Optical properties of some complex defects in silicon

Abstract: lnfrared absorption spectroscopy has been used study transitions between electronic states related to some complex defects in silicon. Three different defects have been studied: a complex defect involving carbon as one of its constituents, a divacancy, and a defect found in silicon containing tin. All defects studied were created by means of irradiating the silicon crystal at room temperature with 2.0 MeV electrons. The experimental method used in the study was Fourier Transform lnfrared (FTIR) spectroscopy.The study is focused on the electronic structure of defects and the coupling between the electronic states and vibrational states. Furthermore, the formation of the divacancy in silicon is found to very likely be formed by the pairing of single vacancies for irradiation of silicon with 2.0 MeV electrons at room temperature.A previously unknown absorption spectrum of a defect in silicon is presented. The absorption spectrum is interpreted as being due to the transitions between a non-degenerate ground state of the defect to hydrogen-like single-particle electron states. The final states of the transition are found to be s­ states. Transitions to p-states are not observed. The final electronic states are referred to as pseudodonor states. The term "pseudo" is used to emphasize that only the final states of the transitions can be referred to as donor states. The ionization energy of the excited and final pseudo-donor s-states with large amplitudes at the defect site (A1-symmetry), is found to be much larger than that for s-states with a node or a small amplitude at the defect site. The s-states with a node at the defect site are found to have ionization energies that are close to the energies estimated with the effective mass approximation. A comparison is made of this new spectrum with spectra related to other defects in silicon, and principal similarities are found that indicate that excited electron states of pseudo-donor character are common for defects in silicon. The defect is also found to be metastable. A change in configuration of the defect is observed via the increase in absorption of the spectrum of the defect versus time of optical excitation at temperatures of the crystal below 65K. The spectrum is found to disappear when the sample is heated in darkness at temperatures exceeding 70K. This is interpreted as being a thermally-induced change from the metastable to the stable configuration of the defect. The activitation energy for the disappearence of the absorption spectrum is found to be 0.21 eV. An Auger recombination of excitons at the defect is found to be the most likely process for initiating the change in configuration of the defect.Furthermore, carbon is found to be one of the constituents of this complex defect. Uniaxial stress measurements have been performed and the effect of the stress on the spectrum is found to be in agreement with the interpretation of the excited states of the defects as being due to a pseudodonor.