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TiNy multilayers by ion beam sputtering
Journal of Crystal Growth 233 (2001) 303–311
Preparation of CNx/TiNy multilayers by ion beam sputtering D.L. Yua, Y.J. Tiana,*, J.L. Hea, F.R. Xiaoa, T.S. Wanga, D.C. Lia, L. Lib, G. Zhengc, O. Yanagisawad a
College of Materials Science and Chemical Engineering, Yanshan University, No. 438, Hebei Street West, Qinhuangdao, Hebei 066004, People’s Republic of China b Institute of Physics, Chinese Academy of Science, P.O. Box 603, Beijing 100080, People’s Republic of China c Institute of Hui Guang Technology, Beijing 100080, People’s Republic of China d Faculty of Engineering, Hiroshima University, Hiroshima, Japan Received 30 March 2001; accepted 10 May 2001 Communicated by M. Roth
Abstract CNx/TiNy multilayers have been prepared by ion beam sputtering and analyzed by X-ray diffraction, transmission electron microscopy, selected area electron diffraction, energy dispersive X-ray and X-ray photoemission spectroscopy. The structure analyses shows that the films may contain several different kinds of CaN crystals embedded in the amorphous matrix. These crystals have hexagonal and cubic crystalline structures. The TiNx layer in multilayers is primarily amorphous and contains only a few dispersed Ti2N crystals. The thickness of CNx and TiNy layers in the CNx/TiNy multilayers is about 20 and 30 nm, respectively. The atomic ratios of [N]/([C]+[N]+[Ti]) in the multilayers vary from 20 to 41 at%. The bonding ratios of CaN/C ¼ N of the CNx layer in multilayers reach 0.447 or 0.585 as calculated from the C1s and N1s X-ray photoemission spectra, respectively. r 2001 Published by Elsevier Science B.V. PACS: 68.55.a; 81; 81.15.z; 81.20.n Keywords: A1. Crystal structure; A3. Polycrystalline deposition; B1. Nitrides
1. Introduction Liu and Cohen [1] theoretically predicted in 1989 the existence of a new compound, carbon nitride (C3N4). During the past decade studies of the carbon nitride compound have been of great interest to the field of materials science and engineering. Film deposition techniques [2–11], *Corresponding author. E-mail address: [email protected] (Y.J. Tian).
such as ion beam sputtering, RF diode sputtering, pulsed laser ablation, DC magnetron sputtering, electron cyclotron resonance plasma and chemical vapor deposition, etc., have been used to synthesize CaN films. Some researchers [12–14] have reported that crystalline b-C3N4 and a-C3N4 with sizes from several to hundreds of nanometers have been observed in amorphous CNx films. To our knowledge, sufficiently large CaN single crystals have not been synthesized for accurate evaluation of the carbon nitride compound structure and
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physical properties. For this reason, many groups worldwide are making great efforts to synthesize large CaN crystals. It is widely recognized that crystalline carbon nitride is thermodynamically metastable at ambient pressure and temperature. Therefore, choosing a suitable material as a structural template for nucleating the metastable C3N4 phase (to overcome its energy barrier) is a valid method. Li et al. [15] made the first attempt to produce crystalline carbon nitride using CNx/TiNy multilayers. Based on the same idea, Wu et al. [16] have also prepared CNx/ZrN multilayers with hardness exceeding 40 GPa and elastic modulus of over 400 GPa by DC magnetron sputtering. TiN and ZrN have been chosen as template material because there are good lattice matches between TiN (1 1 1) or ZrN (1 1 1) and b-C3N4 (0 0 0 1). The [N]/[C] ratio appears to be one of the most important characteristics of the carbon nitride films. The properties of carbon nitride films may be improved by increasing the N content [17]. However, most of the researches show that the nitrogen contents of the films are substantially below the stoichiometric composition of the C3N4 crystal (57 at%). In this work, a new method for preparation of CNx/TiNy films has been attempted. CNx/TiNy multilayers have been prepared by ion beam sputtering (IBS) and characterized using X-ray diffraction, transmission electron microscopy, selected area electron diffraction, energy dispersive X-ray and X-ray photoemission spectroscopy. The results obtained and discussed include the structures of crystals embedded in the amorphous matrix in CNx/TiNy multilayers, the average atom ratios of [N]/[C] and the binding ratios of CaN/ C ¼ N.
2. Experimental details TiNy/CNx/TiNy trilayers, CNx/TiNy bilayers and CNx/TiNy multilayers were deposited on NaCl slices and Si (1 1 1) wafers using an ion beam sputtering system. The schematic diagram of the experimental system is presented in Fig. 1. There are three Kaufman ion sources and a
Fig. 1. Schematic diagram of ion beam sputtering system.
