Diamond and Related Materials 9 (2000) 358–363 www.elsevier.com/locate/diamond Nitrogen-doped diamond films selected-area deposition by the plasma-enhanced chemical vapor deposition process Kuoguang Perng a, Kuo-Shung Liu a, I-Nan Lin b, * a Department of Materials Science and Engineering, National Tsing-Hua University, Hsin-Chu, Taiwan 300, R.O.C. b Materials Science Center, National Tsing-Hua University, Hsin-Chu, Taiwan 300, R.O.C. Abstract The nitrogen-doped diamond films have been successfully synthesized by using urea as the nitrogen source. Selected-area deposition of diamond nuclei was formed by using a SiO layer as the masking material. Diamond pads, around 9 mm in diameter, 2 were obtained when the N-doped diamond films were deposited on these patterned diamond nuclei using the chemical vapor deposition process. An emission current density as high as 200 mA/cm2, with a turn-on field of around 8 V/mm, was obtained. However, the diamond emitters broke down easily, which is ascribed to the localized melting of the substrate materials surrounding the diamond pads. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Chemical vapor deposition; Electron field emission; Nitrogen-doped diamonds; Selected-area deposition 1. Introduction Field emission devices ( FED) with a high emission current density have been obtained in metal tip- [1] and silicon tip arrays [2]. They have a great potential for applications as electron emitters in flat panel displays and have attracted thorough investigations. Diamond films have negative electron affinity (NEA) characteristics [3]. They are considered to be highly promising for applications in electron field emission devices, and the related emission properties have been widely investigated [4–12]. Moreover, the inclusion of aliovalent dopants such as nitrogen, phosphor or boron to convert diamonds into semiconducting materials has been found to enhance the field emission characteristics of these films considerably [13–18]. Patterning of diamond films is a technique urgently needed for devices such as electron emitters. However, etching of diamond films is difficult due to their chemical inertness. Therefore, selected-area deposition (SAD) of diamond films, a self-aligned process, is an indispensable alternative. SAD of diamond films is commonly achieved by either enhancing the nucleation rate on selected regions or suppressing the formation rate on the others * Corresponding author. Fax: +886-3-5716977. E-mail address: inlin@mx.nthu.edu.tw (I-Nan Lin) [19–23]. Selective suppression on the formation of diamond nuclei has been obtained by precoating the selected regions with low-nucleating materials such as amorphous silicon oxide layers or nitride layers. Selective enhancement on the rate of diamond nucleation, however, has been successfully achieved by bombarding the selected area using hard materials, or by ion implantation. The ability to grow the diamond films selectively using silicon oxide as a masking material is of practical importance in the development of novel devices using diamond films since SiO layers are the 2 most commonly used masking materials in mechanoelectrical microsystem (MEMS). Therefore, in this study, SiO materials were chosen 2 as masking materials for selective deposition of nitrogendoped diamond films on silicon substrates using the microwave-enhanced CVD method. The electron field emission properties of the SAD diamond films thus obtained were examined, and the possible mechanism was discussed. 2. Experimental Diamond films were grown on silicon substrates by a microwave plasma-enhanced chemical vapor deposition (MPECVD) method, using an ASTeX 5400 system. The substrates, (100) p-type silicon with the resistivity 0925-9635/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S0 9 25 - 9 63 5 ( 9 9 ) 00 2 9 7- 6 K. Perng et al. / Diamond and Related Materials 9 (2000) 358–363 of 10 V-cm, were cleaned by acetone and deionized water prior to deposition. The diamond grains were directly nucleated on a mirror-smooth silicon surface using a −170 V bias voltage for 12 min and then deposited with a bias voltage applied in situ. In addition to the CH and H (CH /H =18/300 sccm) gases used, 4 2 4 2 nitrogen species were incorporated into diamond films by directly vaporizing urea maintained at 23°C (6 sccm). The total pressure and microwave power were controlled at 70 Torr and 2500 W, respectively, and the substrate temperature was maintained at around 900°C. A SiO 2 layer, around 500 nm in thickness, was first deposited on a silicon substrate using the atmospheric pressure chemical vapor deposition (APCVD) process, followed by the standard lithographic process to form 5 mm patterns of exposed Si surface. The patterned Si substrates were prenucleated under bias, followed by etching of the SiO layer to yield patterns of