Optical response of wurtzite and zinc blende GaP nanowire arrays

Abstract

We compare the optical response of wurtzite and zinc blende GaP nanowire arrays for varying geometry of the nanowires. We measure reflectance spectra of the arrays and extract from these measurements the absorption in the nanowires. To support our experimental findings and to allow for more detailed investigations of the optical response of the nanowire arrays than possible in experiments, we perform electromagnetic modeling. This modeling highlights the validity of the extraction of the absorptance from reflectance spectra, as well as limitations of the extraction due to anti-reflection properties of the nanowires. In our combined experimental and theoretical study, we find for both zinc blende and wurtzite nanowires an absorption resonance that can be tuned into the ultraviolet by decreasing the diameter of the nanowires. This peak stops blue-shifting with decreasing nanowire diameter at a wavelength of approximately 330 nm for zinc blende GaP. In contrast, for the wurtzite GaP nanowires, the resonance continues blue-shifting at 310 nm for the smallest diameters we succeeded in fabricating. We interpret this as a difference in refractive index between wurtzite and zinc blende GaP in this wavelength region. These results open up for optical applications through resonant absorption in the visible and ultraviolet wavelength regions with both zinc blende and wurtzite GaP nanowire arrays. Notably, zinc blende and wurtzite GaP support resonant absorption deeper into the ultraviolet region than previously found for zinc blende and wurtzite InP and InAs.

Electronic Structures of Free-Standing Nanowires made from Indirect Bandgap Semiconductor Gallium Phosphide

“…[1][2][3] In recent years, GaP nanostructures have received considerable attention because of their potential applications in miniature optical and optoelectronic devices with enhanced performances and functyionalities. [4][5][6][7] GaP nanowires are often grown by the vapour-liquid-solid growth mechanism, which can provide nanowires with different crystal structures. 8 Typical dimensions of nanowires are diameters of the order of a few to one hundred nanometers and lengths of several micrometers.…”

“…It is possible to tailor both the crystal phase and geometry of nanowire arrays, allowing control of the optoelectronic characteristics of semiconductors. [4][5][6] In addition, nanowires, nanowire heterostructures and superlattices can be doped in a controlled manner. [9][10][11] For further development and optimization of devices containing nanowires, it is important to clearly understand the electronic structures of the nanowires along common crystallographic directions such as the [001] and [111] directions.…”

“…Gallium phosphide (GaP) is an important Group III-V semiconductor material with a wide band gap of 2.272 eV at 300 K, making it attractive for use in optical devices, light-emitting diodes, and photoelectrochemical cells 1 2 3 . In recent years, GaP nanostructures have received considerable attention because of their potential applications in miniature optical and optoelectronic devices with enhanced performances and functyionalities 4 5 6 7 . GaP nanowires are often grown by the vapour–liquid–solid growth mechanism, which can provide nanowires with different crystal structures 8 .…”

“…The techniques used to grow nanowires have been well developed during the last two decades. It is possible to tailor both the crystal phase and geometry of nanowire arrays, allowing control of the optoelectronic characteristics of semiconductors 4 5 6 . In addition, nanowires, nanowire heterostructures and superlattices can be doped in a controlled manner 9 10 11 .…”

High refractive index in wurtzite GaP measured from Fabry-Pérot resonances

“…Due to the confinement of the mode's electric field inside the nanowire, the substrate underneath the nanowire has a negligible effect on the refractive index calculation, with variations smaller than 1%. Recently, a lower refractive index for WZ GaP nanowires with respect to ZB bulk was indirectly estimated from the reflection measurements in the UV range, 19 leading to a similar behaviour in the visible range due to the positive sign of dn E dk . However, a value for the WZ refractive index smaller than the ZB refractive index does not match with the experimentally observed FP mode spacing in the present work.…”