Raman study of optical phonons in ultrathinInAs∕InPsingle strained quantum wells

By means of Raman scattering, photoluminescence, and x-ray diffraction experiments, we have studied a series of ultrathin $\mathrm{In}\mathrm{As}∕\mathrm{In}\mathrm{P}$ single quantum wells grown with nominal well thickness varying between one and four monolayers. The observed Raman peaks are attributed to the first InAs-confined longitudinal optical phonon. When the quantum well is thinner than three monolayers, the measured frequencies of the confined modes depart from the predictions of the standard confined-phonon model but are in good agreement with the results of ab initio calculations based on density-functional theory. This indicates that there is a significant penetration of the InAs layer vibrations in the InP barriers and that thickness fluctuations occur at a scale short enough to further shift the phonon frequencies. When the wells contain more than three monolayers, a doublet Raman structure is observed which parallels the evolution of the photoluminescence spectra from single- to multiple-peak emission. We associate this simultaneous change in Raman and photoluminescence spectra with the two- to three-dimensional growth-mode transition.