| Research Abstract |
In this research project we have developed a new instrument to realize a time resolved (TR) ESR (EPR) measurement in semiconductor solids such as titanium oxide for which measurement had been impossible until now. Previously, we could only observe extremely long lived radicals formed at the semi-conductor surface with CWESR spectroscopy on steady-state photo illumination. With this new instrument, we will be able to detect a short lived radicals by TRESR In conventional TRESR methods, a sudden change in the dielectric constant, which is induced by short-lived caniers formed on laser pulse in semiconductor solids, disturbs the sample cavities to reduce their sensitivity (Q-value) and to give large transient signals due to the dynamic dielectric change (DDC). We use two sample cavities ; one is for sample and the other is for reference. Two microwave signals coming from either of two cavities are mixed to be cancel out in the microwave circuit by phase matching technique. The sample cavity is located in the external magnetic field which is scanned to obtain TRESR spectra In the experiment with titanium oxide micro particle, we could lower the DDC signal to be less than 1/% and therefore the total sensitivity increased about as 1OO time large as before. We added the extemal automatic tequency controller (AFC) to exclude the effect of instability of cavities to obtain low-noise signal. Until now, the sensitivity of this instrument is too low to succeed in TRESR measurement for titanium oxide which is our initial purpose. In principle. however, it is clear that this method is generally applicable to semiconductor solids to detect short-lived paramagnetic species formed from trapping states existing in the band gap. This apparatus provides a new method which can be generally used for the detection of paramagnetic centers (defects, impurities, etc.) formed in semiconductor solids.
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