Highly-tolerant distributed Bragg reflectors (DBRs) based on the same materials consisting of nanoporous/dense titanium dioxide (TiO2) film pair structures with wide-angle and broadband highly-reflective properties at visible wavelengths are reported. For a high refractive index contrast, the two dense and nanoporous TiO2 film stacks are alternatingly deposited on silicon (Si) substrates by a oblique angle deposition (OAD) method at two vapor flux angles (θα) of 0 and 80° for high and low refractive indices, respectively. For the TiO2 DBRs at a center wavelength (λc) of 540 nm, the maximum level in reflectance (R) band is increased with increasing the number of pairs, exhibiting high R values of > 90% for 5 pairs, and the normalized stop bandwidth (∆λ/λc) of ~17.8% is obtained. At λc = 540 nm, the patterned TiO2 DBR with 5 pairs shows an uniform relative reflectivity over a whole surface of 3 inch-sized Si wafer and a large-scalable fabrication capability with any features. The angle-dependent reflectance characteristics of TiO2 DBR at λc = 540 nm are also studied at incident angles (θinc) of 20-70° for p-, s-, and non-polarized lights in the wavelength region of 350-750 nm, yielding high R values of > 70.4% at θinc values of 20-70° for non-polarized light. By adjusting the λc/4 thicknesses of nanoporous and dense films, for λc = 450, 540, and 680 nm, tunable broadband TiO2 DBRs with high R values of > 90% at wavelengths of 400-800 nm are realized.
© 2014 Optical Society of America
Distributed Bragg reflectors (DBRs) have been widely used in specific or broadband wavelength applications for optoelectronic devices including vertical-cavity surface-emitting lasers, light-emitting diodes, and solar cells [1,2]. Conventional DBRs are composed of repeated pairs of low- and high-refractive index (low-n/high-n) materials with optical quarter-wavelength (λ/4) film thicknesses and their performance (i.e., reflectivity and stop bandwidth) is determined by the number of pairs as well as the refractive index contrast between low-n and high-n materials . Hence, larger refractive index contrast and higher number of pairs can lead to a higher reflectivity and a wider stop bandwidth. Additionally, the wavelength region with a high reflectivity can be controlled by adjusting the λ/4 thicknesses of alternating two films . However, these conventional DBRs made of two different materials have fundamental limitations such as material selection, thermal expansion mismatch, and high fabrication cost due to the use of expensive growth equipments. To avoid these disadvantages, it is necessary to develop highly-reflective DBRs with good thermal stability and mechanical durability in an effective way including cost-effective and simple processes. Over the past years, the same material-based DBRs (i.e., silicon (Si) , indium tin oxide , and germanium ) produced by the oblique angle deposition (OAD) technique have been demonstrated for optical and optoelectronic applications. In the OAD method, at higher incident vapor flux angles, the nanoporous films with more inclined columnar structures, which exhibit a lower n value compared to the dense films in the same materials, can be formed due to the further enhanced self-shadowing effect and limited mobility of incoming atoms [8,9].
Meanwhile, titanium dioxide (TiO2) is one of dielectric materials used in various optical coatings such as DBRs, antireflection coatings, and passivation layers in optical and optoelectronic devices due to its high transparency, non-toxicity, good chemical stability, mechanical durability/hardness, and excellent insulation property . However, there is very little or no work on the tunable DBRs consisting of the alternating nanoporous/dense TiO2 films as low-n/high-n pairs by the OAD method for visible wavelength applications. Indeed, TiO2 is almost no absorption in the visible wavelength region. Thus, it is very meaningful to study reflection properties of the TiO2-based DBRs prepared by the OAD. In this work, we fabricated the DBRs with different pairs consisting of alternating nanoporous/dense TiO2 films on Si substrates by the electron-beam (e-beam) evaporation via the OAD method and investigated their optical reflection characteristics at different center wavelengths, together with a theoretical analysis using the rigorous coupled-wave analysis (RCWA) method. For the TiO2 DBR with 5 pairs, the wafer-scale uniformity on a 3 inch Si wafer and the angle dependent reflectance property of incident light were also explored.
