## Photoluminescence (PL) Characterization **Photoluminescence (PL)** is a non-destructive optical technique used to study the electronic and optical properties of semiconductors. It involves exciting a material with a light source and analyzing the emitted light resulting from radiative recombination of electron–hole pairs. It is used to - Bandgap engineering and composition analysis (e.g., GeSn alloys) - Defect and impurity level identification - Quality assessment of epitaxial layers and quantum wells - Evaluation of carrier dynamics with time-resolved PL ### Basic principle 1. **Excitation**: The sample is illuminated with a laser or other light source (above-bandgap energy). 2. **Carrier Generation**: Electrons are excited from the valence band to the conduction band, creating electron–hole pairs. 3. **Radiative Recombination**: These carriers relax and recombine, emitting photons. 4. **Detection**: The emitted light (photoluminescence) is collected and spectrally analyzed. ### Measured parameters - **Peak position (energy or wavelength)**: Indicates the bandgap or transition energies. - **PL intensity**: Reflects the radiative recombination efficiency. - **Full-width at half-maximum (FWHM)**: Provides information about disorder, inhomogeneity, or temperature effects. - **Spectral shape**: Can reveal defect states or indirect transitions. ### Temperature-Dependent PL Temperature-dependent PL is widely used to: - Determine whether a material has a direct or indirect bandgap. - Study bandgap shrinkage with temperature. - Analyze carrier localization, phonon interactions, and non-radiative processes. The typical behavior is listed in the following chart. | Property | Direct Bandgap | Indirect Bandgap | |------------------------|----------------------|-------------------------| | PL intensity (low T) | Strong | Weak or absent | | PL intensity (high T) | Decreases moderately | Rapidly quenched | | Temperature shift | Smooth redshift | Often similar behavior | | Recombination type | Radiative | Phonon-assisted | #### Varshni Equation The temperature dependence of the bandgap is often modeled using the **Varshni equation**, which is described in the famous paper [[Temperature dependence of the energy gap in semiconductors]]: $ E_g(T) = E_g(0) - \frac{\alpha T^2}{T + \beta} $ Where: - $E_g(T)$: Bandgap energy at temperature $T$ - $E_g(0)$: Bandgap at 0 K - $\alpha$, $\beta$: Empirical fitting parameters ### Typical experimental setup - **Excitation Source**: Laser (e.g., 532 nm or 785 nm) - **Cryostat**: Enables measurements from ~10 K to 300 K or higher - **Spectrometer**: Resolves PL emission spectrum - **Detector**: CCD or $\ce{InGaAs}$, depending on emission wavelength >[!Notice] > - PL is **surface-sensitive**; surface passivation can improve signal. > - Complementary to other techniques like absorption, Raman, or TRPL. > - In indirect semiconductors, PL may be detectable only at low temperatures or with high excitation powers.