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Depletion of surface boron of heavily boron-doped diamond films by annealing
Diamond and Related Materials 8 (1999) 1006–1010
Depletion of surface boron of heavily boron-doped diamond films by annealing K.W. Wong a,c, L.J. Huang b, Y. Hung b, S.T. Lee a,*, R.W.M. Kwok c a Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong b Seagate Recording Media Organization, 311 Turquois Street, Milpitas, CA 95035, USA c Department of Chemistry, Chinese University of Hong Kong, Shatin, Hong Kong Received 14 August 1998; accepted 12 November 1998
Abstract Surface boron of heavily boron-doped polycrystalline diamond thin films was found to be depleted upon annealing. Based on analyses using X-ray photoelectron spectroscopy, X-ray photoelectron energy loss spectroscopy, time-of-flight secondary ion mass spectroscopy and Auger electron spectroscopy, surface boron was found to be removed when the diamond film was annealed at 500 °C in vacuum and continued to be eliminated at 900 °C. The loss of surface boron undoubtedly altered the electronic properties of the diamond surface. This phenomenon is important in the fabrication of diamond-based electronic devices. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Annealing; Boron-doped diamond; Depletion; Polycrystalline
1. Introduction Diamond thin films have been shown to have great promise in diamond-based electronic applications [1–5]. Owing to its ease of preparation and its potentially wide range of use, polycrystalline diamond films have attracted a large amount of scientific research interest and motivated studies of polycrystalline diamond films on doping [6 ], etching [7,8] smoothing [9], etc. When the diamond film is properly doped, different useful diamond-based electronic devices can be fabricated to produce diamond-based electron multiplier, field emitter display, cold cathodes, etc. Among various dopants, only boron has been reported to be successfully doped into diamond films. Boron can be doped into the diamond film under as-grown conditions or through ion implantation [10,11]. The doped boron imparts a p-type semiconducting property in the film, and the activation energy for boron in diamond is 0.37 eV [12]. As boron can be easily incorporated into diamond, its potential in electronic applications of diamond films is the greatest at the present stage. This stimulates various studies on the boron-doped polycrystalline diamond films. To real* Corresponding author. Fax: +852 2784-4696. E-mail address: [email protected] (S.T. Lee)
ize the enormous potential of diamond films in electronic applications, a detailed understanding of the electrical and electronic properties must be obtained. Although there are many investigations on the electrical properties of the boron-doped films [13,14], investigations on electronic properties are limited. In this article, the structural properties, content of surface boron, surface Fermi level and their changes upon annealing at 500 and 900 °C were investigated using time-of-flight secondary ion mass spectroscopy ( TOF-SIMS ), X-ray photoelectron spectroscopy ( XPS), X-ray photoelectron energy loss spectroscopy and Auger electron spectroscopy (AES). The temperature dependence of various electronic properties is of critical importance because annealing is an essential process in device fabrication.
2. Experimental In the present study, a heavily boron-doped diamond film was prepared by hot filament assisted chemical vapour deposition at 850 °C from a 0.5% CH /H 4 2 mixture at a pressure of 40 Torr. The substrate used was a p-type mirror-polished silicon wafer with the resistivity of 10–20 mV · cm. Boron was incorporated into the diamond film during growth using diborane (B H ) as 2 6
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 8 ) 0 0 44 5 - 2
K.W. Wong et al. / Diamond and Related Materials 8 (1999) 1006–1010
the boron source. The diamond film prepared was determined as p-type using Hall measurements. The resistivity measured by four-point probe was 1 mV · cm. Prior to analysis, the heavily boron-doped film was cleaned by dipping into a boiling mixture of H SO , HNO and HClO in a ratio of 1:1:1 for 5 min. 2 4 3 4 The film was then immersed into boiling deionized water for 5 min, followed by boiling methanol for a short time, and the film was finally blown dry with nitrogen. The cleaned diamond film was annealed at 200 °C in vacuum before taking any measurements to eliminate surface hydrocarbons and other contaminants. The XPS and X-ray photoelectron energy loss spectroscopy in this experiment were performed using a 180° hemispherical analyser ( HSA). Monochromated AlKa line (1486.6 eV ) was used as the excitation source, and the HSA was operated to give an energy resolution of 0.8 eV. The binding energy was calibrated against the Au 4f peak at 84.00 eV from a sputtered clean gold 7/2 foil. TOF-SIMS measurements were taken by using a 15-keV pulsed Ga liquid metal ion gun with a raster size of 40×40 mm. The mass resolution was about 10−4 for Si+ and 3×10−4 for C−. Before the TOFSIMS experiment, the surface contamination was further reduced through in-situ sputtering, which efficiently removed even traces of heavy metals and hydrocarbon contaminations on the surface. The diamond film was also analysed by Auger electron spectroscopy (AES) with a double pass cylindrical mirror analyser. The diamond film was then annealed at 500 °C and subsequently at 900 °C in a vacuum using an electron beam heater beneath the sample. The pressure during annealing was always below 5×10−9 Torr. The annealed film was then studied in-situ using the same techniques.
