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CH4 microwave plasma
Diamond & Related Materials 20 (2011) 232–237
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Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Investigation in the role of hydrogen on the properties of diamond films grown using Ar/H2/CH4 microwave plasma☆ K.J. Sankaran a, P.T. Joseph b, H.C. Chen b, N.H. Tai a,⁎, I.N. Lin b a b
Department of Materials Science and Engineering, National Tsing-Hua University, Hsin-Chu 300, Taiwan, ROC Department of Physics, Tamkang University, Tamsui 251, Taiwan, ROC
a r t i c l e
i n f o
Available online 13 December 2010 Keywords: Diamond films Electron field emission Optimization
a b s t r a c t The transition of diamond grain sizes from micron- to nano- and then to ultranano-size could be observed when hydrogen concentration is being decreased in the Ar/CH4 plasma. When grown in H2-rich plasma (H2 = 99% or 50%), well faceted microcrystalline diamond (MCD) surface with grain sizes of less than 0.1 μm are observed. The surface structure of the diamond film changes to a cauliflower-like geometry with a grain size of around 20 nm for the films grown in 25% H2-plasma. In the Ar/CH4 plasma, ultrananocrystalline diamond (UNCD) films are produced with equi-axed geometry with a grain size of 5–10 nm. The H2-content imposes a more striking effect on the granular structure of diamond films than the substrate temperature. The induction of the grain growth process, either by using H2-rich plasma or a higher substrate temperature increases the turn-on field in the electron field emission process, which is ascribed to the reduction in the proportion of grain boundaries. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The unique properties of diamond, such as extreme hardness, low friction coefficient, chemical inertness, high electrical resistivity, semiconductivity when doped, excellent thermal conductivity, and good biocompatibility, make it a very attractive material for a wide range of engineering applications [1,2]. Diamond is also proposed as an ideal candidate for electron field emission (EFE) that has potential applications in the areas of flat panel display and microelectronic devices [3]. There are substantial researches carried out on the growth, properties and applications of single-crystalline and MCD in the last few decades. Atomic hydrogen has been recognized to play a crucial role of any gas mixture in the growth of phase-pure MCD films by the chemical vapor deposition (CVD) techniques, typically using hydrocarbons as the carbon source [4–6]. It is the key in a number of vital steps in diamond deposition, from etching away any growing sp2-bonded nuclei, stabilizing the surface of the growing diamond nuclei, and abstracting the surface hydrogen to provide reactive sites for the methyl radical absorption [7]. The ability to control the microstructure and surface morphology of diamond films, therefore, could be important for tailoring this unique material to a variety of applications. However, the high surface roughness of microcrystalline diamond (MCD) films usually from hydrogen-rich plasma makes them inapplicable in some specific applications. The reduction in diamond grain size may increase the ☆ Presented at NDNC 2010, the 4th International Conference on New Diamond and Nano Carbons, Suzhou, China. ⁎ Corresponding author. Tel.: + 886 2 2626 8907; fax: + 886 2 2620 9917. E-mail address: [email protected] (N.H. Tai). 0925-9635/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2010.12.018
conducting pathways; and it is possible to improve diamond EFE by depositing size controlled diamond films. Zhou et. al., [3] reported the growth of nanocrystalline diamond (NCD) films by microwave plasma enhanced CVD, with an emission threshold as low as 1 V/μm. Furthermore, in the microwave induced methane–hydrogen plasma, noble gases were found to have a profound effect on plasma chemistry, including additional ionization and dissociation. Gruen and his coworkers reported the use of noble gas dilution using the microwave plasma enhanced chemical vapor deposition (MPECVD) system [8–14]. Moreover, Lin et. al., [15] presented the growth of NCD from Ar/H2/CH4 systems and well faceted diamond deposition was achieved using Ar concentration of N90%. There is a noticeable transition in morphology and structure from microcrystalline to nanocrystalline with an Ar mixture N95.5%, through to ultrananocrystalline diamond (UNCD) films, in atmospheres greater than 98% Ar. UNCD films possess many excellent properties and several of them actually exceed those of diamond [16]. As the diamond grain size in UNCD film is less than 10 nm, surface smoothness was improved markedly, making it as a promising material for tribiological applications [17]. The decrease in diamond grain size increases the proportion of grain boundaries containing non-diamond carbon, which play a crucial role for applications like electron field emitters [18]. Additionally, the microstructure of UNCD films is extremely sensitive to the growth parameters, especially the substrate temperature. Reports show that the growth of UNCD films synthesized with Ar/CH4 plasma is a thermally activated process, leading to the increase in growth rate with increasing temperature [19]. However a systematic investigation is still talking in this area. In the present work, we focus on understanding better the role hydrogen plays in diamond films grown from Ar/H2/CH4 plasma. The
K.J. Sankaran et al. / Diamond & Related Materials 20 (2011) 232–237 Table 1 Summary of the process parameters used for diamond film depositions from microwave discharges. Gases constituent (sccm)
H2-rich plasma Ar-rich plasma
CH4
Ar
H2
1 1 1 1 1
0.0 49.0 74.0 97.5 99.0
99.0 50.0 25.0 1.5 0.0
Microwave power (W)
Pressure (torr)
1600 1300 1200 1200 1200
50 80 100 110 125
factors leading to a transition of diamond grain size from micron- to nano- and then to ultranano-size when hydrogen concentration is being decreased in the plasma has been investigated. In addition, we have also investigated the effect of substrate temperature on the structural modification and the properties of diamond films. Our observation revealed that changing the ratio of H2 to Ar and the substrate temperature facilitates the control of grain sizes of diamond. 1.1. Experimental details Diamond films with different grain sizes are grown on n-type Si wafers in a MPECVD system (IPLAS Cyrranus, deposition time of 1 h). Substrates were ultrasonically seeded using a mixture of diamond and titanium powders (1:1) in a methanol solution for 45 min. Mixtures of CH4, Ar and H2 were used as the reactant gases for the microwave discharges. The flow rate of CH4 was kept constant at 1 sccm; while the flow rate of H2 was varied from 0 to 99 sccm and supplemented by Ar so as to maintain a 100 sccm total flow rate. The characteristics of diamond films profoundly vary with the process parameters such as pressure and microwave power. We have systematically examined the effect of these parameters on the microstructure and Raman spectra of the materials.
