Photoemission Study of the Electronic Structure of Wurtzite CdSe and CdS

In photoemission studies of single crystals of CdSe and CdS cleaved in vacuum, structure due to both direct transitions (k conserved) and nondirect transitions (k not important) is found. We explicitly separate the contributions to the energy distributions of the photoemitted electrons due to direct transitions from those due to nondirect transitions. By correlating structure in the energy distributions with structure in the reflectivity we determine (1) the energy of the initial and final states for the transitions causing this structure, and (2) the nature of the transitions (direct or nondirect or a combination of both). For CdSe we find that the transitions resulting in the $E_2$ reflectivity peak are direct and have initial states near -1.6 eV and final states near 5.8 eV (both with respect to the valence-band maximum). By comparing these initial and final states with the pseudopotential band structure of Bergstresser and Cohen, we find that these transitions occur at regions of the Brillouin zone around $H$ and $K$. The ${E_1}^{\prime }$ reflectivity peak is due to direct transitions from initial states near -0.9 eV to final states near 7.5 eV; however, the region of the Brillouin zone involved is not certain. Whereas we show that the $E_2$ structure is almost entirely due to direct transitions, only about 20% of the absorption near the ${E_1}^{\prime }$ peak is due to direct transitions, the other 80% being due to nondirect transitions. We suggest that the $F_3$ reflectivity shoulder is due to nondirect transitions from a peak at -1.3 eV in the valence-band optical density of states to a peak at 7.5 eV in the conduction-band effective optical density of states. The nondirect transitions from this valence-band peak at -1.3 eV (and to this conduction-band peak at 7.5 eV) are observed over a wide range of photon energy. Direct transitions from initial states near the valence-band maximum are observed for $\hslash \omega \gtrsim 10.2$ eV. This suggests that there is a $\Gamma$ conduction band near 10.2 eV. A deep valence band, tentatively identified as the cadmium $4d$ band, has been located at -9.9 eV. The results for CdS are similar, except that the relevant conduction-band states lie ∼0.5 eV higher than in CdSe.