rotatable substrate holder in the system. Two ion sources were used as sputtering guns and another one was used for cleaning the surfaces of the substrates. Two targets, graphite and titanium, were used in this work. The distance between the target and the substrate was about 110 mm. The anode-to-cathode potential and the anode potential were held at 50B70 V and 1000B1200 V during sputtering, respectively. The accelerating voltage was about 300 V. The ion beam current densities were 25B30 mA/cm2 and 65B70 mA/ cm2 for sputtering graphite and titanium, respectively. The discharge gas was high-purity nitrogen (99.999%). The total gas pressure was kept at 4B7 102 Pa in the chamber with a base pressure of 5 104 Pa. The substrate temperature was about 70B901C during deposition. All substrates were ultrasonically cleaned in acetone and ethanol before they were introduced into the sputtering system. The titanium and graphite targets were alternatively sputtered to form CNx/TiNy bilayers, TiNy/CNx/TiNy trilayers and CNx/TiNy multilayers (18 layers). The deposition rates were determined by measuring the thickness of films at different times using a profilemeter. The deposition rates for CNx and TiNy layers were 20 and 60 nm/h, respectively. In the bilayers, trilayers and multilayers, the thickness of the CNx layer was about 20 nm and that of TiNy layer was about 30 nm. The TiNy/CNx/TiNy trilayers deposited on freshly cleaved NaCl substrates were characterized using transmission electron microscopy (TEM)
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and selected area electron diffraction (SAED). The investigation was performed using the H-800 microscope. The structure of TiNx/CNy multilayers was also studied by X-ray diffraction (XRD) with Rigaku model D/Max-RB X-ray diffractometer. We varied the spatial angle of the specimen relatively to the incident X-ray beam in order to collect more data. The composition of the multilayers was measured using an energy dispersive X-ray (EDX) spectrometer attached to the scanning electron microscope model AMMARY1000B. The CaN bonding nature on the top CNx layer in the CNx/TiNy bilayer was determined by X-ray photoemission spectroscopy (XPS). XPS was carried out in the ESCALAB 200I-XL spectrometer produced by VG Scientific Company using a Mg Ka X-ray source. The resolution of the XPS was about 0.5 eV. The surfaces of specimens were sputtered by argon ions for about 5 min before XPS measurements.
3. Results and discussion 3.1. Crystal structure Fig. 2 shows the XRD spectrum obtained from a multilayer film of CNx/TiNy (18 layers) deposited on Si (1 1 1). Apparently, many peaks are present in the 2y range of 20B1001 besides an amorphous peak at about 301. The experimental XRD data, the calculated data and the relative
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intensities (I/I0 >1) of the powder diffraction lines based on the latest results of Teter and Hemley [18] are listed in Table 1. We have compared the X-ray diffraction data of Fig. 2 with similar data on all known crystals that can be formed by Ti, C and N. The results show that only four peaks with ( can d-spacings of 3.726, 2.385, 2.078 and 1.620 A be well indexed as the Ti2N phase in the XRD spectra. Yet, the peaks at 2.815, 2.128, 1.944, 1.620 ( may be well fitted to the (2 0 0), (2 1 0), and 1.539 A (1 1 1), (2 1 1) and (1 3 0) reflections of the b-C3N4 phase predicted theoretically [18]. The peaks with d-spacings of 3.378, 2.535, 2.385, 2.078, 1.944, ( can be indexed as the graphite1.675 and 1.539 A C3N4 phase and the peaks at 1.899, 1.716, 1.156 ( coincide with d-spacings of the cubicand 1.049 A C3N4 (2 0 2), (1 0 3), (2 4 0) and (5 0 1) planes. There ( in is one unknown peak with a d-spacing of 4.29 A the XRD spectrum. It should be noted that only the Ti2N phase is found in the CNx/TiNy multilayers, and the sizes of crystals may be too small to present all diffraction peaks of Ti2N. According to our XRD identification results there may be more than one kind of CaN crystals in the film obtained in our experiment. In order to identify further the types of crystals involved, we have performed TEM and SAED experiments using a very thin plane-view sample of TiNy/CNx/TiNy trilayers with the thickness of about 80 nm deposited on NaCl. The trilayer is collected onto a microscope grid by dissolving the NaCl substrate with de-ionized water. The sample
Fig. 2. XRD spectrum of CNx/TiNy multilayers.
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Table 1 Experimental XRD data compared with calculated [18] X-ray diffraction patterns of b-C3N4, graphite-C3N4, cubic-C3N4 and Ti2N () Experimental d (A
() Calculated d (A b-C3N4 hkl
3.726 3.378 2.815 2.535 2.385 2.128 2.078 1.944 1.899 1.716 1.675 1.620 1.539 1.156 1.049
200
Graphite-C3N4 d
2.789
I/I0
Cubic-C3N4
hkl
d
I/I0
002
3.36
100
102 110
2.601 2.371
1.1 1.2
200 201
2.053 1.964
3.8 19.1
hkl
2.108
33.9
111
1.958
55.8
202 103 1.603 1.547
I/I0
hkl
d
101
3.740
103
2.396
200
2.069
105
1.621
100
210
211 130
d
Ti2N
14.0 7.1
004
1.680
7.2
203
1.514
6.1
is then repeatedly cleaned in pure acetone and ethanol. We have found some crystals embedded in the matrix as shown in Fig. 3(a). Moreover, we have also found some regions with different contrast in the matrix, such as the A and B regions. SAED results reveal that both A and B are amorphous as shown in the pattern of Fig. 3(b). A crystal and its five SAED patterns obtained by rotating this crystal in the electron microscope are shown in Fig. 4. The experimental data from five diffraction patterns and calculated data [18] for graphite-C3N4 are listed in Table 2. Apparently, our experimental data are close to the calculation for the graphite-C3N4 phase. We have indexed the diffraction spots and determined each zone axis by means of the CaRIne Crystallographic software based on the structure parameters of the graphite-C3N4 phase. The identification results and the corresponding errors (on two distances and an angle) are also listed in the Table 2. Obviously, the five SAED patterns from this crystal can match well the [0 1 0], [1 3 1], ½6% 3 1 1; ½3% 4 1 and [0 1 3] crystal axes of graphiteC3N4, respectively. Fig. 5(a) shows some small crystals with the shapes of square, rhombus and polygon. The diffraction pattern (Fig. 5(c)) has
240 501
1.909 1.708
1.208 1.059
56.6 1.0
6.0 12.9
Fig. 3. (a) TEM micrograph of the amorphous matrix containing a few crystals in the TiNy/CNx/TiNy trilayer; (b) SAED pattern of the matrix of TiNy/CNx/TiNy trilayer.
