- Email: [email protected]
Suppression of ageing effects in porous silicon interference filters
15 January 1998
Optics Communications 146 Ž1998. 309–315
Full length article
Suppression of ageing effects in porous silicon interference filters M. Kruger ¨ a
a,1
a , S. Hilbrich b, M. Thonissen , D. Scheyen b, W. Theiß b, H. Luth ¨ ¨
a
Institut fur GmbH, 52425 Julich, Germany ¨ Schicht- und Ionentechnik, Forschungszentrum Julich ¨ ¨ b I. Physikalisches Institut, RWTH Aachen, 52066 Aachen, Germany Received 11 July 1997; revised 1 September 1997; accepted 8 September 1997
Abstract Interference filters made from porous silicon can be a useful alternative to conventional dielectric filters because of the fast and cheap fabrication and the compatibility to conventional silicon technology. However, so far the main disadvantages of these structures were ageing effects due to the oxidation of porous silicon in ambient air. In this paper we demonstrate that the problem can be solved by a thermal pre-oxidation of porous silicon. This treatment allows the use of these filters not only at room temperature, but even at high temperatures up to 6008C. Moreover, the pre-oxidation reduces the absorption in the blue and UV which is necessary for future applications in this spectral range. The complex refractive index of the pre-oxidised porous silicon is determined by numerical simulations of reflectance measurements. q 1998 Published by Elsevier Science B.V.
1. Introduction Interference filters were realised in porous silicon ŽPS. for the first time four years ago w1–3x. They are formed from silicon wafers by electrochemical etching in hydrofluoric acid w4x, without the necessity of any expensive deposition process. During this anodisation a part of the silicon is dissolved and the remaining crystalline silicon forms a spongue-like structure with a porosity between some 30% up to more than 90%. The microstructure of the PS depends on the doping level of the silicon wafers: the use of low and moderately doped p-type substrates results in microporous silicon Žpore and crystallite size - 2 nm. and the use of highly doped substrates in mesoporous silicon Ž2–50 nm. w5,6x. In both cases the structures are much smaller than the wavelength of visible light and the material can be treated as a homogeneous, effective medium w7–10x. The effective refractive index w7,10,11x is mainly determined by the porosity which can be varied by several anodisation parameters. The most convenient way
1
E-mail: [email protected].
is changing the anodisation current density, with high current densities resulting in high porosities and low refractive indices. If the current density is modulated during the anodisation, alternating layers of different porosities are formed as the silicon dissolution occurs primarily at the etch front PSrsilicon substrate w12x. The transmission electron microscopy ŽTEM. image in Fig. 1 shows such a layer stack consisting of layers with 34% porosity Ždark layers. and 64% porosity Žbright layers.. Although the interface roughness of 10–20 nm is relatively high w10,13–15x, light scattering at these interfaces turned out to be very low. For this reason such layer stacks can act as optical waveguides w16,17x and interference filters if the refractive indices Žgiven by the anodisation current densities. and the layer thicknesses Žgiven by the etching times. are chosen properly. In the meantime different depth profiles of the refractive index were realised which act as Bragg reflectors Žnoted as ŽHL. a . w1,3,18x, Fabry-Perot ŽFP. filters ŽŽHL. aŽLH. b or ŽLH. aŽHL. b . w2,18x or more complex Rugate filters w19x, with a and b denoting the number of high ŽH. and low ŽL. refractive index layers. The optical thick-
0030-4018r98r$19.00 q 1998 Published by Elsevier Science B.V. All rights reserved. PII S 0 0 3 0 - 4 0 1 8 Ž 9 7 . 0 0 5 1 3 - 0
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Fig. 1. TEM cross-section of a PS Bragg reflector. The dark layers have a low porosity of 34%, the bright layers 64%. The anodisation current densities and times for each layer were 20 mArcm2r2.758 s and 300 mArcm2r0.630 s, respectively.
nesses of the H and L layers are 1r4 of the filter wavelength, so that these structures are usually called quaterwave-stacks. The reflectance spectra in Fig. 2 demonstrate that high reflectivities up to more than 99% and sharp transmission peaks with a FWHM of 11 nm can be realised. These filters can be used as separate optical components, but also new applications in colour-sensitive photodiodes w20,21x, in luminescent devices w2,18,22,23x and in sensors w24x were demonstrated.
Fig. 2. Reflectance spectra of a PS Bragg reflector ŽŽHL. 20 , left. and a PS Fabry-Perot filter ŽŽHL. 4 ŽLH. 4 , right., compared to the reflectance of a silicon wafer Ždotted line.. The anodisation current densities were 20 mArcm2 and 450 mArcm2 . These parameters result in wide reflectance maxima with a high reflectivity, and sharp transmission bands in the spectra of Fabry-Perot filters. Sharp reflectance peaks of Bragg reflectors would be realised by using small differences of the anodisation current densities.