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Only the films, which were grown in optimized parameters with respect to each plasma constituents (Ar/H2/CH4 ratio), were included in this paper for discussion. The optimized growth parameters of the reactant gases used for each film deposition are listed in Table 1. Furthermore, UNCD films with 1.5% of H2 concentration (abbreviated as 1.5% H2UNCD films) were grown for 1 h on various substrate temperatures ranging from 550 to 750 °C. The SEM images were observed. As-grown diamond films were characterized by a field emission scanning electron microscope (FESEM, JEOL 6010) to examine the surface and growth morphologies of the films in terms of decreasing H2 content. Microstructure of the UNCD films grown under various substrate temperatures was examined using transmission electron microscopy (TEM, JEOL-JEM2100F). In order to obtain information on plasma species during the deposition process, optical emissions were measured using Optical Emission Spectroscopy (OES). Crystal quality of the diamond films was investigated by microRaman spectroscopy using 532 nm Nd:YAG laser beam. Room temperature EFE properties of diamond films were measured with an electrometer (Keithley 237) using a parallel cathode–anode setup [20], and the Fowler–Nordheim (FN) theory [21] is used to explain the EFE behavior of materials. The turn-on field is designated as the interception of the straight lines extrapolated from the low field and high field segments of the FN plots, viz. ln(J/E2) vs. 1/E. 2. Results and discussion The FESEM images in Fig. 1(a–e) shows a dramatic change in the surface morphology of the films when decreasing H2 gas in the (Arx-H2 (1 − x))/CH4 microwave plasma. Fig. 1(a) shows the surface morphology of the 99% H2-diamond films, displaying a well-faceted MCD with grain size less than 0.1 μm with rough surface. Fig. 1(b) shows that, for 50% H2-films, small diamond grains start to form on the surface of bigger diamond grains during the deposition process,
Fig. 1. SEM images of as-grown diamond films prepared in H2-rich plasma containing (a) 99% H2 and (b) 50% H2, Ar-rich plasma containing (c) 25% H2, (d) 1.5% H2, and (e) 0% H2 (the CH4 is 1% and the rest is Ar). (f) Variation of grain size and growth rates of the diamond films prepared in H2-rich and Ar-rich plasma.
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suggesting that secondary nucleation or re-nucleation starts. Contrary to the faceted grains for the diamond films grown in H2-rich plasma, the morphology of films synthesized in Ar-rich plasma is changed markedly. Fig. 1(c) shows that when the H2 concentration is decreased to 25% (25% H2-films), faceted micron size crystals disappeared and are replaced by cauliflower-like grains with a grain size of about 20 nm (NCD). Fig. 1(d) and (e) show that the crystal sizes of the NCD can be further reduced to a grain size of 5–10 nm by decreasing the H2-content to 1.5% and 0% and designate as UNCD respectively. These series of SEM micrographs demonstrate the
Intensity (arb. units)
(a) H2-rich plasma D
G
50% H2 99% H2
1000
1200
1400
1600
1800
Raman shift (cm-1)
Intensity (arb. units)
(b) Ar-rich plasma
transition of microstructure of the films from MCD to NCD and then to UNCD when hydrogen concentration is decreased in the plasma. Besides the surface morphology, the growth rate and grain sizes of diamond films has also been found to change noticeably on the ratio of Ar to H2 in the reactant gas because of effects on the microwave discharge chemistry. The growth rate of the diamond films is estimated from the cross-section SEM micrographs of the films (figures not shown). Fig. 1(f) shows that the growth rate is 280 nm/h for H2 = 99% film and tripled for 50% H2 in the plasma. The growth rate decreases rapidly for low H2-content films. Although the decreasing of H2 concentration in the reactant gases from 25 to 0% significantly increases the concentration of C2 dimer in the plasma, the growth rate in this region nonetheless decreases rapidly from 440 nm/h for 25% H2-films to 200 nm/h for 0% H2 ones. Restated, the growth behavior of diamond films synthesized in H2-rich plasma is completely different from that deposited in Ar-rich plasma. To characterize the structure of the diamond films grown at various H2 concentrations, micro-Raman spectroscopy was employed. Raman spectra of the diamond films (Fig. 2(a–b)), reveal the transition from MCD to NCD and then to UNCD films. The spectrum corresponds to H2 = 99% (Fig. 2(a)) which shows a sharp Raman peak at 1332 cm− 1. The 50% H2-spectrum (Fig. 2(a)) also demonstrates the presence primarily of MCD, with the peak slightly broadened. Further decreasing H2 concentration in the plasma results in the shrinking of the diamond peak (25% H2 spectrum, Fig. 2(b)). When there is a low percentage of H2, the spectra show nanocrystalline features (1.5% H2 and 0% H2 spectra, Fig. 2(b)). The diamond peak at 1332 cm− 1 is significantly broadened, and Raman scattering intensity in the 1400– 1600 cm− 1 region is pronounced. The Raman peaks corresponding
G
D
0% H2 1.5% H2
25% H2
1000
1200
1400
1600
1800
Raman shift (cm-1)
(c) Tsub Intensity (arb. units)
G
D
o
750 C 700oC 650oC o
600 C
550oC
1000
1200
1400
1600
1800
Raman shift (cm-1) Fig. 2. Raman spectra of diamond films grown in (a) H2-rich plasma, H2 = 99% and 50%, and (b) Ar-rich plasma, H2 = 25%, 1.5% and 0%. (c) Raman spectra of the diamond films grown under various substrate temperatures from 550 to 750 °C in 1.5% H2-plasma.
Fig. 3. OES spectra of Ar/H2/CH4 microwave plasma (a) H2-rich plasma: H2 = 99% and 50% and (b) Ar-rich plasma: H2 = 25%, 1.5% and 0%.
K.J. Sankaran et al. / Diamond & Related Materials 20 (2011) 232–237
(a) H2-rich plasma -10 0.25
II.
-12
0.20 2
-14
J (mA/cm )
ln (J/E2) ([mA/cm2]/[V/µm])
the sp3-peak (1332 cm− 1) decreases as H2-content decreases and is only barely observable for 0% H2-spectrum. Furthermore, Raman spectra that show the influence of substrate temperature for the 1.5% H2-UNCD films are shown in Fig. 2(c) which will be discussed shortly.
0.15
-16 0.0
0.1
0.10
0.2
1/E [µ m/V ]
I. 0.05
I. 99% H2 II. 50% H2
0.00 0
20
40
60
80
100
120
140
(b) Ar-richplasma I.
-10
2.5
II.
-12
2.0
-14 1.5 -16
III. 0.1 1/E [µm /V]
0.0
0.2
1.0
I. 0% H2 II. 1.5 % H2
0.5
III. 25% H2 0
20
J (mA/cm2)
ln (J/E2) ([mA/cm2]/[V/µm])
E (V/µm)
0.0 40
60
80
100
120
140
(c) Tsub -8 -10
Table 2 Field emission data of (a) as-grown diamond films with varying Ar/H2/CH4 microwave discharges and (b) ultrananocrystalline diamond (UNCD) films with varying substrate temperatures (from 550 °C to 750 °C).
