Field emission characteristics of boron carbon nitride films synthesized by plasma-assisted chemical vapor deposition

Diamond and Related Materials 9 (2000) 1233–1237 www.elsevier.com/locate/diamond

Field emission characteristics of boron carbon nitride films synthesized by plasma-assisted chemical vapor deposition Takashi Sugino *, Haruhiko Hieda Department of Electrical Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan

Abstract Boron carbon nitride (BCN ) films have been synthesized by plasma-assisted chemical vapor deposition. Borontrichloride (BCl ), methane (CH ) and nitrogen (N ) were used as source gases. The BCN films were characterized by transmission electron 3 4 2 microscopic observation, transmission electron diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared absorption and ultraviolet–visible transmission measurements. The BCN films produced were polycrystalline and the band gap varied from 6.0 to 3.4 eV depending on composition. It was found that the incorporation of C atoms in boron nitride (BN ) films is effective in solving the problem of BN films cracking and peeling off the substrate when steeped in water. Electron emission from BCN films is initially detected with an electric field as low as 11 V/mm. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Boron carbon nitride film; Field emission characteristics; Fourier transform infrared absorption; Optical band gap; Plasma-assisted chemical vapor deposition; X-ray photoelectron spectroscopy

1. Introduction The development of a cold cathode is desired as a key device for vacuum microelectronic devices and field emission flat panel displays. The reliability and performance of the cold cathode are strongly related to material properties. Recent attention has been paid to the application of diamond films to cold cathodes because of its negative electron affinity (NEA) property[1–3], in addition to superior properties such as mechanical hardness, chemical inertness and high thermal conductivity. On the other hand, BN has similar properties to diamond. It has been found that NEA appears on cubic (c-) and hexagonal (h-) BN surfaces [4–6 ]. NEA was detected significantly on a BN surface treated with hydrogen (H ) plasma [6 ]. Moreover, NEA 2 was maintained even on the BN surface treated with oxygen plasma [6 ]. Therefore, BN is expected to be a promising material for a cold cathode. There are several reports on field emission characteristics of BN films. It has been reported that electrons are emitted at a low voltage from the C-doped BN film synthesized by laser ablation [7]. The influence of the series resistance of BN * Corresponding author. Tel.: +81-6-6877-5111; fax: +81-6-6879-7774. E-mail address: [email protected] (T. Sugino)

films on field emission characteristics was investigated [8] and it was demonstrated that BN field emitters had the potential for high emission current operation [9]. Electron emission from Si-doped BN films has been also reported [10]. The authors synthesized BN films by plasma-assisted chemical vapor deposition (PACVD) and found that the electron emission occurred at low electric field from sulfur(S)-doped BN films [11]. Moreover, it was reported that the field emission characteristics of the Si tip array were much improved by coating with S-doped BN films [12]. It was also found that hysteresis of field emission characteristics was suppressed on a BN surface treated with O plasma [13]. 2 There is a problem to be solved in fabricating BN field emitters. When BN is steeped in water, the film cracks and peels off the substrate. It was demonstrated that deposition of BN films on GaN improved adhesion between the BN film and the substrate [14]. In the work leading to this paper, BCN films were synthesized by PACVD. It is found that incorporation of C atoms in BN films is effective in solving the adhesion issue. The BCN films are characterized by transmission electron microscopic ( TEM ) observation, transmission electron diffraction ( TED), X-ray photoelectron spectroscopy ( XPS), Fourier transform infrared absorption ( FTIR), and ultraviolet–visible ( UV–visible)

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optical transmission measurements. Also demonstrated are the field emission characteristics of BCN films.

