Friday 20 November
13:00 - 16:00
Structuring of semiconductor materials is utilized in many optoelectronic devices, e.g, in order to make them more efficient, cost-effective, and/or to obtain specific wavelength-engineered responses. Semiconductor materials are widely used in optoelectronic devices due to their favorable optical and electric properties. Surface structuring of semiconductor materials can be applied in order to functionalize the surface by providing specific light manipulation functions (e.g., anti-reflection, trapping, scattering, and/or mode coupling). Thereby, light-matter interactions can be realized that are tuned for specific optoelectronic applications. The choice of materials, geometry, and spacing of the surface structuring are important factors. This thesis work investigates the nanostructuring of several types of semiconductor materials (e.g., c-Si, a-Si, GaInP, TiO2, and ZnO) for applications such as optical sensing, broadband anti-reflection, light extraction enhancement, optical/color filtering, and color conversion.
c-Si nanopillar arrays are used as a platform for optical sensing. Their characteristic reflectance spectrum is used, where a peak shift of the reflectance spectrum is related to the refractive index and thickness of a deposited layer. A fabrication process based on nanoimprint lithography and inductively coupled plasma reactive ion etching was used to fabricate Si nanopillar arrays. Refractive index sensing applications were investigated by measurements and simulation study. Both medium sensing and surface sensing applications were demonstrated. The medium sensing resulted in a refractive index sensitivity of ~250 nm/RIU within the visible(-NIR) wavelength spectrum and a lower bound detection limit of ~2∙10-4 RIU-1. The surface sensing enables to measure lower bound (average) SiO2 and Si3N4 layer thicknesses of ~8.1∙10-2 and ~5.2∙10-2 nm, respectively, which is less than a monolayer of these materials.
a-Si nanodisk arrays are demonstrated for broadband anti-reflection and optical/color filter applications. Their (relative) occurring Mie resonances were utilized, by controlling the nanodisk diameters. Substrate-free a-Si nanodisk arrays were obtained by a top-down method, based on a combination of colloidal lithography, inductively coupled plasma reactive ion etching, and sacrificial layer etching. This resulted in nanodisk arrays with a hexagonal array period of 500 nm, height of 200 nm, and diameters varying between 150-400 nm. Placed on a Si substrate, broadband anti-reflection applications were demonstrated. An average surface reflection of ~7.5% was obtained in the visible-NIR wavelength spectrum for the structures placed on Si. By subsequently embedding the nanodisk arrays in a highly transparent film (PDMS), (reflectance and transmittance) optical/color filter applications were demonstrated in the visible-NIR wavelength spectrum. Spectrophotometry and simulations were utilized for optical characterization. For the optical/color filter applications, tunable reflectance and transmittance peaks were obtained with a shift rate of ~1.5 nm/nm nanodisk diameter increase.
Structured metal-oxide-based optical coatings are demonstrated as non-absorbing broadband anti-reflection coatings and for light extraction enhancement add-on layers for solar cell and LED applications, respectively. Sol-gel-based ZnO nanodisk and nanocone arrays were realized by a combination of a drop-casting, colloidal lithography, and reactive ion beam etching process. TiO2 nanoparticle-based microcone and nanodisk arrays were fabricated based on an in-house developed embossing/soft imprinting method. For both the TiO2 and ZnO stuctures, spectrophotometry and simulation studies were utilized for optical characterization. The fabricated ZnO nanocone and TiO2 nanodisk array structures on Si resulted in a lowest average surface reflection of ~7.5% and ~7% along the visible-NIR wavelength spectrum, respectively. Surface reflections as low as ~4% were shown to be possible for further optimized ZnO nanocone and TiO2 nanodisk array structures by simulation studies. TiO2 nanodisk arrays were realized on pre-fabricated planar single-junction Si, GaAs, and InP solar cells. This resulted in an increase of ~1.3 times of the short-circuit current density, and a slight increase (~10-20 mV) of the open-circuit voltage was observed. TiO2 microcone arrays were realized on vertical thin film GaN-based (planar) light emitting diodes. This resulted in an optical power enhancement of ~2.1 times.
GaInP nanowire/pillar arrays are demonstrated for color conversion applications. This was investigated for light conversion from blue (450 nm) and green (532 nm) to red (~660 nm) light emission (e.g., in order to obtain high brightness red LEDs). Nanopillar/wire structures show the advantage of a better in- and out-coupling of the source and emitted light, respectively. This results in absorbance enhancement as well as higher light extraction for the same type of structuring. Two top-down fabrication processes were demonstrated in order to obtain high quality GaInP nanopillars/wires, obtained from lattice matched GaInP layers grown on GaAs (100). A passivation treatment was used in order to obtain high optical quality GaInP nanopillar arrays, with a carrier lifetime in the order of 0.6 ns and increased photoluminescence intensities of ~5 times. A simulation study was performed to design the optimal GaInP nanowire array structuring for absorption of blue and green light. Similar GaInP nanopillar arrays were fabricated, based on this study. A GaAs sacrificial layer was used below the fabricated structures in order to obtain substrate-free structures. The structures were embedded in a PDMS film in order to study their optical and color conversion properties. The nanowire/pillar structuring showed an almost 100% absorption of (blue) incident light. A photoluminescence study was used to demonstrate the color conversion properties, indicating favorable properties for the nanowire/pillar arrays.