2.0
-12 -14
1.5
II. -16 0.0
The OES of the plasma is examined to investigate how the change in H2 into the plasma modifies the microstructure of the diamond films. Fig. 3 shows typical OES spectra of the plasma used for growing these diamond films with different ratios of H2 and Ar, revealing that OES of H2-rich plasma is completely different from those for Ar-rich ones. In the 99% and 50% H2 plasma (Fig. 3(a)), the major optical emission peak is due to the presence of atomic hydrogen species Hα (656.3 nm). However, the proposed diamond growing species, CH3+, is not observable in these OES spectra [22]. In 25% H2-plasma (Fig. 3(b)), in addition to Hα line, we begin to observe emission from C2 dimers, which is proposed as the diamond growing species in the Ar-rich plasma [1]. The Ar-rich plasma (H2 b 25%) are predominated with the Swan-band near 516.5 nm, that mainly contains the C2 species in addition to the small amount of CH species (431 nm) [23]. There also exist two other C2 bands near 563 nm and 474 nm. The spectral lines corresponding to atomic hydrogen are essentially not observable. Further decreasing the concentration of H2 (1.5% and 0%) increases the emission intensity of C2 species (Fig. 3(b)). Restated, the H2-rich plasma tends to grow faceted large diamond grains, whereas the Ar-rich plasma tends to form nanosize diamond grains. This result is in accordance with the rapid change in the microstructure of the diamond films (Fig. 1(a–e)). The effect of H2/Ar ratio in the plasma on the EFE properties of the diamond films is shown in Fig. 4(a) and (b). The insets show the FN plot of the corresponding field emission data and explain that ln(J/E2) is almost linear with 1/E in the high field segment, indicating that the field emission current is controlled predominantly by the FN tunneling of electrons at the diamond surface [24,25]. The EFE parameters, including the turn-on field and EFE current density are extracted from these figures and are summarized in Table 2(a and b). Fig. 4(a) indicates that the 99% H2-MCD films can be turned on at (E0)H99 = 69.0 V/μm, whereas 50% H2-MCD films can be turned on at (E0)H50 = 53.3 V/μm. The EFE current density for these films are very small, i.e., b0.2 mA/cm2 at 105 V/μm applied field. Faceted granular structure with low conductivity for these MCD films is presumably the prime factor resulting in inferior EFE properties for these films. The turn-on field decreases and the EFE current density increases monotonously with the decreasing H2-content in the plasma used for growing diamond films. Fig. 4(b) shows that, for the 25% H2-NCD films, the turn-on field decreases to 50.0 V/μm (E0)H25, and EFE current density increases to (Je)H25 = 1.15 mA/cm2 at 125 V/μm applied field ((Je)H25 b 0.3 mA/cm2 at 105 V/μm), whereas 1.5% H2-UNCD films could be turned on at (E0)H1.5 = 33.8 V/μm with (Je)H1.5 = 2.31 mA/cm2 (at 105 V/μm applied field). The 0% H2-UNCD diamond films possess the best EFE properties, that is, smallest turn-on field, (E0)H0 = 31.6 V/μm, and highest EFE current density, (Je)H0 = 2.49 mA/cm2 at 105 V/μm applied field. Owing
2.5
I.
III. 0.1
o
I.550 C 1/E o II.600 C o III.650 C o IV.700 C o V.750 C 0
20
0.2
[µm /V]
1.0
IV. V.
J (mA/cm2)
ln (J/E2) ([mA/cm2]/[V/µm])
E (V/µm)
235
0.5
0.0 40
60
80
100
120
140
E (V/µm) Fig. 4. EFE properties of the diamond films grown in (a) H2-rich plasma, H2 = 99% and 50%, and (b) Ar-rich plasma, H2 = 25%, 1.5% and 0%. (c) EFE properties of the diamond films grown under various substrate temperatures from 550 to 750 °C in 1.5% H2-plasma.
(a) Flow rate of gases (sccm) CH4
Ar
H2
1 1 1 1 1
0 49 74 97.5 99
99 50 25 1.5 0
E0 (V/μm)
Je (mA/cm2) at 105 (V/μm)
69.0 53.3 50.0 33.8 31.6
b0.02 0.20 b0.30 2.31 2.49
(b) Temperature (°C)
E0 (V/μm)
Je (mA/cm2) at 94 (V/μm)
550 °C 600 °C 650 °C 700 °C 750 °C
34.5 37.8 52.2 57.5 62.7
2.30 0.40 0.20 b 0.01 b 0.01
236
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to the smaller grain microstructures, the UNCD films contain higher proportion of grain boundaries than those of NCD and MCD films. The higher grain boundary density possibly provides electron conduction networks, which transport the electrons more efficiently to emission sites for field emission. As a result, the 0% H2- and 1.5% H2-UNCD films exhibit better field emission performance than the other large grain diamond films. Since UNCD films exhibits higher EFE properties as compared to that of the NCD and MCD films, the UNCD films were grown at higher substrate temperature so as to modify the microstructure of the films and study their effect on the EFE properties. The SEM morphologies for the 1.5% H2 films grown at different temperatures are very similar with one others (not shown). Fig. 2(c) corresponds to the Raman spectra for the 1.5% H2-UNCD films grown at 550 to 750 °C substrate temperatures. From the Raman spectra, it can be observed that the
characteristic D- and G-peaks increase with the substrate temperature deposition. In the case of the sample grown at the highest temperature (750 °C) the presence of a sharp G-peak confirms that there are especially sp2-hybridized compounds, which are accentuated at higher substrate temperatures. However, the Raman spectra vary with the different substrate in a much less prominent manner than that due to the change in gas composition. Restated, the gas composition in the plasma, rather than the substrate temperature, is the predominating factor modifying the granular structure of the diamond films. The EFE properties for 1.5% H2-UNCD films vary pronouncedly with substrate temperatures, which are shown in Fig. 4(c) and the EFE parameters are listed in Table 2(b). This reveals that, as the substrate temperature increases from 550 to 750 °C, the turn-on field increases monotonically from 34.5 to 62.7 V/μm, whereas the EFE current-
Fig. 5. TEM micrographs of the ultrananocrystalline diamond (1.5% H2-UNCD) films grown under (a) 550 °C, (b) 650 °C, and (c) 750 °C substrate temperature. The insets show the corresponding SAED pattern.
Fig. 6. HRTEM images of the ultrananocrystalline diamond (UNCD) films with 1.5% H2 grown under (a) 550 °C, (b) 650 °C, and (c) 750 °C substrate temperatures. The insets show the corresponding regions from which the images were taken.
