Fabrication and optical properties of periodical structures based
on a water-developable and tunable La
0.7
Sr
0.3
MnO
3
resist†
Ming-Chung Wu,
aChih-Min Chuang,
aYang-Fang Chen*
band Wei-Fang Su*
aReceived 24th September 2007, Accepted 11th December 2007 First published as an Advance Article on the web 8th January 2008 DOI: 10.1039/b714653a
We have fabricated a unique periodic structure with different optical properties in the visible range using a water-developable La0.7Sr0.3MnO3electron beam resist. The La0.7Sr0.3MnO3resist can be
developed under a nontoxic and environmentally friendly manner using pure water. Additionally, the resist is capable of exhibiting both positive and negative resist behavior depending on the electron beam dosage. Thus, these special characteristics were used to fabricate and tune periodic structure thin films having controlled optical reflectance properties in the wavelength range of 300 nm to 800 nm with one fixed design electron beam pattern (i.e. without changing the geometries and lattice constants of the periodical structures) by simply changing the electron beam dosage only.
Introduction
Recently, considerable interest has been focused on light modu-lation using periodic structures such as photonic crystals.1–6
Yablonovitch7 and John8 first described photonic crystals in
1987. The structures exhibit photonic band gaps, similar to the electronic band gaps of materials. Therefore, one can control the propagation of light by restricting its allowable directions at cer-tain frequencies or localizing light in specific areas. The technology is expected to have numerous applications in optical communica-tion. Many methods have been developed to fabricate photonic crystals, such as multilayer lithography,9multibeam holographic
lithography,10self-assembly,11–13step and flash imprint
lithogra-phy,14electrochemical deposition,15etc. Among all the methods,
electron beam lithography16–17 has become the most common
method to fabricate 2D photonic crystals and nanoscale patterns, because of its ability to construct high precision structures.
The formation of photonic band gaps, which can be used to guide the propagation of light as well as to enhance spontaneous emission, is due to periodically varying refractive indices. Thus, an electron beam patternable metal oxide with high refractive index will be useful for this process. The nanostructure fabrica-tion of metal oxide materials generally starts with selective etching or template growth on the pre-patterned resist and then lift-off. While these approaches lack the ability to control the construction of the nanostructure accurately, using an electron beam to generate exact nanopatterned metal oxides like ZnO, TiO2, etc.18,19 can be much easier. The methods and
materials mentioned above all generate negative patterns. There is a zwitter resist,20,21which refers to a resist that can exhibit both
positive and negative properties depending on the applied
electron beam dosage. The resist possesses both positive and negative patterning capabilities and will benefit the direct writing technology of the electron beam. In addition, the solvents used in the resist developing process are often volatile and toxic and thus contribute to health hazards and environmental pollution. The solvents even increase the cost of lithography processing. Many studies are directed towards building multifunctional resists with precise nanostructures and at the same time reducing the process hazards and pollution.
Herein we illustrate one unique feature of a patterned La0.7Sr0.3MnO3film which is able to exhibit both positive and
negative resist characteristics. This unique feature then can be used to control the optical reflectance properties of the film within one fixed design pattern without changing the lattice constant, shape and size of the pattern. Our approach provides a simple and convenient alternative for the fabrication of perio-dical structures with different optical properties in the visible range using a water-developable La0.7Sr0.3MnO3resist.
Experimental
The electron beam resist was prepared by dissolving 8.58 wt% of lanthanum nitrate (La(NO3)3$6H2O, Acros, 98%), strontium
nitrate (Sr(NO3)2, Fluka, 98%), manganese nitrate (Mn(NO3)2$ 4H2O, Fluka, 97%) and 1.75 wt% polyvinyl alcohol (PVA, Acros,
88%, 22000 g mol1) in water with a molar ratio of La : Sr : Mn¼
0.7 : 0.3 : 1.0. The precursor solution was stirred for 48 h at 50C.
