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Vacuum 72 (2004) 321–326
Field emission studies of CVD diamond thin films: effect of acid treatment P.M. Koinkara, P.P. Patila, M.A. Morea,*, V.N. Tondareb, D.S. Joagb b
a Department of Physics, School of Physical Sciences, North Maharashtra University, Jalgaon 425 001, India Department of Physics, Center for Advanced Studies in Materials Science and Solid State Physics, University of Pune, Pune 411 007, India
Received 28 May 2003; received in revised form 26 July 2003; accepted 20 August 2003
Abstract Field emission from CVD diamond thin films deposited on silicon substrate has been studied. The diamond films were synthesized using hot filament chemical vapor deposition technique. Field emission studies of as-deposited and acid-treated films were carried out using ‘diode’ configuration in an all metal UHV chamber. Upon acid treatment, the field emission current is found to decrease by two orders of magnitude with increase in the turn-on voltage by 30%. This has been attributed to the removal of sp2 content present in the film due to acid etching. Raman spectra of both the asdeposited and acid-treated films exhibit identical spectral features, a well-defined peak at 1333 cm1 and a broad hump around 1550 cm1, signatures of diamond (sp3 phase) and graphite (sp2 phase), respectively. However upon acid treatment, the ratio ðId =Ig Þ is observed to decrease which supports the speculation of removal of sp2 content from the film. The surface roughness was studied using atomic force microscopy (AFM). The AFM images indicate increase in the number of protrusions with slight enhancement in overall surface roughness after acid etching. The degradation of field emission current despite an increase in film surface roughness upon acid treatment implies that the sp2 content plays significant role in field emission characteristics of CVD diamond films. r 2003 Elsevier Ltd. All rights reserved. Keywords: CVD diamond; Field emission; Field emission display; Atomic force microscopy; Raman spectroscopy
1. Introduction In the context of cold cathode materials for field emission displays (FEDs), the CVD diamond films have received much attention owing to their low or even negative electron affinity and high surface roughness coupled with robust physical and chemical properties [1–4]. The field electron emis*Corresponding author. Tel.: +91-257-2252187; fax: +91257-2252183. E-mail address: mmore
[email protected] (M.A. More).
sion characterization of CVD diamond has been explained by several models viz., defects model [5], emission from conduction band due to a negative electron affinity [6], conducting channels in insulating diamond matrix [7], dielectric breakdown [8], tunneling [9] and hot-electron injection [10]. Recently, May et al. [11] have discussed the field emission conduction mechanisms in CVD diamond and diamond-like carbon films based on a scheme comprised of four independent models. Several researchers have investigated the influence of process variables, quality of CVD diamond
0042-207X/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2003.08.010
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and post-treatments on the emission characteristics of CVD diamond films. The field emission from CVD diamond is found to vary inversely with the crystalline size [12,13]. Satyanarayana et al. [13] have claimed that with the decrease in grain size of the diamond crystal, there is increase in emission current and decrease in threshold field. They have also studied the effect of methane concentration and have observed that the emission characteristic is improved with increase in methane concentration. In order to achieve better field emission characteristics, attempts have been made by surface/bulk conditioning of CVD diamond. The bulk conditioning of CVD diamond is based on in-situ doping with boron and nitrogen [14–16]. These authors have observed low threshold voltage for the sp3-rich films which have high resistivity. Recently, Yamada et al. [17] have studied the effect of sp2/sp3 ratio on electron emission characteristics of nitrogen-doped CVD diamond films. The various surface conditioning treatments employed are high-energy ion bombardment, thermal annealing and acid etching of nondiamond components in CVD diamond. Upon high-energy ion implantation, Habermann et al. [18] have observed reduction in the turn-on voltage by 30% and 16% for C+ and Si+ ions, respectively. Sahli et al. [19] observed an increase in resistivity of CVD diamond films after thermal annealing at high temperature. Because of its potential to cause surface erosion thereby creating rough surfaces or micro-protrusions, influence of acid treatment has been investigated by few groups. May et al. [20]. have studied the effect of oxidation and observed reduction in the field emission current by two orders of magnitude after acid treatment. In similar studies, carried out by Yuan et al. [21] and Han et al. [22] these authors have observed enhancement in field emission current and reduction in the turn-on voltage. Both the groups, May et al. and Sang Yuan Hang et al. have attributed the change in emission current to formation of O–C bands due to acid treatment, which in turn affects the effective work function of CVD diamond. Yuan et al. have speculated removal of graphite and non-diamond impurities
present in the film and increase in the number of micro-protrusions on the film surface by acid etching. Although these groups have given plausible justification for the observed change in emission current, their results are contradictory. One of the possible reasons for this may be attributed to the ‘chemical nature’ of CVD diamond film, i.e. whether the CVD film has exclusive sp3 phase or does it contain some nondiamond phases. It has been shown by a number of researchers that graphite plays a very important role in emission characteristics of CVD diamond. It is well known that Raman scattering is sensitive to graphite since its scattering efficiency is almost 50 times higher than that of pure diamond. We therefore planned to employ the Raman spectroscopy to gain better understanding of influence of acid treatment on the field emission behavior of CVD diamond films. The surface micro-structure is one of the key parameters governing the field emission characteristics of a given material. The acid etching is expected to ‘damage’ the surface thereby generating micro-protrusions, which in turn influence the emission properties. Therefore, we have used atomic force microscopy (AFM) to examine the surface micro-structure of CVD diamond films. In this paper, we present the investigation on influence of acid etching on field emission characteristics of CVD diamond films. In order to understand the role of graphite content and surface roughness, we have used Raman Spectroscopy and AFM as characterization tools.