3. Results Fig. 1 shows the B 1s XPS spectra of the sample before and after annealing at 500 and 900 °C. Each B 1s XPS signal was composed of two components. The peak at the binding energy of ~187 eV was attributed to the elemental B species doped in the diamond as the binding energy is close to the B 1s of B C (186.5 eV ) [15]. The 4 other peak at ~191 eV, which was close to the B 1s of B O (192 eV ) [15], was considered to originate from a 2 3 boron oxide species, B O . It is obvious that the relative x y amount of B O to elemental boron was found to x y increase after annealing at 500 °C and further increase upon a subsequent 900 °C anneal, whereas the total boron content decreased significantly. The B/C ratio that was calculated from the XPS signals of C 1s and B 1s changed from 2.83×104 ppm before anneal to 1.92×104 ppm after annealing at 500 °C, and then to 0.76×104 ppm at 900 °C. Surface boron within the ˚ ) of XPS is clearly removed sampling depth (~50 A
1007
Fig. 1. B 1s XPS spectra before and after annealing.
upon annealing. In addition, a shifting of C 1s signal towards a higher binding energy is observed. The C 1s signal shifts from 284.3 eV before annealing to 284.7 eV after annealing at 500 °C, and to 285.1 eV at 900 °C. As the diamond film is heavily boron-doped, the charging effect can be excluded. The peak shift towards the high binding energy region implies a raise of Fermi level with respect to the valence band maximum ( VBM ) and/or a surface band bending. As the depletion of surface boron is evident after annealing, the decrease of boron content should play a significant role in increasing the Fermi level of the diamond surface with respect to the VBM and gives a downward shift of C 1s signal in XPS. Fig. 2 shows the X-ray photoelectron energy loss spectra from the C 1s signal of the sample before and after annealing. Feature A (~35 eV ) is the diamond bulk plasmon, whereas the loss at around 25 eV is the diamond surface plasmon and may also be mixed with the plasmons of graphitic phase and/or amorphous carbon as both of them display a prominent feature at around 25 eV [16 ]. As shown in Fig. 2, the diamond bulk plasmon becomes much more prominent and distinctive after annealing at 500 °C. However, after a 900 °C anneal, although the diamond bulk plasmon remains as pronounced, the loss feature at ~25 eV ( Feature B) emerges sharply, together with a small bump at ~5 eV (Feature C ). This indicates an emer-
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K.W. Wong et al. / Diamond and Related Materials 8 (1999) 1006–1010
Fig. 2. X-ray photoelectron energy loss spectra from C 1s signal before and after annealing.
gence of graphitic phase because the loss at ~25 eV can be due to the graphitic phase as mentioned above, and Feature C is, in fact, an additional indication of graphitic carbon because graphite shows an obvious loss signal at ~5 eV [16 ]. These observations agree with the results of the AES study. Fig. 3 shows the AES spectra before and after annealing. The AES profile of the C KLL signal obviously changes after annealing. Feature B becomes more prominent than feature A after annealing, which implies a diamond surface with a better quality
Fig. 3. C KLL AES spectra before and after annealing.
