6 Spectroscopy of differently annealed semiconductor Bragg mirrors
6.2 Optical properties
900 950 1000 1050 1100 1150
0 20 40 60 80 100
Wavelength (nm)
Reflectivity (%)
no RTA 450°C 650°C 700°C
Figure 6.5: Static reflectivity spectra of semiconductor Bragg mirrors annealed at different temperatures.
6.2 Optical properties
Beyond the structural properties the optical properties of Bragg mirrors are also very important. They are even from special interest, since they determine the optical prop-erties and the quality of the device the Bragg mirror is incorporated.
The static reflectivity of a superlattice is based on the reflectivity on every interface and the only influencing parameters are the refractive index and the layer thickness of the periodic structure.
The static reflectivity spectra were measured with the FTIR-spectrometer, described in Section 4.1. We also measured all four Bragg mirrors before the annealing process and obtained identical spectra, indicating the homogeneity of all four samples.
The spectra of all four Bragg mirrors after the annealing process are plotted in Fig.
6.5. Since the Bragg mirrors are optimized for NIR, they exhibit the stopband in the range between 960-1150 nm. This range does not change due to the annealing process.
Additionally, the spectra of all four samples show a shoulder at 950 nm. Simulations showed that shoulders next to the stopband occur if the superlattice period on top of the Bragg mirror differs a few nanometers compared to the superlattice period at the bottom in this way that the mean superlattice period remains constant.
The spectra of the un-annealed sample and BM-450 are almost identical. This might result from the fact that typical growth temperatures for Bragg mirrors are in the range of 500◦C so that the annealing temperature is below the growth temperature.
In contrast to the unaffected stopband region for all annealed samples, the reflectivity
900 950 1000 1050 1100 1150
900 950 1000 1050 1100 1150 0
900 950 1000 1050 1100 1150 0
900 950 1000 1050 1100 1150 0
Figure 6.6: Static reflectivity spectra of semiconductor Bragg mirrors annealed at different temperatures, measured and calculated data. The calculation is based on the transfer matrix method introduced in Section 4.4.2.
spectra outside the stopband are different and show effects due to annealing. For wavelengths below the stopband a blueshift occurs, respectively a shift towards smaller wavelengths, which increases with increasing annealing temperature. This blueshift can be explained by a change in the refractive index. In [Dan94] a decrease of the re-fractive index of low temperature grown GaAs and AlAs due to annealing is observed.
Calculations of the reflectivity spectra of the Bragg mirrors show that a decreased re-fractive index leads to such a blueshift.
For wavelengths above the stopband the spectra exhibit complete different shapes. The spectrum of BM-650 exhibits a large blueshift and a second shoulder. Analogous to the blueshift below the stopband this blueshift can be explained by a decreased refractive index of the superlattice alloys [Dan94]. The shoulder occurs due to an inhomogeneity of the superlattice period.
The spectrum of BM-700 is similar to the un-annealed sample apart from a kind of
’stretching’ of the oscillations towards larger wavelengths. This redshift is an opposite development compared to the blueshift of BM-650 and can be reproduced in calcula-tions by increasing the refractive index of GaAs, which is in contrast to the results given in [Dan94].
As it is described in Section 4.4.2, the static reflectivity of a multilayer structure is given by the layer thicknesses and the refractive indexes of the alternating materials. Since
6.3 Summary and conclusion
the structural analysis of the Bragg mirrors in Section 6.1 showed that the superlattice constants are not affected by annealing, the changes observed in the FTIR-spectra may results from changes in the refractive index or from unobserved structural changes such as modified interface roughness or inhomogeneities in the superlattice structure. To get a deeper understanding about the changes of the refractive indexes we reproduced the measured reflectivity spectra by calculations based on the transfer matrix method, which is shortly introduced in Section 4.4.2.
The measured and the calculated spectra of all four Bragg mirror sample are plotted in Fig. 6.6. The calculations are based on Eq. (4.7). Main parameters influencing the spectra are the layer thicknesses and the refractive index of the materials. Since thickness extractions showed that the periods of all four samples are identical, the thickness of the samples was kept constant. The only parameters that were changed are the refractive indexes of AlAs and GaAs. They are depending on the wavelength and additionally on the annealing temperature [Dan94]. As it can be seen in Fig. 6.6 the shoulders at the edges of the stopband could not be reproduced by the calculation based on the transfer matrix method. However, the shoulders might be due to an inhomogeneity of the layer thicknesses within the heterostructures.
The calculations showed that changes in the static spectra due to annealing result from changes in the refractive indexes. The modifications of the refractive indexes for the calculations of the spectrum of BM-650 are consistent with the predictions of Dankowski et al. [Dan94]: Due to annealing the refractive indexes of GaAs and AlAs decrease leading to a blueshift of the spectrum for wavelengths below and above the stopband. However, a further increasing of the annealing temperature does not lead to a further blueshift. Whereas the spectrum of BM-700 for wavelengths below the stop-band shows a blueshift the spectrum above the stopstop-band exhibits opposite behavior:
The spectrum is redshifted, as it can be reproduced in calculations by increasing the refractive index.
