- Email: [email protected]
Field emission from DLC films
surface science ELSEVIER
Applied Surface Science 111 (1997) 135-139
Field emission from DLC films O. GriSning *, O.M. Kiittel, P. GriSning, L. Schlapbach lnstitut fiir Physik der Universitiit, P~rolles, CH-1700 Fribourg, Switzerland Received 4 June 1996; revised 4 July 1996; accepted 15 August 1996
Abstract
Field emission measurements on diamond like carbon (DLC) films with different amounts of sp 3 and sp 2 carbon were carried out. Depending on the amount of sp 2 carbon in the film, activated and non-activated Fowler-Nordheim like emission could be observed. The emission spots were investigated using a combination of AFM and STM, by simultaneously measuring the topography and the conductivity of the samples. In the case of sp 2 rich DLC films we could observe that the emission originates from highly conducting inclusions of sp 2 carbon in a matrix of insulating sp 3 carbon. These inclusions are already existing on the sp 2 sample by the deposition process itself and are formed by the activation on the sp 3 rich sample.
1. Introduction
Since a decade carbon has been regarded as a very promising base material for the use as electron field-emission cathodes [1,2]. In recent years chemical vapor deposition of (CVD) and natural diamond [3-7] and diamond like carbon (DLC) films [8,9] have attracted great interest due to several outstanding properties including the negative electron affinity (NEA). NEA means that the vacuum level lies below the bottom of the conduction band and hence, electrons in the conduction band gain energy when leaving the surface. NEA has been demonstrated for diamond surfaces [10] and there are indications that the surface of wide band gap DLC films might also have N E A properties [11 ]. Electron field emission from carbon based mate-
* Corresponding author. Tel.: +41-26-3009068; fax: +41-263009747; e-mail: [email protected].
rial can be explained by at least two different mechanism. Emission from a N E A surface or emission from carbon structures (tips, protrusions, conducting channels) due to geometrical field enhancement. Many reports are found in the literature making NEA responsible for field emission from carbon material [12]. However, a clear proof could not be given so far. In most of the cases the Fowler-Nordheim (FN) equation [13] theoretically describes the field emission from carbon material [6]. However, to date it is not clear whether the low work function (few hundred meV) coming out of FN-plots are due to an underestimation of the field enhancement factor/3 or if it is a physically meaningful number. Very recently, we showed that emission from ion etched diamond tips with a field enhancement factor of 7 0 0 - 1 0 0 0 as determined from high resolution scanning electron microscopy (HRSEM) leads to a physically meaningful work function of 4 - 5 eV [14]. We believe that field emission from carbon material is in
0169-4332/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0 169-4332(96)0071 3-1
136
O. Gr6ning et al. / Applied Surface Science 111 (1997) 135-139
most cases rather due to field enhancement than due to very low work functions or NEA. In order to gain an even deeper insight in the emission mechanism we investigate in this work the field emission behavior of DLC films with high and low amounts of sp 2 bonded carbon. FowlerNordheim like emission in the range of 20 to 50 V / / x m could be observed for both cases. The emission spots on both types of films were investigated using a combination of AFM and STM.
the contact mode with the feedback loop regulating a constant force. While scanning the surface a bias was applied to the sample and the current flowing from the conducting tip to the sample was measured and displayed as a current-map, reflecting the local conductivity of the sample. We used silicon AFM tips with a spring constant of 0.9 N / m , which were covered by a 100 nm thick layer of highly boron doped CVD-diamond i in order to get a hard and yet conductive AFM tip.
