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GaAs nanopillar arrays with suppressed broadband reflectance and high optical quality for photovoltaic applications
KTH, School of Information and Communication Technology (ICT), Material Physics, Semiconductor Materials, HMA.ORCID iD: 0000-0002-2069-2820
KTH, School of Information and Communication Technology (ICT).
KTH, School of Information and Communication Technology (ICT), Material Physics, Semiconductor Materials, HMA.
Lund University.
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2012 (English)In: Optical Materials Express, ISSN 2159-3930, Vol. 2, no 11, 1671-1679 p.Article in journal (Refereed) Published
Abstract [en]

We report on fabrication and optical characterization of GaAs nanopillar (NP) arrays, obtained using a combination of low-cost mask generation by self-assembled silica particles (nanosphere lithography) and dry etching. Tapered structures (conical and frustum NP arrays) are fabricated by appropriate optimization of process parameters. Significant suppression of surface reflectance is observed for both geometries over a broad wavelength range. Simulations, based on finite difference time domain (FDTD) method, show good agreement with reflectivity measurements and serve as a guideline for design of NPs and understanding their interaction with light. A combination of wet chemical etching and sulfur-based passivation of GaAs NPs, results in more than one order of magnitude enhancement in PL intensity and recovery of PL line-width, which is very promising for photovoltaic applications.

Place, publisher, year, edition, pages
2012. Vol. 2, no 11, 1671-1679 p.
Keyword [en]
Solar-Cells, Efficiency, Absorption, Sulfide, Limit
National Category
Other Physics Topics Nano Technology
Identifiers
URN: urn:nbn:se:kth:diva-104526DOI: 10.1364/OME.2.001671ISI: 000310647500023Scopus ID: 2-s2.0-84870370508OAI: oai:DiVA.org:kth-104526DiVA: diva2:564981
Funder
Swedish Research CouncilEU, FP7, Seventh Framework Programme
Note

QC 20121210

Available from: 2012-11-05 Created: 2012-11-05 Last updated: 2014-11-04Bibliographically approved
In thesis
1. Top-down Fabrication Technologies for High Quality III-V Nanostructures
Open this publication in new window or tab >>Top-down Fabrication Technologies for High Quality III-V Nanostructures
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

III-V nanostructures have attracted substantial research effort due to their interesting physical properties and their applications in new generation of ultrafast and high efficiency nanoscale electronic and photonic components. The advances in nanofabrication methods including growth/synthesis have opened up new possibilities of realizing one dimensional (1D) nanostructures as building blocks of future nanoscale devices. For processing of semiconductor nanostructure devices, simplicity, cost effectiveness, and device efficiency are key factors. A number of methods are being pursued to fabricate high quality III-V nanopillar/nanowires, quantum dots and nano disks. Further, high optical quality nanostructures in these materials together with precise control of shapes, sizes and array geometries make them attractive for a wide range of optoelectronic/photonic devices.

This thesis work is focused on top-down approaches for fabrication of high optical quality nanostructures in III-V materials. Dense and uniform arrays of nanopillars are fabricated by dry etching using self-assembly of colloidal SiO2 particles for masking. The physico-chemistry of etching and the effect of etch-mask parameters are investigated to control the shape, aspect ratios and spatial coverage of the nanopillar arrays. The optimization of etch parameters and the utilization of erosion of etch masks is evaluated to obtain desired pillar shapes from cylindrical to conical. Using this fabrication method, high quality nanopillar arrays were realized in several InP-based and GaAs-based structures, including quantum wells and multilayer heterostructures. Optical properties of these pillars are investigated using different optical spectroscopic techniques. These nanopillars, single and in arrays, show excellent photoluminescence (PL) at room temperature and the measured PL line-widths are comparable to the as-grown wafer, indicating the high quality of the fabricated nanostructures. The substrate-free InP nanopillars have carrier life times similar to reference epitaxial layers, yet an another indicator of high material quality. InGaAs layer, beneath the pillars is shown to provide several useful functions. It effectively blocks the PL from the InP substrate, serves as a sacrificial layer for generation of free pillars, and as a “detector” in cathodoluminescence (CL) measurements. Diffusion lengths independently determined by time resolved photoluminescence (TRPL) and CL measurements are consistent, and carrier feeding to low bandgap InGaAs layer is evidenced by CL data. Total reflectivity measurements show that nanopillar arrays provide broadband antireflection making them good candidates for photovoltaic applications.  A novel post etch, sulfur-oleylamine (S-OA) based chemical process is developed to etch III-V materials with monolayer precision, in an inverse epitaxial manner along with simultaneous surface passivation. The process is applied to push the limits of top-down fabrication and InP-based high optical quality nanowires with aspect ratios more than 50, and nanostructures with new topologies (nanowire meshes and in-plane wires) are demonstrated.  The optimized process technique is used to fabricate nanopillars in InP-based multilayers (InP/InGaAsP/InP and InP/InGaAs/InP). Such multilayer nanopillars are not only attractive for broad-band absorption in solar cells, but are also ideal to generate high optical quality nanodisks of these materials. Finally, the utility of a soft stamping technique to transfer free nanopillars/wires and nanodisks onto Si substrate is demonstrated. These nanostructures transferred onto Si with controlled densities, from low to high, could provide a new route for material integration on Si.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. xvi, 75 p.
Keyword
III-V nanostructures, colloidal lithography, top-down fabrication, dry etching, quantum well, multilayer structures, nanowires, nanopillars, nanodisks, mono-layer etching, surface passivation, photoluminescence, carrier life time, total reflectivity, photonic crystals, nanomesh
National Category
Physical Sciences Nano Technology
Identifiers
urn:nbn:se:kth:diva-117766 (URN)978-91-7501-633-7 (ISBN)
Public defence
2013-02-25, Sal/Hall C1, KTH-ICT, Electrum, Isafjordsgatan 26, Kista, 10:00 (English)
Opponent
Supervisors
Note

