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Transmission photocathodes based on stainless steel mesh coated with deuterated diamond like carbon films
Nuclear Instruments and Methods in Physics Research A 753 (2014) 14–18
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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Transmission photocathodes based on stainless steel mesh coated with deuterated diamond like carbon films J. Huran a,n, N.I. Balalykin b, A.A. Feshchenko b, A.P. Kobzev b, A. Kleinová c, V. Sasinková d, L. Hrubčín a,b a
Institute of Electrical Engineering, Slovak Academy of Sciences, Dúbravská cesta 9, 84104 Bratislava, Slovakia Joint Institute for Nuclear Research, Joliot-Curie 6, Russian Federation 141980 Dubna, Moscow Region, Russia c Polymer Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 84541 Bratislava, Slovakia d Institute of Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, 84538 Bratislava, Slovakia b
art ic l e i nf o
a b s t r a c t
Article history: Received 11 December 2013 Received in revised form 24 March 2014 Accepted 25 March 2014 Available online 2 April 2014
In this study we report on the dependence of electron emission properties on the transmission photocathodes DC gun based on stainless steel mesh coated with diamond like carbon films prepared at various technological conditions. Diamond like carbon films were deposited on the stainless steel mesh and silicon substrate by plasma enhanced chemical vapor deposition from gas mixtures CH4 þD2 þ Ar, CH4 þH2 þAr and reactive magnetron sputtering using a carbon target and gas mixtures Arþ D2, Arþ H2. The concentration of elements in films was determined by Rutherford backscattering spectrometry (RBS) and elastic recoil detection (ERD) analytical methods simultaneously. Chemical compositions were analyzed by Fourier transform infrared spectroscopy (FT-IR). Raman spectroscopy at visible excitation wavelength was used for the intensity ratio determination of Gaussian fit D-peak and G-peak of Raman spectra. The quantum efficiency was calculated from the measured laser energy and the measured cathode charge. The quantum efficiency of a prepared transmission photocathode was increased with increasing intensity ratio of D-peak and G-peak, which was increased by adding deuterium to the gas mixture and using technology reactive magnetron sputtering. & 2014 Elsevier B.V. All rights reserved.
Keywords: Transmission photocathode Deuterated diamond like carbon film Plasma deposition Reactive magnetron sputtering
1. Introduction There are a number of papers that describe the photosensitivity of various standard photocathodes in both reflection mode (electrons emitted backwards from the illuminated photoemissive surface) and transmission mode (the illumination passes through a transparent window into a photoemissive layer and electrons emitted from the opposite surface). The photo-gun is a key element of injectors for producing high-charge electron bunches. Research continues on various schemes of cathodes. So far a promising technically challenging DC gun with the hollow cathode is proposed by JINR [1]. At the development of this concept, the use of transmission photocathodes in the form of grid was the most perspective direction. Diamond like carbon (DLC) is a disordered carbonaceous material composed of hydrogen and carbon, which is bonded in sp2 and sp3 electronic configurations with a significant C–C sp3 bonds. The properties of DLC are strongly affected by the amount of carbon atoms bonded in sp2
n
Corresponding author. Tel.: þ 421 2 59222778; fax: þ 421 2 54775816. E-mail addresses: [email protected], [email protected] (J. Huran).
http://dx.doi.org/10.1016/j.nima.2014.03.053 0168-9002/& 2014 Elsevier B.V. All rights reserved.
and sp3 electronic hybridizations. Also the amount of incorporated hydrogen and oxygen plays an important role in the final properties of DLC films [2]. Electron field emission from diamond, diamond like carbon, carbon nanotubes and nanostructured carbon is compared [3]. Electron emission from carbon materials has been based on two effects: field enhancement from conducting nanostructures and barrier lowering due to the negative electron affinity of diamond surfaces [4]. The photoemission properties of different samples were rationalized by considering the electron emission process located at the a-C/diamond/vacuum triple border and the quantum efficiency governed by the ratio of amorphous sp2-C to crystalline sp3-C [5]. The electron field emission properties of sulfur-assisted nanocrystalline carbon (n-C:S) thin films grown on molybdenum substrates by the hot-filament CVD technique using methane–hydrogen (CH4/H2) and hydrogen sulfide–hydrogen (H2S/H2) gas mixture were investigated [6]. The results of the external Quantum Efficiency (QE) measurements, in the range 150–210 nm, of Poly-, Nano- and Single-Crystalline Diamond (PCD, NCD and SCD) film photocathodes (PCs) were reported and discussed [7]. DLC films have been prepared by physical vapor deposition (PVD) and chemical vapor deposition (CVD) at low pressure and temperature [8–11]. Deuterated
J. Huran et al. / Nuclear Instruments and Methods in Physics Research A 753 (2014) 14–18
amorphous carbon films (a-C:D) deposited by plasma enhanced chemical vapor deposition (PECVD) using deuterated hydrocarbons as precursor gases were investigated [12] and some specific applications, like storing ultra cold neutron devices [13] or as neutron mirror [14] were published. The careful comparison of the vibrational excitation functions for hydrogen/deuterium termination stretching modes v(sp3-CHx) and v(sp3-CDx), for hydrogen termination bending modes v(CHx) mixed with diamond lattice modes v(CC), for deuterium termination bending modes v(CDx), and for multiple loss 2v(CC) demonstrates the close interplay between three characteristics: (i) the density-of-states of the substrate, (ii) the vibrational excitation mechanisms (dipolar and/or impact scattering including resonant scattering) and (iii) the surface versus lattice character of the excited vibrational modes [15]. In this paper we have focused our attention on the DLC film preparation, following the relationship between DLC film properties and quantum efficency of prepared transmission photocathodes based on stainless steel mesh coated with DLC film. DLC films were prepared by standard PECVD technology and reactive magnetron sputtering with deuterium and hydrogen as reactive gas. The properties of films were investigated by RBS, ERD, IR and Raman measurement techniques. The properties of the prepared photocathodes were performed by measurement of the quantum efficiency.