been obtained from one of these crystals, which is enlarged and shown in Fig. 5(b). From its shape and the distribution of diffraction spots, we infer that it may belong to the cubic crystallographic ( and an system. Two interplanar distances of 2.20 A angle of 70.51 between the two spots are obtained from the diffraction pattern. They are close to the ( and the angle of (2 1 1) plane d-spacing of 2.18 A
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Fig. 4. TEM image of a CaN crystal and its SAED patterns corresponding to five directions. The SAED patterns are identified with structure parameters of the graphite-C3N4 phase. Table 2 SAED measured data compared with the d-spacings and angles calculated from graphite-C3N4 [18] Pattern number The data measured from SAED patterns The data calculated from graphite-C3N4 Sum of errors on d1, d 2 and a b
c
d
e
f
( d1=3.39 A ( d2=3.55 A a=581
( d(0 0 2)=3.36 A ( d(1 0 1)=3.50 A a=58.571 zone axis: [0 1 0]
2.6%
( d1=3.55 A ( d2=2.55 A a=821
( dð1 0% 1Þ ¼ 3:50 A ( dð1% 1 2% Þ ¼ 2:60 A a=82.341 zone axis: [1 3 1]
3.7%
( d1=1.01 A ( d2=1.58 A a=881
( dð4 3% 3Þ ¼ 1:02 A ( d(1 2 0)=1.55 A a=87.321 zone axis: ½6% 3 1 1
3.1%
( d1=4.17 A ( d2=1.52 A a=79.51
( d(1 0 0)=4.11 A ( dð2 3% 1Þ ¼ 1:51 A a=79.391 zone axis: [0 1 3]
2.2%
( d1=2.24 A ( d2=2.10 A a=84.51
( dð1 1 1Þ ¼ 2:24 A ( d(1 0 3)=1.97 A a=84.32 zone axis: ½3% 4 1
6.7%
711 between the (2 1 1) and ð2 1% 1% Þ planes in the cubic-C3N4 phase predicted by Teter and Hemley [18], and the corresponding error is 5%. A TEM micrograph containing a great number of circular
shape crystals and their diffraction rings indexed with the structure parameters of the b-C3N4 phase ( and c=2.4041 A ( ) are shown (P3(143), a=6.4017 A in Fig. 6. The experimental data and the calculated
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Fig. 5. (a) and (b) TEM images of crystals with different shapes and (c) SAED pattern identified with the cubic-C3N4 structure.
In our CNx single layer deposited on a Si substrate by IBS [8], we have obtained only a single kind of CaN crystal with the structure of bC3N4, as deduced from XRD and SAED results. TEM observations show that the crystallinity of the film is poor. The Auger analysis indicate that the average N content of the CNx single layer is about 20%. IR absorption spectrum and XPS results reveal that the carbon nitrogen bonds exist in the film. However, the analyses of both SAED and XRD show that there may be simultaneously several kinds of carbon nitride compounds in our CNx/TiNy multilayer sample. This phenomenon may be related to the small Ti2N crystals, which can act as nucleation sites for growth of crystalline CaN compounds. According to the calculations [18], the total formation energies of CaN compounds are very close (the difference of the total energies between them are below 0.008 eV per C3N4 unit). Therefore, it is possible that carbon nitride compounds with different crystal structures are formed in the same CNx/TiNy multilayer. 3.2. Composition and bonding characterization
Fig. 6. TEM image of small circular shape crystals and SAED rings indexed with structure parameters of the b-C3N4 phase.
Table 3 Comparison of the diffraction rings data with the interplanar spacings of (h k l) phanes calculated from the b-C3N4 structure Miller indices (h k l)
Calculated d () (A
SAED measured d () (A
110 101 111 220 310 221 320 410 420
3.22 2.26 1.96 1.61 1.55 1.35 1.28 1.22 1.05
3.10 2.28 1.97 1.67 1.55 1.36 1.28 1.20 1.01
data for the b-C3N4 phase are listed in Table 3. Clearly, there is a good correspondence between the experimental data obtained from the diffraction rings and the d-spacings calculated for the b-C3N4 phase.
