Abstract
The atomic-scale microstructure and electron emission properties of boron and sulfur (denoted as B-S) codoped diamond films grown on high-temperature and high-pressure (HTHP) diamond and Si substrates were investigated using atom force microscopy (AFM), scanning tunneling microscopy (STM), secondary ion mass spectroscopy (SIMS), and current imaging tunneling spectroscopy (CITS) measurement techniques. The films grown on Si consisted of large grains with secondary nucleation, whereas those on HTHP diamond are composed of well-developed polycrystalline facets with an average size of 10–50 nm. SIMS analyses confirmed that sulfur was successfully introduced into diamond films, and a small amount of boron facilitated sulfur incorporation into diamond. Large tunneling currents were observed at some grain boundaries, and the emission character was better at the grain boundaries than that at the center of the crystal. The films grown on HTHP diamond substrates were much more perfect with higher quality than the films deposited on Si substrates. The local I-V characteristics for films deposited on Si or HTHP diamond substrates indicate n-type conduction.
1. Introduction
Diamond is a semiconductor material with many excellent physical properties, such as a wide bandgap, a high saturation velocity of carriers, and other remarkable electronic properties. These properties make diamond a promising material for applications in high-power, high-temperature, high-frequency electronics, detectors, and electron emitter devices. The ability to produce large-area diamond wafers by chemical vapor deposition (CVD) techniques has aroused intensive interest in the use of the commercial material for active electronics. However, progress in this area has been severely hindered by the polycrystalline nature of such films and the difficulty in efficiently incorporating suitable dopant species [1]. Although homoepitaxial growth of diamond has been widely demonstrated, substrate cost and dopant incorporation limit commercial applications.
Identifying the best donor dopant for diamond is one of the fundamental issues for the development of diamond-based devices. Phosphorus (P) is expected to be a promising n-type dopant candidate for diamond. Katagiri reported that n-type conductivity of homoepitaxial diamond with a Hall mobility of 410 cm2 V−1 s−1 at room temperature [2]. Sally et al. reported on the n-type conductivity of sulfur-doped polycrystalline samples on silicon [3]. However, the distribution of sulfur between grain boundaries and the bulk has still not been reported. Sakaguchi et al. reported an activation energy of 0.38 eV and a Hall mobility of 597 cm2 V−1 s−1 at room temperature for sulfur-doped n-type diamond grown on (100) substrates [4]. Katayama-Yoshida et al. examined the theoretical aspects of codoping for wide-bandgap semiconductors [5]. Codoping has been proved as an effective way to incorporate donor dopants. However, up to now, doped films with such high Hall mobility have not been reported. This may be due to the compensation of donors by defects or residual acceptor impurities. The improvement in electrical properties is strongly related to the improvement of crystalline perfection, surface microstructure characteristics, and the electron surface states.
2. Methods
In our previous studies, the growth of boron and sulfur codoped diamond films on p-type (100) silicon substrates (heteroepitaxy) and HTHP diamond substrates (homoepitaxy) by microwave plasma-assisted chemical vapor deposition (MPCVD) is demonstrated. The diamond films were grown in a microwave reactor using dimethyl disulfide as the sulfur source. Acetone and hydrogen were used as the main reactant gases. The acetone concentration was 0.5%; the S/C atomic ratio in the gas sources was from 0.001 to 0.005, with a B/S ratio of 0.01-0.02; and the total gas pressure and the substrate temperature were 25 Torr and 750°C, respectively.
In previous research, microstructures and electrical properties of CVD diamond films that were investigated by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) were studied [6, 7]. Current imaging tunneling spectroscopy (CITS) [8] was used to compare the structures and electrical properties of the doped diamond films on Si and HTHP diamond substrates. The study of the local conductivity characteristics is expected to provide essential information for the development of a controlled process and the improvement of crystalline perfection. STM measurements were carried out with air-based AFM under a tip bias of 1-2 V with a tip current of 0.1–1 nA. The surface of the diamond was connected to ground with silver paint.
3. Results and Discussion
Figures 1 and 2 show AFM images of films on HTHP diamond and Si substrates with B-S codoping ((a) S/C = 0.001, B/S = 0.01; (b) S/C = 0.005, B/S = 0.02). Figure 1(a) shows well-developed, randomly oriented polycrystalline facets with an average crystal size of 10–50 nm grown on HTHP diamond substrates. The film was flat with a roughness of approximately 1.8 nm. As shown in Figure 1(b), the grain size of this film was about 5–20 nm, and the roughness of this film was approximately 1.6 nm. Figure 2(a) shows that the film grown on the Si substrate consisted of large grains with a high fraction of grain boundaries. The grain size of this film was about 100 nm. The roughness of this film was approximately 18.5 nm, which was due to the successive secondary nucleation. Figure 2(b) shows that the film has relatively small grains with about 50 nm, and the roughness of this film was approximately 16 nm. The comparison of (a) in Figures 1 and 2 and (b) in Figures 1 and 2 shows that the addition of sulfur and boron can decrease the grain size. The increase of boron content can improve the crystal quality and the surface morphology significantly.
