Temperature plays a critical role in photoluminescence (PL) spectroscopy, influencing carrier dynamics, exciton behavior, and defect-related emissions in semiconductor materials. By varying temperature, researchers gain insights into material properties such as bandgap tuning, thermal quenching, and defect states. This article explores the mechanisms behind temperature-dependent PL and the experimental setups used to study these effects.
At the core of temperature-dependent PL is the interaction between thermal energy and charge carriers. As temperature increases, lattice vibrations (phonons) become more pronounced, leading to enhanced electron-phonon coupling. This coupling affects the recombination pathways of excited carriers, altering the PL spectrum. At low temperatures, carriers are more localized, and phonon-assisted processes are minimized, resulting in sharper emission peaks. Conversely, higher temperatures introduce broadening due to increased phonon scattering and thermal dissociation of excitons.
Excitons, bound electron-hole pairs, exhibit strong temperature dependence. In direct bandgap semiconductors, free excitons dominate at low temperatures, producing narrow emission lines. As temperature rises, excitons gain enough thermal energy to dissociate into free carriers, reducing excitonic emission intensity. In materials with strong exciton-phonon coupling, such as perovskites or quantum dots, exciton binding energies can be directly extracted by analyzing the thermal quenching behavior of PL peaks.
Defect-related emissions also show pronounced temperature dependence. Shallow defects, such as donor-acceptor pairs, often exhibit thermal ionization at relatively low temperatures, leading to a decrease in defect-related PL intensity. Deep-level defects, however, may persist to higher temperatures due to larger activation energies. Temperature-dependent PL can thus be used to map defect energy levels and their relative concentrations within the bandgap.
Experimental setups for temperature-dependent PL vary depending on the temperature range of interest. Cryogenic systems, operating between 4 K and 300 K, are essential for studying excitonic behavior and shallow defects. Closed-cycle helium cryostats or liquid nitrogen cryostats are commonly used, providing precise temperature control. For high-temperature PL measurements (300 K to 1000 K), resistive heating stages or furnace-based systems are employed. These setups require careful calibration to avoid blackbody radiation interference, especially above 600 K.
Bandgap tuning is a key application of temperature-dependent PL. In many semiconductors, the bandgap decreases with increasing temperature due to lattice expansion and electron-phonon interactions. This shift, known as the Varshni effect, can be quantified by tracking the PL peak energy as a function of temperature. For example, in GaAs, the bandgap decreases by approximately 0.4 meV/K near room temperature. In contrast, some materials, like ZnO, exhibit anomalous behavior due to competing effects between thermal expansion and electron-phonon coupling.
Thermal quenching is another critical phenomenon studied via temperature-dependent PL. The PL intensity typically decreases with rising temperature due to enhanced non-radiative recombination. By fitting the quenching curve using the Arrhenius model, activation energies for non-radiative pathways can be extracted. For instance, in InGaN quantum wells, thermal quenching analysis reveals defect-related activation energies ranging from 20 meV to 100 meV, depending on material quality.
High-temperature PL measurements are particularly useful for evaluating material stability and performance under operational conditions. Wide-bandgap semiconductors like GaN and SiC are often studied at elevated temperatures to assess their suitability for power electronics. Similarly, perovskite semiconductors exhibit complex thermal degradation pathways that can be probed via temperature-dependent PL, providing insights into their long-term stability in solar cells.
In summary, temperature-dependent PL spectroscopy is a powerful tool for investigating semiconductor properties. By analyzing how thermal energy influences carrier dynamics, exciton behavior, and defect emissions, researchers can extract critical material parameters and optimize device performance. Cryogenic and high-temperature PL setups enable a broad range of studies, from fundamental exciton physics to applied materials engineering. The ability to correlate temperature-dependent PL features with material characteristics makes this technique indispensable in semiconductor research.