An indium tin oxide (ITO)-coated vertically aligned nanowire array is fabricated, and the field emission characteristics of the nanowire array are investigated. An array of vertically aligned nanowires is considered an ideal structure for a field emitter because of its parallel orientation to the applied electric field. In this letter, a vertically aligned nanowire array is fabricated by modified conventional UV lithography and coated with 0.1-μm-thick ITO. The turn-on electric field intensity is about 2.0 V/μm, and the field enhancement factor, β, is approximately 3,078 when the gap for field emission is 0.6 μm, as measured with a nanomanipulator in a scanning electron microscope.
Keywords:Field emission; ITO; Nanowire; Top–down
Field emission is quantum mechanical tunneling of electrons through the surface potential barrier into vacuum , which has been widely exploited in vacuum electronic applications including electron guns, microwave tubes, and flat panel displays . Vertically aligned nanowire array (VANA) shows excellent field emission properties owing to a strong local electric field due to their parallel orientation to the applied electric field . Numerous studies have been carried out on the fabrication of VANAs as field emitters using bottom-up synthesis approaches [4-6]. However, these bottom-up methods have drawbacks such as an expensive process and relatively low fabrication reliability, thus making them unsuitable for mass production.
In this letter, a top–down method of modified conventional UV lithography is suggested for the fabrication of a VANA, thereby resolving the problems of bottom-up methods while allowing control over the VANA’s position and shape. In the lithography process, a photoresist VANA is fabricated with a UV exposure dose control. After the lithography, a carbonization process (pyrolysis process) is followed to provide volume contraction of the array’s structure. The pyrolyzed carbon VANA is coated with a 0.1-μm-thick ITO layer in order to realize a durable field emitter and to provide low turn-on voltage [7,8]. ITO has good transparence characteristics in the visible region of the electromagnetic spectrum while maintaining high electrical conductivity, thermal stability, and oxidation resistance [4,9]. In a field emission experiment, a nanomanipulator in a scanning electron microscope (SEM) is used in order to measure field emission while precisely controlling the distance between the anode and cathode (field emitter).
The fabrication process of the ITO-coated VANA is shown in Fig. 1. Figure 1a shows circular aperture patterns (φ = 1 μm) of a 0.1-μm-thick chrome (Cr) layer on a fused silica wafer. SU-8 50 (Microchem Co.) is deposited on the Cr layer in Fig. 1b. The backside of the fused silica wafer is exposed to UV light filtered by a narrow band-pass filter (λ = 365 nm and bandwidth = 10 nm, OptoSigma Co.) with an exposure dose of 200 mJ/cm2 in Fig. 1c. The intensity of UV light is concentrated to the central axis by diffraction in the SU-8 medium, which defines a sharp, high aspect ratio SU-8 structure . After development of SU-8 and Cr etching, the SU-8 VANA is obtained, as shown in Fig. 1d. Figure 1e shows the pyrolyzed carbon VANA obtained by a pyrolysis process in a quartz tube furnace at 900°C for 30 min . Volume shrinkage to approximately 60% occurs in this process for the SU-8 VANA. Finally, Fig. 1f shows the VANA coated with a 0.1-μm-thick ITO layer. The size of VANA is controlled by the circular aperture patterns of a Cr layer on a fused silica wafer and the aspect ratio by the UV light exposure dose.
Figure 1. Schematic view of fabrication process of ITO-coated VANA. a Circular aperture patterns (φ = 1 μm) of Cr layer on a fused silica wafer. b SU-8 50 on Cr layer. c Backside UV exposure with exposure dose of 200 mJ/cm2 filtered by a narrow band-pass filter. d SU-8 VANA after development of SU-8 and Cr etching. e Pyrolyzed carbon VANA by pyrolysis process at 900°C for 30 min. f ITO-coated VANA with 0.1-μm-thick ITO
Figures 2a–2c are the fabrication results of Fig. 1d–1f, r