Improvements to differential voltage contrast and light and electron scanning beam analysis of solar cells

Differential voltage contrast (DVC) in conjunction with light and electron beam scanning (LEBEAMS) technique were used for measuring the electric potential, field, and charge distribution in solar cells. The DVC is based on enhancement or retardation of secondary electron emission, generated by an electron beam, due to local changes in the potential of a semiconductor device. The information provided by this technique is invaluable to the development of any device. Solar cells have been studied by the DVC technique, both under electrical bias (DVC) and under illumination (DVC in conjunction with LEBEAMS); however, the conditions of the previous did not replicate the normal illumination conditions of a solar cell. The goal of this research was to redesign and expand the previous LEBEAMS experiments to produce accurate profiles of quasi Fermi energies on solar cells.     

High saturation solar light beam induced current scanning of solar cells

The response of the electrical parameters of photovoltaic cells under concentrated solar irradiance has been the subject of many studies performed in recent times. The high saturation conditions typically found in solar cells that are subjected to highly concentrated solar radiation may cause electrically active cell features to behave differently than under monochromatic laser illumination, normally used in light beam induced current (LBIC) investigations. A high concentration solar LBIC (S-LBIC) measurement system has been developed to perform localized cell characterization. The responses of silicon solar cells that were measured qualitatively include externally biased induced cell current at specific cell voltages, I(V), open circuit voltage, V(oc), and the average rate of change of the cell bias with the induced current, DeltaV/DeltaI(V), close to the zero bias region. These images show the relative scale of the parameters of a cell up to the penetration depth of the solar beam and can be obtained with relative ease, qualifying important electrical response features of the solar cell. The S-LBIC maps were also compared with maps that were similarly obtained using a high intensity He-Ne laser beam probe. This article reports on the techniques employed and initial results obtained.     

Reversed scan direction reduces electron beam damage in EBSD maps

The deleterious effects of electron beam damage on high‐resolution electron backscatter diffraction (EBSD) maps of undeformed quartz are significantly reduced by scanning in the direction opposite to that dictated by widely used EBSD acquisition software. Higher quality electron backscatter patterns are produced when the electron beam moves progressively down the sample (the apparent ‘up’ direction in the resulting maps) for all step sizes where beam damage affects EBSD map quality (≤ ～0.4 μm in this study). The relative improvement associated with downward scanning increases as step size is reduced. A comparison of high‐resolution maps made in experimentally deformed quartz demonstrates that downward scanning reduces by a factor of ～2 the lower limit in step size relative to maps scanned in the conventional direction. The electron beam damages quartz at its point of entry, forming ～0.1‐μm diameter bumps visible in Scanning electron microscope (SEM) images. Downward scanning produces better results because it minimizes the flux of electrons through these loci of damaged crystal.     

A systematic investigation of the charging effect in scanning electron microscopy for metal nanostructures on insulating substrates

Scanning electron microscopy is perhaps the most important method for investigating and characterizing nanostructures. A well‐known challenge in scanning electron microscopy is the investigation of insulating materials. As insulating materials do not provide a path to ground they accumulate charge, evident as image drift and image distortions. In previous work, we have seen that sample charging in arrays of metal nanoparticles on glass substrates leads to a shrinkage effect, resulting in a measurement error in the nanoparticle dimension of up to 15% at 10 kV and a probe current of 80 ± 10 pA. In order to investigate this effect in detail, we have fabricated metal nanostructures on insulating borosilicate glass using electron beam lithography. Electron beam lithography allows us to tailor the design of our metal nanostructures and the area coverage. The measurements are carried out using two commonly available secondary electron detectors in scanning electron microscopes, namely, an InLens‐ and an Everhart–Thornley detector. We identify and discriminate several contributions to the effect by varying microscope settings, including the size of the aperture, the beam current, the working distance and the acceleration voltage. We image metal nanostructures of various sizes and geometries, investigating the influence of scan‐direction of the electron beam and secondary electron detector used for imaging. The relative measurement error, which we measure as high as 20% for some settings, is found to depend on the acceleration voltage and the type of secondary electron detector used for imaging. In particular, the Everhart–Thornley detectors lower sensitivity to SE₁ electrons increase the magnitude of the shrinkage of up to 10% relative to the InLens measurements. Finally, a method for estimating charge balance in insulating samples is presented.     