2. Experimental
3. Results and discussion
The 100 nm thick DLC films used in this study were deposited by laser ablation on highly p-doped Si(100) wafers. By changing the deposition parameters different sp2/sp 3 ratios in the films could be obtained. We will discuss two different DLC films in this paper: AC-1, and AC-2 with low respectively high concentration of sp 2 carbon. X-ray photoelectron spectroscopy (XPS) was used to evaluate the sp2/sp 3 ratios of the samples using a VG ESCALAB 5 spectrometer with a nonmonochromatised MgK c~ radiation. Surface contamination (mostly CO) were removed by an ECR (electron cyclotron resonance) hydrogen plasma treatment at 0.003 mbar and 15 s duration. Electron field emission (EFE) measurements were carried out at a pressure of 10-8 mbar by measuring the electron current collected by a highly polished stainless steel sphere of 4 mm diameter with a Keithley 237 instrument having an incorporated high voltage supply ( + 1100 V). The sphere is mounted on a linear piezo drive which allows a reproducible and accurate positioning of the anode with respect to the sample ( + 0 . 5 /zm). An 1 G O resistance is placed in series to limit the emission current. In order to get reproducible results the I - V characteristics were measured as fast as possible, typ. in 2 - 3 s. A test measurement on a cleaned silicon wafer showed an emission current of 0.5 pA up to fields of 170 V / / z m . The electric field are computed by tracing the ratio of the voltage applied to the sample and the anode-cathode distance. The emission spots on the samples were characterized by a combination of AFM and STM. The topography of the samples was measured by AFM in
3.1. Sample AC-1 AC-I is a l o w s p 2 containing DLC film as revealed by XPS. HRSEM measurements in the back scattered electron mode as well as A F M / S T M measurements show no inclusion or inhomogeneities in the film. Fig. 1 shows a typical current-voltage characteristic of this sample. In the first voltage sweep no field emission can be observed up to an applied field of 145 V / / z m (trace 1). Then, a sudden increase of the FE current over several orders of magnitude is observed, simultaneously with a strong discharge current pulse of typically a few 100 mA lasting 50 to 200 ns as measured by an oscilloscope. The saturation of the emission current is given by the current limitation of the measuring instrument which was set to 100 nA. After this activation step a much higher current is observed and stable FE is observed as indicated by trace 2. As an inset the FE behavior after the activation for positive and negative polarity is shown. The activation consists in creating a vacuum arc discharge between anode and cathode [15,16]. Such a discharge leads to a change of the film morphology as can be seen in the AFM image of Fig. 2a. Little /~m-sized hillocks are formed. A more detailed investigation of the emission spot by a combination of A F M / S T M on a smaller scale are presented in Fig. 2b (topography) and Fig. 2c (conductivity) at the location marked by an arrow in Fig. 2a. The initially insulating and flat sp 3 DLC film
~ Coated by CSEM (Centre Suisse de Electronique et Microtechnique), Neuchatel, Switzerland.
137
O. Gr6ning et al. /Applied Surface Science 111 (1997) 135-139
,o.-I\
10 .6 _
_
Fig. 3 shows a conductivity map of this sample on a region where no field emission measurements had been performed. The A F M tip was biased with 500 mV. One can observe highly conductive spots in the film. To every spot seen in Fig. 3 a little correspond-
,o,-1\
1 0 .7 _ 10 4 ~,
10 "8-
10"14-~ i l, -40
I ' 0 40 Field [Vrtm "1]
"E 1 0 " 1 ° -h 1 0 "11 0 1 0 "12 _
a)
1 0 "la _ 10-'~4 _
-i250.0 r~ lOO
-so
o Field [Vp.m "1]
s'o IgS,0 he4
Fig. l. Emission intensity versus the applied field for sample AC- 1. The activation of the emission is nicely illustrated by curve (1) and (2). The inset shows the diode character of the field emission.
0 0aN
0
was partially converted by the arc discharge into conducting sp 2 bonded carbon. This conversion is not homogenous as illustrated in Fig. 2c. Islands of highly conducting regions are separated by regions of low conductivity. The difference in conductivity is at least 2 orders of magnitude. Such islands can only be explained by assuming the formation of conductive channels through the whole thickness of the film down to the silicon substrate. The lateral dimensions of these channels can be estimated from our A F M / S T M measurement to be as small as a few nm. Like a freestanding, conductive tip in the vacuum, a conductive channel in an insulating matrix leads to field enhancement and hence to an enhanced electron emission. A F M and H R S E M investigations showed many protruding features on the surface of the structure leading to further field enhancement. Such an explanation yields a deeper understanding of the activation process. W e have observed this kind of activated field emission also on CVD diamond films and on polymer films with an field emission threshold which is easily in the 10 V / / z m range but which reaches values as low as 3 V / / x m (for emission currents of 10 pA).
