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Growth of nanocrystalline diamond films for low field electron emission
Diamond and Related Materials 8 (1999) 768–771
Growth of nanocrystalline diamond films for low field electron emission Simon S. Proffitt a, Simon J. Probert a, Michael D Whitfield b, John S. Foord a, Richard B. Jackman b,* a Physical and Theoretical Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QZ, UK b Electronic and Electrical Engineering, University College London, Torrington Place, London, WC1E 7JE, UK Received 23 July 1998; accepted 4 November 1998
Abstract Nanocrystalline diamond films have been prepared using a magnetically enhanced RF assisted plasma chemical vapour deposition (CVD) source. Such films show field emission at applied fields below 10 V mm−1. Similar results are obtained using methane–hydrogen and methane–nitrogen gas mixtures, suggesting that the nitrogen promoted enhancement in field emission that has been observed in high quality diamond films does not occur for nanocrystalline layers. The design of the source used is easily scaled up for large area deposition, suggesting that this could be a useful approach for the preparation of nanocrystalline diamond films for practical field emission purposes. © 1999 Elsevier Science S.A. All rights reserved.
1. Introduction It is now widely recognised that the property of negative electron affinity (NEA) that can be displayed by diamond does not alone make this material ideal for low field electron emission applications. Few electrons exist in the conduction band of diamond, which is difficult to dope n-type, making transport of electrons from metallic contacts and emission of electrons from the diamond surface difficult [1]. Thus, highly crystalline diamond films make relatively poor field emitters, with defects and damage sites being thought responsible for much of the emission that is seen [2–4]. However, fine grain, highly defective diamond films have shown more promise, presumably due to the presence of a network of grain boundaries and a higher level of non-diamond carbon which can increase the material’s conductivity. These films, often called nanocrystalline diamond, are now being investigated by a number of teams [5,6 ]. The generation of films with reliable properties over large areas remains difficult and it is this which is the central aim of the current study. Microwave plasma enhanced chemical vapour deposition (MWPECVD) is the pre-eminent method for growing high quality diamond films. However, the nature of * Corresponding author. Fax: +44 171 388 9325; e-mail: [email protected]
the plasma makes it difficult to produce large area homogenous films with this technique. Capacitively coupled radio-frequency (RF ) plasma sources are more readily adapted to large area film growth. A typical system would use two parallel plate electrodes, one powered and the other earthed, with the substrate placed upon it, with the plasma formed between them. An oscillating (RF ) field causes the formation of a plasma where comparatively large energies can be acquired by the charged particles within it. Electrons are much more mobile than ions within the plasma and leave more readily; thus a charged region or ‘‘sheath’’ forms between the plasma and the ground electrode. This gives rise to ion energies at the substrate surface of >100 eV and is the principle reason that such a source is, in fact, a poor choice for the growth of crystalline diamond. Inductively coupled RF plasma sources generate lower sheath potentials, but are difficult to scale up for large area film deposition and suffer power losses as high as 90%. In this paper, we describe films grown with a magnetically enhanced capacitively coupled RF plasma source. Attempts to grow high quality diamond films with such a source have been only partially successful [7,8], but fine grain, nanocrystalline diamond can be routinely produced from methane–hydrogen or methane–nitrogen gas feedstocks. These films have been effective during field emission trials and the approach appears to offer a promising route towards large area homogenous films for this application.
