Synthesis of crystalline carbon nitride thin films by ...

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113. 14 U. Gelius, P. F. Heden, J. Hedman, B. J. Lindberg, R. Manne, R. Nord- berg, R. Nordling, and K. Siegbahn, Phys. Scr. 2, 70 1970. 15 B. J. Lindberg and ...

Synthesis of crystalline carbon nitride thin films by laser processing at a liquid–solid interface A. K. Sharma, P. Ayyub, M. S. Multani, K. P. Adhi, S. B. Ogale et al. Citation: Appl. Phys. Lett. 69, 3489 (1996); doi: 10.1063/1.117261 View online: http://dx.doi.org/10.1063/1.117261 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v69/i23 Published by the American Institute of Physics.

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Synthesis of crystalline carbon nitride thin films by laser processing at a liquid–solid interface A. K. Sharma, P. Ayyub,a) and M. S. Multani Materials Research Group, Tata Institute of Fundamental Research, Homi Bhabha Road, Bombay 400 005, India

K. P. Adhi and S. B. Ogale Department of Physics, Centre for Advanced Studies in Materials Science and Solid State Physics, University of Poona, Pune 411 007, India

M. Sunderaraman, D. D. Upadhyay, and S. Banerjee Metallurgy Division, Bhabha Atomic Research Centre, Bombay 400 085, India

~Received 12 February 1996; accepted for publication 23 September 1996! Pulsed laser induced reactive quenching at a liquid–solid interface was used for the synthesis of tetrahedrally coordinated crystalline carbon nitride on a tungsten substrate. The crystalline phase was identified by transmission electron diffraction. X-ray photoelectron spectroscopy indicated that the carbon atoms are coordinated only tetrahedrally with nitrogen—as expected for C3 N4 . The atomic percentage of N ~considering only those atoms coordinated with C! is about 35%. © 1996 American Institute of Physics. @S0003-6951~96!01149-7#

The possible existence of a class of carbon nitrides isomorphic to Si3 N4 has been suggested some years ago by Cohen.1 From ab initio calculations based on an empirical model for the hardness of covalent solids, a particular phase of carbon nitride ~b-C3 N4 ) was predicted to have a bulk modulus as high as that of diamond, the hardest known solid. The shortness of the C–N bonds and the high degree of covalence in these compounds lead to expectations of a new class of superhard structural ceramics. Total energy and electron structure calculations were later made2,3 for the hypothetical b-C3 N4 . The results were consistent with the earlier prediction. The experimental quest to realize this novel compound has resulted in several publications mainly in the past couple of years.4 Most of these report the deposition of partially or predominantly amorphous CNx (x,1). In a recent review, Fang5 concludes: ‘‘...today there is no credible evidence of this incredible material with chemical composition C3 N4 ... .’’ Since the publication of this review, a few authors have reported preliminary evidence for the partial presence of a crystalline carbon nitride phase.6 An ab initio force field calculation7 suggests that another isomorph of carbon nitride, namely a-C3 N4 , is more stable than b-C3 N4 , and—intriguingly—that it should have a negative Poisson ratio. A recent study indicates that the bulk modulus of a-C3 N4 should be 96% that of diamond, while that of cubic C3 N4 should be much higher.8 The most important issue, therefore, is to convincingly establish the existence of the predicted phase~s! of C–N and devise a practical and reproducible synthesis technique that would allow the predicted properties to be tested. We have recently demonstrated the feasibility of using pulsed laser induced reactive quenching at the interface of an organic liquid and a solid substrate for the synthesis of tetrahedrally coordinated (s p 3 bonded! solids such as diamond and its polytypes.9 The organic liquids ~cyclohexane and a!