and order [17]. However, after the 900 °C anneal, Feature A increases again, indicating the presence of disordered carbon phase [18]. Evidently, both results from X-ray photoelectron energy loss spectroscopy and AES indicate an improvement in diamond quality after annealing at 500 °C, but the quality degraded after a 900 °C anneal. We propose that the improvement at 500 °C should be due to the depletion of surface boron after annealing. Since the boron originally incorporated into the diamond as a substitutional dopant that disrupts the diamond structure, which is evident from the X-ray photoelectron energy loss spectroscopy and AES results, the depletion of surface boron after annealing would undoubtedly improve the diamond crystallinity. However, after the 900 °C anneal, the rather high temperature anneal is believed to induce more structural damage to the diamond surface. As a result, a degradation of the diamond surface is observed from the X-ray photoelectron energy loss spectroscopy and AES. This phenomenon will be further explained with the results of TOF-SIMS. The diamond film before and after annealing was carefully analysed using TOF-SIMS in order to confirm the depletion of surface boron after annealing. The surface boron concentration was estimated using B+ ion intensity. The TOF-SIMS spectra of the sample before and after annealing ( Fig. 4) showed that the boron signal substantially decreased after annealing at 500 °C
Fig. 4. SIMS spectra of B+ ion before and after annealing.
K.W. Wong et al. / Diamond and Related Materials 8 (1999) 1006–1010
and could barely be observed after the 900 °C anneal. The concentration of boron was determined using the sensitivity factors provided by the manufacturer. These sensitivity factors have been checked also with heavily boron-doped silicon wafers with a known boron concentration. It was found that the surface boron concentration dropped from 1.32×1013 cm−2 to 7×1011 cm−2 (decrease of 95%) after the 500 °C anneal and became undetectable upon further annealing at 900 °C. The decrease of surface boron after the first anneal at 500 °C as measured by TOF-SIMS (95%) is much larger than that measured by XPS (32% decrease). Nevertheless, it confirmed that there was a substantial depletion of boron at the surface after annealing because TOF-SIMS is much more surface-sensitive (one or two topmost layers). This substantial decrease of surface boron is further supported by the decrease of the relative intensity of BH+ to C+ illustrated in Fig. 5. In Fig. 5, the BH+ signal decreased sharply after the 500 °C anneal, and no detectable BH+ was observed after annealing in the 900 °C anneal. The decrease of BH+ closely matched the decrease of B+ in Fig. 4. Although the decrease of BH+ can also be due to the elimination of hydrogen after annealing, Fig. 6 shows pronounced signals of CH+ before and after the 500 °C anneal. This indicates that the diamond surface was hydrogen-terminated before annealing, and the surface remained hydrogenated even at 500 °C. Also, other scientists observed
1009
Fig. 6. SIMS spectra of CH+ ion before and after annealing.
the desorption of surface hydrogen at ~900 °C, rather than at 500 °C from the diamond surface [19,20]. We therefore believe that the decrease of BH+ after the 500 °C anneal was mainly due to the decrease of surface boron rather than hydrogen. In fact, our TOF-SIMS analysis found that upon annealing at 900 °C, the CH+ fragment decreases drastically ( Fig. 6). It is consistent with other observations on hydrogen desorption at 900 °C. Owing to the hydrogen desorption and the rather high-temperature annealing at 900 °C, surface reconstruction and local graphitization will occur. This also explains the degradation of the diamond surface as shown by X-ray photoelectron energy loss spectroscopy and AES.
4. Conclusions
Fig. 5. SIMS spectra of C+ and BH+ ion before and after annealing.
The depletion of dopants upon annealing is unusual in common semiconductors like silicon, gallium arsenide, etc. However, in the present study, the depletion of surface boron in heavily boron-doped diamond films is clearly demonstrated using XPS and SIMS. The extraordinary response of heavily B-doped diamond films observed here can be explained by the presence of large amount of structural defects, which assisted the movement of surface boron and enhanced the depletion of surface boron. This plays a vital role in an annealing process in a vacuum. Although the as-grown incorpora-
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K.W. Wong et al. / Diamond and Related Materials 8 (1999) 1006–1010
tion of boron during diamond film deposition was performed at 850 °C, hydrogen, boron and carbon species were continuously fed on to the film. The reaction kinetics was, therefore, fundamentally different from the annealing in vacuum performed here. It is reasonable that the surface boron can be retained in the diamond surface during the growth process. In conclusion, depletion of surface boron from heavily boron-doped diamond films was observed using XPS and SIMS. This is also supported by the improvement in diamond surface quality after annealing, as shown by X-ray photoelectron energy loss spectroscopy and AES. In addition, the analysis by XPS revealed that the loss of surface boron affected the surface Fermi level position. As a result, the physical characteristics like Fermi level position and dopant concentration, which critically control the electronic properties of diamond films, were found to be closely related to the sample treatment. Even a rather mild anneal (500 °C ) was shown in the present study to be very influential. So, for the potential of diamond films in electronic applications to be fully realized, the response of diamond towards various treatments must be carefully characterized.