nuclei. Finally, 2 diamonds were selectively grown on these prenucleated patterns by the CVD process without bias. The morphology and structure of the diamond films were examined using a scanning electron microscope (SEM, Hitachi S-4000) and Raman spectroscope (He– Ne Laser, Renishaw), respectively. The electron field emission behavior of diamond films was characterized by a diode set-up, in which the as-deposited diamond films, without any preconditioning process, were separated from the anode (In Sn )O -coated glass, by 1−x x 2 using 50 mm glass fibers as spacers. The current–voltage characteristics were measured by using an electrometer ( Keithley 237). The electron field emission properties were analyzed using the Fowler–Nordheim model[24], viz. I=aV2 exp(−bW3/2/V ), where a and b are constants. e The turn-on voltage was estimated as the voltage at which the log(I/V2)−1/V curve deviates from the Fowler–Nordheim plot, and the effective work functions (W =W/b) of these films were calculated from the slope e of the Fowler–Nordheim plot, where b is the field enhancement factor, and W is the true work function of these materials. 3. Results and discussion The incorporation of urea into CH /H mixture 4 2 substantially modifies the nucleation rate of diamonds on the mirror surface of the silicon substrate. The bias current induced with a bias voltage of −170 V in the CVD chamber increases with nucleation time and reaches the saturated value at 9 min, when 0 sccm urea was introduced into the chamber (H /CH = 2 4 300/18 sccm). The saturated time interval (t ) increases s monotonously with the urea content in the gas mixture, indicating that the nucleation of diamonds is significantly retarded due to incorporation of nitrogen species 359 Fig. 1. (a) SEM microscopy and (b) Raman spectra of nitrogen-doped diamond films grown with CH /H /urea=18/300/0 sccm (n ) or 4 2 0 18/300/6 sccm (n ). 6 into diamond lattices. However, the SEM micrographs shown in Fig. 1a reveal that, when deposited for 1 h under 0 V bias voltage after completion of the nucleation period under a bias voltage of −170 V, these diamonds grow to a size of around 0.6 mm, which does not vary with the urea concentration in the gas mixture. The size distribution of these diamonds is quite uniform. The Raman spectra shown in Fig. 1b indicate that characteristics of diamond films vary insignificantly with nitrogen content. A sharp diamond characteristic peak, labeled as the D-band, occurs at 1332 cm−1, whereas a diffused graphite characteristic peak, labeled as the G-band, occurs at 1593 cm−1. In addition, a diffused peak (G∞-band) appears near the same location as the D-band 360 K. Perng et al. / Diamond and Related Materials 9 (2000) 358–363 Fig. 2. Electron field emission properties of nitrogen-doped diamond films grown with CH /H /urea=18/300/0 sccm (n ) or 18/300/6 sccm (n ); the 4 2 0 6 inset shows the corresponding Fowler–Nordheim plots. resonance peak, and another diffused resonance peak (D∞-band) is barely observed in the vicinity of 1160 cm−1. These diffused resonance peaks are presumably contributed from the amorphous carbon and nanosized diamond crystals. The SEM microscopic and Raman spectroscopic examination illustrates that the microstructure and crystal structure of the diamond films is insignificantly modified due to the incorporation of nitrogen. However, the electrical resistivity and the electron field emission properties of these diamond films vary markedly with the concentration of urea in plasma. As shown in Fig. 2, the undoped diamonds (n ) exhibit essentially no 0 electron field emission characteristics. The electron field emission capacity of the films improves dramatically due to the incorporation of nitrogen. The electron field emission current density (J ) reaches J =1020 mA/cm2 e e (at 21.6 V/cm) for nitrogen-doped samples (n ). The 6 resistivity of these diamond films, measured by the fourpoint probe technique, decreases from 0.28 V-cm to 0.076 V-cm due to nitrogen doping. The selected-area deposition of diamond nuclei on patterned silicon substrate is much more difficult than the formation of diamond nuclei directly on an unpatterned silicon substrate. An excessively high microwave power induced overheating of the substrate due to the insulating nature of the SiO coating, whereas insuffi2 cient microwave power would result in an insufficient proportion of carbon species to form diamond nuclei. A similar phenomenon occurred when the total pressure of the deposition chamber was changed. The diamond nuclei could be successfully formed only when the plasma was induced in a very narrow range, i.e. 2500 W microwave power and 70 Torr total pressure. Fig. 3a and the corresponding inset illustrate the morphology of the nuclei pattern after the SiO layer had been etched 2 away. Each of the nuclei pads, with