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Synthesis, characterization, growth mechanism, photoluminescence and field emission properties of novel dandelion-like gallium nitride
Applied Surface Science 257 (2011) 10289–10293
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis, characterization, growth mechanism, photoluminescence and field emission properties of novel dandelion-like gallium nitride Ghulam Nabi a , Chuanbao Cao a,∗ , Waheed S. Khan a , Sajad Hussain a , Zahid Usman a , Muhammad Safdar a , Sajjad Hussain Shah b , Noor Abass Din Khattak c a
Research Centre of Materials Science, School of Material Science and Engineering, Beijing Institute of Technology, Beijing 100081, People’s Republic of China Department of Physics, School of Science, BIT, Beijing 100081, People’s Republic of China c Department of Physics, Gomal University, D.I. Khan, NWFP, Pakistan b
a r t i c l e
i n f o
Article history: Received 13 May 2011 Received in revised form 7 July 2011 Accepted 7 July 2011 Available online 19 July 2011 Keywords: Dandelion-like GaN Semiconductor CVD method Field emission properties
a b s t r a c t Dandelion-like gallium nitride (GaN) microstructures were successfully synthesized via Ni catalyst assisted chemical vapor deposition method at 1200 ◦ C under NH3 atmosphere by pre-treating precursors with aqueous ammonia. The as-synthesized product was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). X-ray diffraction analysis revealed that as-synthesized dandelion-like GaN was pure and has hexagonal wurtzite structure. SEM results showed that the size of the dandelion-like GaN structure was in the range of 30–60 m. Dandelion-like GaN microstructures exhibited reasonable field emission properties with the turn-on field of 9.65 V m−1 (0.01 mA cm−2 ) and threshold field of 11.35 V m−1 (1 mA cm−2 ) which is sufficient for applications of electron emission devices, field emission displays and vacuum micro electronic devices. Optical properties were studied at room temperature by using fluorescence spectrophotometer. Photoluminescence (PL) measurements of dandelion-like GaN showed a strong near-band-edge emission at 370.2 nm (3.35 eV) with blue band emission at 450.4 nm (2.75 eV) and 465.2 nm (2.66 eV) but with out yellow band emission. The room-temperature photoluminescence properties showed that it has also potential application in light-emitting devices. The tentative growth mechanism for the growth of dandelion-like GaN was also described. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Among binary nitride semiconductors gallium nitride (GaN) is one of the most attractive and promising semiconductor materials with a wide band gap (Eg = 3.4 eV) at room temperature having Wurtzite structure. GaN is a group III–V semiconducting material having potential applications in different fields of life especially in the field of electronics due to its unique material properties such as direct wide-band gap, high thermal stability and strong resistance to radiation [1]. In the last few years, a great number of studies have been conducted on excellent field emission (FE) properties of different materials. The basic reasons of the growing interest in the synthesis and characterization of nanostructured materials and their study as field electron emitters are the application of these emitters in flat panel display devices and as electron sources in vacuum microelectronic devices. Gallium nitride has also attracted great attention as a material for field emitters due to its high melting point (2600 K), high
∗ Corresponding author. Tel.: +86 10 6891 3792, fax: +86 10 6891 2001. E-mail address: [email protected] (C. Cao). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.07.043
chemical and mechanical stability [2,3] and low electron affinity (2.7–3.3 eV), which is even lower than Si and metals [4,5]. Moreover, FE cathodes made up from GaN have longer lifetimes than either those from Si or other conventional semiconductors. Until now GaN has been used for fabricating the different devices such as high power microwave tubes [6] laser diodes [7] blue–green light emitting diodes (LEDs) [8] detectors [9] field effect transistors [10] and field emitter arrays [11–13] due to its excellent field emission and photoluminescence (PL) properties. Currently one-dimensional and three-dimensional nanoscale materials are under focus due to their unique electronic, optical and magnetic properties as well as potential applications in constructing novel nanodevices [14,15]. In recent years much work has been done on synthesizing one-dimensional and threedimensional morphologies of GaN and using it in different fields of life. Up to now, one-dimensional and quasi-one-dimensional GaN nanorods [16], nanowires [17], nanotubes [18], nanoribbons [19], nanobelts [20] and prismatic rods and cone nanowires [21] have been synthesized. However, only a few reports about threedimensional GaN nanostructures have been published [22,23]. Cactus-like and paint brushes like 3D morphologies of gallium oxides, Cu2 O and ZnO has also been already reported [24–27].
