Propagation of light in ensembles of semiconductor nanowires

Semiconductor nanowires have received a lot of interest during the last years because of their extreme optical properties. Giant polarization anisotropy in the absorption and emission of light fromsingle nanowires, extreme light confinement assisted by exciton polaritons, and enhanced detection sensitivity of analytes are some examples. These properties are leading to novel applications such as nano-light sources, electrical sensors, or quantum emitters. Many applications, such as nanowire solar cells or LEDs, will rely on large areas covered by ensembles of nanowires. Therefore, for devices based on nanowires, the knowledge on how light propagates in these layers is of utmost importance. In this thesis, an experimental study on light propagation in ensembles of GaP nanowires and arrays of InP nanowires is given. First, the growth of GaP and InP nanowires using the vapor-liquid-solid growth mechanism by metal-organic vapor phase epitaxy is introduced in Chapter 2. The vapor-liquid-solid growth mechanism requires a metal catalyst particle. Depending on the catalyst, ordered or disordered ensembles of nanowires can be grown. We describe the growth of disordered ensembles of nanowires from a thin gold film and the growth of arrays of nanowires from nanoparticles patterned by substrate conformal imprint lithography. Further,we discuss the nanowire morphology depending on the growth parameters and the substrate. We show that the nanowires grow vertical on (111) substrates. Nanowires grown on (100) substrates preferentially form an angle of 35¿ with respect to the substrate surface. While the initial diameter of the nanowires is defined by the gold catalyst, increasing the growth temperature allows for growing shells around the nanowires. The growth at a specific temperature results in conically shaped nanowires. Depending on the morphology of the nanowires, light propagates differently in ensembles of nanowires. While layers of thin nanowires form an effective medium that is strongly birefringent, i.e., the refractive index is different for different polarizations, layers of thick nanowires form a strongly scattering medium with a short scattering mean free path. Conically shaped nanowires form a graded refractive index layer that guides the light into the substrate and therewith reduces the reflection at the interface. In Chapter 3, the birefringence of thin layers of nanowires is described. The refractive index of these layers of nanowires is different for light polarized along or perpendicular to the nanowire elongation. The difference between the two refractive indices defines the birefringence parameter. We determine the birefringence by measuring the reflection contrast, i.e., the ratio of reflected light passing through crossed and parallel aligned polarizer and analyzer. We obtain the birefringence parameter of the nanowire layer from fits to the measurements with a transfer-matrix formalism based on Jones calculus. The experimentally determined birefringence is compared to the birefringence parameter determined from Maxwell-Garnett effective medium theory. The birefringence parameter is slightly lower than expected from theory due to bending of the nanowires. We find that the birefringence parameter of layers of nanowires is constant over a broad range of wavelengths. The large birefringence of ensembles of vertically aligned GaP nanowires can be significantly modified by adding a shell as thin as 10 nm of SiO2 around the nanowires. In Chapter 4, the modification of the birefringence is determined experimentally by polarization-dependent reflection measurements. This modification is modeled with Maxwell-Garnett effective medium theory and Jones calculus for anisotropic layers. We show that s-polarized light is more sensitive to changes in the surrounding of the nanowires than p-polarized light. The reflection contrast exhibits large and narrow peaks that shift strongly due to the presence of the thin shell. In contrast to Chapters 3 and 4, where we have determined the birefringence of vertically aligned nanowires, we have investigated the birefringence of nanowire layers that are grown on (100) GaP substrates in Chapter 5. These nanowires are oriented such that they form an angle of 35¿ with respect to the substrate surface. Due to this alignment, dense ensembles of GaP nanowires formbiaxial media. We determine the in-plane birefringence of layers of nanowires with different nanowire diameter by measuring the transmission contrast. We find that a nanowire layer with a certain nanowire diameter forms a ¿/4-waveplate. In Chapter 6, we describe the propagation of light in layers of thick nanowires. Scattering of light influences the propagation of light depending on the nanowire diameter. We determine the scattering mean free path of light, i.e., the mean distance between two scattering events, in layers of vertically aligned nanowires. We show that the scattering is anisotropic and that the scattering mean free path varies with the angle of incidence due to the alignment of the nanowires. GaP nanowires grown on (100) substrates form a stronger scattering medium than vertically aligned nanowires. We find that ensembles of nanowires belong to the strongest scattering media to date. In Chapter 7, we describe that graded refractive index layers reduce the reflection and increase the coupling of light into a substrate by matching the refractive index at the interfaces. For obtaining a graded refractive index layer based on GaP nanowires, the GaP filling fraction needs to be gradually increased from the top to the bottom of the layer. We show that ensembles of GaP nanorods form graded refractive index layers when they are conically shaped. Alternatively, a graded refractive index can be obtained using cylindrically shaped nanorods with a distribution of lengths, which also leads to an increased GaP filling fraction at the bottom of the layer. We model the graded index layers using a transfer-matrix method for isotropic layered media. We find that the coupling of light into a GaP substrate is increased for a broad range of wavelengths and angles. In Chapter 8, we demonstrate experimentally that arrays of base-tapered InP nanowires on top of an InP substrate form a strongly broadband and omnidirectional absorbing medium due to their specific geometry. Almost perfect absorption of light (higher than 97 %) occurs in the system. We explain the strong optical absorption by finite-difference time-domain simulations and we find that the base-tapered geometry of the nanowires strongly enhances the absorption for wavelengths below the electronic bandgap energy of InP. Above the electronic bandgap energy of InP, the light is efficiently coupled into the underlying substrate due to guided optical modes in the nanowires. Based on the findings of Chapters 4, 7, and 8, we propose in Chapter 9 possible applications of ensembles of nanowires. The high sensitivity of layers of nanowires to thin shells around the nanowires that is described in Chapter 4, inspired us to propose a very sensitive gas and bio-sensor. Tapered nanowires form a graded refractive index layer, which we describe in Chapter 7. We propose using graded refractive index layers based on GaP nanowires for increasing light coupling into III/Vmulti-junction solar cells. From the strong absorption of light in arrays of base-tapered InP nanowires (Chapter 8), we propose a novel solar cell concept based on base-tapered nanowire arrays