In a direct band gap material, the valence band maximum (VBM) and conduction band minimum (CBM) occur at the same point in reciprocal space (the same k-vector). In an indirect band gap material, the VBM and CBM are at different k-points.
Practical Implications
- Direct gap materials (GaAs, CdTe, halide perovskites): Efficient light absorption and emission. Preferred for LEDs, lasers, and solar cells because photons can directly excite electrons across the gap.
- Indirect gap materials (Si, Ge, diamond): Weaker optical absorption near the band edge because a phonon (lattice vibration) is needed to conserve crystal momentum. Silicon solar cells work despite this, but require thicker absorber layers (~200 um vs. ~1 um for direct-gap materials).
How to Check
MatCraft displays "Direct" or "Indirect" on each material's detail page. In the band structure plot, a direct gap shows VBM and CBM at the same k-point, while an indirect gap shows them at different positions along the k-path.