15
The stainless steel meshes with DLC films were marked as PD, PH and used for PD and PH transmission photocathodes. 2.2. Reactive magnetron sputtering of diamond like carbon films
2.1. Plasma enhanced chemical vapor deposition of diamond like carbon films
The Si substrate and stainless steel mesh were put into the magnetron sputtering vacuum chamber on the substrate holder. The base pressure of the deposition chamber was 1 10 4 Pa. The Si substrate and mesh were cleaned in argon DC plasma at UDC ¼ 700 V, 10 min prior the film deposition. The magnetron target was a high-purity graphite disk 3 in. in diameter. High purity argon was used as an inert gas and deuterium and hydrogen as reactive gases. The DLC film was deposited at a working pressure 0.6 Pa and magnetron input power combined with RF power 200 W at 13.56 MHz and DC power (UDC ¼ 550 V, IDC ¼150 mA). During magnetron sputtering was the substrate holder biased UDC ¼ 150 V. The substrate holder temperature was 100 1C during the deposition. The DLC film was deposited on both sides of the stainless steel mesh under the same deposition conditions. The DLC film on the Si substrate was used for structural characterization and marked as MD and MH samples. The stainless steel meshes with the DLC films were marked as MD, MH and used for MD and MH transmission photocathodes. The concentrations of elements in films were analyzed using the RBS and ERD analytical method simultaneously. Program SIMNRA was used for simulating RBS and ERD spectra and calculating concentrations of elements in films. This method was described in more detail in [16]. Chemical compositions were analyzed by infrared spectroscopy using an FT-IR Nicolet 8700 spectrometer in absorption mode and the absorption spectra of the prepared substrate were subtracted from the film spectra. The IR spectra were measured from 4000 to 400 cm 1. Raman measurements of the DLC films were performed using a Thermo Fisher Scientific DXR Raman microscope with a 532 nm laser. The G and D peaks were fitted with two Gaussians and the intensity was calculated as the peak's area. The DLC transmission photocathode quantum efficiency testing was performed at JINR Dubna. The photocathode was placed on the hollow cathode assembly of the Pierce structure DC gun to produce photoelectrons as shown in Fig. 1. At one side of the photoinjector test facility vacuum chamber a sapphire viewport is mounted that transmitted UV light. The vacuum condition was 6 10 9 mbar. The 15 ns UV laser pulses (quadrupled Nd:YAG laser, 266 nm) with the laser spot sizer5 mm were used to backside illuminate the DLC film/mesh as a photocathode. To draw electrons from the DLC film coated mesh type photocathode a negative voltage was placed on the cathode. This voltage was kept at roughly 10–12 kV. The bunch charge was measured using a Faraday cup (FC). The FC was connected to the ground through the measuring capacitor with a math cable; voltage on charging capacity (Uc) was monitored by 500 MHz oscilloscope.
The Si substrate and stainless steel mesh were put into the PECVD vacuum chamber on the substrate holder-bottom electrode. Before film deposition the Si substrate and mesh were cleaned in argon RF plasma at Usb ¼ 400 V (Usb, self bias voltage), for 10 min. The methane and deuterium (hydrogen) were introduced into reactor through the shower head. Both gases were flown vertically toward samples on bottom electrode connected with RF power 150 W at a frequency of 13.56 MHz. The flow rates of CH4, D2, Ar and CH4, H2, Ar gases were 15 sccm, 8 sccm and 10 sccm, respectively, and a deposition pressure was 10 Pa. The substrate holder temperature during deposition was 100 1C. DLC films were deposited on both sides of the mesh under the same deposition conditions. DLC films on Si substrate were used for structural characterization and marked as PD and PH samples.
Fig. 1. Principal scheme of the quantum efficency measurement system for transmission photocathodes.
2. Experiment Diamond like carbon films were deposited on silicon (Si) substrate and stainless steel mesh by plasma enhanced chemical vapor deposition from gas mixtures CH4 þD2 þ Ar, CH4 þH2 þAr, and reactive magnetron sputtering (RMS) using a carbon target and gas mixtures Ar þD2, Ar þH2. The mesh grid: stainless steel wire with a diameter of 0.03 mm; the size of a cell was 0.04 mm, the photocathode mesh shape was circular with the diameter of 8 mm. Standard parameters for deposition processes were used on the base of technological experience and published results about DLC films prepared by various technologies for photocathode application. Prior to deposition, standard cleaning was used to remove impurities from the silicon surface and stainless steel mesh. A 5% hydrofluoric acid was used to remove the native oxide from Si surface. The Si substrate was then rinsed in deionized water and dried in nitrogen ambient. The stainless steel mesh was cleaned in the ultrasonic bath containing acetone for 30 min and then rinsed in deionized water and dried in nitrogen ambient.
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3. Results and discussion 3.1. Characterization of prepared diamond like carbon films
MD MH PD PH
The measured and simulated RBS and ERD spectra for samples PD, PH and MD, MH are shown in Fig. 2. The RBS and ERD analysis indicated that films contain carbon, deuterium, hydrogen, and a small amount of oxygen. The concentrations of species for all samples are presented in Table 1. We propose that the concentration of hydrogen in the sample MD was mainly caused by a small amount of hydrogen in deuterium gas. In Fig. 2b, positions of H and D when they are at the sample surface are indicated by arrows. The thickness of all films was in the range of 250–500 nm. The FT-IR spectra, Fig. 3, of samples PD, PH and MD, MH show well defined bending bands (Fig. 3a) and stretching bands (Fig. 3b). Both bands can be wider than in the case of only C–H bond films due to existence of C–D bonds in films and then shift vibrational frequency. As can be seen in Fig. 3, in bending and stretching
Absorbance (a.u.)
FT-IR bending band MD MH PH
PD
1200
1250
1300
1350
1400
MD MH PD PH
FT-IR
Counts per channel
MDexp MDsim MHexp MHsim PDexp PDsim PHexp PHsim
RBS spectra
6000 5000
PD
C
4000
MD
3000
Absorbance (a.u.)
8000 7000
PH
stretching band MH PH
PD MD
Si
2000 MH
1000
2700
2800
2900
3000
3100
3200
-1
0 100
Wavenumber (cm ) 200
300
400
500
600 Fig. 3. FT-IR spectra bending band (a), stretching band (b) of the DLC films deposited by the PECVD technology and reactive magnetron sputtering.
Channel
3000 MDexp MDsim MHexp MHsim PDexp PDsim PHexp PHsim
ERD spectra
2500
Counts per channel
1450
Wavenumber (cm-1)
MD
2000
H
1500
PD
D
1000 PH
500 MH
0 200
300
400
500
600
700
800
900
1000
Channel Fig. 2. Experimental and simulated spectra RBS (a) and ERD (b) for samples PD, PH prepared by the PECVD technology and samples MD, MH prepared by reactive magnetron sputtering.
