Effect of n- and p-type Doping on Coherent Phonons in GaN

TL;DR

This study investigates how n- and p-type doping influence coherent phonons in GaN, revealing doping-dependent phonon dephasing and amplitude changes driven by carrier-plasmon interactions and surface effects.

Contribution

It demonstrates the role of doping in modulating coherent phonon dynamics in GaN, highlighting the contributions of TDFS and ISRS mechanisms under sub-band gap excitation.

Findings

Doping enhances A1(LO) phonon dephasing via plasmon coupling.

Doping increases the coherent A1(LO) phonon amplitude relative to E2.

Surface Franz-Keldysh effect enables interband transitions below the band gap.

Abstract

Effect of doping on the carrier-phonon interaction in wurtzite GaN is investigated by pump-probe reflectivity measurements using 3.1 eV light in near resonance with the fundamental band gap of 3.39 eV. Coherent modulations of the reflectivity due to the E2 and the A1(LO) modes, as well as the 2A1(LO) overtone are observed. Doping of acceptor and more so for donor atoms enhances the dephasing of the polar A1(LO) phonon via coupling with plasmons, with the effect of donors being stronger. Doping also enhances the relative amplitude of the coherent A1(LO) phonon with respect to that of the high-frequency E2 phonon, though it does not affect the relative intensity in Raman spectroscopic measurements. This enhanced coherent amplitude indicates that transient depletion field screening (TDFS), in addition to impulsive stimulated Raman scattering (ISRS), contribute to generation of the coherent polar phonons even for sub-band gap excitation. Because the TDFS mechanism requires photoexcitation of carriers, we argue that the interband transition is made possible at the surface with photon energies below the bulk band gap through the Franz-Keldysh effect.