Contactless microwave study of shallow traps in thin-film CdSe

The contactless microwave technique was used to measure light-induced transients in the power absorption by thin films of polycrystalline CdSe. Because the rise time of the detector was 500 ns, the analysis was limited 1 {μ}sec or longer. Measurement of these transients at a number of fixed frequencies across the ``dark'' resonance frequency made reconstruction of the difference signal possible. This signal, which represents the difference between the ``dark'' and ``light'' Lorentz resonance curves, was determined at various times during the decay. Analysis of these signals provided the changes in the real and imaginary parts of the dielectric constant as a function of time. The decays of these parameters were characterized by the sum of two exponential terms: τ₁=16.9 {μ}sec and τ₂=261.5 {μ}sec for the change in the imaginary part, and τ₁=18.5 {μ}sec and τ₂=283 {μ}sec for the change in the real part. The close agreement of these values indicates the simultaneous presence of both trapped and free electrons having identical decay times. A mechanism consistent with these results involves rapid equilibration between the free electrons and those in the two shallow traps. Decay from each trap is the rate limiting step, and the time to reach thermodynamic equilibrium must be less than 500 ns.The initial electron densities in the two traps were 2.1×{}10¹⁸ and 1.0×{}10¹⁸ cm^-3, and the corresponding concentrations of the free electrons were 3.6×{}10¹⁷ and 1.4×{}10¹⁷ cm^-3. For the free electrons, the values for the drift mobility were 2.1 and 1.8 cm²/V s, respectively. Using the harmonic-oscillator model for trapped electrons, the trap energies were 0.096 eV for the fast decay and 0.152 eV for the slower decay. The initial changes in a real part of the dielectric constant were 44.9 and 11.9, respectively. Thus, the photoinduced concentrations of electrons in naturally existing shallow traps are sufficient to effect large changes in the dielectric constant on a short time scale.