Optical Properties of Strained Wurtzite Gallium Phosphide Nanowires

Abstract

Wurtzite gallium phosphide (WZ GaP) has been predicted to exhibit a direct bandgap in the green spectral range. Optical transitions, however, are only weakly allowed by the symmetry of the bands. While efficient luminescence has been experimentally shown, the nature of the transitions is not yet clear. Here we apply tensile strain up to 6% and investigate the evolution of the photoluminescence (PL) spectrum of WZ GaP nanowires (NWs). The pressure and polarization dependence of the emission together with a theoretical analysis of strain effects is employed to establish the nature and symmetry of the transitions. We identify the emission lines to be related to localized states with significant admixture of Γ7c symmetry and not exclusively related to the Γ8c conduction band minimum (CBM). The results emphasize the importance of strongly bound state-related emission in the pseudodirect semiconductor WZ GaP and contribute significantly to the understanding of the optoelectronic properties of this novel material.

Optical study of the band structure of wurtzite GaP nanowires

“…A possible reason for such a strong increase in the oscillator strength is the significant admixture of U 8c -U 7c symmetry due to the localized nature of the c-line bound-state. 17 When a transition with (partial) U 7c symmetry is involved, the high value for the oscillator strength 20 would result in a short lifetime, as observed in Fig. 4.…”

“…Polarization and pressure-dependent PL measurements on WZ GaP nanowires strongly suggest that the c-transition is neither related to the indirect M-minimum nor to the light hole or split-off band. 17 Instead, significant admixture of U 7c -symmetry due to high localization of the associated state and a low density of states in the conduction band 60 promotes an efficient transition from a resonant level to the U 9v valence band. Thus, the c-line is attributed to a level resonant in the C 8c conduction band.…”

“…Thus, the c-line is attributed to a level resonant in the C 8c conduction band. 17,60 A possible non-radiative recombination channel that dominates the c-line recombination at high temperature (see Fig. 6) is the inter-valley tunnelling of electrons into the indirect M-minimum in the conduction band.…”

“…[10][11][12] Recently, the Auseeded growth of high purity WZ GaP nanowires via vaporliquid-solid (VLS) method was demonstrated, and the optical properties have been investigated. [13][14][15][16][17][18] Photoluminescence (PL) measurements on patterned arrays of WZ GaP wires showed optical emission at 2.09 eV with a short lifetime of $0.8 ns, indicating a direct band gap semiconductor. 13 Strain dependence of the optical emission 17 and estimation of the dielectric constant 18 provide additional information on this material system.…”

“…[13][14][15][16][17][18] Photoluminescence (PL) measurements on patterned arrays of WZ GaP wires showed optical emission at 2.09 eV with a short lifetime of $0.8 ns, indicating a direct band gap semiconductor. 13 Strain dependence of the optical emission 17 and estimation of the dielectric constant 18 provide additional information on this material system. Furthermore, efficient water reduction in WZ GaP nanowires resulting from an enhanced optical absorption further indicates the direct band gap nature of WZ GaP.…”