In contrast to sample A C - I , the sample AC-2 contains to a much higher extent sp 2 bonded carbon.
20.0
b)
O,On~
O
1.00
2.00
2,00
c) 5 nA
1.oo
0
3.2. Sample A C - 2
10.0
1.b0
2.bo
o
I.IN
Fig. 2. AFM/STM topography and conductivity measurements on sample AC-1, (a) topography map of the emission spot on a larger scale; (b) and (c) topography resp. conductivity measurements on the location marked by an arrow in (a).
O. GriSning et aL / Applied Surface Science 11 l (1997) 135-139
138
10
10"7- b -40 -42 "",., -44< -46 ~'-48 ~-50~" -52 -
~
1 0 .8 _
~ ""
b)
"'",
~
10-9_
i 10"'°¢~ 10 "11 10-12
_
3o 4~ 5'o 0'o Fleld [V~tm-1] '%
-541.5
10
5
0
ttm
2:0
2;5 310 3:5 Field"1[Vqem]
4;0 ".Sx~O-6
Fig. 4. Fowler-Nordheim representation of the 1 - V characteristic of the emission from sample AC-2 before (trace a) and after (trace b) rising the field to 50 V / / z m for 1 h of operation. The inset shows the 1 - V characteristics of the curves.
10 nA
Fig. 3. A F M / S T M of sample AC-2 showing conducting inclusions in an otherwise insulating matrix.
ing hillock could be identified in the topographic image measured simultaneously with the conductivity map. This spots are believed to be inclusions of sp 2 bonded carbon in the DLC film. The size of this inclusions ranges between 80 nm and 150 nm. Investigations with an ordinary light microscope showed an uniform distribution over the whole surface. We believe from our measurements that these sp 2 inclusions are as thick as the film and hence are electrically connected to the substrate. Regarding the emission properties the two films behave differently. While the film AC-I needed an activation as discussed in Section 3.1, sample AC-2 showed field emission without any pretreatment. Certainly, the emission could be improved by applying fields in the order of 50 V / / x m for some time interval, however, no vacuum discharge initiating a much higher emission could be observed. This leads us to the conclusion that sample AC-2 showed field emission without any visible activation taking place. Keeping the role of the conducting channels in mind which was discussed in Section 3.1, the emission of this film can be explained by these sp: inclusions. However, in contrast to sample AC-I where channels were formed by the activation itself, sample AC-2
did not need an activation due to the inclusions already existing in the film by the deposition process. Fig. 4 shows Fowler-Nordheim like emission measured on the sample AC-2 before (trace a) and after (trace b) rising the field to 50 V / / x m for 1 h of operation. While the slopes of the Fowler-Nordheim plot remained the same, the emitting surface increased as witnessed by the parallel shift of the plots. A careful investigation by AFM of the emitting spot after one hour of operation showed little bumps formed by the intense electric field (Fig. 5). We
2001
0 rim
21
Fig. 5. AFM picture of an emission site on sample AC-2 after 1 h of emission.
O. Gr6ning et al. / Applied Surface Science 111 (1997) 135-139
believe the electric field induced morphological changes to be responsible for the enhancement of the emitting surface. It has to be stressed out that the work function and the field enhancement factor stay constant and are not affected by the field. Measurements in a plane-parallel diode setup were performed. In this setup the anode (Si wafer) was separated from the cathode by 635 /.tm thick A120 3 insulators. With this setup a surface of 5 × 5 mm 2 is probed. Non-activated field emission of 1 /zA at 1.65 kV, corresponding to a field of 2.6 V / / z m could be observed for the sample AC-2.