In conclusion, on the basis of four samples a general prediction about annealing effects on the reflectivity of Bragg mirrors is hardly possible. One systematic change might be the blueshift of the spectrum for wavelengths below the stopband. This blueshift can be explained by a change in the refractive index, as it is described in [Dan94]. However, the stopband region does not seem to be affected at all. Therefore, the performance of optical devices based on a simple Bragg reflector should not be affected by post-growth annealing processes.
6.3 Summary and conclusion
Post-growth rapid thermal annealing (RTA) is a common process to tailor the optical properties of semiconductor devices, such as SESAMs and semiconductor disk lasers.
Since these devices consist inter alia of a semiconductor Bragg mirror, we investigated the effects of post-growth annealing on AlAs/GaAs Bragg mirrors. Furthermore, modi-fications of the properties of the Bragg mirror due to post-growth heating are of interest, since the optical devices in a laser cavity may be exposed to high temperatures, too.
Consequently, from information about post-growth annealing effects one can even con-clude about the effects of high operating temperatures in a laser cavity.
We investigated four samples of AlAs/GaAs Bragg mirrors: No annealing, RTA for 100 s at 450◦C, 650◦C, and 700◦C. To prevent arsenic out-diffusion during the an-nealing process the samples were covered upside down with a GaAs wafer.
We focused on two aspects: How does annealing change the superlattice structure and how does it change the static reflectivity spectrum.
For structural analysis we used coherent acoustic phonon spectroscopy based on pump-probe spectroscopy with the ASOPS technique, as it is described in Section 4.4.5. Based on these phonon measurements we extracted a superlattice constant of 156.3 nm for all four samples. Whereas this technique is already used in the structural analysis for short-periodic superlattices [Geb10, Bru12, Kra14], it is quite new to use it also for long-periodic structures. Since this method was not tested before for superlattice constants in the range of >100 nm, we also investigated the samples by X-ray spec-troscopy to get information about the accuracy. X-ray measurements conclude that the superlattice constants of all four samples is 155.9 nm. Based on this result two points get clear: Firstly, the superlattice constant extracted by phonon spectroscopy is just 0.4 nm larger than the values extracted by X-ray measurements, so the values are equal within the measurement accuracy. This agreement is absolutely remarkable and shows that with coherent acoustic phonon measurements accuracy in the nm-range is obtained. Secondly, the conclusion of coherent phonon spectroscopy that annealing does not change the superlattice constant is confirmed by the X-ray measurements.
Nevertheless, one has to keep in mind that structural changes due to annealing like a change in the interface roughness or changes in the homogeneity of the layer structure cannot be measured with X-ray spectroscopy or coherent phonon spectroscopy. How-ever, huge structural changes due to the post-growth annealing process can be excluded based on the structural analysis we made.
Apart from structural properties we were also interested in optical properties of the different annealed Bragg mirrors. Therefore, we measured the static reflectivity spec-tra of all four samples with the FTIR-spectrometer and reproduced the experimentally obtained spectra with calculations based on the transfer matrix method, as it is shortly introduced in Section 4.4.2.
The spectra of the un-annealed sample and of BM-450 were almost identical, since typical growth temperatures for Bragg mirrors are above 500◦C so that an annealing temperature of 450◦C is below the growth temperatures. All four samples exhibit the stopband in the range of 960-1150 nm, independent of the annealing temperature.
However, the spectra outside the stopband changed due to the heating: For wave-lengths below the stopband the spectra showed a slight blueshift. This blueshift might be explained by a change of the refractive index of AlAs and GaAs as it was observed from Dankowskiet al. [Dan94]. The spectra above the stopband region do not show a consistent trend. The spectra of BM-650 also shows a blueshift for longer wavelengths, whereas the spectra of BM-700 is redshifted for wavelengths above the stopband.
However, since the reflectivity of a superlattice structure is only determined by the
6.3 Summary and conclusion
layer structure and the refractive indexes, the changes in the spectra must be based on changes of these two parameters. Changes of the superlattice constant could be excluded by means of structural analysis, whereas changes of the superlattice homo-geneity and interface roughness could not be considered. Changes of the refractive index due to annealing processes are obvious, since changes have already been ob-served [Dan94].
Nevertheless, the reflectivity of the stopband of the Bragg mirror did not change due to annealing so that post-growth annealing processes should mainly affect electron-hole recombination time constants instead of the stopband reflectivity of the device. Fur-thermore, we conclude that typical operating temperatures in a laser cavity should not influence the performance of a Bragg mirror permanently. Nevertheless, the reflectivity spectra of heated SESAMs, as they are shown in Section 5.2.2, show that the temper-ature dependent refractive index of GaAs and AlAs influence the reflectivity spectrum of the Bragg mirror leading to a redshift. Thus, one has to distinguish between per-manent and irreversible changes due to annealing and high working temperatures and acute changes due to heating in the moment of the heat impact. The measurements of this Chapter showed that the former can be neglected, whereas the latter occurs as it is verified by the measurements in Section 5.2.2.