References [1] M.W. Geis, J.A. Gregory, B.B. Pate, IEEE Elect. Dev. 38 (1991) 619.
[2] M.W. Geis, N.N. Efremow, J.D. Woodhouse, M.D. McAleese, M. Marchywka, D.G. Socker, J.F. Hochedez, IEEE Elect. Dev. Lett. 12 (1991) 456. [3] M.W. Geis, J.C. Twichell, T.M. Lyszczarz, J. Vac. Sci. Technol. B 14 (1996) 2060. [4] J.D. Shovlin, M.E. Kordesch, D. Dunham, B.P. Tonner, W. Engel, J. Vac. Sci. Technol. A 13 (1995) 1111. [5] W.N. Wang, N.A. Fox, D. Richardson, G.M. Lynch, J.W. Steeds, J. Appl. Phys. 81 (1997) 1505. [6 ] K. Okano, Diamond: Electronic Properties and Applications, Chapter 4, Kluwer Academic, Boston, 1995. [7] B.Y. Liaw, T. Stacy, G. Zhao, E.J. Charlson, E.M. Charlson, J.M. Meese, M.A. Prelas, Appl. Phys. Lett. 65 (1994) 2827. [8] G.S. Sandhu, W.K. Chu, Appl. Phys. Lett. 55 (1989) 437. [9] S.A. Kajihara, A. Antonelli, J. Bernholc, Physica B 185 (1993) 144. [10] K. Okano, Y. Akiba, T. Kurosa, M. Iida, T. Nakamura, J. Cryst. Growth 99 (1990) 1192. [11] F. Fontaine, C. Uzan-Saguy, B. Philosoph, R. Kalish, Appl. Phys. Lett. 68 (1996) 2264. [12] J.F. Prins, Phys. Rev. B 38 (1988) 5576. [13] A. Masood, M. Aslam, M.A. Tamor, T.J. Potter, Appl. Phys. Lett. 61 (1992) 1832. [14] K. Nishimura, K. Das, J.T. Glass, J. Appl. Phys. 69 (1991) 3142. [15] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Appendix B, Physical Electronics Industry, Perkin-Elmer Corporation, Physical Electronics Division, Eden Prairie, Minnesota, USA, 1992. [16 ] S. Haq, D.L. Tunnicliffe, S. Sails, J.A. Savage, Appl. Phys. Lett. 68 (1996) 469. [17] S.V. Pepper, Appl. Phys. Lett. 38 (1981) 344. [18] K.W. Wong, S.T. Lee, R.W.M. Kwok, Y.W. Lam, H. Kawarada, Jpn. J. Appl. Phys. 35 (1996) 5444. [19] B.B. Pate, Surf. Sci. 165 (1986) 83. [20] S.-T. Lee, G. Apai, Phys. Rev. B 48 (1993) 2684.