QC 20130205

Available from: 2013-02-05 Created: 2013-02-04 Last updated: 2013-11-11Bibliographically approved
2. Ensemble and Individual III-V Semiconductor Nanopillars: Optical Properties and Applications
Open this publication in new window or tab >>Ensemble and Individual III-V Semiconductor Nanopillars: Optical Properties and Applications
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Optical properties of semiconductor nanowires (NWs)/nanopillars (NPs), as individual or ensemble, have attracted significant research interest in recent years. Their potential applications range from solid-state lighting, photovoltaics, lasing and nonlinear optics to sensing and life sciences. Many of III-V NWs/NPs are particularly interesting for optoelectronic and photovoltaic applications, because of their direct band gap, high refractive index and superior electrical properties. These properties are beneficial for use in next generation solar cells by reducing the active cell thickness, while maintaining high efficiency. Furthermore, high second order nonlinearity coefficients of many III-V materials, for example GaAs and GaP, enhanced electric fields and tight confinement of optical modes make nanowaveguide geometries ideal for nonlinear effects.

The focus of this thesis is on the fabrication of III-V NPs, their optical properties and applications. Different methods for fabrication of NPs (top-down approach) are proposed. The fabricated NPs show a broadband suppression of reflectance, which is particularly an interesting feature for photovoltaic applications. The effect of the shape and geometry of GaAs NPs on their reflectance spectra is investigated and the experimental data show a very good agreement with the simulations. In order to decrease surface recombination in the fabricated GaAs NPs, a sulfur-based chemical passivation method was used, resulting in the recovery of photoluminescence (PL) linewidth and enhancing the PL intensity for more than an order of magnitude. Moreover, a unique wafer-scale self-organization process for generation of InP NPs is demonstrated. As a proof of concept, the self-organized InP NPs were used to fabricate solar cell devices. For fabrication of InP NP solar cells, epitaxial overgrowth of NPs arrays was used to realize p-n junctions. A significant increase in the open circuit voltage (0.13 V) of the NP solar cell was obtained after surface passivation.

Second-harmonic generation (SHG) was experimentally observed from GaP NP waveguides (single and in arrays) with vertical geometry. The generated second- harmonic light was analyzed with respect to the size of the NP waveguides and the corresponding effects of surface and bulk nonlinearities. In case of individual NPs, SHG was analyzed considering different modal excitations in GaP NPs. It was demonstrated that by varying the NP diameter and changing the pump polarization, it is possible to alter the field distribution of the radiated SHG light. The importance of tight confinement of the pump in the NP waveguides and consequently the longitudinal component of the electric field in this geometry is shown. A method was proposed to distinguish between surface and bulk contributions in SHG, which also addressed how to employ surface SHG to enhance the generated light. The proposed method was used to estimate the nonlinear coefficient and the effective thickness of the nonlinear region at the surface of GaP NP waveguides. Based on these findings, the corresponding nonlinear coefficient at the surface is estimated to be approximately 15 times higher, compared to the bulk. These findings, suggest that NPs/NWs (in this case GaP NPs) are potential alternatives for future nonlinear nanophotonic devices. Additionally, the SHG light from single GaP NPs are promising candidates for ultrafast light sources at nanoscopic scale, with potential applications in sensing, bio and single cell/ molecular imaging.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. xiii, 68 p.
Series
TRITA-ICT/MAP AVH, ISSN 1653-7610 ; 2014:14
Keyword
nanopillar, nanowires, nanophotonics, nanofabrication, III-V semiconductors, photovoltaics, second-harmonic generation, top-down approach, colloidal lithography, antireflection, modal dispersion, polarization, nanowaveguide, indium phosphide, gallium arsenide, gallium phosphide
National Category
Nano Technology
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-155205 (URN)978-91-7595-333-5 (ISBN)
Public defence
2014-11-26, Sal/hall 205, Electrum, KTH-ICT, Kista, 10:00 (English)
Opponent
Supervisors
Note

QC 20141104

Available from: 2014-11-04 Created: 2014-11-03 Last updated: 2014-11-04Bibliographically approved

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