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Optical and electrical properties of a-C:H deposited by magnetic confinement of r.f. PECVD plasma
Diamond and Related Materials 8 (1999) 522–526
Optical and electrical properties of a-C:H deposited by magnetic confinement of r.f. PECVD plasma N. Tomozeiu, A. Hart *, B. Kleinsorge, W.I. Milne Engineering Department, Cambridge University, Cambridge, CB2 1PZ, UK Received 27 July 1998; accepted 25 November 1998
Abstract Amorphous hydrogenated carbon (a-C:H ) thin films have been deposited in an r.f. PECVD chamber using a magnetic multipole system to confine the plasma. The influence of magnetic field on both the plasma parameters and the film properties has been studied. The results are compared with those obtained under similar conditions using a standard PECVD system. Optical emission spectroscopy (OES ) shows an increase in the intensity of the hydrogen and C–H lines in the plasma. EELS, optical, electrical and electron field emission measurements have been used to characterize the deposited films. The sp3/sp2 ratio was increased using the magnetic field and the optical gap was also increased as compared to films grown using a standard process. The electron field emission was found to be improved (higher current density and smaller barrier height) for samples deposited in the presence of the magnetic field. © 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Amorphous carbon; Field emission; Magnetic confinement; Material properties
1. Introduction The optical, electrical and mechanical properties of diamond thin films have recently attracted much attention [1,2]. As a result, a variety of deposition techniques such as ion beams [3], r.f. plasma-enhanced chemical vapor deposition (PECVD) [4] and filtered cathodic vacuum arc (FCVA) [5] have been used to obtain diamond and diamond-like carbon (DLC ) films. Highly tetrahedral amorphous carbon (ta-C or "amorphous diamond") films under high compressive stress have been successfully deposited using the plasma stream from a cathodic vacuum arc [6 ]. In many applications, hydrogenated DLC films (a-C:H with high sp3 content) are used. In order to obtain such films, the most used deposition system is r.f. PECVD. In this work, we concentrate on both the plasma and film properties obtained in an r.f. PECVD system under magnetic multipole confinement conditions. Optical emission spectroscopy (OES) and d.c. bias voltage are used to observe the influence of the magnetic field on the plasma character. The carbon films were deposited onto n-type silicon and optical glass substrates at room temperature. The low-temperature deposition process * Corresponding author. Fax: +44 1223 332662.
conforms with the requirements for thin film technology. Electron energy-loss spectroscopy ( EELS ), optical transmission and reflection, electrical and field emission measurements were performed to characterize the properties of the films.
2. Experimental The hydrogenated DLC films were deposited using a 13.56 MHz capacitively coupled r.f. plasma deposition chamber from CH . A magnetic confinement of the r.f. 4 plasma, obtained with permanent magnets aligned in an alternating north pole–south pole configuration, was used to increase the plasma ionization. The magnetic multipole system configuration has been described from both theoretical and experimental points of view elsewhere [7]. A magnetic field strength of about 0.1 T near the electrode edge, decreasing exponentially towards the center of the electrode, was the radial configuration of the magnetic field. It was shown in Ref. [7] that the broadening of the energy distribution of the ions is smaller in the presence of the magnetic field. This means that more energetically uniform ions are used for deposition. Optical emission spectroscopy was used to character-
0925-9635/99/$ – see front matter © 1999 Published by Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 8 ) 0 0 44 6 - 4
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ize the plasma structure. Spectral analysis of light emitted by glow discharge is a very convenient tool for in situ control of the deposition process. The silicon and glass substrates were cleaned in an ultrasonic bath with methanol, acetone and isopropyl alcohol after a nitric acid dip. Prior to deposition, a 30-s argon plasma clean was carried out to remove the native oxide from the silicon surface. During deposition the lower powered electrode, on which the substrates were placed, was maintained at 20 °C. The pressure was 100 mtorr and the r.f. power was 120 W. Optical transmission and reflection measurements were performed for the films deposited on the glass substrates, in the range of 400–900 nm. The refractive index and the optical gap were determined. Electrical properties were measured on films deposited onto silicon substrates. Sandwich structures were fabricated by thermally evaporating aluminum contacts onto DLC surface at a pressure of 10−6 Torr. The Al contacts were evaporated through a shadow mask and the diameter of the contacts was 1 mm. A back contact was made by connecting the back of the silicon substrate to a large metal contact using highly conductive silver paint. The ohmic nature of this contact was controlled and confirmed. The field effect emission of these films was also measured using a high vacuum chamber (most measurements were performed at 1.10−7 mbar) The emission measurements were carried out in a parallel plate configuration using a PTFE spacer which was 100 mm thick, the anode being an indium tin oxide (ITO) coated glass slide and the emissive cathode being a-C:H deposited on silicon substrates. The computer program ramped the voltage from 0 to 2000 V at a rate of 1 V s−1 and then down to 0 V. This is considered as one cycle of measurement.