2. Experiments BCN films were synthesized by PACVD under application of DC negative bias (V ) to the substrate. (100)DC oriented Si and fused silica were used as substrates. The growth apparatus is shown in Fig. 1. The substrate was placed on a stainless steel holder in a horizontal quartz reactor and heated by an external furnace. The reactor was evacuated to a base pressure of 1×10−3 Torr, followed by the introduction of gases to the total gas pressure of 1 Torr. Prior to depositing BCN films, the substrate was treated with H plasma for 3 min in the 2 reactor to clean the surface. BCl , CH and N were 3 4 2 used as source gases. Plasma consisting of CH and N 4 2 gases was produced 15 cm away from the substrate by supplying RF power (13.56 MHz) to the turn coil which was installed around the reactor. The RF power was fixed at 40 W. BCl was carried with H near the 3 2 substrate without mixing with the CH and N . The 4 2 N/B flow rate ratio was regulated at 2.5 under the condition that BCl was fixed at 0.8 sccm. BCN films 3 were deposited with various flow rates of CH . The DC 4 negative bias of 200 V was applied to the substrate. The substrate temperature was typically 650°C. We also attempted growth at reduced temperatures. The growth rate was estimated to be 100 nm/h under the above conditions. TEM observation and TED measurement were carried out to examine the structure of BCN films. XPS, FTIR and UV–visible optical transmission measurements were performed to characterize the BCN films. Current–voltage characteristics were measured at room temperature for BCN films on which Au electrodes were provided. In addition, field emission characteristics were measured for a BCN film deposited on an Si substrate. An Si wafer measuring 1×3 mm was used as an anode. The anode was set 125 mm away from the sample surface using two pieces of glass fiber as a spacer. A resistor of 10 MV was connected to the cathode to protect the

Fig. 1. Growth apparatus.

measurement equipment. The field emission measurements were carried out at a pressure of 4~8×10−7Torr. An image of the electron emission was not observed in this measurement. Prior to field emission measurements, sample surface was treated with a remote H plasma which made a 2 mild process possible [15]. H plasma treatment was 2 carried out under the following conditions: H plasma 2 was produced by supplying RF power of 50 W to the inductive coil after introducing H gas at a flow rate of 2 15 sccm; the gas pressure was regulated at 0.5 Torr; the sample temperature and treatment duration were kept constant at 95°C and 60 min respectively.

3. Results and discussion TEM observation was carried out for a BCN film as thin as 100 nm. Fig. 2 shows a TEM micrograph. A number of nanocrystalline grains can be seen on the TEM micrograph. The grain size is about 3 nm. Moreover, diffraction ring patterns are observed on the TED photograph — this is also shown on Fig. 2. These ring patterns mean that the polycrystalline BCN film is synthesized by PACVD.

Fig. 2. TEM and TED photographs.

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XPS measurements were carried out on BCN films. In order to estimate the values of x and y in BCxNy, XPS signal intensities of C 1s and N 1s were normalized to that of B 1s taking account of the photoionization cross-section. The formation of BMN, CMN and BMC bonds was confirmed through deconvolution of XPS spectra [16 ]. Fig. 3 shows the FTIR spectra of BCN films 300 nm thick. Also indicated is the FTIR spectrum of an h-BN film produced by PACVD for comparison [11]. The spectrum of the h-BN film consists of a strong absorption band at 1380 cm−1 and a weaker band at 780 cm−1. There have been several reports on IR absorption measurements of BN films [17–20]. The strong absorption band at 1380 cm−1 and the weaker band at 780 cm−1 are attributed to the BMN stretching mode and the BMNMB bending mode respectively. Although it has been reported that the absorption band due to c-BN appears at 1070 cm−1 [21], it is not detected in our FTIR spectrum. On the other hand, the spectrum of the BCN film features not only a strong absorption band at 1380 cm−1 and a weaker band at 780 cm−1, but also an increasing absorption band around 1250 cm−1. It is inferred that the increasing absorption band is possibly due to the bonds: CMB at 1070 cm−1; CMN at 1210 cm−1 and 1260 cm−1 [22]; and CMC at 1250 cm−1 [23]. The optical absorption spectrum in the UV-visible region was measured for BN and BCN films deposited on fused silica substrates. The film thickness was 300 nm. The absorption coefficient at 6.2 eV increased to 7.5×105 cm−1, and 1.6×106 cm−1 for the BN and BC N films respectively. An absorption coefficient 0.23 0.91