The advantages of PS interference filters compared to conventional ones are: Ø low-cost experimental set-up ŽF $20000. and fabrication; Ø fast fabrication Žsome seconds to some minutes.; Ø compatibility to existing silicon technology w20,25–27x; Ø the refractive index can be varied continuously over a wide range which allows the fabrication of Rugate filters w10,19x. Moreover, in contrast to filters made from organic materials the PS is stable at high temperatures, as will be shown below. However, one of the main problems concerning the use of PS interference filters is the ageing of the PS: due to the large inner surface of about 600–700 m2rcm3 for microporous and about 200–250 m2rcm3 for mesoporous silicon w28,29x the material oxidises very fast compared to bulk silicon. This phenomenon is well-known from light-emitting PS w30,31x. For PS interference filters the natural oxidation is disturbing as well as it causes a change of the refractive index and of the PS layer thickness w32,33x, which results in the following ageing effects: Ø a blue-shift of the filter wavelength; Ø a decrease of the filter performance, if the change of the optical thickness is different for the H- and L-layer; Ø a continuos decrease of reflectivity Žwhich depends on the refractive index ratio n Hrn L .. A first investigation for microporous FP filters revealed that the filter wavelength shifts for more than 4% in 100 days storage in ambient air at room temperature w11,34x, without reaching saturation. This effect could be strongly
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reduced to about 1% by a thermal pre-oxidation step at 3008C. However, the pre-oxidation temperature was too low for totally preventing ageing, and no measurement of the refractive index of this pre-oxidised porous silicon was reported. In this paper we will present optimised pre-oxidation parameters, and the determination of the refractive index of this oxidised porous silicon. In contrast to the first investigations of microporous silicon interference filters w11,34x, we focus on mesoporous silicon which has the advantage that a much wider range of porosities Žand of the refractive index. can be utilised compared to microporous silicon w10,19x. This is advantageous for the fabrication of high-reflective mirrors as the reflectivity increases with increasing refractive index ratio n Hrn L w19x. Moreover this results in a larger freedom in filter design for the construction of Rugate filters w10,19x. For oxidised PS this fact is even more important than for as-prepared PS because the oxidation reduces the refractive index, and thereby also the available refractive index range. In addition for highly-doped substrates no metal backside contact is required during PS formation, as opposed to the microporous silicon used in Refs. w11,34x. This backside contact would contaminate the furnace during the high temperature pre-oxidation step.
2. Sample preparation and characterisation The PS samples were formed on highly doped Ž9–11 m V cm. p-type substrates, a doping level very common for the fabrication of mesoporous silicon. The wafers were Czochralski grown Ž²100: orientation, diameter 100 mm. and boron doped. Before anodisation the substrates were cleaned in propanol in an ultrasonic bath and rinsed in deionised water. Anodisation was performed in the dark using an electrolyte composition of HF:H 2 O:C 2 H 5 OH 1:1:2, and after PS formation the samples were rinsed in pure ethanol. The current was supplied by a computer controlled Keithley 238 current source which allows switching of the current in less than 500 ms. For all filters except those in Fig. 2 we used anodisation current densities of j H s 20 mArcm2 for high refractive index layers and j L s 300 mArcm2 for low refractive index layers, which results in porosities of 34% and 64%, respectively. In principle the whole range between these current densities can be used in order to produce layers with porosities Žand refractive indices. in between, but for this investigation we concentrated on these parameters. The homogeneity of the samples was very satisfying, except for a small circle at the edge of the sample. For example, using an etch cell with a diameter of 20 mm resulted in a homogeneous circle with a diameter of 15 mm, surrounded by a small ring which exhibits a blue shift. This problem is well known and results from the
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increasing current density at the edge of the etch cell w27x. However, for large etch cells the area of this outer region becomes negligible compared to the homogeneous inner region. The thermal oxidation of the PS was performed in a rapid thermal annealing ŽRTA. oven ŽAddax RM3. in dry oxygen under atmospheric pressure. The oxygen flux was 5.0 lrmin. As will be discussed extensively in the last section, the complex refractive index n˜ s n q i k of the PS was determined by measuring reflectance spectra of single PS layers and fitting them. The numerical simulations were carried out using the software SCOUT w35x. We used the Looyenga effective medium theory instead of the more general Bergman theory as it turned out to be very well suited for microporous and mesoporuos silicon w9x. Reflectance measurements of single layers and interference filters were performed in the range from 200 to 1100 nm using a Perkin-Elmer l2 spectrometer Žangle of incidence: 88.. Moreover, for fast measurements of lower accuracy an Atos Micromap 512 interferometer was used in the range from 400 to 800 nm Žusing vertical incidence..
3. Ageing of as-prepared samples The ageing of PS was investigated by using FP filters because their filter wavelength gives directly the optical thickness of the cavity layer, without the necessity of fitting reflectance spectra which is necessary for single layers. Fig. 3 gives the frequency shift of as-prepared samples as a function of time for storage in ambient air at different temperatures. At room temperature the filter wavelength decreases very slowly for some 3% after 100 days, compared to 4% for the microporous silicon described in Refs. w11,34x. For the mesoporous as well as for the microporous samples no saturation of the filter wave-
Fig. 3. Shift of the filter wavelength of as-prepared FP filters ŽŽHL. 4 ŽLH. 4 . due to ageing in ambient air. The anodisation current densities for the H- and L-layers were 20 mArcm2 and 300 mArcm2 , the anodisation times 1.887 s and 0.445 s.
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length shift was found, although the observation of the microporous sample lasts for more than 600 days up to now. As expected, the use of higher temperatures accelerates the oxidation, and also the ageing, very strongly. For example, after 2 days at 3008C the filter wavelength has shifted by 5%, and at 6008C even by 16%. This means that, even more than for room-temperature applications, for using at high temperatures Žin front of strong illumination sources or in laser beams. a stabilisation of the optical properties is required. For this reason in the following all ageing investigations were performed at 6008C in ambient air. This approach has the additional advantage that the ageing investigation times are strongly reduced compared to ageing at room temperature.