Compositional analysis of the CNx/TiNy multilayers has been performed by EDX. The results show that the samples contain silicon and a small amount of oxygen besides the main elements, such as carbon, nitrogen and titanium. Silicon originates from the substrate, and the oxygen impurity stems from the surface pollution of the film. The multilayers composition is characterized by the ratio [N]/([C]+[N]+[Ti]) that does not depend on the concentration of the impurities. This ratio measured in different regions of the multilayers varies from 20 to 41 at%. The variation of the [N]/ ([C]+[N]+[Ti]) ratio indicates that various crystalline nitride compounds may exist in the multilayers leading to the uneven N distribution. This phenomenon is also observed in Fig. 3(a). In the process of structure analysis of TiNy/CNx/TiNy trilayers and CNx/TiNy multilayers we have found only a single titanium nitride phase (Ti2N) besides the crystalline carbon nitride compound. Yet, it seems that just a small amount of the Ti2N phase cannot seemingly cause the observed large fluctuations in the [N]/([C]+[N]+[Ti]) ratio. Therefore,
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we presume that the crystalline CaN compound is most likely responsible for the uneven nitrogen distribution. This has prompted our studies of Ncontent and the nature of carbon and nitrogen chemical bonds in the top CNx layer of CNx/TiNy bilayers by XPS. The XPS spectra are shown in Fig. 7. They do not reveal any significant amount of oxygen on the surface of the film. This indicates that the small amount of oxygen detected by EDX spectroscopy may originate from the surface pollution of the film. The average N-content in the top CNx layer of our CNx/TiNy bilayers ranges from 23 to 27 at% as calculated from the XPS spectra. These
values differ from the results obtained by EDX. The N-content measured by EDX spectroscopy represents the average N-content of the multilayers over a certain area in the film including the CNx and TiNy layers. In contrast, XPS represents the average N-content of only the CNx layer in the CNx/TiNy multilayers. This is an additional manifestation of the fact that the multilayer is made up of CNx layers and TiNy layers. The C1s and N1s spectral bands are deconvoluted into six peaks and four peaks, respectively. The bond energies to which every peak corresponds and the ratio of the CaN/C ¼ N bonds in the CNx layer are listed in Table 4. Within the C1s band, the
Fig. 7. XPS spectra of the CNx layer in CNx/TiNy multilayers.
Table 4 Quantitative data calculated from C1s and N1s spectra C1s Binding energy (eV) 291.36 289.17 287.39 285.69 284.45 281.99
CaN/C ¼ N=0.447
N1s [AT]% 1.791 5.564 21.701 48.585 15.774 6.585
Bonding type *
p–p (C) CO CaN C¼N Pure C Metal carbides
Binding energy (eV)
[AT]%
Bonding type
402.01 400.86 399.56 398.39
3.285 13.947 52.211 30.557
NO NO C¼N C¼N
CaN/C ¼ N=0.585
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287.39 and 285.69 eV peaks correspond to the bonding energies of CaN and C ¼ N, respectively. Among the other peaks, 291.36 eV may be a satellite peak corresponding to the p–p* bond of the carbon network. The 289.17 eV peak reflects the CO-type bonds, and the 281.99 eV peak is a contribution of metal carbides on the surface of the sample bracket. The 284.45 eV peak can be identified as originating from adventitious or surface carbon due to surface exposure and is often observed in XPS [19]. The N1s band is composed of four peaks at 402.01, 400.86, 399.56 and 398.39 eV, respectively. The bond energies of 399.56 and 398.39 eV can be attributed to the C ¼ N and CaN bonds. The peak at 402.01 eV is identified as originating from NaO or NaN bonds [20–22]. However, there are different explanations to the peak at 400.86 eV in the literature. Some researchers [19,23] attribute this peak to the NO bond. Others [24–27] consider it as corresponding to N-sp2. We contemplate that the bond energy of 400.86 eV is too large to correspond to the CaN bond, though it is close to the bond energy (400.6 eV) of pyridine-like N. The content of carbon nitride bonding in the CNx layer is 69 at% for the CaN and 82 at% for the C ¼ N bonds. The intensity ratios of the CaN/C ¼ N bonds reach 0.447 and 0.585 as calculated from the C1s and N1s spectra, respectively.
4. Conclusion We have prepared TiNy/CNx/TiNy trilayers, CNx/TiNy bilayers and CNx/TiNy multilayers by nitrogen ion beam sputtering of graphite and titanium targets. The SAED and XRD analyses indicate that besides the Ti2N phase there are several modifications of CaN crystals with hexagonal or cubic structure in the CNx/TiNy multilayers. The composition analyses from EDX spectroscopy and XPS show that the [N]/ ([C]+[N]+[Ti]) ratio in CNx/TiNy multilayers changes from 20 to 41 at%, and the average Ncontent in the CNx layer is about 23B27 at%. The intensity ratios of the CaN/C ¼ N bonds, as calculated from the C1s and N1 s spectra, are 0.447 and 0.585, respectively.
Acknowledgements We acknowledge financial support from Natural Science Foundation of Hebei Province, P.R. China.
