High throughput metrology for nanowire energy harvesting devices
Energy harvesting from renewable sources (solar, heat and movement) is a prominent solution to create small amounts of electrical energy in areas of difficult access, and energy harvesting devices have much potential to address our world energy problems. Nanowire (NW) based energy harvesting systems have achieved encouraging progress, but due to nanometre (nm) dimensions of the wires and large size (m2) of the devices, they also bring challenges for testing and characterisation. Average properties of energy harvesting devices can be measured, but a quantitative link and correlation between the performance of single NWs and that of the overall device is lacking. This project aims to develop reliable and high throughput metrology for the quality control of NW energy harvesting systems.
Limited fossil fuel-based energy resources and their negative effect on the environment have resulted in enormous efforts being made over several decades to make energy supply and consumption more sustainable. Scavenging energy from renewable sources like solar, waste heat and mechanical movement is seen as a prominent solution to our world energy problems. The overall market for energy harvesting is expected to grow from USD 311 Million in 2016 to USD 646 Million by 2023. Nowadays, about 1.7 % of world electricity is provided by solar cells.
Over the past two decades, major efforts have been made to develop energy harvesting devices from macro- and microscales down to nanoscale. Due to their extremely small physical size and high surface to volume ratio, NW based energy harvesting systems, including photovoltaic solar cells, thermoelectrical and electromechanical energy nanogenerators, have gained tremendous interest and encouraging progress has been achieved. In particular, it has been confirmed that the efficiency of NW solar cells can be enhanced from 17.8 % currently to its ultimate limit of 46.7 % by means of nanophotonic engineering.
While novel designs and materials for various energy harvesting devices indeed offer many potential benefits, they also bring challenges for testing and characterisation. For example, the quantitative link and correlation between the performance of a single NW and that of the overall device is still missing. Moreover, no reliable metrology for large area NW arrays (from cm2 to several m2) with diameters between 50 nm and 1 µm is currently available. Quality control of these energy harvesting systems is therefore highly challenging, and high throughput metrology is necessary, which requires the development of traceable measurement methods and models for the characterisation of NW energy harvesters, solar cells and devices.
The overall goal of this project is the traceable measurement and characterisation of energy harvesting devices based on vertical NW. The specific objectives are:
- To develop traceable measurement methods for high throughput nanodimensional characterisation of NW energy harvesters (> 108 NWs/cm2) including 3D form (cylindrical, prismatic, pyramidal) and sidewall roughness.
- To develop traceable measurement methods for high throughput nanoelectrical characterisation of semiconductor NW solar cells using conductive Atomic Force Microscopy (AFM) for current-voltage characteristic in the current range 100 fA to 1 mA, Scanning Microwave Microscopy (SMM) for doping concentration variation (between 1015 and 1020 atoms/cm3 with an accuracy better than 10 %), and Micro-Electro-Mechanical Systems - Scanning Probe Microscopy (MEMS-SPM) for lateral resolution (< 50 nm).
- To develop and validate traceable measurement methods and models for high throughput nanomechanical characterisation of NW devices, and electromechanical energy harvesters taking into account local bending and compression of NWs including the development of a traceable MEMS-SPM (< 10 pm depth resolution) for fast simultaneous nanomechanical and electrical measurement of semiconductor and polymer piezoelectric NWs.
- To develop and validate traceable measurement techniques for thermoelectrical characterisation, based on fast areal thermal imaging, of NWs (thermal conductivity lower than 10 W/(mK) with an uncertainty < 10 %) under different scanning speeds and tip-surface contact.
- To facilitate the take up of the technology and measurement infrastructure developed in the project by the measurement supply chain, standards developing organisations (IEC TC 113 and IEC TC 82) and end users (solar cell and energy generator manufacturers).
These objectives will require large-scale approaches that are beyond the capabilities of individual National Metrology Institutes and Designated Institutes. To enhance the impact of the research, the involvement of the appropriate user community such as industry, standardisation and regulatory bodies is intended, both prior to and during methodology development.
