Measuring cathodoluminescence at deep ultraviolet wavelengths

Gatan instrument used

The Monarc^® Pro system is equipped with UV-grade optics and detectors allowing for the measurement of light down to 185 nm.

Background

Many materials of scientific and commercial interest emit light at ultraviolet (UV) wavelengths when
excited by an electron beam. Examples include ultrawide bandgap semiconductors used for power
electronics, optoelectronics, or photodetectors. Cathodoluminescence (CL) can be a powerful technique
for such materials and can be used to study defects, strain, and compositional variations on the
nanoscale. However, the detection of UV photons can be challenging, requiring specialized optics and
detectors. Consequently, many standard CL systems are unable to measure photons of wavelengths
<300 nm efficiently, refer to Fig. 1. Here we examine how the Monarc Pro system is ideal for working in
the UV regime.

CL systems use optical components to collect and transfer light emitted from a sample to analysis
hardware. It is imperative that transmissive optical components (e.g., fibers, vacuum windows and
lenses) have low absorption that can be achieved using high quality optics of an appropriate UV grade.
However, the optical throughput cannot be optimized for all wavelengths due to the strong variation
in refractive index of transmissive optics which introduce chromatic aberration and reduces the light
entering an optical spectrometer. To overcome this, some systems require the user to exchange transfer optics—and realign the optical system—between UV and visible wavelengths. However, the Monarc Pro system uses chromatic aberration-free reflective optics for spectroscopic analysis preserving high optical throughput from deep UV to infrared wavelengths.

For spectroscopic analysis charge coupled devices (CCD) are capable of measuring a complete spectrum rapidly. However, conventional (front illuminated) CCDs offer poor responsivity at UV wavelengths due to absorption of the light prior to reaching the active region of the sensor. Back illuminated CCDs offer >96% efficiency at visible wavelengths but still suffer from poor responsivity— close to zero—at wavelengths <300 nm due to the low absorption of the UV light within the (thin) active region. The Monarc Pro maximizes efficiency at visible wavelengths by using a back illuminated device but overcomes the deficiencies at short wavelengths by using a UV enhanced coating that provides >30% quantum efficiency at wavelengths below 300 nm.

Photomultiplier tubes (PMTs) can also be used as detectors for CL, and are capable of producing high-resolution images in rapid times. These detectors may be limited by the vacuum windows or the photocathode materials employed. Vacuum windows are a concern for CL measurements in general, as the chambers of SEMs must be operated under high vacuum. Therefore, any light we collect must pass through a vacuum window or optical fibers at some stage. In The Monarc Pro, system, the vacuum window is fabricated from CaF that offers outstanding transmission from 150 – 2,500 nm while the PMT selected offers high detection efficiencies above 185 nm.