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Preparation of CNx/TiNy multilayers by ion beam sputtering D.L. Yua, Y.J. Tiana,*, J.L. Hea, F.R. Xiaoa, T.S. Wanga, D.C. Lia, L. Lib, G. Zhengc, O. Yanagisawad a
College of Materials Science and Chemical Engineering, Yanshan University, No. 438, Hebei Street West, Qinhuangdao, Hebei 066004, People’s Republic of China b Institute of Physics, Chinese Academy of Science, P.O. Box 603, Beijing 100080, People’s Republic of China c Institute of Hui Guang Technology, Beijing 100080, People’s Republic of China d Faculty of Engineering, Hiroshima University, Hiroshima, Japan Received 30 March 2001; accepted 10 May 2001 Communicated by M. Roth
Abstract CNx/TiNy multilayers have been prepared by ion beam sputtering and analyzed by X-ray diffraction, transmission electron microscopy, selected area electron diffraction, energy dispersive X-ray and X-ray photoemission spectroscopy. The structure analyses shows that the films may contain several different kinds of CaN crystals embedded in the amorphous matrix. These crystals have hexagonal and cubic crystalline structures. The TiNx layer in multilayers is primarily amorphous and contains only a few dispersed Ti2N crystals. The thickness of CNx and TiNy layers in the CNx/TiNy multilayers is about 20 and 30 nm, respectively. The atomic ratios of [N]/([C]+[N]+[Ti]) in the multilayers vary from 20 to 41 at%. The bonding ratios of CaN/C ¼ N of the CNx layer in multilayers reach 0.447 or 0.585 as calculated from the C1s and N1s X-ray photoemission spectra, respectively. r 2001 Published by Elsevier Science B.V. PACS: 68.55.a; 81; 81.15.z; 81.20.n Keywords: A1. Crystal structure; A3. Polycrystalline deposition; B1. Nitrides
1. Introduction Liu and Cohen [1] theoretically predicted in 1989 the existence of a new compound, carbon nitride (C3N4). During the past decade studies of the carbon nitride compound have been of great interest to the field of materials science and engineering. Film deposition techniques [2–11], *Corresponding author. E-mail address: [email protected] (Y.J. Tian).
such as ion beam sputtering, RF diode sputtering, pulsed laser ablation, DC magnetron sputtering, electron cyclotron resonance plasma and chemical vapor deposition, etc., have been used to synthesize CaN films. Some researchers [12–14] have reported that crystalline b-C3N4 and a-C3N4 with sizes from several to hundreds of nanometers have been observed in amorphous CNx films. To our knowledge, sufficiently large CaN single crystals have not been synthesized for accurate evaluation of the carbon nitride compound structure and
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physical properties. For this reason, many groups worldwide are making great efforts to synthesize large CaN crystals. It is widely recognized that crystalline carbon nitride is thermodynamically metastable at ambient pressure and temperature. Therefore, choosing a suitable material as a structural template for nucleating the metastable C3N4 phase (to overcome its energy barrier) is a valid method. Li et al. [15] made the first attempt to produce crystalline carbon nitride using CNx/TiNy multilayers. Based on the same idea, Wu et al. [16] have also prepared CNx/ZrN multilayers with hardness exceeding 40 GPa and elastic modulus of over 400 GPa by DC magnetron sputtering. TiN and ZrN have been chosen as template material because there are good lattice matches between TiN (1 1 1) or ZrN (1 1 1) and b-C3N4 (0 0 0 1). The [N]/[C] ratio appears to be one of the most important characteristics of the carbon nitride films. The properties of carbon nitride films may be improved by increasing the N content [17]. However, most of the researches show that the nitrogen contents of the films are substantially below the stoichiometric composition of the C3N4 crystal (57 at%). In this work, a new method for preparation of CNx/TiNy films has been attempted. CNx/TiNy multilayers have been prepared by ion beam sputtering (IBS) and characterized using X-ray diffraction, transmission electron microscopy, selected area electron diffraction, energy dispersive X-ray and X-ray photoemission spectroscopy. The results obtained and discussed include the structures of crystals embedded in the amorphous matrix in CNx/TiNy multilayers, the average atom ratios of [N]/[C] and the binding ratios of CaN/ C ¼ N.
2. Experimental details TiNy/CNx/TiNy trilayers, CNx/TiNy bilayers and CNx/TiNy multilayers were deposited on NaCl slices and Si (1 1 1) wafers using an ion beam sputtering system. The schematic diagram of the experimental system is presented in Fig. 1. There are three Kaufman ion sources and a
Fig. 1. Schematic diagram of ion beam sputtering system.