2. Experimental details
Figure 1 shows the (a) schematic diagram for the fabrication of DBRs consisting of nanoporous/dense (low-n/high-n) TiO2 film pairs by the OAD method via e-beam evaporation at two incident vapor flux angles (θα) of 0 and 80° and (b) cross-sectional scanning electron microscopy (SEM) images of the fabricated TiO2 DBRs with 1, 3, and 5 pairs. To fabricate TiO2 DBRs, single-side polished (100) Si wafers were used. To obtain two TiO2 films with a high refractive index contrast, the deposition process was performed at θα = 0° for high-n and θα = 80° for low-n without substrate rotation by using an e-beam evaporation system at room temperature. The deposition rate was kept at 0.15 nm/s using a quartz crystal thickness monitor and the TiO2 source with 99.95% purity was used. The TiO2 DBRs with different pairs of 1-5 were prepared on Si substrates. Here, one pair of DBRs is made up of the hierarchical structure consisting of dense high-n film at θα = 0° on the top of the nanoporous low-n film at θα = 80°. The λ/4 thicknesses of the TiO2 films at θα = 0 and 80° were estimated to be ~66 and 94 nm, respectively, by nhigh = 2.053 at θα = 0° and nlow = 1.438 at θα = 80° for a center wavelength (λc) of 540 nm. As shown in the SEM images of Fig. 1(b), the TiO2 DBRs with 1, 3, and 5 pairs were well fabricated on Si substrates. For two deposited TiO2 films, the λ/4 thicknesses were ~66 ± 4 and 94 ± 8 nm at θα = 0 and 80°, respectively. The deposited profiles of the fabricated samples were observed by the field-emission SEM (LEO SUPRA 55, Carl Zeiss). The optical reflectance was measured by using a UV-vis-NIR spectrophotometer (Cary 5000, Varian) at normal incidence. Spectroscopic ellipsometry (V-VASE, J. A. Woollam Co.Inc.) was used to determine the refractive index (n) and extinction coefficient (k) of two TiO2 films and to measure the angle-dependent reflectance at incident angles of 20-70° in p-, s-, and non-polarized lights. The relative reflectance mapping was characterized by using a rapid photoluminescence and reflectance mapping system (RPM 2000, Accent).
The theoretical analysis on the optical properties of the fabricated TiO2 DBRs was carried out by the RCWA method using a commercial software (DiffractMOD 3.1, Rsoft Design Group). In simulations, it was assumed that the incident light enters from air into the DBR structure at incident angles of 0-80° and the nanoporous TiO2 film with inclined columns, which was deposited by the OAD at θα = 80°, is a homogeneous medium with an effective refractive index at each wavelength. The optical parameters (i.e., n and k) of the Si used in these calculations were referred from the SOPRA N&K Database .