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 42 4 - 5
S.S. Proffitt et al. / Diamond and Related Materials 8 (1999) 768–771
2. Experimental methods The capacitively coupled plasma source used is magnetically enhanced, with a magnetic field of a few hundred gauss parallel to the surface of the electrodes, such that the mobility of the electrons in the direction of the electric field is radically reduced by their Larmor precession, whilst that of the ions is virtually unaffected. This means that current equilibrium takes place at a lower self-bias voltage, i.e. the sheath potential at the electrode is reduced for a given power [7]. The design of the source is such that the growth substrate is not one of the electrodes, which are instead formed from ‘‘rings’’ encasing the plasma volume, and hence only low energy ion impact on the substrate occurs. This favours the growth of crystalline diamond over amorphous forms of carbon which are more readily produced at the energies usually associated with RF plasma sources. Moreover, the magnetic field considerably reduces the loss of the bulk electrons from the plasma, leading to higher plasma densities, closer to those found in microwave stimulated plasmas. The source was placed within a turbo-molecular pumped vacuum system (base pressure 5×10−6 Torr); at a pressure of 5×10−4 Torr with an RF power of up to 300 W, electrical measurements indicated the presence of a plasma of high density (n ~1011 cm−3), a low ion e energy (10–20 eV ) and an electron energy of 6 eV, which compares well with a microwave plasma. For the feed gases of 1% methane in hydrogen or nitrogen which were used, a discharge could be sustained at pressures of 10−4–20 Torr. Silicon substrates were used for all growth experiments which were placed approximately 40 mm from the end of the last ring electrode on a heated plate. For field emission tests, samples were mounted within a purpose-built holder with the silicon side of each pressed against a tantalum cathode. The edges of each sample were masked with an insulating polythene sheet which defined the field emission area. A flat tantalum plate was placed 15 mm from the surface to act as the anode and the entire device evacuated to <5×10−9 Torr. I–V characteristics were recorded using a Keithley 417 picoammeter and voltage source. Raman spectroscopy was performed with a Renishaw system 2000 with (red ) He–Ne laser light. X-ray photoelectron spectroscopy ( XPS) was carried out using a VG ESCALAB II, with Al K (1486 eV ) radiation. a
769
gas mixtures were used. The nature of the deposited films were relatively insensitive to the growth conditions, apart from an increase in film thickness of 1–5 mm as the growth time increased. A typical SEM image of a grown film is shown in Fig. 1; it can be seen to comprise fine grain (<1 mm) material with no clearly defined crystal faceting. Following prolonged use as field emitters, many films were visibly damaged, with pits appearing as shown in Fig. 1 (5–10 mm in diameter). Raman spectra are illustrated in Fig. 2a; the expected diamond band at 1330 cm−1 is significantly broadened, which can be attributed to the low quality diamond present and the small grain size [9]. This peak cannot be attributed to a graphitic phase which would be expected to appear at around 1350 cm−1 [9,10]. However, a peak is visible at around 1600 cm−1 which is likely to be due to a graphitic component within the film (the so-called G mode [9,10]). Whilst this band is often observed alongside the graphitic D band at around 1350 cm−1 within graphite films, it has also been seen in a number of studies of diamond films where the diamond partcilce size is small [5,6,10]. It is also worth noting that (red ) He–Ne laser light is being used to stimulate Raman spectra in the current study, which is known to significantly enhance the graphitic contributuion to the spectra compared with the more usually deployed green and blue forms of Raman analysis. XPS spectra of the C1s spectral region (Fig. 2b) were consistent with the rather disordered nature of the deposited film, showing a featureless background in the 0–40 eV energy loss region, where characteristic peaks due to graphite ( line 1), SiC ( line 2) or diamond ( line 3) are normally observed. Interestingly, no significant differences were apparent between films grown from methane–hydrogen and methane–nitrogen gas mixtures. Typical field emission data from a grown sample are
3. Results All samples were grown using an RF dissipated plasma power of 180 W, with gas pressures in the range 2.5–5 Torr and a silicon substrate temperature of 750–850 °C. Growth runs between 5 and 20 h were employed and methane in both hydrogen and nitrogen
Fig. 1. SEM image of a typical nanocrystalline diamond film grown during the present study, revealing poorly faceted, fine grain material; following prolonged use as a field emission source, pits often emerged.
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S.S. Proffitt et al. / Diamond and Related Materials 8 (1999) 768–771
(a)
emission is maintained at much lower applied fields below 6 V mm−1. Once ‘‘activated’’ in this way, the sample continued to show field emission at these low applied fields, during upward and downward ramping of the applied electric field, so the changes introduced during the activation process are apparently irreversible. Samples were prepared with varying process parameters (substrate temperature, gas pressure, gas mixture, RF power) to investigate their influence on the field emission properties. However, no clear trends were observed in the films that were tested, all showing initial field emission at applied fields above 10 V mm−1, a value which always reduced as a result of the ‘‘activation’’ processes. However, variations in the long term performance of the various films was apparent with some films remaining active over prolonged periods (a number of days), whilst others degraded after several measurements. Although clear trends were difficult to identify, degenerated films often revealed pitting when studied in SEM, as shown in Fig. 1.
