Accurate determination of homogeneous and inhomogeneous excitonic broadening in ZnO by linear and nonlinear spectroscopies

Excitonic parameters of ZnO have been determined from 5 to 300 K using the combination of time-integrated four-wave mixing, continuous wave reflectivity, autocorrelation of reflectivity at the femtosecond scale, and photoluminescence spectroscopy. These techniques allow an accurate determination of both inhomogeneous and homogeneous linewidths, oscillator strength, and other parameters characteristic for excitonic resonances. It turns out that, due to impurity-exciton scattering, the homogeneous damping of A and B excitons (0.55 and 1.35 meV, respectively) predominates over inhomogeneous one in high-quality ZnO at low temperature. Our data also prove that the strong exciton-phonon coupling enhances the excitonic broadening at room temperature up to 47 meV. This approach, which combines various sophisticated spectroscopy experiments, can be employed for an accurate determination of excitonic parameters in other semiconductor compounds and in any system where excitons are confined.

Figure 1

Ti-FWM signal of ZnO bulk sample recorded at 5 K. The pulse energy is 3380 meV, its linewidth is 14 meV, the excitation density per pulse is 0.2 nJ cm${}^{-2}$.

Figure 2

CW-R spectrum of a ZnO bulk sample recorded under normal incidence at 5 K. Solid black line corresponds to the experimental result and red dashed one is the CW-R simulation using a Gaussian local model. The arrows locate the different excitonic transitions.

Figure 3

On the left appears the AR spectrum. Results obtained at 5 K with a pulse energy of 3377 meV (black) and calculated (red). Top of right figure, the dielectric function (real and imaginary parts) is plotted to determine the LT splitting of A exciton. At the bottom, the PL spectrum recorded at 5 K. The vertical dashed lines identify transverse and longitudinal modes.

Figure 4

(a) Experimental Ti-FWM (solid line) and the results of calculations using Eq. (3) (dashed line). (b) Temperature dependence of CW-R, in solid black line the experiment and in red dashed line the calculation with a Gaussian local model.

Figure 5

Evolution of homogeneous broadening vs temperature. The average homogeneous broadenings between A and B obtained by Ti-FWM are represented by red lozenges and those achieved from CW-R are reported through green circles. The dashed line is the linewidth evolution following Segall's law.