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Figure 3 showed the temperature dependency of the resistivity of B-S codoped diamond films deposited on different substrate with a temperature range of 300–650 K. As shown in Figure 3, the linear relationship between and the inverse temperature indicated that the conductivity of the films was thermally activated. For the films deposited on silicon substrate, the activated energies of conductivity were determined to be 0.16 eV. But for the films deposited on diamond substrate, the activated energies of conductivity decreased to 0.07 eV. This indicated that conductivity of homoepitaxial films is better than that of heteroepitaxial films.
SIMS analyses confirmed that the sulfur was successfully introduced into diamond lattice. Figure 4 showed the SIMS spectrum from codoped sample with a B/S ratio of 0.01 and the surface of the film consisting of sulfur atoms, boron atoms, and hydrogen atoms. From the inset shown in Figure 4, the sulfur concentration of B-S codoped diamond films can increase by almost one order of magnitude of S doped diamond films. This indicated that the addition of boron facilitated sulfur incorporation into diamond.
Figure 5 shows the STM and CITS images for diamond films on a silicon substrate with B-S codoping (S/C = 0.001, B/S = 0.01). This image was taken with a sample bias and a tunneling current of 1.61 V and 0.10 nA, respectively. Small secondary grains or cluster grains with an average size less than 10 nm were observed in Figure 5(a). These small grains were observed in all crystalline facets and indicated continuous secondary nucleation during CVD processing. Figure 5(b) shows the corresponding CITS image of the sample shown in Figure 5(a). The white regions indicate the relatively high current area, and the black regions indicate low emission. High electron emission was observed only at some grain boundaries. These results are in agreement with the results of Frolov et al. [9]. The local I-V characteristics at high and low electron emission positions are also shown in Figures 5(c) and 5(d). It can be seen that the tunneling current at grain boundaries is larger than the current at specific facets. The result shows that the emission efficiency at the grain boundary is higher than the efficiency in the grain facet. This is probably due to the heterogeneity in the composition or defects at the grain boundary. Both results show the typical nonlinear behavior for the tunneling current. Figure 6 shows the STM and CITS images for diamond films on a silicon substrate with B-S codoping (S/C = 0.005, B/S = 0.02). Small grains with an average size less than 5 nm were observed in Figure 6(a). The local I-V characteristics are also shown in Figures 6(c) and 6(d). In these figures, it can be seen that the electrical conductivity of the diamond film increases with the increase of the boron-doping concentration. The tunneling current at a negative voltage applied to the sample is larger than the current at a positive voltage, which indicates that the diamond film has n-type conductivity [10, 11].
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The STM and CITS images of boron-sulfur codoped diamond deposited on an HTHP diamond substrate are shown in Figure 7. Those images were taken with a sample bias and a tunneling current of 2.15 V and 0.11 nA, respectively. It is noted that the average size of grain deposited on HTHP diamond was approximately 40–70 nm. Figure 7(b) shows the corresponding CITS image of the sample in Figure 7(a). No significant difference in the emission efficiency was observed between the crystalline facets and the grain boundaries. Therefore, it indicates that the growth of homoepitaxial diamond film could improve the crystal quality and the crystal perfection. The local I-V characteristics are also shown in Figures 7(c) and 7(d). In these figures, it can be seen that the tunneling current at a negative voltage is also larger than that at a positive voltage, which indicates that the diamond film has an n-type electronic conductivity.
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4. Conclusions
The structure and the electron field emission properties of boron-sulfur codoped diamond films on HTHP diamond substrates and Si substrates were investigated. It shows that the crystal grain size and the surface roughness of homoepitaxial diamond films were much smaller than those of heteroepitaxial diamond films, and films deposited on Si substrates had more grain boundaries than those deposited on HTHP diamond substrates. The electronic field emission efficiency at the grain boundaries is larger than that in the crystalline facets. The electronic structure was identified as an n-type electronic structure at the surface of films grown with sulfur and limited amount of boron.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant nos. 51072113 and 10804071) and the Shanghai Outstanding Academic Leaders Project Funding (12XD1420700) and the Innovation Program of Shanghai Municipal Education Commission (13ZZ143).