Electron Emission from Nano-Structured Diamond
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e important for field emission displays and other vacuum microelectronic devices, as they will allow simpler, cheaper and more efficient designs to be made. EXPERIMENTS We worked with commercially available diamond particles in the size range of 10-100 nm which are traditionally used for polishing hard materials. They are typically produced by highpressure high-temperature processes and are available as dispersed aqueous suspensions. X-ray diffraction patterns and Raman spectra show broadened peaks indicating that the particles are indeed extremely small diamond crystallites. The structure of these nanometer-size crystallites are typically defective because structural evolution is limited by the size.' 8 The dispersed diamond was attached onto an n-type Si substrate surface (p - 1 Q.cm) as a thin, uniform layer by a simple spraying or brushing technique. The diamond layer was dried at ambient temperature and subsequently subjected to a hydrogen plasma heat treatment at a temperature of 650 0C and a pressure of 20 Torr for an hour. Neither the as-deposited layer nor the structure heat-treated in high-purity hydrogen or argon gas atmosphere gave any measurable emission at a reasonable field. Only those samples which underwent the hydrogen plasma heat treatment were "activated" and produced the desirable low-field electron emission. Figure 1 is an SEM micrograph showing the processed diamond emitter layer consisting of nanometer-size particles. The thickness of such a layer is typically around 500 nm. The nanocrystalline nature and blocky, irregular configuration of the diamond particles are evident in both the SEM micrograph and a TEM micrograph shown in Fig. 2(a). The electron diffraction pattern in Fig. 2(b) indicates that the nanometer-size particles possess a crystalline diamond structure. The field emission measurement was carried out in a vacuum chamber with a 10-' Torr base pressure at room temperature. As described previously,9"° a voltage up to 2 kV was applied to a spherical-tipped molybdenum anode probe (tip radius of curvature =1 mm) to collect electrons emitted from the cathode diamond surface. A precision step controller (3.3 [m step size) was used to control the movement of the probe toward the cathode, and the emission current-voltage (I-V) characteristics were measured as a function of the anode-cathode distance. The obtained IV data were analyzed using the Fowler-Nordheim theory' 9 , taking into consideration the variation of electrical field across the cathode surface and assuming a broad distribution in emission properties among the emitters.20 The appropriately fitted parameters were subsequently used to interpolate / extrapolate the I-V characteristics of the emitters to a standard display pixel area (100%tm x 100pm). The threshold field required to produce an emission current density of 10 mA/cm2 (that is, 1 jA over the pixel area, as is typically required for display applications) was subsequently calculated and used as a figure of merit to evaluate and compare various emitter samples. It should
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