Optical characterisation of CNx thin films deposited by reactive pulsed laser ablation

Thin Solid Films 349 (1999) 100±104

Optical characterisation of CNx thin ®lms deposited by reactive pulsed laser ablation A. Zocco a,*, A. Perrone a, A. Luches a, R. Rella b, A. Klini c, I. Zergioti c, C. Fotakis c a

UniversitaÁ di Lecce, Dip. di Fisica and Istituto Nazionale Fisica della Materia, Via per Arnesano CP 193, 73100 Lecce, Italy b Istituto per lo studio dei Nuovi Materiali per l'Elettronica, IME-CNR, Via per Arnesano, 73100 Lecce, Italy c University of Crete, Department of Physics and Foundation for Research and Technology- Hellas (FORTH), Laser and Application Division, P.O. Box 1527, 71110 Heraklion, Crete, Greece Received 18 June 1998; received in revised form 26 February 1999; accepted 27 February 1999

Abstract Optical absorption measurements on CNx thin ®lms produced by reactive pulsed laser ablation (RPLA) at different pressures of nitrogen in the growth chamber were performed. The in¯uence of growth regime on optical properties (n,k) of the CNx ®lms has been examined with IR and UV-VIS spectroscopy. The dependence of the absorption coef®cient a on the photon energy "v at the edge of the absorption band is well described by the relation a"v ˆ B…"v 2 Eopt †2 indicating the presence of allowed indirect transitions in the 0.8±3.0 eV photon energy range. Furthermore, we observed a decrease in the optical gap values with increasing N concentration in the deposited ®lms. Fourier transform infrared (FTIR) spectra were also employed to analyse the chemical bonding state between the different species present in the ®lms. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Nitrides; Optical properties; Laser ablation; Infrared spectroscopy

1. Introduction Present interest in the synthesis of carbon nitride ®lms in particular for b -C3N4 phase starts from the well known theoretical work by Liu and Cohen foreseeing for this new material a bulk modulus comparable to that of diamond [1]. In addition to the hardness, this new compound is expected to have superior compressive strength, high optical transparency, high thermal conductivity and chemical inertness [2]. This material can be used not only as optical but also as a super-hard coating for thermally unstable materials such as glasses and plastics. Many other applications are foreseen in the microelectronics, optics and tribology ®elds [3±6]. Considering the potential application of CNx ®lms as optical coatings, it is unexpected to ®nd so few studies on their optical properties [7±9]. The study and analysis of optical constants of CNx ®lms evaluated near the fundamental absorption edge are interesting for potential application of this compound as a variable gap material into silicon technology [10]. In particular, the study of optical constants in the sub * Corresponding author. Tel.: 1 39-832-320502; fax: 320505. E-mail address: [email protected] (A. Zocco)

1 39-832-

band gap region is of interest for the possible use of this material in solar cells and in optoelectronic devices. Consequently, accurate and adequate knowledge of the refractive index and absorption coef®cient for this material is necessary for the design and exploitation of perspective devices. Based on the above considerations, we deposited carbon nitride thin ®lms on sapphire substrates by laser ablation of graphite target in different N2 gas pressures. In this paper, the results on the optical properties of the ®lms, obtained by transmission and re¯ection spectra in the visible range and by FTIR analysis, are presented and discussed.

2. Experimental details Depositions were performed using a KrF excimer laser (l ˆ 248 nm, tFWHM ˆ 25 ns). Series of 10 4 pulses at a repetition rate of 10 Hz were directed to high purity graphite targets. The laser ¯uence was set at 22 J/cm 2. The laser beam was incident on the target surface with an angle of 458. In order to obtain irradiation conditions as uniform as possible and to reduce drilling, the target was rotated with a 3 Hz frequency during the laser ablation process. The ablated material was collected on sapphire substrates at room temperature. The substrate was placed on a support

0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0022 1-7

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Table 1 Thickness, energy gap and [N]/([N] 1 [C]) values obtained for the ®lms deposited by KrF laser ablation of graphite at F ˆ 22 J/cm 2, as a function of N2 pressure (PN2 ). PN2 (Pa)

Thickness (nm) Eg(eV) [N]/([N] 1 [C])

0.2

1

50

600 1.25 ±

510 1 0.3

320 0.8 0.5

n0 (air) and n2 (sapphire substrate), the latter supposed to be very thick with respect the wavelength. According to this model, the re¯ectance and transmittance may be expressed Fig. 1. Schematic diagram of experimental setup: T, target; S, substrate; L, lens.