2. Experimental Hot filament chemical vapor deposition (HFCVD) system was used for deposition of diamond film on p-type Si (1 1 1) wafer (6 mm 6 mm). Before deposition, the silicon substrates were polished with 0.5 mm diamond paste for 15 min so as to increase the diamond nucleation. The distance between the substrate and the filament was kept about 10 mm. The reactor chamber was evacuated by a rotary pump and hydrogen gas was filled through mass flow
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3. Results and discussion 3.1. Characterization of CVD diamond films The Raman spectra of as-deposited and acidtreated CVD diamond films are shown in Fig. 1. A typical Raman spectrum of as-deposited film (Fig. 1(a)) shows a well-defined peak at B1333 cm1 indicating formation of the diamond
(a)
INTENSITY (Arb. Units)
controller to a background pressure of about 40 Torr. The tungsten filament was resistively heated up to B2200 C and 1% methane gas was mixed with hydrogen after a stable filament temperature was reached. The total flow rate was 400 sccm and the substrate temperature was kept at B900 C. At least four samples were prepared under identical experimental conditions. The acid treatment was employed by dipping the CVD diamond films in a mixture of hydrochloric and nitric acids with a ratio 1:4. The treatment was carried out at temperature B90 C for 15 min. After this, the samples were thoroughly rinsed with deionized water. The field emission measurements were performed in an all metal UHV chamber evacuated at B108 Torr. For the emission studies diode configuration was used, with a phosphor-coated tin oxide glass plate (circular disk of 30 mm diameter) as an anode and CVD film as the cathode. The sample was mounted on a stainless steel disk of same diameter as that of the anode. The distance between anode and cathode was kept at about B1 mm. The CVD diamond films were characterized by laser Raman spectroscopy to evaluate the quality of films. The Raman spectroscopy was carried out at ambient temperature using a Renishaw Spectrometer (Ramascope 1000). The instrument is equipped with a microscope whose focal spot diameter is B2 mm and uses output from an Argon (Ar+) laser for excitation at wavelength l ¼ 514:5 nm. The spectra were recorded using 100 objective and 40 scans were taken for each sample. The surface morphology was studied by AFM (Nanoscope II, Digital Instruments, USA). The AFM was operated in ‘contact mode’ in air.
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(b)
1300
1500 RAMAN SHIFT
1700 (cm-1)
Fig. 1. Raman spectra of: (a) as-deposited, and (b) acid-treated CVD diamond films.
(sp3) phase. In addition to this a broad hump around B1550 cm1 indicative of non-diamond (sp2) phase is also observed. Thus the Raman spectrum clearly reveals presence of both the sp3 and sp2 phases in the film with ðId =Ig Þ ratio=0.89, Id and Ig are the intensities of the diamond and graphite peaks, respectively. The Raman spectrum of acid-treated film (Fig. 1(b)) exhibits spectral features similar to that of as-deposited film with ðId =Ig Þ ratio B1.25. However, in comparison with the as-deposited case, a careful observation reveals—(i) reduction in the intensity of peak corresponding to sp2 phase and (ii) enhancement in the intensity and sharpness of the sp3 peak, and (iii) increase in the ðId =Ig Þ ratio. These observations can be attributed to removal of graphitic content during acid etching. It can be speculated that the removal of nondiamond content surrounding the diamond crystallites will make them ‘well isolated’. These ‘well isolated’ crystallites will thus exhibit better Raman scattering than that of as-deposited case. It should be noted that in case of CVD diamond films,
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Raman scattering from the sp3 phase is influenced by the amount of sp2 phase present therein, since the scattering efficiency of graphite is higher than that of pure diamond. As a consequence, in case of films with more sp2 content, the Raman scattering from sp3 phase will be relatively weaker as compared to that from a film with less sp2 content, for the same number of scans. Thus the observed Raman spectrum clearly supports the speculation of removal of graphitic content by the acid treatment. Raman spectrum clearly supports the speculation of removal of graphitic content by the acid treatment. Fig. 2(a) and (b) show the AFM 3-D surface images of as-deposited and acid-treated CVD diamond thin films. It is apparent from these images that after acid treatment the number of protrusions have been increased and are relatively sharper as compared to that of as-deposited case. As a result, the overall surface roughness has increased from 4.07 nm (for as-deposited case) to 12.5 nm for acid-treated CVD diamond film. The protrusions are due to the randomly oriented diamond crystallites present on the film surface. It is expected that the removal of graphite surrounding the diamond crystallites will ‘expose’ them