Progress beyond the state of the art
High throughput nanodimensional characterisation of NW energy harvesters
A set of demonstrator NW arrays made of different materials (e.g. Si, ZnO and GaN) with diameters below 100 nm, cylindrical shape, aspect ratios between 20 and 100, densities between 108 NWs/cm2 and 109 NWs/cm2 and sidewall roughness ranging from nm to tens of nm will be developed for the first time.
A new high throughput optical measuring method, imaging scatterometry, which works by capturing a series of images of an illuminated sample at several wavelengths, will be used and further developed in this project. Large scale and reliable methods for defect characterisation will be developed, either based on optical ensemble methods on their own or by sophisticated combination with high resolution microscopy methods in a hybrid metrology approach.
The complex 3D geometry of the NW arrays, their different materials and large size set metrological requirements that are beyond state-of the art techniques. The combination of advanced AFM and Scanning Electron Microscopy (SEM) metrology together with sophisticated optical metrology tools provide unique possibilities to tackle this challenge. Access to the geometrical form and size of the nanostructures are possible locally (e.g. AFM) as well as globally (i.e. scatterometry and Mueller matrix ellipsometry - MME).
Efficient modelling capabilities based, e.g. on FEM will be developed. Thus, at the end of this project, for the first time, a high-volume metrology for reliable determination of order parameters of large arrays of coordinated nanosystems such as Si nanowire arrays will be available.
High throughput nanoelectrical characterisation of semiconductor NW solar cells
The project will go beyond the state of art by developing traceable methods to perform I-V spectroscopy on individual NW junctions in the dark and under illumination using a new conductive AFM equipped with a MEMS SPM probe and coupled to a commercial version of a contact-mode AFM. The expected relative uncertainty on the data determination is in the order of 10 %. In addition, using a technique that allows 3D nano-printing conductive tip materials, new optimised tip geometries for nanowire characterisation will be realised, that will allow the nanoelectrical characterisation of NW.
Traceable Scanning Microwave Microscope (SMM) measurements of doping concentration profile will be carried out for the first time by probing individually the top of vertical doped NWs (attached to the substrate on the bottom part) in non-destructive contact mode and under environmental conditions compatible with NW solar cell design. The target relative uncertainty on the doping concentration is below ± 10 %.
High throughput nanomechanical characterisation of NW devices
A MEMS-SPM head capable of mechanical measurements will be extended by the functionality of electrical measurements. This new conductive MEMS-SPM with 10 pm resolution and 10 mN maximum testing force allows traceable measurement of direct and converse piezoelectrical properties of NW used in motion‑to‑electricity EH nanodevices. Then, for the first time, a conductive AFM will be equipped with this new MEMS‑SPM head with conductive diamond probes and coupled to a powerful contact-mode AFM developed for piezo generation measurements.
In addition, innovative diamond AFM probes with moulded pyramid-like tips and novel 3D nano-printed conductive tips will be developed which allow stable simultaneous electric and mechanical measurements of individual NWs. An AFM probe equipped with a standardised (ISO 14577) indenter tip will, for the first time, allow to link the mechanical measurements of nanomaterials using nanomechanical AFMs with those nanomechanical measurements using traditional validated instruments.
To further increase the throughput of mechanical measurements a microprobe-based high-speed contact resonance (CR) microprobe will be developed. The rate of tracked resonance frequency samples shall be increased from 10 to > 100 kSamples/sec.
Fast areal thermal imaging of NWs
The Scanning Thermal Microscopy (SThM) device will be improved to decrease the uncertainty of thermal conductivity measurements from 20 % to 10 % in the case of flat isotropic samples and for materials with a thermal conductivity lower than 10 Wm-1K-1. This will be combined with non-contact techniques for fast imaging of local defects in active devices based on nanowire arrays, like solar cells.
High throughput nanodimensional characterisation of NW energy harvesters
Comprehensive fabrication technologies including bottom-up approach, nanospheres self-assembly and metal assisted chemical etching, optical lithography and cryogenic deep reactive ion etching (cryo-DRIE), Zn sputtering/annealing and CBD, and Metalorganic vapor phase epitaxy (MOVPE) have been, for the first time, utilized to fabricate a set of NW arrays made of typical materials (Si, ZnO and GaN).
