Terahertz time-domain spectroscopy clarifies influence of nitrogen on coherent longitudinal optical phonons in GaAs1−xNx

An Osaka Metropolitan University-led research team investigated the decay time of coherent longitudinal optical (LO) phonons both in a GaAs_1−xN_x epilayer and in a GaAs single crystal, to clarify the effects of dilute nitridation. The team observed in terahertz time-domain spectroscopy that the terahertz electromagnetic waves, which are emitted from the coherent GaAs-like LO phonons, have a relatively long decay time in a GaAs_1−xN_x epilayer in comparison with the terahertz waves from the coherent GaAs LO phonons in a semi-insulating GaAs single crystal.

This implies that alloy effects (mixed crystal effects) on the phonon Raman band broadening, which have a possibility leading to the short decay time, hardly govern the decay time even in the present GaAs_1−xN_x epilayer sample. Taking account of the ultrafast photocurrent generated by femtosecond pulse illumination during the terahertz time-domain spectroscopic measurement, the researchers attribute the origin of the relatively long decay time of the coherent GaAs-like LO phonons to the suppression of the ultrafast photocurrent scattering for the coherent GaAs-like LO phonon.

The research team concludes that the main factor for the above-mentioned suppression originates from relatively heavy electron effective masses in dilute III-V nitride semiconductors, in which the lowest conduction subband has a gentle curvature owing to the band anti-crossing caused by the nitrogen energy level. The relatively heavy electron effective mass results in the reduction of transient electron velocity, and, hence, the scattering effects on the coherent LO phonon are decreased. 

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