Depletion of surface boron of heavily boron-doped diamond films by annealing K.W. Wong a,c, L.J. Huang b, Y. Hung b, S.T. Lee a,*, R.W.M. Kwok c a Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong b Seagate Recording Media Organization, 311 Turquois Street, Milpitas, CA 95035, USA c Department of Chemistry, Chinese University of Hong Kong, Shatin, Hong Kong Received 14 August 1998; accepted 12 November 1998
Abstract Surface boron of heavily boron-doped polycrystalline diamond thin films was found to be depleted upon annealing. Based on analyses using X-ray photoelectron spectroscopy, X-ray photoelectron energy loss spectroscopy, time-of-flight secondary ion mass spectroscopy and Auger electron spectroscopy, surface boron was found to be removed when the diamond film was annealed at 500 °C in vacuum and continued to be eliminated at 900 °C. The loss of surface boron undoubtedly altered the electronic properties of the diamond surface. This phenomenon is important in the fabrication of diamond-based electronic devices. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Annealing; Boron-doped diamond; Depletion; Polycrystalline
1. Introduction Diamond thin films have been shown to have great promise in diamond-based electronic applications [1–5]. Owing to its ease of preparation and its potentially wide range of use, polycrystalline diamond films have attracted a large amount of scientific research interest and motivated studies of polycrystalline diamond films on doping [6 ], etching [7,8] smoothing [9], etc. When the diamond film is properly doped, different useful diamond-based electronic devices can be fabricated to produce diamond-based electron multiplier, field emitter display, cold cathodes, etc. Among various dopants, only boron has been reported to be successfully doped into diamond films. Boron can be doped into the diamond film under as-grown conditions or through ion implantation [10,11]. The doped boron imparts a p-type semiconducting property in the film, and the activation energy for boron in diamond is 0.37 eV [12]. As boron can be easily incorporated into diamond, its potential in electronic applications of diamond films is the greatest at the present stage. This stimulates various studies on the boron-doped polycrystalline diamond films. To real* Corresponding author. Fax: +852 2784-4696. E-mail address: [email protected] (S.T. Lee)
ize the enormous potential of diamond films in electronic applications, a detailed understanding of the electrical and electronic properties must be obtained. Although there are many investigations on the electrical properties of the boron-doped films [13,14], investigations on electronic properties are limited. In this article, the structural properties, content of surface boron, surface Fermi level and their changes upon annealing at 500 and 900 °C were investigated using time-of-flight secondary ion mass spectroscopy ( TOF-SIMS ), X-ray photoelectron spectroscopy ( XPS), X-ray photoelectron energy loss spectroscopy and Auger electron spectroscopy (AES). The temperature dependence of various electronic properties is of critical importance because annealing is an essential process in device fabrication.
2. Experimental In the present study, a heavily boron-doped diamond film was prepared by hot filament assisted chemical vapour deposition at 850 °C from a 0.5% CH /H 4 2 mixture at a pressure of 40 Torr. The substrate used was a p-type mirror-polished silicon wafer with the resistivity of 10–20 mV · cm. Boron was incorporated into the diamond film during growth using diborane (B H ) as 2 6
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 8 ) 0 0 44 5 - 2
K.W. Wong et al. / Diamond and Related Materials 8 (1999) 1006–1010
the boron source. The diamond film prepared was determined as p-type using Hall measurements. The resistivity measured by four-point probe was 1 mV · cm. Prior to analysis, the heavily boron-doped film was cleaned by dipping into a boiling mixture of H SO , HNO and HClO in a ratio of 1:1:1 for 5 min. 2 4 3 4 The film was then immersed into boiling deionized water for 5 min, followed by boiling methanol for a short time, and the film was finally blown dry with nitrogen. The cleaned diamond film was annealed at 200 °C in vacuum before taking any measurements to eliminate surface hydrocarbons and other contaminants. The XPS and X-ray photoelectron energy loss spectroscopy in this experiment were performed using a 180° hemispherical analyser ( HSA). Monochromated AlKa line (1486.6 eV ) was used as the excitation source, and the HSA was operated to give an energy resolution of 0.8 eV. The binding energy was calibrated against the Au 4f peak at 84.00 eV from a sputtered clean gold 7/2 foil. TOF-SIMS measurements were taken by using a 15-keV pulsed Ga liquid metal ion gun with a raster size of 40×40 mm. The mass resolution was about 10−4 for Si+ and 3×10−4 for C−. Before the TOFSIMS experiment, the surface contamination was further reduced through in-situ sputtering, which efficiently removed even traces of heavy metals and hydrocarbon contaminations on the surface. The diamond film was also analysed by Auger electron spectroscopy (AES) with a double pass cylindrical mirror analyser. The diamond film was then annealed at 500 °C and subsequently at 900 °C in a vacuum using an electron beam heater beneath the sample. The pressure during annealing was always below 5×10−9 Torr. The annealed film was then studied in-situ using the same techniques.