NW samples with different nanowire doping types (p- and n-type), diameters (100 nm and 250 nm), heights (from 300 nm up to 8 µm) and periodicity (190 nm and 400 nm) have been characterised by SEM and room temperature photoluminescence that is proportional to the porosity of the nanowires. For the partners who requested “findme” structures, platinum patterns have been deposited by a DualBeam system to identify the best Regions of Interest (RoI). All these samples lay a solid foundation for the consortium.
The partners selected suitable sample parameters for the measurement equipment and agreed with the manufacturing partners to circulate multiple sets of samples fabricated by different technologies and, after inspecting several sets iteratively, choose the best sample set to perform the detailed measurements. So far, the first set of samples were inspected by atomic force microscopy and by scatterometry. Definitions for the measurands have been established and measurement results were provided as feedback to the manufacturing partners. The manufacturing partners have modified their manufacturing process accordingly and will provide a second batch with improved sample properties.
High throughput nanoelectrical characterisation of semiconductor NW solar cells
A comprehensive study is in progress to identify the most stable measurement’s conditions for C-AFM experiments. This includes a study of stable electrical contact area using a large spectrum of AFM probes (single crystal diamond coated probes, fully metallic probes and metal coated probes). To this end, current/resistance measurements have been carried out in contact mode on calibration samples of known resistance values (10 kΩ, 1 MΩ and 100 MΩ). The effects of vertically applied force between the tip and the sample, contact time, applied bias voltage (I-V curves) and friction are all being investigated.
Initial experiments to measure the electrical properties on single pure n-doped and p-doped GaAs nanowires (stabilized in a surrounding polymer matrix) have been made. Measurements show that these NWs in the configuration mentioned here are indeed stable vertically and AFM imaging is straightforwardly achievable in both contact and intermittent contact modes. However, no initial currents could be measured on the nanowires, which is currently a subject of investigation to unravel the origin of such observations. Improvements on the samples’ preparations and/or the experimental measurement’s setup are expected to be identified as an outcome of these investigations.
Assembly of a conductive AFM tip into the passive gripper of the MEMS-SPM for a commercial AFM has been intensively discussed, leading to a practical solution. An ad-hoc 3D-printed adaptor element has been designed and fabricated to allow first tests expected to identify the practical feasibility of the MEMS integration to the Nanoobserver.
GaAs multi-layer samples, with various levels of doping concentrations (in the range of1017 cm-3 - 1019 cm-3), have been designed and fabricated to be used as reference samples for the SMM calibration in terms of dopant concentration. The dopant concentrations of these samples have been measured by Secondary Ion Mass Spectrometry (SIMS) techniques. Additional reference samples based on doped-GaN multi-layers have also been fabricated with various concentrations of n or p-dopants (1016 cm-3 to 1018 cm-3). Associated with these GaN multi-layer samples, corresponding single doped-layers have been systematically grown for quantification of the dopant concentration by the Hall-Effect.
First dopant concentration measurements have been carried out on an “unknown” GaAs multi-layer sample using modified Short Open Load (m-SOL) method to calibrate the SMM. A good agreement has been found within a range of 5 % to 10 % on the dopant concentration ratios between the SMM and SIMS techniques. Investigations are in progress regarding measurements of dopant concentration in absolute values. In addition, a second method based on dC/dV measurements is being currently investigated.
A large set of pure silicon and germanium NW samples has been fabricated with different NW dimensions, doping types and densities (1017 cm-3 – 1019 cm-3), and thin film coatings. Additional samples of silicon micro-wires have been prepared with a Cr/Au metallic contact layer on top of each wire. The wires are embedded in a photoresist matrix without further overall coatings.
Another set of p-type GaAs NW samples has been fabricated and cover various levels of dopant concentrations ranging from 0.5·1018 cm-3 to 3.4·1018 cm-3. The NWs were encapsulated in a matrix to ensure good mechanical stability. No additional metallic coating has been added on the top of these NWs. Instead, chemical etching has been performed to improve a good top electrical contact.
