Hole Photoemission from CdS, CdSe, HgS, and Se into Sulfur Single Crystals

Hole photoemission from CdS, CdSe, HgS, and Se into sulfur single crystals has been observed. The hole quantum efficiency ($Y$) was measured as a function of the photon energy ($\hslash \omega$) in the range 2.11-3.54 eV at room temperature. Our results fit the equation $Y^{\frac{1}{2}}=A\hslash (\omega -{\omega }_0)$ for ${\omega }_0<\omega <{\omega }_1$ and $Y^{\frac{1}{2}}={[A^2{\hslash }^2{(\omega -{\omega }_0)}^2+B^2{\hslash }^2{(\omega -{\omega }_1)}^2]}^{\frac{1}{2}}$ for $\omega >{\omega }_1$, where $A$, $B$, ${\omega }_0$, and ${\omega }_1$ are adjustable parameters. The threshold energies $h{\omega }_0$ and $h{\omega }_1$ vary from one emitter to the other as one would expect for junctions with different band-energy relations at the interface. However the difference $\hslash ({\omega }_1-{\omega }_0)=0.48\mathrm{}\pm 0.04$ eV and the relation $\frac{B}{A}=2.1\mathrm{}\pm 0.3$ seem to be independent of the emitter. These results can be understood if one identifies $\hslash ({\omega }_1-{\omega }_0)$ with the energy difference between the tops of the two overlapping valence bands of sulfur reported previously by Cook and Spear.