K.J. Sankaran et al. / Diamond & Related Materials 20 (2011) 232–237
density at 94 V/μm decreases monotonously from (Je) 550C = 2.30 mA/ cm2 to (Je) 750C b 0.01 mA/cm2. These EFE results reveal that higher temperature degraded the EFE properties for the UNCD films. While the change in EFE properties with the H2-content in plasma can be correlated closely with the modification on the granular structure for the films synthesized in different Ar-H2 plasma, the explanation on the change in EFE properties with the substrate temperature for 1.5% H2-UNCD films is not as straight forward. The EFE properties of the 1.5% H2-UNCD films at various temperatures is further explained using the nanoscale microstructural changes, which are observed from TEM micrographs for the temperatures 550 and 750 °C respectively. Fig. 5 shows that, generally, the 1.5% H2-UNCD films deposited at low temperatures exhibit a very similar nanoscale structure compared to the films grown at higher temperatures. The SAED patterns shown as insets indicate the diamond characteristics of the films. We observed that there are few large clusters formed in all the UNCD films. The size of the clusters is bigger, and larger in population, for the UNCD films grown at higher substrate temperature. However, the more significant effect due to the increase in substrate temperature is the modification on the defect structure of the materials, which is best demonstrated by the structure images. Fig. 6(a) shows the structure image from a typical cluster (inset) of UNCD films deposited at a low temperature (550 °C), showing that the 550 °C-UNCD films contain some stacking faults, i.e., parallel fringes in Fig. 6(a). The size of the clusters is bigger, accompanied with a larger proportion of defects contained in the clusters, for the UNCD films grown at 650 °C (Fig. 6(b)). Further increase in substrate temperature induces an even larger size of the clusters with a more complicated defect structure. Fig. 6(c) shows, for the films grown at 750 °C, that polymorphs of cubic diamonds such as 2H and 8H diamonds even formed. The defects are presumably induced by the abnormal growth of the diamond grains. The larger the grain growth rate is, the more complex the defect structure. In consequence of the cluster formation, the proportion of grain boundaries are very much lowered for the films grown at a high temperature compared with the low temperature grown UNCD films. Accordingly, there is a confinement in the emission of electrons from the emission sites which could be a reason for the decrease in the EFE property of the films grown at high substrate temperatures. 3. Conclusions The modification on microstructure of diamond films due to the incorporation of H2 species into the Ar/CH4 plasma has been systematically investigated. We report that systematic change in the ratio of the reactant gases is the way to control the nucleation rate, surface morphology, grain size, and the growth mechanism of diamond films. There is a strong correlation between the field emission properties and the microstructure of diamond films. The
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high grain boundary density can also provide electron conduction networks, which transport the electrons more efficiently to emission sites for field emission. Consequently, UNCD films have higher densities of grain boundary than those of NCD and MCD films, resulting in the highest field emission enhancement among all the diamond films. In addition, for 1.5% H2-UNCD films, higher substrate temperature (750 °C) ensuing a larger size of the clusters with more complicated defect structure including polymorphs of cubic diamonds as compared with the low substrate temperature grown UNCD films. As a result, there forms a smaller proportion of grain boundary density for the high temperature grown films, which results in fewer emission sites for electrons and leads to inferior field emission properties than those grown at lower temperature (550 °C). Acknowledgements The authors would like to thank the National Science Council, Republic of China for the support of this research through the Project No. NSC99-2119-M-032-003-MY2. References [1] D. Zhou, T.G. McCauley, L.C. Qin, A.R. Krauss, D.M. Gruen, J. Appl. Phys. 83 (1998) 540. [2] C. Popov, W. Kulisch, S. Boycheva, K. Yamamoto, G. Ceccone, Y. Koga, Diamond Relat. Mater. 13 (2004) 2071. [3] D. Zhou, A.R. Krauss, L.C. Qin, T.G. McCauley, D.M. Gruen, T.D. Corrigan, R.P.H. Chang, H. Gnaser, J. Appl. Phys. 82 (1997) 4546. [4] B.V. Deryagin, D.V. Fedoseev, N.D. Polyanskaya, E.V. Statenkova, Sov. Phys. Crystallogr. 21 (1976) 239. [5] S. Matsumoto, Y. Sato, M. Tsutsumi, N. Setaka, J. Mater. Sci. 17 (1981) 3106. [6] P.K. Bachman, D. Leers, H. Lydtin, Diamond Relat. Mater. 1 (1991) 1. [7] D.M. Gruen, Annu. Rev. Mater. Sci. 29 (1999) 211. [8] C.S. Wang, H.C. Chen, H.F. Cheng, I.N. Lin, J. Appl. Phys. 107 (2010) 034304. [9] A. Erdemir, G.R. Fenske, A.R. Krauss, D.M. Gruen, T.G. McCauley, R.T. Csencsits, Surf. Coat. Technol. 565 (1999)8 120/121. [10] R. Krauss, D.M. Gruen, Diamond Relat. Mater. 10 (2001) 1952. [11] L.C. Qin, D. Zhou, A.R. Krauss, D.M. Gruen, Nanostruct. Mater. 10 (1998) 649. [12] V.I. Konov, S.M. Pimenov, A. Erdemir, M. Halter, G.R. Fenske, A. Krauss, D.M. Gruen Surf, Coat. Technol. 94 (1997) 537. [13] C. Zuiker, A.R. Krauss, D.M. Gruen, X. Pan, J.C. Li, R. Csencsits, A. Erdemir, C. Bindal, G. Fenske, Thin Solid Films 270 (1995) 154. [14] A. Erdemir, C. Bindal, G.R. Fenske, C. Zuiker, A.R. Krauss, D.M. Gruen, Diamond Relat. Mater. 5 (1996) 923. [15] T. Lin, G.Y. Yu, A.T.S. Wee, Z.X. Shen, Appl. Phys. Lett. 77 (2000) 2692. [16] J.A. Carlisle, O. Auciello, Electrochem. Soc. Interface 28 (2003)8 spring. [17] C.D. Zuiker, A.R. Krauss, D.M. Gruen, X. Pan, J.C. Li, R. Csencsits, A. Erdemir, C. Bindal, G. Fenske, Thin Solid Films 270 (1995) 154. [18] D. Zhou, A.R. Krauss, T.D. Corrigan, T.G. McCauley, R.P.H. Chang, D.M. Gruen, J. Electrochem. Soc. 144 (1997) L224. [19] D.C. Barbosa, F.A. Almeida, R.F. Silva, N.G. Ferreira, V.J. Trava-Airoldi, E.J. Corat, Diamond Relat. Mater. 18 (2009) 1283. [20] Y.C. Lee, S.J. Lin, I.N. Lin, H.F. Cheng, J. Appl. Phys. 97 (2005) 054310. [21] R.H. Fowler, L. Nordheim, Proc. R. Soc. London Ser. A 119 (1928) 173. [22] J. Ma, N. Michael, R. Ashfold, Y.A. Mankelevich, J. Appl. Phys. 105 (2009) 043302. [23] D. Zhou, D.M. Gruen, L.C. Qin, T.G. McCauley, A.R. Krauss, J. Appl. Phys. 84 (1998) 1981. [24] M.W. Geis, N.N. Efremow, K.E. Krohn, Nature 393 (1998) 431. [25] J.Y. Shim, E.J. Chi, H.K. Baik, K.M. Song, Thin Solid Films 355 (1999) 223.