3. Results Fig. 1 shows the B 1s XPS spectra of the sample before and after annealing at 500 and 900 °C. Each B 1s XPS signal was composed of two components. The peak at the binding energy of ~187 eV was attributed to the elemental B species doped in the diamond as the binding energy is close to the B 1s of B C (186.5 eV ) [15]. The 4 other peak at ~191 eV, which was close to the B 1s of B O (192 eV ) [15], was considered to originate from a 2 3 boron oxide species, B O . It is obvious that the relative x y amount of B O to elemental boron was found to x y increase after annealing at 500 °C and further increase upon a subsequent 900 °C anneal, whereas the total boron content decreased significantly. The B/C ratio that was calculated from the XPS signals of C 1s and B 1s changed from 2.83×104 ppm before anneal to 1.92×104 ppm after annealing at 500 °C, and then to 0.76×104 ppm at 900 °C. Surface boron within the ˚ ) of XPS is clearly removed sampling depth (~50 A
1007
Fig. 1. B 1s XPS spectra before and after annealing.
upon annealing. In addition, a shifting of C 1s signal towards a higher binding energy is observed. The C 1s signal shifts from 284.3 eV before annealing to 284.7 eV after annealing at 500 °C, and to 285.1 eV at 900 °C. As the diamond film is heavily boron-doped, the charging effect can be excluded. The peak shift towards the high binding energy region implies a raise of Fermi level with respect to the valence band maximum ( VBM ) and/or a surface band bending. As the depletion of surface boron is evident after annealing, the decrease of boron content should play a significant role in increasing the Fermi level of the diamond surface with respect to the VBM and gives a downward shift of C 1s signal in XPS. Fig. 2 shows the X-ray photoelectron energy loss spectra from the C 1s signal of the sample before and after annealing. Feature A (~35 eV ) is the diamond bulk plasmon, whereas the loss at around 25 eV is the diamond surface plasmon and may also be mixed with the plasmons of graphitic phase and/or amorphous carbon as both of them display a prominent feature at around 25 eV [16 ]. As shown in Fig. 2, the diamond bulk plasmon becomes much more prominent and distinctive after annealing at 500 °C. However, after a 900 °C anneal, although the diamond bulk plasmon remains as pronounced, the loss feature at ~25 eV ( Feature B) emerges sharply, together with a small bump at ~5 eV (Feature C ). This indicates an emer-
1008
K.W. Wong et al. / Diamond and Related Materials 8 (1999) 1006–1010
Fig. 2. X-ray photoelectron energy loss spectra from C 1s signal before and after annealing.
gence of graphitic phase because the loss at ~25 eV can be due to the graphitic phase as mentioned above, and Feature C is, in fact, an additional indication of graphitic carbon because graphite shows an obvious loss signal at ~5 eV [16 ]. These observations agree with the results of the AES study. Fig. 3 shows the AES spectra before and after annealing. The AES profile of the C KLL signal obviously changes after annealing. Feature B becomes more prominent than feature A after annealing, which implies a diamond surface with a better quality
Fig. 3. C KLL AES spectra before and after annealing.
and order [17]. However, after the 900 °C anneal, Feature A increases again, indicating the presence of disordered carbon phase [18]. Evidently, both results from X-ray photoelectron energy loss spectroscopy and AES indicate an improvement in diamond quality after annealing at 500 °C, but the quality degraded after a 900 °C anneal. We propose that the improvement at 500 °C should be due to the depletion of surface boron after annealing. Since the boron originally incorporated into the diamond as a substitutional dopant that disrupts the diamond structure, which is evident from the X-ray photoelectron energy loss spectroscopy and AES results, the depletion of surface boron after annealing would undoubtedly improve the diamond crystallinity. However, after the 900 °C anneal, the rather high temperature anneal is believed to induce more structural damage to the diamond surface. As a result, a degradation of the diamond surface is observed from the X-ray photoelectron energy loss spectroscopy and AES. This phenomenon will be further explained with the results of TOF-SIMS. The diamond film before and after annealing was carefully analysed using TOF-SIMS in order to confirm the depletion of surface boron after annealing. The surface boron concentration was estimated using B+ ion intensity. The TOF-SIMS spectra of the sample before and after annealing ( Fig. 4) showed that the boron signal substantially decreased after annealing at 500 °C
Fig. 4. SIMS spectra of B+ ion before and after annealing.