4. Discussion (b) Fig. 2. (a) Raman spectrum [Renishaw system 2000 with (red) He–Ne light] typical of the nanocrystalline films grown with methane–hydrogen (A) and methane–nitrogen (B) gas mixtures, revealing a very broad diamond peak at 1330 cm−1, and a strong graphitic component at 1600 cm−1. (b) XPS spectra of the C1s spectral region, showing a featureless background in the 0–40 eV energy loss region for various samples (a–e), where characteristic peaks due to graphite ( line 1), SiC ( line 2) or diamond ( line 3) are expected.
shown in Fig. 3. Little field emission is seen until the applied field is raised to around 15 V mm−1, after which a rapid increase in the field emission current is apparent. An increase in chamber pressure is observed during this process, and irreversible changes to the emitting sample occur since reduction of the applied field shows field
Fig. 3. Typical low field electron emission curves for the nanocrystalline diamond films grown here.
Low field emission from high quality diamond films is known to be limited in part by the difficulty in injecting electrons from the back contact into the conduction band of the diamond. Hence, nitrogen doping is thought to promote field emission through the downward band bending this induces at the metal–diamond interface, as a result of the deep donor states nitrogen introduces in the band gap [11]. The difficulty in achieving reproducible n-type doping in diamond is, however, a disadvantage to this approach. Although reliable p-type doping can be achieved with boron, greatly increasing film conductivity, such defects do little to enhance field emission, except possibly via the structural defects which boron introduces [12]. It is therefore desirable to seek an approach to low field emission which achieves the requisite electronic properties in the deposited diamond film without the need to achieve n-type impurity doping. The current work shows that one approach to this is to utilise nanocrystalline films grown in a magnetically enhanced RF plasma enhanced CVD source. The mechanism by which field emission occurs from such nanocrystalline phases is not yet established. Presumably the defects present at grain boundaries greatly enhance electron conduction from the back contact to the film surface. The exact electronic states from which emission into the vacuum takes place, the spatial location of the emission sites and the role of surface roughness on the nanoscale which these films display, remain to be resolved. However, it is feasible that the grain boundaries present at the surface also provide sites where the
S.S. Proffitt et al. / Diamond and Related Materials 8 (1999) 768–771
energy barrier for electron tunnelling into the vacuum is significantly lowered. Field emission from nanocrystalline diamond has been reported previously [5,6 ]. Such films were prepared using conventional microwave-enhanced plasma CVD, by perturbing the usual feedstock chemistry through the omission of hydrogen, or addition of fullerene, in order to move into a nanocrystalline diamond growth regime. However, MWPECVD is rather difficult to scale up to the large areas which will be required for practical applications, such as field emission-based displays; if the high quality diamond films for which MWPECVD has evolved are not required, then in technological applications it will be more profitable to move to alternative growth approaches. The source used in the present study is suitable for scale-up and hence it may represent an appropriate method for the formation of these films. Previous work [5] used nitrogen as the alternative gas dilutant to hydrogen in MWPECVD to prepare the nanocrystalline diamond. Nitrogen was incorporated into the film at significant concentrations (1020 cm−3) and it was suggested that this substitutional doping was influential in achieving the field emission properties observed. The present work found rather similar field emission properties, irrespective of whether methane– hydrogen or methane–nitrogen plasma were used. Although we have not directly measured impurity content, it is likely that some nitrogen is incorporated into the film grown using the nitrogen plasma. Since such films appeared physically similar to the films grown using the hydrogen plasmas, and performed similarly in field emission, the implication is that nitrogen does not play a significant role in controlling field emission in the present case. This is easily rationalised given the large concentration of other defects present in nanocrystalline diamond materials.