4. Ageing of pre-oxidised samples The ageing due to oxidation of PS could be reduced in several ways: by covering the surface of the sample, filling the pores with another material, or by a pre-oxidation of the PS, which is by far the easiest way. Such pre-oxidation can be performed anodically, by chemical treatment or by thermal oxidation. In the following we report on chemical oxidation in boiling HNO 3 and thermal oxidation in dry oxygen, while anodic oxidation was not investigated. As an example of chemical oxidation samples were stored in boiling HNO 3 Ž70%. for 10 min, cleaned with deionised water and dried in N2 . Afterwards the samples were annealed for 10 min at 7008C in N2 atmosphere. On bulk silicon this results in a 1 nm thick layer of SiO 2 with good stoichiometry w36,37x, thus no total oxidation of the silicon crystallites Ž10–20 nm in diameter. is expected but a complete covering of the inner surface with oxide. For thermal oxidation we used a two-step oxidation procedure in order to prevent undefined changes of the PS microstructure due to heat treatment. First the samples were oxidised for 60 min at 3008C to stabilise the porous structure and prevent coarsening during the following, high-temperature oxidation step w13,29,38x. Such coarsening would strongly slow down the oxidation and result in poor reproducibility w29,38x. Moreover, the formation of large voids of about 1 mm diameter would cause light scattering in the visible spectral range and the thickness of the H- and L-layers Žwhich is typically 50–200 nm. would not be well defined any more. The final oxidation was performed at 8508C and 9508C, respectively, for different times varying from 10 to 60 min. We did not use higher temperatures because in this case viscous flow of silica occurs w29,38x. In order to investigate the ageing of both high and low porosity layers, we fabricated FP filters of the types ŽHL. 4 ŽLH. 4 and ŽLH. 4 ŽHL. 4. The same ageing characteristics were observed for both filter geometries. Fig. 4 shows the filter wavelength shift of an unoxidised, as-pre-
Fig. 4. Filter wavelength of pre-oxidised FP filters ŽŽLH. 4 ŽHL. 4 . as a function of ageing time. The anodisation current densities for the H- and L-layers were 20 mArcm2 and 300 mArcm2 , the anodisation times 2.175 s and 0.508 s. All samples were heated at 6008C in ambient air during this ageing investigation.
pared sample in comparison to a chemically oxidised sample and two thermally oxidised samples Žusing the oxidation parameters 3008Cr60 min q 8508Cr10 min and 3008Cr60 min q 9508Cr10 min, respectively.. The 9508C pre-oxidation decreases the filter wavelength by more than 130 nm Ž21%., much more than for the 8508C pre-oxidation or the chemical treatment. This means that during the chemical oxidation and the thermal pre-oxidation at 8508C by far not the whole PS is oxidised, which results in a further wavelength shift during the ageing investigation on a timescale of several tens of hours. In contrast, the filter wavelength does not shift any more after the 9508C treatment. However, the reflectivity of these samples turned out to decrease continuously Žup to 7% after 6 days., which indicates that the PS still oxidises a little during the ageing sequence. This can be explained as follows: the filter wavelength corresponds to the optical thickness of the cavity, that is the product of refractive index and geometrical layer thickness, while the reflectivity is determined by the refractive index ratio of the layers. As will be shown below, the layer thickness slightly increases during oxidation, which can compensate the decrease of the refractive index. To prevent also this decrease of reflectivity, we subsequently increased the oxidation time at 9508C from 10 to 30 min. Fig. 5 shows reflectance spectra of an oxidised Bragg reflector ŽHL.15, directly after oxidation and after 2 days Žmain diagram. and 16 days Žinset. at 6008C in ambient air. For both ageing times the filter wavelength and the reflectivity of the main peak remained the same, without any ageing effect. The reflectivity in the UV, however, increases slightly between the measurement after 2 days and the one after 16 days. This spectral range is very sensitive to light absorption in the PS, so that a slight further oxidation can cause changes in the UV without noticeable influencing the visible spectral range.
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Fig. 5. Reflectance spectra of an oxidised Bragg reflector ŽŽHL.15 . before the ageing investigation Žsolid lines. and after storage at 6008C in ambient air Ždotted lines.. The layer stack and the anodisation parameters were identical to the ones of the structure in Fig. 1. The ageing times were 2 days Žmain diagram. and 16 days Žinset..
The same results were obtained with FP filters of the type ŽHL. 6 ŽLH. 6 and ŽHL. 8 ŽLH. 8. In order to check the reproducibility of this pre-oxidation process, five identical FP filters were fabricated and oxidised. After the preoxidation, the filter wavelength of the filters varied by just 1.5% from sample to sample, the same value as before the oxidation.
