Lineshape analysis of ZnSe longitudinal optical phonons near the $E_0$ gap excitation of epitaxial $\mathrm{Zn}\mathrm{Se}∕\mathrm{Ga}\mathrm{As}$(001)

Longitudinal optical (LO) phonon lineshape of pulsed laser deposited ZnSe epitaxial films on (001) oriented GaAs with near $E_0$ gap excitation is investigated. Temperature dependence Raman data with $4579\mathrm{\AA }$ excitation show a shift of resonance from 2LO anti-Stokes to the LO Stokes in the temperature range of $150–300\mathrm{K}$. The ZnSe LO lineshape does not fit a Lorentzian profile and is characterized by the presence of large wings on both the lower and higher frequency sides of the LO phonon. The wings on the lower frequency side of the LO phonon are attributed to the Fano interaction of the LO phonon and an electronic continuum due to photogenerated carriers. The electronic continuum is observed in Stokes and anti-Stokes Raman spectra when in resonance. We have proposed an electronic transition scheme based on the presence of donor states below the conduction band to explain the observed electronic continuum. The dependence of the intensity of the continuum with the incident laser intensity is also accounted for in the same scheme. Wings at the higher frequency side of the ZnSe LO phonon are attributed to a combination phonon $[\mathrm{TO}\left(\mathit{q}\right)+\mathrm{TA}\left(\mathit{q}\right)]$ at $\sim 270{\mathrm{cm}}^{-1}$, which is also confirmed by the temperature behavior of the mode.

Figure 1

Raman spectra for different excitations with inset showing FWHM of LO phonon of ZnSe vs temperature.

Figure 2

LO lineshape and continuum (inset) for the LI and HI cases.

Figure 3

The electronic continuum (shown by arrows) and the corresponding Raman transitions for the HI and LI cases are shown in the left and right insets, respectively. SDL refers to shallow donor levels.

Figure 4

Raman spectra showing the $A$ mode, with the inset showing the Lorentzian fit to spectra at 150 and $300\mathrm{K}$.

Figure 5

Representative fit for LO phonon.