thereby generating more protrusions. Thus the observed increase in number of protrusions and surface roughness is attributed to removal of graphitic content during the acid etching. 3.2. Field emission studies The current–voltage (I2V ) characteristics of asdeposited and acid-treated films recorded at a base pressure of 1 108 Torr are shown in Fig. 3. It is apparent from the I2V curves that the field emission current has reduced to a notable level, nearly by two orders of magnitude after acid treatment. Also, the turn-on voltage required to draw B1 mA current has increased by almost 30% than that of as-deposited case. The observed results showing reduction in the field emission current and an increase in the turnon voltage after acid treatment are in good agreement with the reported literature. Similar results were obtained by May et al., who have studied the effect of diamond surface termination species on the field emission behavior of CVD diamond films [20]. The authors have observed that upon oxidation by treating the films in acid bath, the field emission current has reduced by two 100 As-deposited
(µA)
80
FE
CURRENT
60
40
20
Acid-treated
5
7
9
11
VOLTAGE (kV)
Fig. 2. AFM images of: (a) as-deposited, and (b) acid-treated CVD diamond films.
Fig. 3. Field emission current–voltage (I2V ) characteristic of as-deposited and acid-treated CVD diamond films recorded at base pressure 1 108 Torr.
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orders of magnitude. Also, the turn-on voltage increased by B20% than that of un-treated films. It has been attributed to the formation of O–C single, double or bridging bonds at the surface. By oxidation, the negative electron affinity of diamond is removed and oxygen being electronegative, the dipole moments will effectively increase the work function of CVD diamond. Thus the emission current will be reduced. The authors have also investigated the effect of hydrogenation (treatment in hydrogen plasma) and claim that upon hydrogenation hydrogen re-establishes the sp3 configuration necessary for the electron emission. Yuan et al. have studied the influence of acid treatment on field emission characteristics of diamond thin films and observed increase in the emission current with reduction in the turn-on voltage [21]. They have attributed it to the increase in number of protrusions, speculating removal of non-diamond (sp2 phase) from the film, by acid etching. Thus the protrusions are due to the randomly oriented diamond crystallites present on the film surface. They have not experimentally verified the speculation. In our case we too have observed increase in number of protrusions, but the emission current has significantly reduced after acid treatment. In general, it is expected that a rough surface having more protrusions will give more emission current with reduction in threshold voltage. However, this analogy may not hold true in case of CVD diamond thin film as its emission characteristics are strongly influenced by the presence of non-diamond impurities in the films. Kang et al. have investigated the effect of sp2 content on the field emission of CVD diamond films [23]. They observed that by increasing sp2 content in the film, the field emission current is significantly increased and the turn-on field is drastically reduced. Now if the protrusions are due to diamond crystallites, as predicted in our case, then the emission current will be less because diamond requires very high field to emit electrons. Hence in such case, the protrusions will have ‘reverse’ effect on the emission current, which we have observed in our case. Therefore, the observed reduction in current despite of increase in surface roughness upon acid treatment implies that the sp2
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content plays an important role in field emission characteristics of CVD diamond films. Thus it is quite apparent from above that emission characteristic of CVD diamond is influenced by non-diamond species present in the films. In our case, the observed results exhibiting reduction in the emission current and enhancement in the turn-on voltage are due to removal of non-diamond impurities present in the films by acid treatment, as supported by the Raman spectroscopic studies.
4. Conclusions From the observed results, it is clear that the acid treatment of CVD diamond films leads to degradation of its field emission characteristics. Moreover, in field emission behavior of such films, role of sp2 content is very important. The films having more sp2 content show better emission properties than that with lower sp2 configuration.
Acknowledgements We are very grateful to Dr. Ramprakash Gupta and Mr. Ashok Sharma, CEERI, Pilani, for AFM characterization. Thanks are also due to Mr. K.K. Sharma, Director, Indian Diamond Institute, Surat, for his help in Raman spectroscopy work. PMK is very grateful to Council of Scientific Industrial Research (CSIR), New Delhi for awarding Senior Research Fellowship (SRF). This work was supported by Third World Academy of Sciences (TWAS), Italy via research grant no. 00-415 RG/PHYSICS.
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