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Typical reflectance spectra are shown in Fig. 6. In the near IR and in the visible the spectra are dominated by interference fringes since in this spectral range the PS is nearly transparent. This means that the complex refractive index and the layer thickness can be extracted unequivocally from the height of the interferences and from the oscillation frequency by numerical simulations. In the UV no interferences can be observed for the as-prepared sample, because the light absorption is high and just the topmost part of the layer contributes to the reflectance. For the oxidised sample in Fig. 6, however, these interferences appear even in the UV. This indicates that the PS is nearly totally oxidised. The quantitative interpretation of these spectra requires numerical simulations. The dotted lines in Fig. 6 demonstrate how excellent the measurements can be simulated by using the Looyenga effective medium theory w9,10x. The adjustable parameters used in this simulation are the porosity, the layer thickness and the dielectric function of the solid phase in the porous material. The parametrization of the dielectric function by using Brendel oscillators is described in detail in Ref. w9x. From these fits one can exclude depth gradients of the refractive index, that means for these relatively thin layers the formation as well as the oxidation is homogeneous with depth. Table 1 gives the thicknesses of the PS layers, determined from these simulations. In addition, the layer thicknesses were checked by SEM, but no discrepancy was observed. As expected w32,33x, the thicknesses increase by 10–20%, with a saturation after 30 min for the high
5. Investigation of the optimal pre-oxidation process In the previous section it was demonstrated that thermal oxidation at 9508C is very well suited for the stabilisation of PS. Now the modification of the optical properties due to this treatment will be investigated in more detail. For the design of the interference filters it is necessary to know the optical layer thicknesses, that means the product of refractive index and geometrical layer thickness, with high accuracy. So far, this calibration was performed just for as-prepared PS w10,11x. The pre-oxidation, however, changes these quantities due to three effects: Ø the refractive index of the solid phase in the PS decreases; Ø the porosity decreases due to volume expansion of the crystallites, which partly compensates the first effect; Ø the geometrical layer thickness increases. For this reason the change of the refractive index as well as the change of the layer thickness have been investigated by fitting reflectance spectra of single layers. We used samples of about 1 mm thickness, etched with 20 and 300 mArcm2 , respectively. The oxidation parameters were Ž3008Cr60 min q 9508Cr30 min. and Ž3008Cr60 min q 9508Cr60 min..
Fig. 6. Reflectance spectra of single layers, as-prepared Žtop. and oxidised Ž3008Cr60 min q 9508Cr60 min, bottom.. The anodisation current density was 300 mArcm2 , the anodisation time 4.99 s. The solid lines correspond to measurements, the dotted lines to numerical simulations.
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Table 1 Thicknesses of single layers before and after pre-oxidation. The anodisation times were 36.76 s for the samples etched with 20 mArcm2 and 4.99 s for the others 20 mArcm2 300 mArcm2 as-prepared 1.09 mm 3008Cr60 min q 9508Cr30 min 1.20 mm 3008Cr60 min q 9508Cr60 min 1.33 mm
1.03 mm 1.23 mm 1.25 mm
porosity layer etched with a current density of 300 mArcm2. In contrast, no saturation was observed for the low porosity layer. This correlates very well to the fact that the low porosity layer is not fully oxidised even after 60 min at 9508C, as will be shown below. In contrast to these relatively small changes of the layer thicknesses, the refractive index of the PS changes dramatically. Fig. 7 shows the real part n and the imaginary part k as a function of wavelength. After pre-oxidation the imaginary part k of the high porosity layer is almost zero, indicating that the layer is totally oxidised and that light absorption is negligible. Accordingly after the pre-oxidation the real part n exhibits nearly no dispersion any more. The fact that n and k are identical for both oxidation times corresponds well to the saturation of the layer thickness. The low porosity layer, however, is just partially oxidised even after 60 min at 9508C, as can be seen from the relatively high imaginary part k and the typical structures in the UV between 25000 and 40000 cmy1. These structures correspond to the interband transitions of silicon which are just slightly modified in mesoporous silicon w9x and which disappear in oxidised silicon. The fact that the
layer is not totally oxidised is not surprising: Yon et al. w38x report that complete oxidation can be reached just for porosities larger than 46%, while the porosity of the low porosity layers in Fig. 7 was 34%. A partial oxidation means that the material can further oxidise which explains the ageing characteristics discussed above, especially the slight changes in the UV in Fig. 5. However, the oxidation at 6008C Žand even more at room temperature. is so slow that this pre-oxidation at 9508C is sufficient even for long-time usage, as described in the previous section. For future applications in the blue and UV, however, the absorption of the low porosity PS is still too high. For these applications one should use PS with porosities ) 46% for both the H- and L-layer.
6. Conclusion The filter wavelength and reflectivity of PS interference filters decrease with time due to oxidation of the silicon crystallites. At room temperature this ageing occurs on a timescale of days or weeks, while at high temperatures this effect is strongly accelerated. In order to suppress the ageing effects, different pre-oxidation processes were investigated. Chemical oxidation in HNO 3 and thermal oxidation at 8508C were not suitable to achieve high-temperature stability, but very good results were achieved using a thermal pre-oxidation at 9508C. Interference filters treated in this way displayed no wavelength shift and no decrease of reflectivity even after several days at 6008C in ambient air. For these optimal oxidation parameters the complex refractive index of the PS was determined by numerical simulations of reflectance measurements.
Acknowledgements We would like to thank S. Meesters for the TEM pictures, H.-P. Bochem for the SEM thickness measurements and M.G. Berger, R. Arens-Fischer and V. Ganse for very fruitful discussions.
References
Fig. 7. Real part Žtop. and imaginary part Žbottom. of the refractive index of PS, determined from numerical simulations of reflectance spectra. The anodisation current densities were 300 mArcm2 for the high porosity layer Žleft. and 20 mArcm2 for the low porosity layer Žright.. The solid lines correspond to as-prepared samples, the dashed lines to short oxidised samples Ž3008Cr60 min q 9508Cr30 min. and the dotted lines to long oxidised samples Ž3008Cr60 min q 9508Cr60 min..