Table 1 Concentration of species in DLC films determined by RBS and ERD methods. Sample
C (at%)
D (at%)
H (at%)
O (at%)
PD PH MD MH
61 71 59 64
12 – 33 –
26 27 5 33
1–2 1–2 2–3 2–3
regions, several bands can overlap, which makes the vibrational analysis particularly difficult. As shown in Fig. 3a, the region 1250–1500 cm 1 has been decomposed using several vibrations at 1280 cm 1, which can be assigned to sp2 CH aromatic, at 1325 cm 1 and 1370 cm 1, which can be assigned to sp3 CH3 sym., at 1411 cm 1 which can be assigned to sp2 CH2, and at 1425 cm 1, which can be assigned to sp2 CH olefinic [17]. The region 2700–3200 cm 1 has been decomposed using vibrations at 2850 cm 1, which can be assigned to sp3 CH3 sym, at 2920 cm 1, which can be assigned to sp3 CH2 asym, at 3014 cm 1, which can be assigned to sp2 CH olefinic and at 3065 cm 1, which can be assigned to sp2 CH2 asym [17]. In the case of sample MD, the vibrational amplitudes are smaller than in other samples. The authors in [18] wrote that, vibrational amplitudes at carbon atoms in the joint C–D vibrational modes are larger than in the joint C–H modes, due to the mass ratio m(D)/m(C) 4m(H)/m(C). However, the vibrational relaxation rate for the D stretch is smaller than for the H stretch, because the energy is dissipated to an acoustic phonon in the case of C–D rather than an optical phonon as in the case of C–H, and hence, the corresponding phonon density of states is lower in the C–D case. We propose, that in our samples vibrational intensity mostly depends on the IR active C–H bonds than on C–D bonds. Another explanation that can be considered is that difference in bonding energy between the C–H bond and C–D bond. The a-C:H film has C–H bonds from CH, CH2 and CH3 which form the carbon network. Fewer IR active C–D bonds might have formed because the C–D bonds required greater bonding energy than the C–H bonds, leading to a rate of
J. Huran et al. / Nuclear Instruments and Methods in Physics Research A 753 (2014) 14–18
RAMAN
Intensity (a.u.)
MD MH PD PH MD MH PD PH
1000
1200
1400
1600
Raman shift (cm-1)
Peak Sum
Intensity (a.u.)
RAMAN
MD-sample 1557 cm G Peak
Exp. Intensity Peak Sum G Peak D Peak
1397 cm D Peak
-1
-1
17
The intensity ratio I(D)/I(G) ¼1.133. In the case of samples MD, PD the G peak position was found to shift to higher frequencies than in the case of samples MH, PH. This can be attributed to the reduction of sp2 sites of aromatic rings. The D peak position was found to shift to higher frequencies for samples MD, PD than in the case of samples MH, PH. In general, the D peak position will increase with increasing disorder for the double-Gaussian fit. Two factors can shift the D peak. On the one hand, smaller aromatic clusters have higher modes and shift D upwards. On the other hand, a decrease in number of ordered aromatic rings on passing from nanocrystalline graphite to a-C lowers D and reduces its intensity, due to softening of the vibrational density of states [21]. From the Raman spectra of all samples, the I(D)/I(G) ratio is higher for samples MD and PD prepared by both technologies with deuterium gas and the I(D)/I(G) ratio is higher for the samples prepared by reactive magnetron sputtering for both hydrogen and deuterium in the gas mixture. It is known that the ion density and energy which are related to the plasma density and electron temperature are higher in the magnetron plasma discharge than in the PECVD plasma discharge. The raised ion energy which causes weaker bonds to be broken can supply sufficient energy for the forming of stronger bonds. In other words, the s bonds which are stronger than π bonds and are the infrastructure of sp3 bonds could be formed. We propose that the deuterium is more effective for the disordering in the MD sample technology than in the PD sample technology due to higher ion deuterium density in the magnetron plasma discharge and the deuterium is more effective than hydrogen for disordering in both technologies. 3.2. Quantum efficiency calculations
800
1000
1200
1400
Raman shift
1600
1800
(cm-1)
Fig. 4. Experimental (correct baseline) Raman spectra of the DLC films for all samples MD, MH, PD, PH (a) and Raman spectrum of the sample MD (b) with fitting the experimental curve by Gaussian centers G and D.
a reaction for C–H bonds that is typically 6 to 10 times faster than the corresponding C–D bonds [19]. Fig. 4a shows the experimental correct baseline Raman spectra of all samples. All disordered carbons show common features in their Raman spectra in 800–2000 cm 1 region, the so-called G and D peaks, which lie at around 1560 and 1360 cm 1, respectively, for visible excitation. The G peak is due to the bond stretching of all pairs of sp2 atoms in both rings and chains. The D peak is due to the breathing modes of sp2 atoms in rings and it is activated by disorder [20]. According to the three-stage model of Ferrari and Robertson [21], the visible Raman parameters can be used to derive the sp3 fraction. Fig. 4b shows Raman spectrum of the deuterated DLC film on the Si substrate, sample MD (deposited by reactive magnetron sputtering with deuterium as a reactive gas), which were Gaussian-fitted and identified as D and G band conversions. We used only two main peaks Gaussian fitting. The G peak and D peak are located at two positions 1557 cm 1 and 1397 cm 1, respectively. The intensity ratio I(D)/I(G) ¼1.723, and was determined after fitting analysis of the spectrum. For the sample MH (deposited by reactive magnetron sputtering with hydrogen as reactive gas), the G peak and D peak are located at two positions 1544 cm 1 and 1370 cm 1, respectively. The intensity ratio I(D)/I(G)¼ 1.473. For the sample PD (deposited by PECVD technology with deuterium in gas mixture), the G peak and D peak are located at two positions 1553 cm 1 and 1390 cm 1, respectively. The intensity ratio I(D)/ I(G)¼ 1.443. For the sample PH (deposited by PECVD technology with hydrogen in the gas mixture), the G peak and D peak are located at two positions 1544 cm 1 and 1376 cm 1, respectively.
