Controlled Growth of Aluminum Nitride Nanostructures: Aligned Tips

The aluminum nitride (AlN) nanostructures, including aligned tips, brushes, and complex structures, have been synthesized by a chemical vapor depositi...
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J. Phys. Chem. C 2007, 111, 1895-1899

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Controlled Growth of Aluminum Nitride Nanostructures: Aligned Tips, Brushes, and Complex Structures Zhuo Chen, Chuanbao Cao,* and Hesun Zhu Research Center of Materials Science, Beijing Institute of Technology, Beijing 100081, People’s Republic of China ReceiVed: September 11, 2006; In Final Form: NoVember 23, 2006

The aluminum nitride (AlN) nanostructures, including aligned tips, brushes, and complex structures, have been synthesized by a chemical vapor deposition procedure without any catalyst. Both the aspect ratio of the AlN nanotips and the morphologies of the products can be controlled by adjusting the growth temperature, time, and orientation of the substrate. The structure and morphology of the as-synthesized AlN nanostructures are characterized by X-ray diffraction and scanning and transmission electron microscopies. Field emission characterization shows that the turn-on fields for the aligned AlN nanotips grown at 700 and 800 °C are 10.8 and 12.2 V µm-1, respectively.

Introduction One-dimensional (1D) nanostructures not only are interesting for fundamental research due to their unique structural and physical properties relative to their bulk counterparts, but also offer fascinating potential for future technological applications.1-3 During the past few decades, the synthesis of 1D nanostructures has attracted extensive research interest. Owing to intensive and growing efforts in the synthesis, a wide variety of 1D nanostructures including nanotubes,4 nanowires,5 nanobelts,6 branched nanostructures,7,8 and so on have been prepared by various methods. To further realize the assembly of these 1D nanostructures into functional nanodevices, the controlled fabrication of the nanostructures with the desired size, morphology, and orientation should be fulfilled. This means that the process of complete “self-organization” has to be influenced and controlled. However, so far, it still remains a significant challenge, suggesting that more experimental studies are needed. Aluminum nitride (AlN), as an important kind of group III nitride wide band gap (6.2 eV) semiconductor, has high thermal conductivity, a low coefficient of thermal expansion which closely matches that of silicon, excellent mechanical strength, and chemical stability.9 At the same time, AlN is a promising candidate for field emission device applications due to electrons in the materials with small or even negative electron affinity,10-12 which means electrons can be easily extracted from the surface to vacuum when an electric field is applied. The aspect ratio and density of the emitting sites are key factors to influence the field emission performance of nanomaterials. Therefore, synthesis of well-aligned 1D AlN nanostructures is of particular interest.13-15 Recently, several groups have synthesized various aligned 1D AlN nanostructures, such as nanocones,16 nanotips,17 and nanorods with multitipped surfaces,18 and hierarchical comblike nanostructures.19 However, there are few reports about the growth of 1D aligned AlN nanostructures with controlled high aspect ratio shapes and sizes. In addition, novel hierarchical nanoarchitectures have attracted considerable attention because they can potentially be used for further nanosystems20 and self* To whom correspondence should be addressed. E-mail: cbcao@ bit.edu.cn.

assembly studies.7,21 Abundant hierarchical nanostructures have been obtained in another interesting material, ZnO.20 However, there are few reports about novel AlN hierarchical nanostructures.18,19 In this study, we report not only the growth of aligned AlN nanotips but also the first brushes and complex structures by a chemical vapor deposition (CVD) method. The aspect ratio control of the AlN nanotips can be achieved by varying the growth time and temperature, which is important for field emission applications. The brushes and complex structures can be synthesized by varying the growth time and orientation of the silicon substrate. This makes the family of AlN nanostructures richer besides their potential applications for further nanodevices and self-assembly studies. Notably, the brush structures are freestanding, which makes it easier to incorporate them into functional devices. In addition, previous efforts to synthesize aligned AlN nanostructures have used catalysts16-18 or produced polycrystalline materials.18 Compared with these methods, the synthesis technique reported here is quite simple and efficient, without any catalyst. Experimental Section The AlN nanostructures were synthesized in a furnace with a horizontal quartz tube (30 mm outer diameter). A silicon wafer was used as the substrate to collect the products. Before the substrate was loaded into the furnace, it was ultrasonically cleaned in ethanol for 5 min and then rinsed with distilled water. Anhydrous AlCl3 (AR reagent, Beijing Chemical Factory, China) and NH3 were used as aluminum and nitrogen sources, respectively. In a typical process, the substrate was placed in the center of the quartz tube, AlCl3 powders were put into a ceramic boat, and then the boat was placed in the upstream zone, where the temperature was about 150 °C. The distance between the Al source and the Si substrate was about 20 cm. After being purged with Ar gas for 5 min, the furnace was heated to the desired temperature and kept at that temperature for the desired time under NH3 and Ar flow at 100 and 200 sccm (standard cubic centimeters per minute), respectively. The deposition pressure was 1 atm. A series of experiments were performed by changing the three controllable parameters of the experi-