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Optics Communications 146 Ž1998. 309–315
Full length article
Suppression of ageing effects in porous silicon interference filters M. Kruger ¨ a
a,1
a , S. Hilbrich b, M. Thonissen , D. Scheyen b, W. Theiß b, H. Luth ¨ ¨
a
Institut fur GmbH, 52425 Julich, Germany ¨ Schicht- und Ionentechnik, Forschungszentrum Julich ¨ ¨ b I. Physikalisches Institut, RWTH Aachen, 52066 Aachen, Germany Received 11 July 1997; revised 1 September 1997; accepted 8 September 1997
Abstract Interference filters made from porous silicon can be a useful alternative to conventional dielectric filters because of the fast and cheap fabrication and the compatibility to conventional silicon technology. However, so far the main disadvantages of these structures were ageing effects due to the oxidation of porous silicon in ambient air. In this paper we demonstrate that the problem can be solved by a thermal pre-oxidation of porous silicon. This treatment allows the use of these filters not only at room temperature, but even at high temperatures up to 6008C. Moreover, the pre-oxidation reduces the absorption in the blue and UV which is necessary for future applications in this spectral range. The complex refractive index of the pre-oxidised porous silicon is determined by numerical simulations of reflectance measurements. q 1998 Published by Elsevier Science B.V.
1. Introduction Interference filters were realised in porous silicon ŽPS. for the first time four years ago w1–3x. They are formed from silicon wafers by electrochemical etching in hydrofluoric acid w4x, without the necessity of any expensive deposition process. During this anodisation a part of the silicon is dissolved and the remaining crystalline silicon forms a spongue-like structure with a porosity between some 30% up to more than 90%. The microstructure of the PS depends on the doping level of the silicon wafers: the use of low and moderately doped p-type substrates results in microporous silicon Žpore and crystallite size - 2 nm. and the use of highly doped substrates in mesoporous silicon Ž2–50 nm. w5,6x. In both cases the structures are much smaller than the wavelength of visible light and the material can be treated as a homogeneous, effective medium w7–10x. The effective refractive index w7,10,11x is mainly determined by the porosity which can be varied by several anodisation parameters. The most convenient way
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E-mail: [email protected].
is changing the anodisation current density, with high current densities resulting in high porosities and low refractive indices. If the current density is modulated during the anodisation, alternating layers of different porosities are formed as the silicon dissolution occurs primarily at the etch front PSrsilicon substrate w12x. The transmission electron microscopy ŽTEM. image in Fig. 1 shows such a layer stack consisting of layers with 34% porosity Ždark layers. and 64% porosity Žbright layers.. Although the interface roughness of 10–20 nm is relatively high w10,13–15x, light scattering at these interfaces turned out to be very low. For this reason such layer stacks can act as optical waveguides w16,17x and interference filters if the refractive indices Žgiven by the anodisation current densities. and the layer thicknesses Žgiven by the etching times. are chosen properly. In the meantime different depth profiles of the refractive index were realised which act as Bragg reflectors Žnoted as ŽHL. a . w1,3,18x, Fabry-Perot ŽFP. filters ŽŽHL. aŽLH. b or ŽLH. aŽHL. b . w2,18x or more complex Rugate filters w19x, with a and b denoting the number of high ŽH. and low ŽL. refractive index layers. The optical thick-
0030-4018r98r$19.00 q 1998 Published by Elsevier Science B.V. All rights reserved. PII S 0 0 3 0 - 4 0 1 8 Ž 9 7 . 0 0 5 1 3 - 0
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Fig. 1. TEM cross-section of a PS Bragg reflector. The dark layers have a low porosity of 34%, the bright layers 64%. The anodisation current densities and times for each layer were 20 mArcm2r2.758 s and 300 mArcm2r0.630 s, respectively.
nesses of the H and L layers are 1r4 of the filter wavelength, so that these structures are usually called quaterwave-stacks. The reflectance spectra in Fig. 2 demonstrate that high reflectivities up to more than 99% and sharp transmission peaks with a FWHM of 11 nm can be realised. These filters can be used as separate optical components, but also new applications in colour-sensitive photodiodes w20,21x, in luminescent devices w2,18,22,23x and in sensors w24x were demonstrated.
Fig. 2. Reflectance spectra of a PS Bragg reflector ŽŽHL. 20 , left. and a PS Fabry-Perot filter ŽŽHL. 4 ŽLH. 4 , right., compared to the reflectance of a silicon wafer Ždotted line.. The anodisation current densities were 20 mArcm2 and 450 mArcm2 . These parameters result in wide reflectance maxima with a high reflectivity, and sharp transmission bands in the spectra of Fabry-Perot filters. Sharp reflectance peaks of Bragg reflectors would be realised by using small differences of the anodisation current densities.