rotatable substrate holder in the system. Two ion sources were used as sputtering guns and another one was used for cleaning the surfaces of the substrates. Two targets, graphite and titanium, were used in this work. The distance between the target and the substrate was about 110 mm. The anode-to-cathode potential and the anode potential were held at 50B70 V and 1000B1200 V during sputtering, respectively. The accelerating voltage was about 300 V. The ion beam current densities were 25B30 mA/cm2 and 65B70 mA/ cm2 for sputtering graphite and titanium, respectively. The discharge gas was high-purity nitrogen (99.999%). The total gas pressure was kept at 4B7 102 Pa in the chamber with a base pressure of 5 104 Pa. The substrate temperature was about 70B901C during deposition. All substrates were ultrasonically cleaned in acetone and ethanol before they were introduced into the sputtering system. The titanium and graphite targets were alternatively sputtered to form CNx/TiNy bilayers, TiNy/CNx/TiNy trilayers and CNx/TiNy multilayers (18 layers). The deposition rates were determined by measuring the thickness of films at different times using a profilemeter. The deposition rates for CNx and TiNy layers were 20 and 60 nm/h, respectively. In the bilayers, trilayers and multilayers, the thickness of the CNx layer was about 20 nm and that of TiNy layer was about 30 nm. The TiNy/CNx/TiNy trilayers deposited on freshly cleaved NaCl substrates were characterized using transmission electron microscopy (TEM)
D.L. Yu et al. / Journal of Crystal Growth 233 (2001) 303–311
and selected area electron diffraction (SAED). The investigation was performed using the H-800 microscope. The structure of TiNx/CNy multilayers was also studied by X-ray diffraction (XRD) with Rigaku model D/Max-RB X-ray diffractometer. We varied the spatial angle of the specimen relatively to the incident X-ray beam in order to collect more data. The composition of the multilayers was measured using an energy dispersive X-ray (EDX) spectrometer attached to the scanning electron microscope model AMMARY1000B. The CaN bonding nature on the top CNx layer in the CNx/TiNy bilayer was determined by X-ray photoemission spectroscopy (XPS). XPS was carried out in the ESCALAB 200I-XL spectrometer produced by VG Scientific Company using a Mg Ka X-ray source. The resolution of the XPS was about 0.5 eV. The surfaces of specimens were sputtered by argon ions for about 5 min before XPS measurements.
3. Results and discussion 3.1. Crystal structure Fig. 2 shows the XRD spectrum obtained from a multilayer film of CNx/TiNy (18 layers) deposited on Si (1 1 1). Apparently, many peaks are present in the 2y range of 20B1001 besides an amorphous peak at about 301. The experimental XRD data, the calculated data and the relative
305
intensities (I/I0 >1) of the powder diffraction lines based on the latest results of Teter and Hemley [18] are listed in Table 1. We have compared the X-ray diffraction data of Fig. 2 with similar data on all known crystals that can be formed by Ti, C and N. The results show that only four peaks with ( can d-spacings of 3.726, 2.385, 2.078 and 1.620 A be well indexed as the Ti2N phase in the XRD spectra. Yet, the peaks at 2.815, 2.128, 1.944, 1.620 ( may be well fitted to the (2 0 0), (2 1 0), and 1.539 A (1 1 1), (2 1 1) and (1 3 0) reflections of the b-C3N4 phase predicted theoretically [18]. The peaks with d-spacings of 3.378, 2.535, 2.385, 2.078, 1.944, ( can be indexed as the graphite1.675 and 1.539 A C3N4 phase and the peaks at 1.899, 1.716, 1.156 ( coincide with d-spacings of the cubicand 1.049 A C3N4 (2 0 2), (1 0 3), (2 4 0) and (5 0 1) planes. There ( in is one unknown peak with a d-spacing of 4.29 A the XRD spectrum. It should be noted that only the Ti2N phase is found in the CNx/TiNy multilayers, and the sizes of crystals may be too small to present all diffraction peaks of Ti2N. According to our XRD identification results there may be more than one kind of CaN crystals in the film obtained in our experiment. In order to identify further the types of crystals involved, we have performed TEM and SAED experiments using a very thin plane-view sample of TiNy/CNx/TiNy trilayers with the thickness of about 80 nm deposited on NaCl. The trilayer is collected onto a microscope grid by dissolving the NaCl substrate with de-ionized water. The sample
Fig. 2. XRD spectrum of CNx/TiNy multilayers.
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Table 1 Experimental XRD data compared with calculated [18] X-ray diffraction patterns of b-C3N4, graphite-C3N4, cubic-C3N4 and Ti2N () Experimental d (A
() Calculated d (A b-C3N4 hkl
3.726 3.378 2.815 2.535 2.385 2.128 2.078 1.944 1.899 1.716 1.675 1.620 1.539 1.156 1.049
200
Graphite-C3N4 d
2.789
I/I0
Cubic-C3N4
hkl
d
I/I0
002
3.36
100
102 110
2.601 2.371
1.1 1.2
200 201
2.053 1.964
3.8 19.1
hkl
2.108
33.9
111
1.958
55.8
202 103 1.603 1.547
I/I0
hkl
d
101
3.740
103
2.396
200
2.069
105
1.621
100
210
211 130
d
Ti2N
14.0 7.1
004
1.680
7.2
203
1.514
6.1
is then repeatedly cleaned in pure acetone and ethanol. We have found some crystals embedded in the matrix as shown in Fig. 3(a). Moreover, we have also found some regions with different contrast in the matrix, such as the A and B regions. SAED results reveal that both A and B are amorphous as shown in the pattern of Fig. 3(b). A crystal and its five SAED patterns obtained by rotating this crystal in the electron microscope are shown in Fig. 4. The experimental data from five diffraction patterns and calculated data [18] for graphite-C3N4 are listed in Table 2. Apparently, our experimental data are close to the calculation for the graphite-C3N4 phase. We have indexed the diffraction spots and determined each zone axis by means of the CaRIne Crystallographic software based on the structure parameters of the graphite-C3N4 phase. The identification results and the corresponding errors (on two distances and an angle) are also listed in the Table 2. Obviously, the five SAED patterns from this crystal can match well the [0 1 0], [1 3 1], ½6% 3 1 1; ½3% 4 1 and [0 1 3] crystal axes of graphiteC3N4, respectively. Fig. 5(a) shows some small crystals with the shapes of square, rhombus and polygon. The diffraction pattern (Fig. 5(c)) has
240 501
1.909 1.708
1.208 1.059
56.6 1.0
6.0 12.9
Fig. 3. (a) TEM micrograph of the amorphous matrix containing a few crystals in the TiNy/CNx/TiNy trilayer; (b) SAED pattern of the matrix of TiNy/CNx/TiNy trilayer.