4. Conclusions In summary we have investigated the field emission behavior of DLC films containing different amounts of sp 2 bonded carbon. Whereas the film containing only sp 3 bonded carbon had to be activated in order to show field emission, the film containing conductive inclusions of sp 2 bonded carbon showed strong non-activated emission. We explain our results by the fact that conducting channels are formed from the surface down to the substrate by the activation process in the sp 3 film. They act as tip-like structure leading to field enhancement. The sp 2 containing films show already such conducting channels formed during the deposition process and hence no activation could be observed. Our results are in good agreement with the work of Schmidt et al. [17], who found the emitting sites on DLC films to be micro-protrusion and inclusion in the film calling it white blobs. They believed the white blobs to be diamond inclusions in a DLC matrix and the emission mechanism being governed by the NEA properties of these diamond inclusions. Our results support the idea of inclusions being
139
responsible for the emission. However, in our case the inclusions are clearly identified as conductive sp 2 channels which through field enhancement lead to electron emission. References [1] F.S. Baker, A.R. Osborn and J. Williams, J. Phys. D: Appl. Phys. 7 (1974) 2105. [2] S. Bajic and R.V. Lantham, J. Phys. D: Appl. Phys. 21 (1988) 200. [3] M.W. Geis, N.N. Efremow, J.D. Woodhouse, M.D. McAleese, M. Marchywka, D.G. Socker and J.F. Hochedez, IEEE Lett. 12 (1991) 456. [4] C. Wang, A. Garcia, D.C. Ingram, M. Lake and M.E. Krodesch, Electron. Lett. 27 (1991) 1459. [5] N.S. Xu, Y. Tzeng and R.V. Lantham, J. Phys. D: Appl. Phys. 26 (1993) 1776. [6] D. Hong and M. Aslam, J. Vac. Sci. Technol. B 13 (1995) 427. [7] K. Okano, S. Koizumi, S. Ravi, P. Silva and G.A.J. Amaratunga, Nature 38l (1996) 140. [8] C. Xie, C.N. Potter, R.J. Fink, C. Hilbert, A. Krishnan and D. Eichman, '94 Proc. of the 7th IVMC, Grenoble, France (1994) p. 229. [9] G.A.J. Amaratunga, S.R.P. Silva, M. Sutter, W.I. Milne and J. Robertson, 6th Eur. Conf. on Diamond and Related Materials, Barcelona, Spain (1995). [10] F.J. Himpsel, J.A. Knapp and J.A. Van Vechten, Phys. Rev. B 20 (1979) 624. [11] J. Sch~ifer, J. Ristein and L. Ley, J. Non-Cryst. Solids 164 (1993) 1127. [12] M.W. Geis, J.C. Twichell, J. Macaulay and K. Okano, Appl. Phys. Lett. 67, 1328. [13] R.H. Fowler and L.W. Nordheim, Proc. R. Soc. London A 119 (1928) 173. [14] Ch. Ntitzenadel, O.M. Kiittel, O. Gr/Sning and L. Schlapbach, Appl. Phys. Lett. (1996), in press. [15] O.M. Kiittel, O GriSning, E. Schaller, L. Diederich, P. GriSning and L. Schlapbach, Diamond Relat. Mater. 5 (1996) 807. [16] O. Gr~ning, O.M. Kiittel, E. Schaller, P. Gr~Sning and L. Schlapbach, Appl. Phys. Lett. 69 (1996) 476. [17] H.K. Schmidt et al., 6th Eur. Conf. on Diamond and Related Materials, Barcelona, Spain (1995).
Applied Surface Science 111 (1997) 135-139
Field emission from DLC films O. GriSning *, O.M. Kiittel, P. GriSning, L. Schlapbach lnstitut fiir Physik der Universitiit, P~rolles, CH-1700 Fribourg, Switzerland Received 4 June 1996; revised 4 July 1996; accepted 15 August 1996
Abstract
Field emission measurements on diamond like carbon (DLC) films with different amounts of sp 3 and sp 2 carbon were carried out. Depending on the amount of sp 2 carbon in the film, activated and non-activated Fowler-Nordheim like emission could be observed. The emission spots were investigated using a combination of AFM and STM, by simultaneously measuring the topography and the conductivity of the samples. In the case of sp 2 rich DLC films we could observe that the emission originates from highly conducting inclusions of sp 2 carbon in a matrix of insulating sp 3 carbon. These inclusions are already existing on the sp 2 sample by the deposition process itself and are formed by the activation on the sp 3 rich sample.
