Glossary entry

Photoluminescence (PL)

Light emission from a material following optical excitation by photons. Standard characterization technique for semiconductor bandgap, defect density, and material quality.

Photoluminescence occurs when a material absorbs photons of energy $\geq$ bandgap, generating electron-hole pairs that relax (typically by phonon emission) to the band edges and subsequently recombine radiatively, emitting photons. The emission spectrum reveals bandgap energy, defect levels, and carrier dynamics.

Standard PL measurement geometry:

Component	Role
Pump laser	Above-gap excitation (e.g., 532 nm for III–V devices at 1300–1550 nm)
Beam-splitter and focusing optics	Direct pump to sample, collect emission
Long-pass filter	Block scattered pump light
Spectrometer + detector array (or monochromator + lock-in)	Resolve emission spectrum
Cryostat (for low-temperature PL)	Reveal defect transitions otherwise washed out by thermal broadening

PL spectra for III–V semiconductors typically show:

Band-edge emission at the fundamental bandgap (or at the quantum-confined energy in a quantum well)
Defect-related emission at sub-bandgap energies — broad features from deep defect levels
Phonon replicas — discrete peaks shifted below the band edge by optical phonon energies (LO phonon $\sim$ 30 meV for GaAs)
Exciton emission at low temperature — bound electron-hole pairs producing sharp features below the band gap

PL intensity vs pump power. A log-log plot of integrated PL vs pump intensity has slope $\sim$ 1 for excitonic transitions and slope $\sim$ 2 for free-carrier band-to-band recombination at moderate excitation. At very high pump power, slope $<$ 1 due to saturation of carrier capture or Auger recombination dominating.

Time-resolved PL (TRPL) measures the decay lifetime $\tau_{PL}$ after a short pump pulse:

\frac{1}{\tau_{PL}} \;=\; \frac{1}{\tau_{rad}} + \frac{1}{\tau_{nr}},

where $\tau_{rad}$ is the radiative lifetime and $\tau_{nr}$ is the non-radiative lifetime. Comparing $\tau_{PL}$ at room vs cryogenic temperature isolates the non-radiative contribution. High-quality III–V epilayers typically show $\tau_{PL} > 1$ ns at room temperature; defective material shows $< 100$ ps.

Quantum yield (internal radiative efficiency) is the integrated PL output power divided by the absorbed pump power, corrected for the photon energy ratio. High-quality InGaAs/GaAs MQW samples reach $>$ 50% internal quantum yield; mature InGaAsP/InP MQW reach 30–50% at room temperature.

PL is non-destructive and requires no electrical contacts, making it the standard quick-look technique for incoming-material inspection in III–V foundries. Electroluminescence (EL) is the analogous measurement under electrical injection rather than optical pumping and is used once contacts are fabricated.