After filtering the precursor solution with syringe filters (0.22 mm), the precursor solution was spin coated at 3000 rpm for 90 s to give a nominal thickness of about 200 nm, and the resist film appeared to be very smooth with a root-mean-square roughness of 1.02 nm. The thickness of the water-developable La0.7Sr0.3MnO3 resist
coated on the substrate (indium tin oxide conductive glass or silicon wafer) was measured directly by using an alpha-stepper (Veeco, Tektak3). High-resolution nanolithography was performed by using a Hitachi ELS-7500EX machine to write the specific patterns across a 150 150 mm field with a 2.5 nm beam step size. The electron beam writer was operated at 50 kV with a probe current of 1.0 nA. A sample containing a 3 3 array field aDepartment of Materials Science and Engineering, National Taiwan
University, No. 1, Sec. 4, Roosevelt Road, Taipei, Taiwan, 106-17, ROC. E-mail: [email protected]; Fax: +886-2-33664078; Tel: +886-2-33664078 bDepartment of Physics, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei, Taiwan, 106-17, ROC. E-mail: yfchen@phys. ntu.edu.tw; Tel: +886-2-33665125
† Electronic supplementary information (ESI) available: Absorption data. See DOI: 10.1039/b714653a
was irradiated with a starting dose time of 1 ms, then with addi-tional dose increments of 0.01, 0.1 and 1 ms per field. Deionized water was used to develop the La0.7Sr0.3MnO3resist at 25C for
30 s. The refractive indices of the La0.7Sr0.3MnO3 thin films
sintered under different conditions were evaluated using a spectral microreflectometer (Mission Peaks Optics, MP100-M) equipped with an optical microscope. The patterned La0.7Sr0.3MnO3
sam-ples were measured by atomic force microscopy (AFM, Digital Instruments, Dimension-3100 Multimode) for surface morpho-logy and roughness, and the microstructures of the samples were studied by both field-emission scanning electron microscopy (FE-SEM, Elionix, ERA-8800FE, Japan) and confocal micro-scopy. The confocal microscopy images of the patterned La0.7Sr0.3MnO3samples were measured by confocal fluorescence
microscopy. An argon ion laser (wavelength 488 nm) was used to excite the patterned La0.7Sr0.3MnO3 samples through a 100
Nikon plane objective (numerical aperture 0.9). A single photon counting photomultiplier tube (PMT) detector was used to detect the intensity of the reflected laser light. The optical properties of the reflectance of the patterned La0.7Sr0.3MnO3samples were
stu-died using the transmittance mode of a spectral microreflectometer (Mission Peaks Optics, MP100-M) equipped with an optical microscope. Unpolarized light was focused on the composite film under the glass substrate at a spot size <30 mm to measure the interference between the incident and reflected light with wave-lengths ranging from ultraviolet to near infrared (250–1000 nm).
Results and discussion
In our previous report22 we have demonstrated that a water
developable La0.7Sr0.3MnO3electron beam resist exhibits dual
negative patterning and positive patterning functions depending on the electron beam dosage. Fig. 1(a)–(d) are atomic force microscope images of La0.7Sr0.3MnO3patterns on silicon wafer
substrates fabricated using different electron beam dosages. Fig. 1(a) is a negative pattern obtained at a low electron beam dose of 3.2 mC cm2. Fig. 1(b) is a positive pattern obtained at
a medium electron beam dose of 32.0 mC cm2. At a high
electron dose of 233.6 mC cm2, we obtained a negative pattern
again. Although the dose of the exposure of this La0.7Sr0.3MnO3
resist is very high in comparison to most organic resists, the La0.7Sr0.3MnO3 resist generates active electromagnetic
func-tional nanopatterns in one step whereas the use of organic resists to generate La0.7Sr0.3MnO3nanopatterns involves three steps:
(1) fabricate nanopatterns from organic resist, (2) deposit La0.7Sr0.3MnO3 on the nanopatterns, and (3) remove organic
resist. The use of La0.7Sr0.3MnO3will not only reduce the process
steps but also use no environmentally harmful organic chemicals as compared with the organic resist.
We observed a very unique and interesting pattern, featuring solid rod center–hollow ring–solid outer, that is 3 patterns of negative–positive–negative, at a medium–high electron beam dose of 134.4 mC cm2. How does this unique pattern occur?
Fig. 1 Atomic force microscope (AFM) images of the La0.7Sr0.3MnO3patterns on silicon wafer substrates with different electron beam dosages. (a) Negative La0.7Sr0.3MnO3periodical structure with low dosage (3.2 mC cm2) electron beam irradiation. (b) Positive La0.7Sr0.3MnO3periodical structure with medium dosage (32.0 mC cm2) electron beam irradiation. (c) Special unique La
0.7Sr0.3MnO3periodical structure with medium–high dosage (134.4 mC cm2) electron beam irradiation. (d) Negative La
0.7Sr0.3MnO3periodical structure with high dosage (233.6 mC cm2) electron beam irradiation. The schematic sketches of pattern formation mechanisms of La0.7Sr0.3MnO3(e),(f),(g),(h) correspond to AFM images (a),(b),(c),(d) respectively.
