Well-Aligned Single-Crystalline GaN Nanocolumns and Their Field Emission Properties Zhuo Chen,†,‡ Chuanbao Cao,*,† Wai Sang Li,‡ and Charles Surya‡ Research Center of Materials Science, Beijing Institute of Technology, Beijing 100081, People’s Republic of China and Department of Electronic and Information Engineering, Hong Kong Polytechnic UniVersity, Hong Kong, People’s Republic of China
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 2 792–796
ReceiVed March 29, 2008; ReVised Manuscript ReceiVed October 21, 2008
ABSTRACT: Well-aligned GaN nanocolumns on silicon substrate were fabricated by simple and low-cost chemical vapor deposition without any catalyst. The structure and morphology of the as-synthesized GaN nanocolumns were characterized by X-ray diffraction and scanning and transmission electron microscopies. The aligned GaN nanocolumn arrays exhibited excellent field emission properties with a low turn-on field of 2.6 V/µm (0.01 mA/cm2) and high stability at room temperature, which is sufficient for applications of field emission displays and vacuum nanoelectronic devices. The room-temperature photoluminescence emission with a strong peak at 369 nm indicates that the well-aligned GaN nanocolumns have potential application in light-emitting nanodevices.
1. Introduction One-dimensional (1D) semiconductor nanowires as building blocks are considered to have great prospects for nanoscale electronic and optoelectronic devices due to their unique properties associated with their highly anisotropic geometry and size confinement.1-3 Among various 1D semiconductor nanowires, gallium nitride (GaN) nanowires have been the subject of intense research because of not only their interesting properties, such as high melting point, carrier mobility, and electrical breakdown field, but also their many potential applications including laser,4,5 light-emitting diodes,6,7 field emitter,8,9 and detectors,10 etc. To date, extensive efforts have been devoted to the synthesis and characterization of GaN nanowires, especially high ordered nanowires because they can be easily integrated into future nanodevices. Compared with artificial assembly techniques, self-organized growth provides an effective and low-cost approach for realization of nanowires with high alignment. Therefore, considerable efforts have been devoted to fabricate various high-ordered nanostructures by a selforganized approach. Thus far, a few aligned GaN 1D nanostructured arrays have been prepared using metal-organic chemical vapor deposition (MOCVD),11-13 molecular beam epitaxy (MBE),14,15 and hydride vapor-phase epitaxy (HVPE).16,17 For example, aligned GaN nanotube arrays have been fabricated by MOCVD,11 where ordered ZnO nanowires are initially used as templates. Synthesis of GaN nanowire arrays with controlled the crystallographic growth directions has been realized by appropriate substrate selection.18 GaN nanowire arrays with well-defined locations and diameters have been obtained by MOCVD under a selective growth mask condition.19 Although some significant progress has been made in the synthesis of high-ordered GaN 1D nanostructures, exploring simple and economical synthesis methods for vertically aligned GaN nanostructures remains a challenge. In addition, due to the lack of commercially available free-standing GaN substrates, GaN is generally grown on sapphire and SiC substrates. However, growth of GaN nanostructures on silicon substrate has attracted much interest due to potential integration between GaN highpower electronics and mature silicon technologies. * To whom correspondence should be addressed. E-mail:
[email protected]. † Beijing Institute of Technology. ‡ Hong Kong Polytechnic University.
Herein, we report the successful fabrication of well-aligned GaN nanocolumns on silicon substrate via simple and low-cost chemical vapor deposition without any catalyst. High-resolution transmission electron microscopy and X-ray diffractometer analysis indicated the GaN nanocolumns have a preferential growth direction along the c-axis direction. The as-synthesized aligned GaN nanocolumns exhibited an excellent field emission property with a lower turn-on field of 2.6 V/µm (0.01 mA/cm2) and high stability at room temperature, which is sufficient for application of field emission displays and vacuum nanoelectronic devices. The room-temperature photoluminescence emission with a strong peak at 369 nm indicates that the well-aligned GaN nanocolumns have potential application in light-emitting nanodevices. It is convenient to integrate the aligned GaN nanocolumns into the silicon technologies.