A wide range of p-type and n-type GaAs thin films with various carrier concentrations (2·1017 cm−3 - 1·1019 cm−3) have been fabricated and analyzed using cathodoluminescence (CL). These samples were used with a rigorous analysis refinement to calibrate CL measurements for the quantitative assessment of p-type and n-type carrier densities. This has been applied to analyze the doping concentration profiles in single GaAs nanowires. Difficulties in obtaining highly doped n-type GaAs NWs are demonstrated through assessments of electron concentrations’ measurements. Overall, it has been shown how CL mapping can be used to provide a contactless, quantitative assessment of active doping at the nanoscale, for both n-doped and p-doped GaAs nanowires, over a wide range of carrier concentrations. These results were published as a series of two articles.
Electron beam induced current (EBIC) microscopy measurements were carried out on the nanowire samples. Results show that purely axial well-defined structure and low wire-to-wire dispersion is actually obtained by reverting the doping order. More specifically by getting p-base instead on n-base p-n junctions. Additionally, the hole concentration in p-doped segments on a nanowire is quantified at 1018 cm-3 using EBIC profiles. This is reached with no induced morphology degradation of the nanowire.
Conductive Berkovich-like diamond AFM probes with a single crystal tip where fabricated. Improvements on the initially obtained tip heights are being conducted. Additional FIB milling techniques are also used to improve tips’ sharpness.
High throughput nanomechanical characterisation of NW devices
With the help of FEM analysis, two MEMS-SPM heads with the maximum indentation force up to 10 mN for high-throughput nanoelectromechanical measurements of nanowires have been designed and fabricated. A new micro-controller-based drive and sensing system with a resolution of 20 aF for the conductive MEMS-SPMs has been developed. A stand-alone high-side current sensing system with a bandwidth up to 1 kHz has been developed. The approach to integrate the MEMS-SPM head with clamped diamond tip into a Resiscope has been defined. The functionality of the conductive MEMS-SPM has been tested by a home-developed test setup with AFM probes including conductive diamond coated tips (CDT-CONTR, NANOSENSORS™) and single crystal diamond tips (NM-RC, Adama Innovations). First measurement results on a gold-coated silicon <100> sample have been obtained, revealing that the prototypes of conductive MEMS-SPM can achieve a current resolution of sub-nA. Quantitative mechanical calibration of the MEMS-SPM heads using a self-developed micro-force measurement facility has been performed, which validates that the nonlinearity of the MEMS-SPM over the whole indentation range of 10 µm is less than 0.3 %. To traceably characterize the MEMS-SPM head and drivers, a laser interferometer has been designed and constructed.
The fast mass-spring model algorithm has been rewritten to be able to handle different nanowire and probe geometries and calculations of probe-sample interaction started. Models were adapted to the real nanowire systems as discussed. In parallel a validation calculation is being setup using Finite Element Method.
Electronics and firmware for a high-speed CR setup (lock-in amplifier, signal generator, micro controller) have been designed and fabricated. A software has been developed for operating a nano positioning table for measuring area profiles of CR frequencies/spectra and force-distance curves with GaN NW arrays. Preliminary line-scan measurements were performed with silicon wires using the 1st, 2nd and 3rd mode of a CAN50-2-5 piezoresistive cantilever at 14 kHz, 44 kHz and 91 kHz, respectively, providing height and CR profiles. The components of the high-speed CR setup (µC & ADC, signal generator, 2-channel lock-in amplifier) are currently under test showing the expected performance of approx. 14 bits of resolution, a sample rate up to 6 kS/s, and a bandwidth of 1 kHz to 1 MHz. Due to its modular architecture, this setup can be adjusted to future requirements by replacing the limiting components without the need of developing a completely new system.
Modelling of the tip-surface interaction has been performed by fitting quasistatic measurements using the Hertzian contact theory. Static force-dependent finite element modelling (FEM) was done with cantilever probes with diamond tips of stable radius (2 µm) and verified using quasistatic measurements on material with different Young’s moduli.