The advantages of PS interference filters compared to conventional ones are: Ø low-cost experimental set-up ŽF $20000. and fabrication; Ø fast fabrication Žsome seconds to some minutes.; Ø compatibility to existing silicon technology w20,25–27x; Ø the refractive index can be varied continuously over a wide range which allows the fabrication of Rugate filters w10,19x. Moreover, in contrast to filters made from organic materials the PS is stable at high temperatures, as will be shown below. However, one of the main problems concerning the use of PS interference filters is the ageing of the PS: due to the large inner surface of about 600–700 m2rcm3 for microporous and about 200–250 m2rcm3 for mesoporous silicon w28,29x the material oxidises very fast compared to bulk silicon. This phenomenon is well-known from light-emitting PS w30,31x. For PS interference filters the natural oxidation is disturbing as well as it causes a change of the refractive index and of the PS layer thickness w32,33x, which results in the following ageing effects: Ø a blue-shift of the filter wavelength; Ø a decrease of the filter performance, if the change of the optical thickness is different for the H- and L-layer; Ø a continuos decrease of reflectivity Žwhich depends on the refractive index ratio n Hrn L .. A first investigation for microporous FP filters revealed that the filter wavelength shifts for more than 4% in 100 days storage in ambient air at room temperature w11,34x, without reaching saturation. This effect could be strongly
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reduced to about 1% by a thermal pre-oxidation step at 3008C. However, the pre-oxidation temperature was too low for totally preventing ageing, and no measurement of the refractive index of this pre-oxidised porous silicon was reported. In this paper we will present optimised pre-oxidation parameters, and the determination of the refractive index of this oxidised porous silicon. In contrast to the first investigations of microporous silicon interference filters w11,34x, we focus on mesoporous silicon which has the advantage that a much wider range of porosities Žand of the refractive index. can be utilised compared to microporous silicon w10,19x. This is advantageous for the fabrication of high-reflective mirrors as the reflectivity increases with increasing refractive index ratio n Hrn L w19x. Moreover this results in a larger freedom in filter design for the construction of Rugate filters w10,19x. For oxidised PS this fact is even more important than for as-prepared PS because the oxidation reduces the refractive index, and thereby also the available refractive index range. In addition for highly-doped substrates no metal backside contact is required during PS formation, as opposed to the microporous silicon used in Refs. w11,34x. This backside contact would contaminate the furnace during the high temperature pre-oxidation step.
2. Sample preparation and characterisation The PS samples were formed on highly doped Ž9–11 m V cm. p-type substrates, a doping level very common for the fabrication of mesoporous silicon. The wafers were Czochralski grown Ž²100: orientation, diameter 100 mm. and boron doped. Before anodisation the substrates were cleaned in propanol in an ultrasonic bath and rinsed in deionised water. Anodisation was performed in the dark using an electrolyte composition of HF:H 2 O:C 2 H 5 OH 1:1:2, and after PS formation the samples were rinsed in pure ethanol. The current was supplied by a computer controlled Keithley 238 current source which allows switching of the current in less than 500 ms. For all filters except those in Fig. 2 we used anodisation current densities of j H s 20 mArcm2 for high refractive index layers and j L s 300 mArcm2 for low refractive index layers, which results in porosities of 34% and 64%, respectively. In principle the whole range between these current densities can be used in order to produce layers with porosities Žand refractive indices. in between, but for this investigation we concentrated on these parameters. The homogeneity of the samples was very satisfying, except for a small circle at the edge of the sample. For example, using an etch cell with a diameter of 20 mm resulted in a homogeneous circle with a diameter of 15 mm, surrounded by a small ring which exhibits a blue shift. This problem is well known and results from the
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increasing current density at the edge of the etch cell w27x. However, for large etch cells the area of this outer region becomes negligible compared to the homogeneous inner region. The thermal oxidation of the PS was performed in a rapid thermal annealing ŽRTA. oven ŽAddax RM3. in dry oxygen under atmospheric pressure. The oxygen flux was 5.0 lrmin. As will be discussed extensively in the last section, the complex refractive index n˜ s n q i k of the PS was determined by measuring reflectance spectra of single PS layers and fitting them. The numerical simulations were carried out using the software SCOUT w35x. We used the Looyenga effective medium theory instead of the more general Bergman theory as it turned out to be very well suited for microporous and mesoporuos silicon w9x. Reflectance measurements of single layers and interference filters were performed in the range from 200 to 1100 nm using a Perkin-Elmer l2 spectrometer Žangle of incidence: 88.. Moreover, for fast measurements of lower accuracy an Atos Micromap 512 interferometer was used in the range from 400 to 800 nm Žusing vertical incidence..
3. Ageing of as-prepared samples The ageing of PS was investigated by using FP filters because their filter wavelength gives directly the optical thickness of the cavity layer, without the necessity of fitting reflectance spectra which is necessary for single layers. Fig. 3 gives the frequency shift of as-prepared samples as a function of time for storage in ambient air at different temperatures. At room temperature the filter wavelength decreases very slowly for some 3% after 100 days, compared to 4% for the microporous silicon described in Refs. w11,34x. For the mesoporous as well as for the microporous samples no saturation of the filter wave-
Fig. 3. Shift of the filter wavelength of as-prepared FP filters ŽŽHL. 4 ŽLH. 4 . due to ageing in ambient air. The anodisation current densities for the H- and L-layers were 20 mArcm2 and 300 mArcm2 , the anodisation times 1.887 s and 0.445 s.
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length shift was found, although the observation of the microporous sample lasts for more than 600 days up to now. As expected, the use of higher temperatures accelerates the oxidation, and also the ageing, very strongly. For example, after 2 days at 3008C the filter wavelength has shifted by 5%, and at 6008C even by 16%. This means that, even more than for room-temperature applications, for using at high temperatures Žin front of strong illumination sources or in laser beams. a stabilisation of the optical properties is required. For this reason in the following all ageing investigations were performed at 6008C in ambient air. This approach has the additional advantage that the ageing investigation times are strongly reduced compared to ageing at room temperature.