We have illustrated the formation mechanisms of the patterns in the lower row of Fig. 1(e)–(f).
We have previously confirmed the formation of the La0.7Sr0.3MnO3resist pattern is due to the heat from the electron
beam exposure. At the low electron dose, only the exposed area is affected by the crosslinking of the PVA (Fig. 1e) and results in a negative pattern of a regular honeycomb array of La0.7Sr0.3MnO3pillars (Fig. 1a). At the medium dose (28.8 mC
cm2), the nitrate salt components of the La
0.7Sr0.3MnO3resist
react with the PVA at the exposure area (Fig. 1f). An explosive reaction occurs, so-called auto-ignition,23–26and then the resist
structure will change to form loose particles that can be easily removed by water to form a positive pattern of a regular honey-comb array (Fig. 1b). At the medium–high dose, the centre part of the exposed area receives direct intense electron beam exposure (Fig. 1g) so a negative solid rod pattern is obtained (Fig. 1c). The area next to the center rod is exposed to the heat associated with the electron beam energy propagated to the neighbourhood which is equivalent to the medium dose (Fig. 1g). A hollow ring pattern is formed due to the positive resist behaviour at this energy level (Fig. 1c). The area further from the center is expected to receive a low electron beam dosage (Fig. 1g) which exhibits negative resist behaviour with a solid outer area formation (Fig. 1c). Finally, at the high dose, the area exposed to the electron beam (Fig. 1h) shows a negative pattern with the formation of crystalline La0.7Sr0.3MnO3
(Fig. 1d). At or above this electron beam thermal energy thres-hold, the pure phase of La0.7Sr0.3MnO3formed as a solid pattern,
the neighbouring loose La0.7Sr0.3MnO3particles can be removed
by water. Our La0.7Sr0.3MnO3 resist exhibits zwitter resist
characteristics, so the height or depth in different patterns will varying with electron dose. The height of the regular honeycomb array of La0.7Sr0.3MnO3pillars (Fig. 1a) is 15 nm, and the depth
of the positive pattern of the regular honeycomb array is 60 nm (Fig. 1b), Finally, the height of the regular honeycomb array of La0.7Sr0.3MnO3pillars (Fig. 1d) is 75 nm.
The refractive index of the electron beam exposed La0.7Sr0.3MnO3resist is one of the key properties that need to be
measured to understand the optical properties of patter-ned La0.7Sr0.3MnO3. However, the electron beam patterned
La0.7Sr0.3MnO3sample is too small to be measured accurately
using conventional ellipsometer or prism coupler methods. We knew from previous work22that the formation of La
0.7Sr0.3MnO3
patterns is due to the thermal reactions of the components in the resist. Therefore, we measured the refractive indices of La0.7Sr0.3MnO3samples prepared by thermal sintering with an
equivalent energy to the electron beam energy. Table 1 summaries
the refractive indices of La0.7Sr0.3MnO3 thin film samples
pre-pared under different sintering conditions. The negative pattern obtained at low electron beam dose has a refractive index of 2.05 whereas the negative pattern obtained at high electron beam dose has a higher refractive index of 2.38. The medium electron dose sample should have a refractive index of 2.05 because the exposed area is removed and the remaining area is a mixture of PVA and La0.7Sr0.3MnO3 precursor salts which is the same as
the low electron dose irradiated sample. Thus, we can change the refractive index of the resist by varying the electron beam dosage. We have used both scanning electron microscopy and confocal microscopy to further study the morphology of the electron beam irradiated resist as shown in Fig 2a, c, e, g and Fig. 2b,
Table 1 Summary of refractive indexes of La0.7Sr0.3MnO3samples pre-pared under different sintering conditions
Sample of La0.7Sr0.3MnO3 thin film Sintering conditions Equivalent to the electron beam dosage/mC cm2 Refractive index A — 0.0 2.00 B 150C, 0.5 h 1.5–4.0 2.05 C 250C, 0.5 h 15.0–35.0 2.05 D 300C, 0.5 h 35.0–120.0 2.30 E 300C, 30.0 h 120.0–300.0 2.38
Fig. 2 Scanning electron microscopy photographs (a, c, e, and g) and confocal microscopy images (b, d, f, and h) of La0.7Sr0.3MnO3periodical structures fabricated from water developable resists at different electron beam dosages: (a,b) 2.2 mC cm2, (c,d) 16.0 mC cm2, (e,f) 80.0 mC cm2, and (g,h) 288.0 mC cm2.
