Electroluminescence (EL)

Electroluminescence is light emission from a material caused by carriers injected via an external electrical current. Free electrons and holes are driven by the current into a region where they recombine radiatively, emitting photons whose energy matches the energy difference of the recombining states (typically the semiconductor bandgap).

EL is the operating principle of every light-emitting diode (LED) and the sub-threshold (below-threshold) emission of every semiconductor laser. It is the electrically-driven counterpart to photoluminescence, which is driven by an external optical pump.

Fundamental form. The EL spectrum from a homojunction near band-edge emission follows approximately:

$$S(\hbar\omega) \;\propto\; (\hbar\omega - E_g)^{1/2} \exp\!\left[ -\frac{\hbar\omega - E_g}{kT} \right],$$

producing a peak slightly above the bandgap with full-width at half-maximum of $\sim 1.8 kT$ — about 45 meV at room temperature. This is the characteristic spontaneous-emission spectral shape; quantum wells and quantum dots modify this through their density-of-states structures.

EL vs PL distinctions.

Property	EL	PL
Excitation source	Electrical current	Above-gap optical pump
Carrier injection profile	Set by p-n junction or contact geometry	Set by pump beam absorption profile (typically uniform over absorption depth)
Excitation spectrum	Implicit in injection — does not need to match material absorption	Must be above bandgap of material
Spatial selectivity	Confined to current-injection region	Confined to pumped region
Information content	Reveals junction quality, contact quality, AND material quality	Reveals only material quality
Sample preparation	Requires fabricated contacts	Requires only polished surface

Why EL is critical for device qualification. PL diagnoses material quality before fabrication; EL diagnoses material + processing quality after fabrication. A wafer that passes PL but fails EL has a fabrication problem (contact resistance, leakage paths, junction defects) — distinguishing this from a material problem is essential in foundry process development.

Standard EL measurement setup.

Component	Function
DC current source or pulser	Drive the device at controlled current
Microscope objective above device	Collect emitted light from top surface
Spectrometer + cooled CCD/InGaAs array	Spectrally resolve emission
Photodetector + lock-in (for low-EL signals)	Detect low-output devices
Probe needle or wire bond	Make electrical contact to top metallization

For an LED, EL is observed near steady-state at any forward current. For a laser, EL increases linearly with current below threshold and then transitions to stimulated-emission (laser) operation above threshold, where the linewidth narrows dramatically and the output power versus current slope changes by 1 – 4 orders of magnitude.

Quantum efficiency. The fraction of injected carriers that produce a photon is the internal quantum efficiency (IQE). The fraction of photons that escape the device is the extraction efficiency. The product is the external quantum efficiency (EQE) — the photons-per-electron observed at the detector.

For high-quality LED material:

Material system	EQE typical
InGaAsP / InP (1310 / 1550 nm LEDs)	0.1 – 5 %
InGaAs / GaAs (980 nm LEDs)	1 – 10 %
AlGaInP / GaAs (red LEDs, 630 nm)	10 – 30 %
InGaN / GaN (blue LEDs, 450 nm)	20 – 50 %
Si LED (forward-biased PN junction at 1100 nm)	< $10^{-4}$ (indirect bandgap)

Below-threshold EL from a laser is typically quoted as IQE rather than EQE (since most photons remain in the cavity); typical values are 60 – 95 % for mature laser epitaxy.

Applications of EL measurement.

• Pre-threshold laser characterization: extract IQE, internal loss, gain spectrum, and recombination parameters from below-threshold L-I-V and EL spectra
• LED bin testing: every LED in a high-volume production line is tested via EL for color, efficiency, and forward voltage
• EL imaging for defect detection: spatial maps of EL intensity reveal local defects, current crowding, and processing nonuniformities
• Aging diagnostics: changes in EL intensity vs current over device lifetime track degradation mechanisms
• Solar cell characterization: solar cells operated in forward bias produce EL whose intensity and spatial pattern reveal cell quality, broken contacts, and shunt defects

Reciprocity with absorption. The EL spectrum and the absorption spectrum are related by the Würfel reciprocity theorem (a generalization of Kirchhoff's law for radiation):

$$S_\text{EL}(\hbar\omega) \;\propto\; \alpha(\hbar\omega) \, \hbar\omega^2 \exp\!\left[ -\frac{\hbar\omega - qV}{kT} \right],$$

allowing prediction of EL emission lineshape from the absorption spectrum and the applied voltage.

References: Rau, Reciprocity relation between photovoltaic quantum efficiency and electroluminescent emission of solar cells, Phys. Rev. B 76, 085303 (2007); Piprek, Semiconductor Optoelectronic Devices: Introduction to Physics and Simulation (2003), Ch. 4 for the EL spectral analysis.