3. Results and discussion
Figure 2 shows the (a) measured and (b) calculated reflectance spectra of the nanoporous/dense TiO2 DBRs with different pairs and (c) the calculated electric-field (E-field) intensity distributions of the TiO2 DBRs with 1, 3, and 5 pairs for λc = 540 nm at normal incidence. The n and k values of TiO2 films deposited at θα = 0 and 80° are shown in the inset of Fig. 2(a). In the OAD process with larger incident vapor flux angles, the nanoporous films with slanted columnar structures, which have a lower n value compared to that of the dense films in the same materials, are formed because of strong atomic self shadowing under conditions of the limited mobility of the deposited atoms [8,9]. As shown in the inset of Fig. 2(a), therefore, the n and k values of the TiO2 film deposited at θα = 80° are lower than those of TiO2 film at θα = 0° due to the increase of porosity within the film. The porosity within the film can be calculated by a well-known Bruggeman effective medium approximation . The porosity of the TiO2 film deposited at θα = 80° was estimated to be approximately 55% from the parameters, i.e., nlow = 1.438 and nhigh = 2.053 at λ = 540 nm. In this calculation, the normally deposited dense TiO2 film at θα = 0° has a zero porosity. As can be seen in Fig. 2(a), the maximum values in reflectance spectra were gradually increased with increasing the number of pairs from 1 to 5. Particularly, the TiO2 DBR with only 5 pairs exhibited high reflectance (R) values of > 90% over a wavelength range (∆λ) of 505-600 nm and the maximum R values of ~95.1% at wavelengths around λc = 540 nm, leading to the large normalized stop bandwidth (∆λ/λc) of ~17.8% for R values of > 90%. As shown in Fig. 2(b), the theoretically calculated reflectance spectra agree well with the measured data with increasing the number of pairs in TiO2 DBRs. For the TiO2 DBRs with pairs larger than 5, the maximum R values are slightly further increased and almost saturated at wavelengths around λc = 540 nm, exhibiting the maximum R values of ~98.2% for the TiO2 DBR with 8 pairs. For the calculated E-field intensity distributions at λc = 540 nm in Fig. 2(c), as the number of pairs increases from 1 to 5, the E-field intensity in the Si substrate also becomes lower due to the higher reflectivity of TiO2 DBRs with higher number of pairs as can be seen in Fig. 2(b).
To demonstrate the high process tolerance and the large-scalable capability, the nanoporous/dense TiO2 DBRs were also fabricated on the Si wafer with a size of 3 inch. Figure 3 shows the (a) measured reflectance spectra of the patterned TiO2 DBR with 5 pairs at λc = 540 nm on the 3 inch-sized Si wafer for different points, (b) relative reflectance mapping image of the corresponding sample, and (c) influence of thickness deviation of nanoporous TiO2 film at the λ/4 thickness of 94 nm on the reflectance of TiO2 DBR with 5 pairs at λc = 540 nm. The photographic image of the patterned TiO2 DBR with 5 pairs on the 3 inch Si wafer at λc= 540 nm is shown in the inset of Fig. 3(a). To realize the selectively patterned TiO2 DBR with 5 pairs, simple transparent plastic adhesive tapes and metal shadow masks were employed. As shown in Fig. 3(a), for all the points from 1 to 5 on the surface of sample, there exists a slight discrepancy between reflectance spectra at some wavelengths. This is mainly attributed to the thickness error in nanoporous TiO2 films deposited at the high θα = 80° due to the difference of deposition distances between the source and different positions on large-scale wafer in the OAD process without substrate rotation, indicating the thickness error of approximately ± 8 nm between the near and far points on the 3 inch wafer from the source for the nanoporous TiO2 film at θα = 80°. However, for all the points, the R values at λc = 540 nm were almost ~95% and the large ∆λ/λc values of ~15.7-17.8% were obtained while keeping high R values of > 90%. This is because the deposition of the nanoporous TiO2 film was performed with varying the θα value of 80° (i.e., θα = 80° and −80°), as shown in Fig. 1(b), which can suppress the variations of the reflectivity and stop bandwidth of DBRs due to the thickness mismatch compared to the optical λ/4 thickness caused by the deposition speed difference between the near and far wafer positions. Besides, the uniformity of reflectivity on large area can be confirmed by the relative reflectance mapping image in Fig. 3(b). The TiO2 DBR with 5 pairs exhibited the high R value of ~95% whereas the R value on the surface of Si wafer was ~32%. From this image, thus, it is found to show a uniform relative reflectivity over the whole surface of 3 inch Si wafer for λc = 540 nm. In the RCWA calculations of Fig. 3(c), it can be also noted that the reflectivity of the TiO2 DBR is definitely dependent on the film thickness, but it does not have a serious effect at target wavelengths though there is a slight variation of film thickness . Over all, these results show that the highly-tolerant and highly- reflective DBRs with a relative uniformity as well as any features can be realized on large-sized substrates by the OAD using various pattering techniques.
