Infrared Wavelengths

Extending cathodoluminescence detection to infrared wavelengths

Gatan instrument used

The Monarc^® Pro cathodoluminescence (CL) system, equipped with detectors and gratings optimized for high performance at infrared wavelengths, offers the most complete analysis of cathodoluminescence emission, extending the operational range up to 2,300 nm.

Background

CL systems are typically designed to provide optimum sensitivity and analysis capabilities for light emitted at or close to visible wavelengths. The design choices made in the selection of transfer optics, spectrometer configuration, and detectors determine the range of wavelengths that can be analyzed; low-performance systems may be limited to wavelengths between 400 – 800 nm, greatly limiting the scope of applications. High-performance systems such as the Monarc Pro system use optics and detectors that extend analysis capabilities to wavelengths from 200 – 1,100 nm. However, this still limits their potential to analyze many interesting materials and devices that emit light at longer wavelengths, including photovoltaic cells made of copper indium gallium selenide (CIGS) or optoelectronic devices based on group III– arsenide compound semiconductors. For this reason, we offer additional detection solutions to extend sensitivity up to 2,300 nm.

In standard detectors, IR photons may fail to produce the required electron-hole pairs or photoelectrons due to their low energy. The reduced absorption in active layers can also lead to etaloning. As a result, detecting infrared photons demands unique sensor designs and improved cooling to minimize dark noise.

Materials and methods

A Monarc Pro (model 450.P) fitted with an infrared detector (P/N 450.P.U3.2-IR), an appropriate diffraction grating, was used to analyze an InGaAs heterostructure. A layer of quantum dots has been intentionally formed within this structure to create a device capable of delivering efficient optical emissions at telecommunication wavelengths for quantum communication applications. In the figure below, the 1,000 nm emission shows a relatively uniform background as expected; however, at 1,430 nm, bright dots and larger, more diffuse regions can be identified.

Image showing how cathodoluminescence detection is extended into infrared ranges

Figure 1. Wavelength-filtered images acquired at 3 kV shows the origin of different emission wavelengths, with a 1,000 ± 10 nm filter on the and 1,430 ± 10 nm on the right.

Infrared cathodoluminescence performance chart comparing photon count rates across CL detection systems

Figure 2. The chart displays the operational wavelength ranges for various IR detector configurations on Monarc Pro.

Summary

The Monarc Pro system can be configured to operate effectively at wavelengths up to 2,300 nm. In the example analysis shown here, we uncovered the distribution and emission wavelengths of highly localized emission centers and provided critical feedback on the epitaxial process.

Credit(s)

Special thanks to the University of Sheffield for generously providing the specimen.