1. Introduction
Since a decade carbon has been regarded as a very promising base material for the use as electron field-emission cathodes [1,2]. In recent years chemical vapor deposition of (CVD) and natural diamond [3-7] and diamond like carbon (DLC) films [8,9] have attracted great interest due to several outstanding properties including the negative electron affinity (NEA). NEA means that the vacuum level lies below the bottom of the conduction band and hence, electrons in the conduction band gain energy when leaving the surface. NEA has been demonstrated for diamond surfaces [10] and there are indications that the surface of wide band gap DLC films might also have N E A properties [11 ]. Electron field emission from carbon based mate-
* Corresponding author. Tel.: +41-26-3009068; fax: +41-263009747; e-mail: [email protected].
rial can be explained by at least two different mechanism. Emission from a N E A surface or emission from carbon structures (tips, protrusions, conducting channels) due to geometrical field enhancement. Many reports are found in the literature making NEA responsible for field emission from carbon material [12]. However, a clear proof could not be given so far. In most of the cases the Fowler-Nordheim (FN) equation [13] theoretically describes the field emission from carbon material [6]. However, to date it is not clear whether the low work function (few hundred meV) coming out of FN-plots are due to an underestimation of the field enhancement factor/3 or if it is a physically meaningful number. Very recently, we showed that emission from ion etched diamond tips with a field enhancement factor of 7 0 0 - 1 0 0 0 as determined from high resolution scanning electron microscopy (HRSEM) leads to a physically meaningful work function of 4 - 5 eV [14]. We believe that field emission from carbon material is in
0169-4332/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0 169-4332(96)0071 3-1
136
O. Gr6ning et al. / Applied Surface Science 111 (1997) 135-139
most cases rather due to field enhancement than due to very low work functions or NEA. In order to gain an even deeper insight in the emission mechanism we investigate in this work the field emission behavior of DLC films with high and low amounts of sp 2 bonded carbon. FowlerNordheim like emission in the range of 20 to 50 V / / x m could be observed for both cases. The emission spots on both types of films were investigated using a combination of AFM and STM.
the contact mode with the feedback loop regulating a constant force. While scanning the surface a bias was applied to the sample and the current flowing from the conducting tip to the sample was measured and displayed as a current-map, reflecting the local conductivity of the sample. We used silicon AFM tips with a spring constant of 0.9 N / m , which were covered by a 100 nm thick layer of highly boron doped CVD-diamond i in order to get a hard and yet conductive AFM tip.
2. Experimental
3. Results and discussion
The 100 nm thick DLC films used in this study were deposited by laser ablation on highly p-doped Si(100) wafers. By changing the deposition parameters different sp2/sp 3 ratios in the films could be obtained. We will discuss two different DLC films in this paper: AC-1, and AC-2 with low respectively high concentration of sp 2 carbon. X-ray photoelectron spectroscopy (XPS) was used to evaluate the sp2/sp 3 ratios of the samples using a VG ESCALAB 5 spectrometer with a nonmonochromatised MgK c~ radiation. Surface contamination (mostly CO) were removed by an ECR (electron cyclotron resonance) hydrogen plasma treatment at 0.003 mbar and 15 s duration. Electron field emission (EFE) measurements were carried out at a pressure of 10-8 mbar by measuring the electron current collected by a highly polished stainless steel sphere of 4 mm diameter with a Keithley 237 instrument having an incorporated high voltage supply ( + 1100 V). The sphere is mounted on a linear piezo drive which allows a reproducible and accurate positioning of the anode with respect to the sample ( + 0 . 5 /zm). An 1 G O resistance is placed in series to limit the emission current. In order to get reproducible results the I - V characteristics were measured as fast as possible, typ. in 2 - 3 s. A test measurement on a cleaned silicon wafer showed an emission current of 0.5 pA up to fields of 170 V / / z m . The electric field are computed by tracing the ratio of the voltage applied to the sample and the anode-cathode distance. The emission spots on the samples were characterized by a combination of AFM and STM. The topography of the samples was measured by AFM in