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Recently Sharma et al. [28] synthesized ball-like morphology of GaN on tungsten (W) tip via plasma assisted reactive evaporation of gallium in nitrogen environment and studied its field emission properties without and with annealing. Such different and unique morphologies have attracted comprehensive interest due to their potential applications in nano/micro devices. It is well known that structural morphology plays a key role in the property control [29]. In the present work we have synthesized a unique morphology of high quality dandelion-like GaN on nickel coated Si (1 0 0) substrates, which were kept in the ammonia atmosphere at 1200 ◦ C. Chemical vapor deposition (CVD) method was used and the precursors were pre-treated with aqueous ammonia at 120 ◦ C. As synthesized dandelion-like GaN structures were characterized by XRD, SEM and EDX techniques. Field emission measurements were performed on synthesized GaN structures to evaluate their usefulness for field emission displays/devices. These dandelion-like GaN structures exhibited convincing field emission properties with a turn-on field of 9.65 V m−1 (0.01 mA cm−2 ) and threshold field of 11.35 V m−1 (1 mA cm−2 ) which is reasonable for applications in electron emission devices, field emission displays and vacuum micro electronic devices. Optical properties were studied at room temperature by using fluorescence spectrophotometer to explore their potential application in optoelectronic devices and LEDs. The room temperature photoluminescence emission with a strong peak at 370.2 nm (3.35 eV) indicates that dandelion-like GaN microstructures have potential application in light-emitting devices.
2. Experimental procedure Typical chemical vapor deposition method was used for synthesizing the dandelion-like morphology of GaN. Ga2 O3 (99.99% pure) and graphite powder (weight ratio 1:1) were mixed in mortar and pestle and then treated with aqueous ammonia at 120 ◦ C thrice. These aqueous ammonia treated precursors were loaded in an alumina boat as Ga source. Silicon (0 0 1) substrate was ultrasonically cleaned and coated with 0.04 M nickel chloride/ethanol solution. After drying in air the substrate was put on the top of the alumina boat loaded with source materials. The vertical distance between the Ga source and the Si substrate was about 5–6 mm. The boat was transferred into a ceramics tube mounted in a horizontal tube (HT) furnace having size 100 cm long with 6 cm diameter. After sealing the HT residual contents were removed by mechanical evacuating pump and then the HT was heavily flushed with high purity NH3 gas for about 5 min. NH3 gas was used as a nitrogen source and reacting gas. Subsequently, the furnace was heated at ramp rate of 10 ◦ C/min to get the maximum set temperature 1200 ◦ C under 200 sccm (standard cubic centimeter per minute) flow of NH3 gas. When the temperature reached the maximum preset temperature 1200 ◦ C then it was maintained for 2 h (120 min). After the completion of the reaction the furnace was allowed to cool down naturally to room temperature. Product collected on the substrate was analyzed. The structure and the phase purity of the product were determined by X-ray powder diffraction (XRD, Philips X’Pert Pro MPD) with Cu-K␣ radiation source ( = 0.15418 nm) whereas the morphologies of the products were examined by scanning electron microscopy (SEM, TM-1000, Japan). The chemical composition of the product was tested by energy dispersive X-ray spectroscopy (EDX). FE properties of the product were investigated in a vacuum chamber with a pressure of 1.2 × 10−6 Pa at room temperature. Photoluminescence properties at room temperature were studied using PL spectrum with fluorescence spectrophotometer (F-4500).
Fig. 1. (a) Shows XRD and (b) shows the EDX pattern of the as synthesized dandelion-like GaN.
3. Results and discussion Typical XRD pattern (2 20–80) of the dandelion-like GaN synthesized on Si substrate has been shown in Fig. 1(a). All of the diffraction peaks in the pattern can be indexed to hexagonal ˚ wurtzite GaN with lattice parameters a = 3.189 A˚ and c = 5.187 A. These values of parameters for GaN are in good agreement with standard values as listed in the JCPDS card (No. 076-0703). No other peaks of crystalline impurities, such as Ga or Ga2 O3 were detected within the detection limit. Miller indices of hexagonal structure of GaN are also marked in the spectrum. From the XRD analysis it was revealed that the as-synthesized product was of high purity. Quantitative analysis was achieved by EDX of the product as shown in Fig. 1(b). Insight of Fig. 1(b) has shown the wire of the dandelion-like structure whose EDX was conducted. Quantitative analysis (EDX) performed at different positions of the wires of dandelion-like GaN indicated that the product was composed of the elements Ga and N, and the atomic ratio of Ga/N was about 1:1 with in the experimental errors, corresponding to the stoichiometric composition of GaN. So EDX also reveals the purity of the product. 3.1. Structural morphologies Fig. 2(a and b) shows the low magnification and Fig. 2(c and d) shows the high magnification SEM images of the as synthesized product on Si substrate. From the SEM it was observed that a lot of dandelion-like microstructures were collected on the silicon substrate. By a close examination of Fig. 2(d) it showed that the dandelion-like structures were built from nanometer-scale wires and rods, which were packed radially. This kind of 3D structure resembles to dandelion hence named as dandelion-like structure.
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Fig. 2. (a and b) shows the low magnification images and (c and d) shows high magnification of SEM images of dandelion-like GaN microstructures.
