Cathodoluminescence microscopy is a powerful analytical technique for investigating the optical properties of semiconductors with high spatial resolution. By using a focused electron beam to excite luminescence in a material, it provides insights into electronic transitions, defect states, and nanoscale heterogeneities. The technique is particularly valuable for studying low-dimensional semiconductor structures such as quantum wells and nanowires, where local variations in composition, strain, and defects significantly influence optoelectronic performance.

The fundamental principle of cathodoluminescence involves the generation of electron-hole pairs when a high-energy electron beam interacts with a semiconductor. These carriers recombine radiatively, emitting photons with energies corresponding to the material's bandgap or defect-related transitions. The emitted light is collected and analyzed spectrally, spatially, or temporally to extract information about the sample's electronic structure. Unlike photoluminescence, which relies on optical excitation, CL offers superior spatial resolution due to the smaller interaction volume of the electron beam, typically on the order of nanometers to micrometers depending on the beam energy and material properties.

Depth-resolved cathodoluminescence is a critical capability for probing buried interfaces and layered structures. By varying the electron beam acceleration voltage, the penetration depth of the electrons can be controlled, allowing selective excitation of different regions within a sample. Higher beam energies result in deeper carrier generation, enabling the study of subsurface features without physical cross-sectioning. For instance, in a multilayer quantum well structure, depth-resolved CL can distinguish emissions from individual wells by adjusting the beam energy to confine excitation to specific layers. This approach is also useful for investigating carrier diffusion and interface quality in heterostructures.

Panchromatic imaging in CL microscopy involves mapping the total luminescence intensity across a sample without spectral resolution. This mode is particularly effective for visualizing defects, grain boundaries, and strain variations that quench luminescence locally. Panchromatic images reveal spatial inhomogeneities in radiative efficiency, which can be correlated with structural or compositional variations observed through complementary techniques like electron microscopy. For example, in GaN-based materials, panchromatic CL imaging can identify threading dislocations as dark spots due to non-radiative recombination at these defects.

Defect mapping using cathodoluminescence relies on the sensitivity of luminescence to point defects, dislocations, and impurities. By analyzing the spectral signatures of defect-related emissions, CL can identify and localize specific defect types within a semiconductor. In wide bandgap materials like ZnO or GaN, deep-level emissions often appear at energies below the bandgap and can be mapped to assess defect distributions. This capability is crucial for optimizing growth conditions and improving device performance, as defects act as non-radiative recombination centers or charge traps. Quantitative analysis of defect densities can be achieved by correlating CL intensity variations with known defect concentrations from calibrated samples.

Quantum wells represent an important application area for cathodoluminescence microscopy due to their nanoscale thickness and sensitivity to interface quality. CL provides direct access to the optical transitions between quantized energy levels in the wells, allowing measurements of well width fluctuations and alloy inhomogeneities. Spatially resolved CL spectra can reveal variations in emission energy and intensity across a quantum well structure, which may arise from local variations in composition or strain. Time-resolved CL further enables the study of carrier dynamics, including recombination lifetimes and tunneling between wells. These measurements are essential for designing efficient light-emitting devices and laser diodes.

Nanowires present unique challenges and opportunities for cathodoluminescence analysis. Their one-dimensional geometry and high surface-to-volume ratio make them sensitive to surface states and defects. CL microscopy can resolve emission variations along the length of a nanowire, identifying regions with different doping levels, crystal phases, or surface recombination velocities. In core-shell nanowires, depth-resolved CL can distinguish emissions from the core and shell layers, providing insights into strain relaxation and interfacial defects. The technique is also valuable for studying nanowire heterostructures, where axial or radial variations in composition create localized potential profiles that influence carrier transport and recombination.

The combination of spectral imaging and high spatial resolution makes cathodoluminescence indispensable for characterizing semiconductor nanostructures. Advances in detector sensitivity and electron beam technology continue to push the limits of resolution and signal-to-noise ratio, enabling studies of single quantum dots or individual defects. Future developments may integrate CL with in-situ biasing or temperature control to study devices under operating conditions. As semiconductor technologies increasingly rely on nanoscale engineering, cathodoluminescence microscopy will remain a vital tool for understanding and optimizing material properties at the relevant length scales.

Practical considerations for CL measurements include optimizing beam current and acceleration voltage to balance spatial resolution and signal intensity. Beam-induced damage can be mitigated by using lower currents or rastering the beam to distribute exposure. Careful calibration of the detection system is necessary for accurate spectral measurements, particularly when comparing absolute intensities across different samples or instruments. For quantitative defect analysis, reference samples with known defect concentrations provide a basis for interpreting CL intensity maps.

In summary, cathodoluminescence microscopy offers unparalleled capabilities for spatially and spectrally resolved analysis of semiconductor materials. Its applications in quantum wells and nanowires highlight its importance for advancing optoelectronic and quantum technologies. By providing direct access to nanoscale optical properties, CL bridges the gap between structural characterization and functional performance, enabling deeper understanding and better engineering of semiconductor devices.