J Mater Sci: Mater Electron DOI 10.1007/s10854-007-9422-2
Optical properties of amorphous silicon nitride thin-films prepared by VHF-PECVD using silane and nitrogen Wee Chong Tan Æ S. Kobayashi Æ T. Aoki Æ Robert E. Johanson Æ S. O. Kasap
Received: 20 July 2007 / Accepted: 28 September 2007 Ó Springer Science+Business Media, LLC 2007
Abstract Hydrogenated amorphous silicon nitride (a-Si1-xNx:H) thin-films with x between 0.42 and 0.58 were deposited from silane diluted in nitrogen gas by VHF PE-CVD using a novel method for impedance matching. We determine the refractive index dispersion relations and the optical absorption edge information from the transmission spectra and report on the changes in the optical properties as the composition is varied. The optical properties of these films are similar to those of silicon nitride deposited using the more conventional silane-ammonia mixtures.
ammonia, and various researchers have studied the optical properties [4]. Recently, a new method of VHF PE-CVD has been developed that uses a novel impedance matching technique to achieve ultra-low power reflection and uses silane diluted in nitrogen gas to produce a-Si1-xNx:H [3, 5]. This paper reports on the optical properties of a-Si1-xNx:H thin-films prepared by the new method, in particular the refractive index dispersion parameters, optical gaps, and Urbach widths.
2 Experimental procedure 1 Introduction Thin films of amorphous hydrogenated silicon-nitride (a-Si1-xNx:H) are excellent dielectric insulators with a low index of refraction relative to silicon. The material finds use in microelectronic and optoelectronic applications for oxidation masks, passivation layers, gate insulating layers, dielectric layers, and antireflection coatings [1] and more recently shows potential in multilayer optical devices [2] and as a luminescent material with emission in the visible [3]. Fabrication is typically by plasma-enhanced chemicalvapor-deposition (PE-CVD) using a mixture of silane and
W. C. Tan R. E. Johanson (&) S. O. Kasap Department of Electrical and Computer Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK, Canada S7N 5A9 e-mail:
[email protected] S. Kobayashi T. Aoki Department of Electronics and Computer Engineering & the Joint Research Center for High-technology (JRCH), Tokyo Polytechnic University, Atsugi 243-0297, Japan
Films of a-Si1-xNx:H were grown on quartz substrates by VHF PE-CVD from 5% silane in nitrogen; the system uses a novel, precision impedance-matching method that reduces stray electromagnetic fields that can cause material degradation [5]. During deposition, the substrate temperature was 350 °C, the rf frequency was 50 MHz, and the rf power was in the range of 4–10 W. System pressure was 0.5 Torr with a gas flow rate of 30 sccm. Increasing the rf power increases the nitrogen concentration in the resulting film. Five films were prepared with x values of 0.42, 0.46, 0.54, 0.55, and 0.58. Raman spectroscopy and x-ray diffraction confirmed that the films are amorphous. The nitrogen concentrations were inferred from the integral of the broad Si–N vibrational line between 700 and 1,200 cm-1 in the IR spectrum [3]. Optical transmission spectra were measured over the wavelength range 185–2,200 nm using a Perkin–Elmer Lambda 900 spectrophotometer. The transmission spectra have interference oscillations in the long wavelength region typical of thin films and absorption edges at shorter wavelengths. We applied the procedure of Swanepoel [6, 7] to extract the index of refraction and the absorption
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J Mater Sci: Mater Electron 5.0 4.5 4.0
n
coefficient as a function of wavelength as well as the thickness of the films [8, 9]. The accuracy of the extracted parameters was improved using the enhancements to the method described in ref. [10]. Although in this work we are more interested in relative changes rather than absolute values, we calculate that the average error in n and d is 0.40%; this theoretical error is based on Eq. 13 in ref. [11]. The precision was checked by taking ten independent measurements of one sample. The 2r error as a percentage of the mean was 0.14% for the average thickness d; 0.16% for the refractive index at long wavelengths, n(1550 nm), 0.19% for the optical gap Eop obtained from the Tauc plot, 0.19% for the optical gap EU04 obtained at a = 104 cm-1, and 1.21% for the Urbach width DE. There may be systematic errors greater than these especially in Eop.
