Monochromatic Cathodoluminescence Image in the Scanning Electron Microscope

Abstract

A TECHNIQUE has been developed for observing monochromatic Cathodoluminescence from semiconductors in the scanning electron microscope, enabling optical micro-analysis of materials to be carried out in a manner analogous to the electron probe X-ray microanalyser. The experimental arrangement is shown in Fig. 1. A three lens system gives a beam of electrons of 30 keV energy focused to a spot of 500 Å diameter at the specimen with a current density between 1 and 100 A cm⁻². The specimen, in the form of a thin platelet, is mounted on a small movable liquid nitrogen cold finger, and is cooled to 100° K during observation. The Cathodoluminescence is recorded on one of two channels—a sample of the total emitted light is collected by a quartz light pipe, the signal being fed through a photomultiplier and amplifier to the display system of the microscope, to build up a scanning micrograph. Simultaneously an image of the specimen is focused on to the entrance slit of a prism spectrometer by means of a quartz lens in the vacuum wall of the system. The output signal from the spectrometer may then be fed by means of a second photomultiplier and a preamplifier into the detecting system of the microscope, in place of the signal from the total emitted luminescence, thus enabling a scanning micrograph to be built up from radiation of any desired wavelength. (Alternatively the complete emission spectrum may be observed by feeding the amplifier output on to a chart recorder and driving the spectrometer as in conventional optical spectroscopy.)