Electronic mail: [email protected]

Appl. Phys. Lett. 69 (23), 2 December 1996

decalin! were selected from stereochemical criteria: their molecules consist of the same chair and boat units that are the building blocks of the diamond polytypes. The choice of starting material is particularly important when there are other energetically competitive configurations. Since the C3 N4 phases are probably metastable, structures with other stoichiometries ~containing C5N and C[N! and comparable energy are expected to compete in their formation. Stereochemistry suggests the use of hexamethylenetetramine or hexamine (C6 H12N4 ) as the parent system. The structure of this compound closely resembles that predicted for a- or bC3 N4 , since all the C atoms are tetrahedrally coordinated with N, just as in C3 N4 . The same compound was also used as a prototype in cluster model calculations7 of the structure and properties of C3 N4 . Since the N:C ratio is smaller in hexamine than in C3 N4 , additional nitrogen was provided by adding liquid ammonia. A 0.1-mm-thick tungsten foil was kept in a small stainless steel reaction cell. Powdered hexamine was sprinkled on the tungsten foil and kept under a flow of dry nitrogen to prevent condensation of moisture. The substrate temperature was maintained at about 260 °C by keeping the reaction cell in thermal contact with an acetone bath held close to its freezing point ~'295 °C! by controlled cooling with liquid nitrogen. An impinging jet of ammonia (b.p. 5233 °C, f .p.5277 °C) gets liquified on coming in contact with the substrate at 260 °C and partially dissolves the hexamine powder. Pulses from an excimer laser operating at 5 Hz ~l5248 nm, pulse width520 ns, energy density '3.5 J/cm2 ) were fired at the solid-liquid interface, while taking care to maintain a steady supply of ammonia and hexamine. The laser beam was made to scan a 6 mm36 mm area ~approximately! with several pulses shot at every point on the surface. Samples prepared in this manner were characterized by x-ray photoelectron spectroscopy ~XPS! and transmission electron microscopy ~TEM!. The most important aspects of the characterization of CNx films are ~a! to determine the bonding state between C

0003-6951/96/69(23)/3489/3/$10.00

© 1996 American Institute of Physics

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FIG. 1. X-ray photoelectron spectrum of C-1s electrons from a carbon nitride film grown by laser processing at the liquid–solid interface.

and N ~whether single, double, or triple bonds!, and ~b! to evaluate the N:C ratio in each phase present. The XPS technique is unique in providing both types of information. The chemical environment of an atom is obtained from the core level chemical shifts, and the technique is believed to be accurate to within 5%–10% for quantitative analysis. For CNx films, XPS has been used for both purposes by a number of workers.10,11 Figure 1 shows the C-1s spectrum recorded from the laser-processed CNx films. The C-1s spectrum can be deconvoluted into two peaks. The major peak at 284.6 eV can be identified as originating from adventitious or surface carbon which has lost its nitrogen neighbors due to surface exposure and is observed often in XPS.11 The other peak at 286.1 eV can be ascribed to the tetrahedral C–bond.10,12,13 A line at the same position ~285.5 eV!—observed by Su et al.6 in their ‘‘b-C3 N4 phase’’—was ascribed to s p 3 bonded carbon. Marton et al.11 have attributed a feature at 287.7 ~60.2! eV to tetrahedrally bonded carbon but their films are amorphous with no clear signature of C3 N4 , and the small upshift in the C-1s peak may arise from multiple phases with different stoichiometries. Note also that other authors13 have assigned a peak at 288.1 eV to a C5O bond, which is likely to be present at the sample surface. We therefore conclude that the only hybridized form of carbon present in our films is sp 3 bonded. The N-1s results are consistent with the above interpretation. Of the three component peaks in the spectrum ~Fig. 2!, the dominant one at 398.6 eV appears to originate from the N atoms that form C–N single bonds—the only type expected in a- or b-C3 N4 . In a hexamine molecule, both the C and N atoms are tetrahedrally coordinated; but in crystalline C3 N4 , the C atoms occupy tetrahedral sites while the N atoms are trigonally coordinated. The C- and N-1s binding energies in hexamine are 286.9 and 399.4 eV, respectively.14,15 The pair of nonbonded electrons in the trigonally coordinated N atoms of C3 N4 screen the core level; so the N-1s and possibly also the C-1s binding energies would be a little lower than in hexamine. The binding energies for these two species obtained from our XPS data, therefore, appear to clearly indicate the presence of C–N. Other bonding configurations such as C5N or C[N were not observed. The XPS peaks at 400.8 and 396.9 eV can be 3490