5. Concluding remarks Nanocrystalline diamond films have been prepared using a magnetically enhanced RF assisted plasma CVD
771
source. Such films show field emission at applied fields below 10 V mm−1. Similar results are obtained using methane–hydrogen and methane–nitrogen gas mixtures, suggesting that the enhancement in field emission in high quality diamond films induced by the presence of nitrogen does not occur for nanocrystalline layers. The design of the source used is easily scaled up for large area deposition, suggesting this could be a useful approach for the preparation of nanocrystalline diamond films for field emission purposes.
Acknowledgement DERA (Malvern) are gratefully acknowledged for access to Raman facilities.
References [1] P.K. Baumann, R.J. Nemanich, Diamond Relat. Mater. 7 (1998) 612. [2] T. Asano, Y. Oobuchi, S. Katsumata, J. Vac. Sci. Technol. B13 (1995) 431. [3] B. Baral, J.S. Foord, R.B. Jackman, Diamond Relat. Mater. 6 (1997) 869. [4] A. Gohl, T. Habermann, G. Muller, D. Nau, M. Wedel, M. Christ, M. Schreck, B. Stritzker, Diamond Relat. Mater. 7 (1998) 666. [5] D. Zhou, A.R. Krauss, L.C. Qin, D.M. Gruen, J. Appl. Phys. 82 (1997) 4547. [6 ] D. Zhou, A.R. Krauss, T.D. Corrigan, T.G. McCauley, R.P.H. Chang, D.M. Gruen, J. Electrochem Soc. 144 (1997) L224. [7] J. Beckmann, R.B. Jackman, J.S. Foord, Diamond Relat. Mater. 3 (1994) 602. [8] R.B. Jackman, J. Beckman, J.S. Foord, Diamond Relat. Mater. 4 (1995) 735. [9] D.S. Knight, W.B. White, J. Mater. Res. 4 (1989) 385. [10] W. Kalss, R. Haubner, G. Lippold, B. Lux, Diamond Relat. Mater. 7 (1998) 158. [11] K. Okano, S. Koizumi, S.R.P. Silva, G.A.J. Armaratunga, Nature 381 (1996) 140. [12] W. Zhu, G.P. Kochaanski, S. Jin, L. Seibles, D.C. Jacobsen, M. McCormack, A.E. White, Appl. Phys. Lett. 67 (1995) 1057.
Growth of nanocrystalline diamond films for low field electron emission Simon S. Proffitt a, Simon J. Probert a, Michael D Whitfield b, John S. Foord a, Richard B. Jackman b,* a Physical and Theoretical Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QZ, UK b Electronic and Electrical Engineering, University College London, Torrington Place, London, WC1E 7JE, UK Received 23 July 1998; accepted 4 November 1998
Abstract Nanocrystalline diamond films have been prepared using a magnetically enhanced RF assisted plasma chemical vapour deposition (CVD) source. Such films show field emission at applied fields below 10 V mm−1. Similar results are obtained using methane–hydrogen and methane–nitrogen gas mixtures, suggesting that the nitrogen promoted enhancement in field emission that has been observed in high quality diamond films does not occur for nanocrystalline layers. The design of the source used is easily scaled up for large area deposition, suggesting that this could be a useful approach for the preparation of nanocrystalline diamond films for practical field emission purposes. © 1999 Elsevier Science S.A. All rights reserved.