10.1021/jp065908b CCC: $37.00 © 2007 American Chemical Society Published on Web 01/13/2007

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Figure 1. XRD patterns of the AlN nanotips obtained on Si(111) substrates at 700 and 750 °C for 30 min.

ment: growth temperature (650-800 °C), growth time (1-60 min), and the orientation of the silicon substrate ((111) and (100)). The morphologies and structures of the products were characterized by X-ray diffraction (XRD) (Philips X’pert Pro diffractometer), scanning electron microscopy (SEM) (Hitachi S-4800), and high-resolution transmission electron microscopy (HRTEM) (Tecnai F30). Field emission measurements were conducted in a vacuum chamber with a pressure of 1.2 × 10-6 Pa at room temperature. A rodlike stainless steel probe (1 mm diameter) of 0.78 mm2 area was used as an anode. The sample was used as the cathode. The spacing between these two electrodes was 100 µm. The emission current was measured by a picoammeter (Keithley 485). A ballast resistor of 10 MV was used to protect the apparatus against circuit shorting.

Figure 2. Controllable and aligned AlN nanotips grown for 30 min: (a-d) top-view SEM images of the growth of AlN nanostructures as a function of the growth temperature; (e, f) corresponding side-view SEM images of AlN nanotips grown at (b) 700 and (d) 800 °C, respectively.

Results and Discussion The aligned AlN nanotips were synthesized using a chemical vapor deposition process. After the reaction, the silicon (111) substrate was covered with a layer of gray-yellow product. Figure 1 shows the XRD patterns of aligned AlN nanotips obtained at 700 and 750 °C for 30 min. Both the XRD patterns have only a strong peak at 35.9°, which can be indexed as the (002) reflection of hexagonal-phase AlN, indicating that the AlN nanotips grew preferentially along the c axis direction. A series of experiments were carried out to investigate the effect of growth temperature variation on the shape evolution of the AlN products. The morphology of the as-synthesized products was characterized by SEM. SEM images shown in Figure 2 illustrate the morphological evolution of the AlN nanotips grown for 30 min as a function of the growth temperature. From the top-view (Figure 2a-d) and side-view (Figure 2e,f) images, it can be seen that large-scale, vertically or slantingly aligned 1D AlN nanostructures were uniformly grown in high density on the silicon substrate. Parts a-d of Figure 2 show the variation of the diameter and density of the AlN nanotips with growth temperature. With increasing growth temperature, the diameters at the top of the nanotips increase while their densities decrease. The diameters at the top of the AlN nanotips are plotted as a function of temperature in the range 650-800 °C and are shown in Figure 3a. With increasing temperature, the mean diameter of the nanotips at the top increases from 12 to 40 nm. Figure 3b shows the effect of the growth temperature on the growth rate of the AlN nanotips; i.e., the growth rate increases as the temperature increases. This indicates that the diameters and densities of the products can be controlled by adjusting the growth temperature, which is simpler than by adjusting the thickness of the gold layer reported

Figure 3. (a) Mean diameter at the top and (b) growth rate of the aligned AlN nanotips as functions of the growth temperature.

by Shi et al.17 In addition, the length of the AlN nanotips can be controlled by adjusting the growth time on the basis of the knowledge of the growth rate. This suggests that the aspect ratio control of the AlN nanotips can be achieved by varying the

Controlled Growth of AlN Nanostructures

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Figure 4. (a) TEM image of the AlN nanotip grown for 30 min at 700 °C. (b, c) Corresponding (b) SAED pattern and (c) HRTEM image.