FIG. 2. X-ray photoelectron spectrum of N-1s electrons from the same film as in Fig. 1.

identified with N–N ~or N–O! and W–N, respectively.11 The presence of WN in a few regions ~probably close to the outer boundary of the film! was also indicated by electron diffraction. The atomic fraction of nitrogen in the tetrahedrally bonded, crystalline CNx phase present in our films was calculated by dividing the area under the respective XPS peaks ~286.1 eV for C and 398.6 eV for N! by the appropriate sensitivity factors.16 The nitrogen content in CNx was found to be 35 at. % as against 57% expected in stoichiometric C3 N4 . Zhang et al.17 have made a systematic study of the composition of laser ablated CNx films and claim a maximum value of 47% N. Note, however, that this is the overall N fraction ~obtained from Rutherford backscattering!. The fraction of tetrahedrally bonded CNx phase in their films ~which is what should be compared to our value of 35%! is not known. Yu et al.18 similarly report an overall N content of 30% in a sample claimed to be b-C3 N4 . Note also the recent suggestion19 that the N fraction in b-C3 N4 should be substantially lower than 57% due to repulsion between the nonbonded N atoms and their consequent partial replacement by C–C bonded dimers. The specimen for electron diffraction studies were ion milled from the substrate side till a small perforation was obtained. The deposited surface was cleaned by briefly ion milling it. The samples were examined in a JEOL 2000 FX

FIG. 3. Selected area diffraction pattern showing the presence of crystalline C3 N4 . The d spacings calculated from this pattern and the relative intensities are given in Table I.

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Sharma et al.

TABLE I. Comparison of the transmission electron diffraction data with the predictions in Ref. 8. The lattice constants calculated from our data are: a56.361(14)Å, c54.824(27)Å ~for a-C3 N4 ), and a 56.410(38)Å, c52.375(20)Å ~for b-C3 N4 ). Note: s5 strong, m5 medium, w5 weak, v w5 very weak. Observed reflections

a-C3 N4 ~predicted!

b-C3 N4 ~predicted!

d~Å!

I

d~Å!

~hkl!

I

d ~Å!

~hkl!