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Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Investigation in the role of hydrogen on the properties of diamond films grown using Ar/H2/CH4 microwave plasma☆ K.J. Sankaran a, P.T. Joseph b, H.C. Chen b, N.H. Tai a,⁎, I.N. Lin b a b
Department of Materials Science and Engineering, National Tsing-Hua University, Hsin-Chu 300, Taiwan, ROC Department of Physics, Tamkang University, Tamsui 251, Taiwan, ROC
a r t i c l e
i n f o
Available online 13 December 2010 Keywords: Diamond films Electron field emission Optimization
a b s t r a c t The transition of diamond grain sizes from micron- to nano- and then to ultranano-size could be observed when hydrogen concentration is being decreased in the Ar/CH4 plasma. When grown in H2-rich plasma (H2 = 99% or 50%), well faceted microcrystalline diamond (MCD) surface with grain sizes of less than 0.1 μm are observed. The surface structure of the diamond film changes to a cauliflower-like geometry with a grain size of around 20 nm for the films grown in 25% H2-plasma. In the Ar/CH4 plasma, ultrananocrystalline diamond (UNCD) films are produced with equi-axed geometry with a grain size of 5–10 nm. The H2-content imposes a more striking effect on the granular structure of diamond films than the substrate temperature. The induction of the grain growth process, either by using H2-rich plasma or a higher substrate temperature increases the turn-on field in the electron field emission process, which is ascribed to the reduction in the proportion of grain boundaries. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The unique properties of diamond, such as extreme hardness, low friction coefficient, chemical inertness, high electrical resistivity, semiconductivity when doped, excellent thermal conductivity, and good biocompatibility, make it a very attractive material for a wide range of engineering applications [1,2]. Diamond is also proposed as an ideal candidate for electron field emission (EFE) that has potential applications in the areas of flat panel display and microelectronic devices [3]. There are substantial researches carried out on the growth, properties and applications of single-crystalline and MCD in the last few decades. Atomic hydrogen has been recognized to play a crucial role of any gas mixture in the growth of phase-pure MCD films by the chemical vapor deposition (CVD) techniques, typically using hydrocarbons as the carbon source [4–6]. It is the key in a number of vital steps in diamond deposition, from etching away any growing sp2-bonded nuclei, stabilizing the surface of the growing diamond nuclei, and abstracting the surface hydrogen to provide reactive sites for the methyl radical absorption [7]. The ability to control the microstructure and surface morphology of diamond films, therefore, could be important for tailoring this unique material to a variety of applications. However, the high surface roughness of microcrystalline diamond (MCD) films usually from hydrogen-rich plasma makes them inapplicable in some specific applications. The reduction in diamond grain size may increase the ☆ Presented at NDNC 2010, the 4th International Conference on New Diamond and Nano Carbons, Suzhou, China. ⁎ Corresponding author. Tel.: + 886 2 2626 8907; fax: + 886 2 2620 9917. E-mail address: [email protected] (N.H. Tai). 0925-9635/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2010.12.018
conducting pathways; and it is possible to improve diamond EFE by depositing size controlled diamond films. Zhou et. al., [3] reported the growth of nanocrystalline diamond (NCD) films by microwave plasma enhanced CVD, with an emission threshold as low as 1 V/μm. Furthermore, in the microwave induced methane–hydrogen plasma, noble gases were found to have a profound effect on plasma chemistry, including additional ionization and dissociation. Gruen and his coworkers reported the use of noble gas dilution using the microwave plasma enhanced chemical vapor deposition (MPECVD) system [8–14]. Moreover, Lin et. al., [15] presented the growth of NCD from Ar/H2/CH4 systems and well faceted diamond deposition was achieved using Ar concentration of N90%. There is a noticeable transition in morphology and structure from microcrystalline to nanocrystalline with an Ar mixture N95.5%, through to ultrananocrystalline diamond (UNCD) films, in atmospheres greater than 98% Ar. UNCD films possess many excellent properties and several of them actually exceed those of diamond [16]. As the diamond grain size in UNCD film is less than 10 nm, surface smoothness was improved markedly, making it as a promising material for tribiological applications [17]. The decrease in diamond grain size increases the proportion of grain boundaries containing non-diamond carbon, which play a crucial role for applications like electron field emitters [18]. Additionally, the microstructure of UNCD films is extremely sensitive to the growth parameters, especially the substrate temperature. Reports show that the growth of UNCD films synthesized with Ar/CH4 plasma is a thermally activated process, leading to the increase in growth rate with increasing temperature [19]. However a systematic investigation is still talking in this area. In the present work, we focus on understanding better the role hydrogen plays in diamond films grown from Ar/H2/CH4 plasma. The
K.J. Sankaran et al. / Diamond & Related Materials 20 (2011) 232–237 Table 1 Summary of the process parameters used for diamond film depositions from microwave discharges. Gases constituent (sccm)
H2-rich plasma Ar-rich plasma
CH4
Ar
H2
1 1 1 1 1
0.0 49.0 74.0 97.5 99.0
99.0 50.0 25.0 1.5 0.0
Microwave power (W)
Pressure (torr)
1600 1300 1200 1200 1200
50 80 100 110 125
factors leading to a transition of diamond grain size from micron- to nano- and then to ultranano-size when hydrogen concentration is being decreased in the plasma has been investigated. In addition, we have also investigated the effect of substrate temperature on the structural modification and the properties of diamond films. Our observation revealed that changing the ratio of H2 to Ar and the substrate temperature facilitates the control of grain sizes of diamond. 1.1. Experimental details Diamond films with different grain sizes are grown on n-type Si wafers in a MPECVD system (IPLAS Cyrranus, deposition time of 1 h). Substrates were ultrasonically seeded using a mixture of diamond and titanium powders (1:1) in a methanol solution for 45 min. Mixtures of CH4, Ar and H2 were used as the reactant gases for the microwave discharges. The flow rate of CH4 was kept constant at 1 sccm; while the flow rate of H2 was varied from 0 to 99 sccm and supplemented by Ar so as to maintain a 100 sccm total flow rate. The characteristics of diamond films profoundly vary with the process parameters such as pressure and microwave power. We have systematically examined the effect of these parameters on the microstructure and Raman spectra of the materials.