Position averaged convergent beam electron diffraction: Theory and applications

A finely focused angstrom-sized coherent electron probe produces a convergent beam electron diffraction pattern composed of overlapping orders of diffracted disks that sensitively depends on the probe position within the unit cell. By incoherently averaging these convergent beam electron diffraction patterns over many probe positions, a pattern develops that ceases to depend on lens aberrations and effective source size, but remains highly sensitive to specimen thickness, tilt, and polarity. Through a combination of experiment and simulation for a wide variety of materials, we demonstrate that these position averaged convergent beam electron diffraction patterns can be used to determine sample thicknesses (to better than 10%), specimen tilts (to better than 1 mrad) and sample polarity for the same electron optical conditions and sample thicknesses as used in atomic resolution scanning transmission electron microscopy imaging. These measurements can be carried out by visual comparison without the need to apply pattern-matching algorithms. The influence of thermal diffuse scattering on patterns is investigated by comparing the frozen phonon and absorptive model calculations. We demonstrate that the absorptive model is appropriate for measuring thickness and other specimen parameters even for relatively thick samples (>50 nm).     

Far‐reaching geometrical artefacts due to thermal decomposition of polymeric coatings around focused ion beam milled pigment particles

Focused ion beam and scanning electron microscope (FIB‐SEM) instruments are extensively used to characterize nanoscale composition of composite materials, however, their application to analysis of organic corrosion barrier coatings has been limited. The primary concern that arises with use of FIB to mill organic materials is the possibility of severe thermal damage that occurs in close proximity to the ion beam impact. Recent research has shown that such localized artefacts can be mitigated for a number of polymers through cryogenic cooling of the sample as well as low current milling and intelligent ion beam control. Here we report unexpected nonlocalized artefacts that occur during FIB milling of composite organic coatings with pigment particles. Specifically, we show that FIB milling of pigmented polysiloxane coating can lead to formation of multiple microscopic voids within the substrate as far as 5 μm away from the ion beam impact. We use further experimentation and modelling to show that void formation occurs via ion beam heating of the pigment particles that leads to decomposition and vaporization of the surrounding polysiloxane. We also identify FIB milling conditions that mitigate this issue.     

Orientation and phase mapping in the transmission electron microscope using precession‐assisted diffraction spot recognition: state‐of‐the‐art results

A recently developed technique based on the transmission electron microscope, which makes use of electron beam precession together with spot diffraction pattern recognition now offers the possibility to acquire reliable orientation/phase maps with a spatial resolution down to 2 nm on a field emission gun transmission electron microscope. The technique may be described as precession‐assisted crystal orientation mapping in the transmission electron microscope, precession‐assisted crystal orientation mapping technique–transmission electron microscope, also known by its product name, ASTAR, and consists in scanning the precessed electron beam in nanoprobe mode over the specimen area, thus producing a collection of precession electron diffraction spot patterns, to be thereafter indexed automatically through template matching. We present a review on several application examples relative to the characterization of microstructure/microtexture of nanocrystalline metals, ceramics, nanoparticles, minerals and organics. The strengths and limitations of the technique are also discussed using several application examples.     

A direct and quantitative image of the internal nanostructure of nonordered porous monolithic carbon using FIB nanotomography

A direct study of the shape, size and connectivity of nonordered pores in carbon materials is particularly challenging. A new method that allows direct three-dimensional (3D) investigations of mesopores in monolithic carbon materials and quantitative characterization of their physical properties (surface area and pore size distribution) is reported. Focused ion beam (FIB) nanotomography technique is performed by combination of focused ion beam and scanning electron microscope. Porous monolithic carbon is produced by carbonization of a resorcinol-formaldehyde gel in the presence of a cationic polyelectrolyte as a pore stabilizer.     