I

2.77 2.39 2.21

m s s

1.72

m

2.80 2.40 2.18 2.11 1.93 1.90 1.73 1.51

200 201 102 210 211 112 301 103

m s s m m s m vw

1.40

w

1.22

s

1.09

m

0.92

w

vw m m m w w m w vw w vw

s ••• s s m m ••• w w w w vw vw vw vw w

m

302 222 321 303 313 114 412 501 330 502 323 205

200 ••• 101 210 111 300 ••• 211 310 301 221 410 401 500 202 411

1.00

1.46 1.33 1.24 1.20 1.10 1.10 1.09 1.08 1.01 0.99 0.90

2.77 ••• 2.20 2.09 1.92 1.85 ••• 1.58 1.54 1.46 1.33 1.21 1.20 1.11 1.10 1.08

0.81

m

0.80

433

vw

1.00 0.99 0.92 0.91 0.80

501 510 511 430 332 611

vw vw vw vw w

TEM operated at 160 kV. Selected area diffraction ~SAD! revealed reflection patterns of a tungsten single crystal. These were used to determine the camera constant of the microscope. Several regions of the film revealed ring-type diffraction patterns typical of a crystal ~Fig. 3!. Each ring contains many individual spots suggesting a microcrystalline structure which is also borne out by electron microscopy. Closely spaced reflections were resolved by taking a microdensitometer trace. The ‘‘d’’ spacings were calculated from Fig. 3 using the previously determined camera constant. The observed d spacings and those predicted8 for a- and b-C3 N4 are listed in Table I. The relative intensities were obtained from a microdensitometer plot. A comparison of the intensities and the d spacings of the observed and predicted reflections suggests the presence of crystalline a- as well as b-C3 N4 in the laser processed thin films. However, the reflections observed at 2.39 and 1.72 Å clearly establish the existence of a-C3 N4 . The hexagonal lattice constants calculated form our data ~Table I! are within 2% of the values predicted in Ref. 8. Note that the calculated total energies8 for the a and b phases of C3 N4 differ only by 266 meV per unit cell ~i.e., 0.017%!. So a nonequilibrium synthesis process such as used by us, is likely to lead to the simultaneous presence of both phases. In summary, we find that it is possible to produce crystalline C3 N4 by pulsed laser induced reactive quenching at

Appl. Phys. Lett., Vol. 69, No. 23, 2 December 1996

the interface of a tungsten substrate and an organic solid ~hexamethylenetetramine! which is stereochemically similar to the predicted structure of C3 N4 . M. L. Cohen, Phys. Rev. B 32, 7988 ~1985!. A. Y. Liu and M. L. Cohen, Science 245, 841 ~1989!. 3 A. Y. Liu and M. L. Cohen, Phys. Rev. B 41, 10 727 ~1990!. 4 T.-Y. Yen and C.-P. Chou, Appl. Phys. Lett. 67, 2801 ~1995!, and references therein. 5 P. H. Fang, J. Mater. Sci. Lett. 14, 536 ~1995!. 6 X. W. Su, H. W. Song, F. Z. Cui, and W. Z. Li, J. Phys., Condensed Matter 7, L517 ~1995!. 7 Y. Guo and W. A. Goddard III, Chem. Phys. Lett. 237, 72 ~1995!. 8 D. M. Teter and R. J. Hemley, Science 271, 53 ~1996!. 9 A. K. Sharma, R. D. Vispute, D. S. Joag, S. B. Ogale, S. D. Joag, P. Ayyub, M. Multani, G. K. Dey, and S. Banerjee, Mater. Lett. 17, 42 ~1993!. 10 O. Matsumoto, T. Kotaki, H. Shikano, K. Takemura, and S. Tanaka, J. Electrochem. Soc. 141, L16 ~1994!. 11 D. Marton, K. J. Boyd, A. H. Al-Bayati, S. S. Todorov, and J. W. Rabalais, Phys. Rev. Lett. 73, 118 ~1994!. 12 F. Fujimoto and K. Ogata, Jpn. J. Appl. Phys. 32, L240 ~1993!. 13 C. N. Reilly and D. S Everhart, in Applied Electron Spectroscopy for Chemical Analysis, edited by H. Windawi and F. F.-L. Ho ~Wiley, New York, 1982!, p. 113. 14 U. Gelius, P. F. Heden, J. Hedman, B. J. Lindberg, R. Manne, R. Nordberg, R. Nordling, and K. Siegbahn, Phys. Scr. 2, 70 ~1970!. 15 B. J. Lindberg and J. Hedman, Chem. Scr. 7, 155 ~1975!. 16 G. C. Smith, Surface Analysis by Electron Spectroscopy ~Plenum, New York, 1994!. 17 Z. J. Zhang, S. Fan, and C. M. Lieber, Appl. Phys. Lett. 66, 3582 ~1995!. 18 K. M. Yu, M. L. Cohen, E. E. Haller, W. L. Hansen, A. Y. Liu, and I. C. Wu, Phys. Rev. B 49, 5034 ~1994!. 19 T. Hughbanks and Y. Tian, Solid State Commun. 96, 321 ~1995!. 1 2

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