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Only the films, which were grown in optimized parameters with respect to each plasma constituents (Ar/H2/CH4 ratio), were included in this paper for discussion. The optimized growth parameters of the reactant gases used for each film deposition are listed in Table 1. Furthermore, UNCD films with 1.5% of H2 concentration (abbreviated as 1.5% H2UNCD films) were grown for 1 h on various substrate temperatures ranging from 550 to 750 °C. The SEM images were observed. As-grown diamond films were characterized by a field emission scanning electron microscope (FESEM, JEOL 6010) to examine the surface and growth morphologies of the films in terms of decreasing H2 content. Microstructure of the UNCD films grown under various substrate temperatures was examined using transmission electron microscopy (TEM, JEOL-JEM2100F). In order to obtain information on plasma species during the deposition process, optical emissions were measured using Optical Emission Spectroscopy (OES). Crystal quality of the diamond films was investigated by microRaman spectroscopy using 532 nm Nd:YAG laser beam. Room temperature EFE properties of diamond films were measured with an electrometer (Keithley 237) using a parallel cathode–anode setup [20], and the Fowler–Nordheim (FN) theory [21] is used to explain the EFE behavior of materials. The turn-on field is designated as the interception of the straight lines extrapolated from the low field and high field segments of the FN plots, viz. ln(J/E2) vs. 1/E. 2. Results and discussion The FESEM images in Fig. 1(a–e) shows a dramatic change in the surface morphology of the films when decreasing H2 gas in the (Arx-H2 (1 − x))/CH4 microwave plasma. Fig. 1(a) shows the surface morphology of the 99% H2-diamond films, displaying a well-faceted MCD with grain size less than 0.1 μm with rough surface. Fig. 1(b) shows that, for 50% H2-films, small diamond grains start to form on the surface of bigger diamond grains during the deposition process,
Fig. 1. SEM images of as-grown diamond films prepared in H2-rich plasma containing (a) 99% H2 and (b) 50% H2, Ar-rich plasma containing (c) 25% H2, (d) 1.5% H2, and (e) 0% H2 (the CH4 is 1% and the rest is Ar). (f) Variation of grain size and growth rates of the diamond films prepared in H2-rich and Ar-rich plasma.
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suggesting that secondary nucleation or re-nucleation starts. Contrary to the faceted grains for the diamond films grown in H2-rich plasma, the morphology of films synthesized in Ar-rich plasma is changed markedly. Fig. 1(c) shows that when the H2 concentration is decreased to 25% (25% H2-films), faceted micron size crystals disappeared and are replaced by cauliflower-like grains with a grain size of about 20 nm (NCD). Fig. 1(d) and (e) show that the crystal sizes of the NCD can be further reduced to a grain size of 5–10 nm by decreasing the H2-content to 1.5% and 0% and designate as UNCD respectively. These series of SEM micrographs demonstrate the
Intensity (arb. units)
(a) H2-rich plasma D
G
50% H2 99% H2
1000
1200
1400
1600
1800
Raman shift (cm-1)
Intensity (arb. units)
(b) Ar-rich plasma
transition of microstructure of the films from MCD to NCD and then to UNCD when hydrogen concentration is decreased in the plasma. Besides the surface morphology, the growth rate and grain sizes of diamond films has also been found to change noticeably on the ratio of Ar to H2 in the reactant gas because of effects on the microwave discharge chemistry. The growth rate of the diamond films is estimated from the cross-section SEM micrographs of the films (figures not shown). Fig. 1(f) shows that the growth rate is 280 nm/h for H2 = 99% film and tripled for 50% H2 in the plasma. The growth rate decreases rapidly for low H2-content films. Although the decreasing of H2 concentration in the reactant gases from 25 to 0% significantly increases the concentration of C2 dimer in the plasma, the growth rate in this region nonetheless decreases rapidly from 440 nm/h for 25% H2-films to 200 nm/h for 0% H2 ones. Restated, the growth behavior of diamond films synthesized in H2-rich plasma is completely different from that deposited in Ar-rich plasma. To characterize the structure of the diamond films grown at various H2 concentrations, micro-Raman spectroscopy was employed. Raman spectra of the diamond films (Fig. 2(a–b)), reveal the transition from MCD to NCD and then to UNCD films. The spectrum corresponds to H2 = 99% (Fig. 2(a)) which shows a sharp Raman peak at 1332 cm− 1. The 50% H2-spectrum (Fig. 2(a)) also demonstrates the presence primarily of MCD, with the peak slightly broadened. Further decreasing H2 concentration in the plasma results in the shrinking of the diamond peak (25% H2 spectrum, Fig. 2(b)). When there is a low percentage of H2, the spectra show nanocrystalline features (1.5% H2 and 0% H2 spectra, Fig. 2(b)). The diamond peak at 1332 cm− 1 is significantly broadened, and Raman scattering intensity in the 1400– 1600 cm− 1 region is pronounced. The Raman peaks corresponding
G
D
0% H2 1.5% H2
25% H2
1000
1200
1400
1600
1800
Raman shift (cm-1)
(c) Tsub Intensity (arb. units)
G
D
o
750 C 700oC 650oC o
600 C
550oC
1000
1200
1400
1600
1800
Raman shift (cm-1) Fig. 2. Raman spectra of diamond films grown in (a) H2-rich plasma, H2 = 99% and 50%, and (b) Ar-rich plasma, H2 = 25%, 1.5% and 0%. (c) Raman spectra of the diamond films grown under various substrate temperatures from 550 to 750 °C in 1.5% H2-plasma.
Fig. 3. OES spectra of Ar/H2/CH4 microwave plasma (a) H2-rich plasma: H2 = 99% and 50% and (b) Ar-rich plasma: H2 = 25%, 1.5% and 0%.
K.J. Sankaran et al. / Diamond & Related Materials 20 (2011) 232–237
(a) H2-rich plasma -10 0.25
II.
-12
0.20 2
-14
J (mA/cm )
ln (J/E2) ([mA/cm2]/[V/µm])
the sp3-peak (1332 cm− 1) decreases as H2-content decreases and is only barely observable for 0% H2-spectrum. Furthermore, Raman spectra that show the influence of substrate temperature for the 1.5% H2-UNCD films are shown in Fig. 2(c) which will be discussed shortly.
0.15
-16 0.0
0.1
0.10
0.2
1/E [µ m/V ]
I. 0.05
I. 99% H2 II. 50% H2
0.00 0
20
40
60
80
100
120
140
(b) Ar-richplasma I.
-10
2.5
II.
-12
2.0
-14 1.5 -16
III. 0.1 1/E [µm /V]
0.0
0.2
1.0
I. 0% H2 II. 1.5 % H2
0.5
III. 25% H2 0
20
J (mA/cm2)
ln (J/E2) ([mA/cm2]/[V/µm])
E (V/µm)
0.0 40
60
80
100
120
140
(c) Tsub -8 -10
Table 2 Field emission data of (a) as-grown diamond films with varying Ar/H2/CH4 microwave discharges and (b) ultrananocrystalline diamond (UNCD) films with varying substrate temperatures (from 550 °C to 750 °C).
