Analytical Microscopy and Imaging Science
NREL uses transmission/scanning electron and scanning probe techniques to measure the chemical, structural, morphological, electrical, interfacial, and luminescent properties on the nano to Angstrom scale.
We investigate such properties in a wide range of photovoltaic and semiconducting materials, with particular emphasis on extended defects and interfaces and how these affect device performance. A powerful approach for further device improvements is the linking of nano- and sub-nanoscale material and device properties to macro-scale device performance.
The following capabilities are used in NREL research, as well as in collaborative research with the Colorado School of Mines as part of the International Center for Multiscale Characterization.
We investigate the structure and chemistry of a wide range of materials, with particular emphasis on the structure and chemistry associated with defects and interfaces using transmission electron microscopy (TEM) and scanning TEM (S/TEM). This is particularly useful for determining how the microstructure affects derived material properties. Our microscopes are equipped to perform diffraction contrast, higher-resolution phase-contrast microscopy, high-angle annular dark-field (HAADF) microscopy, nanodiffraction, convergent beam electron diffraction, energy-dispersive X-ray spectroscopy (EDS), and electron energy-loss spectroscopy (EELS).
Our FEI Nova 200 Nanolab is used for multiple tasks at NREL. The primary task is sample preparation for high-resolution TEM/STEM analysis. Coupled with the in-situ electron beam induced current capabilities, this allows for extraction of specific features deemed detrimental to device performance such as grain boundaries, dislocations, and interfaces. This tool is also used for fiduciary marking, preparation of cross-sections for SEM imaging, and preparation of samples for cutting-edge SEM-based analysis. In addition, this instrument is equipped with electron backscatter diffraction (EBSD) and EDS capabilities, and it can operate in the temperature range of 300 K to 80 K.
Our JEOL 8900 Super Probe is used to provide Electron Probe Microanalysis (EPMA) for quantitative compositional analysis. It relies on wavelength-dispersive spectroscopy to identify and quantify elemental composition with a high degree of accuracy. Profiling against standards we have available, the EPMA covers the majority of the periodic table from boron to bismuth. Detection limits and sensitivity for the EPMA are in the 0.5%–1% atomic weight percentage range.
We use field-emission SEM (FESEM) to analyze the morphology/microstructure with high-spatial resolution (up to 1.2 nm). Backscattered-electrons (BSE) mode allows for elemental-sensitive imaging.
We use electron backscatter diffraction (EBSD) to analyze the crystallographic and structural properties, including texture, boundaries, and grain size. Capabilities include orientation maps that produce images of the crystallographic orientation of the sample surface with angle resolution of 0.1°, and spatial resolution as high as 10 nm.
Energy-dispersive X-ray diffraction spectroscopy (EDS) is used for standardless compositional analysis and can identify all elements above beryllium (Z=4), in spot, line, and mapping mode, with sensitivity as low as 0.5 weight %.
Cathodoluminescence, Electron-Beam-Induced Current
Cathodoluminescence (CL) and electron-beam-induced current (EBIC) are SEM-based characterization techniques that use the electron beam to generate electron-hole pairs for imaging the electrical and optical properties of semiconductors with high spatial resolution.
They are used to investigate the distribution of recombination centers in semiconductors, including extended defects such as dislocations and grain boundaries, stress fields, compositional fluctuations, and other important features—with an ultimate spatial resolution of about 50 nm. Our 6K cryogenic stage allows an energy resolution that is comparable to low-temperature photoluminescence (PL).
We can perform surface morphology and structure determination, nm-resolution imaging of electrical inhomogeneity, and nm-scale junction and defect studies. The platforms include atomic force microscopy (AFM) for surface morphology; Kelvin probe force microscopy (KPFM) for electrical potential, scanning spreading resistance microscopy (SSRM)/conductive-AFM (C-AFM) for local resistivity; scanning capacitance microscopy (SCM)/scanning capacitance spectroscopy (SCS) for carrier distribution; scanning tunneling microscopy/spectroscopy (STM/STS) for atomic/electronic structures; and near-field scanning optical microscopy (NSOM) for transport imaging by combining with e-beam exciting of minority carriers.
The Materials Science Center is part of the Materials, Chemical, and Computational Science directorate, led by Associate Lab Director Bill Tumas.