Optical Absorption Model for Direct-Bandgap Semiconductors
The optical absorption edge is a key property for determining the optical emission spectrum in direct-bandgap III-V semiconductors. It reflects the influence of numerous properties including the joint optical density of states, the optical transition strength, and the presence of localized tail states at the band edges. Precise and repeatable measurement of the absorption edge is crucial for assessing material quality and provides insight into the density of states and transition probabilities in the material. Specifically, the critical parameters include the fundamental bandgap energy, the magnitude of the absorption coefficient at the bandgap energy, and the characteristic width of the Urbach tail (that embodies the manifestation of localized states near the band edges due to lattice disorder). Examining the absorption coefficient in terms of these model parameters provides insight into the optical joint density of states, the optical transition strength, and Coulomb enhancement of the optical transition strength. Existing models typically treat interband and tail state absorption separately, which can hinder the extraction of the bandgap energy from the absorption coefficient spectrum.
Researchers at Arizona State University have developed a five-parameter model that describes the key characteristics of the optical absorption edge of direct bandgap semiconductors. Among these parameters are the bandgap energy, the characteristic energy of the Urbach tail, and the absorption coefficient at the bandgap energy. The model provides highly accurate and repeatable measurements of the bandgap energy in addition to providing insight into material quality via the Urbach energy, which quantifies the impact of sub-bandgap tail states, and the absorption knee amplitude. The model is simple and easily applied to any direct-gap semiconductor, enabling direct comparison between materials systems.
• Direct-bandgap semiconductors (including GaAs, GaSb, InAs, and InSb)
• Semiconductor modeling
Benefits and Advantages
• Can be used to model any direct-bandgap semiconductor
• Enables highly accurate and repeatable measurement of bandgap energy
• Makes no assumptions about the energy dependence of the dipole and momentum matrix elements