2.0
-12 -14
1.5
II. -16 0.0
The OES of the plasma is examined to investigate how the change in H2 into the plasma modifies the microstructure of the diamond films. Fig. 3 shows typical OES spectra of the plasma used for growing these diamond films with different ratios of H2 and Ar, revealing that OES of H2-rich plasma is completely different from those for Ar-rich ones. In the 99% and 50% H2 plasma (Fig. 3(a)), the major optical emission peak is due to the presence of atomic hydrogen species Hα (656.3 nm). However, the proposed diamond growing species, CH3+, is not observable in these OES spectra [22]. In 25% H2-plasma (Fig. 3(b)), in addition to Hα line, we begin to observe emission from C2 dimers, which is proposed as the diamond growing species in the Ar-rich plasma [1]. The Ar-rich plasma (H2 b 25%) are predominated with the Swan-band near 516.5 nm, that mainly contains the C2 species in addition to the small amount of CH species (431 nm) [23]. There also exist two other C2 bands near 563 nm and 474 nm. The spectral lines corresponding to atomic hydrogen are essentially not observable. Further decreasing the concentration of H2 (1.5% and 0%) increases the emission intensity of C2 species (Fig. 3(b)). Restated, the H2-rich plasma tends to grow faceted large diamond grains, whereas the Ar-rich plasma tends to form nanosize diamond grains. This result is in accordance with the rapid change in the microstructure of the diamond films (Fig. 1(a–e)). The effect of H2/Ar ratio in the plasma on the EFE properties of the diamond films is shown in Fig. 4(a) and (b). The insets show the FN plot of the corresponding field emission data and explain that ln(J/E2) is almost linear with 1/E in the high field segment, indicating that the field emission current is controlled predominantly by the FN tunneling of electrons at the diamond surface [24,25]. The EFE parameters, including the turn-on field and EFE current density are extracted from these figures and are summarized in Table 2(a and b). Fig. 4(a) indicates that the 99% H2-MCD films can be turned on at (E0)H99 = 69.0 V/μm, whereas 50% H2-MCD films can be turned on at (E0)H50 = 53.3 V/μm. The EFE current density for these films are very small, i.e., b0.2 mA/cm2 at 105 V/μm applied field. Faceted granular structure with low conductivity for these MCD films is presumably the prime factor resulting in inferior EFE properties for these films. The turn-on field decreases and the EFE current density increases monotonously with the decreasing H2-content in the plasma used for growing diamond films. Fig. 4(b) shows that, for the 25% H2-NCD films, the turn-on field decreases to 50.0 V/μm (E0)H25, and EFE current density increases to (Je)H25 = 1.15 mA/cm2 at 125 V/μm applied field ((Je)H25 b 0.3 mA/cm2 at 105 V/μm), whereas 1.5% H2-UNCD films could be turned on at (E0)H1.5 = 33.8 V/μm with (Je)H1.5 = 2.31 mA/cm2 (at 105 V/μm applied field). The 0% H2-UNCD diamond films possess the best EFE properties, that is, smallest turn-on field, (E0)H0 = 31.6 V/μm, and highest EFE current density, (Je)H0 = 2.49 mA/cm2 at 105 V/μm applied field. Owing
2.5
I.
III. 0.1
o
I.550 C 1/E o II.600 C o III.650 C o IV.700 C o V.750 C 0
20
0.2
[µm /V]
1.0
IV. V.
J (mA/cm2)
ln (J/E2) ([mA/cm2]/[V/µm])
E (V/µm)
235
0.5
0.0 40
60
80
100
120
140
E (V/µm) Fig. 4. EFE properties of the diamond films grown in (a) H2-rich plasma, H2 = 99% and 50%, and (b) Ar-rich plasma, H2 = 25%, 1.5% and 0%. (c) EFE properties of the diamond films grown under various substrate temperatures from 550 to 750 °C in 1.5% H2-plasma.
(a) Flow rate of gases (sccm) CH4
Ar
H2
1 1 1 1 1
0 49 74 97.5 99
99 50 25 1.5 0
E0 (V/μm)
Je (mA/cm2) at 105 (V/μm)
69.0 53.3 50.0 33.8 31.6
b0.02 0.20 b0.30 2.31 2.49
(b) Temperature (°C)
E0 (V/μm)
Je (mA/cm2) at 94 (V/μm)
550 °C 600 °C 650 °C 700 °C 750 °C
34.5 37.8 52.2 57.5 62.7
2.30 0.40 0.20 b 0.01 b 0.01
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to the smaller grain microstructures, the UNCD films contain higher proportion of grain boundaries than those of NCD and MCD films. The higher grain boundary density possibly provides electron conduction networks, which transport the electrons more efficiently to emission sites for field emission. As a result, the 0% H2- and 1.5% H2-UNCD films exhibit better field emission performance than the other large grain diamond films. Since UNCD films exhibits higher EFE properties as compared to that of the NCD and MCD films, the UNCD films were grown at higher substrate temperature so as to modify the microstructure of the films and study their effect on the EFE properties. The SEM morphologies for the 1.5% H2 films grown at different temperatures are very similar with one others (not shown). Fig. 2(c) corresponds to the Raman spectra for the 1.5% H2-UNCD films grown at 550 to 750 °C substrate temperatures. From the Raman spectra, it can be observed that the
characteristic D- and G-peaks increase with the substrate temperature deposition. In the case of the sample grown at the highest temperature (750 °C) the presence of a sharp G-peak confirms that there are especially sp2-hybridized compounds, which are accentuated at higher substrate temperatures. However, the Raman spectra vary with the different substrate in a much less prominent manner than that due to the change in gas composition. Restated, the gas composition in the plasma, rather than the substrate temperature, is the predominating factor modifying the granular structure of the diamond films. The EFE properties for 1.5% H2-UNCD films vary pronouncedly with substrate temperatures, which are shown in Fig. 4(c) and the EFE parameters are listed in Table 2(b). This reveals that, as the substrate temperature increases from 550 to 750 °C, the turn-on field increases monotonically from 34.5 to 62.7 V/μm, whereas the EFE current-
Fig. 5. TEM micrographs of the ultrananocrystalline diamond (1.5% H2-UNCD) films grown under (a) 550 °C, (b) 650 °C, and (c) 750 °C substrate temperature. The insets show the corresponding SAED pattern.
Fig. 6. HRTEM images of the ultrananocrystalline diamond (UNCD) films with 1.5% H2 grown under (a) 550 °C, (b) 650 °C, and (c) 750 °C substrate temperatures. The insets show the corresponding regions from which the images were taken.
