Mapping of the efficiency of electron-hole pair separation for a semiconductor electrode: Luminescent properties of graded cadmium sulfoselenide electrodes

Inhomogeneous samples of n-type CdS(X)Se(1-X) (0 or = X or = 1) have been prepared by vapor-phase diffusion of S into a single-crystal CdSe substrate. Characterization by Auger electron spectroscopy (AES) in conjunction with Ar ion sputter etching reveals that S was substituted for Se in the lattice to produce a graded region. The depth profile analysis indicates a composition of CdS over the first approx. 0.05 micron from the surface; over the next approx. 2 micron from the surface, the composition monotonically deceases in X, corresponding to a decline in band gap energy from approx. 2.4 eV for CdS to approx. 1.7 eV for CdSe. Photoluminesence (PL) and electroluminescence (EL) from the graded material appear to derive from the luminescence of the CdS(X)Se(1-X) compositions which comprise the graded region. Emission from approx. 470-750 nm matches the spectral region spanned by PL and EL from homogeneous, single crystal CdS(X)Se(1-X) samples which emit near their band gap energies. This assignment is also supported by chemical etching experiments in which the removal of S-rich, near-surface strata progressively red-shifts the PL spectra.