Determination of the exciton energy from electron beam excited luminescence in direct gap semiconductors

The exciton radiative recombination spectra of CuCl, ZnSe, CuGaS₂ and AgGaSe₂ have been investigated at 300 K. The sample was excited by electron beams of different energies. The reabsorption is determined by the ratio η of the spectra obtained for different electron beam energies such as 5 and 40 kV. This ratio is independant of hv when the absorption coefficient α is small (< 10³cm⁻¹). It increases when α becomes larger than the inverse of the penetration depth ( 3 μ) of the 40 kV electron beam. In this case, η is approximately proportional to the absorption coefficient. In particular, its maxima give the excitonic energies E_ex. We find the following values, in agreement with previous reflectivity and absorption measurements: for CuCl, E_ex(Z₃) = 3.26 eV and E_ex(Z₁₂) = 3.33 eV: for ZnSe, E_ex = 2.70 eV; for CuGaS₂, E_ex(A) = 2.46 eV and E_ex(B) = 2.59 eV. In the case of AgGaSe₂, there is no peak in the η spectrum and we have fitted our data to a broadened exciton model. We obtain E_ex(A) = 1.80 eV.