at a distance d ˆ 4 cm from the target. The depositions were performed inside a stainless steel vacuum chamber evacuated down 10 24 Pa and then ®lled with high purity nitrogen. The ¯ux of very pure nitrogen (electronic grade 99.999%) was continuously blown inside the chamber which was accurately set a chosen value in the 0.2±50 Pa range. A scheme of our experimental apparatus is given in Fig. 1. Optical measurements were carried out by using a Varian Cary 5 UV/VIS/NIR double beam spectrophotometer equipped with an integrating sphere in the spectral range between 200 and 800 nm at normal incidence. The refractive index n and extinction coef®cient k were determined from both transmission and re¯ection measurements. The thickness of the examined samples was evaluated by a TENCOR Alphastep 200 stylus pro®lometer. Table 1 reports the average thickness of different analysed samples. Transmission infrared spectra of ®lms deposited on sapphire substrates (1 mm thick) were recorded by a Perkin Elmer Spectrum 2000 Fourier transform infrared spectrometer using FR-DTGS (fast recovery deuterated tryglycine sulfate) detector in the wavenumber range 1300±4000 cm 21.

3. Results and discussion 3.1. UV-VIS spectroscopy The optical parameters were calculated from transmission T(l ) and re¯ection R(l ) data. Typical transmission and re¯ection spectra obtained onto samples deposited with different N2 pressure conditions are shown in Fig. 2a and Fig. 2b. An abrupt decrease in transmission at high energy indicates the onset of interband absorption (fundamental absorption edge). The refractive index n and extinction coef®cient k vs. wavelength at normal incidence have been calculated from R(l ) and T(l ) data. The optical model used takes into account a parallel-sided absorbing ®lm of complex refractive index between media of index

Fig. 2. UV-VIS spectra of the ®lms deposited on sapphire at 0.2, 1 and 50 Pa: (a) Transmission spectra; (b) re¯ection spectra.

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Fig. 3. Refractive index n vs. wavelength at normal incidence for the ®lms deposited at 0.2, 1 and 50 Pa.

in the following forms: ÿ
Acoshg 1 Bsinhg 2 Ccosd 1 Dsind
R…l† ˆ ÿ Ecoshg 1 Fsinhg 2 Gcosd 1 Hsind h T … l† ˆ ÿ

 i 8n2 n2 1 k2

Ecoshg 1 Fsinhg 2 Gcosd 1 Hsind




…1†

…2†

where g ˆ 4pkd=l and d ˆ 4pnd=l (d being the thickness of the absorbing layer). The terms A-H appearing in Eq. (1) and Eq. (2) are functions of n and k [11]. When the re¯ectance R(l ), the transmittance T(l ), both at normal incidence, and the thickness of the ®lm (absorbing layer) are known, it is possible, by a point-by-point computer ®t procedure, to calculate the optical constants. This method

Fig. 4. Extinction coef®cient k vs. wavelength at normal incidence for ®lms deposited at 0.2, 1 and 50 Pa.

also permits to evaluate the ®lm thickness with good accuracy, because only with the correct value one obtains continuous n(l ) and k(l ) curves over the whole investigated wavelength range. Generally, thickness values are within 3% equal to the values measured with a stylus pro®lometer and reported in Table 1. Fig. 3 reports the calculated refractive index of the different samples. As one can see, it varies with wavelength in the range 1.5±1.7. The results of our calculus, regarding the real part of the refractive index, evidence oscillations that are probably due to the nonuniformity in the composition of the ®lms, because the method is forced to give values of n that follow the interference pattern present in the re¯ection spectra. In effect, in some cases, the analysis can be affected by error (estimated around at 5%), in particular when the ®lm presents nonuniformity in the composition or in the physical thickness. However, the fact that the thickness values found are similar to those measured with the pro®lometer is the best indication that the range of the calculated values is correct. Moreover, they are also in agreement with the values reported by Gonzalez et al. [12] for CNx ®lm prepared by excimer laser ablation. They report a refractive index value between 1.6 and 1.8 for ®lms having the same nitrogen percentage ‰NŠ=…‰NŠ 1 ‰CŠ† ˆ 0:3±0:5 as our samples, which was estimated by EDX (energy dispersion X-ray) spectroscopy. Moreover, we noted that nitrogen incorporation in our ®lms increases with the nitrogen pressure in the reaction chamber [13]. As regarding the extinction coef®cient k, Fig. 4 shows the presence of a maximum centred (within the experimental error) at about 400 nm for all the examined samples. Moreover, the optical absorption increases when increasing the nitrogen concentration in the ®lm. Similar behaviour is already reported by Wang et al [14] for CNx ®lm deposited by ion assisted arc deposition. Furthermore, the calculated k(l ) values give us the possibility to obtain the absorption coef®cient a ˆ 4pk=l, in the same spectral range, in order to have information about direct or indirect interband transitions optically activated near the fundamental absorption edge region. In fact, the simple analysis of the absorption coef®cient spectra, calculated in this way, gives information about the different allowed or forbidden electronic transitions near the region of high optical absorption. The theory of interband absorption shows that the absorption coef®cient a varies with energy "v according to the relation  m a"v ˆ B "v 2 Eg …3† where B is a parameter that depends on the transition probability and m is a number which characterises the transition process [15]. An analysis of the absorption spectrum, obtained for our samples, shows that the spectral variations in a , inside the fundamental absorption region, can be described by Eq. (3) with m ˆ 2, up to a photon energy of