Figure 5. SEM images of the AlN complex structures grown on Si(100) for 1 min at 800 °C: (a) seeds; (b) nanorods; (c) highmagnification complex structures; (d) low-magnification complex structures.

growth time and temperature, which is a key step toward field emission applications. Further structural characterization of the aligned AlN nanotips grown for 30 min at 700 °C was performed by transmission electron microscopy (TEM) and HRTEM. Figure 4a shows a TEM image of the AlN nanotip. Basically, the diameters of the AlN nanotips under TEM observation are consistent with the SEM results. Selected area electron diffraction (SAED) was utilized to investigate the structure and orientation of the synthesized AlN nanostructures. The SAED pattern confirmed the single-crystal nature of the AlN nanotip grown along the 〈001〉 direction, as shown in Figure 4b. The lattice constants (a ) 0.310 nm, c ) 0.498 nm) calculated from the SAED pattern are in good agreement with the reported values of hexagonalphase AlN (a ) 0.3099 nm, c ) 0.4997 nm, JCPDS card no. 79-2497). The HRTEM image shown in Figure 4c gives a lattice fringe of about 0.498 nm, corresponding to the d001 space of the bulk hexagonal AlN. Both the SAED pattern and the HRTEM image confirm that the AlN nanotips grew along the c axis as derived from the XRD pattern. Two novel self-organized hierarchical AlN nanostructures, i.e., complex structures composed of nanorods (Figure 5) and brush structures composed of nanotips (Figure 6), can be synthesized by varying the growth time and the orientation of the silicon substrate and keeping the other growth parameters fixed. When the growth time was decreased to 1 min and Si(100) was used instead of Si(111), the complex structures were

Figure 6. SEM images of the freestanding AlN brush structures grown on Si(100) for 30 min at 700 °C: (a, c) low-magnification images; (b, d) corresponding local-magnification images.

formed at high yield at 800 °C (Figure 5d). Figure 5 shows SEM images that illustrate the formation of the AlN complex structures composed of nanorods. Nanometer-sized seeds were first grown on the Si(100) substrate without catalyst (Figure 5a), then AlN nanorods were grown on the seeds (Figure 5b), and finally AlN complex structures were obtained (Figure 5c). In addition, if the growth time was kept for 30 min at 700 °C and the Si(111) wafer was replaced by Si(100), novel AlN brushlike structures were obtained. Figure 6 shows SEM images with different magnifications of these AlN microbrushes. These brushlike microstructures have lengths of 50-60 µm and diameters of 17 µm as shown in Figure 6a,c. From the localmagnification SEM images shown in Figure 6b,d, it can be seen that the self-assembly structures are composed of high-density nanotips growing like brushes. These novel hierarchical AlN nanoarchitectures could be used as building blocks for a selfassembly study. The formation of these AlN nanostructures could be attributed to a vapor-solid growth process because no metal catalyst was used and no alloy droplets on their tips were found. These AlN nanostructures grown preferentially along the [001] direction have been confirmed by XRD, SAED, and HRTEM results, which is consistent with their crystallographic characteristics.22 Since there is no center of inversion in the hexagonal crystal structure, an inherent asymmetry along the c axis is present which allows anisotropic crystal growth along the [001] direction.4,23 It is a rapid and preferential growth along the axial direction [001] combined with a slow growth along the radial direction that leads to the formation of 1D nanotips. The sizes