d, f, h respectively. The confocal microscopy images are much clearer than the SEM images due to the small topographic contrast in SEM. The pattern was designed and set at 1000 nm lattice constant and 500 nm diameter. However, different charac-teristics of patterned La0.7Sr0.3MnO3resists were obtained by
only varying the electron beam dosage. The results are due to the different chemical reactions occurred in the resist as described above. Fig. 2a,b shows a periodical pillar array with 250 diameter and 1000 nm lattice constant, and the electron beam dosage in this step was 2.2 mC cm2. A periodical hole
array pattern with 800 nm diameter and 1000 nm lattice constant is shown in Fig. 2c,d, which was obtained at a low electron beam dose of 16.0 mC cm2. However, we can observe a special pattern
array with the same processing setting at 800 nm diameter and 1000 nm lattice constant at a medium–high electron beam dose of 80.0 mC cm2due to the heat transfer to the neighboring
area (Fig. 2e,f). Finally, the La0.7Sr0.3MnO3 pillar array with
500 nm diameter and 1000 nm lattice constant was obtained at the high electron beam dose of 288.0 mC cm2 as shown in
Fig. 2g,h.
Traditionally, the optical properties of photonic crystals are modulated either by changing the radius with fixed lattice constant or by changing the lattice constant with fixed radius.
Therefore, we fabricated two types of La0.7Sr0.3MnO3 2D
photonic crystals on silicon wafers. One was a pillar structure fabricated at low electron dose and the other was an air column structure fabricated at medium electron dose. They have the size of about 50 50 mm. For the pillar structure, we studied the reflectance spectra of La0.7Sr0.3MnO3 photonic crystals with
fixed 900 nm lattice constant and different La0.7Sr0.3MnO3pillar
diameters of 200, 225, and 250 nm respectively. The results are shown in Fig. 3a, the samples were prepared using low dose elec-tron beams, so their refractive indices (samples A, B and C) are about 2.05. Each reflectance curve of the samples was obtained by a spectral microreflectometer equipped with an optical micro-scope. The reflectance spectra show a red shift of lmaxfrom 580
nm, to 603 nm and 633 nm upon increasing the pillar diameter from 200 nm to 225 nm and 250 nm respectively. Therefore, the reflectance spectra of the 2D La0.7Sr0.3MnO3photonic
crys-tals can be adjusted by varying the geometric shape or filling ratio. The insert of Fig. 3a shows the relationship of the fre-quency (a/l) versus r/a ratio for the samples, which has a negative slope. The result is consistent with the theoretical prediction.27
We used the Maxwell equation to validate the shifting trend of the reflectance of the patterned La0.7Sr0.3MnO3 samples. By
considering the light propagation in the x–y plane within the triangular lattice of dielectric columns and the refractive index of the La0.7Sr0.3MnO3 resist film, the band structures for
photonic crystals of La0.7Sr0.3MnO3 columns with different
r/a values were calculated by using the BANDSOLVE program (RSOFT Design Group, Inc). This calculation was performed using the plane-wave expansion method and index guiding in the third dimension.28 The refractive index of La
0.7Sr0.3MnO3
pillars irradiated at 1.5–4.0 mC cm2is about 2.05. Hence, the
refractive index of 2.05 for La0.7Sr0.3MnO3pillar samples and
a domain size of 7 11 unit cells were used in this calculation. In the La0.7Sr0.3MnO3pillar structures, the frequency of the gap
is decreased as r/a increases. This is expected, because the frequency is related to 1/O3, where 3 is the dielectric constant of the medium; when r/a increases, the average dielectric constant of the medium increases and as a result the frequency increases.
Moreover, for the air column structure, the reflectance spectra of La0.7Sr0.3MnO3 photonic crystals with a fixed 400 nm air
column diameter and different lattice constants: 500, 700, and 900 nm (Fig. 3b; samples D, E and F respectively) show a red shift from 417 nm, to 586 nm and 755 nm. Thus, the lmax of
the samples increase with increasing lattice constant. The insert in Fig. 3b is the relationship of the frequency (a/l) versus r/a ratio, which is positive. The frequency of the gap increases as r/a increases in the air column structures. This is also an expected feature, as explained above.