Determination of mean free path for energy loss and surface oxide film thickness using convergent beam electron diffraction and thickness mapping: a case study using Si and P91 steel

Determining transmission electron microscope specimen thickness is an essential prerequisite for carrying out quantitative microscopy. The convergent beam electron diffraction method is highly accurate but provides information only on the small region being probed and is only applicable to crystalline phases. Thickness mapping with an energy filter is rapid, maps an entire field of view and can be applied to both crystalline and amorphous phases. However, the thickness map is defined in terms of the mean free path for energy loss (λ), which must be known in order to determine the thickness. Convergent beam electron diffraction and thickness mapping methods were used to determine λ for two materials, Si and P91 steel. These represent best‐ and worst‐case scenario materials, respectively, for this type of investigation, owing to their radically different microstructures. The effects of collection angle and the importance of dynamical diffraction contrast are also examined. By minimizing diffraction contrast effects in thickness maps, reasonably accurate (±15%) values of λ were obtained for P91 and accuracies of ±5% were obtained for Si. The correlation between the convergent beam electron diffraction‐derived thickness and the log intensity ratios from thickness maps also permits estimation of the thickness of amorphous layers on the upper and lower surfaces of transmission electron microscope specimens. These estimates were evaluated for both Si and P91 using cross‐sectional transmission electron microscopy and were found to be quite accurate.     

Application of the focused ion beam technique in aerosol science: detailed investigation of selected, airborne particles

The focused ion beam technique was used to fabricate transmission electron microscope lamellas of selected, micrometre‐sized airborne particles. Particles were sampled from ambient air on Nuclepore polycarbonate filters and analysed with an environmental scanning electron microscope. A large number of particles between 0.6 and 10 µm in diameter (projected optical equivalent diameter) were detected and analysed using computer‐controlled scanning electron microscopy. From the resulting dataset, where the chemistry, morphology and position of each individual particle are stored, two particles were selected for a more detailed investigation. For that purpose, the particle‐loaded filter was transferred from the environmental scanning electron microscope to the focused ion beam, where lamellas of the selected particles were fabricated. The definition of a custom coordinate system enabled the relocation of the particles after the transfer. The lamellas were finally analysed with an analytical transmission electron microscope. Internal structure and elemental distribution maps of the interior of the particles provided additional information about the particles, which helped to assign the particles to their sources. The combination of computer‐controlled scanning electron microscopy, focused ion beam and transmission electron microscopy offers new possibilities for characterizing airborne particles in great detail, eventually enabling a detailed source apportionment of specific particles. The particle of interest can be selected from a large dataset (e.g. based on chemistry and/or morphology) and then investigated in more detail in the transmission electron microscope.     

Three-colour x-ray mapping using digital storage

Computer-controlled scanning of a scanning electron microscope (SEM) fitted with an energy-dispersive x-ray analyser has been carried out. This has been achieved by building an SEM interface module which controls the scanning of the electron beam and also provides a communication link between the x-ray analyser and an enhanced BBC micro. The digitised x-ray data pulses can be collected from three elements simultaneously to produce colour-coded x-ray distribution maps. Some such maps from two different specimens are presented and an effect of x-ray shadowing is clearly demonstrated. A comparison between images acquired using a frame size with 256 × 256 pixels and those with 128 × 128 pixels is made.     