1. Introduction It is now widely recognised that the property of negative electron affinity (NEA) that can be displayed by diamond does not alone make this material ideal for low field electron emission applications. Few electrons exist in the conduction band of diamond, which is difficult to dope n-type, making transport of electrons from metallic contacts and emission of electrons from the diamond surface difficult [1]. Thus, highly crystalline diamond films make relatively poor field emitters, with defects and damage sites being thought responsible for much of the emission that is seen [2–4]. However, fine grain, highly defective diamond films have shown more promise, presumably due to the presence of a network of grain boundaries and a higher level of non-diamond carbon which can increase the material’s conductivity. These films, often called nanocrystalline diamond, are now being investigated by a number of teams [5,6 ]. The generation of films with reliable properties over large areas remains difficult and it is this which is the central aim of the current study. Microwave plasma enhanced chemical vapour deposition (MWPECVD) is the pre-eminent method for growing high quality diamond films. However, the nature of * Corresponding author. Fax: +44 171 388 9325; e-mail: [email protected]
the plasma makes it difficult to produce large area homogenous films with this technique. Capacitively coupled radio-frequency (RF ) plasma sources are more readily adapted to large area film growth. A typical system would use two parallel plate electrodes, one powered and the other earthed, with the substrate placed upon it, with the plasma formed between them. An oscillating (RF ) field causes the formation of a plasma where comparatively large energies can be acquired by the charged particles within it. Electrons are much more mobile than ions within the plasma and leave more readily; thus a charged region or ‘‘sheath’’ forms between the plasma and the ground electrode. This gives rise to ion energies at the substrate surface of >100 eV and is the principle reason that such a source is, in fact, a poor choice for the growth of crystalline diamond. Inductively coupled RF plasma sources generate lower sheath potentials, but are difficult to scale up for large area film deposition and suffer power losses as high as 90%. In this paper, we describe films grown with a magnetically enhanced capacitively coupled RF plasma source. Attempts to grow high quality diamond films with such a source have been only partially successful [7,8], but fine grain, nanocrystalline diamond can be routinely produced from methane–hydrogen or methane–nitrogen gas feedstocks. These films have been effective during field emission trials and the approach appears to offer a promising route towards large area homogenous films for this application.
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 42 4 - 5
S.S. Proffitt et al. / Diamond and Related Materials 8 (1999) 768–771
2. Experimental methods The capacitively coupled plasma source used is magnetically enhanced, with a magnetic field of a few hundred gauss parallel to the surface of the electrodes, such that the mobility of the electrons in the direction of the electric field is radically reduced by their Larmor precession, whilst that of the ions is virtually unaffected. This means that current equilibrium takes place at a lower self-bias voltage, i.e. the sheath potential at the electrode is reduced for a given power [7]. The design of the source is such that the growth substrate is not one of the electrodes, which are instead formed from ‘‘rings’’ encasing the plasma volume, and hence only low energy ion impact on the substrate occurs. This favours the growth of crystalline diamond over amorphous forms of carbon which are more readily produced at the energies usually associated with RF plasma sources. Moreover, the magnetic field considerably reduces the loss of the bulk electrons from the plasma, leading to higher plasma densities, closer to those found in microwave stimulated plasmas. The source was placed within a turbo-molecular pumped vacuum system (base pressure 5×10−6 Torr); at a pressure of 5×10−4 Torr with an RF power of up to 300 W, electrical measurements indicated the presence of a plasma of high density (n ~1011 cm−3), a low ion e energy (10–20 eV ) and an electron energy of 6 eV, which compares well with a microwave plasma. For the feed gases of 1% methane in hydrogen or nitrogen which were used, a discharge could be sustained at pressures of 10−4–20 Torr. Silicon substrates were used for all growth experiments which were placed approximately 40 mm from the end of the last ring electrode on a heated plate. For field emission tests, samples were mounted within a purpose-built holder with the silicon side of each pressed against a tantalum cathode. The edges of each sample were masked with an insulating polythene sheet which defined the field emission area. A flat tantalum plate was placed 15 mm from the surface to act as the anode and the entire device evacuated to <5×10−9 Torr. I–V characteristics were recorded using a Keithley 417 picoammeter and voltage source. Raman spectroscopy was performed with a Renishaw system 2000 with (red ) He–Ne laser light. X-ray photoelectron spectroscopy ( XPS) was carried out using a VG ESCALAB II, with Al K (1486 eV ) radiation. a
