Patterned growth of carbon nanotubes over vertically aligned silicon nanowire bundles for achieving uniform field emission
© Hung et al.; licensee Springer. 2014
Received: 19 June 2014
Accepted: 10 September 2014
Published: 1 October 2014
A fabrication strategy is proposed to enable precise coverage of as-grown carbon nanotube (CNT) mats atop vertically aligned silicon nanowire (VA-SiNW) bundles in order to realize a uniform bundle array of CNT-SiNW heterojunctions over a large sample area. No obvious electrical degradation of as-fabricated SiNWs is observed according to the measured current-voltage characteristic of a two-terminal single-nanowire device. Bundle arrangement of CNT-SiNW heterojunctions is optimized to relax the electrostatic screening effect and to maximize the field enhancement factor. As a result, superior field emission performance and relatively stable emission current over 12 h is obtained. A bright and uniform fluorescent radiation is observed from CNT-SiNW-based field emitters regardless of its bundle periodicity, verifying the existence of high-density and efficient field emitters on the proposed CNT-SiNW bundle arrays.
One-dimensional nanomaterials have attracted a tremendous amount of attention in recent years due to their interesting electronic and optical properties and a diverse array of potential for nanoscale device applications [1, 2]. Among the promising nanoelectronic applications, field emission and related devices have become the subject of intense fundamental and technological investigations because of their great potential for industrial applications, in particular as flat panel displays and electron guns [3–10]. Reports had shown that carbon nanotube (CNT)-based field emitter exhibits excellent field emission properties due to its superior thermal and electrical characteristic . However, the wire number density of as-grown CNTs is usually as high as 1010 CNTs/cm2. Such a dense CNT structure would suffer from the so-called electrostatic screening effect provoked by the proximity of neighboring wires which results in limited field emission performance. Alternatively, a pillar array of aligned CNT bundles had shown better field emission performance due to the release of the electrostatic screening effect . However, it is difficult to maintain the verticality of CNT bundles as the bundle periodicity becomes submicron scale to maximize the number of effective field emitters. Therefore, instead of using CNTs as both field emitter material and the supporting structure to obtain certain aspect ratio in conventional CNT bundle arrays, this work proposes to use vertically aligned silicon nanowires (VA-SiNWs) as the supporting template structures, while CNTs only serve as the emitter material. Recent results also focused on the synthesis of hybrid CNT-SiNW heterojunction array in order to take the advantages from both material systems for breakthrough findings in not only high performance electron field emitters but also many other emerging technologies [14–16]. We had previously demonstrated that VA-SiNWs could be realized with top-down wet chemical etching using a thin silver film as the catalyst . We also developed several fabrication processes to realize an array of VA-SiNW bundles using thin chemical oxide or thick photoresist (PR) as the mask to define the arrayed structure [18, 19]. In this work, mesa-type arrays of VA-SiNW bundles are realized to serve as an arrayed template for the growth of coarse CNT mats atop their surfaces. The proposed hybrid device leverages from the metallic-like property of CNTs and mature and flexible processing technologies available in silicon. The bundle arrangement of SiNW templates is optimized in order to release the electrostatic screening effect and to maximize the field enhancement factor. The material and electrical characterization of as-realized CNT-SiNW field emitter array as well as its emission uniformity and stability tests are conducted and discussed.
Synthesis of CNT-SiNW heterojunction bundle array
Material and electrical characterization
All materials are characterized by using a field emission scanning electron microscope (SEM), an atomic force microscope (AFM), a Raman spectrometer, and a transmission electron microscope (TEM). The field emission characteristics and long-term stability tests of all samples are measured in a stainless chamber in which the samples serve as the cathode electrode. The current-voltage characteristic is determined by manipulating an electrometer under a chamber pressure of around 1.33 × 10−5 Pa. Accordingly, the current density to electric field characteristic of the sample is derived from the measured current-voltage curve, the inter-electrode distance (150 μm), and the sample area (10 × 10 mm2). The field emission performance is analyzed according to the Fowler-Nordheim (F-N) equation: J = A(β2E2/Φ)exp(−BΦ3/2/βE), where J is the field emission current density, β is the overall field enhancement factor, E is the applied electric field (V/μm), A (1.54 × 10−6 AeV/V2) and B (6.8 × 103 eV−3/2 V/μm) are constants, and Φ is the work function (Φ is 4.8 and 4.15 eV for CNTs and SiNWs, respectively). The field enhancement factor is determined by the slope of the F-N plot.
For the electrical resistivity characterization of single SiNW, individual SiNWs are dispersed on the insulating Si3N4/n-Si template with pre-patterned Ti/Au microelectrodes. Electrical contacts of the two-terminal single-nanowire devices are fabricated by focused ion beam deposition using platinum as the metal electrode. Electrical measurements are carried out on an ultralow-current leakage cryogenic probe station. A semiconductor characterization system is utilized to source dc bias and measure current.
Results and discussion
The realization of field emitters based on arrayed CNT-SiNW heterojunctions would benefit from the superior thermal and electrical properties of CNTs while taking the advantages of flexible patterning technologies available for SiNWs. The proposed field emitter is capable of generating >5 mA/cm2 emission current density with a turn-on electric field of 0.9 V/μm. With the proposed fabrication strategy, it is possible to further scale down the periodicity of the CNT-SiNW bundle array while maintaining the verticality of the supporting structure. To investigate the electrical conductivity of as-realized SiNWs, a two-terminal single-nanowire device is fabricated and characterized. The results indicate similar electrical resistivity between individual SiNW and original bulk silicon substrate, meaning that no obvious electrical degradation after SiNW fabrication is observed. Therefore, further reducing the series resistance of the field emitters could be achieved by simply using low-resistivity silicon wafer as the starting substrate. Raman spectra of CNT-SiNW bundle arrays also reveal direct interactions between CNTs and SiNWs. The bundle array of CNT-SiNW heterojunctions exhibits relatively stable emission performance with a current fluctuation of about 10% over a 12-h period, as compared to planar CNT-SiNW counterparts. Finally, a bright and uniform radiation from the fluorescent screens of CNT-SiNW bundle arrays is observed regardless of its bundle periodicity. The results indicate the existence of high-density and efficient field emitters on the proposed CNT-SiNW bundle arrays.
Y-JrH is the assistant professor at the Department of Photonics, National Sun Yat-sen University, Kaohsiung, Taiwan. His research interests include the realization of silicon-based nanostructures for applications in various emerging fields. Y-JH and H-CC are graduate students at the Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan, under the supervision of professor Kuei-Yi Lee. Their researches focus on carbon nanotube- and graphene-based materials and devices. S-LL is the distinguished professor at the Department of Electronic Engineering, National Taiwan University of Science and Technology. His research interests include semiconductor optoelectronic devices and optical networking.
This work was supported in part by the National Science Council, Taiwan, under grant NSC 102-2218-E-110-010 and NSC 102-2218-E-011-002.
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