Scattering of light in TiO2 layers of broad band mirrors

Thin Solid Films, 176 (1989) 25-32 ELECTRONICS AND OPTICS

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SCATTERING OF LIGHT IN Tie 2 LAYERS OF BROAD BAND MIRRORS E. SCHMIDT, H. MULLER, P. PERTSCH A N D D. G~,BLER

Sektion Physik, Friedrich-Schiller-Universitiit Jena und Physikalisch-Technisches Institut der Akademie der Wissenschafien, Jena (G.D.R.)

(Received August 8, 1988; revised January 9, 1989; accepted March 15, 1989)

It is shown by measurements of the scattered light of TiO2/SiO 2 broad band mirrors, that the scattered light must have been generated from the bulk of the Tie2 layers. By the application of two different technological procedures (water and oxygen technology) the structure of these layers can be altered, and this is demonstrated by the electron diffraction photographs. The reason for this scattered light is supposed to be the birefringence of Tie2.

1. INTRODUCTION

The properties of complicated optical interference layer systems depend, in a very complex manner, on the system design and on the real parameters of the applied interference layers received in the technological process used. In this paper we report on the optimization of dielectric broad band mirrors for the visible spectrum on the base of evaporated layers of Tie2 and SiO2. 2. CONSTRUCTION OF DIELECTRIC BROAD BAND MIRRORS

Dielectric broad band mirrors with a high reflectivity in the whole visible spectrum usually consist of two coupled stacks of dielectric layers with alternating refractive indices. The layers of each individual stack have identical optical thicknesses 21/4 and 22/4 respectively. The light reflected from such an optical filter consists of two different spectral components, one part which is reflected from the upper stack and the other part which is reflected from the stack below. In the case of the so-called blue-red mirror, the blue light has to pass the upper stack before being reflected; similarly, in the case of the red-blue mirror the red light passes the upper stack before it is reflected. As such stacks of layers are not absolutely free from losses, smaller reflection values have to be expected always in that part of the spectrum where the light is reflected by the lower stack. 0040-6090/89/$3.50

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26 3.

E. SCHMIDTet al. P R O D U C T I O N A N D P R O P E R T I E S OF T H E B R O A D B A N D M I R R O R S

Our manufactured mirrors consist of S i o 2 / T i O 2 layers. For the production we used two different technologies: (1) Reactive evaporation of the suboxides TiO and SiO in an oxygen partial pressure of ca. 10- 2 Pa on unheated glass substrates and subsequent annealing in air at a temperature of 400 °C for several hours (oxygen technology). (2) Evaporation in a water vapour atmosphere instead of oxygen under the same conditions, also with the subsequent annealing (water technology). Figure l(a) represents the measured reflection (upper curve) and transmission (lower curve; dotted line: extended ordinate) of a blue-red type broad band mirror consisting of 40 SiO2/TiO2 layers manufactured by the oxygen technology. Thus, while the transmission curve describes a useful broad band mirror, the short-wave reflection curve indicates disastrous losses oscillating along with the side bands of the long-wave layer stack. Quite similar effects have been observed in the case of the red-blue version of this mirror type (see Fig. l(b)). In that part of the

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Fig. I. (a) Light traps in the blue-red broad band mirror; (b) light traps in the red-blue broad band mirror.

SCATTERING OF LIGHT IN TiO2 LAYERS

27

spectrum which passes the upper quarter-wave stack twice we obtain real "light traps". Obviously, the broad band mirror increases the total losses in this part of the spectrum. By the application of the water vapour technology we were able to produce broad band mirrors with much less intensive light traps. For comparison, the reflection spectra in Fig. 2(a) represent the results of the two different technologies, reactive deposition in oxygen and water vapour respectively (39 TiO2/SiO 2 layers each, beginning with TiO2) for blue-red mirrors. Such properties of the losses in broad band mirrors are the result of intensive investigations on the origin of light traps and the possibilities to suppress them. 51%1 5

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(b) Fig. 2. (a) Comparison of broad band mirrors produced by oxygen(02) and water technology(H20); (b) comparison of the scattering of broad band mirrors produced by oxygen (02) and water technology (H20). 4. SCATTEREDLIGHTIN THE TiO 2 LAYERSAS THE SOURCEOF THE LIGHT TRAPS By photoacoustic absorption measurements I we found values less than 0.1~ for the absorption losses in the mirrors. F r o m this it follows that the light traps can only be attributed to scattered light. Figure 3 schematically shows the principal set-up of the arrangement used for spectral scattering measurements at a VEB Carl Zeiss J E N A S P E C O R D M 40 tight ebsorber

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28

E. SCHMIDTet al.

spectrophotometer. Figure 2(b) presents the spectral scattered light characteristics corresponding to the reflection spectra of the mirrors in Fig. 2(a). Despite some deviations the measured scattering is in reasonable agreement with the reflection spectra. Qualitatively, Figs. 2(a) and 2(b) express the same fact: when the light is reflected at the uppermost layers (i.e. in the red spectrum), light traps are missing and scattering effects remain small. Intensive scattering occurs only in the bulk of the layers being passed by the light (i.e. in the blue spectrum). There are some explanations for these deviations. The tilted light incidence and the large range of scattering angles cause a short-wave shift and broadening of the scattering maxima with respect to the original light traps. Furthermore, their magnitude is not strictly proportional to the light trap strength, because splitting into the two possible polarization planes leads to intricate relationships. By scattering measurements of annealed single layers of TiO2 and SiO2 we have found that the scattered light originates from the TiO2 films. Electron microscopic studies of the layer structure have demonstrated that there is no essential, optical effective difference in the surface roughness of the TiO2 layers produced by the two different technologies, but that their bulk structures strongly differ from each other. The electron micrographs in Fig. 4(a) and 4(b) show the replicas of the two TiO2 layers on the surface of the mirrors presented in Fig. 2. The results of transmission electron diffraction indicate a coarse-grained oxygen-deposited TiO2 layer (Fig. 5(a)) and a fine-grained water vapour-deposited TiO2 layer (Fig. 5(b)). Further support for this is gained from single-layer electron microscopy. The transmission electron micrographs of an annealed quarter-wave TiO2 deposit on a carbon

