Photoluminescence spectroscopy is a powerful non-destructive technique for probing the electronic structure and defect states in semiconductors. By analyzing the emitted light following photoexcitation, researchers can identify intrinsic and extrinsic defects, quantify their concentrations, and assess their impact on material properties. The method is particularly effective for studying bulk crystals and thin films, where defect-related transitions produce distinct spectral signatures.

When a semiconductor absorbs photons with energy greater than its bandgap, electrons are excited from the valence band to the conduction band. These excited carriers relax through various pathways, including radiative recombination, which produces photoluminescence. Defects introduce additional energy levels within the bandgap, creating alternative recombination channels. The resulting spectra contain peaks corresponding to band-to-band transitions as well as defect-related emissions, allowing for detailed defect characterization.

Deep-level traps, vacancies, and impurities each produce characteristic photoluminescence features. Deep-level traps, often caused by transition metals or intrinsic lattice defects, create broad emission bands due to strong electron-phonon coupling. For example, in gallium arsenide, chromium impurities introduce a deep acceptor level approximately 0.8 eV above the valence band, producing a luminescence peak near 0.8 eV at low temperatures. Similarly, silicon vacancies in silicon carbide generate distinct emission lines at 1.35 eV and 1.40 eV, corresponding to different charge states of the defect.

Vacancies and interstitials typically produce sharper luminescence lines compared to deep-level traps because they involve less lattice distortion. In zinc oxide, oxygen vacancies are associated with a green emission band centered around 2.45 eV, while zinc vacancies contribute to a blue emission near 2.85 eV. The relative intensities of these peaks provide a semi-quantitative measure of vacancy concentrations, though absolute quantification requires calibration against other techniques like electron paramagnetic resonance.

Impurities introduce discrete energy levels that can be identified through their spectral fingerprints. Donor-acceptor pair transitions produce a series of closely spaced lines whose energy depends on the separation between the pairs. In cadmium telluride, chlorine donors and sodium acceptors generate a series of peaks between 1.45 and 1.55 eV, with the exact positions shifting slightly due to varying pair distances. Isoelectronic impurities, such as nitrogen in gallium phosphide, create bound exciton complexes with narrow emission lines whose positions are well-documented for common semiconductor systems.

Quantitative analysis of photoluminescence spectra involves several steps. Peak deconvolution is performed using Gaussian or Voigt profiles to separate overlapping contributions from different defects. The integrated intensity of each peak is proportional to the defect concentration, though this relationship depends on the recombination efficiency and carrier capture cross-sections. Temperature-dependent studies are crucial for distinguishing between different defect types, as thermal quenching behavior varies significantly between shallow and deep levels. For instance, shallow impurity-related emissions typically quench at lower temperatures compared to deep-level emissions due to the smaller thermal activation energy required for carrier escape.

Time-resolved photoluminescence adds another dimension to defect characterization by measuring recombination lifetimes. Defects with large capture cross-sections cause rapid non-radiative recombination, shortening the observed lifetime. In high-quality gallium nitride, the near-band-edge emission exhibits lifetimes exceeding 1 ns at room temperature, while defect-related yellow luminescence shows sub-nanosecond decay due to efficient non-radiative pathways. Lifetimes can also reveal the presence of metastable defects that exhibit slow capture or emission kinetics.

The excitation power dependence of photoluminescence provides additional insights into defect properties. At low excitation densities, defects with large capture cross-sections dominate the spectrum. As the power increases, higher-energy transitions become more prominent as the defect states become saturated. This behavior helps distinguish between competing recombination channels and can reveal the presence of defect complexes that only become active at certain carrier concentrations.

Thin films present unique challenges for photoluminescence analysis due to interference effects and strain-induced modifications of the band structure. Optical interference creates oscillations in the baseline photoluminescence intensity, which must be accounted for during spectral analysis. Strain splits degeneracies in the band structure, leading to additional peaks or peak broadening. In strained silicon-germanium alloys, the photoluminescence spectrum shows a shift in the direct transition energy proportional to the germanium content and strain level, typically following the relationship ΔE = k·x, where x is the germanium fraction and k ranges between 80-100 meV per unit strain depending on the crystal orientation.

Surface recombination significantly impacts photoluminescence in thin films, where the high surface-to-volume ratio enhances non-radiative losses. Passivation techniques, such as sulfur treatment for gallium arsenide or silicon nitride capping for silicon, can reduce surface recombination velocities from over 1,000 cm/s to below 100 cm/s, dramatically improving the photoluminescence yield. The effectiveness of passivation is directly observable in the spectrum through increased near-band-edge emission intensity and reduced defect-related background.

Advanced analysis methods combine photoluminescence with other characterization techniques for comprehensive defect identification. Correlation with deep-level transient spectroscopy data allows matching optical transitions with specific trap levels measured electrically. Secondary ion mass spectrometry provides elemental identification of impurities that may be responsible for observed luminescence features. This multi-technique approach is essential for unambiguous defect assignment, particularly in materials with complex defect landscapes like wide bandgap semiconductors.

In summary, photoluminescence spectroscopy serves as a versatile tool for semiconductor defect analysis, offering both qualitative identification and quantitative assessment of deep-level traps, vacancies, and impurities. The technique's sensitivity to electronic transitions, combined with temperature, time, and power-dependent measurements, provides a detailed picture of defect states in bulk and thin-film materials. While spectral interpretation requires careful consideration of material-specific parameters and potential artifacts, properly executed photoluminescence analysis yields invaluable insights for semiconductor research and development.