3.1. Sample AC-1 AC-I is a l o w s p 2 containing DLC film as revealed by XPS. HRSEM measurements in the back scattered electron mode as well as A F M / S T M measurements show no inclusion or inhomogeneities in the film. Fig. 1 shows a typical current-voltage characteristic of this sample. In the first voltage sweep no field emission can be observed up to an applied field of 145 V / / z m (trace 1). Then, a sudden increase of the FE current over several orders of magnitude is observed, simultaneously with a strong discharge current pulse of typically a few 100 mA lasting 50 to 200 ns as measured by an oscilloscope. The saturation of the emission current is given by the current limitation of the measuring instrument which was set to 100 nA. After this activation step a much higher current is observed and stable FE is observed as indicated by trace 2. As an inset the FE behavior after the activation for positive and negative polarity is shown. The activation consists in creating a vacuum arc discharge between anode and cathode [15,16]. Such a discharge leads to a change of the film morphology as can be seen in the AFM image of Fig. 2a. Little /~m-sized hillocks are formed. A more detailed investigation of the emission spot by a combination of A F M / S T M on a smaller scale are presented in Fig. 2b (topography) and Fig. 2c (conductivity) at the location marked by an arrow in Fig. 2a. The initially insulating and flat sp 3 DLC film
~ Coated by CSEM (Centre Suisse de Electronique et Microtechnique), Neuchatel, Switzerland.
137
O. Gr6ning et al. /Applied Surface Science 111 (1997) 135-139
,o.-I\
10 .6 _
_
Fig. 3 shows a conductivity map of this sample on a region where no field emission measurements had been performed. The A F M tip was biased with 500 mV. One can observe highly conductive spots in the film. To every spot seen in Fig. 3 a little correspond-
,o,-1\
1 0 .7 _ 10 4 ~,
10 "8-
10"14-~ i l, -40
I ' 0 40 Field [Vrtm "1]
"E 1 0 " 1 ° -h 1 0 "11 0 1 0 "12 _
a)
1 0 "la _ 10-'~4 _
-i250.0 r~ lOO
-so
o Field [Vp.m "1]
s'o IgS,0 he4
Fig. l. Emission intensity versus the applied field for sample AC- 1. The activation of the emission is nicely illustrated by curve (1) and (2). The inset shows the diode character of the field emission.
0 0aN
0
was partially converted by the arc discharge into conducting sp 2 bonded carbon. This conversion is not homogenous as illustrated in Fig. 2c. Islands of highly conducting regions are separated by regions of low conductivity. The difference in conductivity is at least 2 orders of magnitude. Such islands can only be explained by assuming the formation of conductive channels through the whole thickness of the film down to the silicon substrate. The lateral dimensions of these channels can be estimated from our A F M / S T M measurement to be as small as a few nm. Like a freestanding, conductive tip in the vacuum, a conductive channel in an insulating matrix leads to field enhancement and hence to an enhanced electron emission. A F M and H R S E M investigations showed many protruding features on the surface of the structure leading to further field enhancement. Such an explanation yields a deeper understanding of the activation process. W e have observed this kind of activated field emission also on CVD diamond films and on polymer films with an field emission threshold which is easily in the 10 V / / z m range but which reaches values as low as 3 V / / x m (for emission currents of 10 pA).
In contrast to sample A C - I , the sample AC-2 contains to a much higher extent sp 2 bonded carbon.
20.0
b)
O,On~
O
1.00
2.00
2,00
c) 5 nA
1.oo
0
3.2. Sample A C - 2
10.0
1.b0
2.bo
o
I.IN
Fig. 2. AFM/STM topography and conductivity measurements on sample AC-1, (a) topography map of the emission spot on a larger scale; (b) and (c) topography resp. conductivity measurements on the location marked by an arrow in (a).
