Raman spectroscopy of wurtzite and zinc-blende GaAs nanowires: Polarization dependence, selection rules, and strain effects

Polarization-dependent Raman scattering experiments realized on single GaAs nanowires with different percentages of zinc-blende and wurtzite structure are presented. The selection rules for the special case of nanowires are found and discussed. In the case of zinc-blende, the transversal optical mode $E_1$ (TO) at $267{\text{cm}}^{-1}$ exhibits the highest intensity when the incident and analyzed polarization are parallel to the nanowire axis. This is a consequence of the nanowire geometry and dielectric mismatch with the environment, and in quite good agreement with the Raman selection rules. We also find a consistent splitting of $1{\text{cm}}^{-1}$ of the $E_1$ (TO). The transversal optical mode related to the wurtzite structure, $E_2{}^H$, is measured between 254 and $256{\text{cm}}^{-1}$, depending on the wurtzite content. The azimuthal dependence of $E_2{}^H$ indicates that the mode is excited with the highest efficiency when the incident and analyzed polarization are perpendicular to the nanowire axis, in agreement with the selection rules. The presence of strain between wurtzite and zinc-blende is analyzed by the relative shift of the $E_1$ (TO) and $E_2{}^H$ modes. Finally, the influence of the surface roughness in the intensity of the longitudinal optical mode on {110} facets is presented.

Figure 1

(Color online) (a) Crystal facets of the reference used for the measurement of the selection rules in GaAs. The axis correspond to the crystallographic directions: $x=\left[0\overline{1}1\right]$, $y=\left[211\right]$, and $z=\left[\overline{1}11\right]$. (b) Parallel polarized Raman spectrum from a bulk GaAs $\left(0\overline{1}1\right)$ in backscattering geometry, obtained using exciting light polarized along the $\left[\overline{1}11\right]$ direction. (c) Theoretical azimuthal dependence of the TO mode of a bulk GaAs $\left(0\overline{1}1\right)$, as in (a). Continuous and dashed lines represent the components along the $\left[\overline{1}11\right]$ $(\parallel )$ and [211] $(\perp )$ of the Raman signal, respectively. (d) Measured azimuthal dependence of the TO mode of a bulk GaAs $\left(0\overline{1}1\right)$. Spheres and open squares represent the components along the $\left[\overline{1}11\right]$ $(\parallel )$ and [211] $(\perp )$ of the Raman signal, respectively. The continuous line is a fit to the data with a squared sine function, which describes dipolar behavior.

Figure 2

(Color online) Schematic drawing of the atomic arrangement in (a) zinc-blende and (b) wurtzite structures, where the arrows indicate the $\left[1\overline{1}1\right]$ and the [0001] nanowire growth axes, respectively. (c) Schematic representation of the phonon dispersion in GaAs. Phonon branches along [111] in the zinc-blende structure are folded to approximate those of wurtzite structure along [0001] (d) Unit cell for the wurtzite structure. Point A corresponds to the edge of the first Brillouin zone in the direction [0001].

Figure 3

(Color online) Sketch of the experimental setup and the configurations used for the measurement of GaAs nanowires in backscattering geometry. The crystal facets of the nanowire and the corresponding set of axis used as indicated: $x=\left[0\overline{1}1\right]$, $y=\left[211\right]$, and $z=\left[\overline{1}11\right]$.

Figure 4

(Color online) (a) BF image of a GaAs nanowire grown under conditions where 100% zinc blende is obtained. (b) HRTEM of a twin in the zinc-blende structure (c) power spectrum of the twined region. The different spots can be identified with the two regions of the twin. The facets on axis are identified by (110) and (011).

Figure 5

(Color online) (a) Raman scan from a nanowire crystallized in ZB structure, obtained using exciting light polarized along the nanowire axis. (b) Parallel polarized Raman spectra from a bulk GaAs $\left(0\overline{1}1\right)$ and a GaAs nanowire crystallized in the ZB structure. For both measurements the exciting and scattered light are polarized along the $⟨-111⟩$ direction. The spectrum of the nanowire is shifted vertically in order to make the comparison easier. (c) Azimuthal dependence of the TO mode related to the ZB structure in the nanowire. Spheres and open squares represent the parallel and perpendicular components of the Raman signal collected with respect to the nanowire axis, respectively. The continuous line is a squared sine fit to the data.

Figure 6

(Color online) (a) BF image of a GaAs nanowire grown under conditions where $\sim 30\%$ wurtzite structure is obtained. (b) and (c) Region of multitwinned zinc-blende in the region B. (d) Bright field TEM image of the tip of region A. (e) HRTEM in the selected square of (d). (f) Power spectrum of the region marked with a square in (e), which allows the identification of the crystalline structure of this region to be wurtzite, where spots C, A1, A2, and A3 correspond to (0001), $\left(1\overline{1}00\right)$, $\left(1\overline{1}01\right)$, and $\left(1\overline{1}02\right)$ planes, respectively.

Figure 7

(Color online) Perpendicularly and parallel polarized Raman scans from a nanowire consisting of 30% wurtzite structure, obtained using different polarization directions of the incident light. (a) Perpendicularly polarized Raman scan from perpendicularly polarized incident light: $x\left(y,y\right)\overline{x}$. (b) Parallel polarized Raman from perpendicularly polarized incident light: $x\left(y,z\right)\overline{x}$. (c) Perpendicularly polarized Raman scan from parallel polarized incident light: $x\left(z,y\right)\overline{x}$. (d) Parallel polarized Raman scan from parallel polarized incident light: $x\left(z,z\right)\overline{x}$.

Figure 8

(Color online) A series of parallel and perpendicularly polarized Raman spectra obtained using exciting light polarized parallel and perpendicularly to the nanowire axis, from (a) the zinc-blende rich segment of a nanowire with 30% of wurtzite structure and (b) a nanowire with 100% zinc-blende structure. The spectra have been shifted vertically. The relative intensities of the spectra obtained for the same sample in different scattering configurations can be compared directly.

Figure 9

(Color online) (a) Azimuthal dependence of the $E_1$ (TO) mode. (b) Azimuthal dependence of the $E_2{}^H$ mode, related to the wurtzite structure. In both polar plots, spheres and open squares represent the parallel and perpendicular components of the Raman signal collected, respectively and the continuous line is a squared sine fit to the data. (c) Representative Raman spectra realized under the main four configurations. For better illustration, the spectra have been normalized and shifted vertically. All spectra have been realized in the same position of the nanowire.

Figure 10

Bright field TEM measurement of the nanowire where one observes both the wurtzite and zinc-blende crystal structures, with a clear gradient of zinc-blende phase from left to right.

Figure 11

(Color online) (a) Waterfall plot of Raman spectra collected along the nanowire, every 100 nm, covering a distance of $1.7\mu \text{m}$, as schematically depicted in (b); $E_2{}^H$ and $E_1$ peak positions and FWHM are plotted as a function of the position along the nanowire in (c) and (e), respectively. (d) Relative intensity of the wurtzite and zinc-blende structure—calculated as the intensity ratio $E_2{}^H/E_1$—as function of the position along the nanowire. Each of the points constitutes an average of three consecutive measurements.

Figure 12

(Color online) (a) HRTEM measurements close to the tip of a nanowire where the “forbidden LO phonon mode is observed; (b) power spectra of the region marked with yellow square in (a), where the diffraction spots of the two orientations of the twins are observed—sublabels 1 and 2. (c) Detail corresponding to the red square in the HRTEM micrograph, in which one can recognize the formation of $\left(\overline{1}\overline{1}\overline{1}\right)$ facets.