Resonance Raman scattering and scanning tunneling spectroscopy of CdS thin films grown by electrochemical atomic layer epitaxy—thickness dependent phonon and electronic properties

The thickness dependence of electron–phonon coupling and the band gap of ultrathin CdS films deposited on Au substrates using electrochemical atomic layer epitaxy (EC-ALE) have been measured by resonance Raman scattering and scanning tunneling spectroscopy (STS). Polarization dependent resonance Raman experiments indicate that electron–phonon coupling is strongly dependent on film thickness when measured using p-polarized radiation, but is essentially thickness independent when measured using s-polarized light. We find that the electron–phonon coupling reaches its limiting value at a film thickness of seven monolayers, yielding an estimated apparent exciton diameter of 2.3 nm. STS measurements of the electronic band gap of the same samples confirm this behavior. The band gap is observed to shift from its bulk value of 2.4 to >2.7 eV for a three monolayer film. The size dependence of the band gap can be described qualitatively using the strong confinement model.