Photoluminescence spectroscopy is a powerful optical characterization technique used to study the electronic and optical properties of semiconductors. It involves exciting a material with photons of energy greater than its bandgap and analyzing the emitted light resulting from radiative recombination processes. The technique provides insights into band structure, exciton dynamics, defect states, and material quality.
Radiative recombination processes in semiconductors occur when excited electrons in the conduction band recombine with holes in the valence band, emitting photons. The primary mechanisms include band-to-band recombination, excitonic emission, and defect-related transitions. Band-to-band recombination produces near-band-edge emission, which is sensitive to the material's bandgap energy. Excitonic emission arises from bound electron-hole pairs (excitons), which can be either free or trapped at impurities or defects. Defect-related emissions occur when carriers recombine through intermediate energy levels within the bandgap, often yielding broad peaks at lower energies than the band-edge emission.
Excitons play a crucial role in semiconductor photoluminescence, particularly at low temperatures where thermal dissociation is minimized. In direct bandgap materials, excitons exhibit strong emission lines, while in indirect bandgap semiconductors, phonon-assisted transitions dominate. The binding energy of excitons varies significantly across materials—for example, in gallium arsenide (GaAs), it is around 4 meV, whereas in transition metal dichalcogenides like MoS2, it can exceed hundreds of meV due to quantum confinement and reduced dielectric screening.
Defect-related emissions provide critical information about impurities, vacancies, and dislocations. Common defects include donor-acceptor pairs, deep-level traps, and surface states. The spectral position and intensity of defect peaks help identify their origin and concentration. For instance, in zinc oxide (ZnO), a green luminescence band around 2.4 eV is often attributed to oxygen vacancies, while a blue emission near 2.9 eV may involve zinc interstitials.
Time-resolved photoluminescence (TRPL) measures the decay dynamics of emitted light, offering insights into carrier lifetimes and recombination pathways. Short lifetimes (nanoseconds to picoseconds) typically indicate efficient radiative or non-radiative recombination, while longer lifetimes suggest carrier trapping or low defect densities. TRPL is particularly useful for studying quantum wells, quantum dots, and perovskite semiconductors, where carrier confinement and interface effects dominate recombination kinetics.
Temperature-dependent PL spectroscopy reveals how thermal energy influences emission properties. As temperature increases, excitons dissociate, and phonon interactions broaden spectral lines. Key observations include the quenching of defect-related emissions at higher temperatures and the redshift of band-edge luminescence due to lattice expansion. Free-to-bound transitions may also become prominent as shallow donors or acceptors ionize. In wide-bandgap semiconductors like gallium nitride (GaN), temperature-dependent PL helps distinguish between donor-bound and free excitons based on their thermal activation energies.
Applications of PL spectroscopy in semiconductor quality assessment are extensive. The technique evaluates material purity, defect density, and crystalline perfection by analyzing emission linewidths, peak ratios, and spectral uniformity. Narrow linewidths indicate high crystallinity, while broad or multiple peaks suggest structural disorder or contamination. In epitaxial films, PL mapping can detect spatial variations in composition and strain.
Band structure analysis benefits from PL spectroscopy through the identification of direct and indirect transitions, exciton binding energies, and spin-orbit coupling effects. For layered materials like MoS2, the technique reveals layer-dependent electronic properties due to quantum confinement. In alloy semiconductors such as AlGaN or InGaN, PL helps determine composition by correlating emission peaks with bandgap bowing parameters.
The versatility of PL spectroscopy makes it indispensable for semiconductor research and development. By probing radiative recombination processes, exciton behavior, and defect states, the technique aids in optimizing material synthesis, device performance, and reliability. Time-resolved and temperature-dependent measurements further enhance its diagnostic capabilities, providing a comprehensive understanding of carrier dynamics under varying conditions.
In summary, photoluminescence spectroscopy serves as a fundamental tool for investigating semiconductor properties. Its ability to resolve band structure, exciton interactions, and defect emissions enables precise material characterization, essential for advancing optoelectronic and quantum technologies. The continued refinement of PL techniques ensures their relevance in both academic research and industrial applications.