Glossary entry

Quantum well

A thin semiconductor layer (5–20 nm) sandwiched between higher-bandgap barrier layers, confining carriers in one dimension. The standard gain medium for modern semiconductor lasers.

A quantum well is a thin semiconductor layer (typically 5–20 nm thick) of low-bandgap material sandwiched between two thicker higher-bandgap barrier layers. The confinement potential in the thin direction is comparable to the carrier de Broglie wavelength, producing quantized energy levels rather than a continuous band:

E_n \;=\; \frac{n^2 \pi^2 \hbar^2}{2 m^* L_w^2},

where $n = 1, 2, 3, \ldots$ is the subband index, $m^*$ is the effective mass, and $L_w$ is the well width.

Real wells have finite barriers, so the spectrum approaches the bulk continuum at higher energies; only the lowest few subbands are typically occupied at room temperature.

Carrier-confinement effects:

Effect	Bulk	Quantum well
Density of states	$\rho \propto \sqrt{E}$	Step function (constant within each subband)
Effective bandgap	Bulk value	Increased by confinement energy
Transition strength	Volume-averaged	Concentrated at subband edges
Polarization selection	None	TE/TM coupling differs strongly

The step-function density of states is the key advantage for laser applications — gain rises more steeply with carrier density than in bulk, giving lower threshold current and higher differential gain. Higher differential gain also produces shorter relaxation oscillation period and higher modulation bandwidth.

Multiple quantum well (MQW) structures stack 3–10 wells separated by thin barriers (10–20 nm), producing optical confinement across the active region while preserving quantum-well carrier confinement. MQW is the dominant active-region design for modern semiconductor lasers and modulators.

Standard material systems and emission wavelengths:

Quantum well composition	Barriers	Emission $\lambda$
InGaAs / GaAs	AlGaAs	940 – 1100 nm
InGaAsP / InP	InGaAsP (wider gap)	1300 – 1550 nm
InGaAlAs / InP	InAlAs or InGaAlAs	1300 – 1550 nm
AlGaAs / GaAs	AlAs	750 – 870 nm
GaN / InGaN	AlGaN	405 – 470 nm (blue/violet)
InGaN (high In)	GaN	510 nm (green, low efficiency)

InGaAlAs/InP MQW devices typically have higher characteristic temperature than InGaAsP/InP because the AlInGaAs system has deeper confinement of holes, reducing thermal carrier escape from the well.

Quantum-confined Stark effect (QCSE) describes the modulation of MQW absorption by applied electric field — the active mechanism in electro-absorption modulators.