2. Experimental Details The aligned GaN nanocolumns were synthesized in a furnace with a horizontal quartz tube (30 mm in outer diameter). A p-type silicon (111) wafer was used as the substrate to collect the products. Before the substrate was loaded into the furnace, it was etched by HF acid (5%) for removal of the native oxide layer, ultrasonically cleaned in ethanol for 5 min, and then rinsed with distilled water. Anhydrous GaCl3 and NH3 were used as gallium and nitrogen sources, respectively. In a typical process, the substrate was placed in the center of the quartz tube, GaCl3 powders were put into a ceramic boat, and then the boat was located at the upstream zone, where the temperature was about 150 °C. After purging 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 200 and 100 sccm (standard cubic centimeters per minute), respectively. The morphologies and structures of the products were characterized by XRD (Philips X’pert Pro diffractometer), SEM (Hitachi S-4800), and 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 rod-like stainless steel probe (1 mm in diameter) of 0.78 mm2 in area was used as an anode. The sample was used as the cathode. The spacing between these two electrodes is 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. The photoluminescence spectrum was collected at room temperature with a Hitachi FL-4500 spectrofluorometer.
10.1021/cg800321x CCC: $40.75 2009 American Chemical Society Published on Web 12/10/2008
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Figure 1. XRD pattern taken from the as-synthesized GaN nanocolumn arrays grown at 800 °C.
Figure 3. (a) TEM image of a GaN nanocolumn. (b) Corresponding SAED pattern taken along the [010] zone axis. (c and d) HRTEM image of the GaN nanocolumn.
Figure 2. Different-magnification SEM images of the aligned GaN nanocolumns grown at 800 °C.
3. Results and Discussion The aligned GaN nanocolumns were synthesized using a chemical vapor deposition process. After reaction, the silicon (111) substrate was covered with a layer of light yellow product. Figure 1 shows the X-ray diffraction (XRD) pattern of aligned GaN nanocolumns obtained at 800 °C for 30 min. The XRD pattern has only a strong peak at 35.9°, which can be indexed as the (002) reflection of hexagonal phase GaN, indicating that the GaN nanocolumns grew preferentially along the c-axis direction. The morphology of the products was investigated by SEM. From Figure 2 it can be seen that large-scale, vertically, or slantingly aligned 1D GaN nanocolumns were uniformly grown in high density on the silicon substrate. The diameters of the 1D GaN nanocolumns are about 50-150 nm. Most of the GaN nanocolumns are of hexagonal cross-section. The chemical composition of the products was investigated by energydispersive spectroscopy (EDS), which reveals the presence of Ga and N within the product with an approximate atomic ratio of 1.00:0.99, as shown in the inset in Figure 2b. Further structural characterization of aligned 1D GaN nanocolumns was performed by TEM and HRTEM. Figure 3a shows a TEM image of the GaN nanocolumn. It can be seen from Figure 3a that there is a layer of oxide coating the surface. Also, the dark lines along the radial direction result from defects such as stacking fault. Basically, the diameters of the GaN nanocolumns
Figure 4. (a and b) Different-magnification top-view SEM images of the aligned GaN nanostructures grown at 750 °C. (c) Tilted-view SEM image. (d) Side-view SEM image.
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 GaN nanostructures. The SAED pattern confirmed the single-crystal nature of the GaN nanocolumn grown along the 〈001〉 direction, as shown in Figure 3b. The lattice constants (a ) 0.318 nm, c ) 0.512 nm) calculated from the SAED pattern are in good agreement with the reported values of hexagonal phase GaN (a ) 0.3190 nm, c ) 0.5189 nm, JCPDS card No. 76-0703). The HRTEM image shown in Figure 3c gives a lattice fringe of about 0.51 nm, corresponding to the d001 space of the bulk hexagonal GaN. Both the SAED pattern and the HRTEM image confirm that the GaN nanocolumns grew along the c axis as derived from the XRD pattern. Further, the effect of growth temperature on 1D GaN nanostructures was investigated while keeping other experimental conditions the same. When the growth temperature was
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Figure 5. (a and b) Top-view SEM images of the aligned GaN nanocolumns grown at 850 and 900 °C, respectively.
