CdSe Thin Film

Here is an abstract from an article in J.Appl.Phys.79 (2), 1996, p.p.786-793

CONTACTLESS MICROWAVE STUDY OF DISPERSIVE TRANSPORT IN THIN FILM CdSe

Serguei Yu. Grabtchak and Michael Cocivera*

Guelph-Waterloo Centre for Graduate Work in Chemistry

University of Guelph

Guelph, Ontario, Canada N1G 2W1

ABSTRACT

The contactless microwave technique was used to measure light-induced transients in the power absorbed by thin films of polycrystalline CdSe. Because the rise time of the microwave cavity was 60 ns, the analysis was limited 100 ns 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 time dependence for the changes in the real and imaginary parts of the dielectric constant, which correspond to the densities of the trapped and free electrons. The decays of these parameters were characterized by three time domains. At the shortest times, the two parameters did not have the same time dependence. At intermediate times, the densities of both the trapped and free electrons had the same time dependence characterized by a power law decay, and a mechanism consistent with these results involves rapid equilibration between the free electrons and those in the shallow traps. Decay in this region was consistent with a dispersive transport mechanism. Intensity effects indicate saturation of the shallow traps. The third region occurred at the break in the power law dependence indicating a bimolecular recombination process. Measurements at higher temperatures indicate a change from a bimolecular to a monomolecular recombination mechanism.

The AMTMP revealed a never observed phenomena in CdSe, a dispersive transport. In contrast to TOF (time-of-flight) and PC (transient photocurrent) methods the motion of charge carriers through the sample as whole was not a necessary condition for a dispersive transport to be observed in the AMTMP. The conduction band electrons observed were in a thermal equilibrium with shallow traps. It was another distinction from above mentioned methods where the studied traps are deeper and the thermal equilibrium is not established during the experiment.

The corresponding time dependence of the light-induced shift of the resonance frequency (cyan) and the change of the bandwidth (read) (related to the cavity quality factor) are presented as solid curves. Both curves demonstrates a gradual transition from t^(a-1), corresponding to the thermalization process to ((t^-1) lnt) behavior corresponding to a bimolecular recombination with initial non-uniform excitation. The fast component is absent in the shift of the resonance frequency. This indicates that the initial fast process involves conduction band electrons only (while the same time dependence means that the thermal equilibrium is established between conduction band and trapped electrons). The square root intensity dependence of the amplitude of the fast component allows to connect it with a direct band-to-band electron-hole recombination.

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