DBRs were fabricated from alternating SiO2 and Ta2O5 films by magnetron sputtering. DBRs of high reflectivity can only be obtained if all films have a homogeneous thickness over the en- tire surface, negligible surface roughness and all layers have a thickness of exactly λ0
4n. Here, λ0 is the central wavelength of the stopband and n is the refractive index of the oxide lay- ers. Any deviation from this ideal cavity will result in a reduction inQ-factor. The process, once optimised, was set up for maximum reproducibility. This was obtained when using con- stant sputtering power (instead of constant rates), always the same gas mixture and flows and no heating of the substrate. Generally, for sputtering high quality or even crystalline films, it is advised to keep the substrate at an elevated temperature, Ts >300 ◦C.247 However, as mentioned before, sputtering at ambient temperature turned out to be necessary to avoid degradation of the organic film, which requires two remarks. (1) The overall temperature on the substrate was found to heat up by∼15◦C during the sputtering process. The maximum measured substrate temperature, Ts =35 ◦C, is still well below the glass transition tempera- ture of C545T at ambient temperatures (Tg =100 ◦C)226, so that the properties of the film were expected to be largely maintained throughout the sputtering process. In terms of reor- ganisation of the sputtered particles on the surface of the substrate, the inherent increase inTs corresponds only to a small increase in kinetic energy (∼5%). Hence, the gradient of material properties in the sputtered film appearing during the heating process over the first 10–20 nm of the DBR stack is expected to be small. (2) The effect of lowering the sputtering tempera- ture from 80◦C to ambient temperature (i.e., 35◦C) was investigated via the transmission of
Figure 5.6: Comparison ofQ-factors as a function of the number of DBR pairs: simulated vs real microcavities. The black line shows transfer matrix simulations of empty cavities with the mode positioned in the centre of the stop band. The grey dashed line shows the analytic calculation from Equations 2.10 and 2.14, which describes the microcavity in the limit of a large number of mirror pairs. Open symbols show the measuredQ-factors from microcavities containing 3% C545T (red) and 1% C545T where the cavity mode is far off the absorption band (‘empty’, green). Solid red and green symbols represent theQ-factors obtained from TMM calculations matching the spectra of the real cavities (i.e., assuming the presence of absorbers inside the cavity and mode positions away from the centre of the stop band).
fully grown DBRs and the surface roughness of the top layer of these DBRs. Upon decreasing the substrate temperature, the minimum of transmission in the stopband in comparable DBRs increased by 27% (from 2.2% to 2.9% for DBRs with 7.5 mirror pairs). This confirms the expectation of poorer film quality at lower substrate temperatures. At the same time, mea- surements by atomic force microscopy of the surface roughness, for which I acknowledge Nils Kronenberg, showed that the surface roughness barely increased upon lowering the tempera- ture (1.5±0.4 nm vs 1.6±0.4 nm). Hence, the reason for the increase in transmittance is not fully understood.
The quality of DBRs sputtered at room temperature was further assessed via theQ-factors of microcavities. In ideal DBRs—consisting of only perfectly transparent, plane parallel films of a thickness of exactly λ0
4n—the reflectivity in the stopband can be enhanced infinitely by only increasing the number of layers (see Equation 2.10). However, any amount of absorption or scattering inside the mirrors will reduce the coherence and thus the maximum reflectivity. TMM simulations serve as ‘ideal’ microcavities to which the measured transmittance spectra of real microcavities are compared in order to evaluate the performance of the DBRs, see Figure
5.6. While the simulations take into account the potential absorption in SiO2 and Ta2O5 via the measured refractive indices, they correspond to ideal microcavities as they assume perfect, parallel interfaces and have the mode in the centre of the stop band. The analytic formulae presented in Chapter 2.2 are also used to calculate theQ-factors expected from ideal DBRs. Yet, these underestimate theQ-factor because they are only valid for a large number of mirror pairs, as it can be seen from the convergence of the analytical and the TMM result for largeN.
Additionally, Figure 5.6 compares experimentally measuredQ-factors (open symbols) toQ- factors from simulated cavities with corresponding structures (filled symbols). The difference between these simulations and simulations of ideal cavities (black line) is the spectral position of the cavity mode with respect to the stopband. Clearly, the experimentalQ-factors are lower than the simulated reference. The two sets of experimental microcavities illustrate two points why an experimental approach might under-estimate the maximumQ-factor achievable with these sputtered mirrors in an empty microcavity. It is important to acknowledge this differ- ence because while the experimental approach is straight forward, the relevant quantity for the properties of the uncoupled cavity photon is theQ-factor of an empty microcavity. In the microcavities represented by red symbols, the cavity mode was in a spectral region of absorp- tion of C545T although this only made up 3% of the material inside the microcavity. Once this absorption was considered in the TMM calculations (red, filled symbols), the experimentalQ- factor nearly matched the simulation. These additional losses by absorption are not inherent to the mirrors so that an experimental determination of theQ-factor is not appropriate here.
The second series of microcavities (green symbols) can be considered to be empty, since the cavity mode was red-shifted with respect to the absorption of the material in the micro- cavity. Here, however, the cavity mode was not centred in the stop band. Comparative TMM calculations (green, filled symbols) were performed for cavities containing an adjusted thick- ness of the cavity material so that the mode position coincided with that of the experimental microcavity. Accounting for the mode position in the stop band reduced the Q-factor with respect to the TMM calculation of an ideal cavity. This reduction inQ-factor is real—at the position of the cavity mode, the mirrors are less reflective and hence, more energy is lost in each round trip. It can be seen that a discrepancy betweenQ-factors from TMM simulations and from measurements decreases when accounting for the offset between cavity mode and centre of the stop band, but does not vanish. This will be due to the scattering introduced by non-zero surface roughnesses and not perfectly identical optical thicknesses in the layers
throughout the stack.
From Figure 5.6, two trends are deduced. First, the larger the number of DBR pairs and the predicted Q-factor, the larger the deviation of the quality of the real microcavity with respect to an ideal cavity. This is because the larger the number of layers in the DBR, the larger the effect of any irregularity that introduces decoherence. Second, even though being smaller than the values of ideal microcavities,Q-factors of real microcavities reached values up toQ=1, 600 (with 10.5 mirror pairs). Previous reports on polariton lasing using SiO2and Ta2O5 as materials for the DBRs use six mirror pairs on top and bottom (reportingQ=600) or six and nine mirror pairs for top and bottom mirrors (Q not specifed).60,61 The mirror investigation shown here thus proves that the process established within this PhD produced microcavities with properties that are compatible with polariton lasing.