Individual nanorods and nanowires were of hundredth of nanometers in length, and tenth of nanometers in diameter. The size of the dandelion-like structures varies from 30 to 60 m in diameter, exhibiting a highly flocky appearance. This kind of dandelion-like GaN microstructures exhibited a higher surface area due to its special structure. So they have potential applications in fabricating the inorganic or organic functional materials with improved performances. 3.2. Growth mechanism With a view to investigate the growth mechanism of the product, the time dependent experiments were performed at different reaction times at 30 min, 60 min and 120 min at same other conditions. At 30 and 60 min reaction time the dandelion-like morphologies were not observed. But performing the reaction at 1200 ◦ C and for 120 min a novel morphology of dandelion-like GaN was achieved as shown in Fig. 2(a–c). On basis of these experimental results, vapor–liquid–solid (VLS) growth mechanism was proposed. According to proposed growth mechanism when source material was heated up then gallium droplets were made up because gallium has less melting temperature. So Ga droplets evaporate and collect on the silicon substrate on energy
favorable sites as in Fig. 3(b and c). After that as temperature of the furnace increased and also due to long reaction time these droplets reacts with nitrogen, gradually these droplets consume and converted into nanowires as in Fig. 3(d). Finally it fully converted into dandelion-like structure as in Fig. 3(e) and it look like that nanowires are emerging from a single point. Schematic diagram for growth mechanism has been shown in Fig. 3. The ammonia decomposition has a slow kinetic rate at low reaction temperature as compared to the open flow system. Ammonia decomposition in similar procedure was estimated in less than 10% of total ammonia inflow [30]. This fact would prevent total decomposition of NH3 (g) and consequently reduces formation of non-reactive specie N2 (g) in synthesis procedure. High temperature causes NH3 gas to decompose gradually into various parts like NH2 , NH, N2 , N, H2 and H [31]. Therefore, NH3 gas can act as a nitriding agent for converting Ga2 O3 powder into gallium nitride product. Under the conditions of ammonia atmosphere and high temperature, the starting materials, Ga2 O3 and graphite powder, dissociated into Ga2 O gas: Ga2 O3 + 2C → Ga2 O(vapors) + 2CO(vapors)
Fig. 3. Shows the growth mechanism of formation of dandelion-like GaN microstructures.
(1)
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At the same time, Ga2 O3 reacted with NH3 to form the GaN: Ga2 O3 + 2NH3(vapors) + 3C → 2GaN + 3CO(vapors) +3H2(vapors)
(2)
The Ga2 O gas generated from the Ga2 O3 powder would further react with the incoming CO gas to form metal gallium: Ga2 O(vapour) + CO(vapour) → Ga(g) + Ga(liquid) + CO2
(3)
On the other hand, the Ga2 O would also react with the atomic N decomposing from the ammonia to form GaN. Ga2 O(vapors) +3N(atom) → 2GaN + NO
(4)
During the growth process the Ga2 O3 converted into metal gallium droplets that transport and adhere to the substrate as shown in Fig. 3(b and c). These gallium droplets did not react with N due to low temperature and condense to gallium liquid droplets (Eq. (3)). After that when temperature was increased these droplets reacted with N at high temperature and also initiated a nucleation process, which resulted in an axial growth of the solid GaN rods/wires of dandelion-like structures as shown in Fig. 3(d and e). In the axis growth, the gallium droplet not only acted as the reactant but also provided an energetically favored site for the absorption of N atom and other reactive gases such as Ga(g) and Ga2 O(g) [32]. Ni also served as an effective catalyst in the process of GaN nanowires growth [33]. So in this work nickel chloride also decomposed into nanometer-sized Ni particles when temperature was raised. These Ni nano-particles continuously adsorbed the gaseous Ga and N atoms as energy-favorable sites to from the GaN. We also performed the experiment using a catalyst-free silicon wafer under the same reaction conditions. As a result, few nanowires were formed, indicating that Ni also played an important role in the growth of GaN dandelion-like structures. 3.3. Field emission properties Field emission measurements of dandelion-like GaN synthesized at 1200 ◦ C were performed in a vacuum chamber with a pressure of 1.2 × 10−6 Pa at room temperature. A rod-like stainless steel probe (1 mm in diameter) of 0.78 mm2 in area was used as an anode and product was used as cathode. The spacing between these two electrodes was 200 m in our experiment. Exponential dependence of the emission current density (J) on the applied electric field (E) has been depicted in Fig. 4(a). By definition, the turn-on field is the electric fields required to produce emission current density of 0.01 mA cm−2 where as threshold fields is the electric fields required to produce emission current density of 1 mA cm−2 [34,35]. The turn-on field (Eto ) of 9.65 V m−1 (0.01 mA cm−2 ) and threshold field (Ethr ) of 11.35 V m−1 (1 mA cm−2 ) were obtained for dandelion-like GaN structures as depicted in Fig. 4(a). This turn-on field value is less and also comparable with many reported values of GaN such as 12 V m−1 for GaN nanowires [33], 9.5 V m−1 for prismatic submicro rods and cone nanowires [21], 8.5 V m−1 for nanowires with protrusions [36] and also 8.5 V m−1 for thin nanowires [37]. The turn-on field value reported for dandelionlike GaN is also less then many other materials such as 29 V m−1 for random SiC nanowires arrays [38] 19.8 V m−1 for tantalum disulfide (TaS2 ) nanobelts [39], 18 V m−1 for ZnO nanowires [40], 14.9 V m−1 for AlN flower like nanostructures [41], 14.15 V m−1 for Zn/ZnO core–shell microstructures [42], 12.6 V m−1 for cactuslike Ga2 O3 nanostructures [24]. So the reported turn-on field value is quite moderate and reasonable as compared with the above referred values of the turn-on field. The admirable field emission properties mainly have been ascribed to the tips of the non-aligned wires/rods of dandelion-like GaN microstructures. The random orientation of the wires/rods of dandelion-like structures causes the strong screening effects. Other factors which can effect the field
Fig. 4. (a) Shows the field emission J–E curve of the dandelion-like GaN microstructures and (b) shows the corresponding Fowler–Nordheim (FN) graph.