4. Ageing of pre-oxidised samples The ageing due to oxidation of PS could be reduced in several ways: by covering the surface of the sample, filling the pores with another material, or by a pre-oxidation of the PS, which is by far the easiest way. Such pre-oxidation can be performed anodically, by chemical treatment or by thermal oxidation. In the following we report on chemical oxidation in boiling HNO 3 and thermal oxidation in dry oxygen, while anodic oxidation was not investigated. As an example of chemical oxidation samples were stored in boiling HNO 3 Ž70%. for 10 min, cleaned with deionised water and dried in N2 . Afterwards the samples were annealed for 10 min at 7008C in N2 atmosphere. On bulk silicon this results in a 1 nm thick layer of SiO 2 with good stoichiometry w36,37x, thus no total oxidation of the silicon crystallites Ž10–20 nm in diameter. is expected but a complete covering of the inner surface with oxide. For thermal oxidation we used a two-step oxidation procedure in order to prevent undefined changes of the PS microstructure due to heat treatment. First the samples were oxidised for 60 min at 3008C to stabilise the porous structure and prevent coarsening during the following, high-temperature oxidation step w13,29,38x. Such coarsening would strongly slow down the oxidation and result in poor reproducibility w29,38x. Moreover, the formation of large voids of about 1 mm diameter would cause light scattering in the visible spectral range and the thickness of the H- and L-layers Žwhich is typically 50–200 nm. would not be well defined any more. The final oxidation was performed at 8508C and 9508C, respectively, for different times varying from 10 to 60 min. We did not use higher temperatures because in this case viscous flow of silica occurs w29,38x. In order to investigate the ageing of both high and low porosity layers, we fabricated FP filters of the types ŽHL. 4 ŽLH. 4 and ŽLH. 4 ŽHL. 4. The same ageing characteristics were observed for both filter geometries. Fig. 4 shows the filter wavelength shift of an unoxidised, as-pre-
Fig. 4. Filter wavelength of pre-oxidised FP filters ŽŽLH. 4 ŽHL. 4 . as a function of ageing time. The anodisation current densities for the H- and L-layers were 20 mArcm2 and 300 mArcm2 , the anodisation times 2.175 s and 0.508 s. All samples were heated at 6008C in ambient air during this ageing investigation.
pared sample in comparison to a chemically oxidised sample and two thermally oxidised samples Žusing the oxidation parameters 3008Cr60 min q 8508Cr10 min and 3008Cr60 min q 9508Cr10 min, respectively.. The 9508C pre-oxidation decreases the filter wavelength by more than 130 nm Ž21%., much more than for the 8508C pre-oxidation or the chemical treatment. This means that during the chemical oxidation and the thermal pre-oxidation at 8508C by far not the whole PS is oxidised, which results in a further wavelength shift during the ageing investigation on a timescale of several tens of hours. In contrast, the filter wavelength does not shift any more after the 9508C treatment. However, the reflectivity of these samples turned out to decrease continuously Žup to 7% after 6 days., which indicates that the PS still oxidises a little during the ageing sequence. This can be explained as follows: the filter wavelength corresponds to the optical thickness of the cavity, that is the product of refractive index and geometrical layer thickness, while the reflectivity is determined by the refractive index ratio of the layers. As will be shown below, the layer thickness slightly increases during oxidation, which can compensate the decrease of the refractive index. To prevent also this decrease of reflectivity, we subsequently increased the oxidation time at 9508C from 10 to 30 min. Fig. 5 shows reflectance spectra of an oxidised Bragg reflector ŽHL.15, directly after oxidation and after 2 days Žmain diagram. and 16 days Žinset. at 6008C in ambient air. For both ageing times the filter wavelength and the reflectivity of the main peak remained the same, without any ageing effect. The reflectivity in the UV, however, increases slightly between the measurement after 2 days and the one after 16 days. This spectral range is very sensitive to light absorption in the PS, so that a slight further oxidation can cause changes in the UV without noticeable influencing the visible spectral range.
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Fig. 5. Reflectance spectra of an oxidised Bragg reflector ŽŽHL.15 . before the ageing investigation Žsolid lines. and after storage at 6008C in ambient air Ždotted lines.. The layer stack and the anodisation parameters were identical to the ones of the structure in Fig. 1. The ageing times were 2 days Žmain diagram. and 16 days Žinset..
The same results were obtained with FP filters of the type ŽHL. 6 ŽLH. 6 and ŽHL. 8 ŽLH. 8. In order to check the reproducibility of this pre-oxidation process, five identical FP filters were fabricated and oxidised. After the preoxidation, the filter wavelength of the filters varied by just 1.5% from sample to sample, the same value as before the oxidation.