Fig. 5(a) shows the bunch charge as a function of the laser energy for all prepared photocathodes. The bunch charge increased from 1156 to 2740 pC as the laser energy increased from 2.4 to 4.8 mJ for the MD photocathode. In the case of the MH photocathode, the bunch charge increased from 870 to 1666 pC. For the PD and PH photocathode the bunch charge increased from 804 to 1523 pC and 782 to 1230 pC, respectively, as the laser energy increased from 2.4 to 4.8 mJ. The increase in the laser energy indicates the increase in the number of photons causing photoemission, which leads to the increase in the number of photoelectrons produced at the cathode surface. The bunch charge intensity trend of the four photocathodes shown in Fig. 5 evidences a dependence of the photoemission properties on the graphitic component inside the DLC films. We define the quantum efficiency (QE) as the ratio of the number of electrons measured at the exit of the DC-gun (Fig. 1) to the number of photons arriving at the cathode. The number of photons is measured with a calibrated joulemeter. The transmission of the vacuum window and the mesh type cathode are taken into account. Usually, we measure the extracted charge Q as a function of the laser energy EL. The QE is calculated from a linear fit to the data points at low charge densities, where space charge effects are negligible and the relation Q (EL) is linear. The quantum efficiency was calculated using the formula QE ¼ N e =N υ where Ne is the number of electrons and Nυ is the number of photons. The quantum efficiency results for all transmission photocathodes with related I(D)/I(G) ratio of DLC films are presented in Table 2. Fig. 5b shows the quantum efficiency and the I(D)/I(G) ratio as a function of the type (technology parameters) of the prepared photocathode. On the basis of the present results it can be concluded that the QE from the MD photocathode is practically two times higher than the QE from the PD photocathode. Moreover, these results can be directly connected with the intensity I(D)/I(G) ratio of D and G peaks in Raman spectra. The intensity I(D)/I(G) ratio of the
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J. Huran et al. / Nuclear Instruments and Methods in Physics Research A 753 (2014) 14–18
3000 PH PD MH MD
Bunch charge (pC)
2500
2000
1500
1000
500 2.5
3.0
3.5
4.0
4.5
5.0
Laser energy (mJ/pulse)
3.0x10-4 1.8
QE(%)
2.0x10-4
I(D)/(G)
1.6
2.5x10-4
1.4 1.2 1.0 PH
PD
MH
Acknowledgment
MD
1.5x10-4
1.0x10-4
the films contained carbon, hydrogen, deuterium and small amount of oxygen. The FT-IR spectra of samples PD, PH and MD, MH show well defined bending bands and stretching bands. We propose that in our samples vibrational intensity mostly depends on the IR active C–H bonds than on the C–D bonds. Raman spectra of the DLC films show the D and G band conversion which were Gaussian-fitted and identified. In the case of samples MD and PD the G peak position was found to shift to higher wave number than in samples MH and PH. The D peak position was found to shift to higher frequencies for samples MD, PD than in the case of samples MH, PH. In general, the D peak position increases with increasing disorder for the double-Gaussian fit. The I(D)/I(G) ratios were higher in the case of samples MD, PD. The emission process from diamond like carbon films, excited with near-UV radiation, seems to be governed mainly by the intensity ratios I(D)/I(G). The quantum efficiency was 2.68 10 4 for the MD photocathode and 1.20 10 4 for the PD photocathode at laser wavelength 266 nm. A further study is underway to investigate the nature of deuterium on the structural and surface properties of a deuterated DLC film together with the quantum efficiency of the prepared transmission mesh photocathode. The development of photocathodes in the form of grid is perspective direction for DC gun transmission cathode type.
PH
PD
MH
MD
Fig. 5. Bunch charge versus laser energy for all photocathodes (a), quantum efficiency and I(D)/I(G) ratios as a function of type and technology parameters of the prepared photocathodes (b).
Table 2 Quantum efficiency results for all transmission photocathodes with related data intensity ratio I(D)/I(G) of DLC films. Sample
Quantum efficiency
Intensity ratio I(D)/I(G)
PD PH MD MH
1.49 10 4 1.20 10 4 2.68 10 4 1.63 10 4
1.443 1.133 1.723 1.473
MD, MH films are higher than for the PD, PH films. The I(D)/I(G) ratio can indirectly determine the content of sp2 and sp3 bonds. However, higher value of the I(D)/I(G) ratio corresponds with the higher quantum efficiency of the diamond like carbon film coated mesh type transmission photocathode. The emission mechanism in DLCs has been difficult to understand. Experimental evidence suggests that the main barrier to emission in a DLC is large and at the front surface. The barrier's lowering requires some types of heterogeneity [3]. In our case, the obtained results show that heterogeneity can be deliberately introduced into a DLC by deuterium mix in working gases in both technologies. 4. Conclusions We investigated the structural properties of the DLC films prepared by plasma enhanced chemical vapor deposition and reactive magnetron sputtering with deuterium and hydrogen in the gas mixture on the Si substrate and stainless steel mesh for the transmission photocathode. RBS and ERD analyses indicated that
