Microplasma illumination enhancement of vertically aligned conducting ultrananocrystalline diamond nanorods
© Sankaran et al.; licensee Springer. 2012
Received: 18 July 2012
Accepted: 2 September 2012
Published: 25 September 2012
Vertically aligned conducting ultrananocrystalline diamond (UNCD) nanorods are fabricated using the reactive ion etching method incorporated with nanodiamond particles as mask. High electrical conductivity of 275 Ω·cm−1 is obtained for UNCD nanorods. The microplasma cavities using UNCD nanorods as cathode show enhanced plasma illumination characteristics of low threshold field of 0.21 V/μm with plasma current density of 7.06 mA/cm2 at an applied field of 0.35 V/μm. Such superior electrical properties of UNCD nanorods with high aspect ratio potentially make a significant impact on the diamond-based microplasma display technology.
Microplasma science and technology is an intersection of plasma science, photonics, and materials science, which offers not only a realm of plasma phenomenology but also device functionality [1–4]. Such plasma-based devices exhibit great potential for a broad spectrum of applications in microdisplays, on-chip frequency standards, materials synthesis, elemental analysis, and detectors of environmentally hazardous or toxic gases or vapors [5–11] But due to the insufficient luminous efficiency of the plasma devices , development of a cathode material with efficient emission of secondary electrons for improving the initiation efficiency of plasma illumination is thus called for. Among carbon-based materials, diamond is a promising material for applications in various electronic and microelectromechanical devices due to its unparalleled intrinsic properties such as wide energy band gap, chemical inertness, extreme hardness, high thermal conductivity, and negative electron affinity [13–16]. Moreover, diamond materials have large secondary electron emission efficiency which is especially adept for serving as cathode materials in microplasma devices . Recent reports of n-type conductivity  and high electron field emission (EFE) characteristics in N2-incorporated ultrananocrystalline diamond (UNCD) films exhibited the potential of such films for cold cathode emitters [19, 20]. These materials are expected to be beneficial for maintaining the plasma excitation when serving as cathode materials for the plasma devices.
It is not the intrinsic properties alone but the surface geometry also which serves in defining the properties for potential applications of materials. In spite of retaining the same chemical composition, nanostructured materials exhibit pronounced variations in the properties in comparison with their bulk and film forms. For instance, it has been demonstrated that diamond coatings on silicon nanostructures significantly reduce the turn-on field (E0) of EFE [21, 22]. Processing of materials to a desired geometry will depend entirely on the intrinsic properties such as hardness, chemical and/or mechanical stability, etc., of the materials. Nanostructures of extremely hard and chemically inert materials such as diamond and other wide-bandgap materials (GaN, Si) have been obtained by the top-down methods including reactive ion etching (RIE) process with or without mask focus ion beam milling and bottom-up approaches [23–33]. Owing to practical applications, it is still of great interest to formulate a low-cost, flexible, and relevant method to fabricate diamond nanostructures with high areal density and high uniformity in a desired geometry.
In this letter, we report the fabrication of vertically aligned UNCD nanorods from n-type UNCD films by RIE using nanodiamond (ND) particles as a hard etching mask. We observed that the plasma illumination characteristics of a microplasma cavity were markedly enhanced when the UNCD nanorods were used as the cathode materials, as compared with those using the as-grown UNCD films as cathode. The detailed mechanism of the improvement of the plasma illumination characteristics of the nanorods is investigated.
UNCD films were grown on Si substrates in a microwave plasma-enhanced chemical vapor deposition system (2.45 GHz 6″ IPLAS-CYRANNUS, Troisdorf, Germany). Prior to deposition, the substrates were ultrasonicated in methanol solution containing the mixture of ND powders (approximately 5 nm) and titanium powders (approximately 325 nm) for 45 min to facilitate the nucleation. The UNCD films were deposited on substrates using N2 (94%)/CH4 (6%) plasma with a microwave power of 1,200 W for 1 h. The pressure and the flow rate were maintained at 50 Torr and 100 sccm, respectively. An external heater was used to heat the substrate to a temperature of about 700°C, where the substrate temperature is measured using a thermocouple (K type) embedded in the substrate holder. The obtained UNCD films were designated as N2-UNCD films. The N2-UNCD films were then immersed in a pseudo-stable suspension (ND particles (8 to 10 nm in diameter) and deionized water) and sonicated for 10 min to seed ND particles on the N2-UNCD films surface. The ND particle layer on the N2-UNCD films is dense, which depends on the suspension quality and time of sonication. After masking, the N2-UNCD films were then etched using the RIE process in an O2 (80%)/CF4 (20%) gas mixture at rf power of 150 W for 30 min. In the process, ND particles acted as etching mask for fabricating vertically aligned N2-UNCD nanorods.