Figure 7. Field emission J-E curve of the aligned GaN nanostructures gorwn at (a) 600 and (b) 800 °C. (Inset) Corresponding FN plot.
Figure 6. Idealized stacking of {GaN4} tetrahedra along the [001] direction.
decreased to 750 °C, the morphology of 1D GaN nanostructures varied from column-like to needle-like, as shown in Figure 4. These needle-like GaN nanostructures have a needle shape at their top, which is unlike with GaN nanocolumns obtained at 800 °C with a smooth shape at their top. The morphologies of the products grown at 850 and 900 °C are shown in Figure 5a and 5b, which is similar to that grown at 800 °C. Additionally, it can be seen from Figure 5 that the size of GaN nanocolumns increases with increasing growth temperature, which is similar to the situation of AlN nanostructures.20 Formation of these GaN nanocolumns could be attributed to the vapor-solid growth process because no metal catalyst was used and no alloy droplets on their tips was found. These GaN nanocolumns grown preferentially along the [001] direction have been confirmed by XRD, SAED, and HRTEM results, which is consistent with their crystallographic characteristics. In the wurtzite-type GaN structure each Ga3+ ion is surrounded by four N3- ions and vice versa. Thus, a hexagonal phase GaN crystal can be regarded as stacked {GaN4} tetrahedra that share
their corners. Growth of a given GaN crystal is governed by the stacking of growth units {GaN4} tetrahedral on various crystal faces, and the stacking in a particular direction is strongly dependent on the bonding force of atoms within the tetrahedra at the interface. In the GaN crystal structure, as shown in Figure 6, the terminal vertex of a corner of the coordination tetrahedron can bond with three growth units; the terminal vertex of the edge of the coordination tetrahedron can bond with two growth units; a terminal vertex of the face of the coordination tetrahedron can only bond with one growth unit. This shows that the terminal vertex of the corner of the coordination of the tetrahedron has the strongest bonding force; the terminal vertex of the edge of the coordination tetrahedron has the second strongest binding force; the terminal vertex of the face of the coordination tetrahedron has the smallest binding force. Thus, the crystal face with the corner of the coordination tetrahedron present at the interface has the biggest growth rate. From Figure 6 one can see that each coordination tetrahedron has a corner in an [001] orientation, which thus favors growth of GaN nanocolumns along the [001] direction. Additionally, with increasing growth temperature the surface roughness at the lateral face will increase, giving rise to the increase in the growth rate of the radial direction. Therefore, the sizes of the GaN nanocolumns at their top increase as the growth temperature increases, similar to the situation of AlN nanostructures.20 A possible formation process of the aligned GaN nanocolumns can be described as follows: heterogeneous nuclei are formed first on the Si(111) substrate and then epitaxial growth of nanocolumns takes place on these nuclei combined with
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Figure 8. Schematic band diagram of the p-Si-GaN heterojunction. ECP, EVP, EFP, ECN, EVN, and EFN are the conduction band, valence band, and Fermi level of p-Si and GaN, respectively.
Figure 9. Field emission current stability of the aligned GaN nanocolumn.
