WP1: High throughput nanodimensional characterisation of NWs
The aim of this work package is to develop traceable methods for high throughput nanodimensional characterisation of NW energy harvesters (> 108 NWs/cm2) including 3D form (cylindrical, prismatic, pyramidal) and sidewall roughness.
The fabrication of the NW structures required for this work package will be carried out in Task 1.1. In order to allow a meaningful characterisation of NW energy harvesters, the nanodimensional characterisation of individual NWs must be performed. For this, traceable methods will be developed for the measurement of diameter, height, surface roughness, and pitch (Task 1.2), and sidewall roughness, vertical angle and 3D form (Task 1.3). These will be compared with ensemble measurement methods (scatterometry, MME), applying suitable scanning strategies to ensure comparability and sufficient statistical relevance of the local methods. Arrays of silicon NWs will be fabricated with different densities, average nanowire diameters (20 nm to 100 nm), average lengths and aspect ratios using VLS growth, to enable the development of methods for an accurate determination of all statistical parameters (mean values and standard deviations). Since it is not clear which ensemble method provides sufficient and optimum sensitivity with respect to the different individual structure parameters, different methods such as goniometric scatterometry, imaging scatterometry), coherent scanning Fourier scatterometry, and spectroscopic and angle-resolved Mueller ellipsometry will be tested for each relevant structure parameter, and the optimum metrology method will be identified. Methods for high throughput characterisation will be developed in Task 1.4. The effect of combining the nanodimensional, nanoelectrical, nanomechanical, and thermoelectrical characterisation of NW energy harvesters on the measurement uncertainty will be studied in Task 1.5.
WP2: High throughput reliable nanoelectrical characterisation of NW solar cells
The aim of this work package is to develop traceable measurement methods for high throughput nanoelectrical characterisation of semiconductor NW solar cells based on conductive AFM and MEMS-SPM techniques for current-voltage (I-V) characteristics to determine the efficiency of individual NWs with an expected uncertainty of 10 %. SMM will be used for doping concentration variation (between 1015 atoms/cm3 to 1020 atoms/cm3) with an accuracy better than 10 % and MEMS-SPM will be used for lateral resolution below 50 nm for a scanning speed vtip up to 1 mm/s, a lateral scanning range of 50 µm and the MEMS electronic bandwidth of 1 kHz. The reliability of the nanoelectrical characterisation will be confirmed by correlation to measurements performed by CL, EBIC, Photoluminescence (PL) and contactless capacitance voltage (CV) techniques.
This will require the development and validation of suitable conductive AFM techniques to perform traceable measurements of I-V characteristics on individual pure doped NWs and individual NW junctions (Task 2.1). It will also require a precise calibration of SMM involving the development of reference materials and CL, EBIC, PL and contactless CV techniques to carry out accurate measurements of doping concentration profile on individual NWs (Task 2.2). In Task 2.3, the design and fabrication of reference materials (multilayer samples and arrays of pure doped NW) and NW junction samples used for Task 2.1 and Task 2.2, will be carried out. In addition, complete NW solar cell devices with interconnected NW junctions will be fabricated using the same batch of NW junctions to make possible the comparison of the photovoltaic parameters between individual NW junctions and the NW solar cells.
WP3: High throughput nanoelectromechanical measurement methods
The aim of this work package is to develop contact-based high throughput methods for nanomechanical and nanoelectrical characterisation of i) motion-to-electricity energy harvesting nanodevices and ii) nanoelectromechanical properties of piezoelectric nanowires used for mechanical energy harvesting nanodevices in particular.
A new traceable MEMS-SPM with a depth resolution of 10 pm and an indentation force up to 10 mN will therefore be developed in Task 3.1 for fast areal topography measurement (up to 1 mm/s) and simultaneously nanomechanical and electrical measurements. The tip-object interaction at high scanning speed will be modelled in Task 3.2, and thereafter utilised to estimate and compensate for the measurement errors in SPM nano-dimensional and nanomechanical measurements. In Task 3.3, the afore mentioned measurement system and models will be validated with the dataset generated by the new MEMS-SPM and commercial nanomechanical AFMs with different modes. In Task 3.4, relevant reference materials and artefacts, innovative moulded pyramidal AFM probes for nanomechanical testing and conductive AFM probes fabricated by nano‑printing for topography and electrical measurement will be developed to serve the whole work package. To macroscopically evaluate the efficiency of energy harvesting nanodevices based on mechanical‑to‑electrical transducers, and to link the physical properties of nanowires and the performance of energy harvesting EH nanodevices at macro scale, in Task 3.5 a new micro-shaker testing platform will be developed.
WP 4: Fast areal thermal imaging of NWs
The aim of this work package is to develop and validate traceable measurement techniques for thermoelectrical characterisation, based on fast areal thermal imaging, of NWs with a target uncertainty of 10 % for flat isotropic samples under different scanning speeds and tip-surface contact. In Task 4.1, contact measurements based on SThM will be used to characterise thermal properties of individual NWs. In Task 4.2, a MEMS platform will used as an alternative for thermal properties measurements with higher accuracy but also higher experimental demands. In Task 4.3, the non-contact techniques like thermoreflectance and infrared imaging will be adapted for fast areal imaging of temperature and thermal properties. As a demonstrator for these techniques, in Task 4.4 solar cell samples will be used, on which the local defects will be visualised.