been obtained from one of these crystals, which is enlarged and shown in Fig. 5(b). From its shape and the distribution of diffraction spots, we infer that it may belong to the cubic crystallographic ( and an system. Two interplanar distances of 2.20 A angle of 70.51 between the two spots are obtained from the diffraction pattern. They are close to the ( and the angle of (2 1 1) plane d-spacing of 2.18 A
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Fig. 4. TEM image of a CaN crystal and its SAED patterns corresponding to five directions. The SAED patterns are identified with structure parameters of the graphite-C3N4 phase. Table 2 SAED measured data compared with the d-spacings and angles calculated from graphite-C3N4 [18] Pattern number The data measured from SAED patterns The data calculated from graphite-C3N4 Sum of errors on d1, d 2 and a b
c
d
e
f
( d1=3.39 A ( d2=3.55 A a=581
( d(0 0 2)=3.36 A ( d(1 0 1)=3.50 A a=58.571 zone axis: [0 1 0]
2.6%
( d1=3.55 A ( d2=2.55 A a=821
( dð1 0% 1Þ ¼ 3:50 A ( dð1% 1 2% Þ ¼ 2:60 A a=82.341 zone axis: [1 3 1]
3.7%
( d1=1.01 A ( d2=1.58 A a=881
( dð4 3% 3Þ ¼ 1:02 A ( d(1 2 0)=1.55 A a=87.321 zone axis: ½6% 3 1 1
3.1%
( d1=4.17 A ( d2=1.52 A a=79.51
( d(1 0 0)=4.11 A ( dð2 3% 1Þ ¼ 1:51 A a=79.391 zone axis: [0 1 3]
2.2%
( d1=2.24 A ( d2=2.10 A a=84.51
( dð1 1 1Þ ¼ 2:24 A ( d(1 0 3)=1.97 A a=84.32 zone axis: ½3% 4 1
6.7%
711 between the (2 1 1) and ð2 1% 1% Þ planes in the cubic-C3N4 phase predicted by Teter and Hemley [18], and the corresponding error is 5%. A TEM micrograph containing a great number of circular
shape crystals and their diffraction rings indexed with the structure parameters of the b-C3N4 phase ( and c=2.4041 A ( ) are shown (P3(143), a=6.4017 A in Fig. 6. The experimental data and the calculated
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Fig. 5. (a) and (b) TEM images of crystals with different shapes and (c) SAED pattern identified with the cubic-C3N4 structure.
In our CNx single layer deposited on a Si substrate by IBS [8], we have obtained only a single kind of CaN crystal with the structure of bC3N4, as deduced from XRD and SAED results. TEM observations show that the crystallinity of the film is poor. The Auger analysis indicate that the average N content of the CNx single layer is about 20%. IR absorption spectrum and XPS results reveal that the carbon nitrogen bonds exist in the film. However, the analyses of both SAED and XRD show that there may be simultaneously several kinds of carbon nitride compounds in our CNx/TiNy multilayer sample. This phenomenon may be related to the small Ti2N crystals, which can act as nucleation sites for growth of crystalline CaN compounds. According to the calculations [18], the total formation energies of CaN compounds are very close (the difference of the total energies between them are below 0.008 eV per C3N4 unit). Therefore, it is possible that carbon nitride compounds with different crystal structures are formed in the same CNx/TiNy multilayer. 3.2. Composition and bonding characterization
Fig. 6. TEM image of small circular shape crystals and SAED rings indexed with structure parameters of the b-C3N4 phase.
Table 3 Comparison of the diffraction rings data with the interplanar spacings of (h k l) phanes calculated from the b-C3N4 structure Miller indices (h k l)
Calculated d () (A
SAED measured d () (A
110 101 111 220 310 221 320 410 420
3.22 2.26 1.96 1.61 1.55 1.35 1.28 1.22 1.05
3.10 2.28 1.97 1.67 1.55 1.36 1.28 1.20 1.01
data for the b-C3N4 phase are listed in Table 3. Clearly, there is a good correspondence between the experimental data obtained from the diffraction rings and the d-spacings calculated for the b-C3N4 phase.