Fig. 1. The OES intensity lines of CH plasma with and without mag4 netic field confinement.
and maximum cross-section) [8]: CH +e−CH+H+H +e− 4 2 (10.0 eV and 0.313×10−16 cm−2)
(1a)
H +e−2H+e− (8.93 eV and 0.8978×10−16 cm−2) 2 (1b) H+e−H++e−
(13.9 eV and 6.88×10−16 cm−2) (1c)
3. Results and discussion
As was shown in Ref. [7], the voltage across the plasma sheath is decreased by a multipole magnetic field because this magnetic field acts as a reflecting barrier for charged particles and their mobilities are decreased. In such a way, the ion energy at the deposition electrode is smaller with a magnetic field, and the energy distribution broadening of ions is also smaller. We can conclude that the magnetic field gives rise to more energetically uniform ions. Some information about the structural characteristics of the deposited films can be obtained from the EELS spectra. Fig. 2 shows the spectra at low loss energy, with
Fig. 1 shows the main results of the OES measurements. From the CH plasma, the emission band of CH 4 (430 nm) and the hydrogen Blamer lines H (486 nm) b and H (656 nm) were observed. The influence of the a magnetic confinement is shown in Fig. 1. First, it must be pointed out that the emission is increased by the multipole magnetic field for all wavelengths. This is an effect of the confinement of the plasma. The higher the plasma density, the higher the number of collisions, which means a higher emission intensity. On the other hand, the increase in the C–H line intensity due to the magnetic field is different to that which corresponds to the hydrogen lines. The relative increase of the OES intensity is 60% for both lines of that atomic hydrogen, while the same amount for the C–H line is about 20%. This could be related to dissociation and ionization reactions and their parameters (the threshold energy
Fig. 2. Low-energy EELS spectra for samples deposited with and without a magnetic field.
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N. Tomozeiu et al. / Diamond and Related Materials 8 (1999) 522–526
the plasmon energy E indicated. A smaller value of this p parameter is found for samples deposited with the magnetic field, i.e. 24.25 eV in comparison with 25.44 eV for samples deposited without magnetic plasma confinement. The density r was derived from the energy of the valence plasmon E by p 앀e2n (2) E = p me 0 where n is the valence electron density, m is the electron mass and e is the permittivity of free space. 0 Approximating that four electrons come from each C and one electron comes from each H, the valence electron density is related to the mass density by 3x +1 c n= (3) M 11x +1 C c where x is the fraction of C in the film. Using Eqs. (2) C and (3), the mass density was found to be 2.22 g cm−3 for the sample deposited in the presence of the magnetic field and 2.01 g cm−3 for the sample deposited without the magnetic field. EELS measurements can be used to determine the fraction of sp2 and sp3 bonding in the film. The carbon K-edge spectrum consists of a peak at around 285 eV due to excitations from the 1s level to the empty p* states of the sp2 sites and a step at 290 eV due to excitation from the same initial level to the s* states. The fraction of the sp2 sites can be determined for each film from the peak area. Using this technique, sp2/(sp2/sp3) was found to be 22% for the sample deposited under standard conditions and 13% for the sample deposited with the magnetic field. This means that the magnetic field gives rise to more sp3 bonds in the film structure. It is known that the sp3 content of a film determines its optical and electrical properties. The higher the sp3 content, the higher the optical gap (the tails of the valence and conduction bands are narrow when the density of the p* states decreases). Also, from an electrical conductivity point of view, a greater sp2 content means high values for conductivity. One of the most important parameters is the optical gap. It has been defined in many ways, and the most common definition uses the photon energy, where the absorption coefficient is 104 cm−1 (also known as the E gap). In order to determine this, the transmission 04 and reflection spectra have been measured, and the spectral distribution of the absorption coefficient is present in Fig. 3. The E optical gap is 2.15 eV for 04 samples deposited under standard conditions and 2.52 eV for samples deposited from magnetically confined plasmas. These values are in good agreement with the trend for the sp3 content. It is known that amorphous materials are characterized by defect states (disorder and dangling bonds) contributing to both band tails r