This research has been supported by the Slovak Research and Development Agency under the contracts APVV-0443-12 and has been executed in the framework of the Topical Plan for JINR Research and International Cooperation (Project 02-0-1067-2013/ 2017). References [1] A. Dudarev, U.A. Budagov, N. Balalykin, V..Kobets, M.V. Lyablin, B.M. Sabirov, G. Shirkov E. Syresin, G.V. Trubnikov, Z. Dai, C. Petit-Jean-Genaz, V.R.W. Schaa, C. Zhang (Eds.), in: Proceedings of IPAC 2013, Shanghai, (2013), p. 1631. [2] D. Caschera, P. Cossari, F. Federici, S. Kaciulis, A. Mezzi, G. Padeletti, D.M. Trucchi, Thin Solid Films 519 (2011) 4087. [3] J. Robertson, IEICE Transaction on Electronics 5 (2003) 787 (E86-C). [4] R.J. Nemanich, G.L. Bilbro, E.N. Bryan, F.A. Koeck, J.R. Smith, Y. Tang, in: Paul K. Chu(Ed.), Proceedings of the 3rd International Nanoelectronics Conference, (IEEE 2010), Hong Kong, 2010, p. 56. [5] V. Sessa, S. Orlanducci, A. Fiori, M.L. Terranova, F. Tazzioli, C. Vicario, I. Boscolo, S. Cialdi, M. Rossi, P. Lugli, L.B. Kish, J. Mateos (Eds.) in: Proccedings of SPIE-The International Society for Optical Engineering 5838, (2005) p. 216. [6] S. Gupta, B.R. Weiner, B.L. Weiss, G. Morell, in: Proceedings of the Materials Research Society Symposium San Francisco, 2001, Vol. 675, (2001 p. W6.9.1. [7] M.A. Nitti, M. Colasuonno, E. Nappi, A. Valentini, E. Fanizza, F. Bénédic, G. Cicala, E. Milani, G. Prestopino, Nuclear Instruments and Methods, A 595 (2008) 131. [8] J.C. Pu, S.F. Wang, C.L. Lin, J.C. Sung, Thin Solid Films 519 (2010) 521. [9] M. Kihel, R. Clergereaux, D. Escaich, M. Calafat, P. Raynaud, S. Sahli, Y. Segui, Diamond and Related Materials 17 (2008) 1710. [10] C. Pardanaud, E. Aréou, C. Martin, R. Ruffe, T. Angot, P. Roubin, C. Hopf, T. Schwarz- Selinger, W. Jacob, Diamond and Related Materials 22 (2012) 92. [11] A. Pardo, C. Gómez-Alexandrie, J.O. Orwa, A. Cimmino, S. Prawer, Diamond and Related Materials 26 (2012) 39. [12] M.E.H. Maia da Costa, F.L. Freire Jr., Surface and Coatings Technology 204 (2010) 1993. [13] M.G.D. van der Grinten, J.M. Pendlebury, D. Shiers, C.A. Baker, K. Green, P.G. Harris, P.S. Iaydjiev, S.N. Ivanov, P. Geltenbort, Nuclear Instruments and Methods, A 423 (1999) 421. [14] Y. Kawabata, M. Hino, T. Horie, S. Tasaki, K. Yoshida, I. Kanno, M. Nakayama, Nuclear Instruments and Methods, A 529 (2004) 84. [15] L. Amiaud, I. Martin, A.R. Milosavljevic, S.h. Michaelson, A. Hoffman, R. Azria, A. Lafosse, Physical Chemistry Chemical Physics 13 (2011) 11495. [16] A.P. Kobzev, J. Huran, D. Maczka, M. Turek, Vacuum 83 (2009) S124. [17] T. Haitz, B. Drévillon, C. Codet, J.E. Boureé, Physical Review 58 (1998) 13957. [18] S. Sakong, P. Kratzer, The Journal of Chemical Physics 135 (2011) 114506. [19] K. Ozeki, D. Sekiba, T. Suzuki, K. Kanda, M. Niibe, K.K. Hirakuri, T. Masuzawa, Applied Surface Science 265 (2013) 750. [20] C. Casiraghi, Diamond and Related Materials 20 (2011) 120. [21] A.C. Ferrari, J. Robertson, Physical Review B 61 (2000) 14095.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Transmission photocathodes based on stainless steel mesh coated with deuterated diamond like carbon films J. Huran a,n, N.I. Balalykin b, A.A. Feshchenko b, A.P. Kobzev b, A. Kleinová c, V. Sasinková d, L. Hrubčín a,b a
Institute of Electrical Engineering, Slovak Academy of Sciences, Dúbravská cesta 9, 84104 Bratislava, Slovakia Joint Institute for Nuclear Research, Joliot-Curie 6, Russian Federation 141980 Dubna, Moscow Region, Russia c Polymer Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 84541 Bratislava, Slovakia d Institute of Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, 84538 Bratislava, Slovakia b
art ic l e i nf o
a b s t r a c t
Article history: Received 11 December 2013 Received in revised form 24 March 2014 Accepted 25 March 2014 Available online 2 April 2014
In this study we report on the dependence of electron emission properties on the transmission photocathodes DC gun based on stainless steel mesh coated with diamond like carbon films prepared at various technological conditions. Diamond like carbon films were deposited on the stainless steel mesh and silicon substrate by plasma enhanced chemical vapor deposition from gas mixtures CH4 þD2 þ Ar, CH4 þH2 þAr and reactive magnetron sputtering using a carbon target and gas mixtures Arþ D2, Arþ H2. The concentration of elements in films was determined by Rutherford backscattering spectrometry (RBS) and elastic recoil detection (ERD) analytical methods simultaneously. Chemical compositions were analyzed by Fourier transform infrared spectroscopy (FT-IR). Raman spectroscopy at visible excitation wavelength was used for the intensity ratio determination of Gaussian fit D-peak and G-peak of Raman spectra. The quantum efficiency was calculated from the measured laser energy and the measured cathode charge. The quantum efficiency of a prepared transmission photocathode was increased with increasing intensity ratio of D-peak and G-peak, which was increased by adding deuterium to the gas mixture and using technology reactive magnetron sputtering. & 2014 Elsevier B.V. All rights reserved.
Keywords: Transmission photocathode Deuterated diamond like carbon film Plasma deposition Reactive magnetron sputtering
1. Introduction There are a number of papers that describe the photosensitivity of various standard photocathodes in both reflection mode (electrons emitted backwards from the illuminated photoemissive surface) and transmission mode (the illumination passes through a transparent window into a photoemissive layer and electrons emitted from the opposite surface). The photo-gun is a key element of injectors for producing high-charge electron bunches. Research continues on various schemes of cathodes. So far a promising technically challenging DC gun with the hollow cathode is proposed by JINR [1]. At the development of this concept, the use of transmission photocathodes in the form of grid was the most perspective direction. Diamond like carbon (DLC) is a disordered carbonaceous material composed of hydrogen and carbon, which is bonded in sp2 and sp3 electronic configurations with a significant C–C sp3 bonds. The properties of DLC are strongly affected by the amount of carbon atoms bonded in sp2
n
Corresponding author. Tel.: þ 421 2 59222778; fax: þ 421 2 54775816. E-mail addresses: [email protected], [email protected] (J. Huran).
http://dx.doi.org/10.1016/j.nima.2014.03.053 0168-9002/& 2014 Elsevier B.V. All rights reserved.
