Chemical Properties
Chapter 8 Optical Properties
8-1 Introduction
Both a-C:F film and a-C:H film consist of carbon sp3 and sp2 bonds.
Therefore, the optic properties of a-C:F films are similar to that of a-C:H films. The sp2 content and the fluorine concentration would affect the photoluminescence lifetime, as well as energy band gap of the a-C:F films.
A lifetime of around 0.34 µsec and an energy gap of ~ 2.75 eV were observed in both the as-deposited and after annealing conditions. The higher sp3 content and the fluorine concentration make the PL peak site of a-C:F film blue-shift, and make energy band gap become higher. The short carrier lifetime in the a-C:F film makes the PL peak blue-shift. The annealing changed both the structure and composition of the a-C:F film.
The type of fluorocarbon bonds and electronic structure characterized the mechanical and physical properties of a-C:F film.
8-2 Experiment
In order to get stronger spectra signal of the a-C:F films, the thickness of a-C:F films was deposited 1 µm on silicon wafers in ECR-CVD which is for PL, pulse laser and n&k measurements. We also deposited 1 µm a-C:F films on 7059 glasses in ECR-CVD for UV/VI measurement. The fluorescence was measured by the PL apparatus using a helium-cadmium (He-Cd) laser (λ= 325 nm) as the excitation source. An
Excimer pulse laser (λ=193 nm) was used to evaluate photoluminescence lifetime. The UV/VI and n&k spectrometers were used to measure the optical band gap of a-C:F films. The experiment flowchart was shown in Fig. 8-1.
Figure 8-1 The experiment flowchart for optical measurement.
8-3 Results and Discussion
The a-C:F film has a broad band luminescence characteristic of an amorphous semiconductor with broad band tails. Figure 8-2 shows PL spectra for R = 0.97 and R = 0.90 a-C:F films in both as-deposited and after annealing conditions. The PL peak band is like rough Gaussian Peak, and changes with the CF4 flow ratio. The PL peak site tends to be blue-shifted as the fluorine concentration increases. It is also noticed that annealing helps to reduce the FWHM of the PL spectra. The broad spectrum results from structural disorder of the a-C:F film. Fluorinated amorphous carbon films after the annealing treatment will be structurally relaxed because the disappearance of electron-hole pairs reduces the probability of the radiative recombination1. The higher fluorine concentration, which generates more sp3 in the film, makes the as-deposited a-C:F films blue-shift at high flow ratio.
Fig. 8-2. PL spectra produced at R=0.97 and at R=0.90 of a-C:F films for both as-deposited and after annealing.
The excimer pulse laser spectrometer was used to measure the lifetime (τ) of the photoluminescence. The results are shown in Fig. 8-3.
The lifetime is directly proportional to both fluorine concentration and sp2
% for both as-deposited and after annealing films. The lifetime is longer in a-C:F(10-7 s) film than in a-C:H(10-8 s)2,3 one, but is much shorter than in a-Si:H(10-3 s)4,5 one. The short lifetime carriers reduce their energy relaxation into lower energy states and then increase the average energy of carriers in the band tail state6. The short photoluminescence lifetime causes a blue-shift of the lowest energy accessible to carriers for radiative recombination in the band tail state7. Therefore, we can observe the pronounced blue-shift of the PL peak in a-C:F film, because of the short carrier lifetime.
Fig. 8-3 illustrates the photoluminescence lifetime of as-deposited and after annealing a-C:F films.
The spectrophotometry of the UV/VI bands of the a-C:F films is plotted in Fig. 8-4. The optical band gap showed in Fig. 8-5 was determined by the Tauc mode8. Since the C-F bond energy (102 kcal/mole) is higher than the C-C bond energy (80 kcal/mole), the greater number of fluorocarbon bonds in the a-C:F film produced at higher CF4 flow ratios will give rise to a higher optical band gap. In addition, the C=C bonds or graphite-like sp2 structures will lower the optical band gap9, so the optical band gap of the as-deposited a-C:F film is larger than the one after annealing. This is also the reason why the R = 0.97 film has less blue-shift than the R = 0.90 film. The optical properties are closely correlated to the amount of fluorine incorporated into the films. The increase in the optical band gap energy indicates that the fluorine incorporated into the a-C:F film
0.90 0.92 0.94 0.96 0.98
has modified the chemical structure of the film towards the higher sp3 bonding fraction9. These properties are similar to those of hydrogenated amorphous carbon10-12.
Fig. 8-4. UV/VI spectra of both R=0.97 and 0.90 of the a-C:F films as-deposited and after-annealed at 300oC.
Fig. 8-5 shows the energy gap of as-deposited and 300℃ annealing a-C:F films.
8-4 Summary
The high fluorine concentration of a-C:F films makes PL peak blue-shift, and it has short lifetime carriers. In addition, the higher fluorine concentration of a-C:F films generates more C-F bonds in the films, and is helpful to form higher energy band gap in the a-C:F films. The higher fluorine concentration will promote a greater number of fluorine carbon bonds in the film and therefore produce a higher optical band gap.
Furthermore, annealing will induce the sp2 structure in the a-C:F film, which will extend the photoluminescence lifetime. The short carriers lifetime in the a-C:F film makes PL peak blue-shift, which is similar to a-Si:H films.
Reference
1 S. R. P. Silva, J. Robertson, Rulsli, G. A. J. Amaratunga, and J. Schwan, Phil. Mag. B, 74 (1996) 369.
2 Rusli, Gehan A. J. Amaratunga, and J. Roberson, Phys. Rev. B, 53 (1996) 16306.
3 W. Lormes, M. Hundhausen, and L. Ley, J. Non-Cryst. Sol. 227-230 (1998) 570.
4 W. Siebert, R. Carius, W. Fuhs, and K. Jahn, Phys. Stat. Sol. B, 140 (1987) 311.
5 S. Liedtke, K. Lips, M. Bort, K. Jahn, and W. Fuhs, J. Non-Cryst. Sol., 114 (1989) 522.
6 Y. Kanemitsu, M. Liboshi, and T. Kushida, Appl. Phys. Lett., 76, 2200
(2000).
7 M. J. Estes, and G. Moddel, Appl. Phys. Lett., 68, 1814 (1996).
8 Richard Zallen, The Physics of Amorphous Solids (Second Edition), Wiley, New York, 1998.
9 A. Weber, R. Pöckelmann, and C.-P. Klages, J. Vac. Sci. Technol. A. 16, 2120 (1998).
10 M. Weiler, S. Sattel, T. Giessen, K. Jung, H. Ehrhardt, V. S. Veerasamy and J. Robertson, Phys. Rev. B, 53 (1996) 1594.
11 Th. Frauenheim, G. Jungnickel, U. Stephan, P. Blaudeck, S.
Deutschmann, M. Weriler, S. Sattel, K. Jung, and H. Ehrhardt, Phys.
Rev. B, 50 (1994) 7940.
12 M. Hakovirta, X. M. He and M. Nastasi, J. Appl. Phys., 88, 1456 (2000).