Fig. 3. Absorption coefficient spectra and optical gap E . 04
and to localized states around the Fermi level in the band gap. In such way, the energy gap (also known as the ‘‘mobility gap’’) is different from the true gap of the crystalline material. The optical gap of the DLC films is determined by the separation between the s and s* bands created by s electrons. If the sp2 fraction is higher, the p bands close the gap. Moreover, the local configurations of the sp2 sites (isolated bonds, pair sites or large clusters) modify the optical gap of the film. Theoretical models for ta-C with a high sp3 content predict that the optical gap depends on the nature of the sp2-bonded atoms [9]. In our case, the sp3 content was modified by the magnetic confinement: the results of the EELS measurements are relevant. The electrical properties of the DLC films were studied using the I–V characteristics of the DLC Si heterojunction diode. Typical current–voltage characteristics are shown in Fig. 4 for films deposited under standard PECVD and using magnetic confinement. The rectifying ratio of the studied structures was not so high (only one order of magnitude) from the I–V characteristics. The
Fig. 4. I–V characteristics of an Al/DLC/Si n-type/Al diode structure.
N. Tomozeiu et al. / Diamond and Related Materials 8 (1999) 522–526
resistivity values of the carbon layers were found to be around 3×109 and 8×1010 V · cm for samples deposited without and with magnets, respectively. The high resistivity values for samples deposited from confined plasmas are related to the high value of the sp3 content in the film. This corresponds to materials with high bandgap values. Field emission from DLC is generally easier than from diamond. Conducting grain boundaries were invoked for field emission from polycrystalline diamond [10]. It is known that hydrogen absorption on the surface creates negative electron affinity spots on the surface of the films. The results of the field emission measurements are presented in Fig. 5. First, we can observe a higher value of the emitted current density (around one order of magnitude) for a sample deposited with magnetic confinement. Secondly, this sample started to emit after 20 cycles of measurements, whereas the sample deposited under standard conditions took 34 cycles of measurements before any emission occurred. The field emission current density for both types of sample obeys the Fowler–Nordheim equation: J=aE2 exp
A B bW3/2 bE
(4)
where E is the applied field, W is the barrier height, b is a dimensionless geometric field enhancement factor, and a and b are constants (b=6.8×109). Fig. 6 shows the Fowler–Nordheim plots which allow us to determine the barrier height W. The values obtained are 58 meV for the sample deposited without magnets and 46 meV for the sample deposited with magnetic confinement, indicating that the improved emission may be due to the lower barrier height. Robertson [11] has shown that the electron field emission from DLC occurs because of surface groups such as C–H, which can produce changes in electron affinity. The electric field from the anode is focused
525
Fig. 6. Fowler–Nordheim plots and barrier high values.
towards surface areas with high affinity. In these regions, a downwards band-bending and very large field result, which cause Fowler–Nordheim emission. If we take into account the higher value of the C–H intensity line in the OES spectra for plasmas confined with a multipole magnetic field (considering that in the deposited film there should normally be a similar situation), the better emission from DLC deposited with a magnetic field may arise from a higher fraction of surface hydrogen.
4. Conclusion It was shown that using a multipole magnetic confinement of the r.f. PECVD plasma, high-quality hydrogenated amorphous carbon can be obtained. The diamond-like characteristics of the films obtained under such conditions (high optical gap, high resistivity, high mass density) is explained by a high content of sp3 sites. The electron field emission is higher for samples deposited with the magnetic field. This is correlated with a higher intensity line of C–H groups in the spectrum of the magnetically confined plasma.
