Observation of local-interfacial optical phonons at buried interfaces using time-resolved second-harmonic generation

Abstract

We have used an all-optical technique to observe optical phonons localized at a buried interface. The technique is based on coherent excitation of the interfacial vibrational mode by femtosecond laser irradiation and detection of the free-induction decay of the coherent oscillation by time-resolved second-harmonic generation. For native oxide-covered GaAs ~100!, we observe a single-optical phonon mode at 8.4860.04 THz localized to a few monolayers on the semiconductor side of the interface. The assignment is based on changes in the phonon spectrum as a function of pump-laser intensity and during in situ oxidation. The mode frequency shifts from 8.48 to 8.29 THz due to coupling with holes driven to the interface by the depletion field. @S0163-1829~99!00619-0# Despite the importance of understanding and controlling the structure and electronic properties of interfaces between dense media ~buried interfaces!, few experimental probes are sensitive to the local properties of the few monolayers of atoms arranged differently at the interface than they are arranged in the bulk. Vibrational ~phonon! spectroscopy provides an important microscopic probe of structure, bonding, and dynamics. Coherent studies of surface acoustic-wave velocities have been reported, 1 however, optical phonons in the range 3‐10 THz are more sensitive to local ~atomic-scale! geometry because these modes involve counter-propagating atomic motion analogous to vibrational modes in molecules. Optical techniques such as Fourier transform infrared ~IR! spectroscopy, free-electron laser-based IR spectroscopy, and visible-IR sum-frequency generation and Raman spectroscopy have been developed and demonstrated for adsorbate molecules at vacuum and gas/solid interfaces. In principle, extension to buried interfaces is straightforward if at least one side of the interface is optically transparent. However, IR phonon or plasmon absorption in the surrounding bulk media prevents all but Raman from being generally applicable to buried interfaces. Raman spectroscopy is used to measure high-frequency vibrational modes of adsorbate molecules at liquid/solid interfaces and bulklike phonon features at buried interfaces. Despite the almost routine application of Raman, only one case of monolayer-scale local phonons at buried interfaces has been reported: spontaneous Raman 2 and resonance Raman spectroscopy of Sb monolayers on GaAs. 3 Helium atom scattering and high-resolution electron-energyloss spectroscopy ~HREELS! in the impact regime have submonolayer sensitivity to surface adsorbates. In the longrange dipole interaction regime, HREELS has been used to