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Typical reflectance spectra are shown in Fig. 6. In the near IR and in the visible the spectra are dominated by interference fringes since in this spectral range the PS is nearly transparent. This means that the complex refractive index and the layer thickness can be extracted unequivocally from the height of the interferences and from the oscillation frequency by numerical simulations. In the UV no interferences can be observed for the as-prepared sample, because the light absorption is high and just the topmost part of the layer contributes to the reflectance. For the oxidised sample in Fig. 6, however, these interferences appear even in the UV. This indicates that the PS is nearly totally oxidised. The quantitative interpretation of these spectra requires numerical simulations. The dotted lines in Fig. 6 demonstrate how excellent the measurements can be simulated by using the Looyenga effective medium theory w9,10x. The adjustable parameters used in this simulation are the porosity, the layer thickness and the dielectric function of the solid phase in the porous material. The parametrization of the dielectric function by using Brendel oscillators is described in detail in Ref. w9x. From these fits one can exclude depth gradients of the refractive index, that means for these relatively thin layers the formation as well as the oxidation is homogeneous with depth. Table 1 gives the thicknesses of the PS layers, determined from these simulations. In addition, the layer thicknesses were checked by SEM, but no discrepancy was observed. As expected w32,33x, the thicknesses increase by 10–20%, with a saturation after 30 min for the high
5. Investigation of the optimal pre-oxidation process In the previous section it was demonstrated that thermal oxidation at 9508C is very well suited for the stabilisation of PS. Now the modification of the optical properties due to this treatment will be investigated in more detail. For the design of the interference filters it is necessary to know the optical layer thicknesses, that means the product of refractive index and geometrical layer thickness, with high accuracy. So far, this calibration was performed just for as-prepared PS w10,11x. The pre-oxidation, however, changes these quantities due to three effects: Ø the refractive index of the solid phase in the PS decreases; Ø the porosity decreases due to volume expansion of the crystallites, which partly compensates the first effect; Ø the geometrical layer thickness increases. For this reason the change of the refractive index as well as the change of the layer thickness have been investigated by fitting reflectance spectra of single layers. We used samples of about 1 mm thickness, etched with 20 and 300 mArcm2 , respectively. The oxidation parameters were Ž3008Cr60 min q 9508Cr30 min. and Ž3008Cr60 min q 9508Cr60 min..
Fig. 6. Reflectance spectra of single layers, as-prepared Žtop. and oxidised Ž3008Cr60 min q 9508Cr60 min, bottom.. The anodisation current density was 300 mArcm2 , the anodisation time 4.99 s. The solid lines correspond to measurements, the dotted lines to numerical simulations.
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Table 1 Thicknesses of single layers before and after pre-oxidation. The anodisation times were 36.76 s for the samples etched with 20 mArcm2 and 4.99 s for the others 20 mArcm2 300 mArcm2 as-prepared 1.09 mm 3008Cr60 min q 9508Cr30 min 1.20 mm 3008Cr60 min q 9508Cr60 min 1.33 mm
1.03 mm 1.23 mm 1.25 mm
porosity layer etched with a current density of 300 mArcm2. In contrast, no saturation was observed for the low porosity layer. This correlates very well to the fact that the low porosity layer is not fully oxidised even after 60 min at 9508C, as will be shown below. In contrast to these relatively small changes of the layer thicknesses, the refractive index of the PS changes dramatically. Fig. 7 shows the real part n and the imaginary part k as a function of wavelength. After pre-oxidation the imaginary part k of the high porosity layer is almost zero, indicating that the layer is totally oxidised and that light absorption is negligible. Accordingly after the pre-oxidation the real part n exhibits nearly no dispersion any more. The fact that n and k are identical for both oxidation times corresponds well to the saturation of the layer thickness. The low porosity layer, however, is just partially oxidised even after 60 min at 9508C, as can be seen from the relatively high imaginary part k and the typical structures in the UV between 25000 and 40000 cmy1. These structures correspond to the interband transitions of silicon which are just slightly modified in mesoporous silicon w9x and which disappear in oxidised silicon. The fact that the
layer is not totally oxidised is not surprising: Yon et al. w38x report that complete oxidation can be reached just for porosities larger than 46%, while the porosity of the low porosity layers in Fig. 7 was 34%. A partial oxidation means that the material can further oxidise which explains the ageing characteristics discussed above, especially the slight changes in the UV in Fig. 5. However, the oxidation at 6008C Žand even more at room temperature. is so slow that this pre-oxidation at 9508C is sufficient even for long-time usage, as described in the previous section. For future applications in the blue and UV, however, the absorption of the low porosity PS is still too high. For these applications one should use PS with porosities ) 46% for both the H- and L-layer.
6. Conclusion The filter wavelength and reflectivity of PS interference filters decrease with time due to oxidation of the silicon crystallites. At room temperature this ageing occurs on a timescale of days or weeks, while at high temperatures this effect is strongly accelerated. In order to suppress the ageing effects, different pre-oxidation processes were investigated. Chemical oxidation in HNO 3 and thermal oxidation at 8508C were not suitable to achieve high-temperature stability, but very good results were achieved using a thermal pre-oxidation at 9508C. Interference filters treated in this way displayed no wavelength shift and no decrease of reflectivity even after several days at 6008C in ambient air. For these optimal oxidation parameters the complex refractive index of the PS was determined by numerical simulations of reflectance measurements.
Acknowledgements We would like to thank S. Meesters for the TEM pictures, H.-P. Bochem for the SEM thickness measurements and M.G. Berger, R. Arens-Fischer and V. Ganse for very fruitful discussions.
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Fig. 7. Real part Žtop. and imaginary part Žbottom. of the refractive index of PS, determined from numerical simulations of reflectance spectra. The anodisation current densities were 300 mArcm2 for the high porosity layer Žleft. and 20 mArcm2 for the low porosity layer Žright.. The solid lines correspond to as-prepared samples, the dashed lines to short oxidised samples Ž3008Cr60 min q 9508Cr30 min. and the dotted lines to long oxidised samples Ž3008Cr60 min q 9508Cr60 min..
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