Acknowledgements Thanks are due to Dr. J. Robertson for helpful discussions. N.T. would like to thank the Royal Society–NATO for financial support during his stay at the Engineering Department of Cambridge University.
References
Fig. 5. Field emission: J–E curves for samples deposited with and without a magnetic field.
[1] D.R. McKenzie, Rep. Prog. Phys. 59 (1996) 1611. [2] V.S. Veerasamy, G.A.J. Amaratunga, W.I. Milne, P. Hewitt, P.J. Fallon, D.R. McKenzie, C.A. Davis, Diamond Relat. Mater. 2 (1993) 782. [3] S. Aisenberg, R. Chabot, J. Appl. Phys. 42 (1971) 2953.
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N. Tomozeiu et al. / Diamond and Related Materials 8 (1999) 522–526
[4] Z. Has, S. Mitura, M. Clapa, J. Szmidt, Thin Solid Films 136 (1985) 161. [5] M.M.M. Bilek, Y. Yin, D.R. McKenzie, IEEE Trans. Plasma Sci. 24 (3) (1996) 1165. [6 ] D.R. McKenzie, D. Muller, B.A. Pailthorpe, Phys. Rev. Lett. 67 (1991) 773. [7] N. Tomozeiu, W.I. Milne, J. Non-Cryst. Solids, to be published.
[8] H. Tahara, K. Minami, A. Murai, T. Yasui, T. Yoshikawa, Jpn. J. Appl. Phys. 34 (1995) 1972. [9] G. Jungnickel, Th. Kohler, M. Haase, Th. Frauenheim, P. Blaudeck, U. Stephen, Diamond Relat. Mater. 5 (1996) 175. [10] M.W. Geis, J.C. Twichell, J. Macaulay, K. Okano, Appl. Phys. Lett. 67 (1995) 1328. [11] J. Robertson, Diamond Relat. Mater. 6 (1997) 212.
Optical and electrical properties of a-C:H deposited by magnetic confinement of r.f. PECVD plasma N. Tomozeiu, A. Hart *, B. Kleinsorge, W.I. Milne Engineering Department, Cambridge University, Cambridge, CB2 1PZ, UK Received 27 July 1998; accepted 25 November 1998
Abstract Amorphous hydrogenated carbon (a-C:H ) thin films have been deposited in an r.f. PECVD chamber using a magnetic multipole system to confine the plasma. The influence of magnetic field on both the plasma parameters and the film properties has been studied. The results are compared with those obtained under similar conditions using a standard PECVD system. Optical emission spectroscopy (OES ) shows an increase in the intensity of the hydrogen and C–H lines in the plasma. EELS, optical, electrical and electron field emission measurements have been used to characterize the deposited films. The sp3/sp2 ratio was increased using the magnetic field and the optical gap was also increased as compared to films grown using a standard process. The electron field emission was found to be improved (higher current density and smaller barrier height) for samples deposited in the presence of the magnetic field. © 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Amorphous carbon; Field emission; Magnetic confinement; Material properties
1. Introduction The optical, electrical and mechanical properties of diamond thin films have recently attracted much attention [1,2]. As a result, a variety of deposition techniques such as ion beams [3], r.f. plasma-enhanced chemical vapor deposition (PECVD) [4] and filtered cathodic vacuum arc (FCVA) [5] have been used to obtain diamond and diamond-like carbon (DLC ) films. Highly tetrahedral amorphous carbon (ta-C or "amorphous diamond") films under high compressive stress have been successfully deposited using the plasma stream from a cathodic vacuum arc [6 ]. In many applications, hydrogenated DLC films (a-C:H with high sp3 content) are used. In order to obtain such films, the most used deposition system is r.f. PECVD. In this work, we concentrate on both the plasma and film properties obtained in an r.f. PECVD system under magnetic multipole confinement conditions. Optical emission spectroscopy (OES) and d.c. bias voltage are used to observe the influence of the magnetic field on the plasma character. The carbon films were deposited onto n-type silicon and optical glass substrates at room temperature. The low-temperature deposition process * Corresponding author. Fax: +44 1223 332662.
conforms with the requirements for thin film technology. Electron energy-loss spectroscopy ( EELS ), optical transmission and reflection, electrical and field emission measurements were performed to characterize the properties of the films.