The reflectivity of DBRs also depends strongly on the incident angle (θinc) of light. For the fabricated TiO2 DBRs with 5 pairs, angle-dependent reflectance characteristics were studied for different polarized lights. Figure 4 shows the (a) measured reflectance spectra at θinc = 20-70° and (b) contour plots of variations of the calculated reflectance spectra at θinc = 0-80° for the TiO2 DBR with 5 pairs at λc = 540 nm in (i) p-, (ii) s-, and (iii) non-polarized lights. For both the p- and s-polarized lights, the high R bands are shifted toward shorter wavelengths with increasing the θinc. However, the maximum R values and bandwidth for the p-polarized light are considerably decreased while those are slightly increased for the s-polarized light. For the non-polarized light, therefore, as the θinc value becomes larger, the high R band is moved to the shorter wavelength region and its values and bandwidth also decreased, indicating the maximum R values of ~94.5% at λ = 540 nm and θinc = 20° and ~82.1% at λ = 490 nm and θinc = 70°. This may be due to the variation of film thickness for inclined incident lights . Nevertheless, at λc = 540 nm, the R values of > 70.4% were kept in a broad θinc range of 20-70°. For theoretical optical analysis of incident light angle-dependent reflectance of the TiO2 DBR with 5 pairs, the simulation results show a similar trend to the measured data in wide ranges of wavelengths and incident angles for all the (i) p-, (ii) s-, and (iii) non-polarized lights, as shown in Fig. 4(b).
The high R band of TiO2 DBRs can be controlled by varying the λc/4 thicknesses of alternating two TiO2 nanoporous/dense (low-n/high-n) film stacks . Figure 5 shows the (a) measured reflectance spectra of the fabricated TiO2 DBRs with 5 pairs for λc = 450, 540, and 680 nm, (b) contour plot of variations of the calculated reflectance spectra of TiO2 DBR with 5 pairs as functions of λc and wavelength, and (c) photographic image of the corresponding samples at λc = 450, 540, and 680 nm. For both λc = 450 and 680 nm, the λc/4 thicknesses of TiO2 films at θα = 0/80° were estimated to be ~53 nm/75 nm and ~84 nm/120 nm by n = 2.134/1.494 and n = 2.03/1.421 at θα = 0/80°, respectively. As shown in Fig. 5(a), for each TiO2 DBR with 5 pairs, the high R bands of > 90% were shifted toward the longer wavelength region and its bandwidth became slightly broader with increasing the λc value of TiO2 DBRs, exhibiting the increased ∆λ values (i.e., R> 90%) from 76 nm for λc = 450 nm to 102 nm for λc = 680 nm. In RCWA simulations, the thicknesses of two nanoporous/dense TiO2 films were set to be λc/4 thicknesses for each λc. As shown in Fig. 5(b), it can be observed that there is also a similar tendency with the experimentally measured data, indicating that the high R region of > 90% is shifted toward the longer-wavelength range and its bandwidth becomes wider when the λc value increases. This phenomenon can be verified from the photographic image in Fig. 5(c). For the TiO2 DBRs with 5 pairs at λc = 450, 540, and 680 nm, each blue-, green-, red-like reflected color is matched with the corresponding wavelength region with maximum R values in Fig. 5(a).
The same material-based DBRs consisting of alternating nanoporous/dense TiO2 film stacks were fabricated by the OAD method via the e-beam evaporation. To obtain the TiO2 films with a high refractive contrast, two dense and nanoporous films were alternatingly deposited at different θα values of 0 and 80° for high-n and low-n, respectively. By controlling the λc/4 thickness for each film, the tunable broadband TiO2 DBRs with high R values of > 90% for 5 pairs were realized for λc = 450, 540, and 680 nm, respectively. The ∆λ/λc of ~17.8% at λc = 540 nm was obtained while maintaining high R values of > 90% at normal incidence and R values of > 70.4% in a wide θinc range of 20-70° for non-polarized light. Also, for λc = 540 nm, the TiO2 DBR with 5 pairs was well-uniformly formed on 3 inch Si wafer. The theoretically calculated reflectance results by the RCWA method showed similar tendencies with the experimentally measured data in wide ranges of wavelengths and incident light angles. These same material-based highly-tolerant TiO2 DBRs made of the alternating nanoporous/dense pairs can provide wide-angle and broadband highly-reflective characteristics for visible wavelength applications.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (No. 2013-068407).