due to the allowed direct transition between p bands has been reported for BN films [24,25]. In order to estimate the optical band gap (E ), the hn dependence g of a2 was plotted, as shown in Fig. 4, where Bn is the photon energy and a is the absorption coefficient. The energy gap of the BN film was evaluated to be as wide as 6.0 eV. The absorption edge shifts to low photon energy for BCN films with increasing C incorporation. It is not unreasonable to consider that the optical band gap of BCN films with a low C composition can be estimated in a similar manner. The optical band gap is reduced to 3.4 eV for the BC N film. 0.71 0.67 From the current–voltage characteristics, the electrical resistivity was evaluated to be as high as 2×1011 V cm for undoped BN films with Au electrodes. On the other hand, the electrical resistivity of the BC N film was estimated to be 2.0×107 V cm. The 0.23 0.91 electrical characteristics of BCN films with various compositions will be examined in the future work. Fig. 5 depicts field emission characteristics measured for undoped BCN films of various compositions. The BCN films were deposited on Si substrates for 90 min. Prior to field emission measurements, all sample surfaces were hydrogenated by remote H plasma treatment. The 2 measurement limit of the emission current was 1×10−12 A in our apparatus. The voltage at an emission current of 1×10−11 A is designated as the turn-on voltage in this paper. The electric field was obtained by

Fig. 3. FTIR spectra of BN and BCN films.

Fig. 4. Plot of a2 vs. Bn to determine the energy gap.

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Fig. 5. Emission current vs. electric field characteristics of undoped BN and BCN films.

normalizing the voltage to the spacing (125 mm) between the anode and the sample. The turn-on voltage was estimated to be 1400 V and 1700 V for the BC N 0.13 0.86 and BC N films respectively. The turn-on electric 0.23 0.91 field was measured as 11 and 14 V/mm for the BC N and BC N films respectively. No sig0.13 0.86 0.23 0.91 nificant variation in the field emission characteristics is observed in comparison with those of BN film. Fig. 6 shows the field emission characteristics of BC N film doped with sulfur, along with those of 0.88 0.43 the S-doped BN film. Both films were deposited on Si substrates for 180 min. The turn-on electric field of the BN film was estimated to be 8 V/mm, while the turn-on

electric field of the BCN film increased to 11 V/mm. It should be noted that it is important to observe an image of the electron emission on the sample surface to characterize the field emission properties of polycrystalline BCN films as well as polycrystalline diamond and diamond-like carbon films. It is also important to understand the factors which have an influence on field emission characteristics. In the present experiment, the influence of the cathode metal on field emission characteristics was investigated by depositing the S-doped BCN film on TiN. These field emission characteristics are shown in Fig. 6. It is found that an increase of the turn-on electric field and a reduction in the emission current occur for the BCN film on TiN. This means that a cathode metal in contact with a BCN film has an influence on field emission characteristics. Therefore, a suitable choice of cathode metal may make it possible to improve the field emission characteristics of the BCN film. Although a cathode metal has been found to influence field emission characteristics, further investigations are needed to understand the electron emission mechanism.

4. Conclusion Polycrystalline BCN films have been synthesized by PACVD with the application of DC bias to substrates. It is observed that the energy gap of the BCN film is changed from 6.0 eV to 3.4 eV with increasing C composition. The electrical resistivity of the BC N film 0.23 0.91 is 2.0×107 V cm. The electron emission current is detected when the electric field is higher than 11 V/mm for BCN films.

References

Fig. 6. Emission current vs. electric field characteristics of S-doped BN and BCN films. The BCN films were deposited on Si and TiN.

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