769
gas mixtures were used. The nature of the deposited films were relatively insensitive to the growth conditions, apart from an increase in film thickness of 1–5 mm as the growth time increased. A typical SEM image of a grown film is shown in Fig. 1; it can be seen to comprise fine grain (<1 mm) material with no clearly defined crystal faceting. Following prolonged use as field emitters, many films were visibly damaged, with pits appearing as shown in Fig. 1 (5–10 mm in diameter). Raman spectra are illustrated in Fig. 2a; the expected diamond band at 1330 cm−1 is significantly broadened, which can be attributed to the low quality diamond present and the small grain size [9]. This peak cannot be attributed to a graphitic phase which would be expected to appear at around 1350 cm−1 [9,10]. However, a peak is visible at around 1600 cm−1 which is likely to be due to a graphitic component within the film (the so-called G mode [9,10]). Whilst this band is often observed alongside the graphitic D band at around 1350 cm−1 within graphite films, it has also been seen in a number of studies of diamond films where the diamond partcilce size is small [5,6,10]. It is also worth noting that (red ) He–Ne laser light is being used to stimulate Raman spectra in the current study, which is known to significantly enhance the graphitic contributuion to the spectra compared with the more usually deployed green and blue forms of Raman analysis. XPS spectra of the C1s spectral region (Fig. 2b) were consistent with the rather disordered nature of the deposited film, showing a featureless background in the 0–40 eV energy loss region, where characteristic peaks due to graphite ( line 1), SiC ( line 2) or diamond ( line 3) are normally observed. Interestingly, no significant differences were apparent between films grown from methane–hydrogen and methane–nitrogen gas mixtures. Typical field emission data from a grown sample are
3. Results All samples were grown using an RF dissipated plasma power of 180 W, with gas pressures in the range 2.5–5 Torr and a silicon substrate temperature of 750–850 °C. Growth runs between 5 and 20 h were employed and methane in both hydrogen and nitrogen
Fig. 1. SEM image of a typical nanocrystalline diamond film grown during the present study, revealing poorly faceted, fine grain material; following prolonged use as a field emission source, pits often emerged.
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S.S. Proffitt et al. / Diamond and Related Materials 8 (1999) 768–771
(a)
emission is maintained at much lower applied fields below 6 V mm−1. Once ‘‘activated’’ in this way, the sample continued to show field emission at these low applied fields, during upward and downward ramping of the applied electric field, so the changes introduced during the activation process are apparently irreversible. Samples were prepared with varying process parameters (substrate temperature, gas pressure, gas mixture, RF power) to investigate their influence on the field emission properties. However, no clear trends were observed in the films that were tested, all showing initial field emission at applied fields above 10 V mm−1, a value which always reduced as a result of the ‘‘activation’’ processes. However, variations in the long term performance of the various films was apparent with some films remaining active over prolonged periods (a number of days), whilst others degraded after several measurements. Although clear trends were difficult to identify, degenerated films often revealed pitting when studied in SEM, as shown in Fig. 1.
4. Discussion (b) Fig. 2. (a) Raman spectrum [Renishaw system 2000 with (red) He–Ne light] typical of the nanocrystalline films grown with methane–hydrogen (A) and methane–nitrogen (B) gas mixtures, revealing a very broad diamond peak at 1330 cm−1, and a strong graphitic component at 1600 cm−1. (b) XPS spectra of the C1s spectral region, showing a featureless background in the 0–40 eV energy loss region for various samples (a–e), where characteristic peaks due to graphite ( line 1), SiC ( line 2) or diamond ( line 3) are expected.
shown in Fig. 3. Little field emission is seen until the applied field is raised to around 15 V mm−1, after which a rapid increase in the field emission current is apparent. An increase in chamber pressure is observed during this process, and irreversible changes to the emitting sample occur since reduction of the applied field shows field
Fig. 3. Typical low field electron emission curves for the nanocrystalline diamond films grown here.
