Microscope Setup

The practicalities of preparing for a cathodoluminescence (CL) experiment vary between a scanning electron microscope (SEM) and a transmission electron microscope (TEM). Please refer to the appropriate section below.

For TEM

SEM

To get ready to perform a CL experiment, please take the following steps:

  1. Secure the sample to a stub suitable for the SEM; for insulating specimens, ensure the sample surface has a good electrical path to the SEM stage.

  2. Insert your sample into the microscope chamber.

  3. Set the electron microscope to the desired analysis conditions (refer to specimen-dependent recommendations).

  4. Find the region of interest you want to investigate.

  5. Insert the collection mirror after ensuring that it is safe, e.g., that the sample (or another detector) does not occupy the space that the CL mirror occupies.

  6. Turn the electron beam on and perform a basic alignment of the electron column.

  7. Align the CL detector and sample; this step is critical to achieving the highest-quality CL results (see the Alignment section).

  8. Perform final alignment of the electron microscope to help maximize image brightness and minimize image distortions.

Selecting the best SEM settings

How a user sets up the microscope is particularly important when considering CL experiments. The selection of the optimum operating parameters, such as accelerating voltage and probe current, is highly dependent on the sample(s) for analysis.

The table below provides guidance for a range of specimens. To learn why and how to optimize these settings for your specimen(s), please refer to the optimization discussion in the Mapping, Spectroscopy, and Emission Pattern portions of the How To section.

Sample type Strategy for sharper images Possible limiting factors Typical accelerating voltage (kV) Typical beam currents (nA)
Insulators Lower accelerating voltages
  • Signal-to-noise ratio
  • Sample preparation
 5 – 20 1 – 100
Semiconductors, direct band gap Lowest accelerating voltage
  • Signal-to-noise ratio
  • Sample structure
  • Minority carrier diffusion
 0.5 – 10 0.001 – 20
Semiconductors, indirect band gap Lowest accelerating voltage
  • Signal-to-noise ratio
  • Sample structure
  • Minority carrier diffusion
 5 – 30 1 – 100
Metals Higher accelerating voltages
  • Signal-to-noise ratio
  • Skin depth of material
  • Propagation of surface plasmon polaritons
 20 – 30 0.1 – 20

TEM

Before you start an experiment, the TEM must be well aligned. Perform the emitter, condenser, and optic axis adjustments typical for your TEM; this will help maximize image brightness and minimize lens aberrations.

It is important to note that the CL results that you can obtain will depend on your sample type and the accelerating voltage selected. The table below suggests TEM operating conditions for a range of specimens. 

Sample type Possible limiting factors Typical accelerating voltage (kV)
Insulators
  • Signal-to-noise ratio
  • Sample preparation
  • Cherenkov and transition radiation (at high accelerating voltages)
 40 – 120
Semiconductors, direct band gap
  • Signal-to-noise ratio
  • Sample structure
  • Minority carrier diffusion
  • Cherenkov and transition radiation (at high accelerating voltages)
 40 – 80
Semiconductors, indirect band gap
  • Signal-to-noise ratio
  • Sample structure
  • Minority carrier diffusion
  • Cherenkov and transition radiation (at high accelerating voltages)
 40 – 80
Metals
  • Signal-to-noise ratio
  • Skin depth of material
  • Propagation of surface plasmon polaritons
 200 – 300

To get ready to perform a CL experiment, please follow the below steps:

  1. Set the TEM to the desired accelerating voltage for your specimen.

  2. Load the sample into the Vulcan® holder.

  3. Set the sample so the region of interest is within the lateral focal range of the Vulcan holder's optics. This step is critical to achieving the highest-quality CL results (see the Alignment section).

  4. Plasma clean your sample using a Solarus® II plasma cleaner or similar system.

  5. Insert your sample into the column. 

  6. Find the region of interest you want to investigate.

  7. Set the TEM to an eucentric focus value and bring the sample to the eucentric height.

  8. Insert and align the hard x-ray aperture (some TEMs only).

  9. Set the TEM to scanning (STEM) mode.

  10. Perform the electron microscope's final alignment, as the aperture in the collection mirrors may have disturbed the optimum alignment conditions.