(a) (b) Fig. 4. Electron micrographs of the surface replicas of the mirrors in Fig. 2: (a) produced by oxygen technology;(b) producedby the watertechnology(Scale:500nm.)

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SCATTERING OF LIGHT IN T i O 2 LAYERS

(a) (b) Fig. 5. TED pattern ofa TiO2-SiO 2 mirror: (a) oxygen technology (b) water technology.

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i (a) (b) Fig. 6. Transmission electron micrographs of annealed 2/4 TiO 2 deposit on carbon: (a) oxygen technology; (b) water technology. (Scale: 500 nm.)

substrate (Fig.6(a) and 6(b)) show the same result as Fig. 5(a) and 5(b): water technology yields considerably smaller crystallites than the oxygen technology. From this we have concluded that the observed scattered light, responsible for the light traps, originates from the bulk of the annealed TiO2 layers.

30

E. SCHMIDTet al.

5. HYPOTHESIS FOR THE FINE-GRAINED WATER VAPOUR DEPOSITED T i O 2 LAYERS

The phenomenon of the formation of small crystallites in the water technology can be explained by the following hypothesis which considers the passivation of the hydroxides originating at the boundaries of the layer crystallites. These hydroxidic layers prevent the individual layer from forming extended uniform crystals through annealing, thus causing a fine-grained layer structure. Quite similar, but undesirable, phenomena have already been reported on the weathering of germanium layers 2. This physico--chemical process can be applied positively to our problem to produce fine-grained layers by controlled weathering. Schematically, this process is illustrated by a TiO2 single layer (Fig. 7(a)-7(e)), the

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(e) Fig. 7. Model of the weathering mechanisms in TiO2 layers: (a) fresh layer after dep°siti°n; (b) f°rmati°n of a hydroxidic gel through water attack; (c) annealing at T >/200 °C; formation of microcrystallites through gel decomposition; (d) repeated hydration; (e) repeated gel decompositio".

SCATTERING OF LIGHT IN T i O 2 LAYERS

31

surface of which is entirely covered by a thin hydroxidic film formed by penetrated water (Fig. 7(b)). Annealing at temperatures higher than 200 °C makes the gelatinous film decompose into dioxide and escaping water molecules. The gelatinous film has now converted into a large number of microcrystallites (Fig. 7(c)). Repeated cooling down and addition of water again forms a gelatinous film (Fig. 7(d)) which in turn can be annealed as described above (Fig. 7(e)). The weathering model is supported by the following results of the scattered light measurements of two red-blue broad band mirrors (Fig. 8) of identical construction: s u b s t r a t e / 1 8 x 2 1 / 4 / 1 9 x 2 2 / 4 TiO2 SiO 2, starting with TiO2 (21 = 600 nm; 42 = 500 nm). s [~J 5 4

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The mirror designated with 0 2 w a s made by the normal oxygen technology, whereas the H 2 0 mirror was made by the water technology, followed by two subsequent weathering cycles. Both mirrors were annealed for 2h in air at a temperature of 420 °C. As we can see, the scattered light of the second mirror is very weak and only insignificantly differs from the boundary scattering background because of coated roughness peaks and dust particles on the substrate surface. This means that by applying the weathering technology bulk scattering can be reduced by more than one order of magnitude. 6. DISCUSSION OF THE SOURCE OF THE SCATTERING LOSSES IN T i O 2 LAYERS

TiO 2 is known to be a birefringent material with quite different refractive indices of the ordinary and the extraordinary beam (e.g. at 600 nm the refractive indices of rutile are no 2.6 and neo = 2.88). Obviously, annealing produces extended domains of random orientation that do not cause any geometric layer deformations. Hence, due to the statistical distribution of the refractive indices (according to the individual crystal orientations), each plan wave front will inevitably be statistically deformed, which has to be referred to as light scattering. Further, the statistical distribution of the crystallites will lead to a statistically distributed rotation of the polarization plane of the passing light. This can be proved

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E. SCHMIDTe t al.

to be correct by looking through the laser mirror in the transmission band between crossed polarizers. Figure 9 shows the photography of the expected statistical lateral brightening of an oxygen-deposited mirror. It should be noted that in mirrors manufactured by the water technology this effect is so weak that it cannot be observed.

Fig. 9. Annealed mirrors between crossed polarizers (oxygen technology); the scrape was used for tocusing. (Scale:0.2 mm.) We have also found similar scattering losses in other optically anisotropic layer materials, e.g. in ZrO2-SiO2 broad band mirrors, while in optically isotropic or quasi-amorphous substances, e.g. in Ta205 or SiO2, these were not observed. REFERENCES 1 E. Welsch, H.-G. Walter, D. Sch/iferand R. Wolf, Thin Solid Films, 152 (1987) 433~142. 2 E. Schmidt, Jenaer Rundschau, 1 (1977) 26.