References and links
1. W. W. Chow, K. D. Choquette, M. H. Crawford, K. L. Lear, and G. R. Hadley, “Design, fabrication, and performance of infrared and visible vertical-cavity surface-emitting lasers,” IEEE J. Quantum Electron. 33(10), 1810–1824 (1997). [CrossRef]
2. M. Y. Kuo, J. Y. Hsing, T. T. Chiu, C. N. Li, W. T. Kuo, T. S. Lay, and M. H. Shih, “Quantum efficiency enhancement in selectively transparent silicon thin film solar cells by distributed Bragg reflectors,” Opt. Express 20(S6), A828–A835 (2012). [CrossRef]
3. O. Blum, I. J. Fritz, L. R. Dawson, A. J. Howard, T. J. Headley, J. F. Klem, and T. J. Drummond, “Highly reflective, long wavelength AlAsSb/GaAsSb distributed Bragg reflector grown by molecular beam epitaxy on InP substrates,” Appl. Phys. Lett. 66(3), 329–331 (1995). [CrossRef]
4. P. Kurt, D. Banerjee, R. E. Cohen, and M. F. Rubner, “Structural color via layer-by-layer deposition: layered nanoparticle arrays with near-UV and visible reflectivity bands,” J. Mater. Chem. 19(47), 8920–8927 (2009). [CrossRef]
5. S. J. Jang, Y. M. Song, C. I. Yeo, C. Y. Park, and Y. T. Lee, “Highly tolerant a-Si distributed Bragg reflector fabricated by oblique angle deposition,” Opt. Mater. Express 1(3), 451–457 (2011). [CrossRef]
6. M. F. Schubert, J. Q. Xi, J. K. Kim, and E. F. Schubert, “Distributed Bragg reflector consisting of high- and low-refractive-index thin film layers made of the same material,” Appl. Phys. Lett. 90(14), 141115 (2007). [CrossRef]
7. J. W. Leem and J. S. Yu, “Broadband and wide-angle distributed Bragg reflectors based on amorphous germanium films by glancing angle deposition,” Opt. Express 20(18), 20576–20581 (2012). [CrossRef]
8. C. Charles, N. Martin, M. Devel, J. Ollitrault, and A. Billard, “Correlation between structural and optical properties of WO3 thin films sputter deposited by glancing angle deposition,” Thin Solid Films 534, 275–281 (2013). [CrossRef]
10. B. S. Richards, “Comparison of TiO2 and other dielectric coatings for buried-contact solar cells: a review,” Prog. Photovolt. Res. Appl. 12(4), 253–281 (2004). [CrossRef]
11. SOPRA, http://www.sopra-sa.com, Accessed 1 November (2013).
12. Y. Zhong, Y. C. Shin, C. M. Kim, B. G. Lee, E. H. Kim, Y. J. Park, K. M. A. Sobahan, C. K. Hwangbo, Y. P. Lee, and T. G. Kim, “Optical and electrical properties of indium tin oxide thin films with tilted and spiral microstructures prepared by oblique angle deposition,” J. Mater. Res. 23(9), 2500–2505 (2008). [CrossRef]
13. K. M. Chen, A. W. Sparks, H. C. Luan, D. R. Lim, K. Wada, and L. C. Kimerling, “SiO2/TiO2 omnidirectional reflector and microcavity resonator via the sol-gel method,” Appl. Phys. Lett. 75(24), 3805–3807 (1999). [CrossRef]