In situ nanomechanical testing in focused ion beam and scanning electron microscopes

The recent interest in size-dependent deformation of micro- and nanoscale materials has paralleled both technological miniaturization and advancements in imaging and small-scale mechanical testing methods. Here we describe a quantitative in situ nanomechanical testing approach adapted to a dual-beam focused ion beam and scanning electron microscope. A transducer based on a three-plate capacitor system is used for high-fidelity force and displacement measurements. Specimen manipulation, transfer, and alignment are performed using a manipulator, independently controlled positioners, and the focused ion beam. Gripping of specimens is achieved using electron-beam assisted Pt-organic deposition. Local strain measurements are obtained using digital image correlation of electron images taken during testing. Examples showing results for tensile testing of single-crystalline metallic nanowires and compression of nanoporous Au pillars will be presented in the context of size effects on mechanical behavior and highlight some of the challenges of conducting nanomechanical testing in vacuum environments.     

Fabrication of various shaped tungsten micro pin arrays using micro carving technology

This paper describes a state of the art in micro-structuring high strength metallic materials. Tungsten micro pin arrays in a variety of shapes are fabricated using a micro carving technology, which combines laser beam machining and electrochemical etching processes. First, micro pin arrays were rough-shaped by laser beam machining along a pre-defined scanning path to control their structural shape. The micro pin array in this stage had near-conical shape of structures due to a recast layer. Next, the genuine shape of micro pin arrays came to the surface via electrochemical etching process to elute the recast layer into electrolyte. Quantitative elemental analysis with energy-dispersive spectroscopy (EDS) was implemented to characterize the formation of recast layer on a micro pin structure after the laser beam machining process. The atomic percentage EDS maps indicated that higher percentage of tungsten was detected on the core micro pin structure, whereas relatively large percentage of oxygen was found on the recast layer (O 9%, W 91% in the center area, and O 53%, W 47% in the outer area).     

Application of transmission electron microscopes to nanometre-sized fabrication by means of electron beam-induced deposition

Electron beam‐induced deposition was carried out using a scanning transmission electron microscope with a field emission gun to fabricate nanometre‐sized structures. A small amount of a metal–organic gas was introduced near the substrate in the microscope chamber, and focused electron beams were irradiated. Two‐ and three‐dimensional structures were fabricated by scanning the beam position. The minimum line width of the freestanding structures was 8 nm at a constant gas flux used. This line width of 8 nm is considered to be achieved by employing a high accelerating voltage, which leads to a small probe size, and the optimum scanning speed.     

晶体硅光伏电池的标准测试

为了建立太阳电池的标准测试规范流程,对影响太阳电池标准测试不确定度的各类因素进行了评价和筛选。基于太阳光模拟器、分光感度仪、IV测试仪、标准太阳电池等二级太阳电池标准测试设备与器件,开展了多项标准测试技术的研究。对电池测量过程中太阳电池模拟光源的空间不均匀性、时间不稳定性、仪器测量重复性、扫描方向导致的不确定度、电池反射率和透射率、面积测量不确定度、量子效率等各类影响因素进行了测量,给出了高效晶体硅太阳电池测量不确定度的测量流程,最终导出在现有实验室测量条件下的扩展测量不确定度为±3.94%。基于对常规太阳电池测试数据的比较,对常规电池测量方法进行了改进,将测量不确定度降低了0.19%。最后,提出了双面电池的精确测试流程和方法,为其它双面光电池的标准化测量提供了借鉴。     

Towards the imaging of Weibel–Palade body biogenesis by serial block face‐scanning electron microscopy