emission properties are tip radius of the emitters, density of emitters, aspect ratio, density of surface states, morphology, intrinsic properties (such as electronic affinity) and dimensions of the emitters [42]. According to the Fowler–Nordheim (FN) model, [34] the relationship between the emission current density and the applied electric field can be expressed as
J=
Aˇ2 E 2 ˚
exp
−B˚−3/2 ˇE
where ˇ is the field enhancement factor, ˚ is the work function of the electron emitter, which is estimated to be 4.1 eV for GaN [35]. Where as A and B are constants with the values of 1.56 × 10−1 A˚ V−2 eV and 6.83 × 103 V eV−3/2 m−1 , respectively. The corresponding FN [ln (I/V2 ) − 1/V] plot of durian-like GaN has been depicted in Fig. 4(b). The linear behavior of semi conducting GaN at high field reveals that the electron emission is due to the vacuum and quantum tunneling effect [43–45]. Finally, it was revealed that dandelion-like GaN microstructures showed a reasonably lower turn-on field 9.65 V m−1 (0.01 mA cm−2 ), which is enough for application of field emission displays and vacuum micro electronic devices [37]. Due to these reasons it can be considered as a promising material for field emission displays and vacuum micro electronic devices.
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Acknowledgements This work was supported by National Natural Science Foundation of China (50972017) and The Research Fund for the Doctoral Program of Higher Education of China (20060007024). References
Fig. 5. Shows the PL spectrum of dandelion-like GaN measured at room temperature.
3.4. PL properties For exploring the optical behavior of as synthesized dandelionlike GaN the photoluminescence spectrum of the product at room temperature was measured. Ultraviolet (UV) light used to excite the product was obtained from xenon lamp and its excitation wavelength was set at 325 nm. The room-temperature photoluminescence emission spectrum of dandelion-like GaN synthesized at 1200 ◦ C has been depicted in Fig. 5. A strong emission peak near-band-edge emission has been observed at 370.2 nm (3.35 eV), which can be attributed to band edge-related emission. The narrow emission peak near band-edge-emission reveals highly crystalline nature of the product. The well-known yellow band emission was not observed, which is a good agreement with the PL of pure GaN [35,46]. The strong peak at 370.2 nm (3.35 eV) is red shifted about 51 meV with respect to the band edge of GaN. Two weak peaks at 450.4 nm (2.75 eV) and 465.2 nm (2.66 eV) have also been observed which fall in the blue luminescence (BL) region. This blue luminescence is mostly attributed to the vacancies of N atoms, deep levels and defects levels [35,47]. The intensive PL emission spectrum shown in Fig. 5 indicated that the dandelion-like GaN microstructures have potential application in light emitting devices and LED’s. 4. Conclusion Dandelion-like GaN with size of 30–60 m have been successfully synthesized on Si substrate at 1200 ◦ C via Ni catalyst assisted CVD method in NH3 atmosphere and pre-treating precursors with aqueous ammonia. XRD and EDX analysis showed that the product was pure. The dandelion-like GaN microstructures exhibited a reasonable field emission property with a turn on field of 9.65 V m−1 (0.01 mA cm−2 ) and threshold field of 11.35 V m−1 (1 mA cm−2 ). That means dandelion-like GaN microstructures have potential applications in the electron emission devices, field emission displays and vacuum micro electronic devices. Photoluminescence measurements of dandelion-like GaN showed a strong near-bandedge emission at 370.2 nm (3.35 eV) with blue band emission peaks at 450.4 nm (2.75 eV) and 465.2 nm (2.66 eV) but with out yellow band emission. PL properties of dandelion-like GaN having unique morphology showed that it has enormous applications in optical devices in UV and visible regions.