2. Experimental The hydrogenated DLC films were deposited using a 13.56 MHz capacitively coupled r.f. plasma deposition chamber from CH . A magnetic confinement of the r.f. 4 plasma, obtained with permanent magnets aligned in an alternating north pole–south pole configuration, was used to increase the plasma ionization. The magnetic multipole system configuration has been described from both theoretical and experimental points of view elsewhere [7]. A magnetic field strength of about 0.1 T near the electrode edge, decreasing exponentially towards the center of the electrode, was the radial configuration of the magnetic field. It was shown in Ref. [7] that the broadening of the energy distribution of the ions is smaller in the presence of the magnetic field. This means that more energetically uniform ions are used for deposition. Optical emission spectroscopy was used to character-
0925-9635/99/$ – see front matter © 1999 Published by Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 8 ) 0 0 44 6 - 4
523
N. Tomozeiu et al. / Diamond and Related Materials 8 (1999) 522–526
ize the plasma structure. Spectral analysis of light emitted by glow discharge is a very convenient tool for in situ control of the deposition process. The silicon and glass substrates were cleaned in an ultrasonic bath with methanol, acetone and isopropyl alcohol after a nitric acid dip. Prior to deposition, a 30-s argon plasma clean was carried out to remove the native oxide from the silicon surface. During deposition the lower powered electrode, on which the substrates were placed, was maintained at 20 °C. The pressure was 100 mtorr and the r.f. power was 120 W. Optical transmission and reflection measurements were performed for the films deposited on the glass substrates, in the range of 400–900 nm. The refractive index and the optical gap were determined. Electrical properties were measured on films deposited onto silicon substrates. Sandwich structures were fabricated by thermally evaporating aluminum contacts onto DLC surface at a pressure of 10−6 Torr. The Al contacts were evaporated through a shadow mask and the diameter of the contacts was 1 mm. A back contact was made by connecting the back of the silicon substrate to a large metal contact using highly conductive silver paint. The ohmic nature of this contact was controlled and confirmed. The field effect emission of these films was also measured using a high vacuum chamber (most measurements were performed at 1.10−7 mbar) The emission measurements were carried out in a parallel plate configuration using a PTFE spacer which was 100 mm thick, the anode being an indium tin oxide (ITO) coated glass slide and the emissive cathode being a-C:H deposited on silicon substrates. The computer program ramped the voltage from 0 to 2000 V at a rate of 1 V s−1 and then down to 0 V. This is considered as one cycle of measurement.
Fig. 1. The OES intensity lines of CH plasma with and without mag4 netic field confinement.
and maximum cross-section) [8]: CH +e−CH+H+H +e− 4 2 (10.0 eV and 0.313×10−16 cm−2)
(1a)
H +e−2H+e− (8.93 eV and 0.8978×10−16 cm−2) 2 (1b) H+e−H++e−
(13.9 eV and 6.88×10−16 cm−2) (1c)
3. Results and discussion
As was shown in Ref. [7], the voltage across the plasma sheath is decreased by a multipole magnetic field because this magnetic field acts as a reflecting barrier for charged particles and their mobilities are decreased. In such a way, the ion energy at the deposition electrode is smaller with a magnetic field, and the energy distribution broadening of ions is also smaller. We can conclude that the magnetic field gives rise to more energetically uniform ions. Some information about the structural characteristics of the deposited films can be obtained from the EELS spectra. Fig. 2 shows the spectra at low loss energy, with
Fig. 1 shows the main results of the OES measurements. From the CH plasma, the emission band of CH 4 (430 nm) and the hydrogen Blamer lines H (486 nm) b and H (656 nm) were observed. The influence of the a magnetic confinement is shown in Fig. 1. First, it must be pointed out that the emission is increased by the multipole magnetic field for all wavelengths. This is an effect of the confinement of the plasma. The higher the plasma density, the higher the number of collisions, which means a higher emission intensity. On the other hand, the increase in the C–H line intensity due to the magnetic field is different to that which corresponds to the hydrogen lines. The relative increase of the OES intensity is 60% for both lines of that atomic hydrogen, while the same amount for the C–H line is about 20%. This could be related to dissociation and ionization reactions and their parameters (the threshold energy
Fig. 2. Low-energy EELS spectra for samples deposited with and without a magnetic field.
