Microwave Transient Photoconductivity Studies in Porous Semiconductors
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Microwave Transient Photoconductivity Studies in Porous Semiconductors Horia-Eugen Porţeanu, Elisaveta Konstantinova1, Vladimir Kytin1, Oleg Loginenko, Victor Timoshenko1, Thomas Dittrich, and Frederick Koch T.U. München, Physik-Department E16, Garching, GERMANY 1 perm. address: Lomonosov Moscow State University, Moscow, RUSSIA. ABSTRACT The dynamics of the photogenerated carriers in porous silicon and TiO2 anatase was studied at 35 GHz by measuring the change in time of the conductivity σ and dielectric constant εr. Localization of carriers leads to a positive change of εr, while quasifree carriers to a negative change. Size reduction in Si shortens the recombination time as long as the surface traps are not significant. Magnetic field investigations show opposite variation of conductivity in porous silicon compared with TiO2. INTRODUCTION Microwave complex conductivity measurements of semiconductors traces its history from the ’60 [1]. In 1969 Hartwig and Hinds [2] determined the change in time of the complex conductivity in CdS putting in evidence carrier trapping and photodielectric effects, with a time resolution of one minute. Microwave reflection investigations with a time resolution of few ns have been done in ’80 [3]. However, the change in the reflection coefficient could not resolve the contribution of the real and imaginary part of conductivity. Grabtchak and Cocivera proposed in ’94 an original method to measure the transient complex photoconductivity with a time resolution of 1 µs [4]. They evidenced the photodielectric effect due to shallow traps in CdSe. We developed a similar method, working at 35 GHz, low temperatures, in magnetic field up to 1.4 T and with a time resolution of 50 ns. The aim of this work was to investigate the dynamics of photogenerated carriers in porous silicon and TiO2 anatase. APPARATUS AND PRINCIPLE OF ACQUISITION The transient complex photoconductivity is measured with a setup presented in the Fig. 1. It consists mainly of a phase locked loop microwave oscillator on 35.1 GHz, a cylindrical cavity running in the TE011 mode, inserted in a He-bath cryostat, a nitrogen laser giving pulsed light of 5 ns, 0.5 mJ at 337 nm, and a digitizing oscilloscope. Essential for this measuring method is the frequency stability of the oscillator, ∆f/f≈10-10. The sample is placed with help of a small quartz tube in the center of the cavity. The empty cavity (with the quartz holder) has a Q-factor of about 7000 at 300 K and 11000 at 4 K. Most of the low temperature measurements were done at 7 K in order to avoid the condensation of He inside the cavity. A special software commands the setup. It is recorded first a number of resonance curves in dark, then there are measured transients (small changes in time of the microwave amplitude) at 50 frequencies around the resonance (see Fig. 1 bottom). Knowing the changes of the dark resonance curve at different times, one reconstruct mathematically the resonance curves as a function of time. Using a special Lorentz-fit procedure, it is possible afterward
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