Cathodoluminescence (CL) spectroscopy is a powerful tool for investigating the optical properties of wide bandgap semiconductors such as gallium nitride (GaN), silicon carbide (SiC), and zinc oxide (ZnO). These materials are critical for high-power electronic and optoelectronic applications due to their superior thermal stability, high breakdown voltages, and efficient light emission. CL provides high spatial resolution and deep penetration, enabling detailed analysis of defect-related emissions, doping effects, and material quality, which are essential for optimizing device performance.
Wide bandgap semiconductors exhibit distinct luminescence features under electron beam excitation. In GaN, the near-band-edge (NBE) emission typically dominates the CL spectrum, appearing around 3.4 eV for undoped material. However, defect-related transitions are also prominent, particularly the yellow luminescence (YL) band centered near 2.2 eV, attributed to gallium vacancies or carbon impurities. The blue luminescence (BL) band around 2.8 eV is often associated with donor-acceptor pairs involving zinc or silicon dopants. Doping significantly alters the CL response; for instance, magnesium-doped GaN shows a broad emission near 3.0 eV due to transitions involving Mg acceptors.
SiC displays a complex CL spectrum influenced by its polytypism. The 4H-SiC polytype, widely used in power devices, exhibits NBE emission at approximately 3.3 eV. Defect-related bands include the D1 center near 1.8 eV, linked to carbon vacancies, and the L1 band around 2.0 eV, associated with silicon vacancies. Nitrogen doping introduces donor-bound exciton peaks near the NBE, while aluminum doping creates acceptor-related transitions. The intensity ratios of defect peaks to NBE emission serve as indicators of crystal quality, which is crucial for high-voltage applications where defects can degrade device reliability.
ZnO has a strong NBE emission at 3.37 eV, corresponding to free exciton recombination. The green luminescence (GL) band near 2.4 eV is commonly observed and often attributed to oxygen vacancies or zinc interstitials. The red luminescence (RL) band around 1.8 eV is associated with oxygen-related defects. Doping with group-III elements like aluminum or gallium enhances n-type conductivity and modifies the CL spectrum by increasing NBE intensity while suppressing deep-level emissions. Conversely, p-type doping with nitrogen or phosphorus introduces new acceptor-related transitions, though achieving stable p-type ZnO remains challenging.
The spatial resolution of CL allows mapping of defect distributions and inhomogeneities in these materials. For example, dislocations in GaN often act as non-radiative recombination centers, visible as dark spots in CL images. In SiC, stacking faults and micropipes produce distinct CL contrast, revealing structural imperfections that can affect device breakdown characteristics. ZnO nanowires and thin films exhibit variations in defect emissions depending on growth conditions, which can be correlated with electrical properties.
Applications of wide bandgap semiconductors in high-power devices benefit from CL insights. GaN-based high-electron-mobility transistors (HEMTs) require low defect densities to minimize current leakage and improve breakdown performance. CL mapping helps identify regions with high defect concentrations that could lead to premature device failure. SiC power diodes and MOSFETs rely on high-purity substrates with minimal deep-level defects to ensure efficient operation at high temperatures and voltages. CL analysis provides feedback for optimizing growth and processing conditions to reduce detrimental defects.
ZnO is promising for ultraviolet (UV) optoelectronic devices, but defect-related emissions can degrade efficiency. CL studies guide efforts to minimize deep-level defects through improved growth techniques or post-growth treatments. In light-emitting diodes (LEDs) and laser diodes based on these materials, CL helps assess radiative efficiency and identify non-radiative loss mechanisms.
The table below summarizes key CL emissions in GaN, SiC, and ZnO:
| Material | NBE Emission (eV) | Defect-Related Bands (eV) | Common Defects |
|----------|-------------------|---------------------------|----------------|
| GaN | 3.4 | 2.2 (YL), 2.8 (BL) | V_Ga, C impurities |
| SiC | 3.3 | 1.8 (D1), 2.0 (L1) | V_C, V_Si |
| ZnO | 3.37 | 2.4 (GL), 1.8 (RL) | V_O, Zn_i |
In conclusion, cathodoluminescence spectroscopy is indispensable for understanding the optical and electronic properties of wide bandgap semiconductors. By analyzing defect-related emissions and doping effects, CL provides critical feedback for material development and device optimization. As demand for high-power and high-frequency devices grows, CL will remain a vital tool for advancing the performance and reliability of GaN, SiC, and ZnO-based technologies.