and sp3 electronic hybridizations. Also the amount of incorporated hydrogen and oxygen plays an important role in the final properties of DLC films [2]. Electron field emission from diamond, diamond like carbon, carbon nanotubes and nanostructured carbon is compared [3]. Electron emission from carbon materials has been based on two effects: field enhancement from conducting nanostructures and barrier lowering due to the negative electron affinity of diamond surfaces [4]. The photoemission properties of different samples were rationalized by considering the electron emission process located at the a-C/diamond/vacuum triple border and the quantum efficiency governed by the ratio of amorphous sp2-C to crystalline sp3-C [5]. The electron field emission properties of sulfur-assisted nanocrystalline carbon (n-C:S) thin films grown on molybdenum substrates by the hot-filament CVD technique using methane–hydrogen (CH4/H2) and hydrogen sulfide–hydrogen (H2S/H2) gas mixture were investigated [6]. The results of the external Quantum Efficiency (QE) measurements, in the range 150–210 nm, of Poly-, Nano- and Single-Crystalline Diamond (PCD, NCD and SCD) film photocathodes (PCs) were reported and discussed [7]. DLC films have been prepared by physical vapor deposition (PVD) and chemical vapor deposition (CVD) at low pressure and temperature [8–11]. Deuterated
J. Huran et al. / Nuclear Instruments and Methods in Physics Research A 753 (2014) 14–18
amorphous carbon films (a-C:D) deposited by plasma enhanced chemical vapor deposition (PECVD) using deuterated hydrocarbons as precursor gases were investigated [12] and some specific applications, like storing ultra cold neutron devices [13] or as neutron mirror [14] were published. The careful comparison of the vibrational excitation functions for hydrogen/deuterium termination stretching modes v(sp3-CHx) and v(sp3-CDx), for hydrogen termination bending modes v(CHx) mixed with diamond lattice modes v(CC), for deuterium termination bending modes v(CDx), and for multiple loss 2v(CC) demonstrates the close interplay between three characteristics: (i) the density-of-states of the substrate, (ii) the vibrational excitation mechanisms (dipolar and/or impact scattering including resonant scattering) and (iii) the surface versus lattice character of the excited vibrational modes [15]. In this paper we have focused our attention on the DLC film preparation, following the relationship between DLC film properties and quantum efficency of prepared transmission photocathodes based on stainless steel mesh coated with DLC film. DLC films were prepared by standard PECVD technology and reactive magnetron sputtering with deuterium and hydrogen as reactive gas. The properties of films were investigated by RBS, ERD, IR and Raman measurement techniques. The properties of the prepared photocathodes were performed by measurement of the quantum efficiency.
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The stainless steel meshes with DLC films were marked as PD, PH and used for PD and PH transmission photocathodes. 2.2. Reactive magnetron sputtering of diamond like carbon films
2.1. Plasma enhanced chemical vapor deposition of diamond like carbon films
The Si substrate and stainless steel mesh were put into the magnetron sputtering vacuum chamber on the substrate holder. The base pressure of the deposition chamber was 1 10 4 Pa. The Si substrate and mesh were cleaned in argon DC plasma at UDC ¼ 700 V, 10 min prior the film deposition. The magnetron target was a high-purity graphite disk 3 in. in diameter. High purity argon was used as an inert gas and deuterium and hydrogen as reactive gases. The DLC film was deposited at a working pressure 0.6 Pa and magnetron input power combined with RF power 200 W at 13.56 MHz and DC power (UDC ¼ 550 V, IDC ¼150 mA). During magnetron sputtering was the substrate holder biased UDC ¼ 150 V. The substrate holder temperature was 100 1C during the deposition. The DLC film was deposited on both sides of the stainless steel mesh under the same deposition conditions. The DLC film on the Si substrate was used for structural characterization and marked as MD and MH samples. The stainless steel meshes with the DLC films were marked as MD, MH and used for MD and MH transmission photocathodes. The concentrations of elements in films were analyzed using the RBS and ERD analytical method simultaneously. Program SIMNRA was used for simulating RBS and ERD spectra and calculating concentrations of elements in films. This method was described in more detail in [16]. Chemical compositions were analyzed by infrared spectroscopy using an FT-IR Nicolet 8700 spectrometer in absorption mode and the absorption spectra of the prepared substrate were subtracted from the film spectra. The IR spectra were measured from 4000 to 400 cm 1. Raman measurements of the DLC films were performed using a Thermo Fisher Scientific DXR Raman microscope with a 532 nm laser. The G and D peaks were fitted with two Gaussians and the intensity was calculated as the peak's area. The DLC transmission photocathode quantum efficiency testing was performed at JINR Dubna. The photocathode was placed on the hollow cathode assembly of the Pierce structure DC gun to produce photoelectrons as shown in Fig. 1. At one side of the photoinjector test facility vacuum chamber a sapphire viewport is mounted that transmitted UV light. The vacuum condition was 6 10 9 mbar. The 15 ns UV laser pulses (quadrupled Nd:YAG laser, 266 nm) with the laser spot sizer5 mm were used to backside illuminate the DLC film/mesh as a photocathode. To draw electrons from the DLC film coated mesh type photocathode a negative voltage was placed on the cathode. This voltage was kept at roughly 10–12 kV. The bunch charge was measured using a Faraday cup (FC). The FC was connected to the ground through the measuring capacitor with a math cable; voltage on charging capacity (Uc) was monitored by 500 MHz oscilloscope.
The Si substrate and stainless steel mesh were put into the PECVD vacuum chamber on the substrate holder-bottom electrode. Before film deposition the Si substrate and mesh were cleaned in argon RF plasma at Usb ¼ 400 V (Usb, self bias voltage), for 10 min. The methane and deuterium (hydrogen) were introduced into reactor through the shower head. Both gases were flown vertically toward samples on bottom electrode connected with RF power 150 W at a frequency of 13.56 MHz. The flow rates of CH4, D2, Ar and CH4, H2, Ar gases were 15 sccm, 8 sccm and 10 sccm, respectively, and a deposition pressure was 10 Pa. The substrate holder temperature during deposition was 100 1C. DLC films were deposited on both sides of the mesh under the same deposition conditions. DLC films on Si substrate were used for structural characterization and marked as PD and PH samples.
Fig. 1. Principal scheme of the quantum efficency measurement system for transmission photocathodes.
2. Experiment Diamond like carbon films were deposited on silicon (Si) substrate and stainless steel mesh by plasma enhanced chemical vapor deposition from gas mixtures CH4 þD2 þ Ar, CH4 þH2 þAr, and reactive magnetron sputtering (RMS) using a carbon target and gas mixtures Ar þD2, Ar þH2. The mesh grid: stainless steel wire with a diameter of 0.03 mm; the size of a cell was 0.04 mm, the photocathode mesh shape was circular with the diameter of 8 mm. Standard parameters for deposition processes were used on the base of technological experience and published results about DLC films prepared by various technologies for photocathode application. Prior to deposition, standard cleaning was used to remove impurities from the silicon surface and stainless steel mesh. A 5% hydrofluoric acid was used to remove the native oxide from Si surface. The Si substrate was then rinsed in deionized water and dried in nitrogen ambient. The stainless steel mesh was cleaned in the ultrasonic bath containing acetone for 30 min and then rinsed in deionized water and dried in nitrogen ambient.
