Band gap of wurtzite GaAs: A resonant Raman study

We report resonant Raman scattering (RRS) by the TO, LO, and 2 LO modes of single wurtzite and zinc-blende GaAs nanowires. The optical band gap of wurtzite GaAs is $1.460\mathrm{eV}\pm 3\mathrm{meV}$ at room temperature, and $35\pm 3\mathrm{meV}$ larger than the GaAs zinc-blende band gap. Raman measurements using incoming light polarized parallel and perpendicular to the wire $c$ axis allowed us to investigate the splitting of heavy ${\Gamma }_9$ and light-hole ${\Gamma }_7$ band at the $\Gamma$ point of $65\pm 6\mathrm{meV}$.

Figure 1

Raman spectra of a single WZ GaAs NW with 50-nm diameter and 2.2-$\mu \mathrm{m}$ length at room temperature at 1.56-eV excitation.

Figure 2

Alternative schematic band structure at the $\Gamma$ point of wurtzite GaAs. (a) The valance bands are formed by the ${\Gamma }_{9V}$ heavy-hole and the ${\Gamma }_{7V}$ light-hole states. One of the conduction bands has ${\Gamma }_{7C}$ symmetry. The energetic position of the ${\Gamma }_{8C}$ conduction band is above the ${\Gamma }_{7C}$ conduction band. Optical transitions are allowed for ${\Gamma }_{9V}\to {\Gamma }_{7C}$ under perpendicular and ${\Gamma }_{7V}\to {\Gamma }_{7C}$ under both parallel and perpendicular polarization of the incoming light. (b) The valance band is formed by the ${\Gamma }_{9V}$ state; the lowest conduction band has ${\Gamma }_{8C}$ symmetry.

Figure 3

Resonant Raman profile of the TO mode from a single wurtzite GaAs nanowire for incident light polarized perpendicular to the NW $c$ axis (black circles and solid line). For comparison the TO resonance profile (blue triangles and solid line) for zinc-blende GaAs NW is shown with light polarized along the wire axis. Both profiles were measured at room temperature.

Figure 4

(a) Resonant Raman profiles of the LO mode (incident light polarized perpendicular to NW $c$ axis) from a single WZ GaAs NW (black circles and solid line) and ZB GaAs reference sample (blue triangles and solid line). (b) 2 LO resonance profile for incident light polarized perpendicular to the NW $c$ axis. All profiles were collected at room temperature.

Figure 5

lh-c resonance profile from a single WZ GaAs NW at room temperature for light polarized parallel to the nanowire axis. TO intensity, squares; LO intensity, solid circles; 2LO intensity, open circles.