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Synthesis, characterization, growth mechanism, photoluminescence and field emission properties of novel dandelion-like gallium nitride Ghulam Nabi a , Chuanbao Cao a,∗ , Waheed S. Khan a , Sajad Hussain a , Zahid Usman a , Muhammad Safdar a , Sajjad Hussain Shah b , Noor Abass Din Khattak c a
Research Centre of Materials Science, School of Material Science and Engineering, Beijing Institute of Technology, Beijing 100081, People’s Republic of China Department of Physics, School of Science, BIT, Beijing 100081, People’s Republic of China c Department of Physics, Gomal University, D.I. Khan, NWFP, Pakistan b
a r t i c l e
i n f o
Article history: Received 13 May 2011 Received in revised form 7 July 2011 Accepted 7 July 2011 Available online 19 July 2011 Keywords: Dandelion-like GaN Semiconductor CVD method Field emission properties
a b s t r a c t Dandelion-like gallium nitride (GaN) microstructures were successfully synthesized via Ni catalyst assisted chemical vapor deposition method at 1200 ◦ C under NH3 atmosphere by pre-treating precursors with aqueous ammonia. The as-synthesized product was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). X-ray diffraction analysis revealed that as-synthesized dandelion-like GaN was pure and has hexagonal wurtzite structure. SEM results showed that the size of the dandelion-like GaN structure was in the range of 30–60 m. Dandelion-like GaN microstructures exhibited reasonable field emission properties with the turn-on field of 9.65 V m−1 (0.01 mA cm−2 ) and threshold field of 11.35 V m−1 (1 mA cm−2 ) which is sufficient for applications of electron emission devices, field emission displays and vacuum micro electronic devices. Optical properties were studied at room temperature by using fluorescence spectrophotometer. Photoluminescence (PL) measurements of dandelion-like GaN showed a strong near-band-edge emission at 370.2 nm (3.35 eV) with blue band emission at 450.4 nm (2.75 eV) and 465.2 nm (2.66 eV) but with out yellow band emission. The room-temperature photoluminescence properties showed that it has also potential application in light-emitting devices. The tentative growth mechanism for the growth of dandelion-like GaN was also described. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Among binary nitride semiconductors gallium nitride (GaN) is one of the most attractive and promising semiconductor materials with a wide band gap (Eg = 3.4 eV) at room temperature having Wurtzite structure. GaN is a group III–V semiconducting material having potential applications in different fields of life especially in the field of electronics due to its unique material properties such as direct wide-band gap, high thermal stability and strong resistance to radiation [1]. In the last few years, a great number of studies have been conducted on excellent field emission (FE) properties of different materials. The basic reasons of the growing interest in the synthesis and characterization of nanostructured materials and their study as field electron emitters are the application of these emitters in flat panel display devices and as electron sources in vacuum microelectronic devices. Gallium nitride has also attracted great attention as a material for field emitters due to its high melting point (2600 K), high
∗ Corresponding author. Tel.: +86 10 6891 3792, fax: +86 10 6891 2001. E-mail address: [email protected] (C. Cao). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.07.043
chemical and mechanical stability [2,3] and low electron affinity (2.7–3.3 eV), which is even lower than Si and metals [4,5]. Moreover, FE cathodes made up from GaN have longer lifetimes than either those from Si or other conventional semiconductors. Until now GaN has been used for fabricating the different devices such as high power microwave tubes [6] laser diodes [7] blue–green light emitting diodes (LEDs) [8] detectors [9] field effect transistors [10] and field emitter arrays [11–13] due to its excellent field emission and photoluminescence (PL) properties. Currently one-dimensional and three-dimensional nanoscale materials are under focus due to their unique electronic, optical and magnetic properties as well as potential applications in constructing novel nanodevices [14,15]. In recent years much work has been done on synthesizing one-dimensional and threedimensional morphologies of GaN and using it in different fields of life. Up to now, one-dimensional and quasi-one-dimensional GaN nanorods [16], nanowires [17], nanotubes [18], nanoribbons [19], nanobelts [20] and prismatic rods and cone nanowires [21] have been synthesized. However, only a few reports about threedimensional GaN nanostructures have been published [22,23]. Cactus-like and paint brushes like 3D morphologies of gallium oxides, Cu2 O and ZnO has also been already reported [24–27].