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density at 94 V/μm decreases monotonously from (Je) 550C = 2.30 mA/ cm2 to (Je) 750C b 0.01 mA/cm2. These EFE results reveal that higher temperature degraded the EFE properties for the UNCD films. While the change in EFE properties with the H2-content in plasma can be correlated closely with the modification on the granular structure for the films synthesized in different Ar-H2 plasma, the explanation on the change in EFE properties with the substrate temperature for 1.5% H2-UNCD films is not as straight forward. The EFE properties of the 1.5% H2-UNCD films at various temperatures is further explained using the nanoscale microstructural changes, which are observed from TEM micrographs for the temperatures 550 and 750 °C respectively. Fig. 5 shows that, generally, the 1.5% H2-UNCD films deposited at low temperatures exhibit a very similar nanoscale structure compared to the films grown at higher temperatures. The SAED patterns shown as insets indicate the diamond characteristics of the films. We observed that there are few large clusters formed in all the UNCD films. The size of the clusters is bigger, and larger in population, for the UNCD films grown at higher substrate temperature. However, the more significant effect due to the increase in substrate temperature is the modification on the defect structure of the materials, which is best demonstrated by the structure images. Fig. 6(a) shows the structure image from a typical cluster (inset) of UNCD films deposited at a low temperature (550 °C), showing that the 550 °C-UNCD films contain some stacking faults, i.e., parallel fringes in Fig. 6(a). The size of the clusters is bigger, accompanied with a larger proportion of defects contained in the clusters, for the UNCD films grown at 650 °C (Fig. 6(b)). Further increase in substrate temperature induces an even larger size of the clusters with a more complicated defect structure. Fig. 6(c) shows, for the films grown at 750 °C, that polymorphs of cubic diamonds such as 2H and 8H diamonds even formed. The defects are presumably induced by the abnormal growth of the diamond grains. The larger the grain growth rate is, the more complex the defect structure. In consequence of the cluster formation, the proportion of grain boundaries are very much lowered for the films grown at a high temperature compared with the low temperature grown UNCD films. Accordingly, there is a confinement in the emission of electrons from the emission sites which could be a reason for the decrease in the EFE property of the films grown at high substrate temperatures. 3. Conclusions The modification on microstructure of diamond films due to the incorporation of H2 species into the Ar/CH4 plasma has been systematically investigated. We report that systematic change in the ratio of the reactant gases is the way to control the nucleation rate, surface morphology, grain size, and the growth mechanism of diamond films. There is a strong correlation between the field emission properties and the microstructure of diamond films. The
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high grain boundary density can also provide electron conduction networks, which transport the electrons more efficiently to emission sites for field emission. Consequently, UNCD films have higher densities of grain boundary than those of NCD and MCD films, resulting in the highest field emission enhancement among all the diamond films. In addition, for 1.5% H2-UNCD films, higher substrate temperature (750 °C) ensuing a larger size of the clusters with more complicated defect structure including polymorphs of cubic diamonds as compared with the low substrate temperature grown UNCD films. As a result, there forms a smaller proportion of grain boundary density for the high temperature grown films, which results in fewer emission sites for electrons and leads to inferior field emission properties than those grown at lower temperature (550 °C). Acknowledgements The authors would like to thank the National Science Council, Republic of China for the support of this research through the Project No. NSC99-2119-M-032-003-MY2. References [1] D. Zhou, T.G. McCauley, L.C. Qin, A.R. Krauss, D.M. Gruen, J. Appl. Phys. 83 (1998) 540. [2] C. Popov, W. Kulisch, S. Boycheva, K. Yamamoto, G. Ceccone, Y. Koga, Diamond Relat. Mater. 13 (2004) 2071. [3] D. Zhou, A.R. Krauss, L.C. Qin, T.G. McCauley, D.M. Gruen, T.D. Corrigan, R.P.H. Chang, H. Gnaser, J. Appl. Phys. 82 (1997) 4546. [4] B.V. Deryagin, D.V. Fedoseev, N.D. Polyanskaya, E.V. Statenkova, Sov. Phys. Crystallogr. 21 (1976) 239. [5] S. Matsumoto, Y. Sato, M. Tsutsumi, N. Setaka, J. Mater. Sci. 17 (1981) 3106. [6] P.K. Bachman, D. Leers, H. Lydtin, Diamond Relat. Mater. 1 (1991) 1. [7] D.M. Gruen, Annu. Rev. Mater. Sci. 29 (1999) 211. [8] C.S. Wang, H.C. Chen, H.F. Cheng, I.N. Lin, J. Appl. Phys. 107 (2010) 034304. [9] A. Erdemir, G.R. Fenske, A.R. Krauss, D.M. Gruen, T.G. McCauley, R.T. Csencsits, Surf. Coat. Technol. 565 (1999)8 120/121. [10] R. Krauss, D.M. Gruen, Diamond Relat. Mater. 10 (2001) 1952. [11] L.C. Qin, D. Zhou, A.R. Krauss, D.M. Gruen, Nanostruct. Mater. 10 (1998) 649. [12] V.I. Konov, S.M. Pimenov, A. Erdemir, M. Halter, G.R. Fenske, A. Krauss, D.M. Gruen Surf, Coat. Technol. 94 (1997) 537. [13] C. Zuiker, A.R. Krauss, D.M. Gruen, X. Pan, J.C. Li, R. Csencsits, A. Erdemir, C. Bindal, G. Fenske, Thin Solid Films 270 (1995) 154. [14] A. Erdemir, C. Bindal, G.R. Fenske, C. Zuiker, A.R. Krauss, D.M. Gruen, Diamond Relat. Mater. 5 (1996) 923. [15] T. Lin, G.Y. Yu, A.T.S. Wee, Z.X. Shen, Appl. Phys. Lett. 77 (2000) 2692. [16] J.A. Carlisle, O. Auciello, Electrochem. Soc. Interface 28 (2003)8 spring. [17] C.D. Zuiker, A.R. Krauss, D.M. Gruen, X. Pan, J.C. Li, R. Csencsits, A. Erdemir, C. Bindal, G. Fenske, Thin Solid Films 270 (1995) 154. [18] D. Zhou, A.R. Krauss, T.D. Corrigan, T.G. McCauley, R.P.H. Chang, D.M. Gruen, J. Electrochem. Soc. 144 (1997) L224. [19] D.C. Barbosa, F.A. Almeida, R.F. Silva, N.G. Ferreira, V.J. Trava-Airoldi, E.J. Corat, Diamond Relat. Mater. 18 (2009) 1283. [20] Y.C. Lee, S.J. Lin, I.N. Lin, H.F. Cheng, J. Appl. Phys. 97 (2005) 054310. [21] R.H. Fowler, L. Nordheim, Proc. R. Soc. London Ser. A 119 (1928) 173. [22] J. Ma, N. Michael, R. Ashfold, Y.A. Mankelevich, J. Appl. Phys. 105 (2009) 043302. [23] D. Zhou, D.M. Gruen, L.C. Qin, T.G. McCauley, A.R. Krauss, J. Appl. Phys. 84 (1998) 1981. [24] M.W. Geis, N.N. Efremow, K.E. Krohn, Nature 393 (1998) 431. [25] J.Y. Shim, E.J. Chi, H.K. Baik, K.M. Song, Thin Solid Films 355 (1999) 223.