Cathodoluminescence

NREL uses cathodoluminescence (CL) 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.

How It Works

CL is photon emission stimulated by an electron beam. It has enabled imaging of the electronic and optical properties of semiconductor structures with an ultimate resolution of about 20 nm.

Both spectroscopy and imaging modes are accessible to most CL instruments. In spectroscopy mode, a spectrum of the emitted light is obtained over a selected area under observation in the electron microscope. In imaging mode, a map of the photon intensity (energy resolved with a spectrograph) is acquired instead.

Because these modes cannot be operated simultaneously, information is inevitably lost. At NREL, we have combined spectroscopy and imaging in one single mode: spectrum imaging. In simple terms, spectrum imaging acquires a spectrum for each pixel on the image. This is achieved by synchronizing the scanning of the electron beam with the spectrum acquisition by a multichannel photodetector. In our setup, we can select a silicon CCD or an InGaAs photodetector array, depending on the wavelength of interest (180 nm to 1800 nm). These cryo-cooled detectors offer high sensitivity and superior performance. Our approach to synchronization, based on linescan control, allows acquisition times of 10-20 ms per pixel. For a 125×125 pixel image, the time to acquire the entire spectrum series is about five minutes. When the acquisition is completed, a spectrum-imaging file is generated. The software developed for spectrum imaging (C2DIMAG) processes the spectrum series to reconstruct images of the photon emission (energy resolved) or to extract the spectrum from a selected area. C2DIMAG provides more advanced processing capabilities such as:

Mapping of the photon energy and full-width-half maximum of selected transitions
ASCII output
Quantitative-imaging mode
Pixel-to-pixel correlation
Spectrum linescan
Spectrum fitting routines.

Cathodoluminescence Capability Examples

Cathodoluminescence involves photon emissions stimulated by a scanning electron microscope, which uses electrons to form high-resolution images like this black and white image on the left of faceted pits in a sample semiconductor made of gallium arsenide and phosphorus on silicon; the image shows dark gray diamond shapes scattered on a lighter gray background. The image on the right is a color image of cathodoluminescence emission of a sample from a layer made of gallium arsenide and phosphorus on silicon; light blue areas, corresponding to dark gray areas in the image at left, are on a dark blue background.

(Left) Faceted pits found in the epitaxy of GaAsP on Si. (Right) Color imaging from the cathodoluminescence emission, revealing the redshift of the spectrum over the facets.

The upper left image is a black and white high-resolution image produced by a scanning electron microscope and shown on a computer screen; the image of a sample semiconductor material appears as a striated oval raised area on a darker background. The lower left image is a spectral image of the semiconductor material sample shown above; the image is a high-contrast light and dark oval on a dark background and was produced by cathodoluminescence; it is shown on a computer screen. Spectrograph of high-resolution image at top left of copper indium gallium selenide semiconductor material sample; the image is shown on a computer screen. Spectrograph of high-resolution spectral image shown at bottom left of a sample of copper indium gallium selenide semiconductor material; the image is shown on a computer screen.

Left image: Gray, black, and white image showing microscopic clusters of transition areas identified on the emission spectrum of a copper indium gallium diselenide thin film sample. Right image: Microscopic black and white clusters, less distinct than in the image at left, of transitions identified on the emission spectrum of a copper indium gallium diselenide thin film sample.

Photon intensity maps for different transitions identified on the emission spectrum of CIGS thin films. Grain boundaries show a distinct electronic behavior.

High-resolution color image of spectrum linescan across a heterostructure device sample of gallium indium diphosphide on gallium arsenide; the image features large red, yellow, blue, and green areas at bottom left of the linescan and smaller areas of the same colors at right.

Cathodoluminescence spectrum linescan across a GaInP₂/GaAs heterostructure. It shows a resolution better than 43 nm @2keV.