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Recently Sharma et al. [28] synthesized ball-like morphology of GaN on tungsten (W) tip via plasma assisted reactive evaporation of gallium in nitrogen environment and studied its field emission properties without and with annealing. Such different and unique morphologies have attracted comprehensive interest due to their potential applications in nano/micro devices. It is well known that structural morphology plays a key role in the property control [29]. In the present work we have synthesized a unique morphology of high quality dandelion-like GaN on nickel coated Si (1 0 0) substrates, which were kept in the ammonia atmosphere at 1200 ◦ C. Chemical vapor deposition (CVD) method was used and the precursors were pre-treated with aqueous ammonia at 120 ◦ C. As synthesized dandelion-like GaN structures were characterized by XRD, SEM and EDX techniques. Field emission measurements were performed on synthesized GaN structures to evaluate their usefulness for field emission displays/devices. These dandelion-like GaN structures exhibited convincing field emission properties with a turn-on field of 9.65 V m−1 (0.01 mA cm−2 ) and threshold field of 11.35 V m−1 (1 mA cm−2 ) which is reasonable for applications in electron emission devices, field emission displays and vacuum micro electronic devices. Optical properties were studied at room temperature by using fluorescence spectrophotometer to explore their potential application in optoelectronic devices and LEDs. The room temperature photoluminescence emission with a strong peak at 370.2 nm (3.35 eV) indicates that dandelion-like GaN microstructures have potential application in light-emitting devices.
2. Experimental procedure Typical chemical vapor deposition method was used for synthesizing the dandelion-like morphology of GaN. Ga2 O3 (99.99% pure) and graphite powder (weight ratio 1:1) were mixed in mortar and pestle and then treated with aqueous ammonia at 120 ◦ C thrice. These aqueous ammonia treated precursors were loaded in an alumina boat as Ga source. Silicon (0 0 1) substrate was ultrasonically cleaned and coated with 0.04 M nickel chloride/ethanol solution. After drying in air the substrate was put on the top of the alumina boat loaded with source materials. The vertical distance between the Ga source and the Si substrate was about 5–6 mm. The boat was transferred into a ceramics tube mounted in a horizontal tube (HT) furnace having size 100 cm long with 6 cm diameter. After sealing the HT residual contents were removed by mechanical evacuating pump and then the HT was heavily flushed with high purity NH3 gas for about 5 min. NH3 gas was used as a nitrogen source and reacting gas. Subsequently, the furnace was heated at ramp rate of 10 ◦ C/min to get the maximum set temperature 1200 ◦ C under 200 sccm (standard cubic centimeter per minute) flow of NH3 gas. When the temperature reached the maximum preset temperature 1200 ◦ C then it was maintained for 2 h (120 min). After the completion of the reaction the furnace was allowed to cool down naturally to room temperature. Product collected on the substrate was analyzed. The structure and the phase purity of the product were determined by X-ray powder diffraction (XRD, Philips X’Pert Pro MPD) with Cu-K␣ radiation source ( = 0.15418 nm) whereas the morphologies of the products were examined by scanning electron microscopy (SEM, TM-1000, Japan). The chemical composition of the product was tested by energy dispersive X-ray spectroscopy (EDX). FE properties of the product were investigated in a vacuum chamber with a pressure of 1.2 × 10−6 Pa at room temperature. Photoluminescence properties at room temperature were studied using PL spectrum with fluorescence spectrophotometer (F-4500).
Fig. 1. (a) Shows XRD and (b) shows the EDX pattern of the as synthesized dandelion-like GaN.
3. Results and discussion Typical XRD pattern (2 20–80) of the dandelion-like GaN synthesized on Si substrate has been shown in Fig. 1(a). All of the diffraction peaks in the pattern can be indexed to hexagonal ˚ wurtzite GaN with lattice parameters a = 3.189 A˚ and c = 5.187 A. These values of parameters for GaN are in good agreement with standard values as listed in the JCPDS card (No. 076-0703). No other peaks of crystalline impurities, such as Ga or Ga2 O3 were detected within the detection limit. Miller indices of hexagonal structure of GaN are also marked in the spectrum. From the XRD analysis it was revealed that the as-synthesized product was of high purity. Quantitative analysis was achieved by EDX of the product as shown in Fig. 1(b). Insight of Fig. 1(b) has shown the wire of the dandelion-like structure whose EDX was conducted. Quantitative analysis (EDX) performed at different positions of the wires of dandelion-like GaN indicated that the product was composed of the elements Ga and N, and the atomic ratio of Ga/N was about 1:1 with in the experimental errors, corresponding to the stoichiometric composition of GaN. So EDX also reveals the purity of the product. 3.1. Structural morphologies Fig. 2(a and b) shows the low magnification and Fig. 2(c and d) shows the high magnification SEM images of the as synthesized product on Si substrate. From the SEM it was observed that a lot of dandelion-like microstructures were collected on the silicon substrate. By a close examination of Fig. 2(d) it showed that the dandelion-like structures were built from nanometer-scale wires and rods, which were packed radially. This kind of 3D structure resembles to dandelion hence named as dandelion-like structure.
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Fig. 2. (a and b) shows the low magnification images and (c and d) shows high magnification of SEM images of dandelion-like GaN microstructures.
