## 3 Raman Line Shape in Dilute Nitrides: Recent Evolution  
The effects of the H dose on LO phonon frequencies have been recently investigated in uniformly hydrogenated layers of a GaAsN alloy with N concentration equal to 1% [39]. Measurements were made on samples which received H doses comprised between 1.2 and 3.5 c (1 c = 1.0 \(\times\) 10\({}^{18}\) ions/cm\({}^{2}\)), corresponding to the average in-plain biaxial strain ranging from 1.0 \(\times\) 10\({}^{-3}\) to \(-\)8.0 \(\times\) 10\({}^{-3}\), as estimated from PR spectra [39]. The frequency evolution with an increasing H dose strongly depends on the polarization geometry of the Raman scattering experiment Table 1.  
Figure 5: (**a**) Calculated 3D distribution of the uniaxial strain field component \(\varepsilon_{xx}\), that is in the direction perpendicular to the axis of a 2.0-\(\upmu\)m wide wire (see section “Experimental and analytical procedure”). Deformation is 100 times exaggerated. (**b**) Calculated \(\varepsilon_{xx}\) along the normal to the wire axis (black line), see Ref. [11]. Note the agreement with the experimental polarization degree \(\rho\) (thick blue line, right axis). Reproduced from [11] ([PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON], Appl. Phys. Lett. **94**, 261905 (2009) [[https://doi.org/10.1063/1.3157838](https://doi.org/10.1063/1.3157838)]([https://doi.org/10.1063/1.3157838](https://doi.org/10.1063/1.3157838))), with the permission of AIP Publishing. While in the typically used geometry (incident and scattered light polarization [110]) the linear relationship between strain and frequency is confirmed [39] as supported by the comparison with the PR data for increasing doses (and increasing strain), in other geometries the behavior might be different. The most intriguing case corresponds to incident light polarization along the [100] crystallographic axis and scattered light polarization along the same [100] axis. In this geometry, Raman scattering is forbidden in GaAs, whereas selection rules are known to relax in strained GaAsN [40]. In-plane strain is also the main parameter responsible of the relaxation of selection rules in in GaAsN:H layers. The low scattering intensity in the 1.2 c sample [39] is amenable to the vanishing strain in this particular sample. Scattering intensities in more heavily irradiated samples are larger, in agreement with a larger (compressive) strain, as confirmed by systematic PR measurements [30; 39]. In contrast to other geometries, frequencies decrease with an increasing H dose, which is accompanied by an increasing compressive strain. In all micro-Raman experiments on H-irradiated samples, laser power must not exceed the threshold above which dissociation of N-H centers starts to occur [41]. With the instrumental setup typically used in our experiments, such threshold can be prudently set at 1 mW [16]. However, a marked dependence of line shapes (peak position, broadening, symmetry) on power was observed (see Figure 6) starting from about 0.3 mW, especially in more heavily hydrogenated GaAsN samples. It is worth noting that the data reported in Table 1 for GaAsN:H layers were obtained by using an excitation power of 0.1 mW. No comparable effect is observed in GaAs, while it was found negligible in GaAs\({}_{1-\mathrm{x}}\)N\({}_{\mathrm{x}}\) (at least for x \(\leq\) 0.01). This effect is completely reversible, so that it is reasonable to believe that it cannot be ascribed to N-nH center dissociation [41; 42]. Although further investigation (actually in progress) will be required to fully explain these results, we may notice that the temperature increase due to the laser beam illumination could enhance the free carriers (originating from ionized impurities) activity and locally modify the pre-existing charge density distribution around the N-nH complexes, so adding disorder in the host matrix [20; 43].