Electron microscopy is used in biological research to study the ultrastructure at high resolution to obtain information on specific cellular processes. Serial block face‐scanning electron microscopy is a relatively novel electron microscopy imaging technique that allows three‐dimensional characterization of the ultrastructure in both tissues and cells by measuring volumes of thousands of cubic micrometres yet at nanometre‐scale resolution. In the scanning electron microscope, repeatedly an image is acquired followed by the removal of a thin layer resin embedded biological material by either a microtome or a focused ion beam. In this way, each recorded image contains novel structural information which can be used for three‐dimensional analysis. Here, we explore focused ion beam facilitated serial block face‐scanning electron microscopy to study the endothelial cell–specific storage organelles, the Weibel–Palade bodies, during their biogenesis at the Golgi apparatus. Weibel–Palade bodies predominantly contain the coagulation protein Von Willebrand factor which is secreted by the cell upon vascular damage. Using focused ion beam facilitated serial block face‐scanning electron microscopy we show that the technique has the sensitivity to clearly reveal subcellular details like mitochondrial cristae and small vesicles with a diameter of about 50 nm. Also, we reveal numerous associations between Weibel–Palade bodies and Golgi stacks which became conceivable in large‐scale three‐dimensional data. We demonstrate that serial block face‐scanning electron microscopy is a promising tool that offers an alternative for electron tomography to study subcellular organelle interactions in the context of a complete cell.     

A unique determination of boundary condition in quantitative electron diffraction: Application to accurate measurements of mean inner potentials

We combine off-axis electron holography and electron shadow imaging to accurately determine the specimen thickness and the incident electron beam direction over the illuminated area of a crystal. We, furthermore, quantify the variations in diffraction intensity with position over the same area. This unique solution to the experimental boundary condition problem enables us to make precise measurements of mean inner electrostatic potentials and structure factors that are sensitive to the bonding characteristics of materials. In this paper, we present the results of mean-inner potential determination from silicon and the newly discovered magnesiumdiboride superconductor.     

Energy-dispersive X-ray microanalysis of aqueous biologic samples on bulk supports

The present investigation describes a modification of the liquid droplet technique that allows for the quantitative elemental analysis of small volumes (< 100 picoliters) of aqueous biologic samples using a scanning transmission electron microscope (Philips 400 HTG-STEM) equipped with an EDAX energy dispersive detector. Aliquots of samples and standards were micropipetted onto solid beryllium supports under paraffin oil. The oil was washed with organic solvents and the samples frozen and freeze-dried. The samples were excited in a Philips 400-HTG-STEM by scanning a 1-μm, 20-kV electron beam over the surface of the droplets, and the X-ray spectra were collected. Measured X-ray intensities in characteristic peaks were found to be linearly related to the concentration of various elements in the sample. This work demonstrates the feasibility of performing quantitative elemental analysis of minute samples and cells in a scanning transmission electron microscope equipped with an energy dispersive X-ray detector.     

Beam damage of polypropylene in the environmental scanning electron microscope: an FTIR study

The environmental scanning electron microscope allows the examination of virtually any specimen in a gaseous environment without the need for coating or drying. Experimental evidence, however, suggests that significant electron beam damage occurs in hydrated specimens. It is thought that water molecules, ionized by the electron beam, produce hydrogen and hydroxyl free radicals which attack the organic material of the sample. In order to elucidate the beam damage mechanisms, areas of polypropylene films were exposed to the electron beam at varying doses and exposure times under both hydrating and dehydrating conditions. The chemical changes occurring as a result of electron-beam irradiation were determined using Fourier transform infra-red microscopy. Direct interaction of the electron beam with the polymer results in extensive cross-linking. In the presence of water, free-radical-initiated reactions lead to hydrolysis and oxidation of the polymer.     

Spatially resolved cathodoluminescence of luminescent materials using an EDX detector

Panchromatic cathodoluminescence (CL) maps were collected in a scanning electron microscope equipped with an EDX (energy dispersive x-ray analysis) detector. These CL maps can readily be correlated with elemental maps obtained by EDX. Although EDX detectors are designed to be insensitive to light and therefore not optimized for high sensitivity CL measurements, high-resolution images can be obtained from luminescent materials without the need for additional hard- or software. The method was tested on highly luminescent BaAl₂S₄ : Eu²⁺ thin films that have a potential use in flat panel displays. The spectral response and linearity of the overall system was determined by means of monochromatic light sources, illuminating the sample through an optical fibre. We studied the response of the EDX detector to the intensity of the incoming light as well as the influence of the detector settings. The observations were explained by numerical simulations.