Low field emission from high quality diamond films is known to be limited in part by the difficulty in injecting electrons from the back contact into the conduction band of the diamond. Hence, nitrogen doping is thought to promote field emission through the downward band bending this induces at the metal–diamond interface, as a result of the deep donor states nitrogen introduces in the band gap [11]. The difficulty in achieving reproducible n-type doping in diamond is, however, a disadvantage to this approach. Although reliable p-type doping can be achieved with boron, greatly increasing film conductivity, such defects do little to enhance field emission, except possibly via the structural defects which boron introduces [12]. It is therefore desirable to seek an approach to low field emission which achieves the requisite electronic properties in the deposited diamond film without the need to achieve n-type impurity doping. The current work shows that one approach to this is to utilise nanocrystalline films grown in a magnetically enhanced RF plasma enhanced CVD source. The mechanism by which field emission occurs from such nanocrystalline phases is not yet established. Presumably the defects present at grain boundaries greatly enhance electron conduction from the back contact to the film surface. The exact electronic states from which emission into the vacuum takes place, the spatial location of the emission sites and the role of surface roughness on the nanoscale which these films display, remain to be resolved. However, it is feasible that the grain boundaries present at the surface also provide sites where the
S.S. Proffitt et al. / Diamond and Related Materials 8 (1999) 768–771
energy barrier for electron tunnelling into the vacuum is significantly lowered. Field emission from nanocrystalline diamond has been reported previously [5,6 ]. Such films were prepared using conventional microwave-enhanced plasma CVD, by perturbing the usual feedstock chemistry through the omission of hydrogen, or addition of fullerene, in order to move into a nanocrystalline diamond growth regime. However, MWPECVD is rather difficult to scale up to the large areas which will be required for practical applications, such as field emission-based displays; if the high quality diamond films for which MWPECVD has evolved are not required, then in technological applications it will be more profitable to move to alternative growth approaches. The source used in the present study is suitable for scale-up and hence it may represent an appropriate method for the formation of these films. Previous work [5] used nitrogen as the alternative gas dilutant to hydrogen in MWPECVD to prepare the nanocrystalline diamond. Nitrogen was incorporated into the film at significant concentrations (1020 cm−3) and it was suggested that this substitutional doping was influential in achieving the field emission properties observed. The present work found rather similar field emission properties, irrespective of whether methane– hydrogen or methane–nitrogen plasma were used. Although we have not directly measured impurity content, it is likely that some nitrogen is incorporated into the film grown using the nitrogen plasma. Since such films appeared physically similar to the films grown using the hydrogen plasmas, and performed similarly in field emission, the implication is that nitrogen does not play a significant role in controlling field emission in the present case. This is easily rationalised given the large concentration of other defects present in nanocrystalline diamond materials.
5. Concluding remarks Nanocrystalline diamond films have been prepared using a magnetically enhanced RF assisted plasma CVD
771
source. Such films show field emission at applied fields below 10 V mm−1. Similar results are obtained using methane–hydrogen and methane–nitrogen gas mixtures, suggesting that the enhancement in field emission in high quality diamond films induced by the presence of nitrogen does not occur for nanocrystalline layers. The design of the source used is easily scaled up for large area deposition, suggesting this could be a useful approach for the preparation of nanocrystalline diamond films for field emission purposes.
Acknowledgement DERA (Malvern) are gratefully acknowledged for access to Raman facilities.
References [1] P.K. Baumann, R.J. Nemanich, Diamond Relat. Mater. 7 (1998) 612. [2] T. Asano, Y. Oobuchi, S. Katsumata, J. Vac. Sci. Technol. B13 (1995) 431. [3] B. Baral, J.S. Foord, R.B. Jackman, Diamond Relat. Mater. 6 (1997) 869. [4] A. Gohl, T. Habermann, G. Muller, D. Nau, M. Wedel, M. Christ, M. Schreck, B. Stritzker, Diamond Relat. Mater. 7 (1998) 666. [5] D. Zhou, A.R. Krauss, L.C. Qin, D.M. Gruen, J. Appl. Phys. 82 (1997) 4547. [6 ] D. Zhou, A.R. Krauss, T.D. Corrigan, T.G. McCauley, R.P.H. Chang, D.M. Gruen, J. Electrochem Soc. 144 (1997) L224. [7] J. Beckmann, R.B. Jackman, J.S. Foord, Diamond Relat. Mater. 3 (1994) 602. [8] R.B. Jackman, J. Beckman, J.S. Foord, Diamond Relat. Mater. 4 (1995) 735. [9] D.S. Knight, W.B. White, J. Mater. Res. 4 (1989) 385. [10] W. Kalss, R. Haubner, G. Lippold, B. Lux, Diamond Relat. Mater. 7 (1998) 158. [11] K. Okano, S. Koizumi, S.R.P. Silva, G.A.J. Armaratunga, Nature 381 (1996) 140. [12] W. Zhu, G.P. Kochaanski, S. Jin, L. Seibles, D.C. Jacobsen, M. McCormack, A.E. White, Appl. Phys. Lett. 67 (1995) 1057.