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3. Results and discussion 3.1. Characterization of prepared diamond like carbon films
MD MH PD PH
The measured and simulated RBS and ERD spectra for samples PD, PH and MD, MH are shown in Fig. 2. The RBS and ERD analysis indicated that films contain carbon, deuterium, hydrogen, and a small amount of oxygen. The concentrations of species for all samples are presented in Table 1. We propose that the concentration of hydrogen in the sample MD was mainly caused by a small amount of hydrogen in deuterium gas. In Fig. 2b, positions of H and D when they are at the sample surface are indicated by arrows. The thickness of all films was in the range of 250–500 nm. The FT-IR spectra, Fig. 3, of samples PD, PH and MD, MH show well defined bending bands (Fig. 3a) and stretching bands (Fig. 3b). Both bands can be wider than in the case of only C–H bond films due to existence of C–D bonds in films and then shift vibrational frequency. As can be seen in Fig. 3, in bending and stretching
Absorbance (a.u.)
FT-IR bending band MD MH PH
PD
1200
1250
1300
1350
1400
MD MH PD PH
FT-IR
Counts per channel
MDexp MDsim MHexp MHsim PDexp PDsim PHexp PHsim
RBS spectra
6000 5000
PD
C
4000
MD
3000
Absorbance (a.u.)
8000 7000
PH
stretching band MH PH
PD MD
Si
2000 MH
1000
2700
2800
2900
3000
3100
3200
-1
0 100
Wavenumber (cm ) 200
300
400
500
600 Fig. 3. FT-IR spectra bending band (a), stretching band (b) of the DLC films deposited by the PECVD technology and reactive magnetron sputtering.
Channel
3000 MDexp MDsim MHexp MHsim PDexp PDsim PHexp PHsim
ERD spectra
2500
Counts per channel
1450
Wavenumber (cm-1)
MD
2000
H
1500
PD
D
1000 PH
500 MH
0 200
300
400
500
600
700
800
900
1000
Channel Fig. 2. Experimental and simulated spectra RBS (a) and ERD (b) for samples PD, PH prepared by the PECVD technology and samples MD, MH prepared by reactive magnetron sputtering.
Table 1 Concentration of species in DLC films determined by RBS and ERD methods. Sample
C (at%)
D (at%)
H (at%)
O (at%)
PD PH MD MH
61 71 59 64
12 – 33 –
26 27 5 33
1–2 1–2 2–3 2–3
regions, several bands can overlap, which makes the vibrational analysis particularly difficult. As shown in Fig. 3a, the region 1250–1500 cm 1 has been decomposed using several vibrations at 1280 cm 1, which can be assigned to sp2 CH aromatic, at 1325 cm 1 and 1370 cm 1, which can be assigned to sp3 CH3 sym., at 1411 cm 1 which can be assigned to sp2 CH2, and at 1425 cm 1, which can be assigned to sp2 CH olefinic [17]. The region 2700–3200 cm 1 has been decomposed using vibrations at 2850 cm 1, which can be assigned to sp3 CH3 sym, at 2920 cm 1, which can be assigned to sp3 CH2 asym, at 3014 cm 1, which can be assigned to sp2 CH olefinic and at 3065 cm 1, which can be assigned to sp2 CH2 asym [17]. In the case of sample MD, the vibrational amplitudes are smaller than in other samples. The authors in [18] wrote that, vibrational amplitudes at carbon atoms in the joint C–D vibrational modes are larger than in the joint C–H modes, due to the mass ratio m(D)/m(C) 4m(H)/m(C). However, the vibrational relaxation rate for the D stretch is smaller than for the H stretch, because the energy is dissipated to an acoustic phonon in the case of C–D rather than an optical phonon as in the case of C–H, and hence, the corresponding phonon density of states is lower in the C–D case. We propose, that in our samples vibrational intensity mostly depends on the IR active C–H bonds than on C–D bonds. Another explanation that can be considered is that difference in bonding energy between the C–H bond and C–D bond. The a-C:H film has C–H bonds from CH, CH2 and CH3 which form the carbon network. Fewer IR active C–D bonds might have formed because the C–D bonds required greater bonding energy than the C–H bonds, leading to a rate of
J. Huran et al. / Nuclear Instruments and Methods in Physics Research A 753 (2014) 14–18
RAMAN
Intensity (a.u.)
MD MH PD PH MD MH PD PH
1000
1200
1400
1600
Raman shift (cm-1)
Peak Sum
Intensity (a.u.)
RAMAN
MD-sample 1557 cm G Peak
Exp. Intensity Peak Sum G Peak D Peak
1397 cm D Peak
-1
-1
17
The intensity ratio I(D)/I(G) ¼1.133. In the case of samples MD, PD the G peak position was found to shift to higher frequencies than in the case of samples MH, PH. This can be attributed to the reduction of sp2 sites of aromatic rings. The D peak position was found to shift to higher frequencies for samples MD, PD than in the case of samples MH, PH. In general, the D peak position will increase with increasing disorder for the double-Gaussian fit. Two factors can shift the D peak. On the one hand, smaller aromatic clusters have higher modes and shift D upwards. On the other hand, a decrease in number of ordered aromatic rings on passing from nanocrystalline graphite to a-C lowers D and reduces its intensity, due to softening of the vibrational density of states [21]. From the Raman spectra of all samples, the I(D)/I(G) ratio is higher for samples MD and PD prepared by both technologies with deuterium gas and the I(D)/I(G) ratio is higher for the samples prepared by reactive magnetron sputtering for both hydrogen and deuterium in the gas mixture. It is known that the ion density and energy which are related to the plasma density and electron temperature are higher in the magnetron plasma discharge than in the PECVD plasma discharge. The raised ion energy which causes weaker bonds to be broken can supply sufficient energy for the forming of stronger bonds. In other words, the s bonds which are stronger than π bonds and are the infrastructure of sp3 bonds could be formed. We propose that the deuterium is more effective for the disordering in the MD sample technology than in the PD sample technology due to higher ion deuterium density in the magnetron plasma discharge and the deuterium is more effective than hydrogen for disordering in both technologies. 3.2. Quantum efficiency calculations
800
1000
1200
1400
Raman shift
1600
1800
(cm-1)
Fig. 4. Experimental (correct baseline) Raman spectra of the DLC films for all samples MD, MH, PD, PH (a) and Raman spectrum of the sample MD (b) with fitting the experimental curve by Gaussian centers G and D.