O. GriSning et aL / Applied Surface Science 11 l (1997) 135-139
138
10
10"7- b -40 -42 "",., -44< -46 ~'-48 ~-50~" -52 -
~
1 0 .8 _
~ ""
b)
"'",
~
10-9_
i 10"'°¢~ 10 "11 10-12
_
3o 4~ 5'o 0'o Fleld [V~tm-1] '%
-541.5
10
5
0
ttm
2:0
2;5 310 3:5 Field"1[Vqem]
4;0 ".Sx~O-6
Fig. 4. Fowler-Nordheim representation of the 1 - V characteristic of the emission from sample AC-2 before (trace a) and after (trace b) rising the field to 50 V / / z m for 1 h of operation. The inset shows the 1 - V characteristics of the curves.
10 nA
Fig. 3. A F M / S T M of sample AC-2 showing conducting inclusions in an otherwise insulating matrix.
ing hillock could be identified in the topographic image measured simultaneously with the conductivity map. This spots are believed to be inclusions of sp 2 bonded carbon in the DLC film. The size of this inclusions ranges between 80 nm and 150 nm. Investigations with an ordinary light microscope showed an uniform distribution over the whole surface. We believe from our measurements that these sp 2 inclusions are as thick as the film and hence are electrically connected to the substrate. Regarding the emission properties the two films behave differently. While the film AC-I needed an activation as discussed in Section 3.1, sample AC-2 showed field emission without any pretreatment. Certainly, the emission could be improved by applying fields in the order of 50 V / / x m for some time interval, however, no vacuum discharge initiating a much higher emission could be observed. This leads us to the conclusion that sample AC-2 showed field emission without any visible activation taking place. Keeping the role of the conducting channels in mind which was discussed in Section 3.1, the emission of this film can be explained by these sp: inclusions. However, in contrast to sample AC-I where channels were formed by the activation itself, sample AC-2
did not need an activation due to the inclusions already existing in the film by the deposition process. Fig. 4 shows Fowler-Nordheim like emission measured on the sample AC-2 before (trace a) and after (trace b) rising the field to 50 V / / x m for 1 h of operation. While the slopes of the Fowler-Nordheim plot remained the same, the emitting surface increased as witnessed by the parallel shift of the plots. A careful investigation by AFM of the emitting spot after one hour of operation showed little bumps formed by the intense electric field (Fig. 5). We
2001
0 rim
21
Fig. 5. AFM picture of an emission site on sample AC-2 after 1 h of emission.
O. Gr6ning et al. / Applied Surface Science 111 (1997) 135-139
believe the electric field induced morphological changes to be responsible for the enhancement of the emitting surface. It has to be stressed out that the work function and the field enhancement factor stay constant and are not affected by the field. Measurements in a plane-parallel diode setup were performed. In this setup the anode (Si wafer) was separated from the cathode by 635 /.tm thick A120 3 insulators. With this setup a surface of 5 × 5 mm 2 is probed. Non-activated field emission of 1 /zA at 1.65 kV, corresponding to a field of 2.6 V / / z m could be observed for the sample AC-2.
4. Conclusions In summary we have investigated the field emission behavior of DLC films containing different amounts of sp 2 bonded carbon. Whereas the film containing only sp 3 bonded carbon had to be activated in order to show field emission, the film containing conductive inclusions of sp 2 bonded carbon showed strong non-activated emission. We explain our results by the fact that conducting channels are formed from the surface down to the substrate by the activation process in the sp 3 film. They act as tip-like structure leading to field enhancement. The sp 2 containing films show already such conducting channels formed during the deposition process and hence no activation could be observed. Our results are in good agreement with the work of Schmidt et al. [17], who found the emitting sites on DLC films to be micro-protrusion and inclusion in the film calling it white blobs. They believed the white blobs to be diamond inclusions in a DLC matrix and the emission mechanism being governed by the NEA properties of these diamond inclusions. Our results support the idea of inclusions being
139
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