Most surface science technologies offer a wide array of particular subtechniques that offer complementary information to enhance the primary imaging mode. Electron microscopy is certainly no exception. All but the simplest electron microscopes offer a number of ports for the installation of additional analyzers. One particular type of analysis is known as cathodoluminescence (CL), which has important applications in the examination of non-metals, particularly semiconductor materials. CL can be used in both scanning electron microscopes (SEM) and tunneling electron microscopes (TEM).

In a typical SEM, an electron beam (generated by an electron gun or cathode) is directed at the surface of the sample. The interaction of the beam with the surface results in the emission of other electrons from the sample. These secondary electrons are detected and used to build up the SEM image of the surface. However, the sample may also react in other ways, for example, giving off light, or luminesce. Thus, CL is a response to the electron beam from the cathode.

Characteristics of the CL signal depend strongly on the electronic properties of the sample. While metals typically do not have a strong CL signal, non-conducting and semiconducting materials do. The electronic structures of these materials allow for the creation of holes in the electron structure, when the electron beam interacts with it. When a hole is filled with electrons, a photon is emitted, forming part of the CL signal.

A simple CL analyzer detects and measures the CL intensity, while more sophisticated systems carry out spectroscopy on the CL light. Spectroscopic systems are generally cryogenically cooled for elevated performance. Spatial resolution in CL is quite good, typically less than 100 nm.

The electronic structure of semiconducting materials is of primary importance in evaluating their performance in electronic components. Consequently, CL is of great utility in measuring the properties of novel semiconducting materials, doping and defects, as well as the effects of various environmental properties on these materials. By application, semiconductor labs make up the largest part of the demand for CL. Other applications include studies of minerals and geological specimens, and nanomaterials.

CL is not as common a technique as other analyzers used with electron microscopes, like X-ray analyzers. The market for CL analyzers was only about $10 million in 2008. The number of suppliers is relatively limited as well. Gatan and JY Horiba are the major players in CL. Gatan introduced its next-generation MonoCL4 analyzer at the Microscopy and Microanalysis Meeting in July. In addition to its own products, JY Horiba has partnered with Attolight to market a system for ultrafast time-resolved CL. Attolight’s system produces electron pulses of 10 ps in duration. Smaller players in the market include KE Developments and ETP.

CL at a Glance:

Leading Suppliers

• Gatan

• JY Horiba

Largest Markets

• Semiconductors

• Minerals

• Nanotechnology

• Academia

Instrument Cost

• $20,000–$110,000