Heterojunction

A heterojunction is the interface between two crystalline semiconductors with different compositions and (typically) different bandgaps. The discontinuity in band alignment at the interface creates potential steps for electrons and holes that can be engineered for carrier confinement, transport, and recombination control.

Band alignment types. When materials with bandgaps $E_{g1}$ and $E_{g2}$ are joined, the bandgap difference $\Delta E_g$ is split between conduction-band ($\Delta E_C$) and valence-band ($\Delta E_V$) discontinuities. Three alignment regimes exist:

Most III–V active-region heterojunctions are Type I. Common examples:

Double heterostructure (DH) lasers. A thin narrow-gap active layer sandwiched between two wider-gap cladding layers forms a double heterostructure. The heterojunctions:

1. Confine carriers in the active region (both electrons from the n-cladding and holes from the p-cladding accumulate in the narrow-gap layer because they cannot escape the band offsets)
2. Confine light through the index difference between active and cladding (narrow-gap material has higher refractive index, forming a slab waveguide)
3. Enable independent doping of the cladding regions for current injection without absorbing the emitted light

This separation of carrier confinement, optical confinement, and current pathways was the foundational architectural advance that made room-temperature continuous-wave semiconductor lasers practical (Alferov / Kroemer, 1970, Nobel Prize 2000).

Lattice matching. The two materials of a heterojunction should have nearly identical crystal lattice constants ($\Delta a / a < 0.1$ %) for defect-free epitaxial growth. Mismatched layers accumulate strain energy and form misfit dislocations beyond the critical thickness. Common lattice-matched systems:

• GaAs / AlAs / Al$_x$Ga$_{1-x}$As: nearly identical lattice constants for all $x$
• InP / In$_{0.53}$Ga$_{0.47}$As / In$_{0.52}$Al$_{0.48}$As: lattice-matched at specific compositions
• Si / Si$_{1-x}$Ge$_x$: lattice mismatch up to 4 %; used in strained-silicon transistors

Strained heterojunctions intentionally use lattice-mismatched layers thinner than the critical thickness. Compressive strain reshapes the valence band structure to reduce hole effective mass, increasing differential gain and reducing Auger recombination. This was a key technique enabling efficient long-wavelength telecom diode lasers in the 1990s.

Quantum wells and superlattices are arrays of thin heterojunctions stacked at the de Broglie wavelength scale, producing quantum confinement effects in addition to the classical heterojunction band offsets.