A Novel Approach to Semiconductor Electrical Properties - The Advanced Method of Transient Microwave Photoconductivity (
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Guelph-Waterloo Centre for Graduate Work in Chemistry University of Guelph, Guelph, Ontario, Canada NIG 2WI ABSTRACT
The advanced method of transient microwave photoconductivity (AMTMP) represents a new method based on cavity perturbation theory, microwave photoconductivity and harmonic oscillator model analysis. AMTMP provides a direct observation of changes to the complex dielectric constant, and free and trapped electron decays can be studied separately. The results obtained for polycrystalline CdSe thin films clearly indicate that a multiple trapping model developed for amorphous materials does not provide a satisfactory description. For SI GaAs the harmonic oscillator model provides a quantitative interpretation. The limitations are discussed for the application of this method to porous Si. INTRODUCTION
Perturbation theory, developed by Slater [1], relates changes in cavity parameters directly to c' and E", the real and the imaginary parts of the dielectric constant, and it was used for steadystate measurements [2]. Transient microwave photoconductivity (TMP), on the other hand, assumes
the signal is proportional to the light induced changes in the conductivity [3, 4]. The interpretation of the kinetics in this case was based on a single measured decay that was assumed equal to 5s" [5], which may not be valid.
To avoid this assumption, we have developed the advanced method of transient microwave photoconductivity (AMTMP) [6-9], which includes three major components (cavity perturbation theory, microwave photoconductivity and harmonic oscillator model analysis). In this method two separate kinetic decays are registered: the bandwidth and frequency shift of the resonance signal. These parameters can be related to 6e' and 8E" by treating the light induced changes as a second
perturbation. As was shown earlier [9], the harmonic oscillator model can be used to relate S,' and 8E " to photogenerated free and trapped electrons quantitatively. In the present paper we outline briefly the major theoretical aspects of AMTMP and present experimental results obtained for various samples (two types of polycrystalline CdSe thin films (CdSe(I) and CdSe(II)), SI (semiinsulating) GaAs, porous Si). The transient behavior is compared with the carrier transport model used for amorphous semiconductors. THEORY Second Cavity Perturbation Theory Insertion of a semiconductor or dielectric material into a microwave cavity (resonator) causes a perturbation, i.e. a change in the resonance frequency,f 0, and the cavity quality factor, QL. Cavity perturbation theory, which relates these changes to real and imaginary parts of the complex dielectric constant, ," = e' - jf", of the material, was developed by Slater [1]. A second perturbation of cavity parameters occurs when a sample already in the cavity is subjected to photons,
electric current, temperature, X-rays etc. to produce excess electrons and cause a change in the complex dielectric constant, 8c* =WS'-fi". This second perturbation is applied to AMTMP. 57 Mat. Res. Soc. Symp. Pro
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