Spin dependent phenomena and magnetic resonance studies of point defects in silicon and silicon structures
1. In general, spin dependent recombination (SDR) is the well known physical phenomenon when the recombination rate of non-equilibrium electrons and holes in semiconductors excited by light or injected in p-n junctions depends on the relative spin orientations of carriers and recombination centers. Spin orientation can be changed by excitation of electron paramagnetic resonance (EPR) increasing the recombination rate and, consequently, decreasing the conductivity of samples. The most of experiments on EDMR were performed using standard EPR spectrometers for excitation of magnetic resonance whereas the EPR signals were detected monitoring dc-current through the sample. In the present work the contact free method was used for detection of SDR-EPR spectra of recombination centers. This method is based on the absorption of the electrical component of microwave field by free carriers whereas the magnetic component in microwave cavity is used for magnetic resonance excitation.
2. It was shown that the detection EPR spectra of recombination centers in silicon using the spin dependent microwave photoconductivity has four orders higher sensitivity compare to the traditional EPR spectroscopy. The developed methods can be applied for monitoring, diagnostic and investigations of different defects at their low concentration created by fabrication processes and during the operation of semiconductor devices.
3. Physical mechanisms of spin dependent recombination of photo excited (or injected in p-n junctions) carriers via the excited triplet (spin S=1) states of recombination centers were investigated and new SDR detected spin S=1 EPR spectra were found. Particularly, new SDR-EPR spectra of low symmetry modifications of well known complexes of oxygen-vacancy (O+V) and carbon related centers in silicon were observed under annealing of irradiated silicon in the temperature range of 350 - 500 oC. The models of low symmetry configurations of the considered centers were suggested.
4. The excited triplet states of recombination centers are metastable and have relative long life time, up to milliseconds, and form the energy levels in silicon band gap. Analyzing the temperature dependences of SDR-EPR spectra The energy level positions of EC - 0.095 eV and EC – 0.25 eV were estimated for the excited triplet states of the O+V and carbon related centers, respectively.
5. The processes of spin SDR included the creation of the excited triplet states of recombination centers were considered in the space-charge layer of silicon p-n junctions. The SDR-EPR spectra were detected monitoring the forward current through p-n junctions without illumination. It was shown that paramagnetic recombination centers and their excited triplet states can be detected even without excitation of magnetic resonance when sharp lines of the conductivity changes are observed at the points of magnetic field corresponding to the anticrossing of magnetic sublevels or to the coincidences of Zeeman frequencies of different paramagnetic recombination centers.
6. SDR effects were applied for study the surface recombination centers in commercially produced silicon wafers. It was found that such centers are always observed even without any additional treatments of surface. New centers on (100) oriented wafers were found and investigated. Analysis of their SDR-EPR spectra allows us to suggest the model of new centers as the closed pairs of dangling Si bonds in Si/SiO2 layer. It was found that such parameters of new spectra as a line width, line positions, fine and hyperfine structure splittings depend on the thickness of SiO2 layer. These parameters are changed gradually during 10-12 days after removal SiO2 by chemical etching in fluoride acid and following slow surface oxidization on air at room temperature.