Individual nanorods and nanowires were of hundredth of nanometers in length, and tenth of nanometers in diameter. The size of the dandelion-like structures varies from 30 to 60 m in diameter, exhibiting a highly flocky appearance. This kind of dandelion-like GaN microstructures exhibited a higher surface area due to its special structure. So they have potential applications in fabricating the inorganic or organic functional materials with improved performances. 3.2. Growth mechanism With a view to investigate the growth mechanism of the product, the time dependent experiments were performed at different reaction times at 30 min, 60 min and 120 min at same other conditions. At 30 and 60 min reaction time the dandelion-like morphologies were not observed. But performing the reaction at 1200 ◦ C and for 120 min a novel morphology of dandelion-like GaN was achieved as shown in Fig. 2(a–c). On basis of these experimental results, vapor–liquid–solid (VLS) growth mechanism was proposed. According to proposed growth mechanism when source material was heated up then gallium droplets were made up because gallium has less melting temperature. So Ga droplets evaporate and collect on the silicon substrate on energy
favorable sites as in Fig. 3(b and c). After that as temperature of the furnace increased and also due to long reaction time these droplets reacts with nitrogen, gradually these droplets consume and converted into nanowires as in Fig. 3(d). Finally it fully converted into dandelion-like structure as in Fig. 3(e) and it look like that nanowires are emerging from a single point. Schematic diagram for growth mechanism has been shown in Fig. 3. The ammonia decomposition has a slow kinetic rate at low reaction temperature as compared to the open flow system. Ammonia decomposition in similar procedure was estimated in less than 10% of total ammonia inflow [30]. This fact would prevent total decomposition of NH3 (g) and consequently reduces formation of non-reactive specie N2 (g) in synthesis procedure. High temperature causes NH3 gas to decompose gradually into various parts like NH2 , NH, N2 , N, H2 and H [31]. Therefore, NH3 gas can act as a nitriding agent for converting Ga2 O3 powder into gallium nitride product. Under the conditions of ammonia atmosphere and high temperature, the starting materials, Ga2 O3 and graphite powder, dissociated into Ga2 O gas: Ga2 O3 + 2C → Ga2 O(vapors) + 2CO(vapors)
Fig. 3. Shows the growth mechanism of formation of dandelion-like GaN microstructures.
(1)
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At the same time, Ga2 O3 reacted with NH3 to form the GaN: Ga2 O3 + 2NH3(vapors) + 3C → 2GaN + 3CO(vapors) +3H2(vapors)
(2)
The Ga2 O gas generated from the Ga2 O3 powder would further react with the incoming CO gas to form metal gallium: Ga2 O(vapour) + CO(vapour) → Ga(g) + Ga(liquid) + CO2
(3)
On the other hand, the Ga2 O would also react with the atomic N decomposing from the ammonia to form GaN. Ga2 O(vapors) +3N(atom) → 2GaN + NO
(4)
During the growth process the Ga2 O3 converted into metal gallium droplets that transport and adhere to the substrate as shown in Fig. 3(b and c). These gallium droplets did not react with N due to low temperature and condense to gallium liquid droplets (Eq. (3)). After that when temperature was increased these droplets reacted with N at high temperature and also initiated a nucleation process, which resulted in an axial growth of the solid GaN rods/wires of dandelion-like structures as shown in Fig. 3(d and e). In the axis growth, the gallium droplet not only acted as the reactant but also provided an energetically favored site for the absorption of N atom and other reactive gases such as Ga(g) and Ga2 O(g) [32]. Ni also served as an effective catalyst in the process of GaN nanowires growth [33]. So in this work nickel chloride also decomposed into nanometer-sized Ni particles when temperature was raised. These Ni nano-particles continuously adsorbed the gaseous Ga and N atoms as energy-favorable sites to from the GaN. We also performed the experiment using a catalyst-free silicon wafer under the same reaction conditions. As a result, few nanowires were formed, indicating that Ni also played an important role in the growth of GaN dandelion-like structures. 3.3. Field emission properties Field emission measurements of dandelion-like GaN synthesized at 1200 ◦ C were performed in a vacuum chamber with a pressure of 1.2 × 10−6 Pa at room temperature. A rod-like stainless steel probe (1 mm in diameter) of 0.78 mm2 in area was used as an anode and product was used as cathode. The spacing between these two electrodes was 200 m in our experiment. Exponential dependence of the emission current density (J) on the applied electric field (E) has been depicted in Fig. 4(a). By definition, the turn-on field is the electric fields required to produce emission current density of 0.01 mA cm−2 where as threshold fields is the electric fields required to produce emission current density of 1 mA cm−2 [34,35]. The turn-on field (Eto ) of 9.65 V m−1 (0.01 mA cm−2 ) and threshold field (Ethr ) of 11.35 V m−1 (1 mA cm−2 ) were obtained for dandelion-like GaN structures as depicted in Fig. 4(a). This turn-on field value is less and also comparable with many reported values of GaN such as 12 V m−1 for GaN nanowires [33], 9.5 V m−1 for prismatic submicro rods and cone nanowires [21], 8.5 V m−1 for nanowires with protrusions [36] and also 8.5 V m−1 for thin nanowires [37]. The turn-on field value reported for dandelionlike GaN is also less then many other materials such as 29 V m−1 for random SiC nanowires arrays [38] 19.8 V m−1 for tantalum disulfide (TaS2 ) nanobelts [39], 18 V m−1 for ZnO nanowires [40], 14.9 V m−1 for AlN flower like nanostructures [41], 14.15 V m−1 for Zn/ZnO core–shell microstructures [42], 12.6 V m−1 for cactuslike Ga2 O3 nanostructures [24]. So the reported turn-on field value is quite moderate and reasonable as compared with the above referred values of the turn-on field. The admirable field emission properties mainly have been ascribed to the tips of the non-aligned wires/rods of dandelion-like GaN microstructures. The random orientation of the wires/rods of dandelion-like structures causes the strong screening effects. Other factors which can effect the field
Fig. 4. (a) Shows the field emission J–E curve of the dandelion-like GaN microstructures and (b) shows the corresponding Fowler–Nordheim (FN) graph.
