Cathodoluminescence (CL) Spectroscopy Analysis Technique

Principles and Applications of CL

Cathodoluminescence (CL) is a spectroscopic technique that involves the emission of ultraviolet, visible, or infrared light due to electron transitions in the band gap or defect positions of oxide or semiconductor materials when bombarded by an electron beam. By combining CL with scanning electron microscopy (SEM), the electron beam spot size is extremely small, enabling spatial resolutions down to the nanometer level. This provides significant advantages for studying sub-nanometer defect distributions, carrier dynamics, and band structures in materials.

CL can be used to test various aspects, including: dislocation defects, carrier dynamics, interface roughness analysis, stress/strain analysis, and the detectable signal range typically falls between 200 and 800 nm, with wider ranges possible, up to 200-1600 nm.

• Spatial resolution down to 10 nm: Provides a comprehensive description of nanostructures.
• Picosecond-level time resolution: Allows analysis of carrier dynamics.
• Spectral resolution down to 0.5 nm: Studies the types and distributions of defects in semiconductor materials.

Figure 1:Schematic illustration of SEM-CL system for acquiring CL spectra

Types of CL Detectors

CL detectors are generally classified based on their installation methods, which can be categorized into flat-mount and tilt-mount configurations.

Flat-mount: The advantage lies in its high-efficiency condenser lens and light guide, resulting in high collection efficiency. However, its disadvantage is that it occupies space, which makes it impossible to use other accessories such as BSE simultaneously.

Tilt-mount: It saves space and is suitable for simultaneous CL and BSE acquisition.

Other mirror design methods include the use of parabolic mirrors, elliptical mirrors, special curved surfaces, and the addition of light guides.

What's the difference between color CL and black-and-white CL?

Color CL: After collecting fluorescence through the condenser system, the signal is separated using tricolor filters, then amplified and processed, and finally synthesized into a color image using software.

Black-and-white CL: After the sample signal is collected through the condenser system, it is directly amplified and processed into an image, with only variations in brightness.

Factors Affecting CL Testing Results

Major influencing factors include: selection of testing software parameters, SEM beam current size, sample preparation conditions, and choice of CL detector.

For example, accelerating voltage can affect the depth of surface detection in CL samples. Higher voltage can excite more short-wavelength electron transitions, thus affecting the resulting spectra.

For instance, non-conductive or weakly conductive samples may cause electron beam drift, affecting material luminescence and mapping imaging.

For example, if the beam current is too small, the luminescence signal will be lower than the background noise and there won't be peaks; if the beam current is too large, electron beam drift will occur.

The selection of testing software parameters is also important.

Applications:

• Performance and reliability of LEDs: Electron beam bombardment induces luminescence on the sample surface, measuring the average fluorescence signal over a certain period of time.

• GaN power transistors: Measuring the sample luminescence signal collection rate.

• Total dislocation density (TDD): Dislocations in the sample cause changes in luminescence performance, measuring the mapping imaging of luminescence spectra within a certain area of the sample, where dark area density represents dislocation density.

• Solar cell efficiency: Measuring the sample luminescence signal collection rate.

• Trace element detection in materials: The introduction of trace elements into the material matrix will introduce defect levels, which will cause CL spectra under electron beam bombardment. CL is extremely sensitive to trace elements, far exceeding EDS.

Sample Requirements for CL Detection Analysis

Sample Types: Semiconductor samples, mineral samples, micro/nano-device samples

Sample Preparation Requirements: For semiconductor samples, the size may vary depending on the sample stage, but generally, a diameter of 25mm*1.5mm is suitable. For mineral samples, the surface should be flattened. Non-conductive samples should be treated for conductivity. If the surface is too dirty, it can be wiped with alcohol. If the surface roughness is poor, gold or carbon coating may be applied.

Case Studies of CL

Representative SEM-CL images of zircon

Figure 2: Representative SEM-CL images of zircon

The SEM-CL and back-scattered electron (BSE) image showing part of a quartz vein from the Mokrsko-East deposit

Figure 3:The SEM-CL and back-scattered electron (BSE) image showing part of a quartz vein from the Mokrsko-East deposit

SEM and μ-CL images (orthogonal view) recorded at 5 K from an InN sample with the cup-cavities using different CL regimes

Figure 4: SEM and μ-CL images (orthogonal view) recorded at 5 K from an InN sample with the cup-cavities using different CL regimes.