an irregular shape and with a size of around 10 mm, contains numerous submicron-sized diamond grains (~0.5 mm). Fig. 3b indicates that, after CVD growth for 1 h, a regular array of these diamond pads, with a circular geometry and a size of about 9 mm, was obtained. The enlarged micrograph shown in inset of this figure reveals that these diamond pads consist of submicron-sized diamond grains, which are not significantly different from the size of the grains in the nuclei pads. Field emission characteristics of the diamond pattern thus obtained are shown in Fig. 4a, revealing that an emission current as large as 4 mA was achieved for an anode electrode area of about 0.1 cm2. It should be noted that these diamond pads occupy only about 20% of the substrate area. The electron field emission capacity of the diamond pads is thus calculated to be around 200 mA/cm2, which is slightly smaller than the emission capacity of planar N-doped diamond films shown in Fig. 2. The probable cause is the uneven distribution of the electron field emission current density, which will be discussed shortly. The Fowler–Nordheim plot of the J– E characteristics indicates that the field emission can be turned on at 10 V/mm with an effective work function around W =0.045 eV, which is comparable with that of e planar N-doped diamond films. Although the emitters made of N-doped diamond pads can provide a large electron density under a K. Perng et al. / Diamond and Related Materials 9 (2000) 358–363 361 Fig. 3. SEM micrographs of (a) as-nucleated diamond patterns and (b) CVD-grown diamond pads (CH /H /urea=18/300/6 sccm). 4 2 relatively low applied field, these emitters break down easily during long-term tests. Fig. 5a indicates the morphology of diamond pads after the occurrence of the breakdown phenomenon. Fig. 5 reveals that the breakdown occurs only along the periphery of the emitter diamond pads, whereas diamond grains inside the pads remain intact. The implication of these results is that the substrate material (Si) surrounding the emitting pads was overheated during continuous flow of field emission electrons. There are two probable factors, which will lead to a non-uniform distribution of field emission current den- sity, viz. the geometric effect and surface conduction process. A detailed examination of Fig. 5a reveals that grains at the periphery of diamond pads are slightly higher than those at the center, which results in a localized increase on the field enhancement factor and an uneven distribution in the electric field undergone by the top surface of the diamonds in these two regions. Electrons are thus extracted preferentially from the periphery of the diamond pads. Moreover, the side walls of the diamond pads, which are lying in parallel with the applied field, are reported to conduct the electrons more efficiently than bulk materials[25], as schematically 362 Fig. 4. Electron field emission CH /H /urea=18/300/6 sccm. 4 2 K. Perng et al. / Diamond and Related Materials 9 (2000) 358–363 properties and Fowler–Nordheim plot of the nitrogen-doped diamond pads grown with illustrated in Fig. 5b. the emission current density (J ) is e mostly contributed by the surface current density (J ) s rather than the bulk current density (J ). The substrate– b diamond interfaces surrounding the diamond pads thus have a larger current density and are easily overheated. Restated, the uneven distribution of emission current density not only reduces the total emission current available, but also induces localized melting of substrate materials, resulting in a breakdown of emitters. 4. Conclusion Nitrogen-doped diamond films, which possess a high emission density, J =500 mA/cm2, with a turn-on field e E =5.0 V/mm, were synthesized using the CVD process 0 with CH /H /urea=18/300/6 sccm. The emission array 4 2 made up of N-doped diamond pads was successfully obtained by forming the diamond nuclei pattern using SiO as masking materials, followed by CVD deposition 2 of diamonds. The diamond pads thus obtained show an emission current density of around 200 mA/cm2 with a turn-on field, E =8.0 V/mm. Inferior emission properties 0 of diamond pads, as compared with those of planar N-doped diamond films, are ascribed to the uneven distribution of emission current density. Acknowledgement Fig. 5. (a) Typical SEM micrographs of the nitrogen-doped diamond pads, after the occurrence of breakdown phenomenon and (b) schematics showing the flow of surface (J ) and bulk current (J ) density, s b which contribute to total field emission current density (J ). e Financial support of National Science Council, R.O.C. through the project No. NSC89-2112-M-007-046 is gratefully acknowledged by the authors. K. Perng et al. / Diamond and Related Materials 9 (2000) 358–363 References [1] C.A. 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