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N. Tomozeiu et al. / Diamond and Related Materials 8 (1999) 522–526
the plasmon energy E indicated. A smaller value of this p parameter is found for samples deposited with the magnetic field, i.e. 24.25 eV in comparison with 25.44 eV for samples deposited without magnetic plasma confinement. The density r was derived from the energy of the valence plasmon E by p 앀e2n (2) E = p me 0 where n is the valence electron density, m is the electron mass and e is the permittivity of free space. 0 Approximating that four electrons come from each C and one electron comes from each H, the valence electron density is related to the mass density by 3x +1 c n= (3) M 11x +1 C c where x is the fraction of C in the film. Using Eqs. (2) C and (3), the mass density was found to be 2.22 g cm−3 for the sample deposited in the presence of the magnetic field and 2.01 g cm−3 for the sample deposited without the magnetic field. EELS measurements can be used to determine the fraction of sp2 and sp3 bonding in the film. The carbon K-edge spectrum consists of a peak at around 285 eV due to excitations from the 1s level to the empty p* states of the sp2 sites and a step at 290 eV due to excitation from the same initial level to the s* states. The fraction of the sp2 sites can be determined for each film from the peak area. Using this technique, sp2/(sp2/sp3) was found to be 22% for the sample deposited under standard conditions and 13% for the sample deposited with the magnetic field. This means that the magnetic field gives rise to more sp3 bonds in the film structure. It is known that the sp3 content of a film determines its optical and electrical properties. The higher the sp3 content, the higher the optical gap (the tails of the valence and conduction bands are narrow when the density of the p* states decreases). Also, from an electrical conductivity point of view, a greater sp2 content means high values for conductivity. One of the most important parameters is the optical gap. It has been defined in many ways, and the most common definition uses the photon energy, where the absorption coefficient is 104 cm−1 (also known as the E gap). In order to determine this, the transmission 04 and reflection spectra have been measured, and the spectral distribution of the absorption coefficient is present in Fig. 3. The E optical gap is 2.15 eV for 04 samples deposited under standard conditions and 2.52 eV for samples deposited from magnetically confined plasmas. These values are in good agreement with the trend for the sp3 content. It is known that amorphous materials are characterized by defect states (disorder and dangling bonds) contributing to both band tails r
Fig. 3. Absorption coefficient spectra and optical gap E . 04
and to localized states around the Fermi level in the band gap. In such way, the energy gap (also known as the ‘‘mobility gap’’) is different from the true gap of the crystalline material. The optical gap of the DLC films is determined by the separation between the s and s* bands created by s electrons. If the sp2 fraction is higher, the p bands close the gap. Moreover, the local configurations of the sp2 sites (isolated bonds, pair sites or large clusters) modify the optical gap of the film. Theoretical models for ta-C with a high sp3 content predict that the optical gap depends on the nature of the sp2-bonded atoms [9]. In our case, the sp3 content was modified by the magnetic confinement: the results of the EELS measurements are relevant. The electrical properties of the DLC films were studied using the I–V characteristics of the DLC Si heterojunction diode. Typical current–voltage characteristics are shown in Fig. 4 for films deposited under standard PECVD and using magnetic confinement. The rectifying ratio of the studied structures was not so high (only one order of magnitude) from the I–V characteristics. The
Fig. 4. I–V characteristics of an Al/DLC/Si n-type/Al diode structure.