K.W. Wong et al. / Diamond and Related Materials 8 (1999) 1006–1010
and could barely be observed after the 900 °C anneal. The concentration of boron was determined using the sensitivity factors provided by the manufacturer. These sensitivity factors have been checked also with heavily boron-doped silicon wafers with a known boron concentration. It was found that the surface boron concentration dropped from 1.32×1013 cm−2 to 7×1011 cm−2 (decrease of 95%) after the 500 °C anneal and became undetectable upon further annealing at 900 °C. The decrease of surface boron after the first anneal at 500 °C as measured by TOF-SIMS (95%) is much larger than that measured by XPS (32% decrease). Nevertheless, it confirmed that there was a substantial depletion of boron at the surface after annealing because TOF-SIMS is much more surface-sensitive (one or two topmost layers). This substantial decrease of surface boron is further supported by the decrease of the relative intensity of BH+ to C+ illustrated in Fig. 5. In Fig. 5, the BH+ signal decreased sharply after the 500 °C anneal, and no detectable BH+ was observed after annealing in the 900 °C anneal. The decrease of BH+ closely matched the decrease of B+ in Fig. 4. Although the decrease of BH+ can also be due to the elimination of hydrogen after annealing, Fig. 6 shows pronounced signals of CH+ before and after the 500 °C anneal. This indicates that the diamond surface was hydrogen-terminated before annealing, and the surface remained hydrogenated even at 500 °C. Also, other scientists observed
1009
Fig. 6. SIMS spectra of CH+ ion before and after annealing.
the desorption of surface hydrogen at ~900 °C, rather than at 500 °C from the diamond surface [19,20]. We therefore believe that the decrease of BH+ after the 500 °C anneal was mainly due to the decrease of surface boron rather than hydrogen. In fact, our TOF-SIMS analysis found that upon annealing at 900 °C, the CH+ fragment decreases drastically ( Fig. 6). It is consistent with other observations on hydrogen desorption at 900 °C. Owing to the hydrogen desorption and the rather high-temperature annealing at 900 °C, surface reconstruction and local graphitization will occur. This also explains the degradation of the diamond surface as shown by X-ray photoelectron energy loss spectroscopy and AES.
4. Conclusions
Fig. 5. SIMS spectra of C+ and BH+ ion before and after annealing.
The depletion of dopants upon annealing is unusual in common semiconductors like silicon, gallium arsenide, etc. However, in the present study, the depletion of surface boron in heavily boron-doped diamond films is clearly demonstrated using XPS and SIMS. The extraordinary response of heavily B-doped diamond films observed here can be explained by the presence of large amount of structural defects, which assisted the movement of surface boron and enhanced the depletion of surface boron. This plays a vital role in an annealing process in a vacuum. Although the as-grown incorpora-
1010
K.W. Wong et al. / Diamond and Related Materials 8 (1999) 1006–1010
tion of boron during diamond film deposition was performed at 850 °C, hydrogen, boron and carbon species were continuously fed on to the film. The reaction kinetics was, therefore, fundamentally different from the annealing in vacuum performed here. It is reasonable that the surface boron can be retained in the diamond surface during the growth process. In conclusion, depletion of surface boron from heavily boron-doped diamond films was observed using XPS and SIMS. This is also supported by the improvement in diamond surface quality after annealing, as shown by X-ray photoelectron energy loss spectroscopy and AES. In addition, the analysis by XPS revealed that the loss of surface boron affected the surface Fermi level position. As a result, the physical characteristics like Fermi level position and dopant concentration, which critically control the electronic properties of diamond films, were found to be closely related to the sample treatment. Even a rather mild anneal (500 °C ) was shown in the present study to be very influential. So, for the potential of diamond films in electronic applications to be fully realized, the response of diamond towards various treatments must be carefully characterized.
References [1] M.W. Geis, J.A. Gregory, B.B. Pate, IEEE Elect. Dev. 38 (1991) 619.
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