x x x x x
= 42% = 46% = 54% = 55% = 58%
1600
1800
3.5 3.0 2.5 2.0 1.5
400
600
800
1000 1200
1400
λ (nm)
Fig. 2 The calculated dispersion curves for the index of refraction for a-Si1-xNx :H samples with x between 0.42 and 0.58 using a Wemple– DiDominico dispersion equation
3 Results and discussion Figure 1 shows the measured transmission spectrum Texp(k) for the a-Si0.54N0.46:H sample. The Swanepoel procedure the index of refraction disperfinds the average thickness d; sion curve n(k) and the absorption coefficient a(k) that produces a calculated spectrum Tcalc(k) that best fits the experimental curve; Tcalc(k) is shown as the solid line in Fig. 1. The agreement between Texp(k) and Tcalc(k)is excellent over the entire spectrum. For the fitting procedure, n(k) is modeled either by the Wemple-DiDominico dispersion equation n2 ¼ 1 þ E0 Ed =ðE02 ðhmÞ2 Þ where hm is the photon energy, E0 is the oscillator energy, and Ed is an energy dispersion parameter, or equivalently by the single-oscillator Sellmeier equation n2 ¼ 1 þ A1 k2 =ðk2 k21 Þ where A1 and k1 are the fitting parameters [10]. There are two parameters in
either case that are determined by the fit to Texp(k). Figure 2 shows the resulting dispersion curves for each sample. As expected, the index of refraction decreases with increasing nitrogen content and the oscillator energy increases as the absorption edge moves to higher energies. Two parameters are extracted from the absorption curve, the Urbach width and the optical bandgap. The Urbach width is the exponential slope DE of the absorption curve a / expðhm=DEÞ for a below 5 9 103 cm-1, seen in Fig. 3 as the straight sections in the curves. The bandgap is obtained in two ways: Eop from the energy intercept of the Tauc plot, (ahm)1/2 versus hm, using a values greater than
80
10000
α per cm
T (% )
60
40
1000
x x x x x
Experimental Calculated Glass Substrate
20
= 42% = 46% = 54% = 55% = 58%
0 500
1000
1500
λ (nm)
Fig. 1 Experimental (open symbols) and calculated (solid line) transmission spectra of an a-Si0.54N0.46:H sample with an average thickness of 2,433 nm
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1.5
2.0.
2.5
3.0
3.5
4.0
4.5
hυ in eV Fig. 3 The absorption coefficient versus photon energy of a-Si1-xNx :H samples with x between 0.42 and 0.58 in the Urbach region. The lines are fits to the exponential regions
J Mater Sci: Mater Electron Table 1 Optical properties of a-Si1-xN x:H thin films Nx (x in %)
d (lm)
Eop (eV)
EU04 (eV)
DE (meV)
N (1550 nm)
E0 (eV)
Ed (eV)
A1
k1 (nm)
42%
3.554
2.13
2.46
162
2.2099
4.99
18.90
3.7838
248
46%
2.433
2.41
2.80
207
1.9583
6.04
16.82
2.7851
205
54%
1.698
2.78
3.27
253
1.8266
7.33
16.93
2.3085
169
55%
2.832
3.29
3.85
281
1.7912
8.43
18.44
2.1885
147
58%
1.608
3.85
4.04
283
1.7826
8.63
18.63
2.1590
144
104 cm-1 and EU04 from the extrapolation of the Urbach curve to the energy where a = 104 cm-1. The optical gaps and Urbach widths for all samples are listed in Table 1 along with the dispersion parameters. Not surprisingly, the optical gap increases with the nitrogen concentration. The Urbach width also increases with nitrogen concentration. In amorphous materials, the Urbach edge results from the joint density of tail states, and an increase in DE means that either the valance or conduction band tail has broadened. Such broadening is usually a sign of increasing ‘‘disorder’’ in the amorphous material, such as greater structural deviations from an ideal random network or larger potential fluctuations. The increase in DE with x is surprising for two