N. Tomozeiu et al. / Diamond and Related Materials 8 (1999) 522–526
resistivity values of the carbon layers were found to be around 3×109 and 8×1010 V · cm for samples deposited without and with magnets, respectively. The high resistivity values for samples deposited from confined plasmas are related to the high value of the sp3 content in the film. This corresponds to materials with high bandgap values. Field emission from DLC is generally easier than from diamond. Conducting grain boundaries were invoked for field emission from polycrystalline diamond [10]. It is known that hydrogen absorption on the surface creates negative electron affinity spots on the surface of the films. The results of the field emission measurements are presented in Fig. 5. First, we can observe a higher value of the emitted current density (around one order of magnitude) for a sample deposited with magnetic confinement. Secondly, this sample started to emit after 20 cycles of measurements, whereas the sample deposited under standard conditions took 34 cycles of measurements before any emission occurred. The field emission current density for both types of sample obeys the Fowler–Nordheim equation: J=aE2 exp
A B bW3/2 bE
(4)
where E is the applied field, W is the barrier height, b is a dimensionless geometric field enhancement factor, and a and b are constants (b=6.8×109). Fig. 6 shows the Fowler–Nordheim plots which allow us to determine the barrier height W. The values obtained are 58 meV for the sample deposited without magnets and 46 meV for the sample deposited with magnetic confinement, indicating that the improved emission may be due to the lower barrier height. Robertson [11] has shown that the electron field emission from DLC occurs because of surface groups such as C–H, which can produce changes in electron affinity. The electric field from the anode is focused
525
Fig. 6. Fowler–Nordheim plots and barrier high values.
towards surface areas with high affinity. In these regions, a downwards band-bending and very large field result, which cause Fowler–Nordheim emission. If we take into account the higher value of the C–H intensity line in the OES spectra for plasmas confined with a multipole magnetic field (considering that in the deposited film there should normally be a similar situation), the better emission from DLC deposited with a magnetic field may arise from a higher fraction of surface hydrogen.
4. Conclusion It was shown that using a multipole magnetic confinement of the r.f. PECVD plasma, high-quality hydrogenated amorphous carbon can be obtained. The diamond-like characteristics of the films obtained under such conditions (high optical gap, high resistivity, high mass density) is explained by a high content of sp3 sites. The electron field emission is higher for samples deposited with the magnetic field. This is correlated with a higher intensity line of C–H groups in the spectrum of the magnetically confined plasma.
Acknowledgements Thanks are due to Dr. J. Robertson for helpful discussions. N.T. would like to thank the Royal Society–NATO for financial support during his stay at the Engineering Department of Cambridge University.
References
Fig. 5. Field emission: J–E curves for samples deposited with and without a magnetic field.
[1] D.R. McKenzie, Rep. Prog. Phys. 59 (1996) 1611. [2] V.S. Veerasamy, G.A.J. Amaratunga, W.I. Milne, P. Hewitt, P.J. Fallon, D.R. McKenzie, C.A. Davis, Diamond Relat. Mater. 2 (1993) 782. [3] S. Aisenberg, R. Chabot, J. Appl. Phys. 42 (1971) 2953.
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N. Tomozeiu et al. / Diamond and Related Materials 8 (1999) 522–526
[4] Z. Has, S. Mitura, M. Clapa, J. Szmidt, Thin Solid Films 136 (1985) 161. [5] M.M.M. Bilek, Y. Yin, D.R. McKenzie, IEEE Trans. Plasma Sci. 24 (3) (1996) 1165. [6 ] D.R. McKenzie, D. Muller, B.A. Pailthorpe, Phys. Rev. Lett. 67 (1991) 773. [7] N. Tomozeiu, W.I. Milne, J. Non-Cryst. Solids, to be published.
[8] H. Tahara, K. Minami, A. Murai, T. Yasui, T. Yoshikawa, Jpn. J. Appl. Phys. 34 (1995) 1972. [9] G. Jungnickel, Th. Kohler, M. Haase, Th. Frauenheim, P. Blaudeck, U. Stephen, Diamond Relat. Mater. 5 (1996) 175. [10] M.W. Geis, J.C. Twichell, J. Macaulay, K. Okano, Appl. Phys. Lett. 67 (1995) 1328. [11] J. Robertson, Diamond Relat. Mater. 6 (1997) 212.