emission properties are tip radius of the emitters, density of emitters, aspect ratio, density of surface states, morphology, intrinsic properties (such as electronic affinity) and dimensions of the emitters [42]. According to the Fowler–Nordheim (FN) model, [34] the relationship between the emission current density and the applied electric field can be expressed as
J=
Aˇ2 E 2 ˚
exp
−B˚−3/2 ˇE
where ˇ is the field enhancement factor, ˚ is the work function of the electron emitter, which is estimated to be 4.1 eV for GaN [35]. Where as A and B are constants with the values of 1.56 × 10−1 A˚ V−2 eV and 6.83 × 103 V eV−3/2 m−1 , respectively. The corresponding FN [ln (I/V2 ) − 1/V] plot of durian-like GaN has been depicted in Fig. 4(b). The linear behavior of semi conducting GaN at high field reveals that the electron emission is due to the vacuum and quantum tunneling effect [43–45]. Finally, it was revealed that dandelion-like GaN microstructures showed a reasonably lower turn-on field 9.65 V m−1 (0.01 mA cm−2 ), which is enough for application of field emission displays and vacuum micro electronic devices [37]. Due to these reasons it can be considered as a promising material for field emission displays and vacuum micro electronic devices.
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Acknowledgements This work was supported by National Natural Science Foundation of China (50972017) and The Research Fund for the Doctoral Program of Higher Education of China (20060007024). References
Fig. 5. Shows the PL spectrum of dandelion-like GaN measured at room temperature.
3.4. PL properties For exploring the optical behavior of as synthesized dandelionlike GaN the photoluminescence spectrum of the product at room temperature was measured. Ultraviolet (UV) light used to excite the product was obtained from xenon lamp and its excitation wavelength was set at 325 nm. The room-temperature photoluminescence emission spectrum of dandelion-like GaN synthesized at 1200 ◦ C has been depicted in Fig. 5. A strong emission peak near-band-edge emission has been observed at 370.2 nm (3.35 eV), which can be attributed to band edge-related emission. The narrow emission peak near band-edge-emission reveals highly crystalline nature of the product. The well-known yellow band emission was not observed, which is a good agreement with the PL of pure GaN [35,46]. The strong peak at 370.2 nm (3.35 eV) is red shifted about 51 meV with respect to the band edge of GaN. Two weak peaks at 450.4 nm (2.75 eV) and 465.2 nm (2.66 eV) have also been observed which fall in the blue luminescence (BL) region. This blue luminescence is mostly attributed to the vacancies of N atoms, deep levels and defects levels [35,47]. The intensive PL emission spectrum shown in Fig. 5 indicated that the dandelion-like GaN microstructures have potential application in light emitting devices and LED’s. 4. Conclusion Dandelion-like GaN with size of 30–60 m have been successfully synthesized on Si substrate at 1200 ◦ C via Ni catalyst assisted CVD method in NH3 atmosphere and pre-treating precursors with aqueous ammonia. XRD and EDX analysis showed that the product was pure. The dandelion-like GaN microstructures exhibited a reasonable field emission property with a turn on field of 9.65 V m−1 (0.01 mA cm−2 ) and threshold field of 11.35 V m−1 (1 mA cm−2 ). That means dandelion-like GaN microstructures have potential applications in the electron emission devices, field emission displays and vacuum micro electronic devices. Photoluminescence measurements of dandelion-like GaN showed a strong near-bandedge emission at 370.2 nm (3.35 eV) with blue band emission peaks at 450.4 nm (2.75 eV) and 465.2 nm (2.66 eV) but with out yellow band emission. PL properties of dandelion-like GaN having unique morphology showed that it has enormous applications in optical devices in UV and visible regions.
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