**5. Assessment of the chemical composition and strain field in nanowires**

In order to demonstrate the power of Raman spectroscopy to probe the chemical composition of NWs, it is worth focusing on alloyed NWs, while to investigate the strain state heterostruc‐ tured NWs (e.g., core‐shell NWs) are ideal. Here, we focus on core‐shell In*x*Ga1‐*x*As/GaAs NWs as they fulfill both requirements. These NWs are called nanoneedles (NNs) because they are highly tapered. They feature a highly pure WZ phase [28].

The chemical composition *x* of the In*x*Ga1‐*x*As core can be determined by Raman spectroscopy as displayed in **Figure 9**. Raman spectra of NNs with different indium composition capped with 30 nm of GaAs are shown in panel (a). Samples are labeled with the nominal In content and spectra were taken in the configuration. In agreement with the observations in **Figure 8(b)** for WZ GaAs, in both A1 (TO) and E2 H modes are observed (the E2 H peak here is more intense than the A1), while in the geometry (not shown here) only the A1 (TO) is observed. The shoulder at about 235 cm−1 can be attributed either to a WZ InAs‐like TO mode or to an interface mode. The Raman shift of all modes as determined by Lorentzian fits to the data (see for example the green solid lines in (a)) is reported in (c) as full squares (for data taken in ) and open circles (for data) as a function of the nominal In content. Solid lines represent the theoretical estimates of the GaAs‐like and InAs‐like modes energies obtained for ZB In*x*Ga1‐*x*As. No theoretical estimate is available for WZ In*x*Ga1‐*x*As, and so the energy of E2 H, present in WZ only, has no theoretical comparison. The effective indium compositions as extracted by the comparison between experiment and calculation for the GaAs‐like TO modes are *x* = (0.22–0.24) for the sample with nominal *x* = 0.20, *x* = (0.17–0.22) for nominal *x* = 0.16, and *x* = (0.12–0.15) for nominal *x* = 0.12 [28].

**Figure 9.** (a) Raman spectra (open circles) of In*x*Ga1‐*x*As nanoneedles (with the indicated nominal In content, *x*) capped with 30 nm of GaAs. Spectra were taken in the scattering geometry and were normalized to the GaAs‐like E2 H phonon peak, whose energy is indicated by a dashed line in the spectra of the *x* = 0.12 sample. Red solid lines are Lorentzian fits to the data. The single contributions to a Lorentzian fit are represented by green solid lines in the *x* = 0.12 sample as an example. (b) Same as (a) for samples with the indicated GaAs shell thickness *t* and fixed In content (*x* = 0.16). The energy of the GaAs‐like E2 H phonon is indicated by a dashed line in the spectrum of the sample without shell. (c) Frequency of various optical phonons as a function of indium composition. Symbols represent experimental data points taken under the (open circles) and (full squares) scattering configurations. Solid lines indi‐ cate theoretical estimations for ZB In*x*Ga1‐*x*As and dashed lines are linear fits to the data for the GaAs‐like and InAs‐like E2 H modes. (d) Energy of phonon modes for an In*x*Ga1‐*x*As NN with *x* = 0.16 a function of shell thickness. Symbols rep‐ resent experimental data points taken under the (full squares) and (open circles) scattering configura‐ tions. Dashed lines indicate the energy of the GaAs‐like A1/E1 (TO) and E2 H modes for In*x*Ga1‐*x*As without shell. Reproduced with permission from Ref. [28]. © 2014, American Chemical Society.

In these NNs, the possible presence of strain was also probed by Raman spectroscopy. In **Figure 9(b)**, we display spectra recorded in configuration from NNs with different GaAs shell thickness *t* and same indium composition (nominal *x* = 0.16). The peaks' attribution is the same as in panel (a). Here, the GaAs LO phonon mode is also observed, but only in the sample with the thickest shell. Its energy (∼287 cm−1) is downshifted by ∼2 cm−1 with respect to the bulk, which could be due to tensile strain in the shell. The Raman shift of all modes as determined by Lorentzian fits to the spectra (the green solid lines in panel (b) are an example) are reported in panel (d) as full squares for data and open circles for data as a function of the nominal shell thickness. We do not observe any clear and consistent energy variation of the phonon modes with increasing shell thickness, neither an increase in their FWHM, which allow us to exclude the presence of strain in the core. Possible compositional or shell thickness variations along the NN length can be also excluded, because Raman measurements performed on different points of a same NN do not display significant changes [28].

In this section, it is also worth mentioning that Raman spectroscopy in semiconductor NWs may be used to monitor the incorporation of dopants. At variance with electrical measure‐ ments, Raman measurements are not affected by spurious effects coming from the fabrication of contacts. Information on the type and concentration of dopants can be obtained by meas‐ uring, respectively, the energy and the intensity of the local vibrational modes associated with the impurities. As an example, we mention Reference [29], where *p*‐type GaAs NWs were grown with different silicon doping concentrations and, depending on the growth condi‐ tions, the SiAs local mode (indicative of Si incorporation into As lattice) and the mode due to the formation of neutral SiAs‐SiAs pairs were observed with different relative weights.