1898 J. Phys. Chem. C, Vol. 111, No. 5, 2007 of the tips should be dominated by the growth rate along the radial direction. According to theories of crystal growth,24 with raising the growth temperature, the surface roughness at the lateral face will increase, giving rise to an increase in the growth rate of the radial direction. Therefore, the sizes of the tips increase as the growth temperature increases. In addition, the formation of the two novel AlN nanostructures is also interesting. A possible formational process of the AlN complex structures can be described as follows: heterogeneous nuclei are formed first on the Si(100) substrate, then the epitaxial growth of nanorods takes place on these nuclei, and finally the AlN complex structures are formed. Subsequently, through comparison with these experimental conditions, it is not difficult to find that the formation of the novel AlN brushlike structure results from varying the orientation of the substrate. The lattice mismatch between AlN(001) and Si(100) is much larger than that between AlN(001) and Si(111).25,26 It is well-known that the larger the mismatch, the larger the stress introduced during the growth process. Under our experimental conditions, the growth of AlN nanostrutures on the Si substrate is not rigid epitaxial growth due to the presence of the native oxide. However, the stress resulting from the lattice mismatch will still influence the growth of AlN because the layer of the native oxide is very thin. On the basis of the above analysis, it is reasonable to conclude that the stress leads a layer of the products to break away from the substrate. Further, the layer of the products formed a curly microwire to lower the surface energy, i.e., the formation of the freestanding brushlike structures. Field emission measurements of the aligned AlN nanotips grown at 700 and 800 °C for 30 min, were conducted in a vacuum chamber with a pressure of 1.2 × 10-6 Pa at room temperature. A rodlike stainless steel probe (1 mm diameter) of 0.78 mm2 area was used as an anode. The aligned AlN nanotips were used as the cathode. The spacing between these two electrodes was 100 µm in our experiment. The emission current was measured by a picoammeter (Keithley 485). A ballast resistor of 10 MV was used to protect the apparatus against circuit shorting. The curves of emission current density versus applied field (J-E) are depicted in Figure 7. The insets are the Fowler-Nordheim (FN) plots. By definition, the turnon field and the threshold field are the electronic fields required to generate an emission current density of 10 and 1 mA cm-2, respectively. A turn-on field of 10.8 V µm-1 was obtained for the aligned AlN nanotips grown at 700 °C and one of 12.2 V µm-1 for the aligned AlN nanotips grown at 800 °C. The threshold fields for the products obtained at 700 and 800 °C were found to be about 13.6 and 15.2 V µm-1, respectively, lower than the previously reported threshold fields of 32 V µm-1 for quasi-aligned AlN nanocones16 and that of 34 V µm-1 for heavily Si doped AlN,27 but higher than that of 3.67-5.17 V µm-1 for the comblike AlN nanostructure19 and that of 7 V µm-1 for aligned AlN nanorods with multitipped surfaces.18 The samples prepared by our simple strategy can easily reach an emission current density of 10 mA cm-2, a basic emission requirement for a flat panel display. Since the aligned AlN nanotips grown at 700 °C have smaller tips compared to those grown at 800 °C, it is reasonable to conclude that smaller tips have smaller turn-on and threshold fields. The inset graphs of Figure 7 show the FN plots of the field emission of the aligned AlN nanotips. Both the FN plots exhibit linear dependence at high fields, which reveals that the emission current is really caused by the quantum tunneling effect. According to the FN theory,28 the relationship between the

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Figure 7. Field emission current density versus electric field curves of the aligned AlN nanotips grown on the Si(111) substrate at (a) 700 °C and (b) 800 °C. The insets show the corresponding FowlerNordheim relationships (ln(J/E2)-1/E plots).

current density, J, and the applied electric field, E, can be described as follows:

J ) (Aβ2E2/φ) exp[-Bφ3/2(βE)-1] 10-10

V-2

(1)

A eV, B ) 6.83 × V m-1 where A ) 1.56 × -3/2 eV , and φ is the work function, which is estimated to be 3.7 eV for AlN.29 Here, β is the field enhancement factor, which can be determined using the slope of the ln(J/E2)-1/E plot. The field enhancement factors, β, have been calculated to be about 367 and 317 for the aligned AlN nanotips grown at 700 and 800 °C, respectively, indicating that the local electric field at the tip can be sharply strengthened due to the small radius of curvature of the nanotip.13 109

Conclusion In summary, the AlN nanostructures, including aligned tips, brushes, and complex structures, have been synthesized by a chemical vapor deposition procedure without any catalyst. Both the aspect ratio of the AlN nanotips and the morphologies of the products can be controlled by adjusting the growth temperature, time, and orientation of the substrate. Both TEM and SAED investigations indicate the single-crystal nature of such nanostructures grown preferentially along the c axis. Field emission measurements show that the aligned AlN nanotips grown at 700 and 800 °C, respectively, have turn-on fields of 10.8 and 12.2 V µm-1, which have potential applications in field emission nanodevices. In addition, the novel hierarchical complex structures and freestanding brushlike structures can be used as building blocks for self-assembly studies and further nanodevices. Acknowledgment. This work was supported by the National Science Foundation of China via Grant No. 20471007.

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