (3?39 eV). This blue shifted PL emission provides evidence for both quantum confinement and strain effects in these samples and suggests the possibility of adapting this reaction for production of GaN quantum dots.21,22 As shown in the inset of Fig. 4, the sample prepared at higher temperature gave a peak shape similar to those observed in the lower temperature samples and the intensity increases along with increasing preparation temperature. This is consistent with the result reported by Ogi et al.23 No defect related structures of yellow to red luminescence are observed obviously, which imply that the authors’ GaN nanocrystalline powders have few defects and high quality.16
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Conclusion In summary, by decomposition of gallium nitrate followed by nitrogenising with ammonia under different temperatures, GaN nanocrystalline powders with different average sizes are synthesised. XRD and the TEM
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