reasons. First, as nitrogen increases, the average coordination number decreases reaching 3.4 at stoichiometry, still far from a good glass former but fewer constraints should allow for fewer defects. Second, the stoichiometric material Si3N4 has x = 0.57, and thus the two samples closest to stoichiometry have the largest DE, again opposite to expectations. However, samples with higher nitrogen content are produced using greater rf power that might lead to more damage due to ion bombardment during growth. More work is needed to resolve this issue. The measured optical parameters are similar to those reported previously for a-SiN:H films prepared in 6
6
5
Reported : n(λ = 1500nm) Calculated: n(λ = 1550nm) Reported : EU04 Calculated: EU04
3 2 2
1
0.40
0.45
0.50
Support for this work came from NSERC
References EU04 (eV)
n (λ)
3
0.35
We have reported the optical properties of a-Si1-xNx:H thin films prepared from silane diluted in nitrogen in a VHF PECVD system that uses a novel impedance matching scheme. As expected, the optical gap increases and the index of refraction decreases with increasing nitrogen content. The deposition method used has two advantages over conventional, lower frequency CVD systems using silane and ammonia: a larger deposition rate gained from the higher frequency used and avoiding the disposal problems associated with using ammonia. However, using nitrogen can potentially be detrimental because the large energy required to dissociate N2 leads to higher ion energies and more damaging bombardment during growth. The new system minimizes this problem and produces a-Si1-xNx:H with optical properties essentially the same as those of material produced in the conventional way. Acknowledgement Canada.
4
0.30
4 Conclusions
5
4
0.25
conventional PE-CVD systems using silane-ammonia mixtures [4, 12]. In Fig. 4, n and EU04 are plotted against nitrogen content for our samples (solid symbols) and conventional samples (open symbols) [4]. Given the scatter in the data, the agreement is quite good.
0.55
0.60
0.65
x
Fig. 4 Dependence of n and EU04 on the nitrogen content. Filled points are from this work, and open points are from samples prepared by a conventional method
1. K. Coates, S. Morrison, S. Narayanan, A. Madan, Proceedings of the 16th European Photovoltaic Solar Energy Conference, Glasgow, 1279 (2000) 2. C. Ricciardi, V. Ballarini, M. Galli, M. Liscidini, L.C. Andreani, M. Losurdo, G. Bruno, S. Lettieri, F. Gesuele, P. Maddalena, F. Giorgis, J. Non-Cryst. Solids. 352, 1294 (2006) 3. S. Kobayashi, N. Ohrui, Y.C. Chao, T. Aoki, H. Kobayashi, T. Asakawa, J. Mater. Sci. Mater. Electron. 18, S29 (2007), and references therein 4. F. Giorgis, C.G. Pirri, E. Tresso, Thin Solid Films 307, 298 (1997), and references therein 5. T. Aoki, K. Fukasawa, Y. Nishikawa, N. Mikoshiba, Jpn. J. Appl. Phys. 35, 4799 (1996) 6. R. Swanepoel, J. Phys. E 16, 1214 (1983)
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J Mater Sci: Mater Electron 7. R. Swanepoel, J. Phys. E 17, 896 (1984) 8. E. Marquez, J.M. Gonzalez-Leal, Prieto-Alcon, Jimenez-Garay, M. Vlcek, J. Phys. D 32, 3128 (1999) 9. V.A.C. Popta, R.G. DeCorby, C.J. Haugen, T. Robinson, J.N. McMullin, D. Tonchev, S.O. Kasap, Optics Express 10, 639 (2002)
123
10. W.C. Tan, K. Koughia, J. Singh, S.O. Kasap, in Optical Properties of Condensed Matter and Applications, ed. by Jai Singh (Wiley & Sons, Chichester, UK, 2006) Chapter 1 11. C. Manifacier, J. Gasiot, J.P. Fillard, J. Phys. E 9, 1002 (1976) 12. H. Fritzsche, Appl. Phys. Lett. 65, 2824 (1994)