of 0.01 and 1 mA/cm2, respectively.21 The turn-on fields of 2.5 V/µm and threshold fields of 4.7 V/µm were obtained for the aligned GaN nanoneedles grown at 600 °C. The turn-on fields of 2.6 V/µm and threshold fields of 5.1 V/µm were obtained for the aligned GaN nanocolumns grown at 800 °C. Since the aligned GaN nanoneedles grown at 600 °C have smaller tips compared to that grown at 800 °C, it is reasonable that the former show smaller turn-on and threshold fields than the latter. The present reported turn-on field of 2.5-2.6 V/µm is lower than that of 5.1 V/µm for P-doping GaN nanowires:22 7.5 V/µm for needlelike bicrystalline GaN nanowires,23 8.5 V/µm for thin GaN nanowires,24 and 12 V/µm for GaN nanowires.25 Even the present field emission properties of the GaN nanocolumn arrays are comparable with that of carbon nanotubes.26 The excellent field emission properties of the aligned GaN nanocolumn arrays have mainly been attributed to the following two factors: One is high alignment of GaN nanocolumns; the other is easier electron flow across the p-type Si-GaN interface under an applied field. The electron affinity of wide band-gap GaN (3.4 eV) is 2.8 eV,27 and that of low band-gap Si (1.1 eV) is 4.01 eV.28 Thus, a straddled heterojunction was formed at the Si-GaN junction. After thermal equilibration is established, the band bending will occur at the junction as shown in Figure 8, which results in a “well” instead of a barrier, allowing easier electron flow across this junction under an applied field. The inset of Figure 7 displays the corresponding Fowler-Nordheim (F-N) plot. The linearity of the F-N curve indicates that electron emission for the aligned GaN nanocolumns results from a quantum mechanical tunneling process. According to FN theory29 the field enhancement factor β can be calculated as follows
β ) -6.83 × 109φ3⁄2 ⁄ S
Figure 10. Room-temperature PL emission spectrum of the well-aligned GaN nanocolumns grown at 800 °C.
preferential growth along the [001] direction; so, finally the aligned GaN nanocolumns were formed. Field emission measurements of the aligned GaN nanostructures grown at 600 and 800 °C were conducted 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. The aligned GaN nanostructures were used as the cathode. The spacing between these two electrodes was 100 µm in our experiment. The curve of emission current density versus applied field (J-E) is depicted in Figure 7. By definition, the turn-on field and threshold field are the electronic fields required to generate an emission current density
(1)
where S is the slope of the ln(I/V2) - 1/V plot, d is the spacing between two electrodes, and φ is the work function, which is estimated to be 4.1 eV for GaN.24 The field enhancement factors, β, have been calculated to be about 3959 and 9725 for the aligned GaN nanostructures grown at 600 and 800 °C. The FE stability of the aligned GaN nanocolumns was investigated further at a fixed applied electric field (2.5 V/µm). Figure 9 shows the result of emission current density versus time for a period 60 min under a pressure of 1.2 × 10-6 Pa. The average current density and standard deviation are calculated to be 2.96 and 0.22 µA/cm2, respectively. The ratio of them is as small as 7.4%, which proves the high stability of the aligned GaN nanocolumn arrays. The aligned GaN nanocolumns show a lower turn-on field and high stability and can be considered as a promising candidate for electron emission devices. The room-temperature photoluminescence emission spectrum of the well-aligned GaN nanocolumns grown at 800 °C was obtained using a Xe lamp with excitation at 325 nm as shown in Figure 10. A strong emission peak is observed at 369 nm
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(3.36 eV) at room temperature, which can be attributed to bandedge-related emission. This peak is red shifted, by ∼70 meV, with respect the band edge of hexagonal GaN. The slight red shift can be attributed to strain in GaN nanocolumns.30 A wellknown yellow luminescence (YL) band is often observed in GaN.31 Three weak peaks are observed at 451 (2.75 eV), 468 (2.65 eV), and 524 nm (2.37 eV). Those emission peaks are probably associated with the deep level or defect levels. More detailed work is needed to clarify its origin. The weak defectrelated signal implies that the GaN nanocolumns contain few defects and high quality. The intensive PL emission indicates that the well-aligned GaN nanocolumns have potential application in light-emitting nanodevices.
4. Conclusion The well-aligned GaN nanocolumns on silicon substrate have been fabricated by simple and low-cost chemical vapor deposition without any catalyst. The results obtained by XRD, SAED, and HRTEM indicated that the GaN nanocolumns grew along the [001] direction. The aligned GaN nanocolumn arrays exhibited an excellent field emission property with a lower turnon field of 2.6 V/µm (0.01 mA/cm2) at room temperature. The fluctuation of the FE current is as small as 7.4% for 60 min. This means that the aligned GaN nanocolumn arrays have potential applications in field emission displays and vacuum nanoelectronic devices. The room-temperature PL emission with a strong peak at 369 nm indicates that the well-aligned GaN nanocolumns have potential application in light-emitting nanodevices.
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