Fabrication of specialized AFM probes using a focused ion beam (FIB) facility and 3D nano-printing technology has been started. The world-first silicon AFM Berkovich tip has been fabricated using FIB and was characterised using SEM. World-first molded single crystal Berkovich-like tips have been successfully fabricated on silicon wafers.
Six samples of ZnO NW arrays on Si and one sample of GaN NW array on sapphire have been produced. Planar Si and GaN reference samples of different doping concentrations were produced and sent to PTB.
A new microshaker with a bandwidth up to 10 kHz has been designed to characterize the averaged performance of nanowire-based energy harvesting devices. A first prototype based on additive manufacturing has been realized. Digital lock-in based measurement circuit for the new microshaker has been developed.
Fast areal thermal imaging of NWs
The specifications of sample configurations and MEMS designs have been defined and agreed. The first types of nanowires have been selected. Silicon nanowires with n+ type doping, length over 8 µm and diameter of 100 nm for the suspension platform have been prepared for thermal transport measurements. Two samples with plenty of solar cells based on SiNWs have been fabricated and delivered. ZnO nanowires were produced and their Young's modulus was initially tested.
The calibration facility for SThM probes has been updated to perform probe calibration for traceable thermal measurements with less influence of the calibration oven on the temperature gradients in the probe. FEM modelling of new suspended platforms has been performed to minimize the uncertainty of the thermal conductance measurement of individual nanowires below 10 %. The new design will enable measuring the conductance of the NW without contribution of the thermal contact resistance. SThM experiments using advanced sampling via force-distance-energy measurements were performed, showing less dependence on the probe-sample contact formation and topography artefacts. A two-probe setup has been modified for better control of the heating probe position in the z direction and measurements on simple microscale structures were performed to test the novel setup.
An initial set of measurements has been performed on an already traceable infrared microscopy setup, where the different operating conditions of solar cell samples are tested and discussed. Novel sample configurations for thermal measurements were suggested. In parallel, the thermoreflectance setup was created for the local temperature measurements and initial measurements were performed on simple reference samples.
To promote the uptake of project results and to share insights generated throughout the project, results have been shared broadly with scientific and industrial end-users. Five papers reporting project results have been published in peer-reviewed international journals and two others submitted. In addition, an article was published in Metrology and Hallmarking, the Bulletin of the Central Office of Measures. Fifteen presentations have been given at conferences and the project was presented to over 100 participants at two plenary sessions of NanoMaterials for Energy Applications. A stakeholder workshop has been held at the 21st Annual Metrology Congress CIM2021 in September 2021 in Lyon, France. 35 attendees representing industry, universities and national metrology institutes attended this hybrid workshop. Project activities were presented at Micro and Nano Engineering, Turin, in September 2021 and to NanoInnovation Rome in September 2021 by two oral contributions. The project and its preliminary mechanical research were also presented at the 53th Inter-University Conference on Metrology MKM2021 in September 2021 in Warsaw, Poland. The project has been presented at national and international standardization committees and posts regularly on social media through Instagram and LinkedIn. A website containing all the posts appearing in social media has also been set up and is periodically updated, in parallel to the official project site, for a wider public. The first edition of a webinar devoted to single nanowires nanomanipulation has also been broadcasted in August 2021 by INRIM with remote and in-presence project partners attendees. The number of people trained amounted to about 26 to 50.
Impact on industrial and other user communities
New contact-based measurement modes will enable industry to simultaneously measure dimensions, electrical, thermal and mechanical properties of surfaces such as elasticity, stress, adhesion and thickness of nanomaterials. Conductive AFM and SMM techniques combined with EBIC and CL techniques have the strong potential to describe overall performance, unwanted loss mechanisms, optimal operating frequencies and aging within one device. Together, these techniques will certainly fit the need of upcoming industrial production. Advanced diamond probe technology will provide the robustness and longevity currently lacking from contact sensors today.