Interestingly, our dual function La0.7Sr0.3MnO3resist exhibits
additional unique characteristics that can modulate the optical properties of photonic crystals by changing the electron dose only with a fixed column diameter and lattice constant. Fig. 4 shows the reflection spectra of patterned La0.7Sr0.3MnO3
fabri-cated with different electron doses using a fixed design of 500 nm diameter and 1000 nm lattice constant. Each curve represents a sample obtained at a specific electron irradiation as shown on the right. The thickness of the La0.7Sr0.3MnO3resist coated on
the silicon wafer substrate was about 200 nm. The sample size was 50 50 mm. By increasing the electron beam irradiation, Fig. 3 Reflectance spectra changes of patterned La0.7Sr0.3MnO3 with
different geometries. (a) The patterns with a fixed 900 nm lattice constant and different La0.7Sr0.3MnO3pillar diameters: 200, 225, and 250 nm. (b) The patterns with a fixed 400 nm air hole diameter and different lattice constants: 500, 700, and 900 nm. Inserts are the plots of frequency versus r/a.
we can obtain different geometries of patterned La0.7Sr0.3MnO3
that result in the changes in the reflectance spectra.
First of all, the negative patterned La0.7Sr0.3MnO3irradiated
at 1.6 and 3.2 mC cm2show l
maxat 445 and 480 nm (Curve
01 and Curve 02) respectively in the low dose region (I). When the positive patterned La0.7Sr0.3MnO3 was obtained by the
increasing electron beam dosage in the range from 16.0 to 32.0 mC cm2, the reflectance spectra are shown in the medium
dose region (II) (Curve 05–Curve 10). Nevertheless, the patterned La0.7Sr0.3MnO3in the medium–high region (III) was
transformed from a positive pattern to a negative pattern gradually, when the electron beam dosage was increased to the range of 48.0–112.0 mC cm2. The l
maxof the reflectance peaks
in the region (III) are blue shifted initially (11 to Curve-12) and then they are red shifted (Curve 12–Curve 15). These behaviors are due to the changes of air column geometry as shown in Fig 2f. In other words, the depth of the air column in patterned La0.7Sr0.3MnO3 samples increases gradually from
Curve 11 to Curve 12 due to the positive characteristics of the resist, then decreases gradually from Curve 12 to Curve 15 due to the gradual transformation of the negative characteristics of the resist. The results reveal that the La0.7Sr0.3MnO3resist can
easily modulate 2D photonic crystal reflectance spectra in the visible range by varying the z-axis using different electron beam dosages without changing the design pattern of lattice constant and the periodicity along the x- and y-axes. Finally, the negative patterned La0.7Sr0.3MnO3was obtained in region
(IV) with the electron beam dosage range of 128.0–304.0 mC
cm2, and their reflectance spectra are shown in Curve 16–
Curve 22. Therefore, we can easily use a water-developable La0.7Sr0.3MnO3 resist to fabricate periodical structures with
different optical properties. This unique feature of the periodic structure of La0.7Sr0.3MnO3 is replotted in Fig. 5 for clear
demonstration of visible light modulation.
Conclusions
In summary, we have created unique periodical structures using a water-developable La0.7Sr0.3MnO3electron beam resist in an
environmentally friendly, one-step method. Both positive and negative patterns are readily fabricated by only varying the electron beam dosage. Different reflectance spectra of patterned La0.7Sr0.3MnO3in the visible light range can be obtained. We
would like to stress that our study provides a simple and convenient approach for the fabrication of photonic crystals with adjustable optical properties, which may underpin many optoelectronic or magnetic applications awaiting future exploration.
Acknowledgements
Financial support from the National Science Council of Taiwan (NSC-96-2628-E-002-017-MY3 and NSC 95-3114-P-002-003-MY3) is highly appreciated. The authors would also like to thank Prof. C. H. Kuan, Prof. C. W. Chen, Ms S. Chen and Mr Y. Y. Lin of National Taiwan University and Prof. K. C. Cheng of National Taipei University for helpful discussions, and Mr A. J. Su of University of Pittsburgh for editing the manuscript. The electron beam lithography was carried out using the Elinox’s facility located at the National Taiwan University Center for Information and Electronics Technologies.
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Fig. 4 Reflectance spectra of patterned La0.7Sr0.3MnO3were changed by varying the electron beam dosage, and the design pattern is fixed at 1000 nm lattice constant and 500 nm diameter. Four ranges of electron beam dose were used. Low dose region (I): 1.6–3.2 mC cm2, medium dose region (II): 12.8–32.0 mC cm2, medium–high dose region (III): 48.0–112.0 mC cm2, and high dose region (IV): 128.0–304.0 mC cm2
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