Compositional analysis of the CNx/TiNy multilayers has been performed by EDX. The results show that the samples contain silicon and a small amount of oxygen besides the main elements, such as carbon, nitrogen and titanium. Silicon originates from the substrate, and the oxygen impurity stems from the surface pollution of the film. The multilayers composition is characterized by the ratio [N]/([C]+[N]+[Ti]) that does not depend on the concentration of the impurities. This ratio measured in different regions of the multilayers varies from 20 to 41 at%. The variation of the [N]/ ([C]+[N]+[Ti]) ratio indicates that various crystalline nitride compounds may exist in the multilayers leading to the uneven N distribution. This phenomenon is also observed in Fig. 3(a). In the process of structure analysis of TiNy/CNx/TiNy trilayers and CNx/TiNy multilayers we have found only a single titanium nitride phase (Ti2N) besides the crystalline carbon nitride compound. Yet, it seems that just a small amount of the Ti2N phase cannot seemingly cause the observed large fluctuations in the [N]/([C]+[N]+[Ti]) ratio. Therefore,
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we presume that the crystalline CaN compound is most likely responsible for the uneven nitrogen distribution. This has prompted our studies of Ncontent and the nature of carbon and nitrogen chemical bonds in the top CNx layer of CNx/TiNy bilayers by XPS. The XPS spectra are shown in Fig. 7. They do not reveal any significant amount of oxygen on the surface of the film. This indicates that the small amount of oxygen detected by EDX spectroscopy may originate from the surface pollution of the film. The average N-content in the top CNx layer of our CNx/TiNy bilayers ranges from 23 to 27 at% as calculated from the XPS spectra. These
values differ from the results obtained by EDX. The N-content measured by EDX spectroscopy represents the average N-content of the multilayers over a certain area in the film including the CNx and TiNy layers. In contrast, XPS represents the average N-content of only the CNx layer in the CNx/TiNy multilayers. This is an additional manifestation of the fact that the multilayer is made up of CNx layers and TiNy layers. The C1s and N1s spectral bands are deconvoluted into six peaks and four peaks, respectively. The bond energies to which every peak corresponds and the ratio of the CaN/C ¼ N bonds in the CNx layer are listed in Table 4. Within the C1s band, the
Fig. 7. XPS spectra of the CNx layer in CNx/TiNy multilayers.
Table 4 Quantitative data calculated from C1s and N1s spectra C1s Binding energy (eV) 291.36 289.17 287.39 285.69 284.45 281.99
CaN/C ¼ N=0.447
N1s [AT]% 1.791 5.564 21.701 48.585 15.774 6.585
Bonding type *
p–p (C) CO CaN C¼N Pure C Metal carbides
Binding energy (eV)
[AT]%
Bonding type
402.01 400.86 399.56 398.39
3.285 13.947 52.211 30.557
NO NO C¼N C¼N
CaN/C ¼ N=0.585
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287.39 and 285.69 eV peaks correspond to the bonding energies of CaN and C ¼ N, respectively. Among the other peaks, 291.36 eV may be a satellite peak corresponding to the p–p* bond of the carbon network. The 289.17 eV peak reflects the CO-type bonds, and the 281.99 eV peak is a contribution of metal carbides on the surface of the sample bracket. The 284.45 eV peak can be identified as originating from adventitious or surface carbon due to surface exposure and is often observed in XPS [19]. The N1s band is composed of four peaks at 402.01, 400.86, 399.56 and 398.39 eV, respectively. The bond energies of 399.56 and 398.39 eV can be attributed to the C ¼ N and CaN bonds. The peak at 402.01 eV is identified as originating from NaO or NaN bonds [20–22]. However, there are different explanations to the peak at 400.86 eV in the literature. Some researchers [19,23] attribute this peak to the NO bond. Others [24–27] consider it as corresponding to N-sp2. We contemplate that the bond energy of 400.86 eV is too large to correspond to the CaN bond, though it is close to the bond energy (400.6 eV) of pyridine-like N. The content of carbon nitride bonding in the CNx layer is 69 at% for the CaN and 82 at% for the C ¼ N bonds. The intensity ratios of the CaN/C ¼ N bonds reach 0.447 and 0.585 as calculated from the C1s and N1s spectra, respectively.
4. Conclusion We have prepared TiNy/CNx/TiNy trilayers, CNx/TiNy bilayers and CNx/TiNy multilayers by nitrogen ion beam sputtering of graphite and titanium targets. The SAED and XRD analyses indicate that besides the Ti2N phase there are several modifications of CaN crystals with hexagonal or cubic structure in the CNx/TiNy multilayers. The composition analyses from EDX spectroscopy and XPS show that the [N]/ ([C]+[N]+[Ti]) ratio in CNx/TiNy multilayers changes from 20 to 41 at%, and the average Ncontent in the CNx layer is about 23B27 at%. The intensity ratios of the CaN/C ¼ N bonds, as calculated from the C1s and N1 s spectra, are 0.447 and 0.585, respectively.
Acknowledgements We acknowledge financial support from Natural Science Foundation of Hebei Province, P.R. China.
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