In the first half of the project, industrial users have been contacted and relative key persons have been invited to join the Stakeholder committee. Additionally, the commercial exploitation of the novel microshaker tool is being explored, following a complete characterisation of the new tool using standard reference materials, and additionally through a review of the market. The application of the new high-speed CR setup on NW arrays developed within this project for commercially available products (e. g., KlettWelding, KlettSintering and ZnO/Si solar cell on glass) has also been assessed and explored.
Impact on the metrology and scientific communities
In the first half of the project, the partners have interacted with various scientific, metrological and industrial networks in order to present the preliminary results. Three new EMPIR European Metrology Networks (EMN) networks have also been contacted through the project representatives in these communities: Quantum Technologies, Advanced Manufacturing and Clean Energy Networks.
Experiences with advanced NW fabrication methods were documented in a “Report on the fabrication of NW array artefacts for demonstration of traceable measurement methods for high throughput nanodimensional characterization of NW energy harvesters (> 108 NWs/cm2) including 3D form”. First results have been presented by INRIM to VAMAS TWA2 Surface analysis in September 2021 and EURAMET TC Length in October 2021. This report will also be presented at NanoWire Week in April 2022, to more than 200 attendees, and MNE2022 in September 2022 to more than 100 attendees A set of process parameters for cryogenic etching of silicon NWs established at TUBS has been transferred to INRIM to accelerate the introducing procedure of their new Oxford cryo etcher.
Experiences of NW fabrication were exploited with MBE-grown highly boron- and phosphorus-doped silicon for diffusion experiments in NWs of 300-1200 nm in diameter at WWU Münster. Furthermore, for Li-ion-battery applications silicon wires of large height (up to 15 µm) and aspect ratio (up to 22) were fabricated. Detailed results have been published in a paper (https://doi.org/10.1038/s41598-021-99173-4), and will be presented at the Physical Colloquium, TU Kaiserslautern, in April 2022, and at the conference Sensoren und Messsysteme in May 2022, to more than 100 attendees.
A training course for members of the consortium on nanomanipulation and positioning of single nanowires on MEMS structures was held in July 2021. Measurement and training services/courses on nanodimensional characterization of NW arrays using scatterometry are under organization.
A one-to-one training for the consortium has been held in September 2021 at DFM, with the aim to demonstrate/investigate the in-situ performance of the MEMS-SPM head with integrated readout electronics developed by PTB, and to integrate the MEMS-SPM head into the commercial AFM (NEX20, Park) at DFM.
Impact on relevant standards
The metrological outputs of this project in the fields of NW solar cells have been presented to standardisation committees e.g. IEC TC 113 ‘Nanotechnology for electrotechnical products and systems’ and IEC TC 82 “Solar photovoltaic energy systems” to foster the creation of new standards. Good Practice Guides will be developed and disseminated to IEC TC 47 “Semiconductor devices”, ISO TC 164 “Mechanical testing of metals”, and the German VDI/VDE-GMA Technical Committee 3.41 “Surface Measurement Technology in the Micro- and Nanometer range” followed by standardisation of the new measurement modes.
LNE has presented the project at the IEC TC113 WG13 "Nano-enabled energy storage" meeting in November 2021. WG11 has recommended the establishment of one Preliminary Work Item (PWI) on “Metrology for nanowire energy harvesting devices”, which will establish a framework for standardization in these areas and initiate a minimum of one NWIP at the next meeting. At the plenary meeting in November 2021, IEC TC113 approved this PWI, to be led by LNE. .
Longer-term economic, social and environmental impacts
The metrology developed within the framework of this project will contribute to quality control of newly developed devices for energy harvesting and storage, and consequently help to promote and accelerate the development and fabrication and enable new nanotechnologies for renewable energy industry. This will strengthen Europe’s response to human-induced climate change.
The high throughput metrology for quality control of innovative energy harvesting and storage devices will substantially improve the competitiveness of the European semiconductor and small energy industries. The developed high throughput SPM techniques can also be applied for ultrafast quality control of ultra-precision workpieces, therefore enhancing the competitiveness of European manufacturing industry.