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matrix of the CNx deposited thin ®lms. Really, the decrease in the calculated optical band gap with increasing PN2 suggests a decrease in the structural order with introduction of localised energetic levels near the edge of the band gap which produce p ±p * transitions in the investigated spectral range [16]. 3.2. IR spectroscopy

Fig. 5. Calculated values of …a"v†1=2 as a function of the energy h¤v for the ®lms deposited at different N2 pressures.

about 3 eV (Fig. 5). This dependence is typical of indirect allowed transition, with an energy gap Eg depending on the deposition conditions, in our case on nitrogen pressure. In particular, we obtain a decrease in the optical energy gap by increasing nitrogen percentage in the ®lms. In Table 1 are also reported the optical gap Eg values obtained from the intercept of the extrapolation of the linear part of the curve …a"v†1=2 ˆ 0 with the energy axis, at different nitrogen pressures. It means that the absorption mechanisms that take place in our samples depend on the nitrogen pressure. This behaviour indicates a modi®cation in the structural order by increasing nitrogen atoms concentration in the

The structural properties of the ®lms were studied using FTIR analysis. Fig. 6 shows the FTIR spectra of the ®lms grown at different nitrogen pressures (0.2, 1 and 50 Pa). In the spectrum of sample deposited at 50 Pa we identi®ed the following bands: 3330 and 3200 cm 21 due to NH2 asymmetric and symmetric stretching modes, respectively; 2910 and 2850 cm 21 due to CH2 asymmetric and symmetric stretching modes, respectively [16]; the doublet at 2325 and 2350 cm 21 associated to CO2; 2200 cm 21 attributed to C ; N triple bond stretching mode [17]. At 1800 cm 21 a strong absorption band starts and it is due to sp 2 carbon and normally IR forbidden. The appearance of this feature suggests that the incorporation of nitrogen into carbon breaks the sp 2 symmetry and makes this feature IR active [17,18]. This broad band is peaked at 1500 cm 21 indicating the absorption due to CvN double bond stretching mode. Finally, the absorption band observed at 1350 cm 21 corresponds to CZN single bond stretching mode [19]. By comparison of the spectra of the samples deposited at three different N2 pressures we can observe the appearance of the NH2, CH2, C ; N and CZN bands only at the highest N incorporation, as already observed in [16]. These bands are clearly absent in the spectra of the ®lms deposited at 0.2 and 1 Pa. The only feature observable in these last spectra is the broad band peaked at 1630 cm 21, attributed to CvC double bond stretching mode [20]. The presence of CH2 and NH2 radicals in the ®lms could be explained with the high reactivity of water vapour and hydrogen present, even if in low concentration, in the vacuum chamber [21]. Water vapour could come from the residual gas or it could be introduced together with the continuously ¯owing N2 gas. 4. Conclusions

Fig. 6. FTIR transmission spectra of the ®lms deposited at N2 pressure of 0.2, 1 and 50 Pa.

We deposited carbon nitride ®lms by RPLA of pure graphite targets in nitrogen atmosphere. The deposited ®lms are uniform and adherent to the substrate. They contain nitrogen bonded to carbon in simple, double and triple con®gurations as deduced from FTIR analysis. It has been demonstrated that the nitrogen incorporation in the carbon matrix has a signi®cant effect on the structural and optical properties of the CNx ®lms. The refractive index n and extinction coef®cient k were determined as a function of wavelength. The values of n for all samples are in the range 1.5±1.7, while the k value presents a maximum at about 400 nm. The dependence of the absorption coef®cient from the energy is typical of indirect allowed

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transition, with Eg depending on the N2 pressure. The increase of the N2 pressure decreases the optical band gap of the ®lms and increases the CN concentration. It is clearly evident that the absorption band shift from 1630 to 1500 cm 21 is due to the incorporation of nitrogen into sp 2-bonded carbon. Acknowledgements The authors are grateful to Prof S. Fonti for the FTIR measurements and thank the Material Science Department of Lecce for the use of UV-VIS spectrometer. Authors were also supported by the TMR-Access to Large Scale facilities (Contract #ERBFMGECT950021) during their visit at the UV Laser Facility (ULF) at FORTH. References [1] [2] [3] [4]

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