a reaction for C–H bonds that is typically 6 to 10 times faster than the corresponding C–D bonds [19]. Fig. 4a shows the experimental correct baseline Raman spectra of all samples. All disordered carbons show common features in their Raman spectra in 800–2000 cm 1 region, the so-called G and D peaks, which lie at around 1560 and 1360 cm 1, respectively, for visible excitation. The G peak is due to the bond stretching of all pairs of sp2 atoms in both rings and chains. The D peak is due to the breathing modes of sp2 atoms in rings and it is activated by disorder [20]. According to the three-stage model of Ferrari and Robertson [21], the visible Raman parameters can be used to derive the sp3 fraction. Fig. 4b shows Raman spectrum of the deuterated DLC film on the Si substrate, sample MD (deposited by reactive magnetron sputtering with deuterium as a reactive gas), which were Gaussian-fitted and identified as D and G band conversions. We used only two main peaks Gaussian fitting. The G peak and D peak are located at two positions 1557 cm 1 and 1397 cm 1, respectively. The intensity ratio I(D)/I(G) ¼1.723, and was determined after fitting analysis of the spectrum. For the sample MH (deposited by reactive magnetron sputtering with hydrogen as reactive gas), the G peak and D peak are located at two positions 1544 cm 1 and 1370 cm 1, respectively. The intensity ratio I(D)/I(G)¼ 1.473. For the sample PD (deposited by PECVD technology with deuterium in gas mixture), the G peak and D peak are located at two positions 1553 cm 1 and 1390 cm 1, respectively. The intensity ratio I(D)/ I(G)¼ 1.443. For the sample PH (deposited by PECVD technology with hydrogen in the gas mixture), the G peak and D peak are located at two positions 1544 cm 1 and 1376 cm 1, respectively.
Fig. 5(a) shows the bunch charge as a function of the laser energy for all prepared photocathodes. The bunch charge increased from 1156 to 2740 pC as the laser energy increased from 2.4 to 4.8 mJ for the MD photocathode. In the case of the MH photocathode, the bunch charge increased from 870 to 1666 pC. For the PD and PH photocathode the bunch charge increased from 804 to 1523 pC and 782 to 1230 pC, respectively, as the laser energy increased from 2.4 to 4.8 mJ. The increase in the laser energy indicates the increase in the number of photons causing photoemission, which leads to the increase in the number of photoelectrons produced at the cathode surface. The bunch charge intensity trend of the four photocathodes shown in Fig. 5 evidences a dependence of the photoemission properties on the graphitic component inside the DLC films. We define the quantum efficiency (QE) as the ratio of the number of electrons measured at the exit of the DC-gun (Fig. 1) to the number of photons arriving at the cathode. The number of photons is measured with a calibrated joulemeter. The transmission of the vacuum window and the mesh type cathode are taken into account. Usually, we measure the extracted charge Q as a function of the laser energy EL. The QE is calculated from a linear fit to the data points at low charge densities, where space charge effects are negligible and the relation Q (EL) is linear. The quantum efficiency was calculated using the formula QE ¼ N e =N υ where Ne is the number of electrons and Nυ is the number of photons. The quantum efficiency results for all transmission photocathodes with related I(D)/I(G) ratio of DLC films are presented in Table 2. Fig. 5b shows the quantum efficiency and the I(D)/I(G) ratio as a function of the type (technology parameters) of the prepared photocathode. On the basis of the present results it can be concluded that the QE from the MD photocathode is practically two times higher than the QE from the PD photocathode. Moreover, these results can be directly connected with the intensity I(D)/I(G) ratio of D and G peaks in Raman spectra. The intensity I(D)/I(G) ratio of the
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3000 PH PD MH MD
Bunch charge (pC)
2500
2000
1500
1000
500 2.5
3.0
3.5
4.0
4.5
5.0
Laser energy (mJ/pulse)
3.0x10-4 1.8
QE(%)
2.0x10-4
I(D)/(G)
1.6
2.5x10-4
1.4 1.2 1.0 PH
PD
MH
Acknowledgment
MD
1.5x10-4
1.0x10-4
the films contained carbon, hydrogen, deuterium and small amount of oxygen. The FT-IR spectra of samples PD, PH and MD, MH show well defined bending bands and stretching bands. We propose that in our samples vibrational intensity mostly depends on the IR active C–H bonds than on the C–D bonds. Raman spectra of the DLC films show the D and G band conversion which were Gaussian-fitted and identified. In the case of samples MD and PD the G peak position was found to shift to higher wave number than in samples MH and PH. The D peak position was found to shift to higher frequencies for samples MD, PD than in the case of samples MH, PH. In general, the D peak position increases with increasing disorder for the double-Gaussian fit. The I(D)/I(G) ratios were higher in the case of samples MD, PD. The emission process from diamond like carbon films, excited with near-UV radiation, seems to be governed mainly by the intensity ratios I(D)/I(G). The quantum efficiency was 2.68 10 4 for the MD photocathode and 1.20 10 4 for the PD photocathode at laser wavelength 266 nm. A further study is underway to investigate the nature of deuterium on the structural and surface properties of a deuterated DLC film together with the quantum efficiency of the prepared transmission mesh photocathode. The development of photocathodes in the form of grid is perspective direction for DC gun transmission cathode type.
PH
PD
MH
MD
Fig. 5. Bunch charge versus laser energy for all photocathodes (a), quantum efficiency and I(D)/I(G) ratios as a function of type and technology parameters of the prepared photocathodes (b).
Table 2 Quantum efficiency results for all transmission photocathodes with related data intensity ratio I(D)/I(G) of DLC films. Sample
Quantum efficiency
Intensity ratio I(D)/I(G)
PD PH MD MH
1.49 10 4 1.20 10 4 2.68 10 4 1.63 10 4
1.443 1.133 1.723 1.473
MD, MH films are higher than for the PD, PH films. The I(D)/I(G) ratio can indirectly determine the content of sp2 and sp3 bonds. However, higher value of the I(D)/I(G) ratio corresponds with the higher quantum efficiency of the diamond like carbon film coated mesh type transmission photocathode. The emission mechanism in DLCs has been difficult to understand. Experimental evidence suggests that the main barrier to emission in a DLC is large and at the front surface. The barrier's lowering requires some types of heterogeneity [3]. In our case, the obtained results show that heterogeneity can be deliberately introduced into a DLC by deuterium mix in working gases in both technologies. 4. Conclusions We investigated the structural properties of the DLC films prepared by plasma enhanced chemical vapor deposition and reactive magnetron sputtering with deuterium and hydrogen in the gas mixture on the Si substrate and stainless steel mesh for the transmission photocathode. RBS and ERD analyses indicated that
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