The morphologies and microstructures of the samples were examined using field emission scanning electron microscopy (FESEM; JEOL-6500, JEOL Ltd., Tokyo, Japan) and transmission electron microscopy (TEM; JEOL 2100; operated under 200 eV), respectively. The visible Raman (λ = 632.8 nm; Lab Raman HR800; Jobin Yvon, Inc., NJ, USA) spectroscopic measurements were performed at room temperature. Hall measurements were carried out in a van der Pauw configuration (ECOPIA HMS 3000, Bridge Technology, USA) to confirm n-type conductivity of the films. EFE characteristics of the samples were measured using a molybdenum rod with a diameter of 2 mm as anode, and I V characteristics were acquired using Keithley 237 electrometer (Keithley Instruments, Inc., OH, USA). The EFE behavior of the materials was explained using Fowler-Nordheim (F-N) theory . The plasma illumination characteristics of a microcavity, in which an indium tin oxide (ITO)-coated glass was used as anode and the N2-UNCD nanorods were used as cathode, were also investigated. The cathode-to-anode separation was fixed by a Teflon spacer (1.0 mm in thickness). A circular hole about 8.0 mm in diameter was cut out from the Teflon spacer to form a microcavity. The plasma was triggered using a pulsed direct current voltage in bipolar pulse mode in Ar environment at a pressure of 100 Torr.
Results and discussion
FESEM image of the N2-UNCD films shows highly dense and uniformly distributed needle-like granular structures in the films (not shown). The root-mean square roughness of the surface is about 7 to 10 nm, and the thickness of the films is about 1 μm. The Hall measurements conducted in the van der Pauw configuration showing the electrical conductivity of the N2-UNCD films are found to be 186 Ω·cm−1. Vertically aligned N2-UNCD nanorods are fabricated by subjecting the N2-UNCD films to the RIE process.
It should be noted that the electric field required to trigger the Ar plasma is much smaller than the E0 for inducing the EFE process for both the N2-UNCD nanorods and N2-UNCD films. The primary reason for such a phenomenon is that the Ar plasma can be triggered whenever the electrons emitted from the cathodes reach a kinetic energy larger than the ionization energy of the Ar species (14.7 eV). Superior EFE properties provide the low ignition threshold for the microplasma easily. After the initiation of the Ar plasma, the cathode materials mainly serve as the source of secondary electrons for maintaining the ignition of the plasma. Better EFE properties of the N2-UNCD nanorods no longer show significant superiority in maintaining the plasma in the microcavity.
In summary, ND particles dispersed on smooth and highly conducting N2-UNCD films can be utilized as an etching mask for the fabrication of vertically aligned N2-UNCD nanorods. These N2-UNCD nanorods show superb plasma illumination characteristics of low threshold field = 0.21 V/μm with high current density of 7.06 mA/cm2 at an applied field of 0.35 V/μm. The excellent performance of the N2-UNCD nanorods as cathode for the microplasma devices is mainly attributed to the unique granular structure of nanorods and a high proportion of graphitic phase surrounding each nanorod. The utilization of N2-UNCD nanorods enhances the illumination performance of the microplasma devices that can be applied to a broad spectrum of applications in microplasma display technologies.
KJS is a Ph.D. student of the Department of Materials Science and Engineering, National Tsing-Hua University, Hsinchu, Taiwan. SK is a Ph.D. student of Department of Engineering and System Science of the same university. SCL is a Ph.D. student of the Department of Photonics Engineering, Yuan Ze University, Chung-Li, Taiwan. JK is a post doctoral fellow in the Department of Physics, Tamkang University, Tamsui, Taiwan. HCC is a post doctoral fellow in the Department of Materials Science and Engineering, National Tsing-Hua University, Hsinchu, Taiwan. CYL and NHT are professors in the Department of Materials Science and Engineering of the same university. KCL is a professor in the Department of Engineering and System Science of the same university. CC is a professor in the Department of Photonics Engineering, Yuan Ze University, Chung-Li, Taiwan. INL is a professor in the Department of Physics, Tamkang University, Tamsui, Taiwan.
The authors would like to thank the National Science Council, Republic of China, for the support of this research through the project numbers NSC99-2119-M-032-003-MY2 and NSC98-2221-E-007-045-MY3.
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